Background:  The combination of efavirenz (EFV), emtricitabine (FTC), and tenofovir disoproxil fumarate (an oral prodrug of tenofovir [TFV]), is the first once-daily single-tablet regimen for the treatment of HIV-infected patients. The efficiency of this treatment depends on drug interactions that may limit their access to the HIV-1 replication site. Active efflux of drugs might produce subtherapeutic drug levels and favor resistant viral strains and the emergence of sanctuary sites. This study was performed to investigate the effect of these 3 drugs on Pgp and MRP1 function in peripheral blood mononuclear cells (PBMC) from healthy donors (n = 7), and particularly the effect of EFV on FTC and TFV accumulation.

Methods:  Quantitative real-time polymerase chain reaction (qRT-PCR) was used to determine which efflux transporters are present on PBMC. Following 20-h incubation with FTC, TFV, EFV, FTC+TFV, TFV+EFV, or FTC+TFV+EFV (drug concentrations were 5 µM), functional ability was assessed by measuring fluorescent dye efflux either with or without specific inhibitors (cyclosporine A, MK571). Intracellular drug concentrations were measured by liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS). Statistical comparisons were made using 2-tailed Student's t tests and were accepted at the p <0.05 significance level.

Results:  qRT-PCR showed that PBMC express high levels of both Pgp and MRP1 mRNA copy number whereas MRP2 and MRP3 were not detectable. mRNA level of MRP1 is 5-fold higher than Pgp (p = 0.0265). Following the 20-h incubation, our findings indicated a decrease in MRP1 function after exposure to FTC (p = 0.0050), to TFV (p = 0.0049), to EFV (p = 0.0072), to FTC+TFV (p = 0.0270), to TFV+EFV (p = 0.0001), and to FTC+TFV+EFV (p <0.0001). Compared to FTC alone, FTC concentration increased of 35.7±8.2% when combined with TFV, and 73.5±21.0% when combined with TFV+EFV. Compared to TFV alone, TFV concentration increased of 30.6±11.0% when combined with FTC; 45.8±13.0% when combined with EFV, and 91.6±25.0% when combined with FTC+EFV.

Conclusions:  Our results showed that FTC, TFV, EFV, alone or combined, decreased MRP1, but not Pgp function, in lymphocytes. This effect was most pronounced for the combinations TFV+EFV and FTC+TFV+EFV. Intracellular drug concentrations were increased in combination, and particularly EFV seemed to increase FTC and TFV concentrations. This in vitro study suggests the implication of efflux transporters and drug interactions impairing HIV treatment and the superior effectiveness of a combination therapy.