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Abstract
To investigate if the cross-linking of transferrin receptor (TfR) induced by Tf-oligomers can alter the endocytosis of receptor-ligand complexes and hence increases intracellular drug release. Pulse chase and cellular uptake studies have demonstrated that the intracellular retention and degradation of Tf-oligomers were significantly higher than that of monomeric Tf in cultured tumor cells. To further demonstrate that this alteration of TfR-trafficking can facilitate the intracellular release of the drug from the Tf-conjugated form, methotrexate (MTX) was conjugated to Tf-oligomer (Agg-Tf-MTX) or monomeric-Tf (Mono-Tf-MTX). Agg-Tf-MTX was 2- to 3-fold more potent than that of Mono-Tf-MTX in tumor cell lines. In addition, Agg-Tf-MTX was more potent than Mono-Tf-MTX in overcoming the drug resistance. Therefore, Tf-oligomers are potentially effective TfR-targeting carriers for intracellular delivery of anti-cancer drugs.
Tf-oligomer was also used to investigate the effect that cross-linking of TfR has on intracellular trafficking of the Tf-TfR complex in epithelial cells 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 of the Tf-oligomer was higher than that of monomeric Tf. In vivo studies in CF-1 mice showed that the plasma concentrations of Tf from orally administered Tf-oligomer were higher than that of monomeric Tf. 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-S-S-In, a delayed onset and prolonged hypoglycemic effect were observed. These results demonstrate that the cross-linking of TfR induced by binding of the oligomeric Tf increases the intracellular retention of Tf-TfR complexes in polarized Caco-2 cells and alters the TfR-trafficking which 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.
