Immunoliposomes directed toward VCAM-1 interact specifically with activated endothelial cells - A potential tool for specific drug delivery

Purpose. Immunoliposomes can be potentially used as carriers for drug delivery to specific cells. The aim of this paper was to exploit the overexpression of vascular cell adhesion molecule-1 (VCAM-1) on activated human endothelial cells (HEC) for targeting of anti-VCAM-1 coupled liposomes with the intent for further use as drug carriers. Methods. TNF-alpha-activated HEC were exposed to liposomes, either plain or coupled with antibodies to VCAM-1 (L-VCAM-1) or to irrelevant IgG (L-IgG); nonactivated HEC subjected to the same conditions were used as control. For binding studies, the cells were incubated with fluorescently labeled liposomes at 4 degrees C, and after 2 h, fluorescence intensity was assessed by flow cytometry; specificity of binding was determined by performing the experiments in the presence of excess anti-VCAM-1. Cellular internalization of liposomes was studied employing radioactively or fluorescently labelled liposomes; to detect the mechanisms of uptake, experiments were performed in the presence of agents that interfere in the endocytotic pathway. Transmigration of liposomes was monitored in a two-chamber culture model. The effect of L-VCAM-1 binding to HEC on intracellular calcium ([Ca2+](i)) and distribution of actin was determined by fluorimetry and fluorescence microscopy. Results. (1) L-VCAM-1 binds selectively and specifically to TNF-alpha activated HEC. (2) Approximately 50% of L-VCAM-1 is taken up by receptor-mediated endocytosis via clathrin-coated vesicles. (3) Binding of L-VCAM-1 to HEC surface induces a rise in [Ca2+](i) and reorganization of actin filaments. (4) A small percentage of liposomes migrates across HEC. Conclusion. The data indicate that VCAM-1 may be an appropriate target for specific drug delivery to activated HEC using immunoliposomes.