P-glycoprotein (P-gp) and multi drug resistance protein 1 (MRP1) have been shown to influence the cellular accumulation of a range of drugs including antiretrovirals and anticancer agents (Jones et al 2001, van Tellingen 2001). The aim of the present study is to examine the cellular accumulation of rifampicin in P-gp and MRP1 overexpressing cell lines, and to determine if there is a correlation between the transporters and rifampicin accumulation in healthy human peripheral blood mononuclear cells (PBMC).

Flow cytometry and Western blot analysis were used to determine the expression of P-gp and MRP1 on CEM (parental), CEM_VBL100 (P-gp overexpressing) and CEM_E1000 (MRP1 overexpressing) T-lymphoblastoid cell lines. The expression of P-gp and MRP1 was also determined on healthy volunteer PBMCs following isolation by density gradient centrifugation. Using [³H]-rifampicin, the cellular accumulation ratios (CAR) were determined for both the T-lymphoblastoid cell lines (10µM [³H]-rifampicin) and PBMCs (1µM [³H]-rifampicin) (30 min, 37 ^oC). Simple linear regression was used to test the significance of correlations and all other statistical analyses were carried out using the Mann-Whitney test.

Flow cytometry and Western blot of the T-lymphoblastoid cell lines confirmed the overexpression of P-gp in CEM_VBL100 and MRP1 in CEM_E1000 compared to the parental CEM cells. There was a significant decrease in the cellular accumulation ratio of [³H]-rifampicin in both the CEM_VBL100 (2.19 ± 0.07) and CEM_E1000 (3.54 ± 0.19) compared to the CEM (5.55 ± 0.59; p < 0.05; n=4). There was a significant positive correlation between the expression of P-gp and MRP1 in PBMCs (R²=0.375; P < 0.05; n=12). However, no significant correlation was observed between the expression of the P-gp or MRP1 and the cellular accumulation ratio of rifampicin in PBMCs (R²=0.028; P = 0.60 and R²=0.034; P = 0.56 respectively n=12)

These findings confirm that rifampicin is a substrate for both P-gp and MRP1. There is also a clear indication of a positive correlation between the basal expression levels of P-gp and MRP1 on PBMCs. However, the cellular accumulation of rifampicin does not correlate directly with the expression of either of the transporters for which it is a substrate. This lack of correlation may be due to the heterogeneity of the PBMC cell population in which the CAR was measured. We cannot rule out that other efflux transporters may account for the differential rifampicin accumulation reported here.

Jones, K., et al. (2001). Aids, 15, 675-81.
van Tellingen, O. (2001). Toxicol Lett, 120, 31-41.