Metabolism and disposition of buspirone

The metabolism and disposition of buspirone have been studied in the rat, the monkey, and in more than 150 human subjects. Buspirone is well absorbed, but is subject to first-pass metabolism. The mean systemic availability is approximately 4 percent. Buspirone is eliminated primarily by oxidative metabolism, which produces several hydroxylated metabolites, including 5-hydroxy-buspirone and 1-pyrimidinylpiperazine. The latter metabolite is from 1 to 20 percent as potent as buspirone in a variety of pharmacologic tests; 5-hydroxybuspirone is essentially inactive. In humans, the systemic exposure to buspirone increases linearly in relation to the oral dose. Food increases the bioavailability of buspirone by decreasing first-pass metabolism; absorption is not markedly altered. The pharmacokinetics of buspirone were not significantly different in men and women or in individuals 21 to 40 years old compared with those over 65 years of age. Half-life values observed in healthy volunteers ranged from two to 33 hours. Mean half-life values observed in healthy volunteers in the 14 studies conducted to date ranged from 2 +/- 1 to 11 +/- 3 hours. The half-life in women tended to be slightly longer than in men, but the difference was not significant. Hepatic cirrhosis resulted in a marked decrease in the clearance of buspirone, which correlated with serum alkaline phosphatase activity. Renal disease produced a modest decrease in buspirone clearance, which could not be correlated with an objective clinical measurement reflecting the severity of renal impairment. Buspirone was not removed by hemodialysis. Buspirone is highly protein bound (more than 95 percent), interacting with both albumin and alpha-acid glycoprotein. However, buspirone did not displace dilantin, propranolol, digoxin, or warfarin from plasma proteins. In rats, buspirone neither inhibited nor induced hepatic mixed-function oxidases. Co-administration of buspirone with amitriptyline or diazepam did not alter the disposition of these agents or their demethylated metabolites.