Effect of product inhibition on elimination kinetics of ethoxybenzamide in rabbits. Analysis by physiological pharmacokinetic model.

In our previous study [Lin, Sugiyama, Awazu, and Hanano: J. Pharmacokin. Biopharm. 10,649 (1982)], we successfully applied a physiological pharmacokinetic model to quantitative prediction of the elimination and distribution kinetics of ethoxybenzamide in rats and rabbits. The predictions of the time course of ethoxybenzamide concentrations in plasma were good at lower doses (10 and 20 mg/kg), whereas those at high dose (80 mg/kg) were poor. In the present study, therefore, product inhibition was suspected and examined. Product inhibition of ethoxybenzamide deethylation by its metabolite, salicylamide, was demonstrated both in vivo and in vitro studies. The plasma disappearances of ethoxybenzamide after a 20 mg/kg iv injection were determined both in the control and the salicylamide-treated rabbits. In the salicylamide-treated rabbits, the plasma disappearance of ethoxybenzamide was significantly delayed compared to that in control rabbits. This delay was quantitatively explained by the physiological pharmacokinetic model taking the competitive-type of product inhibition into consideration. The apparent dissociation constant for the salicylamide-enzyme complex in vivo was estimated as 0.14 mM. The inhibition of ethoxybenzamide de-ethylation by salicylamide was observed also in in vitro study using liver microsome of rabbits.