Comparison of monomeric and oligomeric transferrin as potential carrier in oral delivery of protein drugs.

Oligomeric transferrin (Tf) was used to investigate the effect that cross-linking of transferrin receptors (TfR) has on intracellular trafficking of the Tf-TfR complex and to determine whether or not the Tf-oligomer would be a better carrier than monomeric Tf for the oral delivery of protein drugs. The intracellular retention and transcytosis of the Tf-oligomer was determined by performing pulse chase studies on enterocyte-like Caco-2 cells. The intracellular retention of the Tf-oligomer was 2-fold higher than that of monomeric Tf while there was no significant difference in transcytosis. However, in vivo studies in CF-1 mice showed that the plasma concentrations of Tf from the orally administered Tf-oligomer were approximately 2-, 3- and 60-fold higher than that of orally administered monomeric Tf at 24, 48 and 72 h post-administration, respectively. In addition, the retention of the Tf-oligomer in the intestine was higher than that of monomeric Tf, which was consistent with in vitro studies. Insulin (In), when conjugated to the Tf-oligomer (Agg-Tf-S-S-In), was more effective than monomeric Tf-In conjugate (Mono-Tf-S-S-In) in reducing blood glucose levels when orally administered to streptozotocin (STZ)-induced diabetic rats. Post-oral administration of Agg-Tf-In, a delayed onset and prolonged hypoglycemic effect was observed. These results demonstrate that the cross-linking of TfR induced by the binding of the oligomeric Tf alters the intracellular trafficking and increases the intracellular retention of Tf-TfR complexes in polarized Caco-2 cells. The alteration of TfR trafficking could conceivably have caused the increase of insulin transport across the intestinal barrier when Agg-Tf-S-S-In was administered orally to STZ-induced diabetic rats. The delayed onset and prolonged effect of Agg-Tf-S-S-In in hypoglycemia strongly suggests that the Tf-oligomer can act as a sustained release carrier in the oral delivery of protein and peptide drugs.