Study Objective. To evaluate P-glycoprotein (P-gp)–mediated renal drug interactions in an in vitro model of tubular secretion.

Design. In vitro experiment. Setting. University-affiliated pharmacokinetics laboratory. Cell Lines. Madin-Darby canine kidney (MDCK), multidrug-resistant-1 (MDR1)-MDCK, and human colon carcinoma (Caco-2) cells. Intervention. Transepithelial transport (basolateral-to-apical and apical-tobasolateral) of cimetidine was assessed in the absence and presence of various concentrations of the P-gp inhibitors itraconazole and PSC-833 in a renal P-gp cell culture model (MDR1-MDCK). Measurements and Main Results. Apparent permeability of cimetidine was characterized, and level of P-gp expression was determined by Western blot analysis, in MDCK (wild type), MDR1-MDCK, and Caco-2 cells (for relative comparison). In the presence of PSC-833, cimetidine’s apparent permeability value for basolateral-to-apical transport decreased from 2.96 to 1.15 x 10-6 cm/second, coupled with a decrease in efflux ratio from 2.36 to 1.80. The effect of itraconazole was concentration dependent, with cimetidine’s apparent permeability value for basolateral-to-apical transport decreasing from 3.96 to 1.92 x 10-6 cm/second (p<0.05), resulting in a 50% decrease in efflux ratio. Expression of P-gp was negligible in MDCK (wildtype) cells, but high-level expression was confirmed in both MDR1-MDCK and Caco-2 cells. Conclusion. P-glycoprotein plays a significant role in the renal tubular secretion of organic cations such as cimetidine, and the high level of P-gp expression in MDR1-MDCK cells makes this a well-suited model for evaluating mechanisms of renal drug interactions.