Lecture 3 - 2024 Antimetabolites PDF

1/26/2024 ANTIMETABOLITE CHEMOTHERAPEUTICS Mimic natural metabolites Interfere with biosynthetic pathways Inhibit critical steps in nucleic acid synthesis – Anti-folates – Pyrimidine analogues – Purine analogues Antimetabolites Mo...

1/26/2024 ANTIMETABOLITE CHEMOTHERAPEUTICS Mimic natural metabolites Interfere with biosynthetic pathways Inhibit critical steps in nucleic acid synthesis – Anti-folates – Pyrimidine analogues – Purine analogues Antimetabolites Most are cell cycle specific (maximal cytotoxic effects in S-phase) Toxicity reflects effects on proliferating cells – Gastrointestinal mucosa – Bone marrow – ? hair loss Why are these drugs not associated with the carcinogenic problems associated with alkylating agents? – Duration of exposure more critical than peak concentrations – continuous infusion 1  1/26/2024 The Folate Cycle Folate “Reduced Folate” SHMT – serine hydroxymethyltransferase TK – Thymidine kinase TYMS – Thymidylate synthase DHFR – Dihydrofolate reductase Methotrexate (MTX) Inside cells, folates and MTX are converted to polyglutamates 2  1/26/2024 pG-MTX MTX METHOTREXATE One of the most widely used antimetabolites in cancer chemotherapy Breast cancer (CMF combination) Childhood Acute Lymphoblastic Leukemia (ALL) Osteogenic sarcoma (HDMTX + Leucovorin rescue) – ∗Oral and gastrointestinal ulceration – ∗Myelosuppression – ∗Pulmonary infiltrates and fibrosis 3  1/26/2024 Leucovorin is used to rescue normal tissue from MTX toxicity dUMP dTMP Thymidylate synthase FH 4 FH2 H DHFR H 2N N N H COOH 2 O CH 2 H N 10 CH 2 4 5 N CH 2 HN C NH CH OH COOH LEUCOVORIN rescue CHO DHFR-independent conversion to FH4 5 N -Formyltetrahydrofolic acid (folinic acid, leucovorin, citrovorum factor) “Selectivity” of leucovorin rescue in the treatment of osteosarcoma Tumor cell Normal cell MTX-resistant full complement of folate transporters deficient in folate transporters HD-MTX MTX HD-MTX MTX Tumor Normal cell cell Leucovorin Leucovorin 4  1/26/2024 Possible Mechanisms of Resistance to MTX Reduce uptake via folate transporters Increased MTX efflux – ATP-binding cassette (ABC) family of transporters Altered target enzymes Reduction in polyglutamation – folypolyglutamate synthetase Possible Mechanisms of Resistance to MTX Increased thymidine uptake (salvage pathway) 5  1/26/2024 Pyrimidine Analogues Antimetabolites which mimic the structure of metabolic pyrimidines Fluorouracil – nucleobase analogue Cytosine arabinoside – nucleoside analogue Gemcitabine – nucleoside analogue 5-FLUOROURACIL (5-FU) O F HN O N H RNA slower growing tumors FUMP FUTP FdUMP FdUTP DNA dUMP dTMP thymidylate synthase CH2FH4 FH 2 block dTMP FdUMP is a suicide-substrate synthesis → inhibit DNA synthesis FH 4 rapidly growing tumors 6  1/26/2024 5-Fluorouracil (5-FU) colorectal cancers (with leucovorin, 20% response) Breast cancer (CMF) – ∗ Myelosuppression – ∗ GI Toxicity G2 M S Cell Cycle Specific Go G1 Prodrug of ? O F HN O N H capecitabine Oral administration (as opposed to IV for 5-FU) Largely replaced 5-FU, especially for gastric cancers 7  1/26/2024 Capecitabine Liver Liver & tumour Tumour deoxythymidine phosphorylase Specific thymidylate synthase inhibitors Raltitrexed – A product of rational drug design based on the structure of the TS substrate binding pocket – Transported into cells via folate transporters – Fully active after polyglutamylation Potent inhibitor of TS Raltitrexid – IC50 = 9 nM in vitro – Competes with N5N10methylene THF – inhibit TS without significantly influencing other folate-dependent enzymes Inhibits DNA Synthesis – Deplete dTMP – S phase specific Used in colorectal cancer – acceptable toxicity profile Folate 8  1/26/2024 Cytidine Analogs Pyrimidine Nucleoside Analogs Cytarabine Gemcitabine Cytosine arabinoside (araC) 2',2'-difluoro-2'-deoxycytidine (dFdC) Both prodrugs araC cytidine deaminase deoxycytidine kinase (dCK) araU ? Interfere with RNA synthesis araCMP Significant differences in patient sensitivity araCDP NDK G2 M competitive inhibitor S of DNA polymerase araCTP Cell Cycle S-phase Specific DNA-araC G1 Go 9  1/26/2024 Clinical Resistance to araC Decreased uptake via nucleoside transporters Increased activity of cytidine deaminase Decreased activity of dCK Change in the activity of DNA polymerase 10  1/26/2024 Cytarabine (araC) Acute myeloid leukemia (AML) – Together with an anthracycline drug Long, intravenous infusions (5-10 days) Rapid deamination and clearance S-phase specificity ∗Myelosuppression Gemcitabine (dFdC) Same activation pathway as araC dFdCTP incorporated into DNA – Inhibition of DNA polymerization dFdCDP inhibits ribonucleotide reductase – Irreversible inhibition – Diphosphate form binds to active site Activity in solid tumors – Pancreas, lung, breast ∗Myelosuppression 11  1/26/2024 Fludarabine Purine Analogues – nucleotide analogue – used mainly in CLL Inhibits various enzymes – DNA polymerase, Ribonucleotide reductase – DNA ligase, RNA polymerase – Prevents elongation of DNA strands through direct incorporation into DNA as a false nucleotide Active against both dividing and resting cells, but most cytotoxic during S-phase Profoundly immunosupprssive – used in non-myeloablative allogenic transplants Purine Analogues Nucleobases 42-10 Converted into fraudulent nucleotides Incorporated into DNA Inhibits various enzymes Used mainly for acute lymphoblastic leukemia 12  1/26/2024 Metabolism of Thiopurines Abbreviations: 6-MP, 6-mercaptopurine; 6-TG, 6-thioguanine; GMPS, guanosine monophosphate synthase; HPRT, hypoxanthine phosphoribosyltransferase; IMPDH, inosine monophosphate dehydrogenase; 6-TGN, thioguanine nucleotides; TIMP, thioinosine 5′- monophosphate; TPMT, thiopurine methyltransferase; XO, xanthine oxidase. Pharmacogenomics of TPMT Certain genetic variations within the TPMT gene can lead to decreased or absent TPMT enzyme activity Increased levels of TGN metabolites - increased risk of severe bone marrow suppression (myelosuppression) TPMT polymorphisms that result in decreased or absent TPMT activity occur with a frequency of approximately 5% TPMT genetic test can identify patients with reduced TPMT activity, allowing for the adjustment of 6-MP dose or avoidance of the drug entirely. 13