Mercaptopurine

Antineoplastic agent Purine analog (thiopurine)

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 1.5-2.5 mg/kg q24h or q48h Duration: Variable; often long-term for chronic conditions
Notes: Dose may be adjusted based on response and toxicity. Monitor CBC and liver enzymes.
Cat PO 1.5-2.5 mg/kg q24h or q48h Duration: Variable; often long-term
Notes: Cats may be more sensitive to myelosuppression; start at lower end of dose range.
Horse PO 1-2 mg/kg q24h Duration: Variable
Notes: Limited data; use with caution and monitor for toxicity.

Clinical Indications & Species Uses

General Indications
  • Immunosuppression for immune-mediated diseases
  • Antineoplastic therapy
Dog (Canine)
  • Treatment of immune-mediated diseases (e.g., inflammatory bowel disease, immune-mediated hemolytic anemia, immune-mediated thrombocytopenia)
  • Adjunct therapy for certain neoplasms (e.g., lymphoma, leukemia)
Cat (Feline)
  • Treatment of inflammatory bowel disease
  • Adjunct therapy for certain neoplasms (e.g., lymphoma)

Pharmacology & Mechanism of Action

Drug Class: Antineoplastic agent | Pharmacological Group: Purine analog (thiopurine)

Mechanism of Action: Mercaptopurine is a prodrug that requires intracellular activation to its active metabolites, 6-thioguanine nucleotides (6-TGNs). It inhibits de novo purine synthesis and incorporates into DNA and RNA, leading to cytotoxicity and immunosuppression. It also inhibits several enzymes involved in purine metabolism, including amidophosphoribosyltransferase, which disrupts nucleic acid synthesis.

Pharmacodynamics: Mercaptopurine is cell-cycle specific, acting primarily in the S-phase. It suppresses both humoral and cell-mediated immunity. In veterinary medicine, it is used for its immunosuppressive effects in conditions like inflammatory bowel disease and immune-mediated diseases, as well as for its antineoplastic activity in certain cancers.

⚡ Pharmacokinetics Summary

Absorption: Mercaptopurine is variably absorbed after oral administration; bioavailability is incomplete and subject to first-pass metabolism. Food can affect absorption.
Distribution: Widely distributed throughout body tissues; penetrates into cerebrospinal fluid but at low concentrations. Protein binding is approximately 19%.
Metabolism: Extensively metabolized in the liver and gastrointestinal tract. Key enzymes include xanthine oxidase, thiopurine methyltransferase (TPMT), and hypoxanthine-guanine phosphoribosyltransferase (HGPRT).
Excretion: Renal excretion of metabolites; a small amount is excreted unchanged. Elimination is primarily via urine.
Half-Life: Dogs: approximately 1.5 hours (parent drug); active metabolites have longer half-lives. Cats: not well documented.
Bioavailability: Variable, approximately 5-37% in humans; in dogs, oral bioavailability is about 10-20%.
Protein Binding: Approximately 19%

Available Formulations & Strengths

Oral Tablet 50 mg (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to mercaptopurine or any component
  • Severe bone marrow depression
  • Pregnancy (teratogenic potential)
  • Concurrent use with allopurinol (unless dose adjusted, as it inhibits metabolism)
Warnings & Clinical Precautions:
  • Myelosuppression is dose-limiting; monitor complete blood count regularly.
  • Hepatotoxicity can occur; monitor liver enzymes.
  • Immunosuppression may increase susceptibility to infections.
  • Use with caution in patients with renal or hepatic impairment.
  • May be carcinogenic in the long term.
  • Do not handle crushed tablets without protection; avoid contact with skin or inhalation.

Adverse Effects & Reactions

Common:

  • Myelosuppression (leukopenia, thrombocytopenia, anemia)
  • Gastrointestinal upset (vomiting, diarrhea, anorexia)
  • Elevated liver enzymes

Serious / Severe:

  • Severe bone marrow suppression
  • Hepatotoxicity (cholestasis, necrosis)
  • Pancreatitis
  • Secondary infections

Rare:

  • Pulmonary fibrosis
  • Teratogenicity
  • Carcinogenesis

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Allopurinol Inhibits xanthine oxidase, reducing metabolism of mercaptopurine, leading to increased toxicity. Reduce mercaptopurine dose to 25-33% of usual dose. High
Warfarin May decrease anticoagulant effect; monitor prothrombin time. Moderate
Aminosalicylates (e.g., mesalamine, sulfasalazine) May inhibit TPMT, increasing risk of myelosuppression. Moderate
Other immunosuppressive agents (e.g., azathioprine, cyclosporine) Additive immunosuppression and myelosuppression. Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Severe myelosuppression (pancytopenia)
  • Gastrointestinal toxicity (vomiting, diarrhea)
  • Hepatotoxicity
  • Potential for secondary infections

Emergency Treatment Protocol: Discontinue drug immediately. Provide supportive care including IV fluids, antiemetics, and blood transfusions if needed. Consider granulocyte colony-stimulating factor (G-CSF) for severe neutropenia. There is no specific antidote; hemodialysis may be considered in severe cases.

Food Animal Withdrawal Times

Not approved for food animals; no withdrawal times established. Use in food animals is prohibited or requires extended withdrawal periods.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (15-30°C)

Handling & Special Conditions: Protect from moisture; keep in tightly closed container.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for human use; not FDA-approved for veterinary use, but used off-label in veterinary medicine.

Extra-Label (Off-Label) Use: In the US, extra-label use in food animals is prohibited due to lack of withdrawal times and potential for residues. In companion animals, extra-label use is permitted under AMDUCA with appropriate veterinary oversight.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Mercaptopurine is an immunosuppressive and antineoplastic agent used in veterinary medicine primarily for immune-mediated diseases and certain cancers. It is a prodrug that requires intracellular activation. Close monitoring of hematologic parameters and liver function is essential due to significant toxicity. Dose adjustments may be necessary when used with allopurinol. The drug is not commonly used in food animals due to lack of withdrawal data. In cats, caution is advised due to potential sensitivity. Always handle with care to avoid exposure to the drug.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)