The bidirectional permeation characteristics of rhodamine 123 and Hoechst 33342, fluorescence probes of the binding sites on P-glycoprotein (P-gp), across monolayers of MDCK cells transfected with the human MDR1 gene (MDCK-MDR1) were investigated. The ratios of the apparent permeability coefficients (P(app)) of rhodamine 123 and Hoechst 33342 flux measured in the basolateral (BL) to apical (AP) direction versus the flux in the AP-to-BL direction (P(app BL-to-AP)/P(app AP-to-BL)) were 115 and 177, respectively. The P-gp inhibitor GF-120918 could significantly reduce the polarized efflux of both rhodamine 123 and Hoechst 33342. Rhodamine 123 appeared to "stimulate" the polarized efflux of Hoechst 33342 across MDCK-MDR1 cell monolayers. In contrast, Hoechst 33342 partially inhibited the polarized efflux of rhodamine 123 across these cell monolayers whereas daunorubicin partially inhibited the polarized efflux of both rhodamine 123 and Hoechst 33342. The uptake characteristics of rhodamine 123 and Hoechst 33342 in MDCK-MDR1 cells were measured in the absence and presence of GF-120918 and known P-gp substrates (Hoechst 33342, rhodamine 123, and daunorubicin). The uptake of rhodamine 123 and Hoechst 33342 in MDCK-MDR1 cells was enhanced more than twofold by inclusion of GF-120918 (2 microM) in the incubation medium. Daunorubicin (160 microM) increased the relative fluorescence unit (RFU) values of cytoplasm-associated rhodamine 123 by up to 30%. However, daunorubicin (40 microM) and rhodamine 123 (5 microM) decreased the RFU values of cell membrane-associated Hoechst 33342 by 70% and 40%, respectively. To further explore what appears to be a "stimulatory" effect of daunorubicin and rhodamine 123 on the uptake of Hoechst 33342 and a stimulatory effect of daunorubicin on Hoechst 33342 transport across cell monolayer, uptake of Hoechst 33342 into liposomes in the presence and absence of GF-120918, daunorubicin, and rhodamine 123 was determined. GF-120918 exhibited no effect on the RFU values of liposome-associated Hoechst 33342. In contrast, rhodamine 123 and daunorubicin decreased the fluorescence of liposome-associated Hoechst 33342 suggesting these molecules were either quenching the fluorescence of this chemical probe or displacing it from the lipid bilayer. In conclusion, these bidirectional transport data indicate that rhodamine 123 and Hoechst 33342 are excellent substrates of P-gp in MDCK-MDR1 cells. The ability of Hoechst 33342 to partially inhibit the polarized efflux of rhodamine 123 is consistent with these substrates binding to the same site on P-gp. In contrast, the ability of rhodamine 123 to apparently "stimulate" the efflux of Hoechst 33342 in both the transport and uptake experiments suggests the substrates might bind to different sites on P-gp. However, experimental results using liposomes suggested that this "stimulation" phenomenon by rhodamine 123 on Hoechst 33342 uptake and efflux might simply be an artifact. Thus, the use of Hoechst 33342 to probe the binding sites on a membrane-bound protein such as P-gp might be problematic.
Copyright 2004 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 93:1185-1194, 2004