Anticancer agents ANTIMETABOLITES Antimetabolites The chemical structure of antimetabolites is similar to that of metabolites which participate in biochemical processes observed in the body. This similarity enables the body to use them in biosynthesis instead of normal metabolites. This replacement leads to the incorporation of antimetabolites into DNA or RNA and causes misreading. 2 The term antimetabolites is applied to two groups of drugs. The first group comprises drugs which are built as nucleotide analogs into the biopolymers of cells (DNA, RNA). This group consists mainly of pyrimidine and purine analogs and their nucleosides. These analogs are incorporated into DNA and make its replication impossible or inhibit competitively the incorporation of deoxyribonucleoside trisphosphate into DNA. 3 The second group involves drugs inhibiting the synthesis of metabolites, necessary for cell life. Their competetive action does not depend on their participation in the building of cellular biopolymers. Inhibitors of mureine synthesis, nucleoside precursors, folic acid antagonists and tetrahydrofolic acid are classified into this group. 4 Many antimetabolite antineoplastics are categorized by the class of nucleotide they inhibit. Purine antagonists inhibit the synthesis of the purine-based nucleotides adenosine monophosphate (AMP) and guanosine monophosphate (GMP), and the pyrimidine antagonists stop the production of the pyrimidine-based nucleotides, primarily deoxythymidine monophosphate (dTMP). Three groups of antagonists are distinguished: . Tetrahydrofolic acid . Pyrimidine analogs . Purine analogs 5 OH 9 10 O N CH N COOH 4 5 2 Folic acid N 3 6 H COOH 2 N 1 8 7 H H2N N N CH O NH2 3 _ _ Methotrexate N CH C Glu N 2 N H2N N N 6 Antimetabolites of folic acid CH3 O NH2 _ _ As antagonists of N CH CH C Glu CHN O 2 dihydrofolic acid NwhichH2 3 _H N _ N CH 2 N N C Glu block the enzymeN 2 N Edatrexate dihydrofolateH N reductaseN N 2 OCH3 CH (DHFR) are classified NH2 3 H CH2 N OCH3 some anticancer agents N H N OCH3 and immunosupressants: 2 N Trimetrexate H C N CH 3 3 O COOH HN N S N COOH H O Methotrexate, METHOTREXAT, TREXAN Raltitrexed, TOMUDEX 7 Methotrexate binds competitively with dihydrofolate reductase irreversibly. This bond is 10, 000 times stronger than with a CH O normal metabolite.NH 2 3 _ _ Glu N CH2 N C The biological Nhalf-time of methotrexate is 3.5 h. H2N N N Methotrexate is used mainly in the treatment of leukemia, breast, head and neck carcinomas, pulmonary carcinoma. It permeates into the cerebrospinal fluid. 8 Methotrexate • Is given orally • The sodium salt form is marketed for intravenous, intramuscular, intra-arterial, or intrathecal injection • It dergoes intracellular folyl polyglutamate synthase, (FPGS)- catalysed polyglutamation, which adds several anionic carboxylate groups to the molecule and traps the drug at the site of action • Polyglutamation is more efficient in tumor cells than in healty cells, which promotes the selective toxicity of this drug • The polyglutamated drug will be hydrolized back to the parent structure before renal elimination • Up to 90% of an administered dose of M. is excreted unchanged in the urine within 24 hours 9 Methotrexate is a potent but very toxic drug. CH O NH2 3 _ _ N CH C Glu Unwanted actionN affects the 2hematopoietic N system, gastrointestinal tract and kidneys.H2N N N Nephrotoxicity is caused by a methotrexate metabolite – 2,4-diamino-N-methylpteroinic acid. It is created under the influence of bacterial peptidases present in the stomach. They cleave the glutamic acid rest from the parent compound. 10 CH3 O Edatrexate demonstrates greater anticancerNH2 activity against cancers _ _ N CH CH C Glu of various organs and lower toxicityN than methotrexate.2 H2N N N Its greater activity is caused by the intensive intracellular metabolism of cancerous cells to polyglutamated forms. Edatrexate is the most effective in the treatment of pulmonary microcellular carcinoma. 11 Trimetrexate - the analog of methotrexate - acts also in the S phase of the cell cycle. It differs from the precursor only in the lack of a glutamic acid rest. It acts synergistically together with fluorouracil and calcium folate. OCH3 CH NH2 3 H CH N OCH It is effective in the treatment ofN colorectal2 carcinoma,3 breast OCH carcinoma, head and stomachH2N carcinomasN and soft3 tissue sarcoma. 12 Pemetrexed Disodium • a new multitarget antifolate administered by the intravenous route for the treatment non-small cell lung cancer and in combination with cisplatin in melignant pleural mesothelioma • Patients on P. must take folic acid and vitamin B12 to reduce the risk of bon marrow suppression and use-limiting GI side effects like mucositis and stomatitis • Pretreatment with corticosteroids can reduce the risk of drug- induced skin rash 13 Pralatrexate • Also more extensively polyglutamated than mothotrexate. • Used in the treatment of non-small cel lung cancer and in combination with the DNA polymerase inhibitor gemcitabine, in NHL. • When used in combination with gemcytabine, the synergistic apoptotic effect is maximized when the antifolate is administered before polymerase inhibitor. 14 Raltitrexed is a new analogue of folates, inhibitors of thymidylate synthetase, which is necessary in the synthesis and repair process of DNA. In the body it undergoes a rapid transformation to polyglutamated forms which act 100 times more strongly than the parent compound. Polyglutamated forms act toxically on the normal cells. Raltitrexed is effective in the treatment of colorectal and rectal carcinomas, ovarian carcinoma, breast carcinoma, pancreas H C N CH carcinoma and non-microcellular3 pulmonary3 O carcinoma.COOH HN N S N COOH H O 15 Antimetabolites of pyrimidine analogues Purine and pyrimidine analogs, the basic elements of the chemical structure of DNA and RNA, play the main role in their normal function. Cytosine, thymine and uracil are pyrimidine analogs. Those bases, bound with pentoses – D-ribose ((Ryb), 2-deoxyribose (d- Ryb) or cytosine (C) create nucleosides – cytidine, uridine, thymidine. 16 cytosine cytidine deoxycytidine 17 • Those compounds create esters with phosphoric acid and are converted into ribonucleotides: uridine monophosphate (UMP), cytidine monophosphate (CMP), thymidine monophosphate (TMP). • If the sugar fragment of those nucleotides is 2-deoxyribose, those derivatives are called deoxythymidine monophosphate (dTMP) and deoxycytidine monophosphate (dCMP). 18 Synthetic fluorine derivatives of pyrimidine analogs and their nucleotides play an important role in cancer therapy. Those antimetabolites act mainly as anticancer or antiviral agents and their activity is the strongest in the S phase. They differ in the mechanism of action and toxicity. 19 The mechanism of action depends on the size of substituents at position 5 in uracil. Analog uracylu Fluorouracyl (Fluorouracil, FLUROBLASTIN) 5-Fluorouracil derivatives (5-FdUMP) O inhibit the activity of thymidylate HN F synthetase. O N H O O Their affinity for that enzyme is several Thymidylatesyntetaza H CH3 thousand times greater than for natural HN synthetasetymidylowa HN O dUMP. O N N d-Ryb-P d-Ryb-P InIncorporationkorporacja dtoo DDNANA 5-FdUMP acts as an enzyme inhibitor and w formasi ed -dRyb-Ry-Pb--PP--PP-P similarly to the natural substrate creates a O O CF covalently bound intermediate product. HN I HN 3 O N O N This product is different from the natural Dd--RiboseRyboza D-dRibose-Ryboza substrate because of the presence of IdoIdoxuridineksyurydyna TTrifluridineriflurydyna fluorine, which is why it is not cleaved into AThyminenalogi tyanalogsminy the enzyme and the end reaction product. DNA The action mechanism of pyrimidine analogs. 20 Derivatives with large substituents at position 5 (e.g.5-iodo- and 5- trifluoromethyl analogs of thymidine) are incorporated into DNA during biosynthesis. As a result of this process DNA functions are disturbed. The inhibition of the ability of DNA to replicate decreases or interrupts the development of cells. Idoxuridine and trifluridine, which is better soluble in water, are incorporated by viral and cellular DNA. 21 O R R = 5-Fluorouracil (5-FU) was designed in N COOH N 1957. The development of this compound N NH H O was based on the observation that some O N COOH H tumors preferentially use uracil rather than HN HN O NH2 O N N5,N10-metyleno-THF orotic acid for pyrimidine biosynthesis. P O N5,N10 –Methylene-THF HO O OH S H SyThymidylatentetaza tymidylan synthetaseowa Thymidine synthesis from uracil involves HO thymidylate synthetase and in this process dUMP a thiol group of a cysteine residue in the R enzyme (E-SH) adds to the C-6 position of O N H N NH deoxyuridylic acid and with subsequent HN 5 10 O addition of the C-5 carbon to the N ,N - O N O N P O S HN 5 10 HO O methylene of N ,N -methylenetetrahydro- NH2 folate. OH SThymidylateyntetaza tymidyla nsynthetaseowa HO The resulting intermediate product transfers the C5-hydrogen to the N10 O CH R HN 3 N position of the folate creating O H O N N NH deoxythymidylic acid, dihydrofolate, and HO P O + O O N the regenerated enzyme. OH HN NH HO 2 + The synthesis of deoxythymidylic acid from dTMP S H SThymidylateyntetaza tymidyla nsynthetaseowa deoxyuridylic acid. 22 In vivo, 5-FU must first be activated by conversion to 5- fluoro-2’- deoxyuridylic acid (5-FdUMP). In general this occurs by conversion of 5-FU to 5-fluorouracil riboside which is then transformed directly into 5-FdUMP by ribonucleotide reductase. The 5-FdUMP then binds to thymidylate synthetase to give an intermediate product that resembles the intermediate product formed with uridylic acid. 23 However, the intermediate compound has fluorine at C-5 instead of a hydrogen atom and the latter is unable to transfer the fluorine. Thus, the intermediate compound cannot break down and the enzyme is inhibited. This can lead to a deficiency of thymidine which is essential for the synthesis of DNA.