Abstract

Zolpidem [N,N-6-trimethyl-2-(4-methylphenyl)imidazo[1,2-a]pyridine-3- acetamide] administered as the hemitartrate salt has proven to be an effective hypnotic agent in animals and humans. This study describes the pharmacokinetic behavior of zolpidem in plasma and brain of rat after i.v. and p.o. administration of 2.63 mg.kg-1 of [14C]zolpidem (dose expressed as the base). Autoradiography was used to examine the regional distribution of the compound and the metabolic profile of zolpidem in the plasma and brain was also investigated. The pharmacokinetic data were related to electrocorticogram power spectral analysis. After i.v. administration, the disappearance of zolpidem from plasma fitted a biexponential model with a rapid phase of 0.2 to 0.3 hr and a slower phase of 1.3 to 1.5 hr. After p.o. dosing, peak plasma concentrations where already attained at 15 min (first sampling time). Independent of the route of administration, the concentrations of zolpidem in the brain at shorter times were 30 to 50% those of the plasma values. Furthermore, up to 1 hr, zolpidem accounted for 80 to 90% of brain radioactivity. The rate of disappearance from brain paralleled that from plasma. Autoradiographic studies confirmed the rapid absorption and elimination of zolpidem as well as the relatively homogenous distribution throughout the brain. Electrocorticogram analysis in immobilized rats after i.v. administration of zolpidem showed a rapid onset and a short-acting sedative effect compatible with the kinetic profile of the parent compound. Metabolites of zolpidem displayed a poor penetration into the brain and no significant hypnotic activity. At the dose of zolpidem used, no alteration of the sleep pattern was observed.