Correction for Nonspecific Binding to Various Components of Ultrafiltration Apparatus and Impact on Estimating In Vivo Rat Clearance for a Congeneric Series of 5-Ethyl, 5-n-Alkyl Barbituric Acids

Accurately predicting in vivo metabolic clearance from in vitro liver microsomes or hepatocytes requires a good understanding of the factors contributing to the prediction. Although much work has concentrated on deriving scaling factors and optimizing the metabolic stability techniques for consistency and rigor, it is only relatively recently that the importance of binding to microsomes and hepatocytes has been appreciated. Ultrafiltration is often used to estimate binding to plasma proteins and microsomes, but the level of nonspecific binding (NSB) to the ultrafiltration apparatus has not been adequately described. We derive an equation to correct for NSB and demonstrate that this can significantly affect the estimate of binding to microsomes and improve the accuracy of scaling to in vivo clearance for a series of barbiturates.