Inhibitory effect of efavirenz on MDR1, BCRP, and MRP2 transporters.

Efavirenz was applied at concentrations of 0.01–50 μM to evaluate its inhibition of Hoechst 33342 accumulation in the MDCK-MDR1 (A), MDCK-BCRP (B), and parental MDCK (C) cell lines, as well as its inhibition of calcein efflux from the MDCK-MRP2 (D) and parental MDCK (E) lines. Model inhibitors of MDR1, BCRP and MRP2, namely LY335979 (LY, 1 μM), Ko143 (2.5 μM), and MK571 (10 μM), respectively, were used as positive controls at the minimal concentrations causing maximal inhibition in the corresponding cellular system and assay. Relative and absolute IC₅₀ values were calculated as the concentrations of efavirenz needed to achieve 50% transporter inhibition. The relative IC₅₀ was determined from the maximum and minimum extremes of the relevant non-linear regression plot showing the inhibitory effect of efavirenz alone (maximal inhibition = 100%, non-inhibited control = 0%). The absolute IC50 was determined from the relevant non-linear regression plot of the inhibitory effect of the control inhibitor (100%) and the accumulation observed in control non-inhibited cells (0%). Data were analyzed by one-way ANOVA followed by Bonferroni’s multiple comparison test (* P ≤ 0.05, ** P ≤ 0.01, *** P ≤ 0.001) and are presented as means ± SD of at least three experiments (n = 3–5) performed in triplicate.