Allopurinol plasma proteins

Allopurinol is largely absorbed after oral ingestion, reaching peak blood levels in about 1 hour. In contrast to the uricosuric drugs, allopurinol is not appreciably bound to plasma proteins and is only a minor substrate for renal secretory mechanisms. The formation of oxypurinol and the finding that this metabolite is in part actively reabsorbed in the proximal tubule account for the long half-life of the metabolite (18-20 hours) and permits once-a-day drug administration. 

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. 

Protein Binding. In plasma, allopurinol less than 5%, oxypurinol about 17%. 

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