Abstract 108: Development of antibody drug conjugates targeting chondroitin sulfate proteoglycan 4 (CSPG4)

Abstract Background: Melanoma is the fifth most common cancer in the United States and has a high five-year survival rate when diagnosed at an early stage. However, most patients with metastatic melanoma (Stage IV) ultimately succumb due to drug resistance. Drug-resistant melanoma is linked with intratumoral heterogeneity and enrichment for de-differentiated cell states. Chondroitin sulfate proteoglycan 4 (CSPG4), a tumor associated surface antigen, is notably enriched in the de-differentiated mesenchymal and neural crest-like subpopulations in human melanoma. Moreover, CSPG4 expression is positively associated with melanoma progression, invasion, and metastasis. Given its association with aggressive disease and relatively low expression in normal tissue, CSPG4 has attracted interest as a therapeutic target with a current focus on CAR T cell-based therapies. The immunosuppressive solid tumor microenvironment, however, limits the effectiveness of existing immunotherapies, and CSPG4 expression is enriched in patients who do not respond to immune checkpoint blockade. Antibody-drug conjugates (ADCs) represent an alternative, promising strategy to target drug resistant melanoma cells by delivering cytotoxic agents directly to tumor cells. Methods: We utilized phage and yeast display to discover novel antibodies with cross-reactivity to human and mouse CSPG4. Several candidate antibodies were conjugated with cytotoxic payloads and evaluated for specificity and potency in CSPG4-high versus CSPG4-null melanoma cells from mouse and human. Results: We have developed a large panel of human synthetic antibodies recognizing a total of six distinct epitopes on CSPG4, with high affinity and specificity. Several of these antibodies demonstrated cross-reactivity to human and mouse CSPG4. ADC candidates derived from this panel exhibited cytotoxicity against CSPG4-high mouse and human melanoma cells, while showing no cytotoxicity against CSPG4-null cells. Notably, the ADCs retained potent cytotoxic activity even in the presence of soluble CSPG4 concentrations 100-fold higher than the levels of soluble CSPG4 observed in cell culture media. Because soluble CSPG4 could potentially limit ADC effectiveness by competing for binding, we also identified antibodies capable of reducing soluble CSPG4 through inhibition of cell-surface CSPG4 shedding, which may improve ADC targeting. Conclusions: Our findings highlight CSPG4 as a promising therapeutic target for drug-resistant melanoma. The novel human/mouse-cross-reactive, fully human antibodies and corresponding ADC candidates demonstrated robust, specific tumor cell killing while maintaining activity despite high levels of soluble CSPG4. These molecules are promising candidates for further development of CSPG4-targeting ADCs, including efficacy assessment in immunocompetent syngeneic models, to address therapeutic resistance in metastatic melanoma and other challenging tumor types. Citation Format: Xiaoxuan Zhong, Injin Bang, Akiko Koide, Michelle Wang, Takamitsu Hattori, Amanda Lund, Shohei Koide. Development of antibody drug conjugates targeting chondroitin sulfate proteoglycan 4 (CSPG4) [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr A108.