Phenazone (antipyrine) metabolism and distribution in young and elderly adults

SUMMARY 1. The plasma half‐life and apparent volume of distribution of phenazone (antipyrine) was studied in twenty‐six normal adults (20–40 years of age) and twenty‐six geriatric patients. 2. A wide range of values for plasma half‐life was recorded in both groups (young 7·7–19·9 h, elderly 9·5–30·1 h) but the mean plasma half‐life for the geriatric group (16·8 h; s.e.m. = 5·9) was significantly longer ( P < 0·01) than the normal adult group (12·5 h; s.e.m. = 3·2). 3. No difference in plasma half‐life was noted between sexes in either the geriatric or control groups. 4. Doubling the dose of phenazone from 10 to 20 mg/kg made no difference to the plasma half‐life in a subgroup of the geriatric patients. 5. The volume of distribution of phenazone, representing total body water, was 0·59–0·63 l/kg in all groups except the elderly females, in whom it was 0·48 l/kg. This latter group showed a correspondingly higher plasma concentration extrapolated to time zero. 6. The difference in plasma half‐life phenazone between normal adult and elderly groups is statistically significant, but the wide range of values in each group makes the biological significance less clear. Alteration of drug distribution with resultant higher plasma or tissue concentrations may contribute to adverse drug effects, particularly in the elderly female.