The bioavailability of phenylbutazone in the horse

1. [phenyl-¹⁴C]-Phenylbutazone was administered to 2 horses p.o. and i.v. on separate occasions. Plasma levels and urinary and faecal elimination of ¹⁴C were monitored for up to 7 days after dosing. 2. Phenylbutazone was rapidly and extensively absorbed after oral administration, and its bioavailability was 91% assessed by comparison of plasma AUCs of unchanged drug after p.o. and i.v. administration. 3. The plasma elimination half-life of phenylbutazone was 9.7 h and this was independent of the route of administration. 4. The pattern of elimination of phenylbutazone was independent of the route of administration, with 55% of the dose being found in the urine in 3 days and a further 39% in the faeces in 7 days. 5. These data, which are the first reports of the absolute bioavailability and excretion pathways of phenylbutazone in the horse, are discussed in terms of their significance for the gastrointestinal toxicity of this drug.