Low in vitro permeability of the cyanotoxin microcystin-LR across a Caco-2 monolayer: With identification of the limiting factors using modelling

• The apparent permeability of MC-LR was very low and time-dependant.
• The same kinetic behaviour was observed regardless of the loaded concentration.
• From 0.30 to 1.4%, crossed the epithelium from A-to-B after 24 h.
• The low permeability is due to an efficient secretion combined with a low efflux.
• An asymmetric transport of MC-LR across the Caco-2 monolayer was observed.

Microcystins (MCs) are toxins produced by several cyanobacteria species found worldwide. MC-LR is the most frequent. Here, we used the human Caco-2 cell line grown on semi-permeable filter supports as an in vitro model for determining MC-LR intestinal bidirectional transport. In this study, there was very low and time-dependent apparent permeability of MC-LR. To identify the limiting factors involved in the low permeability of MC-LR, a mathematical model was constructed to get physiologically relevant and informative parameters. The apical-to-basolateral transport was characterised by a rapid and substantial decrease in apical MC-LR concentrations (24–40% of the initial amount). In the basolateral compartment, the concentrations increased slowly after a lag time, but represented only a small fraction of the loaded concentrations (0.3–1.3%) after 24 h. This weak permeability was mainly due to a low clearance of efflux (from the cellular to the basolateral compartment) and effective secretion (from the cellular to the apical compartment). During the basolateral-to-apical transport, we observed a slow decrease in basolateral concentrations and a rapid increase in apical concentrations. In conclusion, modelling has the potential to highlight the key mechanisms involved in the complex kinetics of toxin transport.