TY - JOUR T1 - Kinetics of Valproic Acid Glucuronidation: Evidence for in Vivo Autoactivation JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 1380 LP - 1386 DO - 10.1124/dmd.107.015719 VL - 35 IS - 8 AU - Harvey Wong AU - Vincent Tong AU - K. Wayne Riggs AU - Dan W. Rurak AU - Frank S. Abbott AU - Sanjeev Kumar Y1 - 2007/08/01 UR - http://dmd.aspetjournals.org/content/35/8/1380.abstract N2 - Sigmoidal or autoactivation kinetics has been observed in vitro for both cytochrome P450- and UDP-glucuronosyltransferase-catalyzed enzymatic reactions. However, the in vivo relevance of sigmoidal kinetics has never been clearly demonstrated. In the current study we investigate the kinetics of valproic acid glucuronide (VPAG) formation both in vivo in adult sheep and in vitro in sheep liver microsomes (pool of 10). After a 100 mg/kg i.v. bolus dose of valproic acid (VPA) to adult sheep (n = 5), the majority of the dose was recovered in urine as VPAG (∼79%). Eadie-Hofstee plots of the VPAG formation rate (calculated from urinary excretion rate data for VPAG) were characteristic of autoactivation kinetics and provided estimates of the apparent maximum velocity of an enzymatic reaction (Vmaxapp), the substrate concentration resulting in 50% of Vmaxapp (S50app), and Hill coefficient (n) of 2.10 ± 0.75 μmol/min/kg, 117 ± 56 μM, and 1.34 ± 0.14, respectively. Comparable estimates of Vmaxapp (2.63 ± 0.33 μmol/min/kg), S50app (118 ± 53 μM), and n (2.06 ± 0.47) describing overall VPA elimination from plasma were obtained by fitting VPA unbound plasma concentration-time data to a two-compartment model with elimination described by the Hill equation. Consistent with our in vivo observations, Eadie-Hofstee plots of VPAG formation in sheep liver microsomes were characteristic of autoactivation kinetics. To our knowledge, these data provide the first clear demonstration that autoactivation kinetics observed in vitro in liver preparations can translate to the in vivo situation at least under certain experimental conditions and confirm its relevance. The American Society for Pharmacology and Experimental Therapeutics ER -