The disposition of antipyrine and its metabolites in young and elderly healthy volunteers.

1 Oxidative metabolism of antipyrine (AP) was compared in 11 elderly (greater than 65 years) and 12 young (less than 40 years) volunteers. All subjects were non-smokers, consumed little if any alcohol and were in good health. 2 After a single dose of AP 500 mg, its clearance from saliva and profiles of the parent drug and its major metabolites in urine were determined using high-performance liquid chromatography. 3 Mean total AP clearance from saliva was lower in the elderly (P less than 0.05). Mean weight-normalised volume of distribution was also smaller (P less than 0.01) so that elimination half-life in the elderly was not significantly different from that in the young. 4 The percentage dose excreted in 48 h urine as norantipyrine (NORA) and its clearance for production were lower in the elderly (P less than 0.001 and P less than 0.01 respectively). Urinary 3-hydroxymethylantipyrine (HMA) and free antipyrine were present in greater quantities in 48 h urine in the elderly (P less than 0.001 and P less than 0.05) while the amounts of 4-hydroxyantipyrine (OHA) were almost identical in the two age groups. 5 The findings suggest that there is a selective impairment of N-demethylation in the elderly which may have important implications for dosage of elderly patients with drugs metabolised by this route.