Lack of P-glycoprotein induction by rifampicin and phenobarbital in human lymphocytes.

The efficacy of drugs acting within lymphocytes depends on their intracellular concentrations, which could be modulated by membrane efflux transporters including P-glycoprotein (P-gp), encoded by the MDR1 gene. In particular, P-gp induction may compromise the efficacy of its substrates. Rifampicin and phenobarbital have been shown to induce P-gp in hepatic and intestinal cells through the activation of the nuclear receptors PXR and CAR. However, controversial data exist in human lymphocytes. We investigated the effect of these drugs on P-gp activity and expression in lymphocytes in vitro and ex vivo. CCRF-CEM cells and peripheral blood mononuclear cells (PBMCs) from healthy volunteers were incubated in the presence of rifampicin, phenobarbital, or without any drug. P-gp activity was measured by flow cytometry using DiOC(6) efflux. MDR1, PXR and CAR mRNA expression were measured by quantitative RT-PCR. Neither P-gp activity nor MDR1 mRNA expression were modified by rifampicin or phenobarbital both in CCRF-CEM cells and PBMCs. Moreover, P-gp protein expression at the membrane was neither detectable nor induced. The very weak PXR and CAR mRNA expression levels in these cells could partly explain these results. Therefore, P-gp induction by rifampicin and phenobarbital may play a negligible role in drug interactions occurring within lymphocytes.