Membrane bound transporters such as P-glycoprotein (P-gp; Profit et al., 1999), multidrug resistance associated protein 1 (MRP-1; Meaden et al., 2002) and breast cancer resistance protein (BCRP; Wang et al., 2003) have been demonstrated to efflux several anti-HIV compounds, including the protease inhibitors (PIs) from cells and tissues in which they are expressed. Effective inhibition of these transporters may cause pharmacoenhancement of a substrate compound thus increasing treatment efficacy and decreasing the chance of developing viral resistance. Mefloquine is an anti-malarial compound which interacts with membrane bound transporters (Pham et al., 2000). We have recently demonstrated an interaction between mefloquine and the HIV PI saquinavir in cell lines (Owen et al., 2003). The aim of this study was to determine the effects of mefloquine on the antiretroviral efficacy of each of the PIs.

MT4 cells (2.5 ´ 10⁴ per well; n = 4) were incubated with the PIs (ritonavir, saquinavir, nelfinavir, indinavir and amprenavir; concentration range 100 - 0.015 nM) or a no drug control in a 96 well plate in the presence or absence of mefloquine (10 µM). HIV-IIIB was added to all wells except negative control wells to which media alone was added. Plates were incubated (37 °C, 5 % CO₂, 6 d) before quantification of cell viability by incubation with MTT (20ul, 5 mg.ml^-1, 2 h) followed by cell lysis and analysis by spectrophotometry (570 nm). The concentration of PI preventing 50 % of virus induced cytopathicity (IC₅₀) was calculated from dose response curves. Comparisons were made using the Mann-Whitney test.

10 µM mefloquine decreased the IC₅₀ in MT4 cells for each of the PIs tested. IC₅₀s in the absence and presence of mefloquine were: ritonavir 4.17 ± 1.30 and 0.80 ± 0.71 nM, saquinavir 0.28 ± 0.05 and 0.12 ± 0.07 nM, nelfinavir 0.69 ± 0.31 and 0.19 ± 0.06 nM, indinavir 8.46 ± 2.04 and 3.67 ± 0.81 nM and amprenavir 6.63 ± 1.37 and 2.25 ± 0.85 nM. (mean ± standard deviation; n = 4, p = 0.028 for all analyses).
These findings suggest that the interaction previously observed between mefloquine and saquinavir is applicable to all PIs. The interactions are likely to occur due to transporter inhibition by mefloquine allowing a higher concentration of PI to penetrate intracellularly. Interactions have already been demonstrated via P-gp and the contribution of other transporters expressed in MT4 are currently being evaluated.

Meaden et al., (2002) J Antimicrob Chemother; 50:583-8.
Owen et al., (2003) Federation of Infection Societies Tenth Anniversary Conference, November 19 - 21, Cardiff, UK.
Pham et al., (2000) Biochim et Biophysica Acta; 15: 1524 (2-3):212-19.
Profit et al., (1999) Aids; 13: 1623-27.
Wang et al., (2003) Mol Pharmacol; 63: 65-72.