Concentration-dependent disposition of glucuronide metabolite of valproate

The glucuronide conjugation metabolism of valproate (VPA) has been assessed to be non-linear within the therapeutic concentration range. However, disposition of its metabolite, valproic acid glucuronide (VPAG), in relation to VPA doses is unclear. The purpose of this study was to elucidate the characteristics of dose-related disposition of VPAG. Guinea-pigs were treated with an intravenous bolus dose of sodium valproate at 20, 100, 500 or 600 mg kg(-1). Plasma was sampled on a pre-selected time schedule, and bile and urine were collected. Concentrations of VPA and VPAG in plasma, bile and urine were determined by gas chromatography. The pharmacokinetics of VPA and VPAG both were dose-dependent. However, the plasma concentration-time profiles of VPAG and VPA were not parallel. At a usual dose of VPA (20 mg kg(-1)), plasma VPAG declined with plasma VPA, whereas at a high dose of VPA (>500mg kg(-1)), plasma VPAG was elevated against the decline of plasma VPA, which suggested accumulation of plasma VPAG possibly owing to saturated elimination. The biliary and urinary clearances of VPA (vCLb and vCLu) were independent of dose. However, the clearances of plasma VPA (vCLp), plasma VPAG (gCLp), biliary and urinary VPAG (gCLb and gCLu) all were decreased against the increase in VPA doses. The dose-dependent decrease of gCLu (from 3.19 to 1.12 mL min(-1)) was less pronounced than that of gCLp (from 6.72 to 0.86 mL min(-1)) and the gCLu turned to exceed the gCLp at high doses of VPA (> 500 mg kg(-1)). These results suggest that the excess urinary VPAG might be produced in kidney. In conclusion, at a high dose of VPA, plasma VPAG is accumulated. The concentration-dependent biliary and urinary recovery of VPAG might be governed by a saturable elimination process rather than by saturable hepatic biotransformation rate. Glucuronide conjugation metabolism of VPA in kidney is speculated, which might be minor at low levels of plasma VPA, but more obvious after saturation of hepatic glucuronidation.