Protein binding and hepatobiliary distribution of valproic acid and valproate glucuronide in rats

The protein binding and hepatobiliary distribution of valproic acid (VPA) and its glucuronide conjugate (V-G) were examined in rats with a combination of in vitro and ex vivo protocols. VPA was moderately bound to proteins in both serum and hepatic cytosol, and the degree of binding was lower ex vivo than in vitro. V-G, which was more highly bound than VPA ex vivo in serum, may have displaced the parent drug from its binding sites when VPA was administered in vivo. Examination of ex vivo hepatic subcellular distribution revealed that VPA localization tended to be high in cytosol and low in the microsomal fraction; V-G appeared to be distributed evenly throughout the cell although V-G concentrations within the liver were very low. The steady-state elimination rate of VPA did not increase proportionately with increasing steady-state concentrations of unbound VPA in serum, consistent with saturable systemic elimination of the drug. In contrast, steady-state VPA elimination was related linearly to unbound cytosolic VPA concentrations. Moreover, a nonlinear relationship between the unbound concentrations of VPA in hepatic cytosol and serum was observed, consistent with saturable distribution of the unbound drug between the two compartments in vivo. These observations suggest that the nonlinear elimination of VPA in rats may be due to concentration-dependent penetration of the drug into the liver as opposed to saturable biotransformation.