Importance of the paracellular pathway for the transport of a new bisphosphonate using the human CACO-2 monolayers model.

The transport of a new bisphosphonate, Tiludronate, was investigated on the human adenocarcinoma cell line, CACO-2. Experiments were performed 7-16 days after cells achieved confluence, conditions under which they form well-differentiated monolayers joined by tight junctions. Tiludronate transport rate across CACO-2 monolayers was independent of the temperature (4 degrees versus 37 degrees), of the polarity of the cell membrane (apical-to-basolateral versus basolateral-to-apical), and of the presence of metabolic poisons (sodium azide). Its transport was enhanced by either the presence of EGTA in the incubation buffer, i.e. when extracellular Ca2+ concentration was reduced, or by the pretreatment of monolayers with EGTA, i.e. when the intercellular spaces and the tight junctions were widened. Based on these different observations, we could suggest that Tiludronate mainly used the paracellular pathway to cross the intestinal epithelium. An increase in the Tiludronate permeability coefficient was also observed following treatment of cells with high Tiludronate concentrations, as a consequence of the direct effect of this compound on the extracellular Ca2+ ions. Hence, for high drug concentrations, i.e. 20 mM, we observed a decrease in free extracellular Ca2+ concentration, an increase in the transepithelial electrical resistance and an increase in the transport of [14C]polyethyleneglycol ([14C]PEG400), a probe for the paracellular pathway. The results indicate that Tiludronate is transported across CACO-2 monolayers by the paracellular route. Moreover, it can affect its own transport by its concentration-dependent effect on tight junction widening.