Atazanavir: Effects on P-glycoprotein transport and CYP3A metabolism in vitro

The effect of atazanavir on P-glycoprotein (P-gp) expression and activity, as well as its inhibitory potency against CYP3A activity, was evaluated in vitro. Induction of P-gp activity and expression was studied using LS180V cells. P-gp inhibition was studied using both LS180V cells and Caco-2 cells. P-gp activity was assessed by measuring P-gp-mediated rhodamine 123 (Rh123) transport, and P-gp expression was determined using SDS-polyacrylamide gel electrophoresis/Western blot analysis. CYP3A inhibition was tested using triazolam hydroxylation in human liver microsomes (HLM). Extended (3-day) exposure of LS180V cells to 30 mu M atazanavir caused a 2.5-fold increase in immunoreactive P-gp expression as well as a concentration-dependent decrease of intracellular Rh123 to a mean 45% ( S. D. 5.2%) of control. Acute exposure 2 h) of LS180V cells to atazanavir increased intracellular Rh123 concentrations up to 300% of control at 100 mu M atazanavir. At 30 mu M and above, acute atazanavir exposure reversed P-gp induction caused by 3-day pretreatment with 10 mu M ritonavir. P-gp inhibition was also observed in Caco-2 cells, causing an effect comparable to that observed for the known P-gp inhibitor verapamil (50% of control). In HLM, atazanavir was an inhibitor of triazolam hydroxylation, with inhibitory potency greatly increased by preincubation. IC50 values with and without preincubation were 0.31 mu M( S. D. 0.13) and 5.7 mu M ( S. D. 4.1), respectively. Thus, atazanavir is an inhibitor and inducer of P-gp as well as a potent inhibitor of CYP3A in vitro, suggesting a potential for atazanavir to cause drug-drug interactions in vivo.