The effect of fenfluramine on disposition and rate of antipyrine elimination.

The effect of fenfluramine, administered orally in a daily dose of 1 mg/kg for 40 days, on the disposition and rate of elimination of antipyrine was studied in 15 obese patients. Although the plasma half-life of antipyrine was unchanged, the apparent volume of distribution (1/kg) fell by 11.6% (p less than 0.001) and the plasma metabolic clearance rate (1/kg/h) of antipyrine was reduced by 14.1% (p less than 0.01). No correlation occurred between change in clearance, on the one hand, and change in patient weight or apparent volume of distribution of antipyrine, on the other. In vitro drug-metabolizing enzyme activity in the rat was measured using rate of aminopyrine and hexobarbital metabolism as indices. Fenfluramine inhibited the metabolism of both substrates. It is concluded that fenfluramine can diminish the rate of elimination of drugs which are extensively metabolized by reduction of microsomal enzyme activity. Also, drug clearance may be diminished by reduction of apparent volume of distribution.