Multidrug resistance gene G1199A polymorphism alters efflux transport activity of P-glycoprotein

The significance of the human multidrug resistance gene (MDR1) G1199A polymorphism, resulting in a Ser400Asn modification in P-glycoprotein (P-gp), remains unclear. We have developed stable recombinant LLC-PK1 epithelial cells expressing either MDR1wt or MDR11199 to evaluate functional consequences of G1199A [N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide]. P-gp activity observed in MDR1wt and MDR11199 cells was completely inhibited in the presence of the specific P-gp inhibitor GF120918. Comparable expression of mRNA and protein in the MDR1-expressed cells and correct localization of P-gp in the apical membrane of recombinant cells was verified. Mean intracellular rhodamine-123 (R123) accumulation, measured by flow cytometry, was approximately 4.75-fold higher in MDR11199 recombinant cells than MDR1wt cells. Cytotoxicity studies have shown that MDR1wt and MDR11199 cells exhibited similar resistance, as measured by EC50 values, to doxorubicin (155 +/- 68 versus 120 +/- 32 nM); however, MDR11199 cells were more resistant to vinblastine (1.41 +/- 0.51 versus 15.7 +/- 4.0 nM; p < 0.001) and vincristine (1.18 +/- 0.56 versus 3.41 +/- 1.47 nM; p < 0.05). The apparent transepithelial permeability ratios of R123 in MDR1wt and MDR11199 cells were 3.54 +/- 0.94 and 2.02 +/- 0.51 (p < 0.05), respectively. Therefore, the G1199A polymorphism alters the efflux and transepithelial permeability of a fluorescent substrate and sensitivity to select cytotoxic agents, which may influence drug disposition and therapeutic efficacy of some P-gp substrates.