A patient with gout on allopurinol requires azathioprine for IBD. Which dose adjustment is mandatory and what is the mechanism?

• A Reduce azathioprine to 25–33% of standard dose; allopurinol inhibits xanthine oxidase, which is required to metabolize azathioprine's active metabolite 6-mercaptopurine to inactive 6-thiouric acid — causing 6-MP and 6-thioguanine nucleotide accumulation and severe myelotoxicity ✓
• B Increase allopurinol dose; azathioprine decreases allopurinol bioavailability via intestinal CYP3A4 induction
• C No adjustment needed; drugs have no pharmacokinetic interaction
• D Avoid the combination entirely; substitute febuxostat for allopurinol instead

Explanation

Azathioprine is a prodrug converted to 6-mercaptopurine (6-MP), which is metabolized by xanthine oxidase (XO) to inactive 6-thiouric acid. Allopurinol inhibits XO, blocking 6-MP catabolism; this drives accumulation of 6-MP and cytotoxic 6-thioguanine nucleotides (6-TGN), causing profound bone marrow suppression and hepatotoxicity. The azathioprine dose must be reduced to 25–33% of normal. Febuxostat (selective XO inhibitor) poses the same risk — the combination is equally dangerous and requires the same dose reduction; it cannot be substituted safely without dose adjustment.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.