Pharmacokinetics of buspirone following oral administration to rhesus monkeys

Pharmacokinetics of buspirone and its active metabolite, 1-pyrimidinyl piperazine (1-PP) following oral administration were assessed in rhesus monkeys at doses used in chronic toxicology studies. The study was conducted over four periods in three male and three female rhesus monkeys. In the first three periods, buspirone hydrochloride solution was administered in a randomized manner by oral gavage at doses (expressed as buspirone free base) of 12.5, 25 and 50 mg kg(-1) once a day on days 1 and 7 and twice a day on days 2-6. In the last period, all monkeys received 25 mg kg(-1) buspirone as a single daily dose for 7 days. Serial plasma samples were collected for analysis of buspirone and 1-PP on days 1 and 7 in the first three periods and on day 7 in the last period for assessment of single dose and steady-state pharmacokinetics. Inter-animal variability in the pharmacokinetics of buspirone was high. Examination of Cmin vs time plots revealed that the steady state was attained by day 7 except for one monkey who demonstrated much higher Cmin values. For buspirone, dose proportionality was concluded for both Cmax and AUC on day 1 but not on day 7. The accumulation factor on day 7 for buspirone was nearly 5 for Cmax and 7 for AUC when compared with day 1. For 1-PP, dose proportionality was concluded except for Cmax in male monkeys on day 7. In contrast to buspirone, 1-PP showed less than 2-fold accumulation in Cmax and AUC values on day 7 compared with those on day 1. Exposure at a dose of 25 mg kg(-1) once daily was in between the 125 mg kg(-1) and 25 mg kg(-1) twice-a-day regimens. These results document dose-dependency in the steady-state pharmacokinetics of buspirone in rhesus monkeys.