Structure–Activity Relationship Studies of Hybrid Antitumor
STRUCTURE–ACTIVITY RELATIONSHIP STUDIES OF HYBRID ANTITUMOR
AGENTS FOR THE TREATMENT OF NON-SMALL CELL LUNG CANCER
BY
ZHIDONG MA
A Dissertation Submitted to the Graduate Faculty of WAKE FOREST UNIVERSITY GRADUATE SCHOOL OF ARTS AND SCIENCES
in Partial Fulfillment of the Requirements
for the Degree of
DOCTOR OF PHILOSOPHY
Chemistry
December 2009
Winston–Salem, North Carolina
Approved By:
Ulrich Bierbach, Ph.D., Advisor ______
Examining Committee:
James P. Vaughn, Ph.D. (Chair) ______
Rebecca Alexander, Ph.D. ______
Christa L. Colyer, Ph.D. ______
S. Bruce King, Ph.D. ______
ACKNOWLEDGEMENTS
The pursuit of a Ph.D. degree is a challenging journey, filled with success, failure,
friendship, loneliness, happiness, and frustration. It took me five years of my Ph.D. to
understand the significance of this degree. Reading journals, discussing with colleagues
and thinking independently in the field of chemistry and biomedical research helped me
understand medicinal chemistry and its great contributions to health care and society.
Therefore, I really want to thank my dissertation advisor, Professor Ulrich Bierbach, not
only for teaching me the detailed technical skills everyday in the past five years, but also leading me to explore a new world of medicinal chemistry. Every time I had a question, he is always there to help me out. I really appreciate his “tough love” as my advisor
helping me build my scientific career and his “soft love” as my friend helping me through
my Ph.D. years.
I would also like to acknowledge my committee members, Professor James
Vaughn, Professor Rebecca Alexander, Professor Christa Colyer, and Professor Bruce
King, for reading my dissertation and giving me constructive suggestions during my Ph.D.
study. I would also like to thank Dr. Cynthia Day for her great help with crystallography,
and Dr. Marcus Wright for his patient in teaching me NMR spectroscopy.
Our collaborators helped me new techniques and allowed me to perform my
experiments in their labs. I want to show my appreciation to our collaborators Dr.
Bradley Jones, Carl Young and Dr. Keith Levine at Research Triangle Institute for
allowing me to perform the ICP-MS experiment, Dr. Gregory L. Kucera and Gilda Saluta
at Wake Forest University Baptist Medical School for teaching me how to perform
cytotoxicity assays, and Dr. Trevor Hambley, Dr Nicole Bryce, and Alice Klein at The
I University of Sydney for the opportunity to perform confocal fluorescence microscopy studies in their lab.
My past and present lab members are really precious to me. We have shared so much memorable time together. Thank you Dr. Rajsekhar Guddneppanavar, Dr. Jayati
Roy Choudhury, Lauren Eiter, Lu Rao, Leigh Ann Graham, Amanda Pickard, Partha
Choudhury, Gary Wilson, Chandra Vemulapalli, Tony DeMartino, Bart Steen, Josh
Dworkin, Jennifer Nguyen. Without your company, it would have been painful to be alone in the lab.
My friends also play a very important role during my time in Wake Forest. We always had a lot of fun together. I would like to thank my friends and chemist fellows
Michael Budiman, Subhasis De, Jiyan, Maben, Liqiong, Xiuli, Lei, Ranjan, Saurva,
Salwa, Erika and Veronica.
The funding was also important to me, without which it would have been impossible for me to continue my research. I would like to thank the Richter Scholar
Award, the National Cancer Institute (Grant RO1 CA101880) and Wake Forest
University for supporting my Ph.D. work.
II
This dissertation is dedicated to my beloved grandma, father, mother and brother.
III TABLE OF CONTENTS
Page
List of Tables VIII
List of Figures X
List of Abbreviation XVII
Abstract XXI
Chapter 1 Introduction 1
1.1 Cancer: Lung Cancer 1
1.1.1 Cancer Statistics 1
1.1.2 Non-Small-Cell Lung Cancer 1
1.2 Chemotherapy of Lung Cancer 2
1.2.1 DNA-targeted Cytotoxic Agents 3
1.2.2 Targeted Therapies 17
1.3 Platinum Antitumor Agents 18
1.3.1 Bifunctional Platinum Crosslinkers 18
1.3.2 Monofunctional Platinum Complexes 24
1.3.3 Other Non-classical Platinum Agents 25
1.3.4 Platinum–Intercalator Conjugates 28
1.4 Background and Significance of the Doctoral Research Program 30
1.4.1 The Development of PT-ACRAMTU 30
1.4.2 Synthesis of PT-ACRAMTU 32
1.4.3 Biological Activity and DNA Binding Mechanism 33
1.4.4 Goals of the Doctoral Research Program 35
IV Chapter 2 Synthesis, Characterization, and Biological Evaluation of
Guanidine Derivatives of ACRAMTU
2.1 Background 39
2.2 Experimental Section 43
2.2.1 Materials and Chemicals 43
2.2.2 Syntheses and Characterizations of Guanidine Derivatives 43
2.2.3 Crystallization and Data Collection 48
2.2.4 Biological Characterization: pKa Determinations, DNA
Binding Affinities and Cell Proliferation Assay 48
2.3 Results and Discussion 50
2.3.1 Two Unusual Reactivity Features of the
9-Aminoacridine Chromophore 50
2.3.2 Biological Evaluation: pKa Determinations, DNA
Binding Constants, and Cytotoxicities 57
2.3.3 Attempted Conjugation of ACRAMGU (38) with
Monofunctional Platinum 60
2.4 Conclusions 61
Chapter 3 Synthesis, Characterization, and Biological Evaluation of
Amidine Derivatives of PT-ACRAMTU
3.1 Background 63
3.2 Experimental Section 64
3.2.1 Materials and Chemicals 64
V 3.2.2 Synthesis and Characterization 65
3.2.3 Cell Proliferation Assay 69
3.2.4 Crystallization and Data Collection 70
3.2.5 NMR Spectroscopy 70
3.2.6 Liquid Chromatographic Separation–Mass
Spectrometry Detection 71
3.2.7 Confocal Fluorescence Microscopy 72
3.2.8 H460 Xenograft Study 73
3.2.9 Inductively Coupled Plasma–Mass
Spectrometry Detection of Platinum 74
3.3 Results and Discussion 75
3.3.1 Lewis Acid Assisted Addition Reaction 75
3.3.2 Design Rationale 77
3.3.3 Biological Activity 79
3.3.4 Kinetics of Platinum Binding with
Biologically Relevant Nucleophiles 81
3.3.5 Analysis of Platinum–Amino Acid Products 92
3.3.6 Determination of the Subcellular Localization of
Complex 54 99
3.3.7 Antitumor Activity and Systemic Toxicity 106
3.3.8 Distribution of Platinum in Murine Tissue Samples 108
3.4 Conclusions and Clinical Implications 112
VI Chapter 4 Changing the Connection Site of the Linker in
PT-ACRAMTU from the 9-Position to the 4-Position
4.1 Background 116
4.2 Experimental Section 117
4.2.1 Materials and Methods 117
4.2.2 Synthesis and Characterization 118
4.2.3 Biological Characterization: pKa Determinations
and Cell Proliferation Assay 121
4.3 Results and Discussion 123
4.3.1 Synthesis of Acridine-4-Carboxamides 123
4.3.2 Acid–Base Chemistry 124
4.3.3 Cytotoxicities 125
4.4 Conclusions 126
Chapter 5 Summary and Outlook 129
References 132
Appendix A 155
Appendix B 172
Appendix C 189
Scholastic Vita 191
VII LIST OF TABLES
Page
Table 1. Cytotoxicity of PT-ACRAMTU (31) in NSCLC. 33
Table 2. Summary of Chemical and Biological Characterization for
38, 40 and 43. 58
Table 3. Cytotoxicity Data (IC50, μM) for Platinum–Acridine Conjugates. 80
Table 4. Platinum Concentrations in Various Biological Tissues
(Tumors, Kidneys and Lungs) at Two Different Doses
(0.1 mg/kg and 0.5 mg/kg) Determined by ICP–MS
after 16 Hours of Tissue Digestion. 111
Table 5. Retention of Platinum Per Single Organ Calculated as
a Percentage of the Total Administered Doses (0.1 mg/kg
and 0.5 mg/kg) Determined by ICP–MS after 16 Hours of
Tissue Digestion. 112
Table 6. Acid–Base Properties and In Vitro Cytotoxicity (IC50, μM)
of Acridines and Platinum Acridines in HL-60 and NCI-H460
Cell Lines. 125
Table B1. Summary of X-ray Crystallographic Data for 1-[2-(Acridine-9-
ylamino)- ethyl]-1,3-dimethylguanidine Dihydrochloride
(ACRAMGU·2HCl), 38. 172
Table B2. Bond Lengths [Å] and Angles [°] for 1-[2-(Acridine-9-
ylamino)-ethyl]-1,3-dimethylguanidine Dihydrochloride
(ACRAMGU·2HCl), 38. 173-175
VIII Table B3. Summary of X-ray Crystallographic Data for (Z)-N-
[1-(Acridin-9-yl)-3-methylimidazolidin-2-ylidene]methanaminium
Hydrochloride, 40. 176
Table B4. Bond Lengths [Å] and Angles [°] for (Z)-N-
[1-(Acridin-9-yl)-3-methylimidazolidin-2-ylidene]methanaminium
Hydrochloride, 40. 177
Table B5. Summary of X-ray Crystallographic Data for 2′-
(1H-Imidazol-1-yl)-8′-methyl-7′,8′-dihydro-6′H,10H-spiro-
[acridine-9,4′-imidazo[1,2-a][1,3,5]triazine], 42. 178
Table B6. Bond Lengths [Å] and Angles [°] for 2′-(1H-Imidazol-
1-yl)-8′-methyl-7′,8′-dihydro-6′H,10H-spiro-[acridine-
9,4′-imidazo[1,2-a][1,3,5]triazine], 42. 179-180
Table B7. Summary of X-ray Crystallographic Data for 1-(Acridin-
9-yl)-3-methyl- imidazolidin-2-imine Hydrobromide, 43. 181
Table B8. Bond Lengths [Å] and Angles [°] for 1-(Acridin-
9-yl)-3-methyl-imidazolidin-2-imine Hydrobromide, 43. 182
Table B9. Summary of X-ray Crystallographic Data for
[Pt(EtCN)(C19H22N4)Cl2], 50. 183
Table B10. Bond Lengths [Å] and Angles [°] for
[Pt(EtCN)(C19H22N4)Cl2], 50. 184-185
Table B11. Summary of X-ray Crystallographic Data for
[PtCl(en)(C19H23N4)](NO3)2, 54. 186
Table B12. Bond Lengths [Å] and Angles [°] for
IX [PtCl(en)(C19H23N4)](NO3)2, 54. 187-188
Table C1. Platinum Concentrations in Various Biological Tissues
(Tumors, Kidneys and Lungs) at Two Different Doses
(0.1 mg/kg and 0.5 mg/kg) Determined by ICP–MS
After 16 Hours of Digestion. 189
Table C2. Masses of Various Biological Tissues (Tumors, Kidneys and Lungs). 190
LIST OF FIGURES
Page
Figure 1. Timeline: The history of chemotherapy. 3
Figure 2. Watson–Crick base pairs (GC and AT) with the major
and minor grooves highlighted. 4
Figure 3. Classification of DNA–interactive agents and their
molecular interaction with DNA. 6
Figure 4. Ethidium, doxorubicin and echinomycin are examples of a
simple intercalator, complex intercalator and polyfunctional
intercalator, respectively. 7
Figure 5. Natural products found as DNA binders. 9
Figure 6. Minor groove hydrogen bonding patterns of Watson–Crick
base pairs. 10
Figure 7. The pyrimidine triple helix motif. 11
Figure 8. Chemical structure of a PNA. 12
Figure 9. The development of N-mustard-based cytotoxic agents. 13
X Figure 10. Monoalkylation and cross-linking chemistry of alkylating agents. 14
Figure 11. Structure of bleomycin. 15
Figure 12. DNA degradation products after bleomycin-induced
strand cleavage. 17
Figure 13. Mechanism of action of cisplatin. 19
Figure 14. Binding modes between cisplatin and DNA
(intrastrand and interstrand cross-links) 20
Figure 15. The current platinum drug “family tree” and generalized
rationale underlying their development. 22
Figure 16. Examples of bioactive monofunctional platinum complexes. 25
Figure 17. Interstrand cross-links for cisplatin (left)
and transplatin (right). 26
Figure 18. Structure of the trinuclear platinum complex BBR3464. 27
Figure 19. Schematic drawing of a platinum–intercalator conjugate. 28
Figure 20. Structures of platinum–intercalator conjugates. 29
Figure 21. Design of PT-ACRAMTU. 31
Figure 22. Synthesis of PT-ACRAMTU. 32
Figure 23. Potential reaction pathways of Pt–guanine adduct formation. 34
Figure 24. Structure of ACRAMTU and its guanidine analogue. 39
Figure 25. Slow decomposition of PT-ACRAMTU due to nucleophilicity
of coordinated thiourea sulfur. 41
Figure 26. Mechanistic implications of the kinetic cis-effect. 42
Figure 27. First attempt to generate ACRAMGU (38) by desulfurating 37
XI with phosgene which leads to unexpected cyclized compound 40. 51
Figure 28. Second attempt to generate ACRAMGU (38) by sequential
replacement of imidazole in 41 by 36 and methylamine, which
leads to unexpected spiro compound 42. 52
Figure 29. Third attempt to generate ACRAMGU (38) using cyanogen
bromide, followed by addition of methylamine, which leads to
cyclized compound 43. 53
Figure 30. Synthetic scheme for ACRAMGU, 38. 54
Figure 31. Proposed mechanism of formation of 42. 55
Figure 32. Crystal structures of 40 (chloride salt, left) and
43 (bromide salt, right). 56
Figure 33. Crystal structures of 42 (left) and protonated 38
(right, anions not shown).. 57
Figure 34. pH titrations for compounds 38 and 40 monitored
by UV-visible spectroscopy (pH range 2-12). 58
Figure 35. Reactivity of ACRAMGU (38) with activated [Pt(en)Cl2]. 60
Figure 36. Alternative strategies for generating platinum–imino
analogues of PT-ACRAMTU (31). 63
Figure 37. Test reaction for addition of secondary amine to
platinum-bound nitrile. 76
Figure 38. Molecular structure of trans-[PtCl2(EtCN)(N-
[2-acridin-9-ylamino]ethyl]-N-methylpropionamidine)]
50 with selected atoms labeled. 77
XII Figure 39. Synthesis of target compounds. 78
Figure 40. Molecular structure of 54 with selected atoms labeled. 79
Figure 41. Kinetic study using time-dependent 1H NMR spectra for
the reaction of PT-ACRAMTU (31) and complex 54
with 5´-GMP monitored for aromatic guanine and acridine
protons giving signal assignments. (A) Reaction scheme
with important atoms labeled. (B) Stacked plot for the
reaction monitored for PT-ACRAMTU (31). (C) Stacked
plot for the reaction monitored for 54. 81-82
Figure 42. Progress of the reaction of PT-ACRAMTU (31) (circles) and
54 (triangles) with the mononucleotide 5´-GMP at 37 oC
monitored by 1H NMR spectroscopy. 83
Figure 43. Hydrolysis of complex 54´ detected by 2-D [1H-15N]
HMQC NMR spectroscopy giving cross-peak assignments. 85
Figure 44. Reaction of complex 54´ with 5´-GMP monitored by
2-D [1H-15N] HMQC NMR spectroscopy giving
cross-peak assignments. 86
Figure 45. Time-dependent 1H NMR spectra for the reaction of
PT-ACRAMTU (31) and complex 54 with N-AcCys.
(A) Reaction scheme with protons monitored highlighted.
(B) Stacked plot for the reaction monitored for
PT-ACRAMTU (31). (C) Stacked plot for the reaction
monitored for 54. 88-89
XIII Figure 46. Progress of the reactions of 54 (blue circle) and
PT-ACRAMTU (red square) with one equivalent of
N-AcCys at 25 oC monitored by 1H NMR spectroscopy. 90
Figure 47. (A) Reverse-phase HPLC elution profile monitored at 413 nm
of a mixture of complex 54 incubated with N-AcCys.
Positive-ion electrospray mass spectra of adducts P2 (B) and
P3 (C) showing peaks for the molecular ion, [P3]+ (m/z 725.2)
and [P2-2H]2+ (m/z 643.7, z = 2) and fragments resulting from
in-source collisionally activated dissociation (CAD). 93-94
Figure 48. (A) Reverse-phase HPLC elution profile monitored at 413 nm
of a mixture of PT-ACRAMTU 31 incubated with N-AcCys.
Positive-ion electrospray mass spectra of adducts P1´ (B) and
P2´ (C) showing peaks for the molecular ion, [P1´-2H]2+
(m/z 660.2, z = 2) and [P2´]+ (m/z 903.4) and fragments resulting
from in-source collisionally activated dissociation (CAD). 96-97
Figure 49. Pathways for reaction of PT-ACRAMTU (31) and 54
with N-AcCys. 98
Figure 50. Fluorescence microscopy image of A2780 cells treated with
LysoTracker Red DND-99 (left) and MitoTracker Green
FM (right). 100
Figure 51. Representative fluorescence microscopy image (left) of
A2780 cells incubated with complex 54 for 10 min. Enlarged
image (right) of a single cell . 101
XIV Figure 52. Confocal fluorescent microscopy images of A2780 cells
treated with complex 54 while co-stained with MitoTracker
Green FM (top) and LysoTracker Red (bottom), respectively.
and (A) show images taken through blue emission filter, (II)
shows an image taken through green emission filter,
(III) shows superpositions of blue and green images, (B) shows
an image taken through red emission filter, and (C) shows
superpositions of blue and red images. 103
Figure 53. Representative fluorescence microscopy images of A2780
cells incubated with complex 54 alone (left) and co-treated
with hydrogen peroxide (right) for 10 mins. 105
Figure 54. Evaluation of 55 against H460 NSCLC tumors xenografted
into nude mice. 107
Figure 55. Platinum concentrations in various biological tissues (tumors,
kidneys and lungs) at two different doses (0.1 mg/kg and
0.5 mg/kg) determined by ICP–MS after 16 hours of tissue
digestion. 109
Figure 56. Retention of platinum in each organ calculated as percentage of
the total administered doses (0.1 mg/kg and 0.5 mg/kg)
determined by ICP–MS after 16 hours of tissue digestion. 111
Figure 57. Changing 9-amino acridine into acridine-4-carboxamide. 117
Figure 58. Synthesis of acridine-4-carboxamides 57 and 60 and platinum
conjugate 58. 124
XV Figure A1. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 38 in D2O. 155
Figure A2. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 40 in D2O. 156
Figure A3. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 42 in DMF-d7. 157
Figure A4. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 43 in D2O. 158
Figure A5. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 45 in CDCl3. 159
Figure A6. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 46 in CDCl3. 160
Figure A7. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 47 in MeOH-d4. 161
Figure A8. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 52 in D2O. 162
1 Figure A9. H NMR spectrum of compound 52′ in MeOH-d4. 163
Figure A10. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 53 in D2O. 164
Figure A11. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 54 in DMF-d7. 165
1 Figure A12. H NMR spectrum of compound 54′ in DMF-d7. 166
Figure A13. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
XVI compound 55 in DMF-d7. 167
Figure A14. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 57 in DMSO-d6. 168
Figure A15. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 58 in DMF-d7. 169
1 Figure A16. H NMR spectrum of compound 59 in MeOH-d4. 170
Figure A17. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of
compound 60 in D2O. 171
LIST OF ABBREVIATIONS
A Aden(os)ine
ACRAMTU 1-[2-(Acridin-9-ylamino)ethyl]-1,3-
Dimethylthiourea
ACRAMGU 1-[2-(Acridine-9-ylamino)ethyl]-1,3-
dimethylguanidine
AMD473 cis-Amminedichloro(2-methylpyridine)platinum(II)
B Nucleobase
BBR3464 [(trans-PtCl(NH3)3(trans-Pt(NH3)2-
(NH2(CH2)6NH2)2))](NO3)4
BOC tert-Butylcarbamate
C Cytosine
Carboplatin cis-Diammine-1,1-
cyclobutanedicarboxylatoplatinum(II)
CAD Collisionally activated dissociation
XVII CML Chronic myeloid leukemia
Cisplatin cis-Diamminedichloroplatiunm(II)
COSY Correlated spectroscopy
DACA N-[2-(Dimethylamino)ethyl]acridine-4-carboxamide
DACH 1,2-Diaminocyclohexane
DSS 3-(Trimethylsilyl)-1-propanesulfonic acid sodium
salt
dGuo 2′-Deoxyguanosine
DNA Deoxyribonucleic acid dNTP Deoxyribonuleotide triphosphate ds Double-stranded
DMF N,N′-Dimethylformamide
DMS Dimethylsulfide
DMSO Dimethylsulfoxide
en Ethane-1,2-diamine, ethylenediamine
enplatin Ethylenediaminedichloroplatinum(II), [PtCl2(en)]
EGFR Epidermal growth factor receptor
EtCN Propionitrile
FDA Food and Drug Administration
G Guan(os)ine
GMP Guanosine monophosphate
GSH Glutathione
HIV Human immuno deficiency virus
XVIII HOMO–LUMO Highest occupied molecular orbital–lowest
unoccupied molecular orbital
HPLC High-performance liquid chromatography
HSQC Heteronuclear single quantum coherence
Hp Hydroxypyrrole ip Intraperitoneally
IC50 Inhibitory concentration
ICP Inductively coupled plasma
ICP–MS Inductively-coupled plasma electrospray mass
spectrometry
Im Imidazole
JM216 Bis-acetatoamminedichlorocyclohexylamine
platinum(IV)
JM473 cis-Amminedichloro-2-methylpyridineplatinum(II)
Kapp Apparent binding constant
KOH Potassium hydroxide
LC-ESMS Liquid chromatography-electrospray mass
Spectrometry
MTD Maximum tolerated dose
N-AcCys N-acetylcysteine
NER Nucleotide excision repair
NMR Nuclear magnetic resonance
NSCLC Non-small-cell lung cancer
XIX NOESY Nuclear Overhauser enhancement spectroscopy
Oxaliplatin trans-L-Diaminocyclohexaneoxalatoplatinum(II)
ppb Parts per billion
ppm Parts per million
PBS Phosphate-buffered saline
Picoplatin cis-Amminedichloro-2-methylpyridineplatinum(II)
PNA Peptide nucleic acid
PT-ACRAMTU [Pt(en)Cl(ACRAMTU)](NO3)2
Py Pyridine
ROS Reactive oxygen species
Satraplatin bis-Acetatoamminedichlorocyclohexylamine
platinum(IV)
SAR Structure−activity relationships
SCLC Small-cell lung cancer
SEM Standard error of the mean
SOPs Standard Operating Procedures
T Thymi(di)ne
TFOs Triplex-forming oligonucleotide
THF Tetrahydrofuran
TMS Trimethylsilane
Transplatin trans-Diamminedichloroplatinum(II)
WATERGATE Water suppression by gradient-tailored excitation
WHO World Health Organization
XX ABSTRACT
Structure–Activity Relationship Studies of Hybrid Antitumor Agents for the Treatment of
Non-Small-Cell Lung Cancer
By
Zhidong Ma
Dissertation under the direction of Ulrich Bierbach, Ph. D.
Associate Professor of Chemistry
DNA-directed chemotherapies continue to play an important role in modern
oncology. The cytotoxic complex, [PtCl(en)(ACRAMTU)](NO3)2 (en = ethane-1,2-
diamine; ACRAMTU = 1-[2-(acridine-9-ylamino)ethyl]-1,3-dimethylthiourea) (PT-
ACRAMTU, 31), is a dual platinating/intercalating DNA binder that, unlike clinical platinum agents, does not induce DNA cross-links. The research in this dissertation was
concerned with establishing structure–activity relationships (SAR) in this novel class of
DNA-targeted antitumor agents with the ultimate goal of improving the biological
activity of the prototypical agent, PT-ACRAMTU.
Two ways were explored of tuning the reactivity and target interactions of
platinum, which was considered critical in changing the pharmacological properties of
this pharmacophore. The first approach involved a structurally minimally invasive
modification of the prototype to enhance its chemical stability and reactivity with DNA.
This was achieved by introducing a new inert nonleaving group (imino group) in place of
thiourea. The first attempt involved the synthesis of a guanidine analogue of ACRAMTU,
1-[2-(acridine-9-ylamino)ethyl]-1,3-dimethylguanidine (38), by adding N-acridin-9-yl-N'-
XXI methylethane-1,2-diamine (36) to a Boc-activated carbodiimide (Me-N=C=N-Boc),
obtained by desulfuration of N-methylthiourea (44) with HgCl2. While the study was able
to delineate unusual pathways to novel cyclic and spirocyclic acridine derivatives, the
ultimate goal of generating a guanidine analogue of PT-ACRAMTU was not reached due
to the chelating properties of the guanidinato group (verified by 195Pt NMR). Instead,
platinum-mediated amidination chemistry afforded two amidine derivatives of PT-
ACRAMTU (54 and 55) with greatly enhanced activity (IC50 values of 26 nM and 28 nM,
respectively) in H460 non-small-cell lung cancer (NSCLC). The amidination reaction
involved addition of the secondary amine in 36 across the activated CN bond of
platinum-bound propionitrile (EtCN). Complex 54 proved to be a more efficient DNA
binder (t1/2 = 65 min) than PT-ACRAMTU (31) (t1/2 = 234 min), and showed
considerably reduced reactivity with N-acetylcysteine compared to PT-ACRAMTU,
based on a mechanistic study using time-dependent 1H NMR and 2-D HMQC NMR
spectroscopy, as well as in-line liquid chromatography–electro-spray mass spectrometry
(LC–ESMS). A cellular imaging study using confocal fluorescence microscopy was
performed to study the subcellular distribution of complex 54. The results show higher cellular levels of 54 than of PT-ACRAMTU and suggest that the drug reaches the nucleus, although a significant amount of compound seems to be trapped in the lysosomes. A
H460 mouse xenograft study showed complex 55 slows tumor growth by 40% when administered at a dose of 0.5 mg/kg, but the compound was quite toxic and caused weight loss in the animals. The concentrations of platinum detected using inductively-coupled plasma electrospray mass spectrometry (ICP–MS) in various biological tissues recovered
from the euthanized mice reveal that, while compound 55 reaches the tumors, most of the
XXII detected platinum accumulated in the kidneys, the major organ of excretion. The high
level of platinum in the kidneys most likely contributes to the toxicity observed in the
treated animals. Complex 55 is the first non-cross-linking platinum agent endowed with
promising activity in NSCLC.
In the second approach, novel thiourea- and guanidine-modified acridine-4-
carboxamides and a corresponding platinum–intercalator conjugate have been
synthesized and evaluated as cytotoxic agents. The point of attachment of the platinum-
modified linker was changed from the 9-position to the 4-position of acridine to alter the
DNA binding of the conjugate and its cytotoxicity. The IC50 value of 2.4 μM determined
in H460 lung cancer cells for [PtCl(en)(N-(2-(1,3-dimethylthioureido)ethyl)acridine-4-
carboxamide)](NO3)2 (58) indicates that this strategy has no advantage over the 9-
aminoacridine-based prototype.
XXIII CHAPTER 1
INTRODUCTION
1.1 Cancer: Lung Cancer
1.1.1 Cancer Statistics
Lung cancer has become the number one cause of death from cancer each year,
accounting for one third of all cancer deaths worldwide. Although its incidence has
decreased significantly in men by 37% between 1990 and 2005, because of the reduction
in tobacco use during the past 50 years, lung cancer remains the leading cause of cancer-
related mortality in the United States. Statistically, a total of 219,440 new lung cancer
cases and 159,390 deaths from this disease are projected to occur in the United States in
2009 [1, 2].
The two main types of lung cancer are small-cell lung cancer (SCLC) and non-
small-cell lung cancer (NSCLC), which can be differentiated by the size and appearance
of the malignant cells observed histologically under a microscope [1]. They can also be
distinguished primarily based on their clinical behaviors: SCLC has a particular aggressive clinical course with widespread early metastasis and, in contrast to NSCLC, will show a better response to cytotoxic chemotherapy and radiation [3, 4].
1.1.2 Non-Small Cell Lung Cancer
While SCLC accounts for approximately 15% of new cases of lung cancer diagnosed annually and for up to 25% of lung cancer deaths each year [3], NSCLC,
which includes three main sub-types (adenocarcinomas, squamous carcinomas, and large-
cell carcinomas), comprises 80% of lung cancers [5]. Most patients with advanced
1 metastatic NSCLC, in which cancer cells have gained the ability to invade normal tissue(s) away from their tissue of origin, have a median survival after diagnosis of 4–5 months and a year survival of less than 10%, if left untreated [6].
1.2 Chemotherapy of Lung Cancer
Prior to the 1950s, surgery was the only option in lung cancer treatment.
Radiation therapy became a valuable tool to control local and regional disease after 1960
[7]. However, it became clear that more effective treatments for most patients needed to reach every affected organ in the body. Although the cause of cancer is not yet well understood, it is obvious that various mutations in the nuclear environment occur during the formation of cancer, and some anticancer drugs aim to target a cancer cell’s specific pathways to minimize the adverse effects on normal cells [8]. These mutations, including translocations, deletions, amplifications, as well as other changes, may have a severe impact on normal nuclear processes, such as DNA replication, repair, and expression [8].
Therefore, small-molecule drugs, biological molecules, and immune-mediated therapies have become the focus of current efforts to treat cancer with minimal side effects to normal tissues. The era of chemotherapy began in the 1940s with the use of nitrogen mustards and antifolate drugs [9]. Cancer drug development since then has transformed from a low-budget, government-supported research to a multi-billion dollar industry [7].
Drug design, which includes the traditional cytotoxic agents as well as the targeted therapies, with the assistance of established structure–activity relationship (SAR) and high-throughput industrial screening, remains a challenging field for scientists worldwide
(Figure 1) [7].
2
Figure 1. Timeline: The history of chemotherapy. Taken with permission from reference [7].
1.2.1 DNA-targeted Cytotoxic Agents
Double-stranded B-form DNA consists of four building blocks called nucleotides,
each of which contains a nucleobase, a five-membered deoxyribose sugar ring, and a
phosphate group. The right-handed double helix has approximately 10 base pairs per turn
with the two spiral strands intertwined in an anti-parallel fashion and connected to one
other through hydrogen bonds between adenine (A) and thymine (T) and between
guanine (G) and cytosine (C) (Watson–Crick base pairing, Figure 2). In a Watson–Crick base pair, the two deoxyribose residues point in the same direction, which causes gaps in
between the sugars of consecutive base pairs and creates continuous indentations in DNA.
The continual indentations wind along, parallel to the sugar–phosphate backbone, to
generate two grooves termed the major and the minor groove.
3
Major groove Major groove H H O H N N H O N N 7 7 HO OH N1 H N3 HO N H N3 N9 N 1 N1 9 OH O N3 N3 N1 N H O O O O O HO H OH HO H1' H1' OH H1' Minor groove H ' Minor groove 1 GC AT
Figure 2. Watson–Crick base pairs (GC and AT) with the major and minor grooves highlighted.
The DNA grooves have multiple receptor sites harboring nucleophiles for DNA-
interactive drugs (Figure 2) [10], of which the purine nitrogens are particularly
susceptible to modification. In the major groove, guanine-N7 is the most reactive site,
while the methyl group of thymine, O6 of guanine, and N6 of adenine are also recognized by DNA-binding proteins and small molecules [11, 12]. Nucleophilic attack in the minor
groove mainly occurs at the purine-N3 [10, 13].
There is a recent study by the World Health Organization (WHO) showing that of
the 17 high-priority clinical anticancer drugs, 7 are DNA-binding agents [14]. Agents that
associate with DNA covalently or non-covalently are considered potential inhibitors of
mitotic progression and inducers of apoptotic cell death, and, thus, are of interest in
cancer chemotherapy [15]. DNA as carrier of genetic information is a major target for
drug interaction due to its unique function in transcription (gene expression) and DNA
replication, a major step in cell growth and division. Therefore, after several decades of
successful clinical applications of cytotoxic agents, the targeting of DNA as well as specific DNA-associated enzymes continues to be an important approach in designing
4 cancer therapies [8, 9]. The ultimate goal is to disrupt vital cellular processes at the nuclear level to trigger downstream events ultimately leading to cancer cell death.
Numerous cancer therapeutics produce cytotoxic responses by damaging nuclear DNA, thereby interfering with the replication and transcription of the nucleic DNA.
However, one has to be aware that DNA-targeted cytotoxic agents exert their effects on all proliferating cells, both normal and cancerous. In other words, cytotoxic agents exhibit a selective antiproliferative action rather than selective anticancer properties [16]. It is then understandable that in the development of cytotoxic anticancer agents, toxic side effects and drug resistance are almost inevitable. On the other hand, cytotoxic agents are still entering the clinic and are gaining regulatory approval because of their significant effectiveness, which is the result of cytotoxic damage to rapidly replicating cells, such as abnormal cancer cells.
The interactions between small molecules and DNA have been broadly classified into two major groups: noncovalent and covalent interactions. Intercalators (insertion of planar, or approximately planar, aromatic ring systems into base-pair steps) and code- reading agents (groove binding with the edges of the base pairs in the major or minor groove) belong to the first group, while alkylating agents, DNA–DNA cross-linkers,
DNA-cleaving agents, and metalating agents belong to the second group [17].
(Metalating agents will be discussed in detail in Section 1.3, Platinum Antitumor Agents)
Figure 3 shows various types of drug-induced DNA modifications including, from left to right, cross-links, intercalation, DNA strand cleavage, and code-reading interactions [17]. The concept of targeting DNA–enzyme complexes is also illustrated.
The following section gives an overview of various kinds of DNA targeting agents along
5 with their (potential) therapeutic applications, while some of the agents’ modes of action
remain to be elucidated [18].
DNA-DNA Intercalator Double-stranded Code reading Protein-DNA Crosslinker break complex
Figure 3. Classification of DNA-interactive agents and their molecular interaction with DNA. Taken with permission from reference [17].
Group I: Noncovalent interactions
Group Ia. Intercalators
Intercalators are usually small organic molecules, characterized as planar chromophores (usually fused aromatic rings) that insert between adjacent nucleobase pairs. Polarization forces, van der Waals interactions, and interactions between the
HOMO–LUMO π-orbitals (highest occupied molecular orbital–lowest unoccupied molecular orbital) of the planar chromophore and nucleic acid base pairs are the three major factors contributing to the binding affinity of intercalators [19]. Intercalators lengthen and unwind the DNA helix in order to π stack between two base pairs, thereby
distorting the DNA backbone conformation and interfering with DNA–protein
interactions.
6 Typical intercalators include: (i) simple intercalators with one planar
chromophore, (ii) complex intercalators with planar chromophore attached to other groups (such as sugar residues or peptide units), and (iii) polyfunctional intercalators with multiple planar chromophores.
OH O OH O
H2N NH2 OH
N MeO O OH O CH3 O
OH H2N
Ethidium, 1 Doxorubicin, 2
O R Me N O N N NH H O Me O N O S O N N O N O S O H O Me N O N HN N Me Me O
Echinomycin, 3
Figure 4. Ethidium, doxorubicin and echinomycin are examples of a simple intercalator, complex intercalator and polyfunctional intercalator, respectively.
In Figure 4, ethidium bromide (1) is one of the classic simple DNA-intercalating
agents that has been known for many years for its ability to inhibit nucleic acid synthesis in a variety of parasitic organisms [20]. Doxorubicin (2), on the other hand, represents a complex intercalator, which has made a major impact in the treatment of cancer patients since the 1960s [21]. It produces DNA strand breaks by inhibiting the reannealing mechanism of topoisomerase II [22].
7 The idea of synthesizing bifunctional, or even polyfunctional, intercalators was inspired by the possibility of enhancing the DNA binding compared to that of the corresponding monomers. The quinoxaline antitumor antibiotics are a family of cyclic depsipeptides that bisintercalate into DNA, positioning their peptide backbones in the minor groove [23, 24]. They consist of two planar chromophores, tethered by flexible or semirigid linker groups that are capable of simultaneously inserting into the DNA base stack. Echinomycin (3) is a typical member of this family, which strongly binds DNA and inhibits nucleic acid synthesis [25]. The syntheses of these bifunctional or even polyfunctional intercalators have attracted great attention because of the notion that the pharmacological activity of intercalating drugs can be greatly improved by significantly increasing their DNA binding constants and slowing their DNA dissociation rates [26].
The mode of intercalation has now been established for a large number of polycyclic aromatic systems [27]. Intercalation requires changes in the sugar–phosphate torsional angles in order to accommodate the aromatic compound [26]. Although models of duplex DNA interacting with intercalators at all possible sites suggest that this binding mode is possible, experimental studies in solution indicate that even the simplest intercalators appear to reach saturation at a maximum of one intercalator per two base pairs. This observation has led to the neighbor exclusion principle: intercalators can bind at alternate base-pair sites on DNA at most. In other words, when an intercalator binds at one non-specific site, the exclusion principle states that binding another intercalator at a directly adjacent site is prohibited [28].
Group Ib. Code-reading molecules
8 In 1964, Irving Goldberg and co-workers introduced the idea that small molecules,
such as actinomycin (4), may show sequence selectivity when interacting with DNA
(Figure 5) [29]. However, this notion was not appreciated until the first X-ray structure of
restriction enzyme bound to DNA stimulated an interest in small (synthetic) molecules that are able to recognize DNA [30]. The discovery of minor-groove-recognizing
molecules, such as distamycin and netropsin (Figure 5) [30], paved the way for the design
and synthesis of DNA code-reading molecules that might ultimately target a unique
sequence, followed by the development of major-groove-recognition molecules [31, 32].
H O N O O O O H N N N N N H HN NH N O O O O NH O O O H N N O N NH2 O CH CH O NH HN O O NH NH O NH
N NH2 Distamycin,Distamycin 5 NH H R O O N H2N N H H O N H O N NH2 Actinomycin,Actinomycin 4 O NH Netropsin,Netropsin 6
Figure 5. Natural products found as DNA binders.
The structural similarities shared by most of the minor-groove binders include a
positive charge, carbon–carbon single bond or amide-linked aromatic and/or
heteroaromatic rings (unlike the fused aromatic rings found in intercalators) and a
characteristic crescent shape.
9
O O N N H H N O N N N H H N O H N Py/Im targets C G O N H HO N Py/Hp targets A T OH H O N N N H Hp/Py targets T A N H O N N N H O Im/Py targets G C
H N N O 5' 3' O H N N H
Figure 6. Minor groove hydrogen bonding patterns of Watson–Crick base pairs.
The minor groove binders distamycin and netropsin (Figure 5) directly read out
hydrogen bonding as a specific form of DNA-sequence recognition. For example,
netropsin reads five A•T base-pair tracts, and binding to the minor groove is interrupted by sequences having mixed A•T/G•C base pairs [30]. Considerable efforts have been put into redesigning netropsin analogues so that they can read both A•T and G•C base pairs.
The new motif, side-by-side pyrrole–imidazole amino-acid pairing, is able to distinguish all four Watson–Crick base pairs (A•T, T•A, G•C and C•G) [33]. In Figure 6 [33], the
NH of the carboxamides point towards the minor groove of the helix forming specific hydrogen bonds with base pairs. The unsymmetrical pair, imidazole(Im)/pyridine(Py), is used to distinguish GC from CG, while the newly designed pair, hydroxypyridine(Hp)/pyridine(Py), distinguishes TA from AT. The design revealed that
10 the field of molecular recognition of DNA has matured from serendipity to successful
design [33].
It is well known that proteins can recognize and bind to DNA by reading
sequence code in either groove, and mostly through major groove. Non-peptidyl
compounds show the opposite preference, which means they bind through the minor
groove, allowing proteins to recognize the minor and major grooves simultaneously.
Accordingly, it would be interesting to design major-groove-binding molecules blocking
access to proteins that recognize the same groove, if sufficient affinity and sequence
selectivity can be obtained. Two types of major-groove binders have been developed, the
triplex-forming oligonucleotides (TFOs) [32] and the peptide nucleic acids (PNAs) [34].
Major groove Major groove
H H R N O O R N N N H H H N N NN N O H O H H R O H N H N N N O N N R H O N N N N N R R H TAT base triplet C+GC base triplet
Figure 7. The pyrimidine triple helix motif. Similar base triplets form TAT and C+GC. The additional pyrimidine is bound through Hoogsteen hydrogen bonds in the major groove to the complementary purine strand in the Waston–Crick base pair.
TFOs were first discovered in 1987 by Heinz Moser, who demonstrated that oligodeoxyribonucleotides (15-mer) can form specific triple helical complexes in the major groove of DNA under pH, temperature, and salt conditions not too different from
11 those found in a living cell [35]. The sequence recognition in TFOs requires that T binds
AT and C binds GC (Hoogsteen TAT and C+GC triplexes) (Figure 7) [36]. It was
demonstrated that TFOs inhibit DNA binding proteins and modulate transcription in cell-
free systems [37, 38].
Peptide nucleic acids (PNAs) have been known for more than fifteen years and
were originally recognized as mimics of TFOs which are able to sequence-specifically
interact with double-stranded DNA [39]. They were built with uncharged, achiral, pseudo
peptide backbones and are therefore chemically more closely related to peptides and
proteins than to nucleic acids (Figure 8) [34]. Because of their excellent hybridization properties, PNAs can also interact strongly with DNA strands to form stable duplex hybrids [40]. Like the TFOs, PNAs in complex with double-stranded DNA effectively block transcription initiation [41].
B B B O O O N H H H N H N N N N N
N-terminal O O O
Figure 8. Chemical structure of a PNA. ‘B’ stands for nucleobase.
Even though both TFOs and PNAs recognize the major groove of DNA, there are two major differences between them. First, the backbones of TFOs are modified analogues of oliogonucleotides, whereas PNAs have peptide-like backbones. Second,
TFOs bind with the existing major groove of duplex DNA, whereas PNAs invade the
DNA helix to form a new hybrid duplex [42], as the result of the displacement of the non- complementary DNA strand.
12 Group II: Covalent interaction
GroupIIa. Alkylating agents and DNA–DNA cross-linkers
Among the DNA interacting agents, covalent binders comprise the largest group.
The beginning of cancer chemotherapy is generally accepted to have been the serendipitous discovery of the mustards family in the first half of the twentieth century
[9]. The nitrogen mustards were developed as derivatives of the sulfur-based mustard gas
(Figure 9) [9], which was first synthesized in 1886.
MustardMustard gas gas, 7 Chlormethine,Chlormethine 8 Chlorambucil,Chlorambucil 9 (sulphur mustard) (mustine) (sulfur mustard) (mustine) Cl Cl Cl
S N HOOC N
Cl Cl Cl Too toxic to be used Aliphatic mustards have Aromatic mustards are less in humans sufficient therapeutic electrophilic and react with DNA index to be used in humans more slowly. Can be administered orally
Estramustine,Estramustine 10 Cyclophosphamide,Cyclophosphamide 11 Melphalan,Melphalan 12
OH Cl Cl NH N O P N H2N Cl N O O O COOH Cl Cl Cl Represents an attempt to An attempt to target Represents an attempt to enhance cellular uptake estrogen-dependent release mustard agent through through the phenylalanine- tumor cells enzymatic degradation transport mechanism
Figure 9. The development of N-mustard-based cytotoxic agents. Redrawn from reference [9] with modifications.
In 1946, it was discovered that the toxic effects of sulfur mustard were due to
DNA alkylation [43]. Figure 9 shows the historic development of the mustard family of agents, from mustard gas to molecules that have shown progressively decreasing toxicity
13 to normal cells, to a molecule design that allows mustard analogues to be targeted to
estrogen receptor positive tumor cells. It was realized that, if the electrophilicity of
mustards could be reduced, then less toxic drugs might be obtained that can be
administrated orally. This led to the development of the compounds chlormethine,
chlorambucil, melphalan, cyclophosphamide, and estramustine, which are still in clinical
use today. Melphalan is composed of a phenylalanine group attached to the mustard to
enhance the selective uptake by tumor cells. Estramustine is the combination of mustard
and an estrogen derivative [9].
Monoalkylated DNA DNA Cl R N DNA R N R N Cl Cl Cl
DNA DNA DNA R N R N DNA Crosslinked DNA
Figure 10. Monoalkylation and cross-linking chemistry of alkylating agents.
Certain cancers, such as leukemias and lymphomas, with very high proliferative rates compared with most normal tissues, are particularly susceptible to these agents. In
N-mustard-based alkylating agents, chloride is a good leaving group that favors
intramolecular nucleophilic attack of nitrogen to form a strained, highly reactive
aziridinium cation, which ring-opens in reactions with DNA nucleophiles (Figure 10)
[17]. The regiospecificity of alkylation of DNA by these nitrogen mustards is governed
primarily by the steric and electronic properties of DNA. Aziridinium cations readily
14 alkylate N7 of guanine to form a monoalkylation adduct because N7 on guanine is the
sterically least hindered site and has the most negative electrostatic potential, rendering
this nitrogen the strongest nucleophile in DNA [17]. This process can be repeated to form
the cross-linked DNA (Figure 10). Cross-links in DNA can occur either between two
complementary strands of DNA (interstrand, Figure 3), as generated by chlorambucil and
melphalan, or within one strand of DNA (intrastrand), as generated by cisplatin
(discussed in detail in Figure 15) [17].
b. DNA-cleavage agents
Bleomycin, unlike most DNA-damaging electrophiles, attacks neither the nucleic
bases nor the phosphate linkages, even though products of its attack include both nucleic
base derivatives as well as phosphomonoester termini [44, 45].
H N O 2 NH H 2 N NH2 O O NN CH3 O N H H NH O HO N H2N O NH N H S CH3HN O HO CH H3C H CH3 H 3 S S O N CH3 OH H HO O N H O OH OH OH O OH H2N O
Bleomycin, 13
Figure 11. Structure of bleomycin.
15 It is isolated as a copper complex from the culture medium of Streptomyces
verticillis, but administrated to patients with wide clinical antitumor use [46] in metal-
free form, which minimizes irritation at the site of injection (Figure 11).
The hypothesis that DNA damage is the major contribution of bleomycin’s
cytotoxicity is supported by the finding that cells treated with bleomycin contain cleaved
DNA with the same site specificity observed in vitro [47]. Involvement of iron and
reduced molecular oxygen in the mechanism of action of bleomycin indicated that
bleomycin might attack DNA through reactive oxygen species (ROS), such as hydroxyl
radical (•OH) [48]. The analysis of bleomycin reactions began with the discovery that in
the presence of oxidizing agents like hydrogen peroxide, bleomycin causes DNA
fragmentation [49]. Bleomycin was found to form a 1:1 complex with Fe(II) that reacts
with O2 [48]. Another study [50] showed that Fe(III)•bleomycin reacted with peroxide yields a drug intermediate that cleaves DNA the same way Fe(II)•bleomycin + O2 does
[48].
The reactions between bleomycin and DNA are somewhat different from other cytotoxic agents: even though the reactive bleomycin intermediate interacts with DNA, it is the oxidative damage that triggers cell death but not the bleomycin–DNA complex itself. In one in vitro study, after degradation of DNA treated with activated bleomycin, the products included oligomers terminated by 5′-phosphate [51] and nucleobase
fragments containing deoxyribose carbons 1′–3′ [52], which were identified as base propenals: 3-(pyrimidin-1-yl)-2-propenal and 3-(purin-9-yl)-2-propenal (Figure 12) [48].
16
PO O B PO B O B DNA cleavage PO 4' 4' (i) O2 4' - + O 3' (ii) e , H HO O 3' O OP 3' OP O OP
Figure 12. DNA degradation products after bleomycin-induced strand cleavage. The final products are stable and have been isolated.
1.2.2 Targeted Therapies
While there is continued interest in the discovery of improved cytotoxic agents, molecular and genetic methods of studying cell biology unveiled entirely new pathways
involved in cancer cell proliferation and survival. Researchers set out to study the
molecular defects in cancer cells and how to target them without harming normal cells [7].
The new targets include growth factors, signaling molecules, cell-cycle proteins,
modulators of apoptosis, and molecules controlling angiogenesis [53].
A highlight in the design of targeted anticancer therapy is the development of
imatinib mesylate (Glivec), which was derived from a natural product by Novartis. It is a
moderately potent inhibitor of the kinase BCR–ABL, the fusion protein product of a chromosomal translocation that is involved in the pathgenesis of chronic myeloid leukemia (CML) [7]. Another class of compounds was developed to inhibit the epidermal growth factor receptor (EGFR). Gefitinib, for instance, is a competitive inhibitor of
EGFR, which causes partial remissions in 10–15% of patients affected with NSCLC [54].
Although the discovery of molecular targets and pathways specific to cancer cells is becoming more and more important, it remains uncertain if targeted agents will ever replace existing cytotoxic chemotherapies. Current targeted agents are only beneficial if co-administered in tandem with cytotoxic agents [55]. For example, Avastin
17 (bevacizumab, vascular endothelial growth factor inhibitor, Genentech/Roche) shows improved responses in colorectal cancer patients co-treated with oxaliplatin (trans-1,2- diaminocyclohexaneoxalatoplatinum(II)), but has demonstrated no activity as a single agent [55].
1.3 Platinum Antitumor Agents
1.3.1 Bifunctional Platinum Crosslinkers
In 1845, the platinum complex cis-diamminedichloroplatinum(II), now known as the antitumor agent cisplatin, was first reported by M. Peyrone [14]. Little research was done on this complex until Rosenberg rediscovered it in the 1960s when he found that E. coli cells cannot divide when exposed to it [56]. Detailed chemical analysis identified two active complexes―the neutral cis-complex cisplatin and a platinum(IV) analogue―as the cause of this intriguing biological effect. In 1968, cisplatin was administered intraperitoneally to mice bearing a xenografted tumor at the non-lethal dose of 8 mg/kg and was shown to cause marked tumor size reduction [57].
Inspired by these findings, the first patients were treated with this agent only three years later in 1971, a remarkably short time period, in modern terms, from the original bench discovery to the clinic. Cisplatin underwent extensive clinical trials afterwards and was approved by the US Food and Drug Administration (FDA) in 1978. Since then, cisplatin has revolutionized the treatment of certain cancers, especially malignancies of the urogenital tract [56].
There is strong evidence to support the view that the mechanism of action of cisplatin (Figure 13) [58] involves intracellular activation by aquation of one of the two
18 chloride leaving groups and subsequent adduct formation with nuclear DNA.
Unfortunately, the activated platinum intermediate also exhibits a high affinity for amino acid sulfur and binds to proteins and peptides containing cysteine and methionine. These include the tripeptide glutathione (GSH) and metallothioneins, binding to which contributes to the toxic side effect of cisplatin and also causes the drug’s detoxification, which may lead to tumor resistance. Cisplatin–DNA adducts activate various signal- transduction pathways involved in DNA-damage recognition and repair (nucleotide excision repair pathway), cell-cycle arrest, and cell death/apoptosis [59].
Figure 13. Mechanism of action of cisplatin. Taken with permission from reference [58].
19 Figure 13 shows how platinum enters cells by either trans-membrane transporters
(like the copper transporter CTR1), or by passive diffusion [58]. Once inside the cell, cisplatin is activated by reaction with water to form chemically reactive aqua species.
This step is facilitated by the relatively low chloride concentration in the cytosol. The activated intermediate binds to DNA to form DNA adducts. In some platinum-resistant cancer cells, GSH and metallothionein levels are relatively high, so the activated intermediate is irreversibly trapped before DNA binding can occur, which causes drug resistance. Platinum drugs are exported from the cells by the glutathione-S-conjugate export pump as well as other transporters, which also contributes to platinum drug resistance [58].
7 Guanine N position NH3 NH3 Pt Pt H NH NH N H O N 3 3 H3N Cl N Pt N H N H3N OH2 N N O H N H
Intrastrand Intrastrand cross-link (major) cross-link (minor)
Figure 14. Binding modes between cisplatin and DNA (intrastrand and interstrand cross- links). Taken with permission from reference [58].
Early studies [58] conducted in vitro with salmon sperm DNA exposed to cisplatin showed platinum binding to the N7 position of the imidazole ring of the purine bases of guanine (G), and to a lesser extent, adenine (A), to form either monofunctional
(loss of one leaving group) or bifunctional adducts (loss of both leaving groups) (Figure
20 14) [60]. The majority of adducts are formed on the same DNA strand and involve bases
adjacent to one another, and are therefore known as intrastrand adducts or cross-links.
These include 1,2-GpG intrastrand (60%–65%) and 1,2-ApG intrastrand (20%–25%)
adducts [61]. The less frequently formed adducts are the 1,3-GpXpG intrastrand cross- link [62] and G–G interstrand cross-link on opposite DNA strands of the double helix
[63]. All these structures have been elucidated either by X-ray crystallography or by
solution structures solved using NMR spectroscopy [61-63].
Even though cisplatin is a very effective anticancer drug, it is notorious for its
toxicity to the kidneys (nephrotoxicity) and gastrointestinal tract [64]. In addition to its
systemic toxicity, other disadvantages of cisplatin are its (i) limited applicability in a
narrow range of tumors, (ii) inactivation by increased levels of thiol-containing proteins
and peptides, such as GSH, and enhanced DNA repair (iii) low water solubility, (iv)
inefficient delivery to the tumors, and (v) inconvenient administration (cisplatin cannot
be taken via oral route). Therefore, three major strategies with the purpose of
circumventing these drawbacks have been proposed: first, the design of improved
platinum drugs; second, to enhance delivery of platinum to tumors; and third, to combine
platinum drugs with new molecularly targeted drugs. By following these strategies,
second- and third- generation cisplatin analogues have been developed, and a platinum
drug “family tree” has been generated (Figure 15) [58].
Carboplatin (cis-diammine-[1,1-cyclobutanedicarboxylato]platinum(II)) was
developed and introduced into the clinic in the mid-1980s [65] as a second-generation
derivative of cisplatin. Its design is based on the idea that a more stable leaving group
other than chloride might decrease the toxicity without affecting antitumor efficacy.
21 Compared with cisplatin, carboplatin largely eliminates nephrotoxicity and causes less neurotoxicity. The adducts formed by carboplatin on DNA are essentially the same as those formed by cisplatin, but 20–40-fold higher concentrations of carboplatin are required to achieve the same therapeutic effect, and the rate of adduct formation is about an order of magnitude slower [66].
H3N Cl Pt H3N Cl CisplatinCisplatin (1971)(1971), 14
Improved safety Broader spectrum of antitumor activity O H3N O Pt H3N O O Broader spectrum CarboCarboplatinplatin ((1982)1982), 15 of activity; retention H2 O N O against acquired Pt Improved patient resistance to cisplatin convenience N O O H2 (oral activity) OxaliplatinOxaliplatin (1986) (1986), 16 O O Cl Improved delivery H3N Cl H3N Pt Pt N Cl N Cl O H2 O Picoplatin,Picoplatin (JM473) SatraplatinSatraplatin (JM216) (JM216) (1993), (1993) 17 (1997),(JM473) 18 (1997)
O O R = C9H19 Na
O O O O H2 O H N O N NH Pt NH2 Pt R N N R H H H2N N O O O H O H 2 N OH DACH-L-NDDPDACH-L-NDDP O (Aroplatin) (2004) n (Aroplatin), (2004), 19 AP5346 (ProLindac) (2004) AP5346 (ProLindac) (2004), 20
Figure 15. The current platinum drug “family tree” and generalized rationale underlying their development. The dates indicate when each drug was first given to patients.
22 Oxaliplatin (1R,2R-diaminocyclohexaneoxalatoplatinum(II)) was designed on the basis of the bulky 1,2-diaminocyclohexane (DACH) ligand and went into clinical trails in the early 1990s [67]. Interestingly, unlike for cisplatin and carboplatin, the cellular accumulation of oxaliplatin seems to be less dependent on the copper transporters [68].
There are also several differences between the structures of cisplatin-induced and oxaliplatin-induced 1,2-intrastrand DNA cross-links [69], and oxaliplatin retains activity against some cancer cells with acquired resistance to cisplatin [70].
Satraplatin (bis-acetatoamminedichlorocyclohexylamineplatinum(IV), JM216) was initially developed as an orally active version of carboplatin. Preclinical studies showed that the drug possesses good oral antitumor activity, comparable to that of intravenously administered cisplatin or carboplatin in human ovarian cancer xenografted into mice [71]. Picoplatin (cis-amminedichloro-2-methylpyridineplatinum(II); JM473) was rationally designed to provide steric hindrance around the platinum center [72].
Picoplatin reacts less avidly with thiol-containing species, such as GSH, than cisplatin.
The drug shows good activity against a wide range of cisplatin-resistant cells in vitro [72] and possesses antitumor activity by both intravenous and oral routes in vivo [73]. DACH-
L-NDDP (Aroplatin) and AP5346 (ProLindac) are two newly developed platinum drug formulations, aimed at improving drug delivery. Aroplatin is based on liposomal preparations of cisplatin-like molecules [74], and ProLindac is synthesized by combining a platinum-based drug with a water-soluble, biocompatible co-polymer [75]. Both formulations have entered clinic trials against cisplatin-resistant tumors.
Even though thousands of platinum compounds have been evaluated for their anticancer activities with varied success, the majority obey the classical structure–activity
23 relationships (SAR) for cisplatin-type complexes, summarized by Cleare and Hoeschele
[76]. These relationships state that a Pt(II) complex does not show antitumor activity
unless the Pt(II) complex exhibits a cis geometry, cis-[PtX2(Am)2], where X stands for a
good leaving group and Am is an inert amine with at least one NH moiety. Cisplatin-type drugs show similar toxicity profiles, form the same type of cross-links, and share the same spectrum of bioactivity [76].
Therefore, major efforts have been made to design compounds that act unlike traditional cisplatin-type drugs, with the common goal of discovering agents that offer substantial clinical advantages over the existing drugs and target cancers that previously did not respond to cisplatin-type drugs. A number of researchers have taken different approaches to design Pt drugs based on an improved understanding of the mechanisms of drug resistance. Several of the recent approaches will be discussed in the following sections.
1.3.2 Monofunctional Platinum Complexes
In a search for new drugs that exhibit biological activity complementary to that of cisplatin, several new classes of platinum complexes possessing unique geometries and ligand sets have been developed. Their design is based on the supposition that such non- classical complexes might act through a mechanism of action distinct from that of cisplatin and exhibit improved pharmacological properties. These new platinum complexes include monofunctional platinum complexes, trans-complexes, multinuclear platinum agents, as well as platinum–intercalator conjugates.
24 Several monofunctional platinum(II) complexes showed promise as a new type of
antitumor agent that does not follow the Cleare–Hoeschele SAR [77, 78]. The general
- formula for monofunctional platinum complexes is [PtL3Cl], in which Cl acts as the only
leaving group.
O H O O N O N N N O NH NH2 Pt 2 Pt N Pt S N N Cl Cl Cl 21 22 23
Figure 16. Examples of bioactive monofunctional platinum complexes.
Figure 16 shows examples of bulky planar ligands such as quinolines [77] that
have been introduced as new non-leaving groups in non-classical platinum anticancer
agents. These ligands significantly lower the rates of platinum binding to sulfur-
containing biomolecules [79]. On the other hand, the binding mode of these complexes
with DNA appears to be radically different from that of existing platinum drugs, and their
spectrum of activity is different from the established pattern of cisplatin. It has been
reported that monofunctional platinum(II) complexes significantly destabilize DNA and
modify the conformation of DNA, similar to the cross-linking agents [78].
1.3.3 Other Non-classical Platinum Agents
The trans isomer of [PtCl2(NH3)2], known as transplatin, lacks chemotherapeutic activity [80]. Unlike cisplatin, which forms 1,2-intrastrand crosslinks, geometric constraints prevent transplatin from forming this adduct. Instead, 1,3-intrastrand cross-
25 links are formed [81]. Interstrand cross-links of transplatin are formed between G and C
residues of the same base pair as shown in Figure 17 [80], which highlights the markedly different DNA conformations induced by the two platinum isomers [61, 82].
In addition, transplatin reacts significantly faster with GSH than cisplatin, because of the mutual trans-labilization of the two chloro ligands in the molecule, which causes rapid cellular detoxicification of this isomer. However, several derivatives of trans- platinum complexes have been described in the literature that show improved activity.
These include analogues containing sterically hindered planar nitrogen-containing heterocycles (pyridine, quinoline, etc). These compounds show reduced reactivity with
GSH and form cytotoxic monofunctional DNA adducts [83].
Figure 17. Interstrand cross-links for cisplatin (left) and transplatin (right). Taken with permission from reference [80].
Multinuclear platinum complexes represent another class of compounds that have
shown great potential in cancer chemotherapy [77]. These complexes usually contain two,
three or four platinum centers with cis and trans configurations and bind to DNA in a
pattern different from that of cisplatin. Until recently, over 50 multi-nuclear platinum
26 complexes have been synthesized and tested for cytotoxicity in a variety of cancer cell lines [84]. Most of these complexes react with DNA more rapidly than cisplatin and produce long-range interstrand and intrastrand cross-links [85].
From all the complexes which have been tested, the first and most important multinuclear platinum complex to enter phase II clinical trials based on its excellent performance in vitro and in vivo was the trinuclear complex BBR3464 (Figure 18) [86].
BBR3464 is significantly more active than cisplatin in all of the cancer cell lines tested, including pancreatic, lung, and skin cancers. It shows activity comparable to cisplatin at up to 500-fold lower doses in vitro [87, 88]. In addition, BBR3464 is also capable of overcoming cisplatin resistance and shows activity in several cancer cell lines that have a natural resistance to cisplatin [87].
4+
H2 H3N Cl H N N Pt H2 3 N NH H N N Pt 3 3 H2 Pt N NH3 H2 Cl NH3
BBR3464, 24
Figure 18. Structure of the trinuclear platinum complex BBR3464.
The high positive charge and amine groups of BBR3464 are believed to contribute to the recognition of target sites on DNA through electrostatic and hydrogen- bonding interactions [89]. The rapid binding to BBR3464 leads to various long-range interstrand and intrastrand cross-links, where the interstrand adducts account for about
20% of the DNA adducts [84].
27 1.3.4 Platinum–Intercalator Conjugates
In platinum–intercalator conjugates, a polycyclic planar (usually fused, and positively charged, aromatic rings) group is tethered to a divalent platinum center through one or multiple thermodynamically stable donor–metal bond(s) that is (are) not reversed under physiological conditions or upon binding of one of the two components to DNA.
Therefore, platinum center and intercalator are functionally independent. Platinum bears at least one substitution-labile group that can be replaced with DNA nucleobase nitrogen to form a stable coordinative bond. The linker group that connects the organic and inorganic portions of the conjugate should be somewhat flexible, like polymethylene chains, to facilitate a hybrid binding mode [90]. A schematic drawing summarizing various functionalities is shown in Figure 19 [90].
tor Square-planar ala erc platinum center int A DN
flexible linker chain Substitution- Substitution- inert group labile group
Figure 19. Schematic drawing of a platinum–intercalator conjugate. Adopted from reference [91] with modifications.
The rational design of conjugates combining platination and intercalation was inspired by the early studies of the DNA binding of cisplatin in the presence of intercalating agents using molecular biology and biophysical methods [91, 92]. These include cisplatin complexes conjugated to both non-therapeutic classical intercalators, such as acridine orange [93, 94] and clinical agents, such as the topoisomerase poison
28 doxorubicin (Figure 20) [95, 96]. The platinum–doxorubicin conjugate shows activity in various cancer models in vivo, such as murine leukemias (L1210, P388), which are resistant to both cisplatin and doxorubicin. In the solid tumor cell lines, the overall activity of this conjugate resembled that of doxorubicin more than that of cisplatin [96].
As a continuation of the biophysical studies, a series of monofunctional and bifunctional platinum–intercalator conjugates, including platinum–ethidium and platinum–9-aminoacridine, have been developed (Figure 20).
OH O OH O Me2N N NMe2 OH
MeO O OH O
O Cl NH H2N Cl Pt OH Pt Cl N BuH2N Cl H2 Platinum-AcridinePlatinum-Acridine Orange Orange Conjugate, Conjugate 25 Platinum-DoxorubicinPlatinum-Doxorubicin Conjugate Conjugate, 26
+ 2+ H3N NH3 H3N NH3 Pt Pt HN Cl H2N NH2 Cl
N N H
Platinum-EthidiumPlatinum-Ethidium Conjugate, Conjugate 27 Platinum-9-AminoacridinePlatinum-9-Aminoacridine Conjugate Conjugate, 28
R R N N
HN HN H2 N Cl (CH2)nNH NH2 (CH2)n Pt Cl Pt NH2 Cl Cl Platinum-9-Anilinoacridine Conjugates
Platinum-9-Anilinoacridine Conjugates, 29 and 30
Figure 20. Structures of platinum–intercalator conjugates.
29 Another design rationale is based on the hypothesis that the tethering of cisplatin to DNA-affinic ligands might have favorable pharmacokinetics by reducing the residence time in the cytosol and improving the DNA binding kinetics of the drug. Two types of platinum–anilinoacridine conjugates (Figure 20) have been synthesized that were able to overcome cross-resistance to cisplatin in vitro. Disappointingly, the compounds’ activity in vitro did not translate into anti-tumor activity in vivo, probably due to their poor solubility [97].
1.4 Background and Significance of the Doctoral Research Program
1.4.1 The Development of PT-ACRAMTU
The development of cisplatin–acridine complexes was inspired by the known topoisomerase II poisoning activity of the free 9-aminoacridines and their increased water solubility compared to the aniline derivatives [90]. In addition, it was demonstrated that the DNA-adduct profile of cisplatin can be modulated by an acridine intercalator [98, 99].
Our laboratory has taken a more radical approach of preventing the metal from cross-link formation of consecutive guanines in the major groove by tethering a monofunctional platinum moiety to a minor groove directed intercalating agent, 1-[2-(acridin-9- ylamino)ethyl]-1,3-dimethylthiourea (ACRAMTU, 37) [100].
The rationale behind PT-ACRAMTU ([PtCl(en)(ACRAMTU)](NO3)2, en = ethane-
1,2-diamine, ACRAMTU = 1-[2-(acridin-9-ylamino)ethyl]-1,3-dimethylthiourea, acridinium cation) (31) was to design a platinum complex that contains one labile group that can be substituted by nucleobase nitrogen to form a coordinative bond, and an
30 intercalator attached to the platinum through a flexible linker, which can stack between
adjacent DNA base pairs.
bidentate non-leaving group 2+ dicationic character
H2N NH2 Pt NH chloro leaving group Cl S N N H 9-aminoacridinium, Pt-S linkage NH a classical intercalating unit
semirigid ethylene-thiourea linker unit
PT-ACRAMTU, 31
Figure 21. Design of PT-ACRAMTU (31).
As shown in Figure 21, there are several significant features [90] of the dual binding
mode agent PT-ACRAMTU distinguishing it from cisplatin and its second- and third-
generation derivatives: 1) A single chloride leaving group can be expected to favor monofunctional binding of the metal with nucleobase nitrogen in double-stranded DNA.
2) The idea of introducing a Pt–S bond is based on the strong affinity of sulfur for platinum, which ensures ACRAMTU remains attached to platinum as a non-leaving group. 3) A semirigid linker was chosen to optimize strainless intercalation and platination with minimum spacing between them to favor platination of DNA bases directly adjacent to the intercalation sites. 4) Acridine as a typical intercalator helps the complex pre-associate with DNA via π–π stacking with base pairs and facilitates the coordinative binding of platinum to nucleophilic sites of purines. 5) The 2+ charge enhances electrostatic attraction between the drug and negatively charged DNA. 6) The
31 chelating nonleaving group ethane-1,2-diamine (ethylenediamine, en) mimics the cisplatin structure and enhances the platinum moiety’s stability.
1.4.2 Synthesis of PT-ACRAMTU
O H o 1.CF3COOEt/THF/ <10 C 0.2M NaOH/48h, 35oC N O N N O H N H N o F C N 2 2 2.(Boc)2O/THF/<10 C 3 H O O 32 33 34
N
O O 1.HCl, CH COOH H 3 H 1.MeSCN/EtOH/Δ N N N N THF/Δ N O 2.Base N 2.HNO H 3
35 36 2+ + H N NH S 2 Pt 2 H enplatin/AgNO3 S Cl HN N H N N DMF/dark/14h HN N H N N H
37·H+ 31
Figure 22. Synthesis of PT-ACRAMTU (31).
The synthesis of PT-ACRAMTU (31) shown in Figure 22 utilizes the activated form
+ of enplatin ([PtCl2(en)]), [PtCl(en)(DMF)] , which is reacted with the acridinylthiourea nitrate salt, ACRAMTU·HNO3 (37·HNO3), in dry DMF. The synthesis of 37 nitrate salt involves: a) selective protection of the secondary amine nitrogen in 32 with tert- butylcarbamate, BOC, b) nucleophilic substitution of 9-phenoxyacridine with protected amine 34 [101], c) deprotection to remove the Boc group, and d) addition of
32 methylisothiocyanate to form 37, followed by protonation with one equivalent of nitric
acid to form the desired acridinium salt [102, 103].
1.4.3 Biological Activity and DNA Binding Mechanism
PT-ACRAMTU (31) has shown good biological activity in cell proliferation
assays. In several cell lines, PT-ACRAMTU proved to be more active than cisplatin and the Pt-free ligand ACRAMTU itself. These include HL-60 leukemia, as well as solid tumors, such as NSCLC (Table 1) [104].
Table 1. Cytotoxicity of PT-ACRAMTU (31) in NSCLC.
Cell line A549 Calu-1 SK-LU-1 H358 H460 H522 H596
a IC50 (μM) 0.23±0.05 1.41±0.72 0.26±0.06 0.50±0.20 0.17±0.02 0.095±0.02 0.18±0.03
a IC50 values are concentrations of drug required to inhibit colony growth by 50% compared to controls and represent averages of at least two experiments plus or minus the standard error of the mean. Data adopted from reference [105].
An unusual observation was made when performing enzymatic digestion of calf thymus DNA treated with PT-ACRAMTU. Four platinated DNA fragments were isolated
and characterized by HPLC–MS and X-ray crystallography, some of which were entirely
unexpected. While the majority of adducts are formed with N7 of guanine (major groove,
80%), the three other platinum-modified DNA adducts proved to be quite unusual. PT-
ACRAMTU forms adducts with adenine at three sites [105], N7 (~11%), N3 (~7%), and
N1 (~2%). It can be said that the seemingly simple replacement of one chloride leaving group with a classical intercalator (ACRAMTU) in [PtCl2(en)], results in a dramatic
decrease in the formation of guanine adducts.
33
2+ N N Pt ACR S OH
?
2+ k1 N N 2+ 3+ N N + H2O N N Pt intercalation Pt Pt ACR S Cl + Cl- ACR S Cl ACR S OH2 k-1 di + r dG k2 + dG ec t p at hw ay? ? 3+ N N Pt dG = 2'-deoxyguanosine ACR S N7-dG
Figure 23. Potential reaction pathways of Pt–guanine adduct formation.
The binding mechanism of PT-ACRAMTU (31) in duplex DNA differs from that
of cisplatin. It does not require an activation step before it associates with DNA because
of its positive charge, whereas cisplatin has to form a cationic complex via hydrolysis,
which enables electrostatic association of the drug with nuclear DNA. A major effort has
been devoted to elucidating the DNA-binding mechanism of PT-ACRAMTU utilizing
various techniques, including analytical (HPLC–MS) [105, 106], biochemical (enzymatic
digestion and footprinting assays) [107, 108] and NMR experiments (NMR solution
structure, 1H and 2D [1H–15N] heteronuclear single quantum coherence (HSQC) kinetic
NMR studies) [109, 110] (Figure 23): The formation of the guanine N-7 adduct by PT-
ACRAMTU (31) is neither a simple first-order nor second-order process. The first step involves rapid intercalation from the minor groove. In the subsequent step, nucleophilic
34 substitution of the chloride leaving group involves two possible pathways: reversible hydrolysis followed by replacement of aqua ligand by nucleobase nitrogen or direct replacement of chloride by nucleobase, which might be favored by the cis-effect of thiourea sulfur [111]. Based on the data acquired in the previous study, the latter pathway seems a possibility.
1.4.4 Goals of Doctoral Research Program
The overarching goal of the research presented in this dissertation was to study structure–activity relationships (SAR) in platinum–acridine hybrid antitumor agents. To
achieve this goal, the current work implemented a classical medicinal chemistry approach
involving synthesis, mechanistic studies at the target level, and biological evaluation.
The work was motivated by the discovery of the DNA-targeted anticancer agent PT-
ACRAMTU (31), which has DNA-binding properties different from those observed for
clinical platinum anticancer agents. The design of the compound does not follow the
Cleare–Hoeschele SAR, and it is believed that its cytotoxic properties are mediated by
the formation of DNA adducts different from those induced by cisplatin and other
platinum anticancer agents. PT-ACRAMTU is the first platinum-based anticancer agent
that forms unusual adenine adducts in the minor groove, which may contribute to its
biological activity.
While the cytotoxicity data previously acquired for PT-ACRAMTU are promising,
the agent proves to be only slightly more cytotoxic than cisplatin and, disappointingly, was found to be inactive in vivo (unpublished data). In addition, the sulfur coordination in the prototype was found to give rise to a high reactivity of the metal which leads to
35 slow degradation of the complex in buffered solution. Therefore, a systematic study of
chemically more stable derivatives of this compound seems warranted in order to develop a new conjugate with enhanced activity in cancer cells and antitumor activity in vivo.
Based on the structural and thermodynamic requirements for the recognition and removal of DNA lesions by the NER complex, the monoadducts formed by PT-
ACRAMTU can be expected to be less efficiently repaired than cisplatin–DNA cross- links because they do not bend or destabilize the DNA helix. PT-ACRAMTU has the ideal geometry and conformational flexibility to allow a binding mode that combines nucleobase platination and strainless classical intercalation perpendicular to the helical axis. It is therefore plausible to assume that retaining the previously optimized core
structure of PT-ACRAMTU in the new set of compounds is an appropriate strategy for
identifying a potent analogue whose adducts are able to circumvent NER. Thus, one goal
pursued in Chapters 2 and 3 was to generate nitrogen-donor-based mimics of
ACRAMTU, based on guanidine- and amidine-modified 9-aminoacridine, and to
introduce them as carrier ligands in the corresponding conjugates. The idea behind this
approach was that tuning of the metal’s reactivity by replacing the thiourea-S in
ACRAMTU with an imino-NH donor might have an effect on the conjugate’s target
affinity and selectivity without altering the structure of its cytotoxic DNA adducts. To
achieve this, new organic synthetic methodology and conjugation chemistry were
developed. To establish relationships between chemical structure and biological activity,
the reactivity of the newly designed compounds with biomolecular targets was studied in
model systems using NMR and UV-visible spectroscopy and in-line liquid
chromatography/mass spectrometry. The mechanistic work was complemented by
36 experiments addressing the cytotoxic activity and the subcellular and tissue distribution of the compounds.
In another approach, a platinum–acridine conjugate was developed in which the linkage geometry (the point of attachment of the platinated side chain) deviated from that in the prototype, PT-ACRAMTU. While in the latter structure platinum is linked to the
9-position of the acridine chromophore, the new derivative contains a 4-carboxamide chain. Since the 9- and 4-positions are located on opposite sides of the aromatic system, it was speculated that the directionality of intercalation might affect the DNA sequence and groove specificity of the platinum moiety, leading to an altered DNA adduct profile and, potentially, an altered cellular response. The results of this study, which include synthesis and characterization of the target compounds, DNA binding studies of the carrier ligands, and growth inhibition assays in cancer cell lines, are presented in Chapter
4.
37 CHAPTER 2
SYNTHESIS, CHARACTERIZATION, AND BIOLOGICAL EVALUATION OF
GUANIDINE DERIVATIVES OF ACRAMTU
The work contained in this chapter was initially published in Journal of Organic
Chemistry in 2007. The manuscript, including figures and schemes, was drafted by
Zhidong Ma and Dr. U. Bierbach and edited by Dr. U. Bierbach before submission to the journal. Since the publication, changes in both format and content have been made to maintain a consistent format throughout this work. The experiments described were performed by Zhidong Ma, except for the crystal structure determinations, which were contributed by Dr. C. S. Day, and the cytotoxicity assays which were contributed by Dr.
G. L. Kucera and G. Saluta.
38 2.1 Background
Acridine derivatives have a broad range of applications as chemotherapeutic agents in the treatment of cancer. They show a high affinity for DNA and rapid association rates with this biomolecule. The geometry of the fused aromatic ring systems have approximately the dimension of DNA base pairs with which they tend to π-stack [13, 112,
113]. Thus, the primary cellular target of acridines is genomic DNA, to which they bind through intercalation to inhibit important DNA-associated enzymes, such as topoisomerases [113]. The biological activity of these agents is mainly determined by two factors, namely the DNA-binding affinity and the dynamic properties of the intercalator–DNA complex. These two factors can be optimized by tuning the electronics of the acridine chromophore (which also affects its protonation state), for instance by introducing pendant, non-intercalating groups [114].
PT-ACRAMTU (31), derived from 1-[2-(acridine-9-ylamino)ethyl]-1-3- dimethylthiourea (ACRAMTU, 37), has been developed to combat the notorious resistance of certain tumors to current DNA-targeted therapies (Figure 24) [90, 100].
X H N N N H HN
X = S, thiourea derivative, “ACRAMTU”, 37 X = NH, guanidine derivative, “ACRAMGU”, 38
Figure 24. Structure of ACRAMTU and its guanidine analogue.
39 While the protonated form (pKa ≈ 9.8) of ACRAMTU, which intercalates into DNA with the thiourea moiety residing in the minor groove, proved to be moderately cytotoxic to certain cancer cells [109], it produces its cytotoxic potential as a DNA-targeted carrier ligand in platinum–intercalator hybrid agents, in which thiourea sulfur is modified with a
DNA–metalating group [10, 100]. Several second-generation derivatives of PT-
ACRAMTU have been synthesized, which demonstrate excellent activity in chemoresistant non-small cell lung cancers in vitro (Table 1) [10].
Although some platinum–thiourea conjugates demonstrated remarkable biological activity in vitro, they seem to lack a true advantage over PT-ACRAMTU, whose cytotoxicity did not translate into the anticipated antitumor activity against cancers in vivo. In addition, the platinum–Sthiourea bond is quite reactive, which causes ligand
scrambling in PT-ACRAMTU (31) in aqueous solution. The extra lone pair of electrons
on sulfur is able to replace the leaving group (chloride) on another PT-ACRAMTU
molecule which leads to slow decomposition of the drug. The resulting
bis(acridinylthiourea)platinum(II) complex (39) [115] can be isolated from aged solutions
of the drug prototype (Figure 25). While compound 39 itself is a cytotoxic agent that
strongly binds to native DNA because of its bisintercalative binding mode [115], it is an
undesired impurity in preparations of PT-ACRAMTU. It was, therefore, deemed
necessary to replace the platinum–thiourea bond with a less reactive linkage.
40
2+ 2+ NH H2N 2 H2N NH2 Pt Pt Cl S S Cl H H N N N N N N H H HN HN
buffer, media 31 weeks 31
4+ H2N NH2 Pt H H S S N N + aquated HN NH "[Pt(en)Cl2]" species N N NH HN
39
Figure 25. Slow decomposition of PT-ACRAMTU due to nucleophilicity of coordinated thiourea sulfur.
Another reason that prompted us to make changes to the platinum–acridine
linkage was to explore ways of modulating the kinetics of DNA adduct formation. In
Chapter 1, (Section 1.4.3), we have speculated that the cis-effect of thiourea sulfur may
play an important role in the irreversible binding step of the drug, the step in which
chloride on the metal center is substituted by nucleobase nitrogen. It has been
demonstrated by Tobe and his group (Figure 26) that for the complexes
[PtCl(en)(DMSO-S)]+ and [PtCl(en)(DMS)]+ (DMSO = dimethylsulfoxide, DMS =
dimethylsulfide), a sulfur donor greatly accelerates chloride substitution by an incoming
nucleophile over analogous complexes with a nitrogen donor set. While the relative rate
enhancement (kS/kN) was found to be large for strong incoming nucleophiles, such as
- sulfur ligands (SCN ), it is relatively small for weakly nucleophilic ligands, such as H2O
[116, 117].
41
H H2 + 2 (2-n)+ N Cl n- N Y Pt + Y Pt + Cl N L N L H H2 2
Relative 2nd Order Rates
Y L = NH3 L= DMSO-S
H2O 1 1.1
NH 1000 10000 3 SCN- 15000 250000
Figure 26. Mechanistic implications of the kinetic cis-effect. Data adopted from references [116] and [117].
Thus, to improve the stability of the conjugate and to investigate the possibility of kinetic tuning of the Pt center, we proposed to replace thiourea sulfur (sp2) in
ACRAMTU with imino nitrogen (sp2). Nitrogen is a donor group low in the cis-influence series, which also lacks, unlike sulfur, the reactive lone pair electrons when attached to the metal. The core structure of the prototype will be virtually unchanged in such a derivative (Figure 24). The expectation would be that platinum and imino nitrogen would form a thermodynamically stable bond, such as in analogous iminoether-substituted
(HN=C(OR)R’) complexes, which have been tested for antitumor activity [118]. We were particularly interested in generating a guanidine analogue of ACRAMTU, in which thiourea sulfur has been replaced with imino nitrogen (Figure 24). The synthetic studies leading to this molecule, 1-[2-(acridine-9-ylamino)ethyl]-1,3-dimethylguanidine
(ACRAMGU, 38), which reveal unexpected reactivity features of the 9-aminoacridine chromophore, are summarized in this chapter.
42 2.2 Experimental Section
2.2.1 Materials and Chemicals
1H NMR spectra of the target compounds and intermediates were recorded on
Bruker Advance 300 and DRX-500 instruments operating at 500 and 300 MHz,
respectively. 13C NMR spectra were recorded on a Bruker Advance 300 instrument
operating at 75.5 MHz. Chemical shifts (δ) are given in parts per million (ppm) relative to
internal standards trimethylsilane (TMS), or 3-(trimethylsilyl)-1-propanesulfonic acid
1 13 sodium salt (DSS) for samples in D2O. H and C NMR spectra for the new compounds
(38, 40, 42, 43, 45, 46, and 47) are available in Appendix A. All the target compounds
(38, 40, 42, and 43) were fully characterized by gradient COSY and 1H-detected gradient
HMQC and HMBC spectra recorded on a Bruker DRX-500 MHz spectrometer.
Elemental analyses were performed by Quantitative Technologies Inc., Madison, NJ. All
reagents were used as obtained from commercial sources without further purification
unless indicated otherwise. Solvents were dried and distilled prior to use. Flash chromatography was performed using ACROS (50-200 mesh) neutral alumina.
The following compounds were prepared according to preciously described procedures: N-acridin-9-yl-N'-methylethane-1,2-diamine (36) [100], 1-(2-(acridin-9- ylamino)ethyl)-1,3-dimethylthiourea (ACRAMTU, 37) [100], and di(imidazol-1- yl)methanimine (41) [119].
2.2.2 Synthesis and Characterization of Guanidine Derivatives
(Z)-N-[1-(Acridin-9-yl)-3-methylimidazolidin-2-ylidene]methanaminium
Hydrochloride, 40. To a solution of 1-[2-(acridin-9-ylamino)ethyl]-1,3-
43 dimethylthiourea (ACRAMTU, 37, 127 mg, 0.39 mmol) in 5 mL dry tetrahydrofuran
(THF) was added dropwise phosgene in toluene (1.41 mL, 1.93 M solution) at 0 oC. The mixture was stirred for 2 h at this temperature and for another 1 h at room temperature.
The solvents and excess of phosgene were removed in a vacuum. The hygroscopic residue was dissolved in 120 mL of dry acetonitrile, and into this solution was bubbled ammonia gas for 2 h. Solvent was evaporated, and the residue was recrystallized from ethanol to afford the product as yellow needles. Yield: 98 mg (76%); mp 238 – 239.5 oC.
1 H NMR (D2O): δ 8.09 (4H, two overlapping d), 7.92 (2H, t, J = 8.7 Hz), 7.78
(2H, t, J = 7.5 Hz), 4.17 (H7a and H8a, 4H, m), 3.27 (H6a, 3H, s), 1.98 (H4a, 3H, s).
13 C-{H} NMR (D2O): δ 158.2, 148.8, 140.1, 132.26, 129.0, 128.6, 123.6, 122.3,
51.6, 49.8, 32.9, 29.1.
ESI-MS (MeOH, +ve mode) m/z: 291.9 [M]+.
Anal. (C18H19N4Cl·0.5C2H5OH) C, H, N: calcd, 65.23, 6.34, 16.01; found, 65.52,
6.19, 15.77.
2′-(1H-Imidazol-1-yl)-8′-methyl-7′,8′-dihydro-6′H,10H-spiro-[acridine-9,4′- imidazo[1,2-a][1,3,5]triazine], 42. To precursor 36 (251 mg, 1.0 mmol) in 10 mL of dry
THF was added di(imidazol-1-yl)methanimine (41) (338 mg, 2.1 mmol). The mixture was allowed to stir at room temperature for 10 h, and then stored at - 4 oC for several
days. The resulting yellow crystals were filtered off, washed with cold THF and ether,
and dried under vacuum. Yield: 173 mg (47%); mp 261.5 – 262.5 oC.
1 H NMR (DMF-d7): δ 9.58 (NH, 1H, s), 8.40 (H11a, 1H, t), 7.71 (H13a, 1H, t, J =
1.1 Hz), 7.42 (H1 and H8, 2H, d, J = 7.9 Hz), 7.26 (H3 and H6, 2H, t, J = 7.6 Hz), 7.04
44 (H4 and H5, 2H, d, J = 7.8 Hz), 6.98 (H14a, 1H, t), 6.94 (H2 and H7, 2H, t, J = 7.4 Hz),
3.42 (H3a, 2H, t, J = 8.7 Hz), 3.02 (H5a, 3H, s), 2.95 (H2a, 2H, t, J = 8.1 Hz).
13 C-{H} NMR (DMF-d7): δ 159.6, 149.8, 138.9, 136.6, 129.6, 129.5, 128.8, 121.4,
120.7, 117.3, 114.7, 74.0, 46.6, 42.5, 31.1.
Anal. (C21H19N7·0.25 THF) C, H, N: calcd, 67.50, 5.40, 25.04; found, 67.55, 5.14,
25.40.
1-(Acridin-9-yl)-3-methylimidazolidin-2-imine Hydrobromide, 43. To a solution of compound 36 (251 mg, 1.0 mmol) in 10 mL of dry CH2Cl2 was added 0.2 mL of triethylamine. After the yellow solution was cooled to - 10 oC, BrCN (112 mg, 1.05 mmol) was added slowly to the stirred solution. The temperature was not allowed to rise above - 5 oC. When the addition was complete, the mixture was stirred for another 1.5 h.
Solvent was removed, and the residues was washed with a small amount of ice-cold water and dried in vacuum. After recrystallization from ethanol, compound 43 was obtained as a yellow microcrystalline solid. Yield: 221 mg (62%); mp 270 – 271.5 oC.
1 H NMR (D2O): δ 8.48 (4H, overlapping d), 8.40 (2H, t), 8.09 (2H, t), 4.46 (H7a,
2H, m), 4.32 (H6a, 2H, m), 3.29 (H5a, 3H, s).
13 C-{H} NMR (D2O): δ 157.4, 148.1, 141.4, 138.6, 130.4, 125.3, 124.2, 120.8,
51.3, 49.8, 32.4.
ESI-MS (MeOH, +ve mode) m/z: 277.0 [M-H]+.
Anal (C17H17N4Br·0.1C2H5OH) C, H, N: calcd, 57.10, 4.90, 15.48; found, 56.73,
4.73, 15.28.
45 N-Methyl-N′-tert-butoxycarbonylthiourea, 45 and tert-butyl-tert- butoxycarbonylcarbamothioyl(methyl)carbamate, 46. A mixture of N-methyl-thiourea
(44, 1.69 g, 18.7 mmol) and NaH (0.9 g, 22.5 mmol, 60% in mineral oil) in 550 mL of dry THF was stirred for 15 min under Ar at 0 oC. To the solution was added di-tert-butyl
dicarbonate (4.25 g, 19.5 mmol), and stirring was continued for another 30 min. A white
slurry formed within 30 min, which was stirred for another 3 h at room temperature. The
reaction mixture was quenched with 50 mL of saturated aqueous NaHCO3, poured into
450 mL of water, and extracted with ethyl acetate (5 × 150 mL). The combined organic layers were dried over Na2SO4 and concentrated in a vacuum. The reaction mixture was
separated on an alumina column using ethyl acetate/hexane (1:5) to afford 2.11g (59%) of
45 (Rf = 0.39), and 0.95 g (17%) of 46 (Rf = 0.50) as white and pale yellow solids,
respectively.
Characterization of 45:
1 H NMR (CDCl3): δ 9.68 (1H, br s), 7.92 (1H, br s), 3.17 (3H, d), 1.48 (9H, s).
13 C-{H} NMR (CDCl3): δ 180.5, 151.9, 83.7, 32.0, 28.0.
Characterization of 46:
1 H NMR (CDCl3): δ 12.14 (1H, br s), 3.58 (3H, s), 1.55 (9H, s), 1.51 (9H, s).
13 C-{H} NMR (CDCl3): δ 181.2, 154.4, 150.1, 85.4, 82.4, 31.3, 28.0, 27.9.
tert-Butyl-{[2-(acridin-9-ylamino)ethyl]methylamino(methylamino)- methanimine}carbamate, 47. Boc-protected thiourea 45 (460 mg, 2.41 mmol),
precursor 36 (668 mg, 2.66 mmol), and 0.75 mL of triethylamine were dissolved in 12
mL of dry N, N-dimethylformamide (DMF). The solution was cooled to 0 oC and solid
46 HgCl2 (723 mg, 2.66 mmol) was added. The mixture was stirred for 1 h at this
temperature and another 4 h at room temperature until the yellow slurry had turned black.
DMF was removed at 30 oC in vacuum, and the residue was redissolved in ethyl acetate,
filtered through Celite and dried over Na2SO4 overnight. After the solvent was removed,
the crude product thus obtained was purified by flash chromatography on an alumina
column using methanol/ethyl acetate/hexanes (1:2:2), which yielded 47 as a yellow
powder. Yield: 730 mg (74%).
1 H NMR (MeOH-d4): δ 8.27 (2H, br), 7.51 (4H, br), 7.19 (2H, br), 4.02 (2H, t),
3.70 (2H, t), 2.80 (3H, s), 2.65 (3H, s), 1.37 (9H, s).
13 C-{H} NMR (MeOH-d4): δ 161.1, 132.3, 78.9, 38.0, 30.5, 29.3, 26.0, 24.3.
1-[2-(Acridine-9-ylamino)ethyl]-1,3-dimethylguanidine Dihydrochloride, 38.
Compound 47 (730 mg, 1.79 mmol) was dissolved in 200 mL of 2 M HCl, and the
mixture was stirred at room temperature for 12 h. Acid was removed under vacuum, and
the resulting residue was dissolved in a minimum amount of ethanol. Ether was added
into the solution to precipitate the product as a yellow microcrystalline solid, which was
filtered and dried in a vacuum at 60 oC for 2 days. Yield: 510 mg (73%); mp 260.5–261.5
oC.
1 H NMR (D2O): δ 8.11 (H1 and H8, 2H, d, J = 8.6 Hz), 7.95 (H3 and H6, 2H, t, J
= 6.9 Hz), 7.61 (H4 and H5, 2H, d, J = 8.6 Hz), 7.55 (H2 and H7, 2H, t, J = 6.9 Hz), 4.32
(H2a, 2H, t, J = 5.6 Hz), 3.59 (H3a, 2H, t, J = 5.4 Hz), 2.37 (H5a, 3H, s), 2.12 (H9a, 3H, s).
47 13 C-{H} NMR (D2O): δ 159.3, 156.4, 139.2, 136.1, 124.9, 124.5, 118.9, 112.4,
50.6, 46.2, 36.1, 27.9.
ESI-MS (MeOH, +ve mode) m/z: 308.1[M-H]+.
Anal. (C18H23Cl2N5·H2O·0.5 C2H5OH) C, H, N: calcd, 54.15, 6.69, 16.62; found,
53.69, 6.46, 16.48.
2.2.3 Crystallization and Data Collection
The X-ray intensity data were measured on a Bruker SMART APEX CCD area
detector system using MoKα radiation. The structure was solved and refined using the
Bruker SHELXTL software package (version 6.12). Crystallographic data and selected
bond distances and angles for 38, 40, 42 and 43 are summarized in Appendix B.
2.2.4 Biological Characterization: pKa Determinations, DNA Binding Affinities
and Cell Proliferation Assay
pKa measurements
The pKa values of 38, 40 and 43 were determined spectrophotometrically using a
Hewlett Packard 8354 spectrophotometer equipped with a Peltier temperature control and a cell stirring module. Titrations were carried out at 25 oC in the pH range 2–13 with a micro combination electrode placed inside the cuvette. Solutions of the acridine derivatives (~50 μM) in 10-1 M HCl/100 mM NaCl were titrated with aliquots of 0.4 M
KOH, and UV-Vis spectra were recorded 2 min after each addition of base. The
protonation state of the chromophores was deduced from spectral changes around
isosbestic points. pKa’s were determined graphically from plots of pH vs.
48 + + log[k(Aacr/AacrH )] (k = εacrH /εacr) using suitable absorbance maxima. Reported pKa’s are means of three measurements.
Competitive ethidium displacement (fluorescent intercalator displacement assay)
The fluorometric assays have been described before [120]. The concentration of calf thymus DNA (CT-DNA) was determined by UV-vis spectrophotometry at room temperature, after annealling at 85 oC, to ensure accurate concentration determination.
Each well of a 96-well assay plate was loaded with aqueous buffer (6 mM Na2HPO4, 2
mM NaH2PO4, 0.1 mM Na2EDTA and 500 mM NaCl, pH 7.0) containing 37.8 μM
ethidium bromide and 15 μM(bp) CT-DNA.
The C50 values are defined as the drug concentrations which reduced the
fluorescence of the DNA-bound ethidium by 50% and are reported as the mean from duplicate determinations. Fluorescence intensities were determined on BioRad UV CCD
Camera. Apparent equilibrium binding constants were calculated from the C50 values
7 -1 using Kapp = (1.26/C50) × Ke, where Ke = 10 M for ethidium bromide at pH 7 [120] .
Cytotoxicity Assay
The cytotoxicity studies were carried out using the Celltiter 96 Aqueous Non-
Radioactive Cell Proliferation Assay kit. HL-60 leukemia cells and H460 lung cancer
o cells were kept in a humidified atmosphere of 5% CO2 at 37 C. Cells were plated on 96- well plates with 700 cells/well for H460 and 27500 cells/well for HL-60 cells. Stock solutions of 38, 40 and 43 were prepared in phosphate-buffered saline (PBS). The cells growing in log phase were incubated with appropriate serial dilutions of the drugs in
49 duplicate for 72 h. To each well were added 20 μL of MTS/PMS solution, and the
mixtures were allowed to equilibrate for 3 h. The absorbance at 490 nm was recorded
using a plate recorder (none of the compounds tested absorbs at this wavelength). The
reported IC50 data were calculated from non-linear curve fits using a sigmoidal dose- response equation in GraphPad Prism and are averages of two individual experiments.
2.3 Results and Discussion
2.3.1 Two Unusual Reactivity Features of the 9-Aminoacridine Chromophore
When undertaking a survey of the guanidine synthetic organic literature, it appeared that the target molecule might be accessible via direct transformation of the thiourea sulfur in ACRAMTU into guanidine imine. The thiourea derivative, 37, can be conveniently synthesized by reacting N-acridin-9-yl-N′-methylethane-1,2-diamine (36) with methylisothiocyanate (Figure 27) [100]. Several guanidylation reactions have been described that involve Lewis acid-promoted desulfuration [121], oxidation [122, 123], or alkylation [124] of thiourea sulfur and subsequent reaction of the reactive intermediate with ammonia or a primary/secondary amine. Unfortunately, all attempts to generate the target molecule directly from compound 37 using the above-mentioned methodologies were unsuccessful.
The first attempt to prepare target molecule 38 involves desulfuration of 37 with
phosgene, followed by ammonolysis of the resulting chloroformamidinium salt at low
temperature (Figure 27) [125]:
50 H N NH
H N H2N N Cl 36 N NH MeNCS/EtOH/Δ N /MeC H NH3 H o S N Cl N Cl 0 C N N NH N o NH 38 COCl2/THF/NH3/0 C NH 6a 0 o 3 /M 76% C e 7a CN N 76 N N % 8a N NH Cl 37 4a NH4Cl 7 8 1 2 65 4 3 N 40
Figure 27. First attempt to generate ACRAMGU (38) by desulfurating 37 with phosgene which leads to unexpected cyclized compound 40.
As anticipated, the highly nucleophilic thiourea sulfur in 37 reacts with phosgene
to produce a (very hygroscopic) Vilsmeier-type salt. However, instead of the desired
subsequent reaction with ammonia, the reaction leads to cyclization due to the
intramolecular nucleophilic attack of the exocyclic 9-amino nitrogen on the reactive
formamidinium carbon, giving compound 40 (Figure 27), which was isolated as the
guanidinium chloride salt. The second equivalent of HCl released in this condensation is
bound by the added base, NH3.
Next, the stepwise build-up of the guanidine moiety starting from precursor 36
was attempted. The strategy involved sequential replacement of imidazole in
di(imidazole-1-yl)methanimine (41), a versatile guanidylating reagent [119], by (i) the
secondary amino group in 36 and (ii) methylamine (Figure 28). Di(imidazole-1-
yl)methanimine (41) was obtained by treating cyanogen bromide with imidazole according to a modified literature procedure [126].
51
H N N NH H NH N NH NH N N N Me N N N N NH NH 2 41 NH N THF, r.t. N N 38 36
N 11a 5a 13a N N N N 14a 47% HN 3a N 2a N N NH 7 8 1 2 N N 6 5 4 3 N N N N H 42 41′
Figure 28. Second attempt to generate ACRAMGU (38) by sequential replacement of imidazole in 41 by 36 and methylamine, which leads to unexpected spiro compound 42.
According to the literature [119], sequential reaction of 41 with the appropriate amines was expected to lead to the desired, unsymmetrically substituted guanidine, 38.
Reactions of 41 with one equivalent of secondary alkylamine at room temperature were shown to afford the monosubstituted intermediates in 60–80% yield, which could be isolated and reacted with a second equivalent of amine to give the desired guanidine
[119]. The by-product generated is water-soluble imidazole, which can be easily removed by repeated washings with water and saturated NH4Cl. However, in the case of amine 36,
this reaction leads to the formation of an unusual spirocyclic product, 42 (Figure 28),
which indicated that two equivalents of reagent 41 were consumed during the synthesis
of 42. Retrosynthetic analysis of compound 42 suggests that the first equivalent most
likely yielded a cyclized product, 41′, similar to compound 40, which could not be
isolated because the species reacts readily with a second equivalent of 41 via release of
one imidazole group to afford 42. Initially, equimolar amounts of 36 and 41 were reacted,
52 and the stoichiometry was subsequently optimized by applying two equivalents of
reagent 41.
Another strategy involved conversion of the dangling secondary amino group in
36 into cyanamide using cyanogen bromide [127] at -10 oC in the presence of a base. The
goal was to treat the reactive intermediate with methylamine to produce guanidine 38.
However, as in the previous cases, the secondary amino-group (N9) attached to the
acridine chromophore acted as an internal nucleophile, resulting in cyclization leading to
compound 43 (Figure 29), the hydrobromide salt of the nonisolable intermediate 41′
implicated in the formation of 42 (Figure 28).
H N N NH o C NH -10 H e, N M NH NH 2 ot N e-p N BrCN/CH2Cl2 NH on o N NEt3/-10 C 38 N 5a N 6a N 62 % 7a 36 N NH2 Br
7 8 1 2 6 5 4 3 N
43
Figure 29. Third attempt to generate ACRAMGU (38) using cyanogen bromide, followed by addition of methylamine, which leads to cyclized compound 43.
The only strategy to avoid cyclization and feasible pathway to the target guanidine 38 started with the treatment of N-methylthiourea (44) with (Boc)2O in the
presence of NaH (deprotonating 44) to obtain the Boc-activated N-methylthiourea (45)
[128]. Following a published procedure [129], 45 was desulfurated in the presence of
HgCl2. In this reaction, the Boc group plays an important role as an electron-withdrawing
group rather than that of a protecting group. It activates the carbodiimide intermediate
53 (Me-N=C=N-Boc) for nucleophilic addition of the dangling secondary amino group in
acridine-amine 36. The Boc-modified intermediate 47 can be easily deprotected in dilute
acid, giving the dihydrochloride salt of 1-[2-(acridin-9-ylamino)ethyl]-1,3-
dimethylguanidine (38) (Figure 30).
S O S O S O 1. NaH/THF/0oC + H2N N 2. (Boc) O/THF O N N O N N O H 2 H H H 44 45 (major, 59%) 46 (minor, 17%) H H H2N N 9a O N N 2 Cl H N N 3a NH O N 5a 2a NH NH 2M HCl r.t. 7 8 1 2 o 3 Et3N/HgCl2/DMF/0 C 73% 6 5 4 N N N H 36 47 38·2HCl
Figure 30. Synthetic scheme for ACRAMGU, 38.
During the various attempts of making ACRAMGU (38), two unusual reactivity
features of the 9-aminoacridine chromophore have been revealed, which can be
considered the result of the high affinity of exocyclic N9 for the activated formamidinium, guanidyl, and cyanamide carbon centers, resulting in the formation of compounds 40, 42, and 43, respectively. Cyclization giving a five-membered ring is apparently favored over reaction with external nucleophiles (amines), prohibiting the formation of compound 38.
Furthermore, the inherently basic conditions in these systems produced by the external amines (40, 43) or by the basic guanidyl imino group (42) itself have the potential to deprotonate the 9-amino nitrogen, thereby increasing the nucleophilicity of N9. While formation of five-membered cyclic guanidines from 1,2-diamines has been observed
54 previously [130], cyclization of 36 should be unfavorable because of the loss of
resonance stabilization between the N9 and the aromatic system, as evidenced in the
molecular structures of 40 and 43 (see Figure 32 and related discussion). The disruption of N9 conjugation has profound consequences for the basicity, DNA affinity, and biological activity of these agents.
On the other hand, transformation of C9 in 41′, the free-base form of 43 implicated in the formation of 42, into an sp3-hybridized spiro carbon, demonstrates that
this ring atom is highly susceptible to nucleophilic attack.
N N N N HN N N N N N N - ImH N N NH N N HN N N N 41 9 10 9 N N N H 41′ 42
Figure 31. Proposed mechanism of formation of 42.
Figure 31 shows the proposed mechanism of formation of 42. The first step
involves nucleophilic attack of the guanidine nitrogen in 41′ on reagent 41 and release of
imidazole (Im). This produces an intermediate in which the modified 9-guanidyl residue
and the acridine chromophore are essentially orthogonal to each other, positioning the imino nitrogen in close proximity to the reactive C9. In the final step, (concerted)
proximal nucleophilic attack of the imino nitrogen on C9 and proton transfer to N10 then
leads to the formation of a six-membered dihydrotriazine ring to afford the spiro
compound, 42. Spiro compounds incorporating the (9-amino)dihydroacridine moiety are
55 rare, and only structures modified with five-membered heterocyclic rings at the 9-
position have been reported in the literature [131, 132].
The molecular structures of compounds 38, 40, 42 and 43 were determined by
single-crystal X-ray crystallography (Appendix B). In the structure of 40 (Figure 32), which was first predicted on the basis of scalar connectivities in heteronuclear 2-D NMR
spectra, the acridine and guanidinium rings adopt an almost perpendicular orientation with an angle of 97.88o between mean planes. The 9-amino nitrogen deviated
significantly from planarity, as can be expected for a partial sp2→sp3 rehybridization due
to disruption of conjugation with the polyaromatic ring system. Characteristically, nitrogen is displaced by 0.244 Å from the plane through the adjacent carbon atoms. It is
noteworthy to mention that the exocyclic guanidine nitrogen but not the 9-aminoacridine is protonated in this molecule. Similar structural details are observed in the guanidinium cation of compound 43 (Figure 32).
Figure 32. Crystal structures of 40 (chloride salt, left) and 43 (bromide salt, right).
56 The crystal structure of compound 42 confirms the spirocyclic nature of this
molecule (Figure 33). It indicates the loss of aromaticity of the central ring and the
associated bent geometry of the 9,10-dihydroacridine, in which the angle between the
mean planes through the outermost phenyl rings is 26.93o. The endocyclic N1 atom is protonated and can be considered amine-like rather than pyridine-like. Finally, the structure of dicationic 38 in the solid state confirms the desired open-chain structure containing a protonated classical 9-aminoacridine moiety and a terminal guanidinium group (Figure 33).
Figure 33. Crystal structures of 42 (left) and protonated 38 (right, anions not shown).
2.3.2 Biological Evaluation: pKa Determinations, DNA Binding Constants, and
Cytotoxicities
Compounds 38, 40, and 43 were further characterized in solution to assess their utility as DNA targeted agents, and their cytotoxicities were determined in H460 lung
57 cancer cells. Unlike the acridinium/guanidium salts, spiro compound 42 proved to be
insoluble in biologically relevant buffers, consistent with the fact that none of its nitrogen
atoms is protonable under physiological conditions, and it was not included in this study.
Relevant data are summarized in Table 2.
Table 2. Summary of Chemical and Biological Characterization for 38, 40 and 43.
a b -1 c Compound pKa1, pKa2 Kapp (M ) IC50 (μM)
38 8.9, >12 1.5 × 104 10.26 40 2.6, 9.4 < 103 >100 43 2.7, 7.9 5.1 × 103 >100 a 1 Spectrophotometric and H NMR spectroscopic pH titrations. pKa1 and pKa2 reflect the protonation of the endocyclic nitrogen and guanidine nitrogen, respectively. b Apparent binding constant in calf thymus DNA. c Concentrations needed to inhibit proliferation of H460 lung cancer cells by 50%; average of two experiments.
0.3 0.6
0.25 0.5
Absorbance
Absorbance 0.2 0.4
0.15 0.3
0.1 0.2
0.05 0.1
0 0
300 350 400 450 500 300 350 400 450 500
WavelengthWavelength (nm) WavelengthWavelength (nm) (nm)
[38]2+ [38]+ [40]2+ [40]+ [40] pH = 2 pH = 11 pH = 2 pH = 6 pH = 11
pKa1 = 8.9, pKa2 > 12 pKa1 = 2.6, pKa2 = 9.4
Figure 34. pH titrations for compounds 38 and 40 monitored by UV-visible spectroscopy (pH range 2–12).
58 The pKa values of the guanidinium analogues were determined using UV-visible
spectroscopy. Changes in absorbance were monitored as a function of pH, and pKa values were extracted using a graphical method based on the Henderson–Hasselbalch equation.
A plot of pH-dependent UV-visible traces for compounds 38 and 40 is shown in Figure
34.
An important finding in this study is that compounds 40 and 43 exist as monocations at physiological pH, whereas derivative 38 forms a dication. This can be attributed to the dramatically decreased proton affinities of the acridine chromophores’ endocylic nitrogen in 40 and 43 compared to 38 (Table 2), a consequence of the disruption of conjugation between the acridine chromophores and the 9-amino nitrogens, which have become part of the cyclic guanidinium moieties. Therefore, compound 38 is the only derivative containing an acridine chromophore that is protonated at physiological pH, and it is the most promising candidate for generating a hybrid agent with DNA binding properties similar to PT-ACRAMTU (pKa, ACRAMTU = 9.8) [109].
These critical differences in basicity and protonation state ultimately affect the
DNA binding and biological activity of the molecules. Negatively charged DNA interacts
strongly with highly positively charged drugs under physiological conditions, which
often leads to the most effective inhibition of DNA–enzyme interactions and potent
biological activity. This is in agreement with the observation that the classical 9-
aminoacridine derivative 38 shows the highest DNA-binding affinity and proves to be the
only compound showing an appreciable cytotoxic effect in H460 lung cancer cell in vitro.
In addition to the less favorable electrostatics in 40 and 43, the steric hindrance produced
59 by the guanidyl groups may not allow efficient intercalation of the acridine moiety into
the DNA base stack, potentially limiting the agents’ cytotoxic potential.
2.3.3 Attempted Conjugation of ACRAMGU (38) with Monofunctional Platinum
Although a nitrogen analogue of ACRAMTU (37) could be synthesized, namely by changing its thiourea group to a guanidine group in ACRAMGU (38), several attempts of generating the corresponding platinum conjugate were unsuccessful. Reaction of the free-base form of ACRAMGU, generated from the hydrochloride salt under basic conditions, with silver-activated platinum precursor yields a complex in which both chloro leaving groups have been replaced with a chelating (bidentate) guanidinato ligand.
This undesired binding mode, which has been observed previously in similar complexes formed by N-H-acidic thioureas [116, 117], should render platinum unreactive with DNA nitrogen, because it lacks a chloro leaving group. The unexpected reactivity of
ACRAMGU (38) was established by 195Pt-NMR spectroscopy: the chemical shift of –
2570 ppm found for the reaction product is typically observed for Pt2+ coordinated by 4
nitrogen donors. A value 100–150 ppm further downfield would have been expected for
a [N3Cl] environment of platinum [133].
+ H H2N Cl H2N N 1.Base HN Pt N H N + N H NH N N 2
2. H2N NH2 N Pt (NO3) / DMF Cl DMF N 195 HCl Pt NMR(D2O): δ -2570 ACRAMGU·2HCl (38·2HCl)
Figure 35. Reactivity of ACRAMGU (38) with activated [Pt(en)Cl2].
60 2.4 Conclusions
In conclusion, in a successful effort to synthesize the guanidine analogue of the anticancer active 9-aminoacridine derivative ACRAMTU, unexpected reactivity features of the acridine C9–N9 linkage were encountered. On the basis of our observations, new methodologies were developed that allow the formation of two novel structural motifs in this class of compounds: (i) incorporation of N9 into five-membered cyclic guanidinium group and (ii) transformation of C9 into a spiro carbon as part of a triazine-type heterocycle. Further application of these methodologies may lead to novel acridines of potential biological interest. While interesting chemistry was discovered, the ultimate goal of making PT-ACRAMGU was not reached. Alternative linker groups are therefore needed to overcome the inherent chelating properties of the guanidinato group. In
Chapter 3, it will be demonstrated that this can be achieved by introducing an amidine functionality in place of the guanidine group.
61 CHAPTER 3
SYNTHESIS, CHARACTERIZATION, AND BIOLOGICAL EVALUATION OF
AMIDINE DERIVATIVES OF PT-ACRAMTU
The work contained in this chapter was initially published in Journal of Medicinal
Chemistry in 2008 and 2009. The manuscripts, including figures and schemes, were drafted by Zhidong Ma and Dr. U. Bierbach and edited by Dr. U. Bierbach before submission to the journal. Since the publications, changes in both format and content have been made to maintain a consistent format throughout this work. The experiments described were performed by Zhidong Ma, except for that the crystal structure determinations which were contributed by Dr. C. S. Day and the in vivo evaluation, which was performed by Washington Biotechnology Inc.
62 3.1 Background
In Chapter 2, attempts of generating the corresponding platinum conjugate of the
guanidine-modified acridine, ACRAMGU (38), failed due to the chelating properties of
the guanidinato group. Therefore, our attention was focused on other imine-based
ligands (X = NH in Figure 36), such as isoureas, isothioureas and amidines, which lack
a reactive, deprotonable NH group. In these groups, while the overall structure of
ACRAMTU is conserved, one nitrogen atom of thiourea (Y) is replaced with oxygen,
sulfur, or carbon, respectively (Figure 36). Reaction of N-(acridine-9-yl)-N´-
methylethane-1,2-diamine (36) with cyanate [134] and silicon isothiocyanate [135] and
subsequent selective methylation [136] of urea-oxygen and thiourea-sulfur yields
isourea and isothiourea derivatives, respectively (unpublished data). However, after
successful preparation of these two ligands, conjugation reactions with platinum failed,
which was attributed to either chelation or competitive binding between nitrogen and
sulfur to platinum in the case of the isothiourea.
Pt X H N N Y HN
X = S, Y = NH (thiourea derivative, "ACRAMTU") X = NH, Y = O (isourea derivative) X = NH, Y = S (isothiourea derivative) X = NH, Y = CH2 (amidine derivative)
Figure 36. Alternative strategies for generating platinum–imino analogues of PT- ACRAMTU (31).
63 The goal of designing a compound containing a suitably modified acridine
derivative whose geometry would closely mimic that of ACRAMTU was accomplished
by introducing an amidine group (Figure 36; X = NH, Y = CH2). After undertaking a
survey of the amidine synthetic organic literature, it appeared that the target molecule
should be accessible via addition of acridine–amine 36 to propionitrile. Nucleophilic
additions to nitriles are greatly facilitated by protonating the cyano nitrogen or by binding
the nitrile to transition metal ions (Lewis acids), such as Cu(I) [137] or Sm(II) [138], to increase the electrophilicity of the adjacent carbon atom. Unfortunately, all attempts to generate the desired ligand using the above-mentioned methods were unsuccessful.
The synthesis of the target platinum conjugate succeeded when platinum-bound propionitrile was reacted with precursor 36, which allows the carrier ligand to be synthesized and attached to the platinum moiety, [PtCl(en)]+, in one step [139-141].
Details of the new synthetic approach, which resulted in a platinum–acridine conjugate
exhibiting dramatically improved chemical stability and DNA affinity, and, most
importantly, superior biological activity compared to PT-ACRAMU in vitro and in vivo,
are reported in this chapter.
3.2 Experimental Section
3.2.1 Materials and Chemicals
1H NMR spectra of the target compounds and intermediates were recorded on
Bruker Advance 300 and DRX-500 instruments operating at 500 and 300 MHz,
respectively. 13C NMR spectra were recorded on a Bruker Advance 300 instrument
operating 75.5 MHz. Chemical shifts (δ) are given in parts per million (ppm) relative to
64 internal standards trimethylsilane (TMS), or 3-(trimethylsilyl)-1-propanesulfonic acid
1 13 sodium salt (DSS) for samples in D2O. H and C NMR spectra for the new compounds
(52, 53, 54 and 55) are available in Appendix A. 195Pt NMR spectra were recorded on a
Bruker DRX-500 MHz spectrometer at 107.5 MHz. Aqueous K2[PtCl4] was used as
195 2- external standard, and Pt chemical shifts are reported vs [PtCl6] . The target compounds (54 and 55) were fully characterized by gradient COSY and 1H-detected
gradient HMQC and HMBC spectra recorded on a Bruker DRX-500 MHz spectrometer.
Elemental analyses were performed by Quantitative Technologies Inc., Madison, NJ. All
reagents were used as obtained from commercial sources without further purification
unless indicated otherwise. Solvents were dried and distilled prior to use.
3.2.2 Synthesis and Characterization
Cis-Pt(EtCN)2Cl2, 48. Propionitrile, C2H5CN (1 mL, 14 mmol), was added into a solution of 1.0 g of K2PtCl4 (2.4 mmol) in 15 mL of water. The solution was heated to 70
–80 oC for 4 h. The resulting yellow solid was filtered off, washed, and dried under
vacuum. The dried crude product was then purified on a silica column using
CHCl3/(CH3)2CO (1:1) as eluent, to yield cis-isomer 48 (0.5 g, 63%).
1 H NMR (CDCl3) δ 2.86 (4H, q, J = 7.6 Hz), 1.41 (6H, t, J = 7.5 Hz).
Pt(EtCN)(C19H22N4)Cl2, 50. A suspension of 48 (375 mg, 1 mmol) in 20 mL of
o CH2Cl2 was treated with 36 (251 mg, 1 mmol) at -10 C. After 6 h the solution was
allowed to warm up to room temperature and stirred for another 4 h. The reaction mixture
was concentrated to a small volume and then treated with cold ether. A yellow precipitate
was collected and separated on a preparative alumina plate using hexanes/CH2Cl2 (2:3) as
65 eluent. The band of interest was collected and extracted with CH2Cl2. Slow evaporation
of the solvent gave 50 as single crystals suitable for X-ray crystallography. 1H NMR
spectrum taken of this fraction shows an unexpected doubling of signal sets, indicating
the presence of two geometrical isomers (cis and trans) in solution. From this mixture, the trans-isomer crystallized. No further attempts to separate this mixture were undertaken.
[PtCl(en)(EtCN)]Cl, 52. The complex [PtCl2(en)] 51 (200 mg, 0.613 mmol) was
heated at reflux in dilute HCl (pH 4) with propionitrile (2.7 mL, excess) until the yellow
suspension turned into a colorless solution (~2 h). Solvent was removed by rotary
evaporation, and the pale yellow residue was dissolved in 7 mL of dry methanol. The
solution was passed through a syringe filter, and the colorless filtrate was added directly
into 140 mL of vigorously stirred dry diethyl ether, affording 52 as an off-white
microcrystalline precipitate, which was filtered off and dried in a vacuum. Yield: 210 mg
(90%).
1 H NMR (D2O) δ 2.88 (2H, q, J = 7.5 Hz), 2.64 (4H, m), 1.30 (3H, t, J = 7.5 Hz).
13 C-{H} NMR (D2O) δ 122.9, 48.7, 48.4, 12.3, 9.2.
195 Pt NMR (D2O) δ −2711.
Anal. (C5H13Cl2N3Pt) % C, H, N: calcd 15.76, 3.44, 11.02; found 15.73, 3.06,
10.74.
[PtCl(NH3)2(EtCN)]Cl, 53. This precursor was synthesized analogously to 52
starting from [PtCl2(NH3)2] (300 mg, 1 mmol) and propionitrile (4.2 mL). Yield: 295 mg
(83%).
66 1 H NMR (D2O) δ 2.89 (2H, q, J = 7.5 Hz), 1.31 (3H, t, J = 7.5 Hz).
13 C-{H} NMR (D2O) δ 121.9, 12.3, 9.2.
195 Pt NMR (D2O) δ -2467.
Anal. (C3H11Cl2N3Pt) % C, H, N: calcd 10.15, 3.12, 11.83; found 10.09, 2.90,
11.69.
[PtCl(en)(C19H23N4)](NO3)2. 54. Precursor complex 52 (170 mg, 0.45 mmol)
was converted to its nitrate salt by reaction with AgNO3 (75 mg, 0.44 mmol) in 10 mL of anhydrous DMF. AgCl was filtered off, and the filtrate was cooled to −10 °C. N-
(Acridin-9-yl)-N´-methylethane-1,2-diamine (36) [100] (117.4 mg, 0.47 mmol) was added to the solution, and the suspension was stirred until it turned into an orange-red solution (~7 h). The reaction mixture was added dropwise into 200 mL of cold dichloromethane, and the resulting yellow slurry was vigorously stirred for 30 min. The precipitate was recovered by membrane filtration, dried in a vacuum overnight, and dissolved in 40 mL of methanol containing 1 mol equiv of HNO3. After removal of the
solvent by rotary evaporation, the crude product was recrystallized from hot ethanol,
affording 54 as a microcrystalline solid. Yield: 169 mg (52%).
1 H NMR (DMF-d7) δ 13.92 (1H, s), 9.90 (1H, s), 8.70 (2H, d, J = 8.6 Hz), 8.07
(4H, overl m), 7.63 (2H, t, J = 6.8 Hz), 5.78 (2H, s), 5.48 (2H, s), 4.51 2H, t, J = 6.3 Hz),
4.10 (2H, t, J = 6.7 Hz), 3.21 (3H, s), 3.12 (2H, q, J = 7.4 Hz), 2.68 (4H, s), 1.33 (3H, t, J
= 7.5 Hz).
13 C-{H} NMR (DMF-d7) δ 170.4, 159.0, 140.6, 135.7, 128.2, 124.3, 119.4, 113.5,
50.1, 49.4, 49.2, 47.5, 28.0, 11.4.
195 Pt NMR (DMF-d7) δ −2494.
67 UV/Vis (H2O): λmax 413, ε = 10571.
Anal. (C21H33ClN8O6Pt·H2O) % C, H, N: calcd 34.08, 4.49, 15.14; found 34.16,
4.31, 15.15.
[PtCl(NH3)2(C19H23N4)](NO3)2, 55. This analogue was prepared as described for
52 starting from 293 mg (0.83 mmol) of 53, 132 mg (0.79 mmol) of AgNO3, and 197 mg
(0.79 mmol) of N-(acridin-9-yl)-N´-methylethane-1,2-diamine. Yield: 315 mg (57%).
1 H NMR (DMF-d7) δ 13.93 (1H, s), 9.92 (1H, s), 8.68 (2H, d, J = 8.6 Hz), 8.03
(4H, overl m), 7.62 (t, J = 7.2 Hz), 6.27 (1H, s), 4.53 (3H, s), 4.49 (2H, t, J = 6.8 Hz),
4.16 (3H, s), 4.10 (2H, t, J = 6.3 Hz), 3.20 (3H, s), 3.15 (2H, q, J = 7.6 Hz), 1.33 (3H, t, J
= 7.5 Hz).
13 C-{H} NMR (DMF-d7) δ 170.3, 159.3, 140.8, 135.9, 126.5, 124.5, 119.6, 113.6,
50.8, 47.8, 28.3, 11.5.
195 Pt NMR (DMF-d7) δ −2264.
UV/Vis (H2O): λmax 413, ε = 9224.
Anal. (C19H29ClN8O6Pt·2.5H2O) % C, H, N: calcd 30.79, 4.62, 15.12; found 30.66,
4.14, 15.03.
Complexes 52´ and 54´ containing 15N-en were synthesized accordingly starting
15 from [PtCl2( N-en)] (51´).
[PtCl(15N-en)(EtCN)]Cl, 52´.
1 1 1 15 H NMR (MeOH-d4): δ 6.11 and 5.86 (2H, d of t, NH2 trans to Cl, J( H– N) =
3 1 1 1 1 15 75 Hz, J( H– H) = 5.3 Hz), 6.01 and 5.76 (2H, d of t, NH2 trans to N, J( H– N) = 75
Hz, 3J(1H–1H) = 5.2 Hz), 2.93 (2H, q, J = 7.6 Hz), 2.57 (4H, m), 1.33 (3H, t, J = 7.5 Hz).
68 15 [PtCl( N-en)(C19H23N4)](NO3)2, 54´.
1 H NMR (DMF-d7) δ 13.92 (1H, s), 9.90 (1H, s), 8.70 (2H, d, J = 8.6 Hz), 8.07
(4H, m, overlap), 7.63 (2H, t, J = 6.8 Hz), 6.26 (NH, 1H, s), 5.82 and 5.53 (2H, d of t,
1 1 15 3 1 1 NH2 trans to Cl, J( H– N) = 74.5 Hz, J( H– H) = 5.0 Hz and 5.1 Hz), 5.47 (2H, d of t,
1 1 15 3 1 1 NH2 trans to N, J( H– N) = 74.7 Hz, J( H– H) = 5.1 Hz), 4.51 (2H, t, J = 6.3 Hz),
4.10 (2H, t, J = 6.7 Hz), 3.21 (3H, s), 3.12 (2H, q, J = 7.4 Hz), 2.68 (4H, s), 1.33 (3H, t, J
= 7.5 Hz).
3.2.3 Cell Proliferation Assay
The cytotoxicity studies were carried out according to a standard protocol [142]
using the Celltiter 96 Aqueous Non-Radioactive Cell Proliferation Assay kit. HL-60
leukemia cells and H460 lung cancer cells were kept in a humidified atmosphere of 5%
o CO2 at 37 C. Cells were plated on 96-well plates with a total 700 cells/well for H460 and
27500 cells/well for HL-60 cells.
Stock solutions of PT-ACRAMTU (31), 54 and 55 were prepared in phosphate-
buffered saline (PBS) and serially diluted with media prior to incubation with cancer cells.
The cells growing in log phase were incubated with appropriate serial dilutions of the
drugs in duplicate for 72 h. To each well was added 20 μL of MTS/PMS solution, and the
mixtures were allowed to equilibrate for 3h. The absorbance at 490 nm was recorded
using a plate recorder (none of the derivatives tested absorbs at this wavelength). The
reported IC50 data were calculated from non-linear curve fits using a sigmoidal dose- response equation in GraphPad Prism (GraphPad Software, La Jolla, CA) and are averages of four individual experiments.
69 3.2.4 Crystallization and Data Collection
The X-ray intensity data were collected on a Bruker SMART APEX CCD area
detector system using MoKα radiation. The structures were solved and refined using the
Bruker SHELXTL software package (version 6.12). Crystallographic data and selected bond distances and angles for 50 and 54 are summarized in Appendix B.
3.2.5 NMR Spectroscopy
Time-dependent NMR spectroscopy. NMR spectra in arrayed experiments were collected at 37 °C on a Bruker 500 DRX spectrometer equipped with a triple- resonance broadband inverse probe and a variable temperature unit. Reactions were performed in 5-mm NMR tubes containing 2 mM platinum complex 54 and 6 mM 5´- guanosine monophosphate (5´-GMP) or 2 mM complex 54 and 2 mM N-acetylcysteine
1 (N-AcCys) (10 mM phosphate buffer, D2O, pH* 6.8). The 1-D H kinetics experiments
were carried out as a standard Bruker arrayed 2-D experiment using a variable-delay list.
Incremented 1-D spectra were processed exactly the same, and suitable signals were
integrated. Data were processed with XWINNMR 3.6 (Bruker, Ettlingen, Germany).
The concentrations of platinum complex at each time point were deduced from relative
peak intensities, averaged over multiple signals to account for differences in proton
−x/t −1 relaxation, and the data were fitted to the equation y = A0 × e (where A0 = 1 and t = kobs) using Origin 7 (OriginLab, Northampton, MA).
2-D NMR spectroscopy. 2-D 1H-15N NMR experiments were performed with
15N-(en)-labeled complex, 54´, by setting up an alternating array of 1-D 1H
WATERGATE (water suppression by gradient-tailored excitation) and 1H-15N HMQC
70 experiments. Experiments were performed in 5 mm NMR tubes at 37 °C in 10 mM
1 sodium phosphate buffer made in 90% H2O/10% D2O (pH 6.8). The 1-D H
WATERGATE pulse sequence was modified to incorporate a delay (d31). Each delay
was set to collect data at 20-min intervals, followed by acquisition of a 2-D 1H-15N
HMQC data set. Each HMQC were collected with 2 scans per time increment. A sweep
width of 10,139 Hz in F1 and 5000 Hz in F2 was used in each experiment. All 2-D
spectra were processed to 1024 in F2 and 1024 in F1 using forward linear predication.
15 15 N chemical shifts were referenced to an external NH4Cl standard (25 mM in 0.1 M
DCl). All data sets were processed with Felix 2000 (Accelrys, San Diego, CA).
3.2.6 Liquid Chromatographic Separation–Mass Spectrometry Detection
Incubations. Reactions of 54 or PT-ACRAMTU (31) with N-acetylcysteine were
performed at 1:1 drug-to-amino acid ratio in 10 mM sodium phosphate buffer (pH 7.1).
Incubations were performed at 37 oC for 6 h.
Chromatographic separations. The mixtures were separated by reverse-phase
HPLC using the LC module of an Agilent Technologies 1100 LC/MSD Trap system equipped with a multi-wavelength diode-array detector and autosampler. A 4.6 × 150 mm reverse phase Agilent ZORBAX SB-C18 (5 µm) analytical column was used in all the
assays, which was maintained at 25 oC during separations. One wavelength, 413 nm, was
used to monitor acridine containing fragments. The following eluents were used for the
separations: solvent A, degassed water/0.1% formic acid, and solvent B, methanol/0.1% formic acid. The gradient for separation was solvent A changing from 98% to 30% over
71 30 min at a flow rate of 0.5 mL/min. The integration of peaks was done using the
LC/MSD Trap Control 4.0 data analysis software.
Mass spectrometry. Mass spectra were recorded on an Agilent 1100LC/MSD ion
trap mass spectrometer. After separation by in-line HPLC, adducts were infused into the atmospheric-pressure electrospray source. Ion evaporation was assisted by a flow of N2 drying gas (350 oC) at a pressure of 40 psi and a flow rate of 10 L/min. Positive-ion mass spectra were recorded with a capillary voltage of 2800V and a mass-to-charge scan range
of 150 to 2200 m/z.
3.2.7 Confocal Fluorescence Microscopy
Cell line. The human ovarian carcinoma cell line A2780 was obtained from
Sigma Aldrich and was used for all fluorescence microscopy studies. Cells were
maintained in exponential growth phases as monolayers in Advanced DMEM media
supplemented with 2% fetal calf serum, 1% Antibiotic Antimyotic and 1% glutamine at
o 37 C in 5% CO2. PBS and trypsin were obtained from Invitrogen.
Instrumentation and materials. Confocal images were aquired using a Nikon
DE-Eclipse microscope C1 equipped with Coherent Radius 405-25, Sapphire 488.20 and
Melles Griot 561 optically pumped semiconductor laser systems. Fluorescence imaging
was performed using an Olympus CellR System, equipped with an MT20 illumination
system. The excitation filters used were 403/12, 492/18, 572/23 nm (DAPI/FITC/Texas
RED) and emission filters 435/475, 510/550 and 595/700 nm (DAPI/FITC/Texas RED).
The objective used was an UPLAN Super Apochromat 40X, NA 0.9 with correction
collar (0.11-0.23). LysoTracker Red DND-99 lysosome marker and MitoTracker Green
72 FM were obtained from Invitrogen. Cells were treated with hydrogen peroxide to induce apoptosis.
Incubations. A2780 cells were seeded in 6-well plates containing glass coverslips.
Approximately 2 × 105 cells were seeded in each well with 2 mL of media, before being
o incubated at 37 C in 5% CO2 for 24 h. For colocalization studies, the cells were then
dosed with either PT-ACRAMTU (31) or 54 at a concentration of 20 mM, and incubated for another 10 min. LysoTracker Red DND-99 (1 μM) or MitoTracker Green FM (1 μM) was added to drug-containing cells and untreated cells. For apoptosis studies, cells were treated with either, 1) drug (20 mM), 2) hydrogen peroxide (6.6 μL, 30%), or 3) drug (20 mM) plus hydrogen peroxide (6.6 μL, 30%), and then incubated for 10 min. Coverslips
were mounted on glass slides and sealed with nail polish, and images were collected
within 10 min of slide preparation.
3.2.8 H460 Xenograft Study
H460 xenografts were established in nude athymic female mice via bilateral
subcutaneous injections. Treatment began when the average tumor volume was
approximately 100 mm3. The tumor-bearing mice were randomized depending on tumor
volume into three groups of five test animals each: one control group receiving vehicle
only, one group treated at 0.1 mg/kg 5d/w (A), and one group treated at 0.5 mg/kg q4d
(B). Animal weights and tumor volumes were measured and recorded for 17 days after
the first dose was administered. Tumor volumes were determined using the formula: V
(mm3) = d2 × D/2, where d and D are the shortest and longest dimension of the tumor, respectively, and are reported as the sum of both tumors for each test animal. At the end
73 of the study, all animals were euthanized and disposed off according to Standard
Operating Procedures (SOPs). One tumor per test animal as well as lungs and kidneys
were removed during necropsy and shipped to the Bierbach lab for detection of platinum levels in each tissue. The xenograft study was performed by Washington Biotechnology
Inc. (Simpsonville, MD), PHS-approved Animal Welfare Assurance, # A4912-01.
Statistical analysis of the growth curves was done using a non-linear polynomial random- coefficient model in SAS Proc Mixed (SAS Institute Inc., Cary, NC).
3.2.9 Inductively Coupled Plasma–Mass Spectrometry Detection of Platinum
Drugs and Standards. Nitric acid (70%, trace-metal grade) and hydrochloric acid (35% trace-metal grade) standards were obtained from Fisher. Platinum standard, as well as internal indium and bismuth were obtained from High-Purity Standards. All standards and spike solutions were prepared and stored in PFA containers.
Aqueous calibration standards were prepared by diluting 10 μg/mL platinum single-element ICP–MS stock solutions (High-Purity Standards with 2% HCl).
Quantification of element concentration was performed using external calibration with aqueous standard solutions and internal standardization using 115In and 209Bi (at 10 ppb in
the final solution) as internal standards. The Pt isotopes used for calculation of Pt
concentrations were 194Pt and 195Pt.
Sample preparation. Tissue samples (tumor, kidneys, and lungs) were thawed
and used for determination of total Pt in tissues. The samples were blotted to remove
extra fluid, weighed, and placed in a solution containing 2.5 mL of 70% nitric acid and
2.5 mL of 35% hydrochloric acid. The samples were digested at 60 oC for 16 h. The
74 aqueous digestate was diluted to 25 mL using distilled deionized water. After the samples
were centrifuged for three minutes at 3000 rpm, an aliquot of 4 mL of supernatant was
mixed with 0.04 mL internal standard containing 1 ppm indium and bismuth. The
solution was transferred to 15 mL tubes for measurement of Pt by inductively coupled
plasma mass spectrometry on a Thermo ICP–MS instrument at the Analytic Chemistry
Service facility of the Research Triangle Institute, Research Triangle Park, NC.
3.3 Results and Discussion
3.3.1 Lewis Acid Assisted Addition Reaction
Prior to the successful synthesis of the amidine analogue of PT-ACRAMTU (31),
a test reaction was performed to determine the feasibility and conditions of the platinum- mediated amidination. The susceptibility of platinum-bound nitriles to nucleophilic attack by water, amines, alcohols and thiols yielding amidates, amidines, iminoethers and iminothioethers has been reported in the literature [143-146]. Based on the reported reactivity of acetonitrile complexes of cisplatin [146], a reaction scheme leading to the desired ligand, N-[2-acridin-9-ylamino]ethyl]-N-methylpropionamidine, can be proposed
(Figure 37). The strategy involves addition of the secondary dangling amino group in N-
(acridine-9-yl)-N´-methylethane-1,2-diamine (36) to the propionitrile ligand in complex
48.
75
N Cl Cl NH H2O/C2H5CN Pt HN K PtCl 2 4 o N N 55 C o CH2Cl2 / -10 C
48
C Cl Cl N Cl Pt N Pt N N + Cl NH N NH N C N N H H
49 50
Figure 37. Test reaction for addition of secondary amine to platinum-bound nitrile.
To generate cis-bis(propionitrile)dichloroplatinum(II) (48), propionitrile was added in large excess to an aqueous solution of K2PtCl4, and the reaction mixture was stirred for several hours at 70-80 oC. The product, which was contaminated with the trans-isomer, was purified by column chromatography. One nitrile ligand in complex 48 is then converted into amidine by reaction with N-(acridine-9-yl)-N´-methylethane-1,2- diamine (36) at low temperature to produce a mixture of cis- and trans-[PtCl2(EtCN)(N-
[2-acridine-9-ylamino]ethyl)-N-methylpropionamidine] (49 and 50), indicating cis–trans isomerization. While the geometric isomers could not be separated chromatographically, the trans-isomer, 50, crystallized selectively from the mixture and produced crystals suitable for X-ray crystallography. The crystals structure of complex 50 confirms the formation of desired amidine coordinative linkage (Figure 38). An interesting feature in this structure proves to be the protonation state of the acridine moiety. The proton of the exocyclic 9-amino group (N3) has migrated to endocyclic N4, producing the unusual
76 imino form of the acridine, which is stabilized by an intramolecular hydrogen bond formed between N1 and N3.
The formation of the platinum–amidine bond in complex 50, which confirmed the feasibility of the proposed addition chemistry, encouraged us to pursue the same strategy for the synthesis of the amidine analogue of PT-ACRAMTU from the appropriate monofunctional platinum–nitrile precursors.
Figure 38. Molecular structure of trans-[PtCl2(EtCN)(N-[2-acridin-9-ylamino]ethyl]-N- methylpropionamidine)] 50 with selected atoms labeled.
3.3.2 Design Rationale
Using the methodology developed in the previous section, the desired acridine, N-
[2-acridin-9-ylamino]ethyl]-N-methylpropionamidine, was synthesized and introduced as
77 a ligand in one step in place of ACRAMTU (Figure 39). In analogy to the synthesis of complex 50, the reaction involved addition of the secondary amino group in N-(acridin-9- yl)-N´-methylethane-1,2-diamine (36) across the activated CN bond [147] of platinum- bound propionitrile (EtCN). The amidination reactions were performed with the appropriate platinum precursors (52, 53) derived from complexes [PtCl2L2] (L2 = en, 51;
L = NH3, 14) to afford the corresponding platinum–acridine hybrids, which were isolated in their fully protonated forms as the water soluble dinitrate salts, 54 and 55. NH3, instead of en, was introduced as a nonleaving group to increase the solubility of the conjugate.
2+ N + H N N L L L L H L L Pt i Pt ii NH (NO ) + C2H5CN Cl Pt 3 2 Cl Cl 85% Cl N 50-60% Cl HN N N H
14/51 52/53 54/55
51, 52 and 54: L2 = en; 14, 53 and 55: L = NH3
Figure 39. Synthesis of target compounds. o Reagents and conditions: (i) EtCN/H2O, 70 C, 2 h. (ii) (1) AgNO3, DMF, rt; (2) N- o (acridine-9-yl)-N´-methylethane-1,2-diamine (36), DMF, -10 C, 7 h; (3) HNO3, MeOH
The structure of 54, as determined by X-ray crystallography (Appendix B), confirms the formation of the desired amidine coordinative linkage and overall complex geometry, which shows platinum in a square planar environment defined by a single chloride leaving group, a bidentate en nonleaving group, and amidine nitrogen (Figure
40). A comparison of the molecular structures of 54 and PT-ACRAMTU reveals a critical difference between the amidine and thiourea coordination modes: while the bond angle at
78 the donor atom (N1) in 54 is 129.6(4)o, the corresponding angle at sulfur in PT-
ACRAMTU is considerably more acute (103.8(4)o) [102]. The opening of this bond angle in 54 relieves steric hindrance in the coordination sphere of the metal, which has previously been identified as a rate-limiting factor in reactions of PT-ACRAMTU with
DNA nitrogen [102, 111].
Figure 40. Molecular structure of 54 with selected atoms labeled. Nitrate counterions are not shown. Thermal ellipsoids are drawn at the 50% probability level.
3.3.3 Biological Activity
The newly synthesized conjugates 54 and 55 were studied, along with the prototype, PT-ACRAMTU (31), for their cytotoxic effects in the human leukemia cell line, HL-60, and in the NSCLC cell line, NCI-H460. The results of the cell proliferation assay are summarized in Table 3. In HL-60 cells, complex 54 showed moderate activity
79 similar to PT-ACRAMTU, based on an IC50 value in the micromolar range. Complex 55,
in which the en group was replaced with the solubility-enhancing ammine ligands, was
approximately 6-fold more potent in this cell line than 54. All three compounds
performed significantly better in H460 cells. This cell-line specific enhancement is most
pronounced for compound 54, which proves to be 100-fold more active in the solid tumor
cell line than in the leukemia cell line. Most strikingly, complexes 54 and 55 were an
order magnitude more cytotoxic in H460 cells than PT-ACRAMTU. This is significant
because PT-ACRAMTU has previously demonstrated only slightly better activity in this
cell line than cisplatin (IC50 of 0.63 μM in clonogenic survival assays [103]).
a Table 3. Cytotoxicity Data (IC50 , μM) for Platinum–Acridine Conjugates.
Compound HL-60 (μM) H460 (μM)
PT-ACRAMTU(31) 3.95 ± 0.24 0.35 ± 0.017 54 2.97 ± 0.11 0.028 ± 0.0024 55 0.47 ± 0.064 0.026 ± 0.0022
aConcentrations of compound that reduce cell viability by 50% determined by cell proliferation assays. Cells were incubated with platinum for 72 h. Values are means of four experiments ± the standard errors of the mean.
Complexes 54 and 55 are remarkably cytotoxic in H460 NSCLC cells. The only
platinum-based agent known to inhibit H460 cell growth with similar potency in the
nanomolar concentration range is [(trans-PtCl(NH3)3(trans-Pt(NH3)2
(NH2(CH2)6NH2)2))](NO3)4 (BBR3464, 24) (discussed in Chapter 1, Figure 18), a
trinuclear platinum complex currently in clinic trials against various cancers. Cisplatin is
at least 20-fold less potent than the nonclassical agents in H460 cells, with IC50 values
typically in the micromolar range [103].
80 3.3.4 Kinetics of Platinum Binding with Biologically Relevant Nucleophiles
Kinetics of DNA Platination. To test the effect of the modification made to the
prototypical agent on the metal´s activity with DNA, the reactions of PT-ACRAMTU (31)
or 54 with excess 5´-guanosine monophosphate (5´-GMP) were monitored by 1H NMR spectroscopy in a simple model system mimicking DNA nitrogen. Compound 54 reacts significantly faster with 5´-GMP than PT-ACRAMTU (Figure 41).
A 2+ Cl 23 O 4 23 H2N NH2 1 H2N NH2 4 N NH 1 Pt 10NH + O Pt Cl X 9 8 10NH N N X 9 N N NH2 HN N N 5 N H 876 R' 8 5 Y H 8 6 H N N N Y 7 5'-GMP 2 R'
O NaO P O ONa O R' = OH OH
PT-ACRAMTU 31: X = S, Y = NH 54 : X = NH, Y = CH2
Figure 41. Kinetic study using time-dependent 1H NMR spectra for the reaction of PT- ACRAMTU (31) and complex 54 with 5´-GMP monitored for aromatic guanine and acridine protons giving signal assignments. R´ stands for ribose and phosphate. (A) Reaction scheme with important atoms labeled.
81 B Adduct(↑) C Adduct (↑) G-H8 H1/H8 H3/H6 H4/H5 H2/H7 G-H8(br) H1/H8 H3/H6 H4/H5 H2/H7 Time(h)
---17---
---8---
---2---
---0---
8.6 8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 ppm 8.6 8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 ppm H4/H5 H1/H8 G-H8 H3/H6 H2/H7 H1/H8 G-H8 H3/H6 H4/H5 H2/H7 Reactant(↓) Reactant (↓)
Figure 41 (Continued). (B) Stacked plot for the reaction monitored for PT-ACRAMTU (31). (C) Stacked plot for the reaction monitored for 54.
82 o From pseudo first-order treatment of the kinetic data, rate constants (kobs, 37 C) of 1.77 × 10-4 s-1 and 4.93 × 10-5 s-1 were calculated, which correspond to half-lives of 65
and 234 min for 54 and PT-ACRAMTU, respectively (Figure 42). Detailed analysis of
the 1H NMR spectra indicates that compound 54 forms monoadducts, in which guanine-
N7 has selectively substituted the chloride ligand, whereas the platinum–amidine linkage
proves to be resistant to nucleophilic attack by nucleobase nitrogen in the presence of
excess nucleotide, in complete analogy to PT-ACRAMTU [110, 111].
Rate constant (kobs) Half Life (t1/2) PT-ACRAMTU (31) 4.93 × 10-5 s-1 234 min
-4 -1 54 1.77 × 10 s 65 min
Figure 42. Progress of the reaction of PT-ACRAMTU (31) (circles) and 54 (triangles) with the mononucleotide 5´-GMP at 37 oC monitored by 1H NMR spectroscopy. The inset shows a scheme of the reaction monitored. The data plotted is the mean of two experiments. Rate constants and half lives have been calculated for PT-ACRAMTU and 54, respectively.
83 A 2-D [1H, 15N] heteronuclear multiple quantum coherence (HMQC) NMR study
performed with 15N-labeled complex (54´) supports this mechanism and suggests that adduct formation is most likely preceded by aquation of the Pt–Cl bond, which is commonly observed in platinum drug–DNA interactions [98]. HMQC NMR spectra were recorded at each time point for 54´ to elucidate the substitution chemistry at the platinum center and to detect possible hydrolysis products. 15N-labeled en as a non-leaving group
is a sensitive probe to detect structural changes in the coordination sphere of platinum:
15N chemical shifts of N-donor ligands attached to platinum (and other metals) are
sensitive to changes in the coordination sphere, especially substitution of ligands trans to the observed 15N nucleus.
1 15 The [ H, N] HMQC spectrum recorded of 54´ in phosphate-buffered H2O/D2O
(90%/10%) shows two cross-peaks at δ 4.98/-34.59 ppm and 5.19/-34.61 ppm. These have been assigned to the NH2 group trans to the amidine nitrogen and to the NH2 group trans to the chloro leaving group, respectively, on the basis of the greater 15N deshielding
effect of chloride compared to the nitrogen (Figure 43, left). In the 2-D spectrum
recorded after 4 h of incubation at 37 oC, two new cross-peaks for the putative hydrolysis product are observed (Figure 43, right). Only a small amount of complex 54´ turned into hydrolyzed product, in agreement with a reversible reaction that lies far to the right. The cross-peaks at δ 5.01/-51.91 ppm and 5.38/-34.32 ppm in this spectrum can be assigned to the NH2 group trans to water oxygen (least deshielding) and to the NH2 group trans to
amidine nitrogen, respectively.
84 3+ 2+
B A H N NH H O 2 2 NH H2N NH2 NH 2 Pt Pt H2O HN N Cl HN N N N H H
Figure 43. Hydrolysis of complex 54´ detected by 2-D [1H-15N] HMQC NMR spectroscopy giving cross-peak assignments.
The 2-D [1H, 15N] HMQC study was designed to detect the potential hydrolysis
intermediate in reaction mixtures containing 54´ and excess 5´-GMP in phosphate buffer
(Figure 44). However, detection of such a species was unsuccessful, which might be
attributable to the low abundance of the hydrolyzed intermediate due to rapid substitution
of this weak leaving group by 5´-GMP (see Figure 41). Complex 54´ and its
monofunctional guanine adduct were the only species detected in the time-dependent
HMQC spectra (Figure 44). In the adduct, the NH2 groups trans to G-N7 and trans to
amidine NH experience similar trans-influences from the sp2 nitrogens, resulting in
overlapping cross-peaks.
85 3+ 2+ B A 5'-GMP H2N NH2 NH H2N NH2 NH Pt Pt N7 HN N Cl HN N GMP N N H H
t = 0 min t = 40 min
artifact artifact
-34.59/4.98 NH2 trans to N A&B
-34.61/5.19 NH2 trans to Cl
t = 100 min t = 180 min
artifact artifact
-35.66/5.33 and -35.93/5.37 NH2 trans to G-N7 and amidine NH
A&B A&B
Figure 44. Reaction of complex 54´ with 5´-GMP monitored by 2-D [1H-15N] HMQC NMR spectroscopy giving cross-peak assignments.
86 The simple modification made to PT-ACRAMTU (31) resulted in greatly enhanced DNA binding kinetics. While the exact mechanism for the amidine analogue 54 and the associated rate laws are yet to be determined, amidine nitrogen appears to accelerate associative substitution reactions, most likely due to steric factors favoring nucleophilic attack (either from water or DNA bases) on the metal. Hydrogen bonding between the imino hydrogen of the complex and exocyclic groups of the DNA bases, such as guanine-O6 [98], may also contribute to the rate enhancement. Efficient DNA binding of the new derivatives is most likely a major contributor to the greatly improved biological activity in rapidly proliferating H460 cells.
Reactivity with cysteine sulfur. In complete analogy to the nucleotide binding study discussed in the previous section, the reaction of the new conjugate (54) with cysteine sulfur was tested. In arrayed one-dimensional 1H NMR experiments, the reaction of 54 with one equivalent of N-acetylcysteine (N-AcCys) (25 oC, 10 mM phosphate buffer, D2O, pH* 6.8) was found to proceed considerably more slowly than the reaction of PT-ACRAMTU under the same conditions. The relative rates at which the amino acid reacted with 54 and PT-ACRAMTU were deduced from the change in integral intensities of 1H NMR signals assigned to the platinum complexes (Figure 45).
87 en A 2+ HO H2N NH2 O NH Products Pt + O Cl X N N HS HN (see Fig. 48) H Y N-acetylcysteineN-acetylcysteine
linkage
PT-ACRAMTU 31: X = S, Y = NH 54 : X = NH, Y = CH2 Figure 45. Time-dependent 1H NMR spectra for the reaction of PT-ACRAMTU (31) and complex 54 with N-AcCys. Drug-based proton signals integrated for plotting the concentration–time traces are highlighted with black boxes. (A) Reaction scheme with protons monitored highlighted.
88
B C
CH2CH2 (linkage) CH2CH2 (en) CH (linkage) CH2CH2 (en) 2 Time 18 h
8 h
2 h
0 h
4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 ppm 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 ppm
Figure 45 (Continued). (B) Stacked plot for the reaction monitored for PT-ACRAMTU (31). (C) Stacked plot for the reaction monitored for 54.
89 Under the conditions chosen, the reaction between 54 and N-AcCys follows a
second-order rate law, a typical mechanism that is dominated by direct substitution of
chloride ligand by cysteine sulfur and is not limited by the rate of aquation [148]. In
contrast, the more rapid reaction of PT-ACRAMTU with N-AcCys followed neither
second-order nor first-order kinetics, suggesting that the reaction of this derivative
proceeds via a more complicated mechanism (Figure 46). In both cases, the 1H NMR spectra of the mixture indicated formation of multiple products. Attempts to assign the species formed by 54 and PT-ACRAMTU using 2-D [1H-15N] NMR spectroscopy, a
technique applied in nucleotide binding studies of [15N]-en labeled conjugates, 31 and 54
(see Section 3.3.4), were unsuccessful due to the complexity of the reaction mixtures
[111].
1.0
0.8
0 0.6
[Pt]/[Pt] 0.4
0.2
0.0 0 200 400 600 800 1000 Time(min)
Figure 46. Progress of the reactions of 54 (blue circle) and PT-ACRAMTU (red square) with one equivalent of N-AcCys at 25 oC monitored by 1H NMR spectroscopy. The data shown is the mean of two experiments. The solid line (for 54) represents a curve fit -1 assuming second-order conditions, [Pt]/[Pt]0 = (1 + k[Pt]0t) ; (t1/2 = 180 min). The dashed line (for PT-ACRAMTU, 31) has no physical significance.
90 The reactivity of Pt–Sthiourea bond and its cis-labilizing effect on chloride are most
likely the cause of the increased cysteine reactivity of PT-ACRAMTU compared to
analogue 54. The rationale for incorporating a thiourea-based nonleaving group in the original design was to enhance the reactivity of the metal with DNA nucleobase by
exploiting the cis-activating effect of sulfur. In their pioneering studies of the inorganic
kinetic cis-effect, Tobe and coworkers demonstrated for the complexes [PtCl(en)(dmso-
S)]+ and [PtCl(dms)]+ (dmso = dimethylsulfoxide, dms = dimethylsulfide) that the sulfur
donors greatly accelerate chloride substitution by an incoming nucleophile relative to
analogous complexes with [N3Cl] donor sets [116, 117]. It was also demonstrated that the
relative enhancement (kS/kN) is large for strong incoming nucleophiles, such as sulfur and
some nitrogen ligands, but negligible for weak ligands like water. The comparative
kinetic study of amidine-modified complex 54 and thiourea-based PT-ACRAMTU 31
shows that replacement of sulfur with an imino donor group reduces, as expected, the
metal´s reactivity with sulfur, but enhances its binding with DNA nitrogen. These
findings strongly suggest that, unlike the reaction with cysteine, the reaction with DNA is
not controlled by the cis-effect but most likely by steric hindrance and/or hydrogen
bonding between the imino hydrogen of the complex and exocyclic groups of the DNA
base that favor complex aquation and facilitate nucleophilic attack of nucleobase nitrogen
on the metal center (see discussion of 5´-GMP binding in section 3.3.4).
Thus, efficient DNA binding of the new derivatives (54 and 55) and reduced
levels of detoxification compared to PT-ACRAMTU are most likely the major
contributors to the greatly improved biological activity in rapidly proliferating H460 cells.
However, subtle structural differences at the target level that may render specific lesions
91 induced by 54 and 55 more cytotoxic than those induced by PT-ACRAMTU cannot be
ruled out.
3.3.5 Analysis of Platinum–Amino Acid Products
In the kinetic study of reactions of these two drugs with N-AcCys, the 1H NMR spectra of the mixtures indicated formation of multiple products. But attempts to assign the species formed by PT-ACRAMTU (31) and 54 using two dimensional [1H–15N]
NMR spectroscopy were unsuccessful due to the complexity of the reaction mixtures. To overcome this difficulty, LC-MS was used to analyze the product distribution in mixtures of platinum complexes incubated with one equivalent of sulfur nucleophile (37 oC, 6 h,
10 mM phosphate buffer, pH 7.2). The two most abundant products formed by 54 were successfully separated (Figure 47A) and unambiguously identified as the dinuclear
4+ 4+ complex [{Pt(en)(L)}2(μ-N-AcCys*)] ([P2] , 72%) (Figure 47B) and mononuclear
species [Pt(en)(L)(N-AcCys*)] ([P3]+, 14%) (Figure 47C) in electrospray mass spectra
recorded in positive-ion mode. (The asterisk indicates the dianionic form of metal-bound
N-AcCys at neutral pH. L = N-(2-(acridine-9-ylamino)ethyl)-N-methylpropionamidine,
acridinium cation).
These findings suggest that adduct P3, formed by substitution of chloride by
cysteine sulfur, rapidly reacts with the unreacted complex 54 to form dinuclear species,
P2 (Figure 49). The tendency of cysteine sulfur to readily induce bridged complexes with
platinum(II) drugs has been amply demonstrated [148]. The platinum–amidine linkage
and the en chelate in complex 54 appears to be resistant to nucleophilic attack by cysteine
92 based on the presence of a minor amount of free acridine L (P1, 3%) and the absence of ring-opened adducts.
mAU A P2, 72%
15 Intensity
10
P3, 14%
5 **, 11%
P1, 3%
0 0 5 10 15 20 25 Time [min] 03060803.D: UV Chromatogram, 405-421 nm
Figure 47. (A) Reverse-phase HPLC elution profile monitored at 413 nm of a mixture of complex 54 incubated with N-AcCys. The asterisks indicate an acridine-containing species that could not be assigned on the basis of mass spectral data.
93
106 307.2 674.0 107 490.6 B P2 P3 363.1 252.1 C 6 HN H2N NH2 H2N NH2 NH 221.0 H2N NH2 Pt Pt HN Pt N NH HN N 307.2 S N S N N NH 1.25 H H N 418 H CH2 307 H 726.2 N CH C OH H C 2 5 O H O O C CH N O + 1.00 O [P3] = 723.8 4+ [P2] = 1286.4 4
z = 2 0.75 z = 1 Intensity 3 Intensity Intensity 252.1 725.2 0.50 2 630 640 660 670 680
0.25 418.0 490.6 643.7 1 221.0 674.0 252.1 726.2 562.2 0.00 0 200 300 400 500 600 700 800 900 m/z 200 300 400 500 600 700 800 900 m/z
Figure 47 (Continued). Positive-ion electrospray mass spectra of adducts P2 (B) and P3 (C) showing peaks for the molecular ion, [P3]+ (m/z 725.2) and [P2-2H]2+ (m/z 643.7, z = 2) and fragments resulting from in-source collisionally activated dissociation (CAD). The insets illustrate the observed fragmentation patterns. The discrepancy between the calculated and observed m/z values for some of the platinum containing ions is due to distorted Pt isotope patterns.
94 In contrast, the LC-MS data acquired for PT-ACRAMTU 31 reveal a more
complicated reaction pattern (Figure 48A–C). In analogy to complex 51, the chloride
4+ substitution pathway leads to dinuclear adduct, [{Pt(en)(ACRAMTU)}2(μ-N-AcCys*)]
(P1´, 32%) (Figure 48B), most likely via the intermediate [Pt(en)(ACRAMTU)(μ-N-
AcCys*)]+, which could not be detected in this case. In addition, significant amounts of
4+ ACRAMTU (P3´, 33%) and the bisintercalator complex [Pt(en)(ACRAMTU)2] (P2´,
26%) (Figure 48C), were observed in the HPLC profiles (Figure 48A). Formation of the
latter complex, P2´, can be explained with chloride substitution in PT-ACRAMTU by
free ACRAMTU in the reaction mixture. The release of ACRAMTU by N-AcCys was
unexpected and shows that thiourea, a typical nonleaving group in reactions of PT-
ACRAMTU with DNA nitrogen [107, 110], becomes substitution-labile in the presence
of cysteine sulfur. These reactions have been summarized in Figure 49.
95 mAU A P3’, 33% 10
8
P1’, 32%
6 P2’, 26%
4
**
2
0
0 5 10 15 20 25 30 35 40 45 Time [min] UV Chromatogram, 405-421 nm
Figure 48. (A) Reverse-phase HPLC elution profile monitored at 413 nm of a mixture of PT-ACRAMTU 31 incubated with N-AcCys. The asterisks indicate an acridine- containing species that could not be assigned on the basis of mass spectral data.
96
x106 x106 P1’ B P2’ H N NH C 325 2 2 325.2 325.2 518 6 HN Pt NH H N NH H N NH HN 2 2 2 2 NH N S S N Pt Pt N N N S S N 4 H H N S N NH HN H H NH HN 741 5 704 578 578.3 H2C H HO C CH N + O 3 [P2´] = 903.4 4 O
[P1´]4+ = 1322.4 Intensity Intensity 3 2
291.1 2
1 452.2
1 422.2 578.2 704.1 252.1 420.2 660.2 252.2 484.6 741.2 903.4 221.1 371.2 578.3 221.0 518.2 0 0 200 300 400 500 600 700 800 900 m/z 200 300 400 500 600 700 800 900 m/z
Figure 48 (Continued). Positive-ion electrospray mass spectra of adducts P1´ (B) and P2´ (C) showing peaks for the molecular ion, [P1´-2H]2+ (m/z 660.2, z = 2) and [P2´]+ (m/z 903.4) and fragments resulting from in-source collisionally activated dissociation (CAD). The insets illustrate the observed fragmentation patterns. The discrepancy between the calculated and observed m/z values for some of the platinum containing ions is due to distorted Pt isotope patterns.
97 H N NH 2 2 HN H2N NH2 H2N NH2 NH 31 HN Pt Pt Pt N S S N S S N N N S N CH2 H H H H NH HN NH N CH C OH O H2C O H HO C CH N O H2N NH2 O HN Pt H2N NH2 Cl N-AcCys 37 HN Pt NH N S P1´ N N S S N H N N NH H H NH HN 31 P2´
HN N S N H NH 37, P3´
HN N NH N H
H2N NH2 HN Pt N-AcCys Cl P1 (minor) N NH N H HN H2N NH2 H2N NH2 NH H2N NH2 Pt Pt HN Pt 54 N NH HN N 54 N S N N NH S H H N CH2 H H N CH C OH H2C H O HO C CH O N P3 O O P2 Figure 49. Pathways for reaction of PT-ACRAMTU (31) and 54 with N-AcCys.
98 3.3.6 Determination of the Subcellular Localization of Complex 54
Fluorescence microscopy has become a powerful tool for detecting the subcellular
localization of fluorescent molecules and can provide insight into their cellular
interactions and mechanism of action [149, 150]. It is important to investigate the properties of pharmacologically important compounds in intact cells, because the cellular environment is difficult to simulate. Thus, live-cell imaging has become a popular method in cellular studies of bioactive molecules [150]. The rapid development of
organelle-specific dyes has made the technique particularly powerful, since they can be
used to perform colocalization studies [150]. In this study, experiments were performed
with both PT-ACRAMTU (31) and complex 54. While both compounds shared similar
intracellular distributions, complex 54 showed a significantly higher level of fluorescence
in all of the experiments. Therefore, in the following sections, only representative
imaging data acquired for complex 54 will be discussed.
To locate relevant subcellular organelles, such as lysosomes and mitochondria,
and to study their shapes and patterns, A2780 ovarian cancer cells were treated with
either LysoTracker Red or MitoTracker Green. LysoTracker Red is a lysosomal stain
consisting of a fluorophore attached to a weak base, which readily accumulates in acidic
cellular compartments (low internal pH), and, thus, is useful for investigating the
biosynthesis and pathogenesis of lysosomes [151]. MitoTracker, on the other hand,
contains a thiol-reactive chloromethyl moiety for labeling mitochondria. MitoTracker
Green FM passively diffuses across the plasma membrane and accumulates in active
mitochondria [152]. Another dye, Hoechst 33342, is a useful nuclear tracker because it
binds to the minor groove of DNA [153]. Unfortunately, the emission spectrum of this
99 tracker and that of 54 overlap, which prohibits the detection of colocalized dye and drug
(unpublished data).
Figure 50. Fluorescence microscopy image of A2780 cells treated with LysoTracker Red DND-99 (left) and MitoTracker Green FM (right).
Figure 50 shows the distinct differences in the appearance of the two compartments within the cell. The lysosomes were observed to have a cytoplasmic pattern of staining characterized by dots and points, while the mitochondria were found to form closely packed, kidney shaped bodies.
The next goal was to confirm that complex 54 can be detected in A2780 cells, and, if so, to determine its cellular localization. After incubating the cells with the drug for 10 min, an intense blue fluorescence is observed indicating compound 54 has entered the cell (Figure 51). By comparison with Figure 50, it appears that the complex is localized mainly to the lysosomes, based on the presence of large cytoplasmic punctuate areas, which have been observed before for lysosomes that have accumulated platinum-
100 containing drugs [154-157]. However, there also seems a small amount of the drug
present in the nucleus, which indicates that free (not DNA-bound) platinum–intercalator
conjugate is present in the nucleus whose fluorescence has not been quenched by π-
stacking with DNA bases.
Figure 51. Representative fluorescence microscopy image (left) of A2780 cells incubated with complex 54 for 10 min. Enlarged image (right) of a single cell.
To prove this more conclusively and to reveal the subcelluar localization of the complexes, colocalization studies were conducted, in which platinum-treated cells were co-stained with LysoTracker Red and MitoTracker Green, respectively. Complex 54 was shown in the previous experiments to be visible using the blue emission filter, while
LysoTracker Red can only be detected using the red emission filter, and MitoTracker
Green can only be observed using the green filter. Consequently, these emission filters should allow the fluorescence originating from the platinum complex 54 and that from
MitoTracker Green /LysoTracker Red (Figure 52) to be clearly distinguished.
101 Figure 52 shows the relative distributions of complex 54 and mitochondrial
marker, MitoTracker Green. In particular, the overlay panel (III) indicates that the
distributions for this compound and mitochondria are mutually exclusive. On the other hand, the blue-red color observable in the superposition of images obtained from co- staining with the complex 54 and LysoTracker Red (Figure 52 C) indicates major areas of co-localization of the drug and lysosomes.
102
I II III
A B C
Figure 52. Confocal fluorescent microscopy images of A2780 cells treated with complex 54 while co-stained with MitoTracker Green FM (top) and LysoTracker Red (bottom), respectively. (I) and (A) show images taken through blue emission filter, (II) shows an image through green emission filter, (III) shows superpositions of blue and green images, (B) shows an image through red emission filter, and (C) shows superpositions of blue and red images. The brightness and contrast of the images have been manually modified.
103 One of the possible mechanisms to account for the accumulation of the platinum
complex within the lysosomes is that the presence of protonated acridine will render the
compound susceptible to pH-dependent accumulation within the highly acidic internal
environment of lysosomes.
However, the blue fluorescence of the complexes is not restricted to the regions
showing red fluorescence, indicating that, while the complex has entered the lysosomes, it is also present elsewhere in the cytoplasm or in other organelles. Based on the appearance of the faint blue fluorescent areas, the platinum complex must have entered
the nucleus, which was not observed at lower concentrations (not shown). It might be that
at low drug concentrations, platinum binding to nuclear DNA is quantitative and the
drug-based fluorescence is effectively quenched [156], which renders platinum
unobservable in the nucleus. However, at higher concentrations of the platinum complex,
the DNA might become saturated with drug, resulting in a light blue fluorescence due to
the presence of the unbound conjugate. If this case, the rupturing of the nuclei and
breakdown of the chromatin structure, which releases the fluorophores, should give rise
to increased fluorescence levels. In order to prove this hypothesis, A2780 cells were
dosed with complex 54 and 1 mM hydrogen peroxide simultaneously for 10 mins, after a
period of which it is possible to identify those cells undergoing apoptosis. The results are shown in Figure 53.
104
Figure 53. Representative fluorescence microscopy images of A2780 cells incubated with complex 54 alone (left) and co-treated with hydrogen peroxide (right) for 10 mins.
By comparing the two images in Figure 53, it appears that the cancer cells are
dying and imploding, and the areas of enhanced fluorescence correspond to the dense
circular regions of each cell, which are believed to be the nuclei. This supports the
previously described hypothesis, although it was important to verify that this enhanced
fluorescence was not a result of the distinct changes in cellular autofluorescence that are known to accompany apoptosis [158]. Therefore, a control sample in which cells were treated only with hydrogen peroxide for 10 mins was examined, and it showed no autofluorescence within this time frame, unlike the sample of complex 54.
The fluorescent microscopy data confirm that our platinum–acridine conjugates are able to enter cancer cells despite their cationic charge. The protonability of the acridine chromophore is most likely the cause of accumulation of drug in the lysosomes.
Increased fluorescence in apoptotic cells suggests that some drug must also have entered
105 the nucleus. This supposition should be confirmed by other methods such as ICP–MS to determine the amount of platinum associated with nuclear DNA extracted for the cells.
3.3.7 Antitumor Activity and Systemic Toxicity
The antitumor activity of 55 was evaluated against H460 bilateral tumors implanted into athymic nude mice. Complex 55 was selected for the study because it was slightly more soluble in biological media than 54. Complex 55 was administered intraperitoneally (ip) according to the following dose schedule: (A) 0.1 mg/kg, five days per week for two consecutive weeks (5d/w), and (B) 0.5mg/kg, three doses given at 4-day intervals (q4d). The tumor volumes recorded in both treatment groups and in untreated control animals are plotted vs days of treatment in Figure 54.
106
2000 Control
0.1 mg/kg i.p./5d/w
1500 0.5 mg/kg i.p./q4d
1000
500
0
0 5 10 15 20 Time (days)
Figure 54. Evaluation of 55 against H460 NSCLC tumors xenografted into nude mice. Growth curves are shown for untreated control animals (open squares), and mice treated according to schedule A (filled triangles) and schedule B (filled circles). Measurement of tumor volumes began on day 0, and treatment began on day 4. Each data point represents the mean of 5 tumor volumes ± S.E.M. (A) 0.1 mg/kg, five days per week for two consecutive weeks (B) 0.5 mg/kg, three doses given at 4 days intervals.
At the end of the study, the tumors measured 1834 ± 160, 1789 ± 309 and 1102 ±
319 mm3 (mean ± SEM) for the control animals and animals treated according to schedules A and B, respectively. On the basis of these data, the low-dose treatment A had
no effect on tumor growth. However, treatment at the higher dose B, which is close to the
maximum tolerated dose (MTD) of compound 55, slowed the tumor growth rate
significantly compared to the control group, which led to a reduction in the mean
terminal tumor volume by 40%.
The high cell kill potential of the new hybrids in H460 cells translates into
pronounced antitumor activity. This was demonstrated for compound 55 in the
107 corresponding tumor xenograft in which the agent slowed the tumor growth at a sublethal
dose close to the MTD. The high cytotoxic potency of compound 55 is documented by
the fact that it is tolerated at doses an order of magnitude lower than those commonly
applied for cisplatin when administrated ip [159]. Because complex 55 is quite toxic at a
dose of 0.5 mg/kg, which results in significant weight loss in the treated animals, further
structural modifications are necessary to minimize adverse effects and improve the therapeutic index of complex 55. Nevertheless, our new agent showed significantly improved cytotoxic potential compared to the “classical” monofunctional, complex, cis-
+ [Pt(NH3)(pyridine)Cl] , which requires high doses to produce appreciable antitumor
effect in vivo [160]. A recent study in kidney cell lines demonstrated that relatively
higher intracellular levels of this agent, mediated by organic cation transporters, are
required to achieve a cytotoxic effect similar to that produced by the clinical agent,
oxaliplatin [161].
3.3.8 Distribution of Platinum in Murine Tissue Samples
After completion of the antitumor activity study, biological tissues from
euthanized test animals were investigated to rationalize the high toxicity and potent
activity of 55 in the xenograft model. Specifically, accumulation levels of platinum in
various biological tissues (tumors, kidneys, and lungs) (Appendix C) at the two different
dose levels (0.1 mg/kg and 0.5 mg/kg, see 3.3.7) were determined using inductively
coupled plasma–mass spectrometry (ICP–MS) (Figure 55).
108
250
Kidneys
200
Kidneys 150
14 Lungs 12 Lungs 10
inPt Tissues (ng/g) 8 6 Tumors Tumors 4
2
0 5 0.1 mg/kg 0.5 mg/kg Treatment of Mice with Two Different Doses
Figure 55. Platinum concentrations in various biological tissues (tumors, kidneys and lungs) at two different doses (0.1 mg/kg and 0.5 mg/kg) determined by ICP–MS after 16 hours of tissue digestion. There are significant differences in platinum accumulation levels between various tissues. Plotted data points are the mean of five tissue samples in five independent experiments.
The highest platinum concentration was observed in the kidneys, which was 15–
30-fold higher than in the lungs and tumors. For the low-dose treatment A (0.1 mg/kg), the concentrations of platinum in all tissues are higher than those observed for the high- dose treatment B (0.5 mg/kg), especially in the kidneys, which might be due to the different treatment schedules. As described before, treatment A was performed for five consecutive days, which would require the kidneys to excrete the toxic drug at a high rate.
However, at the higher-dose treatment, B, each injection occurred at four-day intervals,
109 which allows more time for the kidneys to excrete platinum and might lead to lower platinum concentrations in the affected tissue.
In order to obtain a better understanding of platinum distribution, retention of platinum in each organ was calculated as a percentage of the total administered doses
(Figure 56). Assuming an average mass of a test animal of 19 g, at the low dose treatment
A (0.1 mg/kg), a total amount of 6.92 μg of platinum was injected into each mouse, while an average of 2.09 ng of platinum was detected in the tumors, which accounts for 0.30% of the total injected drug. A similar amount, 1.63 ng (0.24%) was detected in the lungs.
By contrast, a dramatically larger amount of platinum, 53.47 ng (7.73%), was found in the kidneys. Therefore, at the same dosage, the majority of platinum accumulated in the kidneys, the major organ of excretion, which most likely contributes to the toxicity observed in the treated group of mice.
On the other hand, at the MTD treatment B (0.5 mg/kg), the retained percentages of platinum in these three biological tissues are all lower than in treatment A. Assuming an average weight of 19 g per mouse, 7.98 μg of platinum was injected, while 1.42 ng
(0.18%), 1.37 ng (0.17%) and 37.57 ng (4.71%) of platinum were detected in tumors, lungs and kidneys, respectively (Tables 4 and 5).
110
Kidneys 9
8 7
6 Kidneys 5
4
Tumors 0.4
0.3 Lungs Tumors Lungs 0.2
Percentage of Pt in Each Tissue 0.1
0.0
0.1 mg/kg 0.5 mg/kg
Treatment of Mice with Two Different Doses Figure 56. Retention of platinum in each organ calculated as percentage of the total administered doses (0.1 mg/kg and 0.5 mg/kg) determined by ICP–MS after 16 hours of tissue digestion. There are significant differences of platinum accumulation levels between various tissues. Plotted data points are the mean of five tissues samples in five independent experiments.
Table 4. Platinum Concentrations (ng/g) in Various Biological Tissues (Tumors, Kidneys and Lungs) at Two Different Doses (0.1 mg/kg and 0.5 mg/kg) Determined by ICP–MS after 16 Hours of Tissue Digestion. Reported Data are the Mean of Five Independent Experiments.
Dose Tumors Lungs Kidneys
0.1mg/kg 4.45 ± 0.24b,c 10.70 ± 0.47 195.46 ± 15.24
0.5mg/kga 3.33 ± 0.45 9.01 ± 1.26 136.51 ± 14.60
a Maximum Tolerable Dose. b Concentrations of platinum in tissues are in ng/g wet weight. c Mean ± Standard error of the mean (SEM).
111 Table 5. Retention of Platinum Per Single Organ Calculated as a Percentage of the Total Administered Doses (0.1 mg/kg and 0.5 mg/kg) Determined by ICP–MS after 16 Hours of Tissue Digestion. Reported Data are the Mean of Five Independent Experiments.
Dose Tumors Lungs Kidneys
0.1mg/kg 0.30 ± 0.07b,c 0.24 ± 0.02 7.73 ± 1.07
0.5mg/kga 0.18 ± 0.06 0.17 ± 0.02 4.71 ± 0.43 a Maximum Tolerable Dose. b Percentage of platinum retained in tumors, kidneys and lungs (% of total doses). c Mean ± Standard error of the mean (SEM).
On the basis of the high platinum levels established in the kidneys of mice treated with compound 55 and the yellow discoloration of the kidneys observed during necropsies (all other organs appeared normal, according to Washington Biotechnology,
Inc.), it seems likely that platinum toxicity to this organ may have a dose-limiting effect and may contribute to the weight loss in the animals. However, the data also suggest that there is no direct relationship between the levels of systemic toxicity and the absolute amount of platinum in the kidneys: while the 0.1 mg/kg doses cause the highest platinum levels and kidney discoloration, this treatment schedule does not lead to weight loss. On the other hand, the kidneys removed from mice treated at the MTD of 0.5 mg/kg did not show any sign of kidney damage. Thus, inefficient platinum excretion may not be the major cause of the toxicity.
3.4 Conclusions and Clinical Implications
In conclusion, we have now demonstrated that a simple structural modification in PT-
ACRAMTU by replacing thiourea sulfur with imino nitrogen leads to greatly enhanced cytotoxicity in H460 non-small cell lung cancer (NSCLC) cells and tumor growth in the
112 corresponding xenograft model. Two complexes, 54 and 55, have been synthesized using
platinum-activated amidination chemistry. Compound 55 significantly slows the tumor
growth rate in a H460 xenograft study in mice by 40% and is the first non-cross-linking
platinum agent endowed with promising activity in NSCLC in vivo.
An important issue to be addressed is the relatively high toxicity of the hybrid agent
in treated animals. The results of a necropsy performed on the test animals treated with
55 revealed “mild to yellow discoloration of the kidneys” while “everything else
appearing normal”. In the ICP–MS study, the highest platinum concentration was observed in the kidneys. It has recently been demonstrated that the nephrotoxicity effect of cisplatin is mediated by its GSH adducts [162]. Thus, it is possible that the GSH adducts formed by complex 55 (see analogous cysteine adducts P2 abd P3, Figure 47 in
Section 3.7.5) contribute to the toxicity of this compound. Although compound 55 showes reduced reactivity with cysteine sulfur in comparison with PT-ACRAMTU, additional modifications may be necessary to suppress this reactivity and improve the compound’s metabolic stability.
It was also demonstrated that a relationship exists between the rate of DNA binding and cytotoxic potential. A simple mononucleotide model monitored by 1H NMR was
employed to study the reactivity of PT-ACRAMTU 31 and 54 with nucleobase nitrogen.
It showed that 54 reacted considerably faster than PT-ACRAMTU did. Rapid formation of irreversible DNA adducts can be considered an important feature in an antitumor agent designed to kill aggressive, rapidly proliferating cancer cells.
Non-cross-linking DNA-targeted agents, such as 54 and 55, have great potential in the treatment of cancers that do not respond to cisplatin because their adducts may evade
113 DNA repair mechanisms. A survey of the recent NSCLC literature suggests that a correlation exists between the expression levels of nucleotide excision repair (NER) genes in NSCLC and the progression of the disease: NSCLC patients with an excision repair-proficient tumor phenotype respond extremely poorly to platinum drugs [163-167].
Overexpression of ERCC1, for instance, seems to be associated with tumor resistance.
The NER pathway is responsible for the repair of platinum–DNA intrastrand cross-links, the putative cytotoxic lesions of cisplatin, which compromises the drug’s efficacy in affected cancers. NER recognizes and removes chemically irreversible bulky lesions that severely distort and destabilize duplex DNA [168]. Unlike cisplatin´s 1,2 intrastrand cross-link, which severely bends DNA and reduces its thermal and thermodynamic stability, the monofunctional intercalative binding mode established for PT-ACRAMTU
(and also expected for 54 and 55) causes local helix unwinding, but no bending, and increases the thermal stability of the modified duplex [169, 170]. Therefore, the latter nonclassical adducts may be poor substrates for NER and circumvent the associated resistance mechanism in affected cancer cells, which might explain why this type of hybrid agent performs better than cisplatin in NSCLC cells.
114 CHAPTER 4
CHANGING THE CONNECTION SITE OF THE
LINKER IN PT-ACRAMTU FROM THE 9-POSITION TO THE 4-POSITION
The work contained in this chapter was initially published in Bioorganic & Medicinal
Chemistry Letters in 2008. The manuscript, including figures and schemes, was drafted by Zhidong Ma and Dr. U. Bierbach and edited by Dr. U. Bierbach before submission to the journal. Since the publication, changes in both format and content have been made to maintain a consistent format throughout this work. The experiments described were performed by Zhidong Ma, except for the cytotoxicity assays which were contributed by
Dr. G. L. Kucera and G. Saluta.
115 4.1 Background
Chapters 2 and 3 were concerned with a structure–activity relationship (SAR) study of PT-ACRAMTU analogues in which sulfur of thiourea attached to the 9-position of acridine has been replaced with imino nitrogen. The approach intended to be minimally invasive to the core structure of the conjugate, while changing its reactivity with biologically relevant nucleophiles.
In another SAR study, we were interested in the effects of repositioning the platinum- modified side chain in PT-ACRAMTU from the 9-position of the acridine to the 4- position. The rationale behind this approach was that variation of the site of attachment might have an impact on the sequence and groove specificity of DNA adducts formed by this agent, which, in turn, might lead to altered biological activity.
Acridine derivatives carrying side chains in the 4-position of the heterocyclic base have been studied extensively [171]. These include acridine-4-carboxamides, such as N-
[2-(dimethylamino)ethyl]acridine-4-carboxamide (DACA), a topoisomerase-targeted anticancer agent currently being considered as a second-line therapy for non-small-cell lung and ovarian cancers [172].
As discussed in Chapter 1, the unique DNA damage profile produced by PT-
ACRAMTU is a consequence of the compound’s sequence preference and directionality of intercalation, which leads to unprecedented platination of adenine bases at N3 (~10% of adducts) in the DNA minor groove [106]. The preclinical success of PT-ACRAMTU and the hypothesis that a relationship exists between its antitumor activity in cisplatin- resistant cell lines and its distinct DNA damage profile have prompted several structure- activity relationship studies.
116 Previously, modifications were made to the linker that connects the acridine and platinum moieties [102, 103], spectator ligands on the metal center were varied [173], and 4-substituted ACRAMTU derivatives were introduced as threading intercalators
[174]. In the present set of compounds (Figure 57), the thiourea and guanidine groups were incorporated into a 4-carboxamide side chain. A new platinum-containing hybrid agent was synthesized and tested along with the platinum-free carriers against HL-60 and
NCI-H460 cells.
N X 4 H 9 O N X N N H HN N N N H 4 9
X = S: thiourea X = N: guanidine
Figure 57. Changing 9-aminoacridine into acridine-4-carboxamide derivatives.
4.2 Experimental Section
4.2.1 Materials and Methods
1H NMR spectra of the target compounds and intermediates were recorded on
Bruker Advance 300 and DRX-500 instruments operating at 500 and 300 MHz, respectively. 13C NMR spectra were recorded on a Bruker Advance 300 instrument operating 75.5 MHz. Chemical shifts (δ) are given in parts per million (ppm) relative to internal standards trimethylsilane (TMS), or 3-(trimethylsilyl)-1-propanesulfonic acid sodium salt (DSS) for samples in D2O. The platinum-free acridines 57 and 60 were fully
117 characterized by gradient COSY and 1H-detected gradient HMQC and HMBC spectra
recorded on a Bruker DRX-500 MHz spectrometer. The 195Pt NMR spectrum of the
target complex 58 was recorded on a Bruker DRX-500 MHz spectrometer at 107.5 MHz.
195 Aqueous K2[PtCl4] was used as external standard, and Pt chemical shifts are reported
2- 1 13 vs [PtCl6] . H and C NMR spectra for the new compounds (57, 58, 59 and 60) are
available in Appendix A. Elemental analyses were performed by Quantitative
Technologies Inc., Madison, NJ. All reagents were used as obtained from commercial
sources without further purification unless indicated otherwise. Solvents were dried and
distilled prior to use. Flash chromatography was performed using ACROS (50-200 mesh)
neutral alumina gel.
The compounds N-(2-(methylamino)ethyl)acridine-4-carboxamide (56) [175-177] and N-methyl-N´-tert-butoxycarbonylthiourea (45) [178] were prepared according to previously described procedures.
4.2.2 Synthesis and Characterization
N-(2-(1,3-Dimethylthioureido)ethyl)acridine-4-carboxamide, 57. A solution of methylisothiocyanate (0.33 g, 4.55 mmol) in 10 mL of absolute ethanol was added dropwise within 10 min to a solution of 1.0 g (3.58 mmol) of 56 in 50 mL of absolute ethanol. The mixture was heated at reflux for 6 h and passed, while hot, through a Celite pad. Ethanol was removed in vacuum and the residue was recrystallized from hot methanol to afford microcrystalline 57. Yield: 0.98 g (77%); mp. 198 oC.
1 H NMR (DMSO-d6): δ 11.43 (H17, t, J = 5.5 Hz), 9.33 (H9, s), 8.73 (H3, d, J =
7.1 Hz), 8.39 (H1, d, J = 8.4 Hz), 8.32 (H5, d, J = 8.8 Hz), 8.24 (H8, d, J = 8.4 Hz), 7.97
118 (H6, t, J = 6.8 Hz), 7.76 (H2, t, J = 8.1 Hz), 7.71 (H7, t, J = 8.0 Hz), 7.58 (H24, NH, b),
4.09 (H19, 2H, t, J = 6.4 Hz), 3.79 (H18, 2H, q, J = 6.1 Hz), 3.18 (H21, 3H, s), 2.91
(H25, 3H, d, J = 4.1 Hz);
13 C-{H} NMR (DMSO-d6): δ 181.9, 165.2, 147.0, 145.4, 138.5, 134.5, 132.9,
131.7, 128.7, 128.4, 128.1, 126.5, 126.4, 125.6, 125.2, 51.7, 38.2, 37.2, 32.6;
Anal. (C19H20N4OS) C, H, N, S: calcd, 64.75, 5.72, 15.90, 9.10; found, 64.42,
5.63, 15.86, 9.02.
[PtCl(en)(C19H21N4OS)](NO3)2, 58. The nitrate salt of 58 was formed by adding
1.4 mL of 1 M HNO3 to 0.5 g of 57 in methanol. The solvent was removed completely in
a vacuum and the residue redissolved in 10 mL of anhydrous DMF. A mixture of 0.46 g
(1.41 mmol) of [PtCl2(en)] and 0.23 g (1.41 mmol) of AgNO3 in 10 mL of anhydrous
DMF was stirred at room temperature in the dark for 14 h. Precipitated AgCl was filtered off through a Celite pad. The yellow solution containing 57 was combined with the filtrate, and the mixture was stirred for 4 h in the dark. DMF was removed in a vacuum at
30 oC yielding a yellow oily residue, which was dissolved in 1.0 L of dry methanol.
Activated carbon was added, and the solution was stirred for 15 min. Carbon was filtered
off, and the solution was concentrated to a final volume of 100 mL. Crude 58 was obtained as a bright yellow solid after the solution was stored at -10 oC for 24 h. The
crude batch was recrystallized from hot methanol. The solution was stored in the
refrigerator for 48 h to afford 58 as a microcrystalline yellow solid, which was dried at
60 oC. Yield: 650 mg (60%); mp. 185 oC.
1 H NMR (MeOH-d4) : δ 9.53 (1H, s), 8.61 (1H, d, J = 7.3 Hz), 8.37 (1H, d, J =
8.5 Hz), 8.27 (1H, d, J = 8.7 Hz), 8.20 (1H, d, J = 8.5 Hz), 8.00 (1H, t, J = 8.4 Hz), 7.70
119 (2H, m), 5.28 (2H, NH2 on ethylenediamine ligand, s), 5.05 (2H, NH2 on
ethylenediamine ligand, s), 4.19 (2H, t, J = 5.9 Hz), 3.82 (2H, t, J = 6.0 Hz), 3.12 (3H, s),
2.38 (4H, C2H4 on ethylenediamine ligand, s);
13 C-{H} NMR (DMF-d7): δ 176.6, 167.0, 147.8, 146.2, 140.23, 135.8, 134.1,
133.0, 129.3, 129.2, 128.3, 127.6, 127.5, 126.9, 126.0, 54.4, 50.5, 48.5, 40.2, 38.3, 34.5;
195 Pt NMR (DMF-d7) δ -2859
Anal. (C21H29ClN8O7PtS · H2O) C, H, N: calcd, 32.08, 3.97, 14.25; found, 32.09,
3.86, 14.03.
ESI-MS (MeOH, +ve mode) m/z: 642.2 [M]+
tert-Butyl-((2-(acridine-4-carboxamido)ethyl)methylamino)(methylamino)- methaniminecarbamate, 59. A mixture of 159 mg (0.92 mmol) of 45, 256 mg of 56
(0.92 mmol), and 257 μL of triethylamine was prepared in 7 mL dry DMF. The solution
o was cooled to 0 C, and 249 mg (0.92 mmol) of HgCl2 were added. The yellow slurry
was stirred for 1 h at 0 oC and for another 4 h at room temperature until the mixture
turned black. DMF was removed in a vacuum, and the residue was dissolved in ethyl acetate, filtered through Celite, and dried over Na2SO4 overnight. After the solvent was
removed, the crude product was purified by flash chromatography on an alumina column
using methanol/ethyl acetate (2:3) as eluent. Yield: 281 mg (70%).
1 H NMR(MeOH-d4) : δ 9.11 (1H, s), 8.78 (1H, d), 8.30 (2H, m), 8.14 (1H, d),
7.92 (1H, t), 7.67 (2H, m), 3.84 (2H, t), 3.75 (2H, m), 3.09 (3H, s), 2.82 (3H, s), 1.41
(9H).
((2-(Acridine-4-carboxamido)ethyl)methylamino)(methylamino)methanimi -
nium chloride, 60. To remove the Boc group, 200 mg (0.46 mmol) of 59 was dissolved
120 in 2.0 M HCl. The mixture was stirred at room temperature overnight. Acid was removed
using a rotary evaporator, and the resulting residue was dissolved in a minimum amount
of ethanol. The product was precipitated with diethyl ether and purified by flash
chromatography on an alumina column using methanol/ethyl acetate (1:1) as eluent.
Yield: 101 mg (59 %); mp: 182 oC.
1 H NMR (D2O): δ 8.25 (H9, s), 8.11 (H3, d, J = 7.1 Hz), 7.71 (H6 and H8, 2H, one triplet and one doublet overlap), 7.66 (H1, d, J = 8.1 Hz), 7.49 (H7, t, J = 7.9 Hz),
7.44 (H5, d, J = 8.4 Hz), 7.27 (H2, t, J = 7.3 Hz), 3.47 (H18, 2H, t, J = 6.0 Hz), 3.38
(H19, 2H, t, J = 6.5 Hz), 2.92 (H21, 3H, s), 2.39 (H25, 3H, s);
13 C-{H} NMR (D2O): δ 168.3, 156.8, 146.6, 144.5, 138.5, 134.8, 134.3, 132.3,
128.7, 128.0, 126.8, 126.0, 125.6, 124.9, 124.7, 49.7, 36.7, 36.4, 28.2;
ESI-MS (MeOH, +ve mode) m/z: 336.4 [M]+
Anal. (C19H22ClN5O · 0.5H2O · 0.5C2H5OH) C, H, N: calcd, 59.47, 6.49, 17.33;
found, 59.04, 6.54, 16.90.
4.2.3 Biological Characterization: pKa Determinations and Cell Proliferation
Assay
pKa Measurements. The pKa value of 57 was determined spectrophotometrically using a
Hewlett Packard 8354 spectrophotometer equipped with a Peltier temperature control and
a cell stirring module. Titrations were carried out at 25 oC in the pH range 2-13 with a
micro combination electrode placed inside the cuvet. Solutions of the acridine derivatives
(~50 μM) in 50%MeOH/50% 10-1 M HCl/100 mM NaCl were titrated with aliquots of
0.4 M KOH, and UV-Vis spectra were recorded 2 min after each addition of base. The
121 protonation state of the chromophores was deduced from spectral changes around
isosbestic points. pKa´s were determined graphically from plots of pH vs.
+ + log[k(Aacr/AacrH )] (k = εacrH /εacr) using suitable absorbance maxima. Reported pKa´s are means of three measurements. The apparent pKa had to be extrapolated to the pKa in
water [179].
The pKa determination of compound 60 was done by NMR measurements on a
Bruker DRX-500 MHz spectrometer. Compound 60 (7 mg) in 550 μL D2O was acidified
to pH 2 by adding 5 M DCl to the NMR tube, and an 1H NMR spectrum was recorded
each time after adding a small amount of NaOD solution, until the pH had reached 12.
The pKa value was determined graphically from a plot of pH vs. proton chemical shift δ.
Cytotoxicity Assay. The cytotoxicity studies were carried out using the Celltiter 96
Aqueous Non-Radioactive Cell Proliferation Assay kit. HL-60 leukemia cells and H460
o lung cancer cells were kept in a humidified atmosphere of 5% CO2 at 37 C. Cells were
plated on 96-well plates with a total 700 cells/well for H460 and 27,500 cells/well for
HL-60 cells. Stock solution of 57 was prepared in DMSO while the stock solutions of 58
and 60 were prepared in phosphate-buffered saline (PBS). Prior to the incubation, the
solution of 58 was diluted with media to a final concentration of less than 0.1% in DMSO.
The cells growing in log phase were incubated with appropriate serial dilutions of the
drugs in duplicate for 72 h. To each well was added 20 μL of MTS/PMS solution, and the
mixtures were allowed to equilibrate for 3 h. The absorbance at 490 nm was recorded
using a plate recorder (none of the derivatives tested absorbs at this wavelength). The
reported IC50 data were calculated from non-linear curve fits using a sigmoidal dose-
response equation in GraphPad Prism and are averages of two individual experiments.
122 4.3 Results and Discussion
4.3.1 Synthesis of Acridine-4-Carboxamides
The new derivatives were synthesized from the common precursor, N-(2-
(methylamino)ethyl)acridine-4-carboxamide (56) (Figure 58) [175, 177]. Transformation
of the secondary amino group into thiourea was accomplished by reacting 56 with
methylisothiocyanate to yield derivative 57 in 70–80% yield. The corresponding platinum complex 58 was synthesized using the conjugation chemistry involving ligand
+ substitution in the activated precursor complex, [Pt(en)(DMF)Cl] , to form a Pt–Sthiourea linkage [100]. Complex 58 was isolated in its fully protonated form as the dinitrate salt.
The guanidine analogue, 60, was generated using guanidinylation chemistry developed for the synthesis of compound 38. The reaction involves condensation of precursor acridine-amine 56 with Boc-activated guanidylating reagent 45 in the presence of HgCl2, followed by removal of the Boc group under mildly acidic conditions [178]. The product was purified on basic alumina to afford 60 as the mono-HCl salt (Figure 58).
123 8 9 1 2+ 12 13 7 2 8 9 6 3 7 1 2 5 4 21 6 3 N 5 10 4 21 H (NO3)2 i H 17 N 11 N 14 ii 19 N N H 15 H O N 18 24 25 N 19 N 22 N H O N O N 18 20 24 S Cl 16 H 25 Pt H 27 17 S 26 H2N NH2 iii 23 56 O S 57 58 28 29 O N N H H + 1 45 7 8 9 2 6 3 N 5 4 21 iv N Cl N N O H O N 17 19 N N H O N 18 24 NH O H 25 NH2 59 60
Figure 58. Synthesis of acridine-4-carboxamides 57 and 60 and platinum conjugate 58. Reagents and conditions: (i) MeNCS/EtOH, reflux; (ii) 1- [Pt(en)Cl2]/AgNO3, DMF, r.t., o 2- 1M HNO3; (iii) HgCl2/ Et3N, DMF, 0 C; (iv) 1- 1 M HCl, r.t., 2- Al2O3.
4.3.2 Acid–Base Chemistry
A major difference exists in the protonation states and overall charges between
the 4- and 9-substituted acridines. For the 9-amino derivatives, 37 and 38, the acridine is
fully protonated at physiological pH (pKa ~ 9–10) [100, 178], leading to + and 2+ overall
charges (pKa > 12 for the guanidinium moiety in 38, Chapter 2) [178]. In contrast, a
dramatic decrease in acridine basicity is observed for the newly synthesized 4-
carboxamides, for which pKa values in the range 3–4 were determined, consistent with
data reported previously for this class of compounds [171]. As a consequence, the 4-
substituted compounds carry a reduced positive charge and prove to be less soluble in aqueous media than their 9-substituted counterparts. Electroneutral compound 57, for instance, had to be dissolved in DMSO prior to serial dilution with PBS buffer in cell proliferation assays.
124 4.3.3 Cytotoxicities
Compounds 57, 58, and 60 were tested for their biological activities in a human
leukemia (HL-60) and a non-small cell lung cancer (NCI-H460) cell line (Table 6) using
a cell proliferation assay. Of the two new acridine-4-carboxamides, only the guanidine
derivative 60 showed appreciable cytotoxicity in HL-60 cells, where it proved to be more active, by an order of magnitude, than its 9-substituted congener, 38. All other 4- substituted derivatives were significantly less active than the 9-substituted derivatives, and both 4- and 9- substituted compounds performed better in the solid tumor cell line. A striking cytotoxic enhancement by >10-fold is observed for the hybrid agent 58 compared to 57 in the NCI-H460 cell line. A similar trend has been observed for the prototypical 9- substituted pair 37/31 (Table 6).
Table 6. Acid–Base Properties and In Vitro Cytotoxicity (IC50, μM) of Acridines and Platinum Acridines in HL-60 and NCI-H460 Cell Lines.
a b Compound pKa1, pKa2 Charge HL-60 NCI-H460
37 9.8 + 11.5 9.5 31 (not determined) 2+ 2.8 0.26 38 8.9, >12 2+ 79.7 10.3 57 3.4 0 >100 30.9 58 (not determined) + 16.0 2.4 60 2.7, >12 + 8.4 >100
a 1 Spectrophotometric and H NMR spectroscopic pH titrations. pKa1 and pKa2 reflect the protonation of the acridine and guanidine moieties, respectively. b Concentrations needed to inhibit proliferation of HL-60 leukemia cancer cells and H460 lung cancer cells by 50%; average of two experiments.
The results of the cell proliferation assay suggest that the 4-substituted derivatives
are less potent than the 9-aminoacridines. Only monobasic compound 60 has an
125 advantage over dibasic compound 38 in HL-60 cells. This observation and the fact that the relative enhancement of activity observed for the platinum conjugates compared to the free ligands is less pronounced in the leukemia cell line than in the solid tumor suggest that reduced uptake of the more highly charged drugs may limit their activity in
HL-60 cells. Derivative 57 was the least active compound in this series, probably due to its poor aqueous solubility. Characteristically, the dual intercalating/platinating agents 31 and 58 showed the lowest IC50 values in H460 cells, which corroborates the view that irreversible DNA adducts formed by the conjugates´ metal moieties contribute considerably to the cell kill mechanism.
4.4 Conclusions
In the present study, acridine-4-carboxamide analogues of the previously studied
DNA-targeted thiourea- and guanidine-modified 9-aminoacridines were prepared. The thiourea-based derivative (57) was introduced as a carrier ligand in the corresponding platinum–intercalator conjugate (58). Differences in the DNA affinities (charge) and binding modes between the 4- and 9-substituted compounds can be predicted. High- resolution structural studies have been reported, which indicate critical differences in the
DNA binding modes of these agents. While in the DNA complexes of both types of acridines the long dimension of the chromophore and the adjacent DNA base pairs are co-aligned, differences exist in the directionality of intercalation: side chains in the 4- position usually reside in the major groove, whereas 9-substituents preferentially protrude into the minor groove [109, 114, 180]. ACRAMTU´s (37) distinct groove specificity of intercalation has been implicated as the driving force in the platination of minor groove
126 sites by PT-ACRAMTU (31). Likewise, the preferred geometry of intercalation of derivative 57 can be expected to affect the base and groove preference of platinum–DNA adduct formation by conjugate 58. Thus, major differences can be predicted between the
DNA damage profiles of this hybrid agent and that of PT-ACRAMTU (31). It is noteworthy to mention that precedent exists for this groove-specific reactivity in the
DNA interactions of analogous dual intercalating/alkylating agents: nitrogen mustard groups attached to the 4-carboxamide residue alkylate guanine-N7 in the major groove, whereas the major target of the analogous 9-anilino-linked mustards is adenine-N3 in the minor groove [181]. It was rationalized that the minor groove adducts formed by PT-
ACRAMTU are intercalator-driven and contribute significantly to its cytotoxic effect
[10]. Finally, differences may exist in the recognition and repair of the DNA adducts formed by 31 and 58. Taken together, the 4-substituted analogues do not appear to have any advantage over the 9-substituted prototype, and the strategy of tuning its reactivity, pursued in Chapter 3, turns out to be far superior to changing the linkage geometry of the hybrids.
127 CHAPTER 5
SUMMARY AND OUTLOOK
128 The research in this dissertation was concerned with establishing structure– activity relationships in a novel class of DNA-targeted platinum antitumor agents with the ultimate goal of improving the biological activity of the prototypical agent, PT-
ACRAMTU (31). Two ways were explored of tuning the reactivity and target interactions of platinum, which was considered critical in changing the pharmacological properties of this pharmacophore. The first approach (Chapters 2 and 3) involved a structurally minimally invasive modification of the prototype to enhance its chemical stability and reactivity with DNA. This was achieved by introducing a new inert nonleaving group in place of thiourea, or, in other words, by simply changing the ligand donor set of the platinum moiety from [PtN3S] to [PtN4]. In the second approach
(Chapter 4), an acridine-4-carboxamide derivative of PT-ACRAMTU was generated, in which the point of attachment of the platinum-modified linker was changed from the 9- position to the 4-position of the planar chromophore. It was reasoned that this change in geometry might have consequences for the DNA groove and sequence selectivity of platinum and, ultimately, the cytotoxicity of the adducts formed.
To investigate the effect of sulfur on the substitution chemistry of platinum in reactions with nucleobase nitrogen and potential consequences of DNA damage profile of the conjugate, a nitrogen analogue of ACRAMTU, 1-[2-(acridine-9-ylamino)ethyl]-1,3- dimethylguanidine (38) was synthesized in which thiourea-S is replaced with guanidine-
NH. While this study was able to delineate unusual pathways to novel cyclic and spirocyclic acridine derivatives, the ultimate goal of generating a guanidine analogue of
PT-ACRAMTU was not reached due to the chelating properties of the guanidinato group.
Instead, two all-nitrogen derivatives of PT-ACRAMTU containing an amidine-NH
129 linkage (54 and 55), based on the ligand N-(2-(acridin-9-ylamino)ethyl)-N-
methylpropionimidamide, could be synthesized using platinum-mediated amidination
chemistry. A relationship was discovered between the rate of DNA binding and
cytotoxic potential: it was found that replacement of thiourea-S with amidine-NH, which
accelerates platinum binding to nucleobase nitrogen, leads to a dramatic increase in cell
toxicity. On the other hand, it was also demonstrated that the change in donor group
reduces the (undesired) reactivity of platinum with cysteine sulfur. These findings
suggest that the direct reaction of platinum with cysteine sulfur, a strong nucleophile, is
controlled by the cis-effect. The fact that the opposite scenario is observed for DNA
nitrogen supports the view that steric effects dominate this reaction, which most likely
requires hydrolytic activation (aquation) of platinum. To test if the increased reactivity of
compound 54 with DNA nitrogen leads to higher nuclear levels of platinum in cancer
cells, a cellular imaging study using confocal fluorescence microscopy was performed.
The study revealed higher intracellular levels of complex 54 than of PT-ACRAMTU.
Unfortunately, direct detection of platinum in the nucleus was complicated by DNA- mediated quenching of acridine fluorescence. A high concentration of drug appears to be located to the lysosomes. The biological relevance of this effect, which is most likely a consequence of the protonation state of acridine in a low-pH environment, is unclear.
Both amidine-modified complexes, 54 and 55, show nanomolar cytotoxicity in H460 cells, and derivative 55 is able to slow the growth of tumors in the corresponding xenograft model. The concentrations of platinum detected in various biological tissues reveal that, while compound 55 reaches the tumors, the majority of platinum accumulated
130 in the kidneys, the major organ of excretion, which most likely contributes to the toxicity
observed in the treated animals.
Unlike the amidine-based derivatives of PT-ACRAMTU, 54 and 55, the compound’s 4-carboxamide analogue, [PtCl(en)(N-(2-(1,3- dimethylthioureido)ethyl)acridine-4-carboxamide)]NO3 (58), did not offer any
improvement compared to the prototype. It may be speculated that the adducts formed
by this complex are less cytotoxic or more easily repaired than those formed by PT-
ACRAMTU. Other factors, such as reduced DNA binding because of the lower charge
on the complex due to the lowered pKa of the acridine carrier, may also contribute to the
relatively low activity of compound 58.
In conclusion, the research in this dissertation has performed two different
structure-activity relationship studies that will have an impact on the future development of these platinum-based therapies into treatments for incurable cancers. In particular, the new amidine-based analogues represent a new lead in the race for a cure of aggressive, chemoresistant cancers, such as NSCLC. As with most cytotoxic agents, one problem that remains to be solved is their high systemic toxicity, which, in the present case, is most likely the result of the promiscuous reactivity of platinum with biological nucleophiles. The future design of analogues of compounds 54 and 55 should therefore
be concerned with improving the therapeutic index of this type of agent. One strategy
might be to reduce the reactivity of the metal with sulfur nucleophiles, which may be
possible by tuning the substitution chemistry of the current lead compounds. In addition,
targeting the new hybrids specifically to cancer cells may be another option of enhancing
the pharmacological properties of these agents.
131 REFERENCES
1. Jemal AS, R.; Ward, E.; Hao, Y.; Xu, J.; Thun, M. J. Cancer Statistics, 2009. CA
Cancer J Clin 2009;59:225-49.
2. Sher T, Dy GK, Adjei AA. Small cell lung cancer. Mayo Clinic Proceed
2008;83:355-67.
3. Heighway JB, DC. Lung: small cell cancer. Atlas of Genetics and Cytogenetics
in Oncology and Haematology 2004.
4. Molina JR, Yang PG, Cassivi SD, Schild SE, Adjei AA. Non-small cell lung
cancer: Epidemiology, risk factors, treatment, and survivorship. Mayo Clinic
Proceed 2008;83:584-94.
5. Sharma SV, Bell DW, Settleman J, Haber DA. Epidermal growth factor receptor
mutations in lung cancer. Nat Rev Cancer 2007;7:169-81.
6. Rapp E, Pater JL, Willan A, Cormier Y, Murray N, Evans WK, et al.
Chemotherapy Can Prolong Survival in Patients with Advanced Non-Small-Cell
Lung-Cancer - Report of a Canadian Multicenter Randomized Trial. J Clin Oncol
1988;6:633-41.
7. Chabner BA, Roberts TG. Timeline - Chemotherapy and the war on cancer. Nat
Rev Cancer 2005;5:65-72.
8. Nelson SM, Ferguson LR, Denny WA. DNA and the chromosome - varied targets
for chemotherapy. Cell Chromosome 2004;3:2.
9. Neidle S, Thurston DE. Chemical approaches to the discovery and development
of cancer therapies. Nat Rev Cancer 2005;5:285-96.
132 10. Guddneppanavar R, Bierbach U. Adenine-N3 in the DNA Minor Groove- An
Emerging Target for Platinum Containing Anticancer Pharmacophores. Anti-
Cancer Agents Med Chem 2007;7:125-38.
11. Neidle S. Nucleic Acid Structure and Recognition. New York: Oxford University
Press, 2002.
12. Wemmer DE, Dervan PB. Targeting the minor groove of DNA. Curr Opin Struct
Biol 1997;7:355-61.
13. Denny WA. DNA minor groove alkylating agents. Curr Med Chem 2001;8:533-
44.
14. Sikora K, Advani S, Koroltchouk V, Magrath I, Levy L, Pinedo H, et al. Essential
drugs for cancer therapy: a World Health Organization consultation. Ann Oncol
1999;10:385-90.
15. Yang XL, Wang AH. Structural studies of atom-specific anticancer drugs acting
on DNA. Pharmacol Ther 1999;83:181-215.
16. Workman P. New drug targets for genomic cancer therapy: successes, limitations,
opportunities and further challenges. Curr Cancer Drug Targets 2001;1:33-47.
17. Hurley LH. DNA and its associated processes as targets for cancer therapy. Nat
Rev Cancer 2002;2:188-200.
18. Chaires JB. Drug--DNA interactions. Curr Opin Struct Biol 1998;8:314-20.
19. Sischka A, Toensing K, Eckel R, Wilking SD, Serald N, Ros R. Molecular
mechanisms and kinetics between DNA and DNA binding ligands. Biophys J
2005;88:404-11.
133 20. Newton BA. Mechanisms of antibiotic action. Annu Rev Microbiol 1965;19:209-
40.
21. Dorr RT, Von Hoff DD. Cancer Chemotherapy Handbook. Norwalk, Connecticut:
Appleton&Lange, 1994.
22. Tewey KM, Rowe TC, Yang L, Halligan BD, Liu LF. Adriamycin-induced DNA
damage mediated by mammalian DNA topoisomerase II. Science 1984;226:466-8.
23. Ward DC, Reich E, Goldberg IH. Base specificity in the intercalation of
polynucleotides with antibiotic drugs. Science 1965;149:1259-63.
24. Searle MS, Wickham G. Hoogsteen versus Watson-Crick A-T basepairing in
DNA complexes of a new group of 'quinomycin-like' antibiotics. FEBS Lett
1990;272:171-4.
25. Waring MJ, Wakelin LP. Echinomycin: a bifunctional intercalating antibiotic.
Nature 1974;252:653-7.
26. Brana MF, Cacho M, Gradillas A, de Pascual-Teresa B, Ramos A. Intercalators as
anticancer drugs. Curr Pharm Design 2001;7:1745-80.
27. Graves DE, Velea LM. Intercalative binding of small molecules to nucleic acids.
Curr Org Chem 2000;4:915-29.
28. Blackburn GM, Gait MJ. Nucleic Acids in Chemistry and Biology. New York:
Oxford University, 1996.
29. Reich E, Goldberg IH. Actinomycin and nucleic acid function. Prog Nucleic Acid
Res Mol Biol 1964;3:183-234.
134 30. Zimmer C, Wahnert U. Nonintercalating DNA-binding ligands: specificity of the
interaction and their use as tools in biophysical, biochemical and biological
investigations of the genetic material. Prog Biophys Mol Biol 1986;47:31-112.
31. Strobel SA, Doucette-Stamm LA, Riba L, Houseman DE, Dervan PB. Site
specific cleavage of human chromosome 4 mediated by triple helix formation.
Science 1991;254:1639-42.
32. Thuong N, Helene C. Sequence sepecific recognition and modification of double
helical DNA by oligonucleotide. Angew Chem Int Ed Engl 1993;32:666-90.
33. Dervan PB. Molecular recognition of DNA by small molecules. Bioorg Med
Chem 2001;9:2215-35.
34. Nielsen PE. Peptide nucleic acids as therapeutic agents. Curr Opin Struc Biol
1999;9:353-7.
35. Moser H, Dervan PB. Sequence-specific cleavage of double helical DNA by triple
helix formation. Science 1987;238:645-50.
36. Povsic TJ, Dervan PB. Triple helix formation by oligonucleotides on DNA
extended to the physiological pH range. J Am Chem Soc 1989;111:3059-61.
37. Maher LJ, Dervan PB, Wold B. Analysis of Promoter-Specific Repression by
Triple-Helical DNA Complexes in a Eukaryotic Cell-Free Transcription System.
Biochemistry 1992;31:70-81.
38. Maher LJ, Wold B, Dervan PB. Inhibition of DNA-Binding Proteins by
Oligonucleotide-Directed Triple Helix Formation. Science 1989;245:725-30.
135 39. Nielsen PE, Egholm M, Berg RH, Buchardt O. Sequence-Selective Recognition
of DNA by Strand Displacement with a Thymine-Substituted Polyamide. Science
1991;254:1497-500.
40. Egholm M, Buchardt O, Christensen L, Behrens C, Freier SM, Driver DA, et al.
Pna Hybridizes to Complementary Oligonucleotides Obeying the Watson-Crick
Hydrogen-Bonding Rules. Nature 1993;365:566-8.
41. Hanvey JC, Peffer NJ, Bisi JE, Thomson SA, Cadilla R, Josey JA, et al. Antisense
and Antigene Properties of Peptide Nucleic-Acids. Science 1992;258:1481-5.
42. Lohse J, Dahl O, Nielsen PE. Double duplex invasion by peptide nucleic acid: a
general principle for sequence-specific targeting of double stranded DNA. Proc
Natl Acad Sci 1999;96:11804-8.
43. Gilman A, Philips FS. The biological actions and therapeutic aplications of beta-
chloroethyl amines and sulfides. science 1946;103:409-15.
44. Henner WD, Rodriguez LO, Hecht SM, Haseltine WA. gamma Ray induced
deoxyribonucleic acid strand breaks. 3' Glycolate termini. J Biol Chem
1983;258:711-3.
45. Janicek MF, Haseltine WA, Henner WD. Malondialdehyde precursors in gamma-
irradiated DNA, deoxynucleotides and deoxynucleosides. Nucleic Acids Res
1985;13:9011-29.
46. Sikic BI, Rosencweig M, Carter SK. Bleomycin Biotherapy. Orlando: Academic
Press, 1985.
47. Murray V, Martin RF. The sequence specificity of bleomycin-induced DNA
damage in intact cells. J Biol Chem 1985;260:10389-91.
136 48. Burger RM. Cleavage of Nucleic Acids by Bleomycin. Chem Rev 1998;98:1153-
69.
49. Nagai N, Yamaki H, Suzuki H, Tanaka N, Umezawa H. On the mechanism of
action of bleomycin: scission of DNA strands in vitro and in vivo. J Antibiot
1969;22:446-8.
50. Burger RM, Peisach J, Horwitz SB. Activated bleomycin. A transient complex of
drug, iron, and oxygen that degrades DNA. J Biol Chem 1981;256:11636-44.
51. Sugiyama H, Kilkuskie RE, Hecht SM, Vandermarel GA, Vanboom JH. An
Efficient, Site-Specific DNA Target for Bleomycin. J Am Chem Soc
1985;107:7765-7.
52. Burger RM, Berkowitz AR, Peisach J, Horwitz SB. Origin of malondialdehyde
from DNA degraded by Fe(II) bleomycin. J Biol Chem 1980;255:11832-8.
53. Hanahan D, Weinberg RA. The hallmarks of cancer. Cell 2000;100:57-70.
54. Kris MG, Natale RB, Herbst RS, Lynch TJ, Prager D, Belani CP, et al. Efficacy
of gefitinib, an inhibitor of the epidermal growth factor receptor tyrosine kinase,
in symptomatic patients with non-small cell lung cancer - A randomized trial. J
Am Med Assoc 2003;290:2149-58.
55. Graham J, Muhsin M, Kirkpatrick P. Oxaliplatin. Nat Rev Drug Discovery
2004;3:11-2.
56. Rosenberg B, Vancamp L, Krigas T. Inhibition of cell division in escherichia coli
by electrolysis products from a platinum electrode. Nature 1965;205:698-9.
57. Rosenberg B, VanCamp L, Trosko JE, Mansour VH. Platinum compounds: a new
class of potent antitumor agents. Nature 1969;222:385-6.
137 58. Kelland L. The resurgence of platinum-based cancer chemotherapy. Nat Rev
Cancer 2007;7:573-84.
59. Siddik ZH. Cisplatin: mode of cytotoxic action and molecular basis of resistance.
Oncogene 2003;22:7265-79.
60. Fichtinger-Schepman AM, Van der Veer JL, Den Hartog JH, Lohman PH,
Reedijk J. Adducts of the antitumor drug cis-diaminedichloroplatinum (II) with
DNA: formation, identification, and quantitation. Biochemistry 1985;24:707-13.
61. Takahara PM, Rosenzweig AC, Frederick CA, Lippard SJ. Crystal structure of
double-stranded DNA containing the major adduct of the anticancer drug cisplatin.
Nature 1995;377:649-52.
62. Teuben JM, Bauer C, Wang AH, Reedijk J. Solution structure of a DNA duplex
containing a cis-diammineplatinum(II) 1,3-d(GTG) intrastrand cross-link, a major
adduct in cells treated with the anticancer drug carboplatin. Biochemistry
1999;38:12305-12.
63. Huang H, Zhu L, Reid BR, Drobny GP, Hopkins PB. Solution structure of a
cisplatin-induced DNA interstrand cross-link. Science 1995;270:1842-5.
64. Kartalou M, Essigmann JM. Mechanisms of resistance to cisplatin. Mutat Res
2001;478:23-43.
65. Harrap KR. Predinical studies identifying carboplatin as a viable cisplatin
alternative. Cancer Treat Rev 1985;12:21-33.
66. Knox RJ, Friedlos F, Lydall DA, Roberts JJ. Mechanism of cytotoxicity of
anticancer platinum drugs: evidence that cis-diamine-(1,1-
138 cyclobutanedicarboxylato)platinum(II) differ only in the kinetics lf their
interaction with DNA. Cancer Res 1986;46:1972-9.
67. Machover D, et al. Two consecutive phase II studies of oxaliplatin (L-OHP) for
the treatment of patients with advanced coloretcal carcinoma who were resistant
to previous treatment with fluoropyrimidines. Ann Oncol 1996;7:95-8.
68. Holzer AK, Manorek GG, Howell SB. Contribution of the major copper influx
transporter CTR1 to the cellular acuumulation of cisplatin, carboplatin and
oxaliplatin. Molec Pharmacol 2006;70:1390-4.
69. Spingler B, Whittington DA, Lippard SJ. 2.4 A Crystal Structure of an Oxaliplatin
1,2-d(GpG) Intrastrand Cross-Link in a DNA Dodecamer Duplex. Inorg Chem
2001;40:5596-602.
70. Raymond E, Faivre S, Chaney S, Woynarowski J, Cvitkovic E. Celluar and
molecular pharmacology of oxaliplatin. Mol Cancer Ther 2002;1:227-35.
71. Kelland LR, al. e. Preclinical antitumor evaluation of bis-acetato-ammine-
dichloro-cyclohexylamine platinum (IV): an orally active platinum drug. Cancer
Res 1993;53:2581-6.
72. Holford J, Sharp SY, Murrer BA, Abrams M, Kelland LR. In vitro circumvention
of cisplatin resistance by the novel sterically hindered platinum complex
AMD473. Br J Cancer 1998;77:366-73.
73. Raynaud FI, al. e. Cis-aminedichloro(2-methylpyridine)platinum (II) (AMD473),
a novel sterically hindered platinum complex: in vivo activity, toxicology and
pharmacokinetics in mice. Clin Cancer Res 1997;3:2063-74.
139 74. Dragovich T, Mendelson D, Kurtin S, Richardson K, Von Hoff D, Hoos A. A
phase 2 trial of the liposomal DACH platinum L-NDDP in patient with therapy-
refractory advanced coloretal cancer. Cancer Chemother Pharmacol 2006;58:759-
64.
75. Rice JR, Gerberich JL, Nowotnik DP, Howell SB. Preclinic efficacy and
pharmacokenetics of AP5346, a novel diaminocyclohexane-platinum tumor-
targeting drug delivery system. Clin Cancer Res 2006;12:2248-54.
76. Cleare MJ, Hoeschele JD. Studies on the antitumor activity of group VIII
transition metal complexes. Part I. Platinum (II) complexes. Bioinorg Chem
1973;2:187-210.
77. Wang X, Guo Z. Towards the rational design of platinum(II) and gold(III)
complexes as antitumor agents. Dalton Trans 2008:1521-32.
78. Peleg-Shulman T, Katzhendler J, Gibson D. Effects of monofunctional platinum
binding on the thermal stability and conformation of a self-complementary 22-
mer. J Inorg Biochem 2000;81:313-23.
79. Okada T, El-Mehasseb M, Kodaka M, Tomohiro T, Okamoto K, Okuno H.
Mononuclear Platinum(II) Complex with 2-Phenylpyridine Ligands Showing
High Cytotoxicity against Mouse Sarcoma 180 Cells Acquiring High Cisplatin
Resistance. J Med Chem 2001;44:4661-7.
80. Aris SM, Farrell NP. Towards antitumor active trans-platinum compounds. Eur J
Inorg Chem 2009:1293-302.
140 81. BernalMendez E, Boudvillain M, Gonzalez Vichez F, Leng M. Chemical
Versatility of Transplatin Monofunctional Adducts within Multiple Site-
Specifically Platinated DNA. Biochemistry 1997;36:7281-7.
82. Paquet F, Boudvillain M, Lancelot G, Leng M. NMR solution structure of a DNA
dodecamer containing a transplatin interstrand GN7-CN3 cross-link. Nucletic
Acids Res 1999;27:4261-8.
83. Bierbach U, Qu Y, Hambley TW, Peroutka J, Nguyen HL, Doedee M, Farrell N.
Synthesis, Structure, Biological Activity, and DNA Binding of Platinum(II)
Complexes of the Type trans-[PtCl2(NH3)L] (L = Planar Nitrogen Base). Effect of
L and Cis/Trans Isomerism on Sequence Specificity and Unwinding Properties
Observed in Globally Platinated DNA. Inorg Chem 1999;38:3535-42.
84. Wheate NJ, Collins JG. Multinuclear platinum complexes as anticancer drugs.
Coord Chem Rev 2003;241:133-45.
85. Hegmans A, Berners-Price SJ, Davies MS, Thomas DS, Humphreys AS, Farrell N.
Long range 1,4 and 1,6-interstrand cross-links formed by a trinuclear platinum
complex. Minor groove preassociation affects kinetics and mechanism of cross-
link formation as well as adduct structure. J Am Chem Soc 2004;126:2166-80.
86. Farrell N, Qu Y, Feng L, Van Houten B. Comparison of chemical reactivity,
cytotoxicity, interstrand cross-linking and DNA sequence specificity of
bis(platinum) complexes containing monodentate or bidentate coordination
spheres with their monomeric analogues. Biochemistry 1990;29:9522-31.
87. Pratesi G, Perego P, Polizzi D, Righetti SC, Supino R, Caserini C, et al. A novel
charged trinuclear platinum complex effective against cisplatin-resistant tumours:
141 hypersensitivity of p53-mutant human tumour xenografts. Br J Cancer
1999;80:1912-9.
88. Roberts JD, Peroutka J, Farrell N. Cellular pharmacology of polynuclear platinum
anti-cancer agents. J Inorg Biochem 1999;77:51-7.
89. Zehnulova J, Kasparkova J, Farrell N, Brabec V. Conformation, recognition by
high mobility group domain proteins, and nucleotide excision repair of DNA
intrastrand cross-links of novel antitumor trinuclear platinum complex BBR3464.
J Biol Chem 2001;276:22191-9.
90. Baruah H, Barry CG, Bierbach U. Platinum-intercalator conjugates: from DNA-
targeted cisplatin derivatives to adenine binding complexes as potential
modulators of gene regulation. Curr Topics Med Chem 2004;4:1537-49.
91. Tullius TD, Lippard SJ. Ethidium bromide changes the nuclease-sensitive DNA
binding sites of the antitumor drug cis-diamminedichloroplatinum(II). Proc Natl
Acad Sci U S A 1982;79:3489-92.
92. Malinge JM, Leng M. Reaction of nucleic acids and cis-
diamminedichloroplatinum(II) in the presence of intercalating agents. Proc Natl
Acad Sci U S A 1986;83:6317-21.
93. Bowler BE, Hollis LS, Lippard SJ. Synthesis and DNA-binding and photonicking
properties of acridine-orange linked by a polymethylene tether to (1,2-
diaminoethane)dichloroplatinum(II). J Am Chem Soc 1984;106:6102-4.
94. Bowler BE, Ahmed KJ, Sundquist WI, Hollis LS, Whang EE, Lippard SJ.
Synthesis, characterization, and DNA-binding properties of (1,2-
diaminoethane)platinum(II) complexes linked to the DNA intercalator acridine-
142 orange by trimethylene and hexamethylene chains. J Am Chem Soc
1989;111:1299-306.
95. Zunino F, Savi G, Pasini A. Synthesis and antitumor activity of a platinum (II)-
doxorubicin complex. Cancer Chemother Pharmacol 1986;18:180-2.
96. Pasini A, Pratesi G, Savi G, Zunino F. A doxorubicin-Pt(II) complex - chemistry
and antitumor-activity. Inorg Chim a-Bioinor 1987;137:123-4.
97. Palmer BD, Lee HH, Johnson P, Baguley BC, Wickham G, Wakelin LP, et al.
DNA-directed alkylating agents. 2. Synthesis and biological activity of platinum
complexes linked to 9-anilinoacridine. J Med Chem 1990;33:3008-14.
98. Gelasco A, Lippard SJ. Anticancer activity of cisplatin and related complexes. In:
Topics in Biological Inorganic Chemistry; Clarke, M. J., Sadler, P. J., Eds.; New
York: Springer, 1999.
99. Gelasco A, Lippard SJ. NMR solution structure of a DNA dodecamer duplex
containing a cis-diammineplatinum(II) d(GpG) intrastrand cross-link, the major
adduct of the anticancer drug cisplatin. Biochemistry 1998;37:9230-9.
100. Martins ET, Baruah H, Kramarczyk J, Saluta G, Day CS, Kucera GL, Bierbach U.
Design, synthesis, and biological activity of a novel non-cisplatin-type platinum-
acridine pharmacophore. J Med Chem 2001;44:4492-6.
101. Xu DQ, Prasad K, Repic O, Blacklock TJ. Ethyl trifluoroacetate - a powerful
reagent for differentiating amino-groups. Tetrahedron Letters 1995;36:7357-60.
102. Ackley MC, Barry CG, Mounce AM, Farmer MC, Springer BE, Day CS, et al.
Structure-activity relationships in platinum-acridinylthiourea conjugates: effect of
143 the thiourea nonleaving group on drug stability, nucleobase affinity, and in vitro
cytotoxicity. J Biol Inorg Chem 2004;9:453-61.
103. Hess SM, Mounce AM, Sequeira RC, Augustus TM, Ackley MC, Bierbach U.
Platinum-acridinylthiourea conjugates show cell line-specific cytotoxic
enhancement in H460 lung carcinoma cells compared to cisplatin. Cancer
Chemother Pharmacol 2005;56:337-43.
104. Hess SM, Anderson JG, Bierbach U. A non-crosslinking platinum-acridine hybrid
agent shows enhanced cytotoxicity compared to clinical BCNU and cisplatin in
glioblastoma cells. Bioorg Med Chem Lett 2005;15:443-6.
105. Barry CG, Baruah H, Bierbach U. Unprecedented monofunctional metalation of
adenine nucleobase in guanine- and thymine-containing dinucleotide sequences
by a cytotoxic platinum-acridine hybrid agent. J Am Chem Soc 2003;125:9629-37.
106. Barry CG, Day CS, Bierbach U. Duplex-promoted platination of adenine-N3 in
the minor groove of DNA: challenging a longstanding bioinorganic paradigm. J
Am Chem Soc 2005;127:1160-9.
107. Budiman ME, Alexander RW, Bierbach U. Unique base-step recognition by a
platinum-acridinylthiourea conjugate leads to a DNA damage profile
complementary to that of the anticancer drug cisplatin. Biochemistry
2004;43:8560-7.
108. Budiman ME, Bierbach U, Alexander RW. DNA Minor Groove Adducts Formed
by a Platinum-Acridine Conjugate Inhibit Association of TATA-Binding Protein
with Its Cognate Sequence. Biochemistry 2005;44:11262-8.
144 109. Baruah H, Bierbach U. Unusual intercalation of acridin-9-ylthiourea into the 5'-
GA/TC DNA base step from the minor groove: implications for the covalent
DNA adduct profile of a novel platinum-intercalator conjugate. Nucleic Acids
Res 2003;31:4138-46.
110. Baruah H, Day CS, Wright MW, Bierbach U. Metal-intercalator-mediated self-
association and one-dimensional aggregation in the structure of the excised major
DNA adduct of a platinum-acridine agent. J Am Chem Soc 2004;126:4492-3.
111. Guddneppanavar R, Wright MW, Tomsey AK, Bierbach U. Guanine binding of a
cytotoxic platinum-acridin-9-ylthiourea conjugate monitored by 1-D 1H and 2-D
[1H,15N] NMR spectroscopy: Hydrolysis is not the rate-determining step. J Inorg
Biochem 2006.
112. Gago F. Stacking interactions and intercalative DNA binding. Methods-a
Companion to Methods in Enzymolo 1998;14:277-92.
113. Denny WA. Acridine derivatives as chemotherapeutic agents. Curr Med Chem
2002;9:1655-65.
114. Adams A, Guss JM, Collyer CA, Denny WA, Wakelin LP. Crystal structure of
the topoisomerase II poison 9-amino-[N-(2-dimethylamino)ethyl]acridine-4-
carboxamide bound to the DNA hexanucleotide d(CGTACG)2. Biochemistry
1999;38:9221-33.
115. Augustus TM, Anderson J, Hess SM, Bierbach U.
Bis(acridinylthiourea)platinum(II) complexes: synthesis, DNA affinity, and
biological activity in glioblastoma cells. Bioorg Med Chem Lett 2003;13:855-8.
145 116. Bonivento M, Canovese L, Cattalini L, Marangoni G, Michelson G, Tobe ML.
Cis effect of dimethylsulfide and leaving group effect in reactions of the cationic
complex chloro(dimethyl sulfide)(1,2-diaminoethane)platinum(II) chloride. Inorg
Chem 1981;20:3728-30.
117. Bonivento M, Cattalini L, Marangoni G, Michelson G, Schwab AP, Tobe ML.
The cis effect of dimethyl sulfoxide in the reactions of the cationic complex
chloro(dimethyl sulfoxide)(1,2-diaminoethane)platinum(II) chloride. Inorg Chem
1980;19:1743-6.
118. Brabec V, Vrana O, Novakova O, Kleinwachter V, Intini FP, Coluccia M, Natile
G. DNA adducts of antitumor trans-[PtCl2(E-imino ether)2]. Nucleic Acids Res
1996;24:336-41.
119. Wu YQ, Hamilton SK, Wilkinson DE, Hamiton GS. Direct synthesis of
guanidines using di(imidazole-1-yl)methanimine. J Org Chem 2002;67:7553-6.
120. Jenkins TC. Drug DNA Interaction Protocols. Totowa, NJ: Humana Press, 1997.
121. Ramadas K, Srinivasan N. An expedient synthesis of substituted guanidines.
Tetrahedron Lett 1995;36:2841-4.
122. Maryanoff CA, Stanzione RC, Plampin JN, Mills JE. A convenient synthesis of
guanidines from thioureas. J Org Chem 1986;51:1882-4.
123. Srinivasan N, Ramadas K. Role of quaternaryammonium permanganates in the
synthesis of substituted guanidines-a comparative study Tetrahedron Lett
2001;42:343-6.
146 124. Rasmussen CR, Villani FJ, Reynolds BE, Plampin JN, Hood AR, Hecker LR, et
al. A Versatile Synthesis of Novel N,N,N″-Trisubstituted Guanidines. Synthesis
1988:460-6.
125. Barton DHR, Elliott JD, Gero SD. The synthesis and properties of a series of
strong but hindered organic bases. Chem Commun 1981:1136-7.
126. Katritzky AR, Rogovoy BV, Chassaing C, Vvedensky V. Di(benzotriazol-1-
yl)methanimine: A New Reagent for the Synthesis of Tri- and Tetrasubstituted
Guanidines. J Org Chem 2000;65:8080-2.
127. Wu YQ, Limburg DC, Wilkinson DE, Hamilton GS. 1-Cyanoimidazole as a Mild
and Efficient Electrophilic Cyanating Agent. Org Lett 2000;2:795-7.
128. Iwanowicz EJ, Poss MA, Lin J. Preparation of N,N'-Bis-Tert-
Butoxycarbonylthiourea. Synth Commun 1993;23:1443-5.
129. Levallet C, Lerpiniere J, Ko SY. The HgCl2-promoted guanylation reaction: The
scope and limitations. Tetrahedron 1997;53:5291-304.
130. Isobe T, Fukuda K, Tokunaga T, Seki H, Yamaguchi K, Ishikawa T. Modified
Guanidines as Potential Chiral Superbases. 2. Preparation of 1,3-Unsubstituted
and 1-Substituted 2-Iminoimidazolidine Derivatives and a Related Guanidine by
the 2-Chloro-1,3-dimethylimidazolinium Chloride-Induced Cyclization of
Thioureas. J Org Chem 2000;65:7774-8.
131. Klika KD, Bernat J, Imrich J, Chomca I, Sillanpaa R, Pihlaja K. Unexpected
Formation of a Spiro Acridine and Fused Ring System from the Reaction between
an N-Acridinylmethyl-Substituted Thiourea and Bromoacetonitrile under Basic
Conditions. J Org Chem 2001;66:4416-8.
147 132. Klika KD, Balentova E, Bernar J, Imrich J, Vavrusova M, Kleinpeter E, et al.
Tautomerism, Regioisomerism, and Cyclization Reactions of Acridinyl
Thiosemicarbazides. Heterocycl Chem 2006;43:633-43.
133. Ismail IM, Sadler PJ. 195Pt and 15N-NMR studies of antitumor complexes. ACS
symp ser 1983;209:171-90.
134. Wieboldt R, Ramesh D, Jabri E, Karplus PA, Carpenter BK, Hess GP. Synthesis
and characterization of photolabile o-nitrobenzyl derivatives of urea. J Org Chem
2002;67:8827-31.
135. Neville RG, McGee JJ. N-Mono- and N,N-disubstituted ureas and thioureas. Org
Synth 1973;5:801-5.
136. Beak P, Lee JK, Mckinnie BG. Methylation of Protomeric Ambident
Nucleophiles with Methyl Fluorosulfonate - Regiospecific Reaction. J Org Chem
1978;43:1367-72.
137. Bae I, Han H, Chang S. Highly efficient one-pot synthesis of N-sulfonylamidines
by Cu-catalyzed three-component coupling of sulfonyl azide, alkyne, and amine. J
Am Chem Soc 2005;127:2038-9.
138. Xu F, Sun JH, Shen Q. Samarium diiodide promoted synthesis of N,N '-
disubstituted amidines. Tetrahedron Lett 2002;43:1867-9.
139. Bokach NA, Kukushkin VY, Kuznetsov ML, Garnovskii DA, Natile G, Pombeiro
AJL. Direct addition of alcohols to organonitriles activated by ligation to a
platinum(IV) center. Inorg Chem 2002;41:2041-53.
148 140. Lasri J, Charmier AJ, da Silva MFCG, Pombeiro AJL. Direct synthesis of
(imine)platinum(II) complexes by iminoacylation of ketoximes with activated
organonitrile ligands. J Chem Soc Dalton Trans 2006:5062-7.
141. Natile G, Intini FP, Bertani R, Michelin RA, Mozzon M, Sbovata SM, et al.
Synthesis and characterisation of the amidine complexes trans-[PtCl(NH3)
(HN=C(NH2)R)2]Cl (R = Me, Ph, CH2Ph) derived from addition of NH3 to the
coordinated nitriles in trans-[PtCl2(NCR)2]. J Organomet Chem 2005;690:2121-7.
142. Guddneppanavar R, Saluta G, Kucera GL, Bierbach U. Synthesis, biological
activity, and DNA-damage profile of platinum-threading intercalator conjugates
designed to target adenine. J Med Chem 2006;49:3204-14.
143. Storhoff BN, Lewis HC. Organonitrile complexes of transition metals. Coord
Chem Rev 1977;23:1-29.
144. Maresca L, Natile G, Intini FP, Gasparini F, Tiripicchio A, Tiripicchio-Camellini
M. Nucleophilic attack of amine and hydroxide to platinum dibenzonitrile
dichloride. Crystal structure of [Pt(NH=CPhN-tert-BuCH2CH2NH-tert-
Bu)Cl(NHCOPh)] and cis-[PtNH=CPhN-tert-BuCH2CH2NH-tert-Bu)Cl2(NCPh)].
J Am Chem Soc 1986;108:1180-5.
145. Fanizzi FP, Intini FP, Natile G. Nucleophilic attack of methanol on
bis(benzonitrile)dichloroplatinum: formation of mono- and bis-imido ester
derivatives. J Chem Soc, Dalton Trans 1989:947-51.
146. Erxleben A, Mutikainen I, Lippert B. Conversion of Acetonitrile into Acetamide
II in the Coordination Spheres of Cis- and Trans-M (Amine)2 (M=Pt or Pd).
Solution and Crystal Structural Studies. J Chem Soc Dalton Trans 1994:3667-75.
149 147. Kukushkin VY, Pombeiro AJ. Additions to metal-activated organonitriles. Chem
Rev 2002;102:1771-802.
148. Wang X, Guo Z. The role of sulfur in platinum anticancer chemotherapy. Anti-
Cancer Agents Med Chem 2007;7:19-34.
149. Stephens DJ, Allan VJ. Light microscopy techniques for live cell imaging.
Science 2003;300:82-6.
150. Haraguchi T. Live cell imaging: approaches for studying protein dynamics in
living cells. Cell Struct Funct 2002;27:333-4.
151. Griffiths G, Hoflack B, Simons K, Mellman I, Kornfeld S. The Mannose 6-
Phosphate Receptor and the Biogenesis of Lysosomes. Cell 1988;52:329-41.
152. Poot M, Zhang YZ, Kramer JA, Wells KS, Jones L, Hanzel DK, et al. Analysis of
mitochondrial morphology and function with novel fixable fluorescent stains. J
Histochem Cytochem 1996;44:1363-72.
153. Sriram M, Van der Marel GA, Roelen HLPF, Van Boom JH, Wang AHJ.
Conformation of B-DNA containing O6-ethyl-G-C base pairs stablized by minor
groove binding drugs: molecular structure of d(CGC[e6G]AATTCGCG)
complexed with Hoechst 33258 or Hoechst 33342. Embo J 1992;11:225-32.
154. New EJ, Duan R, Zhang JZ, Hambley TW. Investigations using fluorescent
ligands to monitor platinum(IV) reduction and platinum(II) reactions in cancer
cells. Dalton Trans 2009:3092-101.
155. Kalayda GV, Jansen BAJ, Molenaar C, Wielaard P, Tanke HJ, Reedijk J.
Dinuclear platinum complexes with N, N'-bis(aminoalkyl)-1,4-
diaminoanthraquinones as linking ligands. Part II. Cellular processing in A2780
150 cisplatin-resistant human ovarian carcinoma cells: new insights into the
mechanism of resistance. J Biol Inorg Chem 2004;9:414-22.
156. Jansen BAJ, Wielaard P, Kalayda GV, Ferrari M, Molenaar C, Tanke HJ, et al.
Dinuclear platinum complexes with N, N'-bis(aminoalkyl)-1,4-
diaminoanthraquinones as linking ligands. Part I. Synthesis, cytotoxicity, and
cellular studies in A2780 human ovarian carcinoma cells. J Biol Inorg Chem
2004;9:403-13.
157. Safaei R, Katano K, Larson BJ, Samimi G, Holzer AK, Naerdemann W, et al.
Intracellular Localization and Trafficking of Fluorescein-Labeled Cisplatin in
Human Ovarian Carcinoma Cells. Clinical Cancer Res 2005;11:756-67.
158. Levitt J, Xylas J, Baldwin A, Munger K, Gerogakoudi I. Instrinsic fluorescence
changes associated with cisplatin-induced apoptosis of human epithelial
keratinocytes. Laser Surg Med 2006:76-.
159. Manzotti C, Pratesi G, Menta E, Di Domenico R, Cavalletti E, Fiebig HH, et al.
BBR 3464: A novel triplatinum complex, exhibiting a preclinical profile of
antitumor efficacy different from cisplatin. Clin Cancer Res 2000;6:2626-34.
160. Hollis LS, Amundsen AR, Stern EW. Chemical and Biological Properties of a
New Series of Cis-Diammineplatinum(II) Antitumor Agents Containing 3
+ Nitrogen Donors - Cis-[Pt(NH3)2(N-Donor)Cl] . J Med Chem 1989;32:128-36.
161. lovejoym KS, Todd RC, Zhang S, McCormick MS, D'Aquino JA, Reardon JT, et
al. Cis-Diamine(pyridine)chloroplatinum(II), a monofunctional platinum(II)
antitumor agent: Uptake, sturcture, function and prospects. Proc Natl Acad Sci
2007;105:8902-7.
151 162. Yao X, Panichpisal K, Kurtzman N, Nugent K. Cisplatin nephrotoxicity: a review.
Am J Med Sci 2007;334:115-24.
163. Gray J, Simon G, Bepler G. Molecular predictors of chemotherapy response in
non-small-cell lung cancer. Expert Rev Anticancer Ther 2007;7:545-9.
164. Weaver DA, Crwaford EL, Warner KA, Elkhairi F, Khuder SA, Willey JC.
ABCC5, ERCC2, XPA, and XRCC1 transcript abundance levels correlate with
cisplatin chemoresistance in non-small-cell lung cancer cell lines. Mol Cancer
2005;4.
165. Fujii T, Toyooka S, Ichimura K, Fujiwara H, Hotta K, Soh J, et al. ERCC1 protein
expression predicts the response of cisplatin-based neoadjuvant chemotherapy in
non-small-cell lung cancer. Lung Cancer 2008;59:377-84.
166. Soria JC. ERCC1 tailored chemotherapy in lung cancer: the first prospective
randomized trial. J Clin Oncol 2007;25:2648-9.
167. Hwang IG, Ahn MJ, Park BB, Ahn YC, Han J, Lee S, et al. ERCC1 expression as
a prognostic marker in N2(+) nonsmall-cell lung cancer patients treated with
platinum-based neoadjuvat concurrent chemoradiotherapy. Cancer
2008;113:1379-86.
168. Zamble DB, Mu D, Reardon JT, Sancar A, Lippard SJ. Repair of cisplatin-DNA
adducts by the mammalian excision nuclease. Biochemistry 1996;35:10004-13.
169. Baruah H, Rector CL, Monnier SM, Bierbach U. Mechanism of action of non-
cisplatin type DNA-targeted platinum anticancer agents: DNA interactions of
novel acridinylthioureas and their platinum conjugates. Biochem Pharmacol
2002;64:191-200.
152 170. Baruah H, Wright MW, Bierbach U. Solution structural study of a DNA duplex
containing the guanine-N7 adduct formed by a cytotoxic platinum-acridine hybrid
agent. Biochemistry 2005;44:6059-70.
171. Denny WA. In DNA and RNA Binders. Germany: Wiley-VCH: Weinheim, 2003.
172. Dittrich C, Couder B, Paz-Ares L, Caponigro F, Salzberg M, Gamucci T, et al.
Phase II study of XR 5000 (DACA), an inhibitor of topoisomerase I and II,
administered as a 120-h infusion in patients with non-small cell lung cancer. Eur J
Cancer 2003;39:330-4.
173. Guddneppanavar R, Choudhury JR, Kheradi AR, Steen BD, Saluta G, Kucera GL,
et al. Effect of the diamine nonleaving group in platinum-acridinylthiourea
conjugates on DNA damage and cytotoxicity. J Med Chem 2007;50:2259-63.
174. Guddneppanavar R, Saluta G, Kucera GL, Bierbach U. Synthesis, Biological
Activity, and DNA Damage Profile of Platinum-Threading Intercalator
Conjugates Designed to Target Adenine. J Med Chem 2006;49:3204-14.
175. Atwell GJ, Cain BF, Baguley BC, Finlay GJ, Denny WA. Potential antitumor
agents. 43. Synthesis and biological activity of dibasic 9-aminoacridine-4-
carboxamides, a new class of antitumor agent. J Med Chem 1984;27:1481-5.
176. Atwell GJ, Rewcastle GW, Baguley BC, Denny WA. Potential Antitumor
Agents .50. Invivo Solid-Tumor Activity of Derivatives of N-[2-
(Dimethylamino)Ethyl]Acridine-4-Carboxamide. J Med Chem 1987;30:664-9.
177. Rewcastle GW, Atwell GJ, Baguley BC, Denny WA. Potential antitumor agents.
42. Structure-activity relationships for acridine-substituted dimethyl
phosphoramidate derivatives of 9-anilinoacridine. J Med Chem 1984;27:1053-6.
153 178. Ma ZD, Day CS, Bierbach U. Unexpected reactivity of the 9-aminoacridine
chromophore in guanidylation reactions. J Org Chem 2007;72:5387-90.
179. Avdeef A, Comer JEA, Thomson SJ. pH-Metric log p.3. glass-electrode
calibration in methanol water, applied to pKa determination of water-insoluble
substances. Anal Chem 1993;65:42-9.
180. Adams A, Guss JM, Denny WA, Wakelin LP. Crystal structure of 9-amino-N-[2-
(4-morpholinyl)ethyl]-4-acridinecarboxamide bound to d(CGTACG)2:
implications for structure-activity relationships of acridinecarboxamide
topoisomerase poisons. Nucleic Acids Res 2002;30:719-25.
181. Fan JY, Ohms SJ, Boyd M, Denny WA. DNA adducts of 9-anilinoacridine
mustards: characterization by NMR. Chem Res Toxicol 1999;12:1166-72.
154 Appendix A
H ClH2N N
N NH
N HCl 38
1 13 1 Figure A1. H NMR (top) and C-{ H} NMR (bottom) spectra of compound 38 in D2O.
155 N
N N H Cl
N
40
1 13 1 Figure A2. H NMR (top) and C-{ H} NMR (bottom) spectra of compound 40 in D2O.
156 N
N N N N N
N H 42
Figure A3. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of compound 42 in DMF-d7.
157
N
N NH2 Br
N 43
1 13 1 Figure A4. H NMR (top) and C-{ H} NMR (bottom) spectra of compound 43 in D2O.
158 O S
O N N H H 45
Figure A5. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of compound 45 in CDCl3.
159 O S O
O N N O H 46
Figure A6. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of compound 46 in CDCl3.
160 H O N N
O N NH
N 47
Figure A7. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of compound 47 in MeOH-d4.
161 +
H2N NH2 Pt Cl Cl N
52
1 13 1 Figure A8. H NMR (top) and C-{ H} NMR (bottom) spectra of compound 52 in D2O..
162
+ 15 15 H2N NH2 Pt Cl Cl N
52´
1 Figure A9. H NMR spectrum of compound 52´ in MeOH-d4.
163 +
H3N NH3 Pt Cl Cl N
53
Figure A10. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of compound 53 in D2O.
164 2+
H2N NH2 NH Pt - (NO3)2 Cl HN N N H
54
Figure A11. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of compound 54 in DMF-d7.
165 2+ 15 15 H2N NH2 NH Pt - (NO3)2 Cl HN N N H
54´
1 Figure A12. H NMR spectrum of compound 54´ in DMF-d7.
166 2+
H3N NH3 NH Pt - (NO3)2 Cl HN N N H
55
Figure A13. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of compound 55 in DMF-d7.
167 N H N N O N H S 57
Figure A14. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of compound 57 in DMSO-d6.
168 2+
N H H N N O N (NO3)2 H S Cl Pt H2N NH2
58
Figure A15. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of compound 58 in DMF-d7.
169 N N N O O N H NH O
59
1 Figure A16. H NMR spectrum of compound 59 in MeOH-d4.
170 N H Cl N N O N H NH2 60
Figure A17. 1H NMR (top) and 13C-{1H} NMR (bottom) spectra of compound 60 in D2O.
171 Appendix B
Table B1. Summary of X-ray Crystallographic Data for 1-[2-(Acridine-9-ylamino)- ethyl]-1,3-dimethylguanidine Dihydrochloride (ACRAMGU·2HCl), 38.
Empirical formula C37H50 N14 O13 Formula weight 898.91 Temperature (K) 193(2) gCrystal system Triclinic
Space group 1 P1 - C i (No. 2) a (Å) 8.1233(17) b (Å) 16.267(4) c (Å) 17.002(4) α (°) 109.081(4) β (°) 100.725(4) γ (°) 94.547(4) V (Å3) 2062.3(8) Z 2 -3 dcalc (g cm ) 1.448 Absorption coefficient (mm-1) 0.112 mm-1 Crystal size (mm3) 0.13 × 0.11 × 0.05 θ range (°) 3.79-25.00 Reflections collected 15189 Unique reflections 7216 [R(int) = 0.0652] Data obsvd (I>2σI), parameters 7216 / 626 Goodness-of-fit on F2 0.869 R1, wR2 (for I>2σI)a,b 0.0560, 0.0950 R1, wR2 (all data) 0.1057, 0.1088 Largest diff. peak and hole (e Å-3) 0.477, -0.285 a R1 = Σ|| Fo|-|Fc||/Σ|Fo| b 2 2 2 2 2 1/2 wR2 = [Σ[w(Fo -Fc ) ]/Σw(Fo ) ]
172 Table B2. Bond Lengths [Å] and Angles [°] for 1-[2-(Acridine-9-ylamino)-ethyl]-1,3- dimethylguanidine Dihydrochloride (ACRAMGU·2HCl), 38.
N(1)-C(1) 1.328(3) N(4A)-C(14A) 1.334(4) N(1)-C(16) 1.458(3) N(4A)-C(18A) 1.443(4) N(1)-H(1N) 0.79(3) N(4A)-H(4NA) 0.87(3) N(2)-C(3) 1.361(3) N(5A)-C(14A) 1.328(4) N(2)-C(4) 1.362(3) N(5A)-H(5N3) 0.94(3) N(2)-H(2N) 0.87(3) N(5A)-H(5N4) 0.87(3) N(3)-C(14) 1.336(3) C(1A)-C(2A) 1.441(4) N(3)-C(15) 1.462(3) C(1A)-C(5A) 1.451(4) N(3)-C(17) 1.466(3) C(2A)-C(3A) 1.408(4) N(4)-C(14) 1.325(4) C(2A)-C(6A) 1.430(4) N(4)-C(18) 1.462(4) C(3A)-C(9A) 1.407(4) N(4)-H(4N) 0.84(3) C(4A)-C(5A) 1.398(4) N(5)-C(14) 1.322(4) C(4A)-C(10A) 1.411(4) N(5)-H(5N1) 0.93(3) C(5A)-C(13A) 1.410(4) N(5)-H(5N2) 0.91(3) C(6A)-C(7A) 1.356(4) C(1)-C(2) 1.440(4) C(7A)-C(8A) 1.401(4) C(1)-C(5) 1.448(4) C(8A)-C(9A) 1.361(4) C(2)-C(3) 1.423(4) C(10A)-C(11A) 1.365(4) C(2)-C(6) 1.427(4) C(11A)-C(12A) 1.394(4) C(3)-C(9) 1.405(4) C(12A)-C(13A) 1.358(4) C(4)-C(10) 1.400(4) C(15A)-C(16A) 1.504(4) C(4)-C(5) 1.407(4) N(6)-O(3) 1.243(3) C(5)-C(13) 1.412(4) N(6)-O(1) 1.245(3) C(6)-C(7) 1.366(4) N(6)-O(2) 1.255(3) C(7)-C(8) 1.392(4) N(7)-O(5) 1.237(3) C(8)-C(9) 1.360(4) N(7)-O(6) 1.245(3) C(10)-C(11) 1.364(4) N(7)-O(4) 1.264(3) C(11)-C(12) 1.396(4) N(8)-O(9) 1.228(3) C(12)-C(13) 1.367(4) N(8)-O(7) 1.248(3) C(15)-C(16) 1.511(4) N(8)-O(8) 1.245(3) N(1A)-C(1A) 1.328(3) N(9)-O(11) 1.211(4) N(1A)-C(16A) 1.469(3) N(9)-O(12) 1.213(4) N(1A)-H(1NA) 0.81(2) N(9)-O(10) 1.241(4) N(2A)-C(4A) 1.358(4) O(1S)-C(1S) 1.405(5) N(2A)-C(3A) 1.363(3) O(1S)-H(1S) 0.95(6) N(2A)-H(2NA) 0.91(3) N(3A)-C(15A) 1.463(3) N(3A)-C(14A) 1.333(4) N(3A)-C(17A) 1.461(4)
173 Table B2, Continued N(1)-C(1)-C(2) 124.7(3) N(1)-C(1)-C(5) 116.8(3) C(1)-N(1)-C(16) 130.1(3) C(2)-C(1)-C(5) 118.4(3) C(1)-N(1)-H(1N) 118(2) C(3)-C(2)-C(6) 115.9(3) C(16)-N(1)-H(1N) 112(2) C(3)-C(2)-C(1) 118.2(3) C(3)-N(2)-C(4) 123.4(3) C(6)-C(2)-C(1) 125.9(3) C(3)-N(2)-H(2N) 118.6(18) N(2)-C(3)-C(9) 118.2(3) C(4)-N(2)-H(2N) 118.0(17) N(2)-C(3)-C(2) 120.3(3) C(14)-N(3)-C(15) 121.3(2) C(9)-C(3)-C(2) 121.5(3) C(14)-N(3)-C(17) 119.7(2) N(2)-C(4)-C(10) 119.3(3) C(15)-N(3)-C(17) 116.7(2) N(2)-C(4)-C(5) 119.6(3) C(14)-N(4)-C(18) 123.9(3) C(10)-C(4)-C(5) 121.1(3) C(14)-N(4)-H(4N) 125(2) C(4)-C(5)-C(13) 117.4(3) C(18)-N(4)-H(4N) 111(2) C(4)-C(5)-C(1) 119.5(3) C(14)-N(5)-H(5N1) 120.4(18) C(13)-C(5)-C(1) 123.1(3) C(14)-N(5)-H(5N2) 121.9(19) C(7)-C(6)-C(2) 121.8(3) H(5N1)-N(5)-H(5N2) 118(3) C(6)-C(7)-C(8) 120.3(3) C(1A)-N(1A)-C(16A) 131.0(3) C(9)-C(8)-C(7) 121.0(3) C(1A)-N(1A)-H(1NA) 115(2) C(8)-C(9)-C(3) 119.6(3) C(16A)-N(1A)-H(1NA) 113(2) C(11)-C(10)-C(4) 119.5(3) C(4A)-N(2A)-C(3A) 122.4(3) C(10)-C(11)-C(12) 120.7(3) C(4A)-N(2A)-H(2NA) 119.9(18) C(13)-C(12)-C(11) 120.2(3) C(3A)-N(2A)-H(2NA) 117.4(18) C(12)-C(13)-C(5) 121.1(3) C(14A)-N(3A)-C(17A) 120.9(2) N(5)-C(14)-N(4) 119.0(3) C(14A)-N(3A)-C(15A) 121.7(3) N(5)-C(14)-N(3) 119.9(3) C(17A)-N(3A)-C(15A) 116.6(2) N(4)-C(14)-N(3) 121.0(3) C(14A)-N(4A)-C(18A) 122.9(3) N(1A)-C(1A)-C(2A) 124.4(3) C(14A)-N(4A)-H(4NA) 117(2) N(1A)-C(1A)-C(5A) 117.0(3) C(18A)-N(4A)-H(4NA) 120(2) C(2A)-C(1A)-C(5A) 118.6(3) C(14A)-N(5A)-H(5N3) 119(2) C(3A)-C(2A)-C(6A) 115.8(3) C(14A)-N(5A)-H(5N4) 120(2) C(3A)-C(2A)-C(1A) 118.2(3) H(5N3)-N(5A)-H(5N4) 120(3) C(6A)-C(2A)-C(1A) 126.0(3) N(3)-C(15)-C(16) 111.0(2) N(2A)-C(3A)-C(2A) 121.1(3) N(1)-C(16)-C(15) 110.2(2) N(2A)-C(3A)-C(9A) 117.1(3) N(3A)-C(15A)-C(16A) 110.5(2) C(2A)-C(3A)-C(9A) 121.8(3) N(1A)-C(16A)-C(15A) 109.2(2) N(2A)-C(4A)-C(5A) 120.6(3) O(3)-N(6)-O(1) 120.9(3) N(2A)-C(4A)-C(10A) 118.4(3) O(3)-N(6)-O(2) 118.8(3) C(5A)-C(4A)-C(10A) 121.0(3) O(1)-N(6)-O(2) 120.2(2) C(4A)-C(5A)-C(13A) 117.2(3) O(5)-N(7)-O(6) 120.8(3) C(4A)-C(5A)-C(1A) 119.0(3)
174 Table B2, Contd. O(5)-N(7)-O(4) 120.3(3) C(13A)-C(5A)-C(1A) 123.8(3) O(6)-N(7)-O(4) 119.0(2) C(7A)-C(6A)-C(2A) 121.5(3) O(9)-N(8)-O(7) 120.0(3) C(6A)-C(7A)-C(8A) 121.2(3) O(9)-N(8)-O(8) 120.6(3) C(9A)-C(8A)-C(7A) 119.5(3) O(7)-N(8)-O(8) 119.2(3) C(8A)-C(9A)-C(3A) 120.0(3) O(11)-N(9)-O(12) 121.0(4) C(11A)-C(10A)-C(4A) 119.3(3) O(11)-N(9)-O(10) 119.4(5) C(10A)-C(11A)-C(12A) 120.6(3) O(12)-N(9)-O(10) 119.5(5) C(13A)-C(12A)-C(11A) 120.1(3) C(1S)-O(1S)-H(1S) 108(3) C(12A)-C(13A)-C(5A) 121.8(3) N(5A)-C(14A)-N(4A) 118.7(3) N(5A)-C(14A)-N(3A) 120.1(3) N(3A)-C(14A)-N(4A) 121.2(3)
175 Table B3. Summary of X-ray Crystallographic Data for (Z)-N-[1-(Acridin-9-yl)-3- methylimidazolidin-2-ylidene]methanaminium Hydrochloride, 40.
Empirical formula C18H19 N4Cl Formula weight 326.82 Temperature (K) 193(2) Crystal system Orthorhombic
Space group 15 Pbca – D 2h (No. 61) a (Å) 11.5196(15) b (Å) 11.3782(16) c (Å) 23.977(3) V (Å3) 3142.7(7) Z 8 -3 dcalc (g cm ) 1.382 Absorption coefficient (mm-1) 0.248 mm-1 Crystal size (mm3) 0.47 × 0.10 × 0.09 θ range (°) 1.70-26.40 Reflections collected 17327 Unique reflections 3223 [R(int) = 0.0882] Data obsvd (I>2σI), parameters 3223 / 214 Goodness-of-fit on F2 0.910 R1, wR2 (for I>2σI)a,b 0.0448, 0.1016 R1, wR2 (all data) 0.0666, 0.1086 Largest diff. peak and hole (e Å-3) 0.253, -0.237 a R1 = Σ|| Fo|-|Fc||/Σ|Fo| b 2 2 2 2 2 1/2 wR2 = [Σ[w(Fo -Fc ) ]/Σw(Fo ) ]
176 Table B4. Bond Lengths [Å] and Angles [°] for (Z)-N-[1-(Acridin-9-yl)-3- methylimidazolidin-2-ylidene]methanaminium Hydrochloride, 40.
N(1)-C(14) 1.358(2) C(1)-C(5) 1.392(3) N(2)-C(3) 1.345(2) C(1)-C(2) 1.396(3) N(2)-C(4) 1.346(2) C(2)-C(6) 1.426(3) N(3)-C(14) 1.325(2) C(2)-C(3) 1.438(3) N(4)-C(14) 1.321(2) C(3)-C(9) 1.435(3) N(1)-C(1) 1.441(2) C(4)-C(10) 1.426(3) N(1)-C(16) 1.482(2) C(4)-C(5) 1.434(3) N(3)-C(17) 1.449(2) C(5)-C(13) 1.428(3) N(3)-C(15) 1.462(2) C(6)-C(7) 1.365(3) N(4)-C(18) 1.462(2) C(7)-C(8) 1.420(3) N(4)-H(4N) 0.90(2) C(8)-C(9) 1.359(3) C(15)-C(16) 1.519(3) C(10)-C(11) 1.352(3) C(12)-C(13) 1.357(3) C(11)-C(12) 1.411(3)
C(14)-N(1)-C(1) 123.40(16) N(2)-C(4)-C(10) 118.32(18) C(14)-N(1)-C(16) 108.66(16) N(2)-C(4)-C(5) 122.98(18) C(1)-N(1)-C(16) 119.19(15) C(10)-C(4)-C(5) 118.70(18) C(3)-N(2)-C(4) 118.33(17) C(1)-C(5)-C(13) 123.92(17) C(14)-N(3)-C(17) 126.36(17) C(1)-C(5)-C(4) 117.51(17) C(14)-N(3)-C(15) 111.62(16) C(13)-C(5)-C(4) 118.50(18) C(17)-N(3)-C(15) 121.13(16) C(7)-C(6)-C(2) 121.08(19) C(14)-N(4)-C(18) 124.82(17) C(6)-C(7)-C(8) 120.0(2) C(14)-N(4)-H(4N) 119.8(14) C(9)-C(8)-C(7) 121.3(2) C(18)-N(4)-H(4N) 113.3(14) C(8)-C(9)-C(3) 120.24(19) C(5)-C(1)-C(2) 120.74(17) C(11)-C(10)-C(4) 120.45(19) C(5)-C(1)-N(1) 119.56(17) C(10)-C(11)-C(12) 121.1(2) C(2)-C(1)-N(1) 119.64(17) C(13)-C(12)-C(11) 120.6(2) C(1)-C(2)-C(6) 124.26(18) C(12)-C(13)-C(5) 120.57(19) C(1)-C(2)-C(3) 117.14(17) N(4)-C(14)-N(3) 123.84(18) C(6)-C(2)-C(3) 118.60(18) N(4)-C(14)-N(1) 124.56(18) N(2)-C(3)-C(9) 118.14(18) N(3)-C(14)-N(1) 111.57(17) N(2)-C(3)-C(2) 123.15(18) N(3)-C(15)-C(16) 102.55(16) C(9)-C(3)-C(2) 118.70(18) N(1)-C(16)-C(15) 104.31(16)
177 Table B5. Summary of X-ray Crystallographic Data for 2′-(1H-Imidazol-1-yl)-8′-methyl- 7′,8′-dihydro-6′H,10H-spiro-[acridine-9,4′-imidazo[1,2-a][1,3,5]triazine], 42.
Empirical formula C21H19 N7 Formula weight 369.43 Temperature (K) 193(2) Crystal system Monoclinic
Space group 5 P2(1)/c - C 2h (No. 14) a (Å) 9.030(9) b (Å) 11.573(12) c (Å) 16.754(17) V (Å3) 1745(3) Z 4 -3 dcalc (g cm ) 1.406 Absorption coefficient (mm-1) 0.090 mm-1 Crystal size (mm3) 0.27 × 0.12 × 0.12 θ range (°) 3.89-25.03 Reflections collected 12280 Unique reflections 3073 [R(int) = 0.1131] Data obsvd (I>2σI), parameters 3073 / 258 Goodness-of-fit on F2 0.981 R1, wR2 (for I>2σI)a,b 0.0512, 0.1172 R1, wR2 (all data) 0.0723, 0.1243 Largest diff. peak and hole (e Å-3) 0.249, -0.205 a R1 = Σ|| Fo|-|Fc||/Σ|Fo| b 2 2 2 2 2 1/2 wR2 = [Σ[w(Fo -Fc ) ]/Σw(Fo ) ]
178 Table B6. Bond Lengths [Å] and Angles [°] for 2′-(1H-Imidazol-1-yl)-8′-methyl-7′,8′- dihydro-6′H,10H-spiro-[acridine-9,4′-imidazo[1,2-a][1,3,5]triazine], 42.
N(1)-C(3) 1.380(3) N(7)-C(20) 1.370(3) N(1)-C(4) 1.392(3) C(1)-C(2) 1.513(3) N(1)-H(1N) 0.99(3) C(1)-C(5) 1.523(3) N(2)-C(14) 1.351(3) C(2)-C(3) 1.389(3) N(2)-C(16) 1.468(3) C(2)-C(6) 1.390(3) N(2)-C(1) 1.494(3) C(3)-C(9) 1.394(3) N(3)-C(14) 1.337(3) C(4)-C(10) 1.390(3) N(3)-C(17) 1.446(3) C(4)-C(5) 1.395(3) N(3)-C(15) 1.455(3) C(5)-C(13) 1.394(3) N(4)-C(14) 1.314(3) C(6)-C(7) 1.371(3) N(4)-C(18) 1.373(3) C(7)-C(8) 1.381(3) N(5)-C(18) 1.276(3) C(8)-C(9) 1.373(3) N(5)-C(1) 1.457(3) C(10)-C(11) 1.366(3) N(6)-C(19) 1.362(3) C(11)-C(12) 1.380(3) N(6)-C(21) 1.375(3) C(12)-C(13) 1.371(3) N(6)-C(18) 1.424(3) C(15)-C(16) 1.523(3) N(7)-C(19) 1.308(3) C(20)-C(21) 1.345(3)
C(3)-N(1)-C(4) 119.5(2) C(2)-C(3)-C(9) 120.1(2) C(3)-N(1)-H(1N) 122.2(14) C(10)-C(4)-N(1) 119.6(2) C(4)-N(1)-H(1N) 117.5(14) C(10)-C(4)-C(5) 120.3(2) C(14)-N(2)-C(16) 109.84(18) N(1)-C(4)-C(5) 120.0(2) C(14)-N(2)-C(1) 117.94(17) C(13)-C(5)-C(4) 118.3(2) C(16)-N(2)-C(1) 122.97(16) C(13)-C(5)-C(1) 122.48(19) C(14)-N(3)-C(17) 124.49(19) C(4)-C(5)-C(1) 119.16(19) C(14)-N(3)-C(15) 110.71(17) C(7)-C(6)-C(2) 120.9(2) C(17)-N(3)-C(15) 122.00(18) C(6)-C(7)-C(8) 119.7(2) C(14)-N(4)-C(18) 110.65(18) C(9)-C(8)-C(7) 120.7(2) C(18)-N(5)-C(1) 117.11(18) C(8)-C(9)-C(3) 119.6(2) C(19)-N(6)-C(21) 106.35(18) C(11)-C(10)-C(4) 119.9(2) C(19)-N(6)-C(18) 126.83(18) C(10)-C(11)-C(12) 120.6(2) C(21)-N(6)-C(18) 126.82(17) C(13)-C(12)-C(11) 119.7(2) C(19)-N(7)-C(20) 104.76(19) C(12)-C(13)-C(5) 121.1(2) N(5)-C(1)-N(2) 108.35(15) N(4)-C(14)-N(3) 124.74(19) N(5)-C(1)-C(2) 110.61(17) N(4)-C(14)-N(2) 125.0(2) N(2)-C(1)-C(2) 107.25(16) N(3)-C(14)-N(2) 110.19(19) N(5)-C(1)-C(5) 111.14(17) N(3)-C(15)-C(16) 102.47(17) N(2)-C(1)-C(5) 110.31(17) N(2)-C(16)-C(15) 102.03(18)
179 Table B6, Contd. C(2)-C(1)-C(5) 109.11(18) N(5)-C(18)-N(4) 131.64(19) C(3)-C(2)-C(6) 118.9(2) N(5)-C(18)-N(6) 116.32(19) C(3)-C(2)-C(1) 119.7(2) N(4)-C(18)-N(6) 112.03(17) C(6)-C(2)-C(1) 121.4(2) N(7)-C(19)-N(6) 112.0(2) N(1)-C(3)-C(2) 120.3(2) C(21)-C(20)-N(7) 111.25(19) N(1)-C(3)-C(9) 119.5(2) C(20)-C(21)-N(6) 105.68(19)
180 Table B7. Summary of X-ray Crystallographic Data for 1-(Acridin-9-yl)-3-methyl- imidazolidin-2-imine Hydrobromide, 43.
Empirical formula C17H17BrN4 Formula weight 357.26 Temperature (K) 193(2) Crystal system Monoclinic
Space group 4 Cc – C s (No. 9) a (Å) 11.124(1) b (Å) 20.633(2) c (Å) 7.1005(8) V (Å3) 1519.8(3) Z 4 -3 dcalc (g cm ) 1.561 Absorption coefficient (mm-1) 2.707 mm-1 Crystal size (mm3) 0.23 × 0.16 × 0.04 θ range (°) 4.21-30.48 Reflections collected 7851 Unique reflections 4117 [R(int) = 0.0533] Data obsvd (I>2σI), restraints, parameters 4117 / 2 / 208 Goodness-of-fit on F2 1.031 R1, wR2 (for I>2σI)a,b 0.0484, 0.1171 R1, wR2 (all data) 0.0511, 0.1193 Largest diff. peak and hole (e Å-3) 0.759, -0.315 a R1 = Σ|| Fo|-|Fc||/Σ|Fo| b 2 2 2 2 2 1/2 wR2 = [Σ[w(Fo -Fc ) ]/Σw(Fo ) ]
181 Table B8. Bond Lengths [Å] and Angles [°] for 1-(Acridin-9-yl)-3-methyl-imidazolidin- 2-imine Hydrobromide, 43.
N(1)-C(13) 1.342(4) C(3)-C(4) 1.418(6) N(1)-C(1) 1.346(4) C(4)-C(5) 1.365(5) N(2)-C(14) 1.356(4) C(5)-C(6) 1.425(4) N(2)-C(7) 1.415(4) C(6)-C(7) 1.400(4) N(2)-C(16) 1.479(4) C(7)-C(8) 1.402(4) N(3)-C(14) 1.331(4) C(8)-C(9) 1.428(4) N(3)-C(17) 1.448(5) C(8)-C(13) 1.431(4) N(3)-C(15) 1.459(5) C(9)-C(10) 1.364(4) N(4)-C(14) 1.319(4) C(10)-C(11) 1.419(5) C(1)-C(2) 1.423(4) C(11)-C(12) 1.355(5) C(1)-C(6) 1.433(4) C(12)-C(13) 1.432(4) C(2)-C(3) 1.362(5) C(15)-C(16) 1.534(5)
C(13)-N(1)-C(1) 118.6(2) C(6)-C(7)-N(2) 118.4(3) C(14)-N(2)-C(7) 125.6(2) C(8)-C(7)-N(2) 121.4(2) C(14)-N(2)-C(16) 110.5(3) C(7)-C(8)-C(9) 123.6(3) C(7)-N(2)-C(16) 121.4(2) C(7)-C(8)-C(13) 117.5(2) C(14)-N(3)-C(17) 125.1(3) C(9)-C(8)-C(13) 118.9(3) C(14)-N(3)-C(15) 111.4(3) C(10)-C(9)-C(8) 120.2(3) C(17)-N(3)-C(15) 119.2(3) C(9)-C(10)-C(11) 120.8(3) N(1)-C(1)-C(2) 118.6(3) C(12)-C(11)-C(10) 120.9(3) N(1)-C(1)-C(6) 122.8(2) C(11)-C(12)-C(13) 120.4(3) C(2)-C(1)-C(6) 118.6(3) N(1)-C(13)-C(8) 123.2(3) C(3)-C(2)-C(1) 120.7(3) N(1)-C(13)-C(12) 118.0(3) C(2)-C(3)-C(4) 120.3(3) C(8)-C(13)-C(12) 118.8(3) C(5)-C(4)-C(3) 121.5(3) N(4)-C(14)-N(3) 126.2(3) C(4)-C(5)-C(6) 119.4(3) N(4)-C(14)-N(2) 123.3(3) C(7)-C(6)-C(5) 122.6(3) N(3)-C(14)-N(2) 110.5(3) C(7)-C(6)-C(1) 117.8(2) N(3)-C(15)-C(16) 103.1(3) C(5)-C(6)-C(1) 119.6(3) N(2)-C(16)-C(15) 102.3(3) C(6)-C(7)-C(8) 120.1(3)
182 Table B9. Summary of X-ray Crystallographic Data for [Pt(EtCN)(C19H22N4)Cl2], 50.
Empirical formula C22H27Cl2N5Pt Formula weight 627.48 Temperature (K) 193(2) Crystal system Monoclinic
Space group 5 P21/c - C 2h (No. 14) a (Å) 13.791(2) b (Å) 12.107(1) c (Å) 14.747(2) V (Å3) 2062.3(8) Z 4 -3 dcalc (g cm ) 1.796 Absorption coefficient (mm-1) 6.297 mm-1 Crystal size (mm3) 0.16 × 0.12 × 0.07 θ range (°) 3.79-27.50 Reflections collected 20212 Unique reflections 5307 [R(int) = 0.0513] Data obsvd (I>2σI), restraints, parameters 5307 / 1 / 282 Goodness-of-fit on F2 1.609 R1, wR2 (for I>2σI)a,b 0.0432, 0.0942 R1, wR2 (all data) 0.0662, 0.1032 Largest diff. peak and hole (e Å-3) 2.672, -1.104
a R1 = Σ|| Fo|-|Fc||/Σ|Fo| b 2 2 2 2 2 1/2 wR2 = [Σ[w(Fo -Fc ) ]/Σw(Fo ) ]
183 Table B10. Bond Lengths [Å] and Angles [°] for [Pt(EtCN)(C19H22N4)Cl2], 50.
Pt(1)-N(1) 1.989(5) C(4)-C(16) 1.468(8) Pt(1)-N(5) 1.993(5) C(4)-C(5) 1.486(8) Pt(1)-Cl(1) 2.299(2) C(5)-C(6) 1.397(9) Pt(1)-Cl(2) 2.290(2) C(5)-C(10) 1.403(8) N(1)-C(1) 1.291(7) C(6)-C(7) 1.385(9) N(2)-C(1) 1.338(8) C(7)-C(8) 1.389(10) N(3)-C(4) 1.284(7) C(8)-C(9) 1.347(11) N(4)-C(11) 1.373(8) C(9)-C(10) 1.393(8) N(4)-C(10) 1.383(8) C(11)-C(12) 1.385(9) N(2)-C(2) 1.463(8) C(11)-C(16) 1.419(8) N(2)-C(19) 1.470(8) C(12)-C(13) 1.364(10) N(3)-C(3) 1.434(7) C(13)-C(14) 1.377(9) N(1)-H(1N) 0.90(6) C(14)-C(15) 1.389(9) N(4)-H(4N) 0.74(2) C(15)-C(16) 1.401(8) C(1)-C(17) 1.512(9) N(5)-C(20) 1.110(8) C(2)-C(3) 1.514(9) C(20)-C(21) 1.471(10) C(17)-C(18) 1.509(10) C(21)-C(22) 1.510(10)
N(1)-Pt(1)-N(5) 178.2(2) N(3)-C(4)-C(5) 117.3(5) N(1)-Pt(1)-Cl(2) 90.31(15) C(16)-C(4)-C(5) 114.7(5) N(5)-Pt(1)-Cl(2) 90.73(17) C(6)-C(5)-C(10) 118.0(6) N(1)-Pt(1)-Cl(1) 88.79(15) C(6)-C(5)-C(4) 121.8(6) N(5)-Pt(1)-Cl(1) 90.13(17) C(10)-C(5)-C(4) 120.2(6) Cl(2)-Pt(1)-Cl(1) 178.59(6) C(7)-C(6)-C(5) 120.9(6) C(1)-N(1)-Pt(1) 129.6(5) C(6)-C(7)-C(8) 119.8(7) C(1)-N(1)-H(1N) 113(4) C(9)-C(8)-C(7) 120.2(7) Pt(1)-N(1)-H(1N) 117(4) C(8)-C(9)-C(10) 121.2(7) C(1)-N(2)-C(2) 121.4(5) N(4)-C(10)-C(9) 121.0(6) C(1)-N(2)-C(19) 124.0(5) N(4)-C(10)-C(5) 119.0(6) C(2)-N(2)-C(19) 114.6(5) C(9)-C(10)-C(5) 120.0(6) C(4)-N(3)-C(3) 123.8(5) N(4)-C(11)-C(12) 120.4(6) C(11)-N(4)-C(10) 123.2(5) N(4)-C(11)-C(16) 119.2(6) C(11)-N(4)-H(4N) 119(4) C(12)-C(11)-C(16) 120.4(6) C(10)-N(4)-H(4N) 117(4) C(13)-C(12)-C(11) 121.1(6) N(2)-C(2)-C(3) 115.6(5) C(12)-C(13)-C(14) 120.2(6) N(3)-C(3)-C(2) 111.3(5) C(13)-C(14)-C(15) 119.2(6) C(18)-C(17)-C(1) 109.7(6) C(14)-C(15)-C(16) 122.3(6) C(20)-C(21)-C(22) 112.7(6) C(15)-C(16)-C(11) 116.1(6) N(1)-C(1)-N(2) 120.5(6) C(15)-C(16)-C(4) 124.1(6)
184 Table B10, Contd. N(1)-C(1)-C(17) 120.4(6) C(11)-C(16)-C(4) 119.7(6) N(2)-C(1)-C(17) 118.9(5) C(20)-N(5)-Pt(1) 177.1(6) N(3)-C(4)-C(16) 127.9(6) N(5)-C(20)-C(21) 178.0(8)
185 Table B11. Summary of X-ray Crystallographic Data for [PtCl(en)(C19H23N4)](NO3)2, 54.
Empirical formula C21H31ClN8O6Pt Formula weight 722.08 Temperature (K) 193(2) gCrystal system Monoclinic
Space group C2/c a (Å) 34.070(7) b (Å) 10.887(2) c (Å) 14.023(3) V (Å3) 5071.7(18) Z 8 -3 dcalc (g cm ) 1.891 Absorption coefficient (mm-1) 5.693 mm-1 Crystal size (mm3) 0.13 × 0.11 × 0.05 θ range (°) 3.79-25.00 Reflections collected 19127 Unique reflections 4439 [R(int) = 0.0930] Data obsvd (I>2σI), restraints, parameters 4439 / 0 / 327 Goodness-of-fit on F2 0.996 R1, wR2 (for I>2σI)a,b 0.0456, 0.0834 R1, wR2 (all data) 0.0790, 0.0933 Largest diff. peak and hole (e Å-3) 1.121, -1.012 a R1 = Σ|| Fo|-|Fc||/Σ|Fo| b 2 2 2 2 2 1/2 wR2 = [Σ[w(Fo -Fc ) ]/Σw(Fo ) ]
186 Table B12. Bond Lengths [Å] and Angles [°] for [PtCl(en)(C19H23N4)](NO3)2, 54.
Pt(1)-N(1) 2.023(3) C(4)-C(16) 1.453(11) Pt(1)-N(5) 2.025(7) C(5)-C(6) 1.410(11) Pt(1)-N(6) 2.013(7) C(5)-C(10) 1.431(12) Pt(1)-Cl(1) 2.313(2) C(6)-C(7) 1.355(11) N(1)-C(1) 1.276(9) C(7)-C(8) 1.382(12) N(2)-C(1) 1.371(8) C(8)-C(9) 1.361(11) N(3)-C(4) 1.320(9) C(9)-C(10) 1.411(11) N(4)-C(10) 1.339(9) C(11)-C(16) 1.404(11) N(4)-C(11) 1.352(9) C(11)-C(12) 1.412(11) N(2)-C(2) 1.450(9) C(12)-C(13) 1.314(12) N(2)-C(19) 1.460(9) C(13)-C(14) 1.402(13) N(3)-C(3) 1.479(8) C(14)-C(15) 1.364(11) N(5)-C(20) 1.475(10) C(15)-C(16) 1.413(11) N(6)-C(21) 1.469(11) N(7)-O(1) 1.255(11) C(1)-C(17) 1.506(11) N(7)-O(2) 1.225(9) C(2)-C(3) 1.524(11) N(7)-O(3) 1.189(10) C(17)-C(18) 1.543(12) N(8)-O(4) 1.209(10) C(20)-C(21) 1.463(13) N(8)-O(5) 1.233(10) C(4)-C(5) 1.453(11) N(8)-O(6) 1.227(10)
N(6)-Pt(1)-N(1) 174.2(2) C(8)-C(9)-C(10) 120.3(9) N(6)-Pt(1)-N(5) 83.3(3) N(4)-C(10)-C(9) 119.0(8) N(1)-Pt(1)-N(5) 90.9(2) N(4)-C(10)-C(5) 121.3(7) N(6)-Pt(1)-Cl(1) 90.5(2) C(9)-C(10)-C(5) 119.7(8) N(1)-Pt(1)-Cl(1) 95.32(12) N(4)-C(11)-C(16) 120.1(8) N(5)-Pt(1)-Cl(1) 173.5(2) N(4)-C(11)-C(12) 120.6(8) C(1)-N(1)-Pt(1) 129.6(4) C(16)-C(11)-C(12) 119.3(8) C(1)-N(2)-C(2) 123.0(6) C(13)-C(12)-C(11) 121.3(9) C(1)-N(2)-C(19) 117.4(5) C(12)-C(13)-C(14) 121.0(8) C(2)-N(2)-C(19) 118.5(5) C(15)-C(14)-C(13) 119.7(9) C(4)-N(3)-C(3) 132.4(6) C(14)-C(15)-C(16) 120.8(8) C(10)-N(4)-C(11) 123.4(6) C(11)-C(16)-C(15) 117.8(8) C(20)-N(5)-Pt(1) 109.4(5) C(11)-C(16)-C(4) 119.5(8) C(21)-N(6)-Pt(1) 110.6(6) C(15)-C(16)-C(4) 122.6(8) N(1)-C(1)-N(2) 122.4(7) N(2)-C(2)-C(3) 113.7(6) N(1)-C(1)-C(17) 119.3(7) N(3)-C(3)-C(2) 108.9(6) N(2)-C(1)-C(17) 118.3(7) C(1)-C(17)-C(18) 113.9(7) N(3)-C(4)-C(5) 125.6(7) C(21)-C(20)-N(5) 110.7(8) N(3)-C(4)-C(16) 116.5(7) C(20)-C(21)-N(6) 108.5(8)
187 Table B12, Contd. C(5)-C(4)-C(16) 117.8(7) O(3)-N(7)-O(2) 122.4(10) C(6)-C(5)-C(10) 116.8(7) O(3)-N(7)-O(1) 120.3(11) C(6)-C(5)-C(4) 125.8(8) O(2)-N(7)-O(1) 117.3(10) C(10)-C(5)-C(4) 117.4(7) O(4)-N(8)-O(6) 121.5(9) C(7)-C(6)-C(5) 121.9(9) O(4)-N(8)-O(5) 119.9(9) C(6)-C(7)-C(8) 120.7(9) O(6)-N(8)-O(5) 118.6(9) C(9)-C(8)-C(7) 120.5(8)
188 Appendix C
Table C1. Platinum Concentrations in Various Biological Tissues (Tumors, Kidneys and Lungs) at Two Different Doses (0.1 mg/kg and 0.5 mg/kg) Determined by ICP–MS After 16 Hours of Digestion.
Dose Sample I.D. Tumor (ppb) Kidney (ppb) Lung (ppb) Control 8788 0.001 0 0 8384 0.004 0 0.001 6970 0.002 0 0 6768 0.003 0 0.001 5758 0 0.001 0
0.1mg/kg 8586 0.368 4.838 0.321 6364 0.101 8.202 0.206 6162 0.506 12.04 0.313 5556 0.205 9.803 0.234 5354 0.494 7.895 0.228
0.5mg/kg 8990 0.018 7.761 0.225 8182 0.279 5.797 0.151 7374 0.426 6.653 0.169 7172 0.349 4.979 0.33 5152 0.063 4.869 0.218
189 Table C2. Masses of Various Biological Tissues (Tumors, Kidneys and Lungs).
Sample I.D. Tumor (g) Kidney (g) Lung (g) 8788 0.5817 0.4100 0.1342 8384 1.1586 0.3477 0.1883 6970 0.6836 0.2881 0.1942 6768 0.6548 0.3083 0.1355 5758 0.7600 0.2972 0.1410
8586 0.5419 0.1494 0.1808 6364 0.1731 0.2917 0.1354 6162 0.6317 0.3029 0.1643 5556 0.2606 0.3181 0.1510 5354 0.6965 0.3120 0.1259
8990 0.0368 0.3091 0.1430 8182 0.6270 0.2901 0.2202 7374 0.5221 0.2272 0.1032 7172 0.7529 0.2940 0.1763 5152 0.1410 0.2720 0.1512
190 SCHOLASTIC VITA
Zhidong Ma
BORN: September 15, 1982 Xining, Qinghai, P. R. China
UNDERGRADUATE Nankai University STUDY: Tianjin, P. R. China B.Sc., Chemistry, 2004
GRADUATE STUDY: Wake Forest University Winston-Salem, North Carolina, USA Ph.D. candidate, 2009
SCHOLASTIC AND PROFESSIONAL EXPERIENCE:
Research Assistant, Wake Forest University 2005-2007 Winston-Salem, North Carolina, USA
Teaching Assistant, Wake Forest University 2004-2005, 2007-2009 Winston-Salem, North Carolina, USA
HONORS AND AWARDS:
American Institute of Chemists Outstanding Graduate Student Award 2009
Richter Scholar Award 2009
1st-runner up prize for poster presentation at the 2008 Wake Forest University Graduate Student Research Day 2008
Graduate Student Oral Presentation Award at the 2008 Cancer Biology Department Research Retreat 2008
WFU Alumni Student Travel Award 2007, 2008
191 PUBLICATIONS:
Z. Ma, L. Rao and U. Bierbach: Replacement of a Thiourea-S with an Amidine-NH Donor Group in a Platinum-Amidine Antitumor Compound Reduces the Metal’s Reactivity with Cysteine Sulfur. Journal of Medicinal Chemistry, 52, 3424. (2009)
Z. Ma, J. Roy Choudhury, M. W. Wright, C. S. Day, G. Saluta, G. L. Kucera and U. Bierbach: A Non-Cross-Linking Platinum-Acridine Agent with Potent Activity in Non- Small-Cell Lung Cancer. Journal of Medicinal Chemistry, 51, 7574. (2008)
Z. Ma, G. Saluta, G. L. Kucera and U. Bierbach: Effect of linkage geometry on biological activity in thiourea- and guanidine- substituted acridines and platinum-acridines. Bioorganic & Medicinal Chemistry Letters, 18, 3799. (2008)
Z. Ma, C. S. Day and U. Bierbach: Unexpected Reactivity of the 9-Aminoacridine Chromophore in Guanidylation Reactions. Journal of Organic Chemistry, 72, 5387. (2007)
PRESENTATIONS:
Z. Ma and U.Bierbach: A Platinum-Acridine Agent with Activity Against Non-Small Cell Lung Cancer (NSCLC) In Vitro and In Vivo, Presented at the 60th Southeast Regional Meeting of the American Chemical Society, Nashville, TN, November 12-15, 2008. (Oral Presentation)
Z. Ma, J. Roy Choudhury, G. Saluta, G. L. Kucera and Ulrich Bierbach: Tuning the reactivity and DNA interactions of platinum-acridine hybrid agents. Presented at the 59th Southeast Regional Meeting of the American Chemical Society, Greenville, SC, October 24-27, 2007. (Poster)
Z. Ma, C. S. Day and U. Bierbach: Tuning the DNA Binding of Cytotoxic Platinum- Acridine Conjugates. Presented at the 3rd Asian Biological Inorganic Chemistry Conference (AsBIC III), Nanjing, China, October 30-November 3, 2006. (Poster)
Z. Ma, C. S. Day and U. Bierbach: Synthesis and Characterization of DNA-Targeted Acridinylguanidines. Presented at the 57th Southeast Regional Meeting of the American Chemical Society, Memphis, TN, November 1-4, 2005. (Poster)
192