Pharmacological Class
- Antimetabolite (purine analogue, thiopurine)
- Cell cycle specific: S-phase
Mechanism of Action
- 6-MP is a prodrug, activated by hypoxanthine-guanine phosphoribosyltransferase (HGPRT) → thioinosine monophosphate (T-IMP).
- T-IMP:
- Inhibits de novo purine synthesis (blocks AMP and GMP synthesis).
- Incorporates into DNA and RNA → faulty nucleic acids.
- Result: inhibition of cell replication, especially in rapidly dividing lymphoblasts.
Indications
- Acute lymphoblastic leukemia (ALL) – maintenance therapy (commonly with methotrexate, vincristine, steroids)
- Occasionally in AML protocols (less common)
- Off-label: Crohn’s disease, ulcerative colitis (as immunosuppressant, though azathioprine more common)
Dosing (Oncology Use)
- Adults/Pediatrics (ALL maintenance):
- 50–75 mg/m² PO once daily (sometimes flat dosing ~50 mg/m² daily in peds).
- Administer consistently at the same time daily, usually in the evening.
- Often combined with methotrexate (synergy).
Do not administer with milk or dairy (xanthine oxidase in milk can degrade the drug).
Pharmacogenomics
- Metabolized by TPMT (thiopurine methyltransferase) and NUDT15.
- Genetic polymorphisms (low or absent enzyme activity) → ↑ risk of life-threatening myelosuppression.
- Guidelines recommend TPMT and/or NUDT15 testing before therapy.
Drug Interactions
- Allopurinol / febuxostat (xanthine oxidase inhibitors):
- Inhibit 6-MP metabolism → ↑ toxicity.
- Must reduce 6-MP dose to ~25–33% if used with allopurinol.
- Avoid concomitant azathioprine (metabolized to 6-MP).
- Warfarin (↓ anticoagulant effect).
Toxicities / Adverse Effects
Dose-limiting:
- Myelosuppression (neutropenia, anemia, thrombocytopenia)
Other important toxicities:
- Hepatotoxicity (cholestasis, ↑ bilirubin, transaminases, veno-occlusive disease in rare cases)
- GI: nausea, vomiting, anorexia, diarrhea, mucositis
- Rare: pancreatitis, rash
Monitoring
- CBC with differential (weekly initially, then monthly during maintenance)
- LFTs (weekly–monthly)
- TPMT/NUDT15 genotyping or phenotyping before therapy if available
- Signs of infection or bleeding
Clinical Pearls
- Take consistently on an empty stomach (1 hr before or 2 hrs after meals; food can reduce absorption).
- Evening dosing improves efficacy in ALL maintenance (circadian biology of DNA repair).
- Adjust dose in TPMT or NUDT15 deficiency (start at 10% of dose or avoid in homozygous deficient).
- Combination with methotrexate is synergistic in ALL maintenance.
- Counsel patients on strict adherence — noncompliance is a major cause of relapse in ALL.
Summary for practice:
6-MP is a purine analogue antimetabolite, used mainly in ALL maintenance. Activated by HGPRT → blocks purine synthesis & incorporates into DNA/RNA. Myelosuppression is dose-limiting, hepatotoxicity is common, and pharmacogenomics (TPMT, NUDT15) strongly influence safety. Requires CBC/LFT monitoring, dose reduction with xanthine oxidase inhibitors, and patient education on adherence + timing.