What is known and objective: Allopurinol (AP) inhibits the xanthine oxidase, which may indirectly lead to myelotoxicity when used in combination with azathioprine (AZA).
Case summary: A 79-year-old female developed symptomatic thrombocytopenia after combination therapy with AZA (75 mg/day) and AP (100 mg/day) – after AP had been stopped. Concentrations of the myelotoxic 6-thioguanine-nucleotides metabolite of AZA were increased. Thrombocyte counts normalized within 8 days of discontinuation of AZA.
What is new and conclusion: The effect of a drug interaction in a patient with decreased elimination capacity may take several weeks to become apparent and may in fact do so even after the drug has been stopped. Concurrent AZA and AP therapy demands cautious use.