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Drug Interaction Azathioprine — ESENeph MCQ

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HardRenal VasculitisDrug Interaction AzathioprineESENeph

A 65-year-old man with ANCA-associated vasculitis is in remission on maintenance Azathioprine and low-dose Prednisolone. He develops allopurinol-induced rash and his GP wants to check interactions. Why is the Azathioprine-Allopurinol combination dangerous?

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Correct answer: D — Allopurinol inhibits xanthine oxidase, blocking Azathioprine (6-MP) metabolism, causing severe myelosuppression

Azathioprine is metabolised to 6-mercaptopurine (6-MP), which is inactivated by xanthine oxidase. Allopurinol inhibits xanthine oxidase, dramatically increasing 6-MP levels and causing severe and potentially fatal myelosuppression. If co-administration is unavoidable, the Azathioprine dose must be reduced by at least 50-75% with close FBC monitoring. This is one of the most important drug interactions in renal medicine.

Reference: BNF 2024 – Azathioprine Interactions; KDIGO 2024 – AAV Guideline