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In Vitro Targeting of NG2 Antigen by ²¹³Bi-9.2.27 -Immunoconjugate Induces Cytotoxicity in Human Uveal Melanoma Cells

¹From the Centre for Experimental Radiation Oncology, Cancer Care Centre, St. George Hospital, Kogarah, NSW, Australia; ²Discipline of Clinical Ophthalmology, Save Sight Institute, University of Sydney, Sydney, NSW, Australia; ³Department of Medicine, University of New South Wales, Kensington, NSW, Australia; and ⁴Department of Ophthalmology, Leiden University Medical Center, Leiden, The Netherlands.

PURPOSE. To examine uveal melanoma cell lines for the expression of human melanoma proteoglycan (NG2) using monoclonal antibody (mAb) 9.2.27 and subsequently to assess the in vitro specificity and cytotoxicity of mAb 9.2.27 conjugated to the -particle–emitting radioisotope ²¹³bismuth (²¹³Bi-9.2.27) for uveal melanoma cells.

METHODS. Immunocytochemistry and flow cytometry were used to examine OCM-1, OCM-3, OCM-8, OMM-1, Mel202 and 92–1 melanoma cell lines for NG2 expression. Melanoma cells were treated with test (²¹³Bi-9.2.27) or control (²¹³Bi-A2) -immunoconjugates (AICs). The specific cytotoxicity of ²¹³Bi-9.2.27 AIC was evaluated using an MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfenyl)-2H-tetrazolium, inner salt) assay. Cell death was also assessed using TUNEL.

RESULTS. OCM-1, OCM-8, OMM-1, and Mel202 cells strongly expressed NG2. OCM-3 cells showed moderate expression and 92–1 cells were NG2-negative. ²¹³Bi-9.2.27 specifically killed NG2-positive OCM-1, OCM-8, and OMM-1 cells in a concentration-dependent manner. D₀ values for 37% cell survival of NG2-positive OCM-1, OCM-8, and OMM-1 cells were 5.8, 5.0, and 5.6 µCi, respectively, and the value was 43.4 µCi for NG2-negative 92–1 cells.

CONCLUSIONS. The specific cytotoxicity of ²¹³Bi-9.2.27 AIC for NG2-positive, but not NG2-negative, cells suggests NG2 is a suitable target for -immunotherapy in uveal melanoma. ²¹³Bi-9.2.27 AIC used directly or as adjunct therapy may be a promising new agent for treating NG2-positive uveal melanomas or metastases.