Antimetabolites
Pharmacology · Chemotherapy · lean revision notes
Antimetabolites
Antimetabolites are structural analogues of normal metabolites (folate, purines, pyrimidines) that masquerade as the real substrate, jam nucleotide synthesis or DNA replication, and thereby kill rapidly dividing cells. They are classically S-phase (cell-cycle) specific and form the backbone of leukaemia, GI cancer, and autoimmune-disease therapy — a perennial NEET PG pharmacology favourite for its mechanism-overlap traps and antidote/rescue timing questions.
Definition & classification
An antimetabolite interferes with the metabolic pathway of an essential intermediate, most often a purine or pyrimidine nucleotide, blocking DNA/RNA synthesis. Because their effect is maximal during DNA synthesis, they are S-phase specific, cell-cycle specific (CCS) agents — a high-yield contrast to alkylating agents (cell-cycle non-specific).
| Class | Prototype drugs | Molecular target |
|---|---|---|
| Folate antagonists | Methotrexate, pemetrexed, pralatrexate | Dihydrofolate reductase (DHFR) ± thymidylate synthase |
| Pyrimidine analogues | 5-Fluorouracil, capecitabine, cytarabine, gemcitabine | Thymidylate synthase / DNA polymerase |
| Purine analogues | 6-Mercaptopurine, 6-thioguanine, fludarabine, cladribine, pentostatin | De-novo purine synthesis / adenosine deaminase |
High-yield: Antimetabolites are S-phase specific. Alkylating agents and platinum compounds are cell-cycle non-specific. Bleomycin is G2/M, vinca alkaloids are M-phase, etoposide is G2 — a commonly tested grid.
Methotrexate (MTX) — the folate antagonist
Mechanism
MTX is a folic acid analogue that competitively and reversibly inhibits dihydrofolate reductase (DHFR), the enzyme that converts dihydrofolate (FH2) to tetrahydrofolate (FH4). Loss of FH4 depletes the reduced-folate cofactors required for:
- Thymidylate synthesis (dUMP → dTMP) — the dominant cytotoxic effect → "thymineless death".
- Purine synthesis (steps requiring N10-formyl-FH4).
- Methionine/serine/glycine inter-conversions.
MTX is polyglutamated intracellularly, trapping it inside the cell and prolonging action; polyglutamates also directly inhibit thymidylate synthase and AICAR transformylase.
Folate flow: Dietary folate → DHFR → FH2 → DHFR → FH4 → cofactors → dTMP & purines → DNA. MTX blocks the DHFR step ⟶ no dTMP ⟶ DNA synthesis arrest.
Leucovorin (folinic acid) rescue — the classic exam point
Leucovorin is N5-formyl-FH4, already a reduced folate, so it bypasses the blocked DHFR step and rescues normal cells.
High-yield: Leucovorin must be given within ~24–42 hours of starting high-dose MTX. Beyond ~48 hours rescue fails because normal cells have already suffered irreversible damage — "rescue too late = no rescue". Tumour cells (poorly transporting reduced folate) are preferentially killed.
High-yield: Folinic acid (leucovorin), NOT folic acid, is the antidote — folic acid still requires DHFR (which is blocked) to be activated.
In MTX overdose / impaired clearance, glucarpidase (carboxypeptidase-G2) rapidly cleaves circulating MTX into inactive metabolites — the modern rescue for renal failure with toxic levels.
Pharmacokinetics & resistance
- Oral absorption saturable; renal excretion (avoid in renal failure & third-space fluid — ascites/pleural effusion act as a reservoir → delayed clearance → toxicity).
- Resistance: ↑DHFR (gene amplification), altered DHFR with low affinity, ↓polyglutamation, defective reduced-folate carrier (RFC) uptake.
Toxicity
Myelosuppression, oral/GI mucositis, hepatotoxicity (chronic → fibrosis/cirrhosis), pneumonitis (idiosyncratic, dose-independent), nephrotoxicity (precipitation in tubules — prevent with alkalinisation + hydration), and teratogenicity (abortifacient, folate antagonist → neural tube defects).
High-yield: NSAIDs, salicylates, sulfonamides, penicillins, probenecid ↓ MTX renal clearance / displace it → ↑ toxicity. Avoid co-administration.
Uses
Choriocarcinoma (curable with single-agent MTX), ALL (maintenance + CNS prophylaxis intrathecal), osteosarcoma (high-dose + leucovorin), NHL, breast; non-oncologic: rheumatoid arthritis, psoriasis, Crohn's, ectopic pregnancy, medical abortion (with misoprostol).
5-Fluorouracil (5-FU) — pyrimidine analogue
Mechanism
5-FU is a uracil analogue activated intracellularly to FdUMP, which forms a stable ternary complex with thymidylate synthase (TS) and the folate cofactor N5,N10-methylene-FH4, irreversibly inhibiting TS → blocks dUMP → dTMP → "thymineless death". It is also incorporated as FUTP into RNA and FdUTP into DNA, causing further damage.
High-yield: Leucovorin POTENTIATES (does NOT rescue) 5-FU — by increasing reduced folate it stabilises the FdUMP–TS ternary complex, enhancing TS inhibition. (Opposite of its rescue role with MTX — a favourite trick question.)
Key clinical points
- Capecitabine = oral prodrug converted to 5-FU; final activating enzyme thymidine phosphorylase is higher in tumour tissue → selectivity.
- DPD (dihydropyrimidine dehydrogenase) deficiency → severe, sometimes fatal 5-FU toxicity (mucositis, myelosuppression, neurotoxicity). Test before therapy.
- Toxicities: myelosuppression, mucositis/diarrhoea, hand-foot syndrome (palmar-plantar erythrodysesthesia, esp. capecitabine), cardiotoxicity (coronary vasospasm), and cerebellar ataxia.
- Uses: colorectal cancer (with leucovorin ± oxaliplatin/irinotecan — FOLFOX/FOLFIRI), breast, GI, head & neck; topical 5-FU for actinic keratosis, superficial BCC.
Cytarabine (ara-C, cytosine arabinoside)
Mechanism
A cytidine analogue (arabinose sugar). Activated by deoxycytidine kinase to ara-CTP, which is incorporated into DNA and inhibits DNA polymerase, terminating chain elongation. It is the most S-phase specific antimetabolite — schedule-dependent, needs continuous infusion.
High-yield: Cytarabine is the single most important drug for AML induction ("7+3" = cytarabine 7 days + daunorubicin 3 days). High-dose ara-C causes cerebellar toxicity (ataxia, nystagmus) and conjunctivitis (prevent with steroid eye drops).
- Inactivated by cytidine deaminase → ara-U (resistance: ↑deaminase or ↓deoxycytidine kinase).
- Gemcitabine is a related analogue (pancreatic, NSCLC, bladder, ovarian); inhibits ribonucleotide reductase and causes "masked chain termination".
6-Mercaptopurine (6-MP) & thiopurines
Mechanism
6-MP is a hypoxanthine analogue. It is a prodrug requiring activation by HGPRT (hypoxanthine-guanine phosphoribosyltransferase) to thio-IMP, which inhibits de-novo purine synthesis (pseudo-feedback inhibition of PRPP amidotransferase) and is incorporated as thioguanine nucleotides into DNA.
High-yield: Lesch-Nyhan syndrome (HGPRT deficiency) → 6-MP is ineffective because it cannot be activated. Classic single-best-answer.
Metabolism & the allopurinol interaction
Three competing pathways inactivate 6-MP:
- Xanthine oxidase (XO) → 6-thiouric acid (inactive).
- TPMT (thiopurine methyltransferase) → methylated metabolite.
- HGPRT → active nucleotides.
High-yield: Allopurinol/febuxostat inhibit xanthine oxidase, so co-administration shunts 6-MP toward the active pathway → marked ↑ toxicity. Reduce 6-MP dose to ~25% (one-quarter) when given with allopurinol. (Febuxostat with thiopurines is generally avoided.)
High-yield: TPMT deficiency (genetic polymorphism) → severe myelosuppression with standard 6-MP/azathioprine doses. Test TPMT before therapy.
- Azathioprine is a prodrug of 6-MP (immunosuppressant — transplant, IBD, autoimmune).
- 6-Thioguanine (6-TG) is not metabolised by XO, so it does not need dose reduction with allopurinol — a frequent distractor.
- Uses: 6-MP — ALL maintenance; 6-TG — AML.
Other purine analogues
| Drug | Distinctive feature / target | Main use |
|---|---|---|
| Fludarabine | Purine nucleotide analogue, inhibits DNA polymerase & ribonucleotide reductase | CLL (part of FCR) |
| Cladribine (2-CdA) | Resistant to adenosine deaminase; accumulates in lymphocytes | Hairy cell leukaemia (drug of choice) |
| Pentostatin | Adenosine deaminase (ADA) inhibitor → dATP accumulation, lymphotoxic | Hairy cell leukaemia, T-cell |
High-yield: Cladribine = drug of choice for hairy cell leukaemia. Pentostatin = ADA inhibitor (mimics genetic ADA-deficiency SCID lymphotoxicity).
Pemetrexed
A multitargeted antifolate inhibiting thymidylate synthase, DHFR and GAR formyltransferase. Used in mesothelioma (with cisplatin) and non-squamous NSCLC. Folic acid + vitamin B12 supplementation reduces its haematologic/GI toxicity — a tested supportive-care fact.
Diagnosis/monitoring & investigation of choice
- MTX: monitor serum MTX levels (high-dose protocols), serum creatinine, urine pH (keep >7), CBC, LFTs. Leucovorin dosing is titrated to serum MTX levels.
- Thiopurines: TPMT genotype/phenotype before initiation; monitor CBC.
- 5-FU/capecitabine: consider DPYD genotyping for DPD deficiency.
Management pearls / drug-of-choice grid
| Indication | Antimetabolite of choice |
|---|---|
| Choriocarcinoma | Methotrexate (single-agent curative) |
| Colorectal cancer | 5-Fluorouracil + leucovorin |
| AML induction | Cytarabine (7+3) |
| ALL maintenance | 6-Mercaptopurine + methotrexate |
| Hairy cell leukaemia | Cladribine |
| Pancreatic cancer | Gemcitabine |
| Mesothelioma | Pemetrexed + cisplatin |
| Rheumatoid arthritis / psoriasis | Methotrexate |
Complications & toxicity overview
- Common to all (S-phase): myelosuppression, mucositis, GI upset, alopecia (mild).
- MTX: hepatic fibrosis, pneumonitis, nephrotoxicity, megaloblastic anaemia.
- 5-FU: hand-foot syndrome, cardiotoxic vasospasm, cerebellar ataxia, DPD-related lethality.
- Cytarabine: cerebellar syndrome, conjunctivitis, "ara-C syndrome" (fever, myalgia).
- 6-MP/azathioprine: dose-dependent marrow suppression (worse with allopurinol/TPMT deficiency), hepatotoxicity.
- Tumour lysis syndrome: rapid leukaemia cell kill → hyperuricaemia, hyperkalaemia, hyperphosphataemia, hypocalcaemia → AKI. Prevent with hydration + rasburicase/allopurinol (remember the 6-MP dose-reduction caveat with allopurinol).
Key differentials / contrast points (don't confuse)
- Leucovorin role: RESCUES MTX toxicity but POTENTIATES 5-FU cytotoxicity.
- Folic vs folinic acid: Only folinic bypasses DHFR; folic acid is useless as MTX antidote.
- Allopurinol: ↓ dose of 6-MP/azathioprine required; 6-TG unaffected.
- HGPRT: needed to activate 6-MP; its deficiency (Lesch-Nyhan) → drug ineffective.
- DPD vs TPMT: DPD deficiency → 5-FU toxicity; TPMT deficiency → thiopurine toxicity.
Mnemonics
- "MAP-C" S-phase antimetabolites: Methotrexate, Antimetabolites generally, Purine/Pyrimidine analogues, Cytarabine.
- 5-FU + leucovorin = "Fuel the Fire" (potentiates). MTX + leucovorin = "Rescue".
- 6-MP + allopurinol → "cut to a quarter (¼)."
- Cladribine → "Claws" → Hairy cell leukaemia.
Recently asked / exam angle
- "Leucovorin must be administered within how many hours of high-dose methotrexate?" → ~24–36 (fails after ~48 h).
- "Antidote for methotrexate toxicity in renal failure with toxic levels?" → Glucarpidase (plus leucovorin).
- "Drug whose dose must be reduced to 25% when allopurinol is co-prescribed?" → 6-Mercaptopurine/azathioprine.
- "Mechanism of 5-FU?" → FdUMP inhibits thymidylate synthase (ternary complex with reduced folate).
- "Most S-phase specific anticancer agent / drug for AML induction?" → Cytarabine.
- "Antimetabolite ineffective in Lesch-Nyhan syndrome?" → 6-MP (no HGPRT).
- "Drug of choice for hairy cell leukaemia?" → Cladribine.
- "Enzyme deficiency causing fatal 5-FU toxicity?" → DPD (DPYD).
- "Single-agent curative chemotherapy for choriocarcinoma?" → Methotrexate.
- "Cerebellar toxicity with which antimetabolite (high dose)?" → Cytarabine (also 5-FU).
Rapid revision
- Antimetabolites are S-phase / cell-cycle specific; alkylators are cell-cycle non-specific.
- MTX inhibits DHFR → no FH4 → no dTMP → thymineless death; effect mainly on thymidylate synthesis.
- Leucovorin (folinic acid) rescues MTX by bypassing DHFR; give within ~24–42 h; folic acid won't work.
- Glucarpidase clears toxic MTX in renal failure.
- Avoid NSAIDs/sulfonamides/penicillins/salicylates with MTX (↑ toxicity); avoid in third-space fluid collections.
- 5-FU → FdUMP inhibits thymidylate synthase; leucovorin potentiates it; DOC region = colorectal cancer.
- Capecitabine = oral 5-FU prodrug (thymidine phosphorylase); causes hand-foot syndrome; DPD deficiency → lethal toxicity.
- Cytarabine (ara-CTP → DNA polymerase inhibition) is the most S-phase specific; AML 7+3; high dose → cerebellar toxicity.
- 6-MP needs HGPRT to activate → ineffective in Lesch-Nyhan; metabolised by XO + TPMT.
- Allopurinol (XO inhibitor) + 6-MP → reduce 6-MP to ¼ dose; 6-TG needs no reduction; TPMT deficiency → severe marrow suppression.
- Cladribine = DOC hairy cell leukaemia; pentostatin = ADA inhibitor; gemcitabine = pancreatic cancer.
- Pemetrexed (mesothelioma) — give folic acid + B12 to cut toxicity.