Pharmacokinetics of tetrabenazine and its major metabolite in man and rat. Bioavailability and dose dependency studies.
The pharmacokinetics of tetrabenazine (TBZ), a catecholamine and serotonin depletor, and its major metabolite, dihydrotetrabenazine (HTBZ), were studied in four patients affected by tardive dyskinesia, who were under treatment with different doses of TBZ (12.5-37.5 mg, t.i.d.), and in the rat. In the patients, the steady-state area under the plasma concentration-time curves (AUCs) of the metabolite were 82.6-199-fold higher than those of TBZ. The drug showed a small and erratic bioavailability (F = 0.06 +/- 0.026, mean +/- SD). It appears to be extensively metabolized, as no unchanged TBZ could be detected in the urine of the patients. Single oral doses of 0.5-10 mg/kg and single iv dose of 1 mg/kg of TBZ were each administered to four to six rats. The clearance of the drug following iv administration to the rat (mean +/- SD, 58.9 +/- 6.01 ml X min-1 X kg-1) was very close to the rat hepatic blood flow indicating a perfusion-limited clearance. An F value of 0.17 was obtained following iv and po doses of 1 mg/kg TBZ in the rat. The oral absorption of TBZ seems to be rapid and almost complete. Plots of the AUCs of TBZ and HTBZ vs. five different po doses (0.5-10 mg/kg) were linear with correlation coefficients of 0.998 and 0.986 for TBZ and HTBZ, respectively, suggesting linear kinetics in the examined dosage range. In both the patients and rats, the plasma profile of TBZ followed characteristics of a multiexponential pharmacokinetic model.(ABSTRACT TRUNCATED AT 250 WORDS)