Comparison of Fraction Unbound Between Liver Homogenate and Hepatocytes at 4°C.

Fraction unbound (f) values obtained from liver or hepatocyte homogenate with equilibrium dialysis (f) or the hepatocyte partition coefficient method at 4°C (f) are both frequently used to estimate unbound drug concentrations (C) and unbound partition coefficient (Kp) of the liver. Literature data are somewhat controversial on this topic: some reported that the two methods gave comparable f values, while others showed that they had no correlation. To better understand the two approaches, 44 structurally diverse compounds with wide ranges of f values were used for the comparison study. The results indicate that f values from the two methods are comparable with an average fold error of 2.9-fold and a bias of 1.0. Although some outliers were observed, the reasons are not entirely clear and further investigations are needed. As the f data from both methods are correlated, f measurement using tissue homogenate is recommended over cells at 4°C (f) in early drug discovery. This is because f method is more reliable, has good in vivo predictability, and feasibility for any tissue types where representative cells may not be readily available. The approach can be used to estimate C and Kp of tissues in order to develop pharmacokinetic/pharmacodynamic relationships, and to estimate therapeutic indices, as well as to predict drug-drug interactions.