Clinical pharmacokinetics of allopurinol

Allopurinol is a widely used drug in the management of hyperuricaemia. It is rapidly and extensively absorbed following oral administration. The major and active metabolite, oxypurinol, is detected in the circulation within 15 minutes of allopurinol administration. Oxypurinol concentrations are higher than those of the parent drug and accumulation occurs during long term administration. Up to 80% of allopurinol is recovered in the urine within 24 hours, mainly in the form of oxypurinol. Allopurinol is negligibly absorbed after rectal administration. In animals, allopurinol is found in highest concentrations in vascular tissue, blood, liver, intestine and heart. It is negligibly bound to plasma proteins. Oxypurinol is eliminated by the kidney and has a much longer elimination half-life than allopurinol. Oxypurinol accumulates in patients with renal dysfunction; hence allopurinol dosages should be adjusted in such patients. Allopurinol inhibits the metabolism of 6-mercaptopurine and azathioprine, which require dosage modifications. The interaction of allopurinol with oral anticoagulants and phenytoin has not been clearly established in clinical practice.