Apical-to-Basolateral Transcytosis of Transferrin-Polylysine Conjugates in Caco-2 Cell Monolayers: A Model of Relay Transport of Protein Drug across Epithelial Cells

While the number of therapeutic biologics on the market has dramatically increased in the past decades, oral delivery of protein/peptide drugs is still one of the challenges for biopharmaceutical scientists. Transferrin (Tf), with its receptor-mediated transcytosis mechanism across intestinal epithelium, has been proven as a potential oral delivery carrier for different protein therapeutics. However, one of the major rate-limiting issues of Tf in oral absorption is that Tf Receptor (TfR) is selectively expressed on the basolateral surface instead of the apical surface of the intestinal epithelium, and thus, limits the mucosal transport capacity of Tf across the intestinal epithelium. To overcome this limitation and develop a more efficient oral delivery carrier, a novel transcytosis mechanism was proposed. The proposed transcytosis mechanism is composed of three major steps: apical binding and endocytosis of Tf with a cationic peptide carrier, intracellular cleavage of the linker between peptide and Tf, and basolateral exocytosis by TfR. The feasibility of the proposed approach was demonstrated when Tf was conjugated to poly-L-lysine (PLL) using a pHsensitive Nitrilo Triacetic Acid (NTA) linker in the presence of nickel (Ni2+). The results of Caco-2 transcytosis assay suggested that the transcytosis rate of histidine-tagged Tf could be unidirectionally increased from apical to basolateral compartment when co-incubated with Ni2+-NTA-PLL.