Development of a minimum-calcium Caco-2 monolayer model: Calcium and magnesium ions retard the transport of pamidronate

Abstract

The oral absorption of some drug molecules (e.g., tetracyclines and quinolones) is limited by complexation with di- and trivalent metal cations in the gastrointestinal tract. Progress in the understanding of these absorption-limiting interactions has been restricted in the absence of a suitable in vitro model. To address this, a modification of the conventional Caco-2 transport model has been developed which has a calcium-free apical donor solution and a basolateral receiver solution containing the minimum calcium concentration required to maintain monolayer integrity (100 μM). The minimum-calcium model is proposed to be a useful universal model for studying the influence of metal cations on the transepithelial transport of drug molecules. The influence of calcium and magnesium ions on the absorption of pamidronate was evaluated by comparing its transport across the conventional and minimum-calcium Caco-2 models. In the conventional Caco-2 model, the ratio of mannitol/pamidronate flux was 5:1, whereas in the minimum-calcium model this ratio was reduced to 3:1. The elevated transepithelial transport rate for pamidronate in the minimum-calcium model cannot be explained by minor changes in the permeability of Caco-2 monolayers. It was concluded that calcium and magnesium ions retard the apical-to-basolateral flux of pamidronate across Caco-2 monolayers.