Studies of the different metabolic pathways of antipyrine in man. Oral versus i.v. administration and the influence of urinary collection time.

The pharmacokinetics of antipyrine in plasma and saliva, and urinary excretion of its major metabolites, were studied following i.v. and oral administration of antipyrine 500 mg to 6 healthy volunteers. Data from both plasma and saliva showed that the oral bioavailability of antipyrine given as an aqueous solution was complete. The saliva/plasma concentration ratio was constant with time from about 3 h onwards, with a mean value of 0.87 after oral and 0.91 after i.v. administration. It is concluded that the pharmacokinetic parameters of antipyrine can be satisfactorily established on the basis of salivary data, although the volume of distribution and clearance values are then slightly too high. After i.v. administration, 3.8 +/- 1.9% of the dose was excreted in urine as unchanged antipyrine in 48 h, 24.9 +/- 6.3% as 4-hydroxyantipyrine, 16.5 +/- 3.2% as norantipyrine, 13.0 +/- 2.2% as 3-hydroxymethyl-antipyrine and 5.8 +/- 1.0% as 3-carboxy-antipyrine. No significant differences were observed following oral administration. The half-lives calculated from the linear part of the urinary excretion rate curves of the metabolites were about the same for oral and i.v. administration, and were of the same order of magnitude as the elimination half-life of parent drug in plasma and saliva. It is important for determination of the ultimate metabolite ratio that urine is collected for at least 36 h, because there is a delay in the excretion of 3-hydroxymethyl-antipyrine in urine.