On the Prediction of Hepatic Clearance Using the Diluted Plasma in Metabolic Stability Assay

It was suggested that in vivo hepatic clearance, CLh, may be predicted rather accurately with the in vitro values of intrinsic clearance, CLint, obtained using the microsomal incubation mix containing diluted plasma, and consequently calculated by the well-stirred model equation. Conceivably the improvement could be due to the direct account of plasma protein binding in the measured values of CLint. It is shown in this article that the prediction of CLh done in this manner may not yield accurate results, both substantial underestimation or overestimation of the true value is possible. The procedure may be useful to reduce the overestirnation of CLh for highly protein bound drugs, though the obtained value of CLh may be far off from the correctly calculated one. The accurate way of calculating CLh, based on the value of CLint obtained in diluted plasma, is presented. It takes into account both the drug protein binding in diluted plasma and microsomal binding, as well as blood-plasma concentration ratio. The prediction of CLh by the suggested calculation using the experimental data on CLint, measured at different plasma dilutions for several drugs, yields consistent (dilution independent) values of hepatic clearance. It does not seem possible to avoid the measurement of plasma protein binding, microsomal binding and blood-plasma concentration ratio for an accurate and consistent prediction of CLh, even if the value of CLint were obtained in the pure (undiluted) plasma. In an early stage screening using plasma in the microsomal incubation mix may be beneficial for fast metabolizing drugs with relatively high protein binding. This would reduce a possible overestimation CLh, and also lead to the increase of the half-life in the microsomal incubation, so that it could be measured more accurately. (C) 2008 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:1922-1927, 2009