Vol. 297, Issue 1, 141-147, April 2001

Probenecid-Associated Alterations in Valproate Glucuronide Hepatobiliary Disposition: Mechanistic Assessment Using Mathematical Modeling

E. Stacy Ward, Gary M. Pollack and Kim L. R. Brouwer

Curriculum in Toxicology, School of Medicine, and Division of Drug Delivery and Disposition, School of Pharmacy, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina

The complexity of processes associated with the hepatobiliary disposition of xenobiotics may require a multiexperimental approach, including pharmacokinetic modeling, to assess mechanisms of drug interactions. The objective of this study was to examine the disposition of valproate glucuronide (VG) in the rat isolated perfused liver (IPL), and to determine the mechanisms of interaction with probenecid (PRB). Livers were isolated and perfused with standard techniques, and valproate (VPA) (20 mg) was administered in the absence and presence of PRB (~75 µg/ml). Concentrations of VPA and VG in perfusate and bile were determined at timed intervals. In the absence of PRB, total recovery of VPA and VG in perfusate and bile was ~80%; PRB significantly increased this recovery to ~100%, suggesting a decrease in oxidative VPA metabolism. Similarly, pharmacokinetic modeling of the IPL data indicated that PRB competitively inhibited formation of oxidative VPA metabolites. PRB also significantly inhibited formation, biliary excretion, and sinusoidal egress of VG. These observations suggest a competitive interaction between PRB and VG for transport across the canalicular and sinusoidal membranes. Despite PRB-associated impairment of VG formation, mathematical modeling of the data revealed that hepatocyte VG concentrations were increased by PRB, presumably due to simultaneous inhibition of VG biliary excretion and sinusoidal egress by PRB. These results demonstrate the utility of pharmacokinetic modeling in elucidating the mechanisms of alteration in the hepatobiliary disposition of xenobiotics.