Pharmacology · Class
Xanthine oxidase inhibitors !
Drugs that cut uric acid production by blocking the last enzyme of purine breakdown. The long-term treatment of gout.
Updated 2026-11-01
What they are
Allopurinol and febuxostat lower serum urate so that urate crystals dissolve and gout flares, tophi and uric acid stones become less frequent. They prevent attacks over months; they do not treat an attack that has already started.
Mechanism of action
Xanthine oxidase turns hypoxanthine into xanthine and xanthine into uric acid, the end product of purine breakdown in humans. Allopurinol is a hypoxanthine analogue whose active metabolite, oxipurinol, stays bound to the enzyme's molybdenum centre; febuxostat is not a purine and blocks the channel that leads to the active site. Hypoxanthine and xanthine are more soluble, are reused or excreted, and urate falls.
Target: Xanthine oxidase (xanthine oxidoreductase)
Drugs in this class
- AllopurinolThe classic tablet that lowers uric acid in gout and in cancer treatment. A rash is a stop sign: rare but severe skin…Xanthine oxidase
- FebuxostatA non-purine xanthine oxidase blocker for gout when allopurinol fails or cannot be used. It carries a warning about…Xanthine oxidase
Test yourself
Think of the answer, then tap the question to see it.
Why can a gout flare happen in the first weeks of allopurinol?
Urate levels change and crystals are mobilised from tissue deposits, which triggers inflammation; flare prophylaxis is given at the start.
Which immunosuppressant becomes dangerous with a xanthine oxidase inhibitor?
Azathioprine (and mercaptopurine): xanthine oxidase inactivates 6-mercaptopurine, so its active metabolites build up and suppress the marrow.
How does febuxostat differ chemically from allopurinol?
Febuxostat is not a purine analogue; allopurinol is a hypoxanthine analogue converted to oxipurinol.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - ALLOPURINOL tablet (DailyMed (U.S. National Library of Medicine), 2023); DailyMed - FEBUXOSTAT tablet (DailyMed (U.S. National Library of Medicine), 2023); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 42 (McGraw Hill, 2023).
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Representative model, for educational purposes only.