No abstract

Human colon adenocarcinoma (Caco-2) cells, when grown on semipermeable filters, spontaneously differentiate in culture to form confluent monolayers which both structurally and functionally resemble the small intestinal epithelium. Because of this property they show promise as a simple, in vitro model for the study of drug absorption and metabolism during absorption in the intestinal mucosa. In the present study, the transport of several model solutes across Caco-2 cell monolayers grown in the Transwell diffusion cell system was examined. Maximum transport rates were found for the actively transported substance glucose and the lipophilic solutes testosterone and salicyclic acid. Slower rates were observed for urea, hippurate, and saliylate anions and were correlated with the apparent partition coefficient of the solute. These results are similar to what is found with the same compounds in other, in vivo absorption model systems. It is concluded that the Caco-2 cell system may give useful predictions concerning the oral absorption potential of new drug substances.

Successful drug development requires not only optimization of specific and potent pharmacological activity at the target site, but also efficient delivery to that site. Many promising new peptides with novel therapeutic potential for the treatment of AIDS, cardiovascular diseases, and CNS disorders have been identified, yet their clinical utility has been limited by delivery problems. Along with metabolism, a major factor contributing to the poor bioavailability of peptides is thought to be inefficient transport across cell membranes. At the present time, the reasons for this poor transport are poorly understood. To explore this problem, we have designed experiments focused on determining the relationship between peptide structure and peptide transport across various biological membranes both in vitro and in vivo. Briefly, peptides that varied systematically in chain length, lipophilicity, and amide bond number were prepared. Permeability results with these solutes support a model in which the principal determinant of peptide transport is the energy required to desolvate the polar amides in the peptide for the peptide to enter and diffuse across the cell membrane. Further impacting on peptide permeability is the presence of active, secretory transport systems present in the apical membrane of intestinal epithelial and brain endothelial cells. In Caco-2 cell monolayers, a model of the human intestinal mucosa, this pathway displayed substrate specificity, saturation, and inhibition. Similar results have been shown in vivo in both rat intestinal and blood-brain barrier absorption models. The presence of such systems serves as an additional transport barrier by returning a fraction of absorbed peptide back to the lumen.

No abstract

The relationship between structure and permeability of peptides across epithelial cells was studied. Using confluent monolayers of Caco-2 cells as a model of the intestinal epithelium, permeability coefficients were obtained from the steady-state flux of a series of neutral and zwitterionic peptides prepared from D-phenylalanine and glycine. Although these peptides ranged in lipophilicity (log octanol/water partition coefficient) from -2.2 to +2.8, no correlation was found between the observed flux and the apparent lipophilicity. However, a strong correlation was found for the flux of the neutral series and the total number of hydrogen bonds the peptide could potentially make with water. These results suggest that a major impediment to peptide passive absorption is the energy required to break water-peptide hydrogen bonds in order for the solute to enter the cell membrane. This energy appears not to be offset by the favorable introduction of lipophilic side chains in the amino acid residues.