Membrane transport mechanisms of mizoribine in the rat intestine and human epithelial LS180 cells

The aim of the present study was to characterize membrane transport mechanisms of mizoribine in the intestinal epithelial cells. We evaluated the contribution of Na⁺-dependent and -independent membrane transporters to mizoribine absorption in the rat intestine using an in situ closed loop method. In addition, we evaluated the effects of structurally related compounds, extracellular Na⁺ concentrations, and an inhibitor of Na ⁺- independent equilibrative nucleoside transporter, nitrobenzylmercaptopurine ribonucleoside (NBMPR), on the uptake of mizoribine in human intestinal epithelial LS180 cells. In the presence and also absence of Na⁺ in rat intestinal loops, more than 60% of the administered dose (50 μg at the concentration of 100 μg/ml=386 μm) of mizoribine was absorbed in 40min. In the LS180 cells, ribavirin and inosine reduced the uptake of 400 μm mizoribine with the increasing concentrations (from 5 to 50 mm) of the inhibitors. The cellular uptake of mizoribine in the absence of extracellular Na⁺ decreased to 72.7% of the uptake in the presence of extracellular Na⁺, whereas 100 μm NBMPR decreased the uptake of mizoribine markedly to 34.7% of that without NBMPR. These findings suggest that Na⁺-independent nucleoside transporters are largely responsible for absorption of mizoribine in the intestine.