With the advent of biotech epoch, more and more proteins and peptides become available for clinical treatment, such as growth hormone , calcitonin , and octreotide . Nevertheless, due to short half-life in the blood circulation, it is inevitable to take the medications subjected to multi-dosage over a long time for chronic diseases. Insulin, a protein secreted by the β cells of the pancreas, is one of the most important therapeutic agents for insulin-dependent (type I) and deteriorative insulin-independent (type II) diabetes mellitus , and commonly administered subcutaneously; however, besides pain, which may bring about unwanted complications, e.g. allergic reactions, hyperinsulinemia, insulin lipodystrophy around the injection site . Problems encountered with insulin injection vitalize the demands to develop alternative delivery systems.
However, to achieve effective oral delivery of insulin, several barriers like instability, gastrointestinal enzymatic degradation, and poor membrane permeability, etc., should be overcome beforehand . Various delivery strategies, especially those based on nanoscaled delivery systems, have been explored to enhance the oral delivery of insulin, including microemulsions , nanospheres , polymeric nanoparticles [9, 10], niosomes , and liposomes [12–14]. However, the state of the art indicates that there seems to have reached a bottleneck in terms of oral bioavailability enhancement of insulin. It is highly recommended to explore novel strategies to ameliorate the performance of nanoscaled drug delivery systems. As known, receptor-mediated endocytosis, a process of internalization of extracellular molecules during which a binding occurs between the molecules and the receptors, is an important absorption mechanism for substances like proteins, hormones, growth factors, and fatty nutrients . In the intestinal cavity, there are a variety of receptors expressing on the membrane of enterocytes such as vitamins, transferrin, amino acids, sugars, and bile salt receptors . Targeted drug delivery to absorptive epithelia by receptor-mediated endocytosis has emerged as a prominent means to improve oral delivery of drugs . Vitamins as ligands, which can specifically bind to enterocytic receptors, have been extensively studied for the oral delivery of poorly permeable molecules [18–22].
Biotin receptors that distribute in the small intestine and partially in the colon are responsible for the essential absorption of biotin by nonspecific receptor-mediated endocytosis . Additionally, biotin plays an important role in maintaining the homeostasis of blood glucose . Improved cellular permeability and higher hypoglycemic effect after oral administration of biotin-conjugated glucagon-like peptide-1 has been observed . Biotin-modified vehicles have been investigated for nonparenteral delivery of active ingredients [26–29]. Our previous report has also proved that biotin-modified liposomes (BLPs) have ability to improve the oral delivery of insulin, and studied the uptake and transport mechanisms in the gastrointestinal tract . However, particular enhanced absorption mechanisms and cytotoxicity of BLPs are not clear enough.
Herein, we performed several experiments to further probe the oral absorption mechanism of BLPs based on previous studies  as well as the cytotoxicity thereof. We evaluated hypoglycemic effects of BLPs of various particles, or with different amounts of biotin-DSPE using normal rats. Meanwhile, the influence of BLPs on tight junctions and internalization process was further investigated by Caco-2 cells.