Nonlinear multiple-dose pharmacokinetics of the dopamine reuptake inhibitor vanoxerine

The human pharmacokinetics of vanoxerine (GBR 12909) were studied in 14 normal subjects with a multiple-dose regimen. In a crossover design, each subject received daily oral doses of 25, 75, and 125 mg for 14 days at each dose level with washout periods of 7 days duration. Drug concentrations in serum during and after dosing were estimated by an HPLC method sensitive to 2 nmol/L (corresponding to 1.04 ng/mL). Drug accumulation was observed during dosing at the two highest dose levels, but near steady-state conditions were attained within 9-11 days of dosing. Estimates of steady-state concentrations all showed statistically significant deviations from dose linearity in the form of disproportionately higher concentrations at higher dose levels than expected from drug concentrations in serum at lower doses. The nonlinear pharmacokinetics was most likely due to increasing bioavailability with dose. The mean elimination half-lives were 53.5 and 66.0 h at 75 and 125 mg/day, respectively, in accordance with the observed time to reach near steady-state conditions. These estimates were higher than previous estimates in less extensive studies.