Investigating the Theoretical Basis for In Vitro-In Vivo Extrapolation (IVIVE) in Predicting Drug Metabolic Clearance and Proposing Future Experimental Pathways

Extensive studies have been conducted to predictin vivometabolic clearance fromin vitrohuman liver metabolism parameters (i.e.,in vitro-in vivoextrapolation (IVIVE)) with little success. Here, deriving IVIVE from first principles, we show that the product of fraction unbound in the blood and the predictedin vivointrinsic clearance determined from hepatocyte or microsomal incubations is the lower boundary condition forin vivohepatic clearance and the prerequisite for IVIVE predictions to be valid, regardless of extraction ratio. For 60-80% of drugs evaluated here, this product is markedly less than thein vivomeasured clearance, a result that violates the lower boundary of the predictive relationship. This can only be explained by (a) suboptimalin vitrometabolic stability assay conditions, (b) significant error in the assumption thatin vitrointrinsic clearance determinations will predictin vivointrinsic clearance simply by scaling-up the amount of enzyme (in vitroincubation toin vivoliver), and/or (c) the methods of determining fraction unbound are incorrect. We further suggest that widely employed organ blood flow values underpredict the effective blood flow within the organ by approximately 2.5-fold, thus impacting IVIVE of high clearance compounds. We propose future pathways that should be investigated in terms of the relationship to experimentally measured clearance values, rather than model-dependent intrinsic clearance. IVIVE outcome can be improved by estimating the ratio of unbound drug concentration in the liver tissue to the liver plasma, examining the assumption of the free drug theory (i.e., there are no transporter effects at the blood cell membrane) and the finding that the upper limit of organ clearance may be greater than blood flow entering the organ.