Hepatocellular binding of drugs: Correction for unbound fraction in hepatocyte incubations using microsomal binding or drug lipophilicity data

Analogous to the fraction unbound in microsomes (fu(mic)), fraction unbound in hepatocyte incubations (fu(hep)) is an important parameter in the prediction of intrinsic clearance and potential drug-drug interactions. A recent study by Austin et al. (Drug Metab Dispos 33: 419-425, 2005) proposed a linear 1:1 relationship between the extent of binding to microsomes at 1 mg/ml and to hepatocytes at 10(6) million cells/ml. The current study collates a fu(mic) and fu(hep) database for 39 drugs to examine the relationship between binding in microsomes and hepatocytes. A new nonlinear empirical equation is proposed as an alternative to the linear relationship to relate binding between the two systems. The nonlinear equation results in higher prediction accuracy and lower bias in comparison to the linear relationship, in particular for drugs with fu(hep) < 0.4. The proposed equation is further extended to allow direct prediction of fu(hep) from drug lipophilicity data by substituting the fu(mic) term by the Hallifax and Houston predictive equation (Drug Metab Dispos 34: 724-726, 2006). The prediction accuracy of this approach is high for relatively hydrophilic drugs (logP/D <= 2.5), whereas less accurate prediction of the fu(hep) is observed for lipophilic drugs (logP > 3), consistent with the limitations observed for microsomal binding predictive tools. In conclusion, the proposed nonlinear equations provide an accurate predictive tool to estimate fu(hep) for the in vitro-in vivo extrapolation of intrinsic clearance and inhibition parameters.