Allopurinol, in contrast to the uricosuric drugs, reduces serum urate levels through a competitive inhibition of uric acid synthesis rather than by impairing renal urate reabsorption. This action is accomplished by inhibiting xanthine oxidase, the enzyme involved in the metabolism of hypoxanthine and xanthine to uric acid. After enzyme inhibition, the urinary and blood concentrations of uric acid are greatly reduced and there is a simultaneous increase in the excretion of the more soluble uric acid precursors, xanthine and hypoxanthine.
Allopurinol itself is metabolized by xanthine oxidase to form the active metabolite oxypurinol, which tends to accumulate after chronic administration of the parent drug. This phenomenon contributes to the therapeutic effectiveness of allopurinol in long-term use. Oxypurinol is probably responsible for the antigout effects of allopurinol. Oxypurinol itself is not administered because it is not well absorbed orally.
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