Phenylbutazone

Abstract

Phenylbutazone is a potent non-steroidal anti-inflammatory drug which may occasionally precipitate cardiac failure by causing fluid retention or may rarely cause agranulocytosis and aplastic anaemia. Use is now therefore limited to hospital treatment of ankylosing spondylitis when other therapy has failed. It is readily absorbed orally or rectally and binds strongly (98%) to plasma proteins. The major metabolic changes are 4-C-glucuronidation and 3- or 4-hydroxylation. The latter forms the active metabolite, oxyphenbutazone. Excretion is slow with some 2/3 of metabolites appearing in urine and 1/3 in faeces. The half-life of phenylbutazone in plasma is 2-5 days and of oxyphenbutazone 2-3 days. Half-life increases in the elderly and in those with renal failure. The therapeutic concentration in plasma is usually 50-100 μg/ml with peak concentrations occurring 1-7 hours after dosing. Toxic effects are associated with plasma concentrations above 100 μg/ml.