A head-to-head Caco-2 assay comparison of the mechanisms of action of the intestinal permeation enhancers: SNAC and sodium caprate (C).

Salcaprozate sodium (SNAC) and sodium caprate (C) are the two leading intestinal permeation enhancers (PEs) in oral peptide formulations in clinical trials. There is debate over their mechanism of action on intestinal epithelia. The aims were: (i) to compare their effects on the barrier function by measuring transepithelial electrical resistance (TEER), permeability of FITC-4000 (FD4) across Caco-2 monolayers, and on immunohistochemistry of tight junction (TJ)-associated proteins; and (ii) to compare cellular parameters using conventional end-point cytotoxicity assays and quantitative high content analysis (HCA) of multiple sub-lethal parameters in Caco-2 cells. C (8.5 mM) reversibly reduced TEER and increased FD4 permeability across monolayers, whereas SNAC had no effects on either parameter except at cytotoxic concentrations. C exposure induced reorganization of three TJ proteins, whereas SNAC only affected claudin-5 localization. High concentrations of C and SNAC were required to cause end-point toxicology changes in vitro. SNAC was less potent than C at inducing lysosomal and nuclear changes and plasma membrane perturbation. In parallel, HCA revealed that both agents displayed detergent-like features that reflect initial membrane fluidization followed by changes in intracellular parameters. In conclusion, FD4 permeability increases in monolayers in response to C were in the range of concentrations that altered end-point cytotoxicity and HCA parameters. For SNAC, while HCA parameters were also altered in a similar overall pattern as C, they did not lead to increased paracellular flux. These assays show that both agents are primarily surfactants, but C has additional TJ-opening effects. While these in vitro assays illucidate their epithelial mechanism of action, clinical experience suggests that they over-estimate their toxicology in the dynamic intestinal environment.