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Pharmacokinetics and plasma binding of diazepam in man, dog, rabbit, guinea pig and rat

The pharmacokinetics and plasma binding of diazepam were compared in man, dog, rabbit, guinea pig and rat. Diazepam (D) and its major metabolite, desmethyl-diazepam, were measured in blood and plasma by a specific and sensitive gas-liquid chromatography assay with an electron capture detector. After an intravenous bolus injection plasma levels of D declined biexponentially in all species examined and the data were analyzed according to the two-compartment open model. The binding of D and desmethyldiazepam has been determined at therapeutic concentrations by equilibrium dialysis in man (96.8 and 96.6%, respectively), dog (96.0 and 95.9%), rabbit (89.9 and 94.7%), guinea pig (91.3 and 78.6%) and rat (86.3 and 90.5%). In man, the elimination half-life, T1/2(beta), increased significantly (P less than .01) with decreasing total plasma clearance (Cl). Plasma binding affected Vd and Cl, but only Cl increased significantly (P less than .05), if more free D was available. This indicates that unbound drug is rate-determining for clearance by the liver, and that D fits into the restrictive elimination class in man. In the four animal species tested, Cl was a direct linear function of the body surface area. T1/2(beta) and the rates of drug clearance were characteristic figures for each species: from 1.1 hours and 81.6 ml/min/kg in the rat to 32.9 hours and 0.35 ml/min/kg in man, whereas T1/2(alpha), the half-life of distribution, varied only approximately 3-fold (0.3-1.0 hours) in the different species. A considerably higher extraction ratio than the unbound fraction of diazepam exists in these animal species, and blood clearance exceeds liver blood flow, giving reason to assume a much higher ability of the liver to metabolize D, and a species-dependent extrahepatic metabolism. The large variations described suggest that pharmacokinetic data or plasma binding results cannot simply be extrapolated to man.