Purpose: It has recently been reported that the permeability of peptides across Caco-2 cells, an in vitro model of the intestinal mucosa, was limited by an apically polarized efflux mechanism. Since surfactants (e.g. Cremophor EL, Polysorbate 80) have been reported to inhibit similar efflux systems in tumor cells, we determined whether they could enhance the permeability of peptides across monolayers of Caco-2 cells.

Methods: The transport studies of [3H]-mannitol and [14C]-model peptides were carried out across the Caco-2 cell monolayers. TEER values were determined using Voltohmmeter with STX-2 electrode and the equilibrium dialysis studies were conducted using side-by-side dialysis apparatus with cellulose ester membranes.

Results: Initially, [3H]-mannitol flux studies were conducted to find concentrations of the surfactants that did not cause damage to the cell monolayer. Based on these studies, Polysorbate 80 and Cremophor EL were selected for further study. The fluxes of [14C]-AcfNH2 (a nonsubstrate for this efflux system) and [14C]-Acf(N-Mef)2NH2 (a substrate for this efflux system) were then measured in the absence and presence of the two surfactants. The permeability of [14C]-AcfNH2 was not affected by the surfactants, while that of [14C]-Acf(N-Mef)2NH2 increased with increasing concentrations of surfactants and then decreased. For example, the Pe values for [14C]-Acf(N-Mef)2NH2 were 3.75 x 10(-6), 8.58 x 10(-6), 10.29 x 10(-6), 7.48 x 10(-6), and 1.46 x 10(-6) cm/sec with Cremophor EL concentrations of 0, 0.01, 0.1, 1 and 10% w/v, respectively. This bimodal effect of surfactants on the Caco-2 cell permeability of this peptide was shown to be due to the interactions between the peptide and micelles at higher concentrations of surfactants, which were demonstrated by the equilibrium dialysis experiments.

Conclusions: These results suggest that surfactants, which are commonly added to pharmaceutical formulations, may enhance the intestinal absorption of some drugs by inhibiting this apically polarized efflux system.