Disposition, distribution, plasma protein binding and biliary excretion of pinazepam after i.p. administration to rat

The pharmacokinetics, distribution, plasma protein binding and the biliary excretion of pinazepam were studied in the rat. The drug was administered (5 mg/kg) by i.p. injection. The chemical analysis of pinazepam and its metabolites was carried out by a gas-chromatographic method. The parent compound was rapidly absorbed, accumulated into the tissues and converted into N-desmethyldiazepam. The highest plasma levels of the parent compound (367 +/- 13 ng/ml) were found 3 min after administration. The volume of distribution and the clearance of the drug were 1315 ml and 7.23 ml/min respectively. The metabolite was detected in the plasma and tissues 3 min after administration. At this sampling time its concentrations were 76 +/- 16 ng/ml in the plasma and 1081 +/- 68 ng/g in the liver. The decay curve of both pinazepam and N-desmethyldiazepam in the plasma, liver, lung, heart, kidney, brain, and gastrochemius muscle were characterized for their Kel, t 1/2, and AUC values. The tissue AUC to plasma AUC ratios indicated a preferential accumulation of pinazepam over its metabolite in the tissues. The plasma protein binding of pinazepam was measured by dialysis at the equilibrium. Rat plasma proteins bound 89.17 +/- 0.20 percent of the drug. The association constant was 2.60 X 10(3) l/mole and the number of sites 0.44 X 10(-6) sites/g. The biliary excretion of pinazepam and N-desmethyldiazepam was poor.