Metabolic rate of phenacetin and of paracetamol in dogs before and after treatment with phenobarbital or skf 525 A

The effect of induction with phenobarbital on the kinetics and bioavailability of antipyrine in the dog

In dogs, the chronic administration of 400 mg/d of phenobarbital produced peak plasma concentrations of 78 micrograms/ml which so markedly induced the metabolism of antipyrine that the conventional schemes by which antipyrine kinetics assess induction were rendered invalid. The bioavailability of antipyrine was only 20% due to first-pass metabolism. Induction raised the observed antipyrine clearance to 31 ml/min/kg, a value approaching hepatic blood flow. If one assumes antipyrine was an ideal test compound, one would calculate that metabolism was induced by a factor of 3.8. Acknowledging that antipyrine is nonideal, one can calculate a correct induction factor of 13.6. Oral, rather than intravenous, antipyrine is the metabolism test of choice under these conditions.

Antipyrine estimations in the rabbit using gas--liquid chromatography: a reliable method for studying factors affecting oxidative drug metabolism

The application of a gas--liquid chromatographic (GLC) method to measurements of antipyrine in plasma for the estimation of antipyrine half-life (T1/2) in the rabbit is described and is compared with previous GLC methods. It has been established that the estimation of antipyrine can be achieved with acceptable precision and that rabbits can act as their own controls in the study of factors that might alter T1/2. Iproniazid phosphate, 50 mg/kg IP, was shown to produce a mean increase of 170% in T1/2.