Relation between time courses of pharmacological effects and of plasma levels of camazepam and its active metabolites in rats.

Camazepam (CZ), a benzodiazepine, was biotransformed to more than ten metabolites. After intravenous and oral administration of these metabolites to rats, CZ, tempazepam (TZ), oxazepam (OZ), and hydroxy CZ (M5) were found to possess pharmacological activities. The brain-to-plasma concentration ratios of CZ and these active metabolites were essentially constant with time after oral administration of CZ. Thus the brain, target organ, was kinetically included in the plasma compartment. The extent of binding of these compounds to plasma protein was independent of concentration tested. Plasma levels of an unchanged drug and its active metabolite(s), and muscle relaxation effect (the impairment of rota rod performance) were measured at 0.5 to 8 h after oral administration of 2 to 3 doses of CZ, TZ, and OZ to rats. When the effect and plasma level data were computer-fitted to a simple Hill equation or a modified Hill equation including competitive factors, the modified Hill equation was found to be adequately applicable to the concentration-effect relation. The parameter values thus obtained could predict the contribution of the administered drug and its active metabolite(s) to the observed pharmacological effect after administration.