NONLINEAR INTESTINAL-ABSORPTION OF 5-HYDROXYTRYPTAMINE RECEPTOR ANTAGONIST CAUSED BY ABSORPTIVE AND SECRETORY TRANSPORTERS

The mechanism of the nonlinear concentration dependence of intestinal absorption of the 5-hydroxytryptamine receptor antagonist azasetron wa s studied by use of rat in situ intestinal perfusion, as well as an in vitro Ussing-type chamber method mounted with rat intestinal tissue a nd cultured monolayers of human adenocarcinoma Caco-2 cells. The intes tinal absorption rate constant of azasetron evaluated by the Doluisio method increased significantly with increasing concentration of azaset ron up to 10 mM in a nonlinear fashion and tended to decrease at highe r concentrations. Mucosal-to-serosal directed permeation of [C-14]azas etron across rat ileal sheets evaluated by the in vitro Ussing-type ch amber method also increased in a nonlinear fashion in a low concentrat ion range, followed by a decrease as the concentration was further inc reased, whereas serosal-to-mucosal directed permeation decreased in a concentration-dependent manner. Vectorial transport of [C-14]azasetron across a Caco-2 cell monolayer was observed, with higher transport in the basolateral-to-apical direction at a trace concentration of azase tron. When the initial uptake rate of azasetron by Caco-2 cells was me asured, it was saturable with an apparent half-saturation concentratio n of 15 mM and was reduced in the presence of several cationic compoun ds, These observations suggest that azasetron is taken up by a carrier -mediated transport mechanism across the intestinal epithelial cells. When the steady-state uptake of [C-14]azasetron was measured, it was i ncreased in the presence of unlabeled azasetron and ondansetron. In ad dition, the steady-state uptake was enhanced in the presence of a P-gl ycoprotein inhibitor, cyclosporin A, and by ATP-depletion of the cells , although these treatments had no effect on the initial uptake of [C- 14]azasetron. Furthermore. the multidrug-resistant Cancer cell line K5 62/ADM that overexpresses P-glycoprotein accumulated azasetron less ex tensively than did the parental drug-sensitive K562 cells. These resul ts strongly suggest that azasetron is secreted into the intestinal lum en predominantly by P-glycoprotein. We conclude that intestinal transp ort of azasetron involves specialized transporters in both the absorpt ive and secretory directions, and the complex nonlinear intestinal abs orption characteristics can be ascribed to the participation of multip le transport mechanisms.