Grapefruit juice activates P-glycoprotein-mediated drug transport

Grapefruit juice (GJ) is known to increase the oral bioavailability of many CYP3A-substrates by inhibiting intestinal phase-I metabolism. However, the magnitude of AUC increase is often insignificant and highly variable. Since we earlier suggested that CYP3A and P-glycoprotein (P-gp) form a concerted barrier to drug absorption, we investigated the role of P-gp in GJ-drug interactions. The transcellular bidirectional flux of drugs that are (i) CYP3A-and/or P-gp substrates (Vinblastine, Cyclosporine, Digoxin, Fexofenadine, Losartan) or that are (ii) primary CYP3A-substrates (Felodipine, Nifedipine) was evaluated across MDCK-MDR1 cell monolayers with or without GJ, verifying monolayer integrity at all times. While both apical-to-basal (A-B) and basal-to-apical (B-A) fluxes of all CYP3A/P-gp substrates tested were increased in the presence of GJ, the resulting net efflux (B-A/A-B) was in all cases significantly greater with GJ than control (Vin, 28.0 vs. 5.1; CsA, 9.9 vs. 2.8; Dig, 22. 9 vs. 14.7, Fex, 22.3 vs. 11.1, Los, 39.6 vs. 26). In contrast, no such GJ flux effect was observed with Fel and Nif, substrates of CYP3A only (2 vs. 1.7 and 1.2 vs. 1.3). GJ significantly activates P-gp-mediated efflux of drugs that are substrates of P-gp, potentially partially counteracting the CYP3A-inhibitory effects of GJ.