The expression of ganglioside GD3, which plays essential roles in normal brain development, decreases in adults but is up regulated in neuroectoderm-derived cancers, where it enhances malignant properties. Antibodies to tumor-associated gangliosides are being used as therapeutic agents. Here, we demonstrated in human (SK-Mel-28) and mouse (B16) melanoma cells and hamster ovary cells (CHO-K1) that the anti GD3 antibody R24 is endocytosed and accumulated in recycling endosomes, precluding its use as a “naked” therapeutic because when internalized it cannot activate pathways of immune-mediated anticancer activity. However, it would be possible to exploit the internalization feature for the selective delivery of cytotoxic agents to GD3+ cancer cells. We have used antibody R24 to deliver saporin toxin-conjugated secondary antibody. We incubated cells in a 96-well plate with primary (3-60 nM) and secondary (1-8 nM) antibodies for 72 h, and cell viability determined. Wells with both antibody showed a decreasing of 70, 40 and 30% of cell viability in CHO-K1, SK-Mel-28 and B16 cells, respectively. The effect of the R24-saporin antibody conjugate was further evident by the lack of any effect on GD3-negative cells. Similar results were obtained using biotin-R24 coupled to streptavidin-saporin in SK-Mel-28 cells. The data suggest that GD3 may be a viable target for drug delivery for tumor cells.
