In Vitro to in Vivo Extrapolation and Physiologically Based Modeling of Cytochrome P450 Mediated Metabolism in Beagle Dog Gut Wall and Liver

The beagle dog is a widely used <i>in vivo</i> model to guide clinical formulation development and to explore the potential for food effects. However, the results in dogs are often not directly translatable to humans. Consequently, a physiologically based modeling strategy has been proposed, using the dog as a validation step to verify model assumptions before making predictions in humans. One current weakness in this strategy is the lack of validated tools to incorporate gut wall metabolism into the dog model. In this study, in vitro to in vivo extrapolation factors for CYP2B11 and CYP3A12 mediated metabolism were established based on tissue enzyme abundance data reported earlier. Thereafter, physiologically based modeling of intestinal absorption in beagle dog was conducted in GastroPlus using <i>V</i>_max and <i>K</i>_m determined in recombinant enzymes as inputs for metabolic turnover. The predicted fraction of absorbed dose escaping the gut wall metabolism (<i>F</i>_g) of all five reference compounds studied (domperidone, felodipine, nitrendipine, quinidine, and sildenafil) were within a two-fold range of the value estimated from <i>in vivo</i> data at single dose levels. However, further in vivo studies and analysis of the dose-dependent pharmacokinetics of felodipine and nitrendipine showed that more work is required for robust forecasting of nonlinearities. In conclusion, this study demonstrates an approach for prediction of the gut wall extraction of CYP substrates in the beagle dog, thus enhancing the value of dog studies as a component in a strategy for the prediction of human pharmacokinetics