Stereoselective interaction of pantoprazole with ABCG2. II. In vitro flux analysis.

(-)Pantoprazole [(-)PAN] accumulated in rat milk stereoselectively, and this accumulation was attributed to rat Abcg2 (rAbcg2). In contrast, flux experiments at 25 μM showed that (+)pantoprazole [(+)PAN] was preferentially transported by rAbcg2. The purpose of the current study was to comprehensively evaluate the transport of PAN isomers in empty-Madin-Darby canine kidney II (MDCKII) and MDCKII cells expressing the human/rat (ABCG2/rAbcg2) isoforms at concentrations ranging from 3 to 200 μM. The apical-to-basolateral and basolateral-to-apical directional flux and the asymmetry efflux ratios were virtually identical for both isomers in empty (mock transfected)-MDCKII monolayers but were concentration dependent for both isomers in ABCG2 (human/rat)-MDCKII. Kinetic analysis using predicted cellular concentrations showed that (-)PAN had an 8-fold lower K(M) compared with (+)PAN for both rAbcg2 (0.25 versus 1.85 μM) and ABCG2 (0.6 versus 5.32 μM). (+)PAN had a 3-fold higher T(Max) compared with the (-)PAN for both rAbcg2 (7.86 versus 2.49 nmol/h · cm(2)) and ABCG2 (10.2 versus 3.29 nmol/h · cm(2)). Effective ABCG2 surface-area permeability of (-)PAN was 9920 and 5480 (μl/h)/cm(2) for rAbcg2 and ABCG2, respectively, compared with the (+)PAN isomer (4250 and 1920 μl/h · cm(2), respectively). These results indicate a stereoselective interaction of PAN with similar kinetic parameters for both human and rat ABCG2. (-)PAN is a better substrate than (+)PAN for ABCG2/rAbcg2 and provide a rationale for the preferential accumulation of (-)PAN into rat milk.