metabolism, mostly to oxypurinol. Oxypurinol is excreted slowly in the urine by glomerular filtration, counterbalanced by some tubular reabsorption. The plasma t/2 of allopurinol is -1-2 hours and of oxypurinol -18-30 hours (longer in those with renal impairment). This allows for once-daily dosing and makes allopurinol the most commonly used antihyperuricemic agent.
Allopurinol and its active metabolite oxypurinol are distributed in total tissue water, with the exception of brain, where their concentrations are about one-thirds of those in other tissues. Neither compound is bound to plasma proteins. The plasma concentrations of the two compounds do not correlate well with therapeutic or toxic effects.
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