mercaptopurine: clinical use

Prescribing considerations

• Initiate and supervise treatment through clinicians experienced in the relevant malignancy or inflammatory condition.
• Assess renal and hepatic function, infection risk, vaccination history and interacting medicines before treatment.
• TPMT assessment is required before starting in routine monitoring guidance; consider NUDT15 testing where clinically appropriate. Neither replaces blood-count monitoring.
• Use particular caution with other myelosuppressive, immunosuppressive or hepatotoxic treatments.

Contraindications and cautions

• Hypersensitivity to mercaptopurine or any tablet excipient.
• Concomitant yellow fever vaccination.
• The selected lactose-containing tablet should not be used in people with specified rare hereditary disorders of galactose metabolism or absorption.

Monitoring

• Baseline full blood count, liver function, renal function and TPMT status; consider viral screening, varicella immunity and NUDT15 genotype according to clinical context.
• Repeat full blood count, liver function and renal tests after initiation or treatment changes and throughout therapy, with intensity determined by indication and protocol.
• Respond promptly to falling cell counts, abnormal liver tests, infection, mucosal reactions, bruising, bleeding or pancreatitis.

Clinical pharmacology

Mercaptopurine is an inactive purine-antagonist prodrug converted intracellularly to thioguanine nucleotides. Metabolism involves TPMT, xanthine oxidase, HPRT and other enzymes, producing substantial interpatient variability and pharmacogenetic susceptibility to myelosuppression.

Formulation and product differences

• The selected product is a pale-yellow, scored tablet containing lactose; interpret the score and suitability for division according to the prescription and product information.
• Oral liquid products also exist, particularly for children, but their administration, measurement, storage and handling instructions differ from tablets.
• Mercaptopurine is cytotoxic: avoid crushing or unnecessary handling, use appropriate precautions and arrange safe disposal.

Tablet

Route: Oral

Strengths: 50 mg