Iowa State University Capstones, Theses and Graduate Theses and Dissertations Dissertations
2015 Supramolecular assemblies of alkane functionalized poly ethylene glycol copolymer for drug delivery Lida Zhu Iowa State University
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Supramolecular assemblies of alkane functionalized poly ethylene glycol copolymer for drug delivery
by
Lida Zhu
A thesis submitted to the graduate faculty
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
Major: Chemical Engineering
Program of Study Committee: Kaitlin Bratlie, Major Professor Martin Thuo Surya Mallapragada
Iowa State University
Ames, Iowa
2015
Copyright © Lida Zhu, 2015. All rights reserved. ii
TABLE OF CONTENTS
Page
LIST OF FIGURES ...... iv
LIST OF TABLES ...... v
NOMENCLATURE ...... vi
ACKNOWLEDGMENTS ...... vii
ABSTRACT………………………………...... viii
CHAPTER I INTRODUCTION ...... 1
Drug Delivery...... 1 Surfactant and its Micelle Structure...... 2 Polyethylene Glycol ...... 4 Macrophages ...... 5
CHAPTER II RELATED WORKS ON PEG ASSOCIATED DRUG DELIVERY ...... 7
PEG Diblock/Triblock Copolymer ...... 7 PEG-D5HIP Polymer ...... 9
CHAPTER III SUPRAMOLECULAR ASSEMBILES OF ALKANE FUNCTIONLIZED POLYETHYLENE GLYCOL COPOLYMERS FOR DRUG DELIVERY ...... 11
Abstract ...... 11 Introduction ...... 12 Experimental ...... 13 Results ...... 18 Discussion ...... 26 Conclusions ...... 30
CHAPTER IV FUTURE WORK ...... 32
REFERENCES ...... 33
APPENDIX A NMR SPECTRUM ...... 38
APPENDIX B CELLS WERE INCUBATED FOR 4 H AT 37°C WITH THE PARTICLES. THE SCALE BAR REPRESENTS 100 μm...... 41
iii
APPENDIX C CELLS WERE INCUBATED FOR 4 H AT 37°C WITH THE PARTICLES. THE SCALE BAR REPRESENTS 100 μm...... 42
iv
LIST OF FIGURES
Page
Figure 1 Micelle formation dependence upon CMC and a plot of solution properties versus surfactant concentration ...... 4
Figure 2 Typical macrophage phagocytic uptake of drugs ...... 6
Figure 3 Ppeg-alkanes form micelles in aqueous solutions. The hydrophilic shell is composed of PEG chains and the hydrophobic core results from the alkyl chains and aromatic rings...... 10
Figure 4 Reaction scheme for polymerization of Ppeg. A) PEG and dimethyl 5- hdroxysiophthalate react under 90°C with novozyme 435 to form poly[(poly-(oxyethylene)-oxy-5-hydroxyisophthaloyl]. B) poly[(poly- (oxyethylene)-oxy-5-hydroxyisophthaloyl] reacts with bromine-alkane at 60°C to form poly[(polyoxyethylene)-oxy-5-(alkane)-isophthaloyl] ..... 14
Figure 5 The relationship between the CMC and drug delivery parameters. A) Thecritical micelle concentration (CMC) of the synthesized polymers in DI water at 5, 25, and 35°C. B) The drug loading capacity of pyrene encapsulated in the micelles at 0.1% in DI water. C) The % drug released percentage at 96 h in pH 5 and pH 7.4 in PBS...... 21
Figure 6 Pyrene is released from Ppeg-alkyl micelles. Pyrene release from Ppeg- alkane in PBS at 37°C, x-axe is hour, y-axe is total amount of pyrene released in µg: A & B) Ppeg-600-alkane and Ppeg-1000-alkane release of pyrene at pH 5. C & D) Ppeg-600-alkane and Ppeg-1000-alkane release of pyrene at pH 7.4...... 23
Figure 7 Cell viability after incubated with naked or pyrene-loaded Ppeg-alkyl particles for (A) 4 h and (B) 24 h...... 24
Figure 8 The amount of pyrene internalization depends of the composition of the micelle. Pyrene loaded Ppeg-alkyl particles were incubated with macrophages at 37°C and 4°C. A) The fluorescence of pyrene internalized by macrophages. B) The fluorescence of pyrene internalized by the cell normalized to the amount of pyrene added to each well...... 25
v
LIST OF TABLES
Page
Table 1 Characterization of Ppeg-600/1000-alkane particles. Zeta potential, micelle size, and % hydroxyl modification with alkyl chain were measured. Errors are given as the standard deviations...... 20
vi
NOMENCLATURE
CMC Critical micelle concentration
D5HIP Dimethyl-5-hydroxyisophthalate
DLS Dynamic light scattering
DSPE 1,2-distearoyl-sn-glycero-3-phosphoethanolamine
HLB Hydrophilic-lipophilic balance
MTT 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide
NMR Nuclear magnetic resonance spectroscopy
PBS Phosphate Buffered Saline
PCL Poly(ɛ-caprolactone)
PDMA Polymethacrylate
PEG Poly ethylene glycol
PLGA Poly (lactide-co-glycolide)
Ppeg-600/1000 Poly[(poly(oxyethylene-600/1000)-oxy-5-hydroxyisophthaloyl]
Ppeg-600-alkane Poly[(poly-(oxyethylene)-600-oxy-5-alkane-hydroxyisophthaloyl]
Ppeg-1000-alkane Poly[(poly-(oxyethylene)-1000-oxy-5-alkane-hydroxyisophthaloyl]
vii
ACKNOWLEDGMENTS
I would like to thank my major professor, Kaitlin Bratlie, and my committee members, Martin Thuo, and Surya Mallapragada, for their guidance and support throughout the course of this research.
This work was supported by the National Science Foundation under Grant No. CBET
1227867, the Roy J. Carver Charitable Trust Grant No. 13-4265, and the Mike and Denise
Mack faculty fellowship to KB
viii
ABSTRACT
The therapeutic effects of many modern drugs were limited owing to their physical properties and half-life in the blood stream. The purpose of this research is to study the relationship between drug delivery performances and chemical properties of the polymer micelle drug carriers.
Polyethylene glycol (PEG) based alternating copolymer poly[(polyoxyethylene)-oxy-
5-hydroxyisophthalic] (Ppeg) with PEG molecular weights of 600 and 1000 were synthesized and modified with different alkanes to study the effects of altering the hydrophobic and hydrophilic chain lengths. The nuclear magnetic resonance (NMR) spectrum, critical micelle concentration (CMC), micelle size, and micelle zeta potential of the synthesized polymers were measured. The resulting polymer particles were able to form micelles in aqueous solution with CMCs lower than 0.04 wt%. Drug delivery studies were performed with a model hydrophobic drug, pyrene. Drug loading data showed the polymer particles were able to encapsulate pyrene and has a loading capacity up to 8 wt%. The sustain release ability was measured and the pyrene release was extended over 5 days. Both loading capacity and sustain release ability were found to be highly dependent on CMC.
Cell culture study was implemented with RAW 264.7 cells in order to determine the polymer micelle’s cytocompatibility, Most Ppeg polymer micelles showed more than 85% cell viability with and without pyrene loading. Cell internalization of the micelles encapsulated drug was measured both quantitatively and qualitatively and was enhanced comparing to unencapsulated drug. The results indicated that the internalization enhancement
ix effect of polymer micelle was mainly affected by hydrophilic chain length; neither hydrophobic chain length nor loading capacity has significant influence on internalization.
1
CHAPTER I
INTRODUCTION
Drug delivery has long been considered an important area in biomedical engineering.
In drug delivery, an active pharmaceutical ingredient is delivered to the infected tissues by various administration routes and modified pharmaceutical dosage forms. According to differences in physical and chemical properties, such as solubility and in vivo half-life, each drug requires a suitable delivery technique to be delivered into targeted tissues1.
Drug Delivery
Many peptides and small molecule drugs suffered from low water solubility, short in vivo half-life, and hydrolysis2–4. Repaid in vivo clearance of drugs will further lead to targeting issues since passive targeting requires extra circulation time in order to increase the possibilities for the drug molecule to pass by the target tissues5. For instance, hydrophobic drug indomethacin, an anti-inflammatory drug commonly used for rheumatoid arthritis, is limited by solubility and has a plasma half-life time of 4.5 h in adults6. Other hydrophobic drug such as anticancer drug paclitaxel, has dose-limiting toxicity and short in vivo half-life time, which as a result, requires frequency drug administration to maintain effective concentration7.
Moreover, for hydrophobic drugs, oral administration sometimes was not appropriate for acute situations because of the long oral absorption time, and thus intravenous administration was required. Due to the solubility limit, cosolvents were added for injection needs8–10. Oftentimes the cosolvent could be toxic, for instance in the case of the antiarrhythmic drug amiodarone,11 which is generally delivered orally but is often combined
2 with hydrochloride acid as amiodarone hydrochloride for emergency injection treatment.
Nifedipine, a cardiotonic, usually solubilized in glycerin, polyethylene glycol (PEG), and peppermint oil mixture8. Other solvents such as ethanol, propylene glycol, and even DMSO8 are often used as cosolvents. Side effects like pain, hemolysis, and irritation commonly occur because of the non-biocompatible cosolvent12.
These disadvantages could be remedied by using appropriate drug delivery method.8,13 It has been reported that the solubility of indomethacin could be significantly enhanced by poly(amidoamine) dendrimer drug delivery system. Preclinical studies also indicated that the terminal half-life time was extended from 2.9 h to 5.0 h in rats14. Micellular drug vehicle Genexol-PM has also been developed for paclitaxel delivery. Clinical studies have shown Genexol-PM increased the half-life time of paclitaxel from 5.8 h to 12.7 h7.
Synthetic polymers with biocompatibility have long been developing for the needs of drug delivery.15 Nanoparticles, especially those can self-assemble under certain driving force such as pH and solvent polarity, have gained an important role in drug delivery field16.
Surfactants and their Micelle Structure
Surfactants are defined as substances that can significantly change the interfacial state of a solution system; they contain both hydrophilic and lipophilic groups, giving them an amphipathic structure17. Surfactants, particularly biosurfactants, present an attractive possibility for drug delivery owing to their biodegradability and chemical sensitivity to temperature and pH18. Base on the hydrophilic-lipophilic balance (HLB), surfactants can be used as solubilizing agents, detergents, emulsifying agents, spreading agents, and antifoaming agents19, etc. Performance of the surfactant is highly dependent on the HLB value. Increasing the length of the lipophilic group will decrease the surfactant’s water
3 solubility and increase the tendency towards aggregation.19,20 In drug delivery, the ability to solubilize hydrophobic drugs is one of the largest concerns, which requires a high hydrophilic to lipophilic ratio.
Polymer surfactants, especially block or graft copolymers, are well known for their unique structure and drug entrapment capability. Due to the amphiphilic nature of the surfactants, hydrophobic drugs can be encapsulated in a micellular form.21 Micelles are organized via auto-assembly.22 It is considered as highly efficient drug carrier23 because their cell membrane-similar structure can enhance internalization.
At low concentrations, surfactant are normally orientated on the interface with the hydrophilic group facing the aqueous media and the hydrophobic groups facing out to minimizing the surface free energy17,20; however, when the surfactant concentration increases to a specific value which saturates the interface, the excess surfactant will aggregating into core-shell structure with hydrophobic groups directed toward the interior and hydrophilic groups toward the solvent in order to reducing free energy24. This specific concentration is defined as the critical micelle concentration (CMC)20. The CMC is not only the critical point for micelle formation, but also the transition point for solution properties such as surface tension, osmotic pressure, conductivity, and particle size (Figure 1). Generally, the CMC is affected by the solution temperature and decreases with the increasing hydrophobicity20,24.
In hydrophilic solvents such as water, with the presence of organic material and small hydrophobic particles, hydrophobic groups of the surfactant will tend to adsorb on the particle surface with the hydrophilic groups of the surfactant facing toward solvent to reduce free energy24,25. As the result, the particles will be encapsulated by surfactants and form a
4 micellular strucutre26. Because of this, the CMC is one of the most important indicators for the ability of a polymer surfactant to encapsulate drug.
C
Osmotic pressure
Surface tension
Interface tension
Conductivity CMC Concentration
Figure 1. Micelle formation upon CMC and a plot of solution properties versus surfactant concentration
Polyethylene Glycol
PEG is well known for its biocompatible, high water solubility, and in vivo chemical stability27. PEG is relatively bioinert, owing to its ability to inhibit protein adsorption and therefore suppress cell attachment.28 It has been reported that drugs loaded in PEG coated particles or PEG-like polymers can extend in vivo circulation time by slowing down opsonization29. Through appropriate synthetic routes, PEG could be fabricated into polymer surfactant16. PEGylated self-assembling drug carriers has been used as a pharmaceutical product for decades. One such development is the first FDA-approved nano-drug, Doxil®.
Doxorubicin was loaded in PEGylated nano liposomes developed by LTI in the early 90s.
5
This technique successfully increased doxorubicin’s in vivo half-life time to 90 h30 from 5 min31,32 . Several efficient copolymer preparation methods have been developed using PEG as a polymeric block33–36. One such development is the stannous octoate catalyst, which is often used in PEGylated polymerization reactions. Many widely used polymers such as
Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3/4HB)37, poly(lactide-co-glycolide)
(PLGA)38 and polycaprolactone (PCL)39 could react with PEG in the presence of stannous octoate to form self-assemble block copolymer. Biocatalyst novozyme 435 was also found to be highly effective in mediating dimethyl isophthalate and PEG condensation reactions.16
Macrophages
Macrophages are a type of white blood cell that differentiate from monocytes, which developed from hematopoietic stem cells40. Macrophages are found in almost all tissues and have tissue dependent behavior41. They play an important role in innate and adaptive immunity, along with tumor angiogenesis and migration42. Macrophages are well known for their highly specialized phagocytic ability, which aids in removing dead cells, pathogens, and digesting drug particles. Particle size less than 10µm are efficiently internalized through phagocytosis.43 Macrophage phagocytosis process is typically including several steps: (1) recognition of particles or pathogens by phagocyte surface receptors; (2) particles are phagocytized; (3) particles are trapped in a phagosome; (4) phagolysosome formation by phagosome and lysosome fusion; (5) particles are degraded or digested inside the phagolysosome; (5) pharmaceutical ingredients or nutrients are absorbed; (6) indigestible materials are excreted.42 By making the advantage of this behavior, intercellular release can be achieved. Figure 2 shows the overview of macrophage digestion of a polymer encapsulated drug particle.
6
Figure 2. Typical macrophage phagocytic uptake of drugs
7
CHAPTER II
RELATED WORKS ON PEG ASSOCIATED DRUG DELIVERY
PEG Diblock/Triblock Copolymer
One family of PEG block polymers that can self-assemble and form micellular or liposomal structures is 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE) – PEG block copolymer.30 Although DSPE-PEG can form sterically stable liposomes, it is typically combined with other polymers to form multicomponent-micelle.44–46 In one study, a mixed poly(histidine)-PEG block polymer and DSPE-PEG block polymer micelle system was developed.44 The resultant micelle system exhibited pH sensitivity from pH 5 to 7.4. Drug release was increased at lower pH, which was shown through micelle swelling and ionization by size distribution and zeta potential, respectively. Cell culture using 4T1 murine breast cancer cells also indicated pH-dependent enhancement of paclitaxel uptake by micelle encapsulation. Matthias and co-workers have studied a liposome composed of dipalmitoyl phosphatidylcholine, cholesterol, and DSPE-PEG in vivo.45 Compared with direct injection of dexamethasone, an anti-inflammatory drug, the PEGylated liposome encapsulated dexamethasone showed a significant reduction of inflammatory response in multiple cell types, such as T-cells and B-cells. Other systems, such as a hydrogenated egg phosphatidylcholine, cholesterol, and DSPE-PEG mixture has been used to study the sustained circulation clearance effect of PEGylated particle.46 The result indicated that
PEGylated particles could extend circulation time in rats by reducing mononuclear phagocyte cell recognition; however, this effect was inhibited by IgM secretion from B cells after the first PEGylated particle injection.46,47
8
Another common combination is with PLGA, which is also a biodegradable and biocompatible polymer.48 The behavior of PLGA-PEG is partially dependent on the molecular weight ratio between PLGA and PEG.38 It was reported that PLGA-PEG nanoparticle with PEG molecular weight of 5000 has a higher degradation rate and lower sustained drug release in vitro when the weight percentage of PLGA is decreased.38
Moreover, PEG-PLGA diblock copolymers and PEG-PLGA-PEG triblock copolymers have been reported to exhibit gelation behavior.49,50 Low molecular weight PEG-PLGA-PEG triblock polymer, PEG and PLGA molecular weights of 550 and 2810 respectively, was shown to form a hydrogel at physiological temperature while existing as a fluid at room temperature thus having the potential to be an injectable hydrogel implant. Drug release behavior with the hydrophobic drug ketoprofen resulted in more than two weeks of sustained release with first order kinetics.50 Alexandra and co-workers developed implantable devices containing indomethacin with a PLGA-PEG coating. With the presence of PEG, the adhesion of blood cells onto the implant surface was inhibited and the drug release was sustained for three weeks35. Some other PEGylated polymers for drug delivery include PEG and poly(N- isopropylacrylamide) star copolymers which can form hydrogel at physiological temperatures, similar to the PLGA-PEG system49.
Poly(ɛ-caprolactone)-PEG-Poly(ɛ-caprolactone ) (PCL-PEG-PCL) triblock copolymer was first synthesized by Ge’s group51 and was shown to have low a CMC and could encapsulate hydrophobic drug . The micelle was found to have around 5 wt% loading capability with nimodipine. Release of nimodipine from the PCL-PEG-PCL micelle could be extended to 6-8 days. A study done by the same group using PCL-PEG-polylactide micelle displayed similar results with a higher loading capability but shorter releasing time.39
9
Despite of spherical micelle structure, polymer particles can also assemble into other structures. polymethacrylate-PEG copolymer synthesized by Cheng has been reported to form multiple structures such as spherical micelles, cylindrical micelles, super-helices, and super-rings in a concentration dependent manner.52
However, most of these ‘traditional’ diblock or triblock copolymers are highly dependent on the component properties and the only modifiable parameter is molecular weight of each component. Other methods for particle optimization were limited into end group reactions or using a copolymer mixture.
PEG-D5HIP Polymer
Several efficient alternating block copolymer preparation methods have been developed. Condensation polymerization between PEG and dimethyl-5-hydroxyisophthalate
(D5HIP) was developed by Kumar et al.53. This reaction system acts under mild conditions and requires no organic solvent. The resulting product, poly[(poly- (oxyethylene)-oxy-5- hydroxyisophthaloyl] (Ppeg), is an amphiphilic polymer53. By attaching different modifiers on to the hydroxyl on phenyl ring, various properties can be achieved. Modification of the hydrophobic chain can impart self-assembly capabilities on the polymer (figure 3).
Further modification of the polymer using different PEG molecular weight of 600,
900, and 1500 (Ppeg-600/900/1500) have been produced with isolated yield reached around
80%.53 Modification of Ppeg-600 with decanoyl chloride and bromodecane attaching to hydroxyl group has also been examined. To measure the interaction of polymer with cations, calcium ions UV adsorption spectrum of polymers with different modifications was generated. Compared with unmodified Ppeg-600, the alkyl and esters modifications showed significantly lower interaction with calcium ions.54
10
Figure 3. Ppeg-alkanes form micelles in aqueous solutions. The hydrophilic shell is composed of PEG chains and the hydrophobic core results from the alkyl chains and aromatic rings
Ppeg-600 has been modified with bromodecane, 12-Bromododecanol, and 11- bromoundecanoic acid to evaluate the polymer’s drug delivery capabilities55. The micelle size and CMC were modification-dependent. The overall efficiency of aspirin encapsulation was found to be near 80% and 7% for naproxen when loading at a 1:5 drug:polymer ratio.
Animal studies exposing the drug loaded micelles to the inner surface of the rats’ inflamed ear and measuring the auricular thickness showed enhanced therapeutic effects55.
However, sustained release has not yet been studied. The effect of side chain length and function groups on polymer micelle behavior remains unclear. Furthermore, because only Ppeg 600 was used in the study, the difference caused by PEG molecular weight is unknown.
11
CHAPTER III
SUPRAMOLECULAR ASSEMBILES OF ALKANE FUNCTIONLIZED
POLYETHYLENE GLYCOL COPOLYMERS FOR DRUG DELIVERY
Abstract
The effects of altering hydrophobic and hydrophilic chain lengths on a poly[(poly-
(oxyethylene)-oxy-5-hydroxyisophthaloyl] (Ppeg) platform for delivering hydrophobic drugs was examined. Ppeg-600/1000 modified with different alkanes were synthesized as drug carriers, and their drug delivery properties and propensity for internalization by macrophages was studied.
The synthesized polymers were characterized by nuclear magnetic resonance spectroscopy (NMR), dynamic light scattering (DLS), and zeta potential. The resulting polymer particles were able to form micelles in aqueous solution and encapsulate pyrene, a highly hydrophobic model drug, with a loading capacity up to 8 wt%, corresponding to a
50% loading efficiency. The ability to sustain drug release over several days was also observed. Most of the particles formed in this study showed cytocompatibility of more than
85%. Cell uptake of the micelles was measured quantitatively and qualitatively to be substantially higher than the unencapsulated drug. The loading capacity of the drug in the various micelles did not correlate with the internalization of the particles into the cells.
12
Introduction
Surfactants, particularly biosurfactants, present an attractive possibility for drug delivery owing to their biodegradability and chemical sensitivity to temperature and pH18.
The drug loading performance of surfactants is highly dependent on the hydrophilic- lipophilic balance (HLB) value19. Delivery of many drugs is limited by water solubility, short in vivo half-life, and hydrolysis. Due to the amphiphilic nature of the surfactants, hydrophobic drugs can be encapsulated in a micellular form. Polymeric delivery vehicles must strike a balance between the binding strength with the drug molecule and the ability to release this material in the cell.56 The ability of these surfactants to complex around drug molecules or to form micelles above the critical micelle concentration (CMC) is shown in
Figure 3.
Polyethylene glycol (PEG) was selected as the hydrophilic block in the amphiphilic polymers synthesized in this research for its stealth capabilities in drug delivery applications in vivo.27 Several efficient copolymer preparation methods have been developed using PEG as a polymeric block33–36. One of these mechanisms couples PEG and dimethyl 5- hdroxysiophthalate to form a copolymer53. The resulting product, Ppeg, is an amphiphilic polymer54. Through further modification tethering alkyl chains to the isophthalate moiety, the polymer can self-assemble in aqueous solution. Based on light scattering experiments,
Kumar et al.55 showed that the synthesized micelle has a hydrophilic PEG shell and a hydrophobic core with aromatic units in close proximity.
Sustained release of model drugs and the impact of these micelles on cells has not yet been studied. Polymer properties influence particle internalization by macrophages.57–59
Macrophages are an important white blood cell, which derive from monocytes and maintain a
13 unique role in innate immunity42, adaptive immunity, and tumor angiogenesis and migration40. In this research, Ppeg modified with bromoalkanes with chain lengths from C6 to C9 were synthesized. By using PEG and alkyl chains of varying molecular weights, the effects of altering the hydrophilic shell and hydrophobic core on internalization by cells could be examined. The polymers synthesized were characterized for their size, pH dependent surface charge, and temperature dependent CMC. The polymers were loaded with a model hydrophobic drug, pyrene, and the release kinetics at pH 5 and 7.4 were measured.
One of the particles was able to decrease cell viability through enhanced delivery of pyrene to the macrophages.
Experimental
Material
All materials were purchased through Sigma and were used as received, unless otherwise stated. Fresh deionized water (Milli-Q, Thermo Scientific Nanopure) was used throughout this study. Poly ethylene glycol MW 600/1000 (PEG 600/1000) was dried under vacuum for 24 h before use. Dimethyl 5-hydroxy isophthalate and Novozyme-435 was dried using desiccator before use.
Polymerization
Equimolar (1 mmol) PEG 600/1000 and dimethyl 5-hydroxy isophthalate were mixed in a round bottom flask. To this mixture Novozyme-435 (10 wt%) was added. The reactor was maintained under vacuum in a 90°C oil bath for 48 h. The product was quenched with water and was filtered under vacuum. The filtrate was then dialyzed for 12 h in Milli-Q water using a 3,500 molecular weight cut off membrane and was lyophilized (Labconco, 4.5L) to obtain the product. The reaction schematic diagram was shown in figure 4. At room
14 temperature, the poly[(poly(oxyethylene-600)-oxy-5-hydroxy-isophthaloyl] (Ppeg-600) obtained was a yellowish mucinous fluid and the poly[(poly(oxy- ethylene-1000)-oxy-5- hydroxyisophthaloyl] (Ppeg-1000) was a white solid.
O O A CH3 CH3 O O O 10wt% novozyme 435 + H OH n 90C oil bath
OH O O
H3C O O O O n
m OH
B O O
H3C O H O O O n + Br x CH3
m x=5,6,7,8
OH
O O
H3C O H O O O n K2CO3
60 C water bath m
O
x CH3
Figure 4. Reaction scheme for polymerization of Ppeg. A) PEG and dimethyl 5- hdroxysiophthalate react under 90°C with novozyme 435 to form poly[(poly-(oxyethylene)- oxy-5-hydroxyisophthaloyl]. B) poly[(poly- (oxyethylene)-oxy-5-hydroxyisophthaloyl] reacts with bromine-alkane at 60°C to form poly[(polyoxyethylene)-oxy-5-(alkane)- isophthaloyl].
15
Equimolar (1 mmol monomer) of Ppeg-600/1000 and bromoalkanes were mixed in
10 mL dry acetone in a round bottom flask. The bromoalkanes used in this study were 1- bromohexane, 1-bromoheptane, 1-bromooctane, and 1-bromononane. To the solution 1 mmol anhydrous potassium carbonate was added. The mixture was maintained in a water bath at
60°C for 12 h. The potassium carbonate was removed through filtration. The acetone was removed by vacuum and the product was dialyzed for 4 h in Milli-Q water and lyophilized.
This yielded poly[(polyoxyethylene-600/1000)-oxy-5-(alkane)-isophthaloyl] (Ppeg-
600/1000- alkane), the reaction schematic diagram was shown in figure 4.
Polymer characterization
NMR was used to determine the structure and % alkyl modification of the polymers.
The 1H spectra were recorded on a Varian MR-400 spectrometer with a sweep width from -2 to 14 ppm, a 90° pulse, and an acquisition time of 2.556 s. All spectra were obtained at room temperature at a concentration of 1 wt% in D2O. VNMRJ 3.0 was used for data acquisition.
Sixteen repetitive scans with 64k points were acquired and the data were processed in
MNova with 128k points, zero filling, and exponential line broadening of 1.0 Hz.
Zeta potential and particle size were measured using a Zetasizer Nano Z (Malvern).
Zeta potential was determined at a concentration of 1 wt% in 100 mM NaCl at 25°C. The size of the micelle was measure using DLS. Ppeg-600/1000-alkane micelles were tested at
1% concentration in Milli-Q water at 25°C. CMC was measured as a change in particle size.
Particles were tested from 0.003% to 1.0% in water and at 5, 25, and 35°C.
Drug loading and release
Pyrene was used as a model hydrophobic drug. Pyrene and Ppeg-600/1000-alkane were mixed in acetone at mass ratio of 1.5:10 (drug: polymer). The solvent was removed
16 under vacuum and the loaded particles were lyophilized. The solid sample was then redispersed in Milli-Q water at 0.1 wt% and mixed using a vortexer. The mixture was centrifuged at 0.2 g for 5 min to separate unloaded pyrene. To a black 96 well plate 50 μL of
0.1 wt% Ppeg-600/1000-alkane was loaded with pyrene. The micelle structure was broken and the encapsulated pyrene was released through the addition of 150 μL of dimethyl sulfoxide (DMSO, Fisher) to each well. A standard curve was made through a serial dilution.
An excitation/emission of 360/460 nm was used to evaluate the encapsulated pyrene. The fluorescence of the pyrene was measured using a plate reader (BioTek Synergy HT
Multidetection Microplate Reader) at an excitation/emission of 360/460 nm. Loading efficiency was calculated by:
푙표푎푑% = 퐴푚표푢푛푡 표푓 푝푦푟푒푛푒 푙표푎푑푒푑 (푝푙푎푡푒 푟푒푎푑푒푟 푑푎푡푎) × 100% (1) 푇표푡푎푙 푎푚표푢푛푡 표푓 푝푦푟푒푛푒
Pyrene loaded Ppeg-alkane (1 mL of 0.1 wt%) was added to a dialysis membrane and placed a in small beaker with 100 mL of phosphate buffered saline (PBS). The pH of the PBS was maintained at pH 5 or 7.4. The beaker was sealed and incubated at 37°C. Aliquots of
1.5mL of the dialysate were taken at 4 h, 12 h, and 24 h and every 24 h for the next 5 days.
After each time point, the dialysate was removed and refreshed to avoid pyrene reaching its maximum solubility and suspending the release. After sampling was complete, 50 μL of each sample (five replicates for each time point) was added to a black 96-well plate with 50 μL
DMSO. The standard curve was made through a serial dilution. The plate was read at
360/460 nm. Drug release amount and percentage were calculated by:
푚푎푠푠 푟푒푙푒푎푠푒푑 = ∑[푑푖푎푙푦푠푎푡푒 푐표푛푐푒푛푡푟푎푡푖표푛 × 푑푖푎푙푦푠푎푡푒 푣표푙푢푚푒 (100푚푙)] (2)
∑[ ( )] 푟푒푙푒푎푠% = 푑푖푎푙푦푠푎푡푒 푐표푛푐푒푛푡푟푎푡푖표푛×푑푖푎푙푦푠푎푡푒 푣표푙푢푚푒 100푚푙 (3) 푡표푡푎푙 푎푚표푢푛푡 표푓 푝푦푟푒푛푒
17
Cell viability and particle internalization
RAW 264.7 cells (ATCC) were cultured at 37°C with 5% CO2 in Dulbecco’s modified Eagle’s medium (Thermo Scientific), which was supplemented with 10% fetal bovine serum, 100 U/L penicillin, and 100 μg/mL streptomycin. Cells (50,000 cells/well in
100 μL of media in every well except the negative control) were seeded into a 96 well plate for 24 h. Particles were added to the plate at concentrations of 0.02 wt% Ppeg-600 alkane and
0.05 wt% Ppeg-1000-alkane, with and without pyrene loading. A control of pyrene without
Ppeg-600/1000-alkane was also tested at a concentration of 0.003 wt %.
For cell viability experiments, after a 4 h incubation with particles described above, the media was aspirated and 10 μL of 5 mg/mL 3-(4, 5-dimethyltiazol-2-yl)-2, 5- diphenyltetrazolium bromide (MTT) and 100 μL media without phenol red were added to each well. The plate was incubated at 37°C for 2 h. A volume of 85 μL was aspirated from each well and 100 μL DMSO were added to dissolve the insoluble formazan crystals. The optical density at 540 nm and a reference of 690 nm were measured with a plate reader. Data was normalized to cells cultured without particles or pyrene and 10 replicates were obtained for each experiment.
For particle internalization experiments, after a 4 h incubation with the particles as described above, the media was aspirated and the plate was read at 360/460 nm. These experiments were done using black 96 well plates. Cold binding experiments were also performed by incubating the particles with cells at 4°C for 4 h, aspirating the media, and reading the fluorescence at 360/460 nm. Blank group, particle adsorption caused by plate, was performed without cell and it’s reading was eliminated from other testing well. Ten replicates were obtained for each polymer.
18
Cell imaging
Cells were seeded at a density of 500,000 cells/mL in 2 mL media in a tissue culture- treated Petri dish (Corning) for 24 h at 37°C. The cells were then incubated for 4 h with 0.02 wt% Ppeg-600 alkane and 0.05 wt% Ppeg-1000-alkane particles loaded with pyrene at 37°C or 4°C. Next, media was aspirated and the Petri dish was washed with PBS. Images were obtained with a FLoid cell imaging station (Life Technologies).
Results
Polymer characterization
NMR characterization of the synthesized polymers confirmed the reaction proceeded as expected. Spectra figures were listed in Appendix A and peak assignments can be found below:
1. Poly[(poly(oxyethylene-600)-oxy-5-hydroxyisophthaloyl] (Ppeg-600)
1H NMR data (in D2O): 3.67-3.79 (PEG main chain), 3.85 (H-4), 3.98 (H-7), 4.55
(H-3), 7.58-7.77 (H-2), 8.11 (H-1);
2. Poly[(poly(oxyethylene-1000)-oxy-5-hydroxyisophthaloyl] (Ppeg-1000)
1H NMR data (in D2O): 3.67-3.80 (PEG main chain), 3.83 (H-4), 3.98 (H-7), 4.54
(H-3), 7.63-7.82 (H-2), 8.27 (H-1);
3. Poly[(polyoxyethylene-600)-oxy-5-(hexane)-isophthaloyl] (Ppeg-600-hex)
1H NMR data (in D2O): 0.95(H-6), 1.35(alkyl chain), 3.71-3.83 (PEG main chain),
3.92 (H-4 and H-8), 4.50 (H-3), 7.40-7.48 (H-2), 8.02 (H-1);
4. Poly[(polyoxyethylene-600)-oxy-5-(heptane)-isophthaloyl] (Ppeg-600-hep)
1H NMR data (in D2O): 0.88 (H-6), 1.38(alkyl chain), 3.68-3.77 (PEG main chain),
3.77-3.99 (H-4 and H-8), 4.53 (H-3), 7.42-7.70 (H-2), 8.03 (H-1);
19
5. Poly[(polyoxyethylene-600)-oxy-5-(octane)-isophthaloyl] (Ppeg-600-oct)
1H NMR data (in D2O): 0.91(H-6), 1.30(alkyl chain), 3.67-3.75 (PEG main chain),
3.89-3.99 (H-4 and H-8), 4.47 (H-3), 7.45-7.78 (H-2), 7.96 (H-1);
6. Poly[(polyoxyethylene-600)-oxy-5-(nonane)-isophthaloyl] (Ppeg-600-non)
1H NMR data (in D2O): 0.94(H-6), 1.33(alkyl chain), 3.68-3.76 (PEG main chain),
3.97 (H-4 and H-8), 4.45 (H-3), 7.44-7.78 (H-2), 7.98 (H-1);
7. Poly[(polyoxyethylene-1000)-oxy-5-(hexane)-isophthaloyl] (Ppeg-1000-hex)
1H NMR data (in D2O): 0.90(H-6), 1.30(alkyl chain), 3.69-3.78 (PEG main chain),
3.88-3.96 (H-4 and H-8), 4.53(H-3), 7.49-7.60(H-2), 8.03(H-1);
8. Poly[(polyoxyethylene-1000)-oxy-5-(heptane)-isophthaloyl] (Ppeg-1000-hep)
1H NMR data (in D2O): 0.90(H-6), 1.31(alkyl chain), 3.69-3.78 (PEG main chain),
3.78-3.97 (H-4 and H-8), 4.53 (H-3), 7.50-7.69 (H-2), 8.04 (H-1);
9. Poly[(polyoxyethylene-1000)-oxy-5-(octane)-isophthaloyl] (Ppeg-1000-oct)
1H NMR data (in D2O): 0.90(H-6), 1.33(alkyl chain), 3.68-3.76 (PEG main chain),
3.89-4.00 (H-4 and H-8), 4.45 (H-3), 7.44-7.78 (H-2), 7.98 (H-1);
10. Poly[(polyoxyethylene-1000)-oxy-5-(nonane)-isophthaloyl] (Ppeg-1000-non)
1H NMR data (in D2O): 0.89(H-6), 1.28(alkyl chain), 3.62-3.77 (PEG main chain),
3.83-3.94 (H-4 and H-8), 4.53 (H-3), 7.49-7.81 (H-2), 8.02 (H-1);
Base on the NMR spectra, the reaction ratio and alkane modification ratio was calculated. Ppeg-600 has a PEG chain to isophthalate mole ratio of 1.06 ± 0.09, and Ppeg-
1000 reached 1.07 ± 0.11. A ratio of 1:1 represents a linear copolymer structure. The alkane chain-attachment percentage was calculated by alkane to be 46-68% (Table 1).
20
Zeta potential measurements of the particles at pH 5 and 7.4 are listed in Table 1.
Since the polymer mainly contains PEG and alkyl chains, in which the PEG chain is slightly negative charged and the alkyl chain is slightly positive charged, the polymer as a whole was relatively neutral. The experimental data shows that all of the micelles have a very slightly negative zeta potential since the zeta potential arises from the chemical moieties present at the surface, which are PEG chains.
Table 1. Characterization result of Ppeg-600/1000-alkane particles. Zeta potential, micelle size, and % hydroxyl modification with alkyl chain were measured. Errors are given as the standard deviations.
Micelle size zeta potential (mv) Modify diameter rate % PH 7.4 PH 5 (nm) 600-hex -2.21±0.17 -2.74±0.37 12.4±6.708 49.4±4.5 600-hep -2.14±0.13 -2.04±0.29 8.619±3.24 46.1±3.7 600-oct -2.28±0.37 -2.10±0.38 9.529±3.168 51.3±1.5 600-non -1.64±0.33 -1.92±0.39 12.66±6.89 68.3±0.8 1000-hex -2.35±0.16 -1.30±0.10 18.51±9.269 51.5±5.6 1000-hep -3.40±0.53 -1.14±0.37 10.89±4.716 51.1±6.4 1000-oct -3.95±0.25 -1.32±0.57 15.72±8.992 58.9±0.9 1000-non -5.91±0.37 -3.20±0.57 17.29±10.69 46.1±0.2 0.1 M NaCl -1.17±0.40 2.10±0.67 solution
The micelle size was determined by DLS using the intensity distribution method
(Table 1). Polydispersity index (PDI) ranged from 0.23 to 0.09 for all of the particles. There were no statistical differences between the sizes of the various micelles.
CMC of Ppeg-alkyl micelles at three different temperatures are shown in Figure 5A with each temperature corresponding to specific cell culture environment: 5°C to cold
21 binding (4°C); 25°C to room temperature; and 35°C to cell culture incubation (37°C). The
CMC of Ppeg-600-hexane and -heptane at 35°C were not listed because they were below the detection limit (less than 0.003%). Temperature is known to affect dipole forces and slightly change the CMC20. Based on the data presented here, most of the micelles synthesized for these experiments did not show significant dependence on temperature.
0.06% a 5C 0.05% 25C 35C 0.04%
0.03%
0.02% concentrationwt% 0.01%
0.00%
0.10 100% b c
0.08 80% Ph 5 PH 7.4 0.06 60%
0.04 40% releasing% 0.02 20%
0.00 0% loading capacityparticle drug/mgloading mg
Figure 5. The relationship between the CMC and drug delivery parameters. A) The critical micelle concentration (CMC) of the synthesized polymers in DI water at 5, 25, and 35°C. B)
22
The drug loading capacity of pyrene encapsulated in the micelles at 0.1% in DI water. C) The
% drug released percentage at 96 h in pH 5 and pH 7.4 in PBS.
Drug loading and releasing
The loading capacity data (Figure 5B) exhibited the reverse trend observed for the
CMCs: particles with lower CMCs have higher loading capacity. The release of pyrene from the micelles is shown for pH 5 and pH 7.4 at body temperature in Figure 6. The total percent released at 96 h is shown in Figure 5C for comparison. Nearly all of the particles release drug in a linear manner after 96 h. All of the Ppeg-600 particles have released less than 20% of their loaded drug at 96 h, which shows sustained release ability. However, Ppeg-1000 particles had shorter drug release times. The % release at pH 7.4 is similar to the CMC. With sufficient core-drug compatibility, a stronger binding ability implies a lower CMC and a more stable micelle since it results in a lower free energy24. Odd-even carbon effects were also observed among Ppeg-1000 particles for drug loading and release. Ppeg-1000-nonane has a low loading capacity considering the trend of other three Ppeg-1000 particles, which could possibly due to odd-even effect. Such effect is more obvious in drug release at pH 7.4, the % release of pyrene from Ppeg-1000 particles with even number carbon alkanes exhibiting a significant decrease comparing to pyrene releasing from Ppeg-1000 particles with odd number carbon alkanes. All Ppeg-alkane particles exhibited a larger % release in pH 7.4 than pH 5. Changes in pH can often affect surfactant performance17,29,60. Low pH can cause ether bond protonation in the PEG chain and change the molecular charge to a positive charge17. The zeta potential showed that all the particles tested here were slightly negatively charged, thus a change in pH could reverse the surface charge of the micelles. Zeta potentials of the particles at pH 5 in 0.1M NaCl solution are listed in Table 1. The Ppeg-600-alkane
23 particles changed very modestly. Ppeg-1000 particles showed significant increase in the zeta potential. Ppeg-600-alkane was less affected since the pH mainly affects PEG chain, which is nearly doubled in Ppeg-1000-alkane.
600-hex 600-hep 1000-hex 1000-hep 600-oct 600-non 1000-oct 1000-non
4 8 a b
3 6
2 4
1 2
0 0 0 50 100 150 0 50 100 150
12 35 c d 10 30
25 8
20 6 15 4 10 2 5
0 0 0 20 40 60 80 100 0 50 100 150 Figure 6. Pyrene is released from Ppeg-alkyl micelles. Pyrene release from Ppeg-alkane in
PBS at 37°C, x-axe is hour, y-axe is total amount of pyrene released in µg: A & B) Ppeg-
600-alkane and Ppeg-1000-alkane release of pyrene at pH 5. C & D) Ppeg-600-alkane and
Ppeg-1000-alkane release of pyrene at pH 7.4.
24
Cell viability and particle internalization
The cytocompatibility of the particles was determined using an MTT cell viability assay. The viability of the cells cultured with the Ppeg-600/1000-alkane particles for 4 h was expressed as a percentage of cell viability of cells not exposed to particles and is shown in
Figure 7A. To ensure micelle formation, the Ppeg-600-alkane particles were tested at 0.02% and Ppeg-1000-alkane particles at 0.05%. With the exception of Ppeg-600-hexane and - heptane, the Ppeg-alkane particles showed cell compatibility >80%. Viability of the cells after 24 h incubation with Ppeg-alkane particles is shown in Figure 7B. Ppeg-600-hexane and
–heptane resulted in extreme low viability as expected, while Ppeg-1000 particles maintained a relatively high cell viability without statistical difference between each modification.
After loading with pyrene, Ppeg-1000-nonane showed more toxicity while significant changes were not observed for the other particles. Unencapsulated pyrene results in nearly
100% viability.
particle only particle with drug 120 120 a b 100 100
80 80
60 60
40 40 cell viability % viability cell 20 20
0 0
Figure 7. Cell viability after incubated with naked or pyrene-loaded Ppeg-alkyl particles for
(A) 4 h and (B) 24 h
25
These results imply that pyrene either has low cytotoxicity or could not be internalized by the cell. The decrease in cell viability for pyrene loaded Ppeg-1000-nonane suggests the internalization of pyrene results in toxicity. The rest of the particles did not show a decrease in viability possibly due to insufficient quantities of pyrene being internalized by the cell necessary to cause toxic effects.
Figure 8A shows the fluorescence level of particles internalized or associated with macrophages. At 4°C, all cell internalization pathways are blocked, meaning the fluorescence signal is generated by particles adsorbed to the cell surface. The results showed a significant increase when incubation temperature was increased from 4°C to 37°C with the exception of pure pyrene, which was below the detection limits at both temperatures. The high fluorescence level for cells cultured with Ppeg-1000-nonane particles agrees with the suggestion that the decrease in viability for this particle results from enhanced internalization.
37C 4 C 14000 5000 a b 12000 4500 4000 10000 3500 8000 3000 2500 6000 2000
4000 1500 Fluorescence level Fluorescence 2000 1000 500
0 0
fluorescence level / pyrene added / pyrene level fluorescence
pyrene
pyrene
600-oct
600-hex 600-hep
600-non
600-oct
1000-oct
600-hep 600-hex
600-non
1000-hep 1000-hex
1000-non
1000-oct
1000-hex 1000-hep 1000-non Figure 8. The amount of pyrene internalization depends of the composition of the micelle.
Pyrene loaded Ppeg-alkyl particles were incubated with macrophages at 37°C and 4°C. A)
26
The fluorescence of pyrene internalized by macrophages. B) The fluorescence of pyrene internalized by the cell normalized to the amount of pyrene added to each well.
In order to compare the performance of each particle, the amount of pyrene added to each well was calculated, and the fluorescent signal was normalized to that amount (Figure
8B). The results demonstrate that the Ppeg-1000-alkane particles have higher cell binding than the Ppeg-600-alkaneparticles. The internalization was also higher for the Ppeg-1000- alkane particles. No significant differences between Ppeg-1000-alkane particles were observed, indicating that cell uptake is not altered by alkane modification for these particles.
Ppeg-600-nonane shows a clear decrease compared to the other Ppeg-600-alkane particles.
Qualitative internalization images of these particles are shown in Appendix B. The blue color indicates pyrene. Pyrene was able to enter the cell when encapsulated in the particles. Pure pyrene showed no detectable cell internalization with the only fluorescence signal arising from a pyrene crystal that formed on the bottom of the Petri dish. Cells appear to ball up in Ppeg-600-hexane, which generally indicates toxicity. The fluorescence images of cell internalization of these particles incubated at 4°C are shown in Appendix C. None of the Ppeg-alkane particles had an observable signal.
Discussion
CMC relationship with drug release
CMC is an important index for micelle mediated drug delivery systems61. The increasing alkyl chain length increases the number of van der Waals interactions present in the core of the micelle, thus decreasing the CMC. The increase in CMC when comparing
Ppeg-600 particles to the Ppeg-1000 particles arises from the increased hydrogen bonding between the water and the PEG chain, preventing particle formation. Lower CMCs indicates
27 enthalpic or hydrophobic dominated micelle assembly, which can compensate for the free energy required for micelle formation and result in micelle stability24. For lower CMCs, the surfactant has a higher tendency to form micelles, leading to high loading capacity. For these more stable micelles, a lower release rate can be achieved, as is observed in Figure 5.
Micelle properties influenced by PEG and alkyl chains
The zeta potential is slightly lower for Ppeg-1000 particles than Ppeg-600 particles.
The main factor altering the zeta potential is the hydrophilic PEG shell, whose surface charge increases with molecular weight20. While the zeta potential across the Ppeg-600-alkane particles did not change significantly, the potential of the Ppeg-1000-alkane particles displayed a linear decrease with increasing alkyl chain length (R = -0.97). Zeta potential is a measure not only of surface charge, but the electric charge on the slipping plane between the bulk solution and the stationary layer62. Ionically separated systems increase in stability when the absolute value of the measured zeta potential increases63, while the stability of sterically separated systems does not depend on zeta potential64,65. Since the polymer itself is neutral and the pH study on drug release behavior suggested that the micelles are more stable for lower absolute values of the zeta potential, the system should be sterically stabilized regardless of the measured zeta potential. However, the negative charge mainly comes from the ether bonds in PEG chain that have a decisive role in hydrogen bonding. The larger negative zeta potential represents a higher activity of the ether bond, which relates to hydrogen bond formation66 or the hydrophilicity of the PEG chain. This leads to increasing hydrophobic forces, which favor micelle stability24. As a result, a more positive zeta potential implies a more stable system. This is further substantiated by the zeta potential data at pH 5, which showed that ether bond protonation caused by the lower pH17 resulted in a less
28 negative zeta potential and enhance micelle stability. One possible explanation is that the
Ppeg-1000 particles are capable of additional hydrogen bonding, requiring additional energy to form a micelle and that the added flexibility of the hydrophilic shell may affect PEG chain arrangement and cause a change in the zeta potential.
In examining the effect of the PEG chain length on the CMC, the results shown here exhibit an increase in CMC with increasing PEG chain. Ppeg-600-alkane particles have a much lower HLB value, meaning that the hydrophobic effect is more significant in these particles and leads to a higher enthalpy change that results in a lower CMC as well as increased drug encapsulation ability29.
Typically, the CMC of surfactants with similar hydrophilic groups decreases logarithmically with increases in the number of carbons in the alkyl chain using the following equation:
log(퐶푀퐶) = 퐴 − 퐵푛 (4) where n is the number of carbons in the alkyl residue and A, B are constants20. The micelles studied here generally followed this trend with the exception of Ppeg-600-nonane. The driving force for assembly into micelles is mediated by free energy and hindered by energy required for nucleation and steric effects. In addition, Ppeg-600-nonane also has the largest modification %, being nearly 20% higher than the other alkyl modifications. This means both hydrophobic forces and steric effects are enhanced, and the outcome increased the CMC of
Ppeg-600-nonane.
Odd-even carbon effect was observed among Ppeg-1000 particles for the CMC, drug loading, and drug release. The particles with odd number carbon alkane modifications tended to have a lower CMC, higher drug loading, and lower drug release than those with even
29 number carbon alkane modifications. Such an effect was most obvious in the % release at pH
7.4 and least evident in the CMC, which suggests that such an effect might depend on pyrene loading. Since the pyrene and alkane chain are both non-polar molecule, the intermolecular interaction is mainly weak van der Waals force. The odd-even effect could possibly be caused by change of instantaneous dipole moment direction at alkane terminal67. Change in the interaction strength is the main reason of odd-even effect for the non-polar self-assemble structure.68 The orientation of the odd-carbon alkane terminal can possibly increase the interaction between alkane chain and pyrene and form more stable micelle.
Cell culture study
Cell viability data demonstrated cytocompatibility at 4 h incubation for the Ppeg- alkane particles with the exception of Ppeg-600-hexane and -heptane. Ppeg-600-hexane and - heptane particles were found to be toxic to the cells. This could be the result of biodegradation. Biodegradation of ethoxylate based linear surfactants is known to have two different mechanisms that occur simultaneously: cracking at the ether group between the
PEG chain and the alkyl group and cracking at the end groups20. After endocytosis by cells, the micelle structure will open up in the lysosome and release the drug encapsulated inside.
Thus, there is a possibility that isophthalate was released as a degradation product.
Furthermore, PEG chains with lower molecular weights can be more easily degraded into oligomers or ethylene glycol monomers and their hydroxyl acids. This will lead to an increase in metabolic acidosis, calcium levels, and serum osmolarity69. Changes in calcium levels and osmolarity will lead to plasma membrane potential decreases and dehydration. The micelle structure of the studied polymers requires a folded PEG chain, which favors intermolecular hydrogen bonding. For Ppeg-600-hexane and -heptane, the degradation
30 process is faster than other particles because the alkyl chain is shorter and the PEG chain also has a lower molecular weight. This is further demonstrated by the 24 h cell viability study: a linear increase in cell viability among the Ppeg-600-hexane, -heptane, and -octane particles was observed, and all the Ppeg-1000 particles exhibited higher viability than Ppeg-600 particles. The only expectation is Ppeg-600-nonane which showed lower viability than Ppeg-
600-octane. One possible reason is that Ppeg-600-nonane micelles open easily inside the cell due to their high CMC and increase the degradation rate of the micelle.
Unencapsulated pyrene was shown to be nontoxic; however, cell imaging and cell uptake data showed that pure pyrene is not internalized. The decrease in Ppeg-1000-nonane viability after loading with pyrene may result from the ability to deliver cytotoxic quantities of pyrene to the cell.
All particles showed a significant improvement in cell uptake compared to pure pyrene. Base on the percent of the hydroxyl modified by the alkyl chain (Table 1), Ppeg-
1000-alkane particles have a higher internalization ratio than Ppeg-600-alkane. The Ppeg-
600-nonane exhibited a significantly lower internalization than the other Ppeg-600-alkanes.
These results suggest that internalization is mainly dependent on PEG chain length rather than alkyl modification.
Conclusions
Here, Ppeg-600/1000 with varying alkyl chain length biosurfactants were studied for their potential as drug delivery systems. Micelle formation was observed for all of the copolymers synthesized here and by adjusting alkyl and PEG chain length, key factors such as the CMC and drug loading and release properties can be programmed while having a negligible effect on micelle size and a small effect on the zeta potential. The CMC was
31 reduced to 0.004 wt% for some of the materials and was able to complex with pyrene, with a loading capacity up to 8 wt% of the polymer. Ppeg-600-alkane particles were able to retard drug release to less than 20% of the loaded pyrene after 96 h. The CMC and drug loading/releasing were also related to each other through micelle stability. With the exception of Ppeg-600-hexane/heptane, the polymer surfactants have a cytocompatibility of more than
85% viability. Cell internalization ability was tested both quantitatively and qualitatively.
Pyrene displayed no detectable internalization by the cells, but was able to be internalized when encapsulated in the polymer micelles. Furthermore, one of the polymers, Ppeg-1000- nonane, was able to reduce cell viability when delivering pyrene. The result indicated that
Ppeg-alkane polymer micelles were highly efficient drug carriers.
32
CHAPTER IV
FUTURE WORK
Both the internalization and release mechanisms for the drug delivery systems studied here remain unclear. Data analysis on drug release behavior exhibited first order release for
Ppeg-1000-octane and Ppeg-1000-hexane at pH 7.4 while Ppeg-1000-hexane at pH 5 followed second order release. The remaining particles displayed behavior between first and second order. The release mechanism could be a combination of diffusion and degradation.
The internalization pathway for these micelles has not yet been studied. Multiple pathways, such as phagocytosis, receptor mediated endocytosis, and caveolae dependent endocytosis, could be involved based on the micelle size of ~10 nm. Specific blocking of these internalization mechanisms did not yield analyzable results. Future approaches could examine endosomal escape through monitoring the lysosome.
33
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APPENDIX A
NMR SPECTRUM
a b
a) Poly[(poly(oxyethylene-600)-oxy-5-hydroxyisophthaloyl] (Ppeg-600) b) Poly[(poly(oxyethylene-1000)-oxy-5-hydroxyisophthaloyl] (Ppeg-1000)
c d
c) Poly[(polyoxyethylene-600)-oxy-5-(hexane)-isophthaloyl] (Ppeg-600-hex) d) Poly[(polyoxyethylene-600)-oxy-5-(heptane)-isophthaloyl] (Ppeg-600-hep)
39
e f
e) Poly[(polyoxyethylene-600)-oxy-5-(octane)-isophthaloyl] (Ppeg-600-oct) f) Poly[(polyoxyethylene-600)-oxy-5-(nonane)-isophthaloyl] (Ppeg-600-non)
g h
g) Poly[(polyoxyethylene-1000)-oxy-5-(hexane)-isophthaloyl] (Ppeg-1000-hex) h) Poly[(polyoxyethylene-1000)-oxy-5-(heptane)-isophthaloyl] (Ppeg-1000-hep)
40
i j
i) Poly[(polyoxyethylene-1000)-oxy-5-(octane)-isophthaloyl] (Ppeg-1000-oct) j) Poly[(polyoxyethylene-1000)-oxy-5-(nonane)-isophthaloyl] (Ppeg-1000-non)
41
APPENDIX B
CELLS WERE INCUBATED FOR 4 H AT 37°C WITH THE PARTICLES. THE SCALE BAR REPRESENTS 100 μm. Ppeg-600-hexane Ppeg-600-heptane
Ppeg-600-octane Ppeg-600-nonane
Ppeg-1000-hexane Ppeg-1000-heptane
Ppeg-1000-octane Ppeg-1000-nonane
Pyrene
100µm
42
APPENDIX C
CELLS WERE INCUBATED FOR 4 H AT 4°C WITH THE PARTICLES. THE SCALE BAR REPRESENTS 100 μm. Ppeg-600-hexane Ppeg-600-heptane
Ppeg-600-octane Ppeg-600-nonane
Ppeg-1000-hexane Ppeg-1000-heptane
Ppeg-1000-octane Ppeg-1000-nonane
Pyrene
100µm