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 was studied by use of rat in situ intestinal perfusion, as well as an in vitro Ussing-type chamber method mounted with rat intestinal tissue and cultured monolayers of human adenocarcinoma Caco-2 cells. The intestinal absorption rate constant of azasetron evaluated by the Doluisio method increased significantly with increasing concentration of azasetron up to 10 mM in a nonlinear fashion and tended to decrease at higher concentrations. Mucosal-to-serosal directed permeation of [14C]azasetron across rat ileal sheets evaluated by the in vitro Ussing-type chamber method also increased in a nonlinear fashion in a low concentration range, followed by a decrease as the concentration was further increased, whereas serosal-to-mucosal directed permeation decreased in a concentration-dependent manner. Vectorial transport of [14C]azasetron across a Caco-2 cell monolayer was observed, with higher transport in the basolateral-to-apical direction at a trace concentration of azasetron. When the initial uptake rate of azasetron by Caco-2 cells was measured, it was saturable with an apparent half-saturation concentration of 15 mM and was reduced in the presence of several cationic compounds. 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 [14C]azasetron was measured, it was increased in the presence of unlabeled azasetron and ondansetron. In addition, the steady-state uptake was enhanced in the presence of a P-glycoprotein inhibitor, cyclosporin A, and by ATP-depletion of the cells, although these treatments had no effect on the initial uptake of [14C]azasetron. Furthermore, the multidrug-resistant cancer cell line K562/ADM that overexpresses P-glycoprotein accumulated azasetron less extensively than did the parental drug-sensitive K562 cells. These results strongly suggest that azasetron is secreted into the intestinal lumen predominantly by P-glycoprotein. We conclude that intestinal transport of azasetron involves specialized transporters in both the absorptive and secretory directions, and the complex nonlinear intestinal absorption characteristics can be ascribed to the participation of multiple transport mechanisms.