Received for publication March 9, 2007.
Revised May 9, 2007.
Accepted for publication May 9, 2007.

Kinetics of Valproic Acid Glucuronidation: Evidence for In Vivo Autoactivation

Abstract

Sigmoidal or autoactivation kinetics have 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. The current study investigates the kinetics of valproic acid-glucuronide (VPAG) formation both in vivo in adult sheep and in vitro in sheep liver microsomes (pool of 10). Following 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 VPAG formation rate (calculated from urinary excretion rate data for VPAG) were characteristic of autoactivation kinetics, and provided estimates of V_max^app, S₅₀^app and n of 2.10 ± 0.75 µmol/min/kg, 117 ± 56 µM and 1.34 ± 0.14, respectively. Comparable estimates of V_max^app (2.63 ± 0.33 µmol/min/kg), S₅₀^app (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 the certain experimental conditions and confirm its relevance.

Key words: enzyme kinetics, glucuronidation, hepatic elimination, in vitro-in vivo prediction, in vitro-in vivo scaling, kinetic modeling, pharmacokinetic modeling, pharmacokinetics, UDP glucuronyltransferases

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