Screening and optimization of ligand conjugates for lysosomal targeting

The use of lysosome-targeted liposomes may significantly improve the delivery of therapeutic enzymes and chaperones into lysosomes for the treatment of lysosomal storage disorders. The aim of this research was to synthesize new potentially lysosomotropic ligands on a base of Neutral Red and rhodamine B and to study their ability to enhance specific lysosomal delivery of surface-modified liposomes loaded with a model compound, fluorescein isothiocyanate-dextran (FD). The delivery of these liposomes and their content to lysosomes in HeLa cells was investigated by confocal immunofluorescent microscopy, subcellular fractionation, and flow cytometry. Confocal microscopy demonstrated that liposomes modified with derivatives of rhodamine B provide a good rate of colocalization with the specific lysosomal markers. The comparison of fluorescence of FD in lysosomes isolated by subcellular fractionation also showed that the efficiency of lysosomal delivery of the liposomal load by liposomes modified with some of synthesized ligands was significantly higher compared to that with plain liposomes. These results were additionally confirmed by flow cytometry of the intact cells treated with liposomes loaded with 5-dodecanoylaminofluorescein di-β-d-galactopyranoside, a specific substrate for the intralysosomal β-galactosidase, using a number of cell lines, including macrophages with induced phenotype of lysosomal enzyme deficiency; two of the synthesized ligands-rhodamine B DSPE-PEG(2k)-amide and 6-(3-(DSPE-PEG(2k))-thioureido) rhodamine B-demonstrated enhanced lysosomal delivery, in some cases, higher than that for commercially available rhodamine B octadecyl ester, with the best results (the enhancement of the lysosomal delivery up to 75% greater in comparison to plain liposomes) shown for the cells with induced lysosomal enzyme deficiency phenotype. Use of liposomes modified with rhodamine B derivatives may be advantageous for the development of drug delivery systems for the treatment of lysosome-associated disorders.