Oncology · Module 3 of 6

Antimetabolites

Folate antagonists, fluoropyrimidines, cytidine analogs, purine analogs, and hypomethylating agents

Antimetabolites are structural analogs of normal cellular substrates — they interfere with DNA synthesis and repair by mimicking purines, pyrimidines, or folate cofactors. All are S-phase specific, making prolonged exposure schedules (continuous infusion, repeated dosing) more effective than bolus administration for most agents in this class.
Methotrexate inhibits dihydrofolate reductase, depleting tetrahydrofolate and halting both thymidylate and purine synthesis. Intracellular polyglutamation by FPGS traps the drug for weeks and extends inhibition beyond plasma half-life. High-dose methotrexate requires leucovorin rescue starting 24–42 hours after infusion and continued until plasma levels fall below 0.1 µmol/L; NSAIDs and proton pump inhibitors reduce renal clearance and can cause 2–10-fold level elevation. Third-space fluids (ascites, pleural effusions) must be drained before high-dose administration.
5-Fluorouracil toxicity is schedule-dependent: bolus dosing causes myelosuppression via RNA disruption, while continuous infusion causes mucositis and hand-foot syndrome via thymidylate synthase inhibition. Leucovorin stabilizes the ternary complex with thymidylate synthase and approximately doubles response rates with infusional 5-FU. Dihydropyrimidine dehydrogenase (DPD) catabolizes more than 85% of 5-FU — complete DPD deficiency causes fatal toxicity at standard doses; DPYD genotyping or plasma uracil level testing is recommended before first administration.
Cytarabine (ara-C) is the cornerstone of AML therapy, administered as a 7-day continuous infusion to maximize exposure of leukemic blasts traversing S phase. High-dose cytarabine causes cerebellar toxicity (ataxia, dysarthria) — neurological examination before each cycle is mandatory. Gemcitabine is self-potentiating: its diphosphate metabolite inhibits ribonucleotide reductase, reducing dCTP competition and increasing gemcitabine triphosphate incorporation.
Two mandatory pharmacogenomic screens govern safe antimetabolite prescribing: TPMT status before 6-mercaptopurine (low TPMT patients require only 10–20% of standard dose; standard dosing causes fatal aplasia; allopurinol co-administration quadruples levels by blocking xanthine oxidase metabolism) and DPD status before any fluoropyrimidine. Fludarabine causes profound CD4 lymphopenia lasting months to years — Pneumocystis jirovecii pneumonia and herpesvirus prophylaxis are required throughout and after therapy.

5-FU = 5-fluorouracil · ALL = acute lymphoblastic leukemia · AML = acute myeloid leukemia · ara-C = cytarabine · CLL = chronic lymphocytic leukemia · DHF = dihydrofolate · DHFR = dihydrofolate reductase · DPD = dihydropyrimidine dehydrogenase · FCR = fludarabine + cyclophosphamide + rituximab · FdUMP = fluorodeoxyuridine monophosphate · FPGS = folylpolyglutamate synthase · HMA = hypomethylating agent · MDS = myelodysplastic syndrome · MTX = methotrexate · THF = tetrahydrofolate · TPMT = thiopurine methyltransferase · TS = thymidylate synthase · XO = xanthine oxidase

Methotrexate — Mechanism and Rescue

Mechanism

DHFR Inhibition & Polyglutamation

MTX competitively inhibits DHFR: DHF cannot be reduced to THF → thymidylate and purine synthesis both halted (S-phase specific)
FPGS polyglutamates MTX intracellularly → traps drug for weeks beyond plasma clearance; polyglutamates also directly inhibit TS and purine synthesis enzymes
Pemetrexed: multi-targeted antifolate (DHFR + TS + GARFT); requires folic acid + vitamin B12 pre-supplementation to reduce toxicity without reducing antitumor effect

High-Dose Safety

Rescue Protocol & Drug Interactions

Leucovorin rescue: start 24–42 h after MTX infusion; continue until plasma MTX <0.1 µmol/L; bypasses DHFR block by providing reduced folate directly
Glucarpidase: enzymatic MTX cleavage; reserved for severe toxicity or failed leucovorin rescue
NSAIDs + proton pump inhibitors reduce renal MTX clearance → 2–10× level elevation → mucositis, myelosuppression, nephrotoxicity
Third-space fluids (ascites, pleural effusions): drain before high-dose MTX — fluid acts as a reservoir releasing drug long after infusion ends

5-Fluorouracil — Schedule-Dependent Toxicity

Feature	Bolus 5-FU	Continuous Infusion 5-FU
Dominant mechanism	RNA disruption via FUTP incorporation into RNA	TS inhibition via FdUMP ternary complex with leucovorin-stabilized THF
Dose-limiting toxicity	Myelosuppression	Mucositis, hand-foot syndrome (palmar-plantar erythrodysesthesia)
Leucovorin benefit	Less pronounced	Stabilizes ternary complex with FdUMP and TS → ~2× response rate improvement
DPD role	DPD catabolizes >85% of administered 5-FU. Complete DPD deficiency → fatal toxicity within days of first dose at standard doses. Partial deficiency (~3–5% of patients) → severe grade 3–4 toxicity. Screen with DPYD genotyping or plasma uracil level before first administration.
Oral prodrug	Capecitabine: converted to 5-FU preferentially in tumor tissue by thymidine phosphorylase; inhibits CYP2C9 → warfarin interaction (INR rises; monitor weekly during capecitabine)

Cytidine Analogs and Purine Analogs

Cytidine Analog

Cytarabine (ara-C)

S-phase specific; 7-day continuous infusion in AML induction ensures exposure of all blasts traversing S phase
Activated by deoxycytidine kinase (dCK); loss of dCK is the primary resistance mechanism
High-dose ara-C: cerebellar toxicity (ataxia, dysarthria, nystagmus) — neurological check before each cycle; discontinue if cerebellar signs develop
Liposomal cytarabine (DepoCyt): intrathecal use for leptomeningeal disease

Cytidine Analog

Gemcitabine

Self-potentiating: gemcitabine diphosphate inhibits ribonucleotide reductase → depletes dCTP → reduces competition for DNA polymerase → more gemcitabine triphosphate incorporated
Indications: pancreatic cancer, non-small cell lung cancer, bladder, ovarian (often in combination with platinum)
Toxicities: thrombocytopenia, pulmonary toxicity (interstitial pneumonitis), rare hemolytic uremic syndrome

Purine Analogs

Fludarabine & Cladribine

Fludarabine: CLL first-line (FCR regimen); causes profound CD4 depletion persisting months to years after completion
Cladribine: hairy cell leukemia; single 7-day continuous infusion course → >90% complete remission rate
Both require Pneumocystis jirovecii pneumonia and herpesvirus prophylaxis throughout and after therapy
Fludarabine: stop immediately if autoimmune hemolytic anemia develops — can be fatal if continued

Pharmacogenomics — Two Mandatory Pre-Treatment Screens

Before 6-Mercaptopurine or Azathioprine

TPMT Status

TPMT metabolizes 6-mercaptopurine to inactive methylated metabolites; low TPMT → toxic thioguanine nucleotides accumulate
Normal TPMT (~90%): standard dose
Intermediate TPMT (~10%): 30–50% dose reduction required
Low or absent TPMT (~0.3%): 10–20% of standard dose — standard dosing causes life-threatening aplasia
Allopurinol (XO inhibitor) + 6-mercaptopurine: blocks alternative catabolism → fourfold level rise → aplasia; reduce 6-MP to 25% of dose

Before 5-FU or Capecitabine

DPD Status

DPD catabolizes >85% of 5-FU; deficiency allows drug to accumulate to toxic levels
Complete DPD deficiency: fatal toxicity (severe mucositis, myelosuppression, neurotoxicity) within days of first standard dose
Partial deficiency (~3–5% of population): severe grade 3–4 toxicity in ~25–30%
Screen with DPYD genotyping or plasma uracil level before first fluoropyrimidine dose; dose-reduce based on phenotype

Clinical Pearl

Hypomethylating Agents — Azacitidine and Decitabine

Azacitidine and decitabine are cytidine analogs that incorporate into DNA and covalently trap DNA methyltransferase, depleting the enzyme and resulting in global DNA hypomethylation. Re-expression of silenced tumor suppressor genes is the proposed mechanism of clinical benefit in myelodysplastic syndrome and AML. Azacitidine also incorporates into RNA, contributing additional cytotoxic effects. Both agents are used for higher-risk MDS and unfit AML, with azacitidine demonstrating an overall survival benefit over conventional care in the AZA-001 trial. Myelosuppression is the primary dose-limiting toxicity; responses are typically gradual and may require 4–6 cycles before assessment.

References

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