Raltegravir is a substrate for SLC22A6: a putative mechanism for the interaction between raltegravir and tenofovir.

The identification of transporters of the HIV integrase inhibitor raltegravir could be a factor in an understanding of the pharmacokinetic-pharmacodynamic relationship and reported drug interactions of raltegravir. Here we determined whether raltegravir was a substrate for ABCB1 or the influx transporters SLCO1A2, SLCO1B1, SLCO1B3, SLC22A1, SLC22A6, SLC10A1, SLC15A1, and SLC15A2. Raltegravir transport by ABCB1 was studied with CEM, CEM(VBL100), and Caco-2 cells. Transport by uptake transporters was assessed by using a Xenopus laevis oocyte expression system, peripheral blood mononuclear cells, and primary renal cells. The kinetics of raltegravir transport and competition between raltegravir and tenofovir were also investigated using SLC22A6-expressing oocytes. Raltegravir was confirmed to be an ABCB1 substrate in CEM, CEM(VBL100), and Caco-2 cells. Raltegravir was also transported by SLC22A6 and SLC15A1 in oocyte expression systems but not by other transporters studied. The K(m) and V(max) for SLC22A6 transport were 150 µM and 36 pmol/oocyte/h, respectively. Tenofovir and raltegravir competed for SLC22A6 transport in a concentration-dependent manner. Raltegravir inhibited 1 µM tenofovir with a 50% inhibitory concentration (IC(50)) of 14.0 µM, and tenofovir inhibited 1 µM raltegravir with an IC(50) of 27.3 µM. Raltegravir concentrations were not altered by transporter inhibitors in peripheral blood mononuclear cells or primary renal cells. Raltegravir is a substrate for SLC22A6 and SLC15A1 in the oocyte expression system. However, transport was limited compared to endogenous controls, and these transporters are unlikely to have a great impact on raltegravir pharmacokinetics.

Maraviroc is a substrate for OATP1B1 in vitro and maraviroc plasma concentrations are influenced by SLCO1B1 521 T>C polymorphism.

BACKGROUND: Organic anion transporting polypeptides (OATPs) are emerging as major determinants of pharmacokinetics for numerous drugs, with the 1B1 isoform-mediating hepatic uptake. The 521 T>C polymorphism has been correlated earlier with higher plasma concentrations of several drugs and the aim of this study was to determine whether this polymorphism influences trough concentrations of maraviroc. METHODS: The uptake of maraviroc by OATP1B1 was assessed using a heterologous Xenopus laevis oocyte expression system and quantified using a novel liquid chromatography-mass spectrometry method. Regression analyses were conducted to identify factors associated with maraviroc Ctrough in 59 patients treated with maraviroc at 150, 300, or 600 mg twice daily. RESULTS: Maraviroc was identified as a substrate for OATP1B1 with a Km of 33.9 µmol/l. A dose of 600 mg of etravirine or efavirenz [odds ratio (OR) = 0.22, 95% confidence interval (95% CI): 0.06-0.76; P = 0.016] and SLCO1B1 521 heterozygosity were both associated with maraviroc Ctrough, above the suggested target concentration of 50 ng/ml (OR = 20.3, 95% CI: 2.2-182; P = 0.007). CONCLUSION: These findings show the importance of OATP1B1 for variability in maraviroc pharmacokinetics. Furthermore, the SLCO1B1 521 T>C polymorphism maybe useful in predicting higher plasma concentrations but these data should be confirmed before prospective clinical studies to define the clinical usefulness.

HIV protease inhibitors are substrates for OATP1A2, OATP1B1 and OATP1B3 and lopinavir plasma concentrations are influenced by SLCO1B1 polymorphisms.

OBJECTIVE: OATP1B1 and OATP1B3 are major hepatic drug transporters whilst OATP1A2 is mainly located in the brain but is also located in liver and several other organs. These transporters affect the distribution and clearance of many endobiotics and xenobiotics and have been reported to have functional single nucleotide polymorphisms (SNPs). We have assessed the substrate specificities of these transporters for a panel of antiretrovirals and investigated the effects of SNPs within these transporters on the pharmacokinetics of lopinavir. METHODS: SLCO1A2, SLCO1B1 and SLCO1B3 were cloned, verified and used to generate cRNA for use in the Xenopuslaevis oocyte transport system. Using the oocyte system, antiretrovirals were tested for their substrate specificities. Plasma samples (n=349) from the Liverpool therapeutic drug monitoring registry were genotyped for SNPs in SLCO1A2, SLCO1B1 and SLCO1B3 and associations between SNPs and lopinavir plasma concentrations were analysed. RESULT: Antiretroviral protease inhibitors, but not non-nucleoside reverse transcriptase inhibitors, are substrates for OATP1A2, OATP1B1 and OATP1B3. Furthermore, ritonavir was not an inhibitor of OATP1B1. The 521T>C polymorphism in SLCO1B1 was significantly associated with higher lopinavir plasma concentrations. No associations were observed with functional variants of SLCO1A2 and SLCO1B3. CONCLUSION: These data add to our understanding of the factors that contribute to variability in plasma concentrations of protease inhibitors. Further studies are now required to confirm the association of SLCO1B1 521T>C with lopinavir plasma concentrations and to assess the influence of other polymorphisms in the SLCO family.

Intracellular accumulation of efavirenz and nevirapine is independent of P-glycoprotein activity in cultured CD4 T cells and primary human lymphocytes.

BACKGROUND: Interaction of antiretrovirals with drug transporters such as P-glycoprotein (P-gp), multidrug resistance-associated protein (MRP), breast cancer resistance protein (BCRP) and solute carrier organic anion transporter (SLCO) may influence the emergence of viral mutants by altering intracellular drug concentrations. Here we characterize the effect of transporter expression in a variety of cell types such as control CEM, CEM(VBL) (P-gp-overexpressing), CEM(E1000) (MRP1-overexpressing), MT4, control MDCKII, MDCKII(MDR1) (P-gp-overexpressing) and peripheral blood mononuclear cells (PBMCs) on the uptake of [(14)C]efavirenz and [(3)H]nevirapine. We also investigated the lipophilicity of [(14)C]efavirenz and [(3)H]nevirapine. METHODS: The expression of P-gp, MRP1, MRP2, SLCO1A2, 1B1, 1B3, 2B1, 3A1 and 4A1 was assessed by PCR. Inhibitors of P-gp (XR9576, GF120918, dipyridamole) and MRP (MK571, frusemide, dipyridamole), and SLCO substrate or inhibitor (estrone-3-sulphate or montelukast, respectively) were used to study the role of drug transporters in the accumulation of [(14)C]efavirenz and [(3)H]nevirapine. Lipophilicity was measured by the octanol/saline partition coefficient. RESULTS: CEM cells, MT4 cells and PBMCs express various SLCO isoforms, with SLCO3A1 detected in all of the cells. XR9576, dipyridamole and GF120918 had no effects on the accumulation of [(14)C]efavirenz, while MK571 and frusemide produced variable effects in the cells. The accumulation of [(14)C]efavirenz was significantly decreased in all the cells by montelukast and estrone-3-sulphate. CONCLUSIONS: P-gp expression had no effect on the accumulation of [(14)C]efavirenz and [(3)H]nevirapine. MRP1/2 expression, lipophilicity and SLCO-like transporters (possibly SLCO3A1) may have greater influence on the accumulation of [(14)C]efavirenz than [(3)H]nevirapine.

Intracellular 'boosting' of darunavir using known transport inhibitors in primary PBMC.

AIMS: ABCB1, some ABCCs and SLCOs have been reported to affect the intracellular accumulation of various protease inhibitors in vitro and ex vivo. Darunavir is the most recently licensed protease inhibitor and we sought to investigate the ability of transport inhibitors to influence its intracellular accumulation in lymphocytes from healthy volunteers. METHODS: The intracellular accumulation of radiolabelled darunavir was assessed using CEM cells and ABCB1-overexpressing CEM(VBL) cells. Apical and basolateral transport of radiolabelled darunavir through MDCKII monolayers was also studied. Finally the ability of known inhibitors to influence intracellular accumulation of darunavir in peripheral blood mononuclear cells (PBMC) was investigated. RESULTS: CEM(VBL) cells (1.4 +/- 0.6, P < 0.001, 95% CI for the difference = 0.46, 0.80, n= 7) had significantly lower accumulation of darunavir compared with CEM cells (5.6 +/- 0.7, n= 7) and this was reversed by addition of tariquidar (30 nm, 4.6 +/- 0.8, P < 0.001, 95% CI =-0.64, -0.41, n= 4). In MDCKII-ABCBI cells, transport from the basal to the apical compartment was observed and this was also reversible with the addition of tariquidar. In PBMCs, dipyridamole (6.9 +/- 1.3, P < 0.01, 95% CI for the difference =-1.16, -0.30, (n= 8) significantly increased whilst montelukast (5.7 +/- 1.0, P < 0.01, 95% CI for the difference = 0.16, 0.79, n= 8) significantly decreased the intracellular accumulation of darunavir when compared with control (6.2 +/- 1.1, n= 8). CONCLUSIONS: Darunavir is a substrate for efflux and influx transporters in PBMC and intracellular concentrations can be manipulated using known inhibitors.