The success of a therapeutic intervention ultimately depends on the ability of the drug molecule to reach its predetermined target inside the body at relevant concentrations. Clearance and active metabolism limit the population and circulation life times of the drug molecules in the blood stream while the physical barrier of the endothelium limits their transport to the organ cells or tissue. In the case of high toxicity drug constructs, specificity in binding to the predetermined target is an additional constraint. A strategy to overcome these limitations would be to conjugate the drug molecule to a chaperone that has high circulation life times on account of slow clearance and metabolism, has predetermined transport pathways for crossing the endothelium out of and back into the circulation and has some specificity in terms of access to distinct compartments in the body. An obvious choice, and with some precedents, would be the use of physiological transport proteins. Multicellular organisms routinely use proteins to transport molecular cargo between cells, and eventually across the vasculature, via a well regulated, highly vectored process of transcytosis. We focused attention on three such proteins, previously implicated in transcytosis, to study their characteristics as chaperones for carrying drug molecules in circulation: human serum albumin (HSA), transferrin (Tf) and immunoglobulin complex type G (IgG).

Initial studies were geared towards understanding the molecular transport of proteins across the vascular endothelium. FITC-labelled dextrans of varying molecular weights were used as model solutes to understand passive diffusion across the endothelium and to establish the validity of the experimental model used. Excellent agreements were reached with published numbers in the literature from ex-vivo studies on microvascular capillary beds. Size selective transport of the endothelium was also very well reproduced in the experimental model. Protein transport across the endothelium was concentration dependent for all the proteins investigated indicating the presence of a receptor mediated transcytosis mechanism. Further the transport was found to be energy dependent indicating the presence of an active transport mechanism over passive diffusion. The active component of albumin transport was observed to be 70-80 % higher as compared to passive transport of a similar molecular weight dextran at sub physiological concentrations. At physiological concentrations, however, the transport is saturated and completely dominated by passive diffusion. Qualitatively similar results were observed for IgG and Tf.

These results provide insights into the proposal of using physiological carrier proteins for delivering systemic therapeutics. Quantitative transport measurements such as those described above will be useful in constructing a framework to explain transport of model proteins across cells and membranes. Such a framework will not only provide insights into transcytosis as an active transport process but will also help in designing drug conjugates using these proteins as chaperones.