Contribution of P-glycoprotein to the enhancing effects of dimethyl-beta-cyclodextrin on oral bioavailability of tacrolimus

We recently reported that of all hydrophilic cyclodextrin (CyD) derivatives examined, 2,6-di-O-methyl-beta -cyclodextrin (DM-beta -CyD) most significa ntly increased the aqueous solubility and the dissolution rate, resulting i n the improvement of oral bioavailability of the immunosuppressive drug tac rolimus in rats. In the present study, we showed that DM-beta -CyD increase d the dissolution rate and oral bioavailability of tacrolimus in rats with increases in the molar ratio of the complexes (DM-beta -CyD: tacrolimus). H owever, nonlinear pharmacokinetic behavior of tacrolimus after oral adminis tration in rats was observed. Thus, an additional mechanism of the solubili zing effect of DM-beta -CyD on oral bioavailability of tacrolimus was postu lated. To gain insight into this additional mechanism of action of DM-beta -CyD, its effects on the efflux of tacrolimus and rhodamine 123, a P-glycop rotein (P-gp) substrate, were examined using both Caco-2 and vinblastine-re sistant Caco-2 (Caco-2R) cell monolayers. Pretreatment of the apical membra nes of the monolayers with DM-beta -CyD decreased the efflux of tacrolimus and rhodamine 123 without an associated cytotoxicity. DM-beta -CyD decrease d the P-gp level in the apical membranes of both Caco-2 and Caco-2R cell mo nolayers, probably by allowing release of P-gp from the apical membrane int o the transport buffer. DM-beta -CyD, however, did not decrease the MDR1 ge ne expression in Caco-2 or Caco-2R cells. These results suggested that the enhancing effect of DM-beta -CyD on the oral bioavailability of tacrolimus is due not only to its solubilizing effect but also, at least in part, to i ts inhibitory effect on the P-gp-mediated efflux of tacrolimus from intesti nal epithelial cells.