(P 42) PRECLINICAL VALIDATION OF M3554, A NOVEL ANTI-GD2 ANTIBODY-DRUG CONJUGATE, AND EPIGENETIC SENSITIZATION STRATEGIES IN SOFT TISSUE SARCOMAS

Objective: GD2, a disialoganglioside is enriched in various malignancies and has recently emerged as a potential therapeutic target in select soft tissue sarcomas (STS). However, approved anti-GD2 antibodies are associated with adverse events, limiting broader applicability. M3554 is a novel anti-GD2 antibody-drug conjugate (ADC) with a ß-glucuronide linker and exatecan payload that has shown strong antitumor activity in neuroblastoma PDX models. The Fc region of M3554 has been engineered to reduce pain associated with anti-GD2 antibodies. In this study, we evaluated the preclinical efficacy of M3554 in GD2-expressing STS models and explored the potential of EZH2 inhibition to upregulate GD2 expression as prior findings showed that EZH2 can silence ganglioside biosynthesis genes.

Methods: GD2 surface expression was assessed via flow cytometry in STS cell lines with or without the EZH2 inhibitor tazemetostat, followed by analysis of ganglioside biosynthetic enzyme expression. The in vitro cytotoxicity of M3554 was evaluated, and synergy between M3554 and tazemetostat was tested using combination treatment assays. In vivo, a dedifferentiated liposarcoma xenograft model was treated with either a single or fractionated dose of M3554, with tumor growth inhibition and toxicity monitored.

Results: GD2 was heterogeneously expressed across sarcoma subtypes. Tazemetostat upregulated GD2 expression across all tested cell lines, particularly in low-GD2-expressing models. M3554 exhibited potent in vitro cytotoxicity that correlated with the percentage of GD2-positive cells. Combination with tazemetostat showed signs of enhanced sensitivity to M3554 in select models tested. In vivo, M3554 significantly inhibited tumor growth without evidence of systemic toxicity.

Conclusion: M3554 is a promising anti-GD2 ADC with strong preclinical activity in GD2-expressing STS cell lines. Epigenetic modulation using EZH2 inhibition effectively upregulates GD2 expression, potentially enhancing therapeutic response and expanding the range of treatable tumors. These findings support the clinical development of M3554 in STS populations and provide a rationale for combining GD2-targeted therapies with epigenetic modulators. A first-in-human study of M3554 in patients with STS and IDH wildtype glioblastoma is ongoing (NCT06641908).