The aim of this study was to explore the mechanisms governing the intra- to extracellular unbound concentration ratio (Kpu,u) for the HIV protease inhibitor atazanavir (ATV) in rat hepatocytes. We had previously proposed a new method to determine Kpu,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 Kpu,u was 0.32, indicating that ATV hepatocellular clearance is uptake rate-limited. This hypothesis was supported by the linear correlation between Kpu,u and active uptake clearance (P = 0.04; R(2)=0.82) in the presence of increasing concentrations of the uptake transport inhibitor losartan. Moreover, in contrast to an expected increase of Kpu,u upon inhibition of ATV metabolism, a decrease of Kpu,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.
Copyright © 2016 by The American Society for Pharmacology and Experimental Therapeutics.