Abstract

The aim of this study was to explore the mechanisms governing the intra- to extracellular unbound concentration ratio (Kp_u,u) for the HIV protease inhibitor atazanavir (ATV) in rat hepatocytes. We had previously proposed a new method to determine Kp_u,u by using the unbound Km values from metabolism studies with suspended rat hepatocytes and rat liver microsomes. Following that method, we determined that the value of ATV Kp_u,u was 0.32, indicating that ATV hepatocellular clearance is uptake rate–limited. This hypothesis was supported by the linear correlation between Kp_u,u and active uptake clearance (P = 0.04; R²=0.82) in the presence of increasing concentrations of the uptake transport inhibitor losartan. Moreover, in contrast to an expected increase of Kp_u,u upon inhibition of ATV metabolism, a decrease of Kp_u,u was observed, suggesting an increased impact of sinusoidal efflux. In summary, involvement of active uptake transport does not guarantee high intracellular accumulation; however, it has a key role in regulating intracellular drug concentrations and drug metabolism. These findings will help improve future in vitro–to–in vivo extrapolations and likewise physiologically based pharmacokinetic models.