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Background:  Rifampicin (RIF) is a potent inducer of CYP P450 and P-glycoprotein (P-gp) in the gut and liver and thereby significantly reduces plasma HIV protease inhibitors (PI) (> 90%) and non-nucleoside reverses transcriptase inhibitors (NNRTI) (30 to 50%). Since both tuberculosis (TB) and HIV are intracellular pathogens, drug accumulation within target cells is important for their activity. We sought to examine the effect of RIF on P-gp and MRP1 expression on peripheral blood mononuclear cells (PBMC) and to characterize the cellular accumulation of RIF in vitro and in vivo.

Methods:  The cellular accumulation ratio of ³H-RIF (1 µM) was determined in CEM, CEM_VBL100 (P-gp overexpressing), and CEM_E1000 (MRP1 overexpressing). Trans-epithelial transport of ³H-RIF (1 µM) was determined in MDCKII, MDCKII_MDR1, and MDCKII_MRP1 cells. All experiments were carried out with or without XR9576 (P-gp inhibitor, 1 µM) and MK571 (MRP1 inhibitor, 50 µM). P-gp, MRP1, and BCRP expression were assessed by flow cytometry. Cellular and plasma pharmacokinetics (AUC) were measured in patients receiving RIF by HPLC-MS. Results were analyzed by Mann-Whitney U test.

Results:  RIF cellular accumulation ratio in CEM_VBL100 (mean cellular accumulation ratio 3.40 ± 0.19 (SD), p < 0.05) and CEM_E1000 (4.63 ± 0.34, p < 0.05) was significantly lower than in CEM (5.54 ± 0.21; n = 4). XR9576 and MK571 significantly increased RIF accumulation in CEM_VBL100 (3.93 ± 0.27, p < 0.05) and CEM_E1000 (4.49 ± 0.54, p < 0.05) compared to the absence of inhibitor (n = 4). Trans-epithelial transport was significantly greater in MDCKII_MDR1 (basal to apical (BL→AP): 11.46 ± 0.51 %/hour, p < 0.05) than in MDCKII (BL→AP: 8.48 ± 0.46 %/hour). XR9576 significantly decreased this transport in MDCKII_MDR1 (BL→AP: 7.38 ± 0.47 %/hour p < 0.05;). AP→BL transport of RIF in MDCKII_MRP1 (AP→BL: 1.13 ± 0.05 %/hour, p < 0.05) was significantly higher than in MDCKII (AP→BL: 0.74 ± 0.07 %/hour) but was not decreased by MK571 (n = 4). A significant increase in the expression of P-gp (ratio of specific vs isotype control fluorescence) was observed in PBMC isolated from RIF-receiving patients (mean ratio ± SD: 2.43 ± 0.40, n = 8, p < 0.0001) vs those from healthy volunteers (1.42 ± 0.23; n = 20). No difference was observed for MRP-1 expression. The cellular accumulation ratio of RIF in patients receiving RIF ranged from 0.75 to 1.44 (n = 6).

Conclusions:  RIF is a substrate for both P-gp and MRP1 and a potent inducer of P-gp expression on PBMC in vivo. Alterations in cellular efflux transporter expression may further compromise HIV drugs and RIF. We have validated an assay for the measurement of cellular RIF accumulation in vivo. Recruitment into this study is ongoing.

Keywords: Rifampicin; P-glycoprotein; MRP1