Interactions of antitumour Sialyl Lewis X liposomes with vascular endothelial cells.

Recently, we showed that tetrasaccharide selectin ligand SiaLe X provided targeted delivery of liposomes loaded in the bilayer with melphalan lipophilic prodrug to tumour endothelium followed by severe injury of tumour vessels in a Lewis lung carcinoma model. Here, we study the impact of SiaLe X ligand on the interactions of liposomes with human umbilical vein endothelial cells (HUVEC) using flow cytometry, spectrofluorimetry and confocal microscopy. Liposomes composed of egg phosphatidylcholine/yeast phosphatidylinositol/1,2-dioleoyl glycerol ester of melphalan, 8:1:1, by mol, and varying percentages of lipophilic SiaLe X conjugate were labelled with BODIPY-phosphatidylcholine. The increase in SiaLe X content in liposomes led to a proportional increase in their uptake by cytokine-activated cells as opposed to non-activated HUVEC: for 10% SiaLe X liposomes, binding avidity and overall accumulation increased 14- and 6-fold, respectively. The early stages of intracellular traffic of targeted liposomes in the activated cells were monitored by co-localisation with the trackers of organelles. Endocytosis of SiaLe X liposomes occurred mostly via clathrin-independent pathways, which does not contradict the available literature data on E-selectin localisation in the plasma membrane. Using dual fluorescence labelling, with rhodamine-labelled phospholipid and calcein encapsulated at self-quenching concentrations, we found that SiaLe X liposomes undergo rapid (within minutes) internalisation by activated HUVEC accompanied by the disruption of liposomes; non-activated cells consumed a negligible dose of liposomes during at least 1.5 h. Our data evidence the selective effect of SiaLe X formulations on activated endothelial cells and indicate their potential for intracellular delivery of melphalan lipophilic prodrug. • Avidity of SiaLe X liposomes to activated HUVEC is much higher than to resting cells. • The higher the SiaLe X content in liposomes, the higher their uptake by activated HUVEC. • This uptake is accompanied by rapid destabilization of liposome membrane. • Resting HUVEC consume a persistent low dose of both targeted and SiaLe X -free liposomes.