Abstract 3524: Exploring both targeted and untargeted (bystander) effects of GSK5764227 antibody-drug conjugate in patient-derived tumor explants (PDEs)

Abstract Background: B7-H3, a member of the B7 family of immune checkpoint molecules, is emerging as a promising therapeutic target due to its overexpression in several cancers and association with poor clinical outcome. GSK5764227 is a fully humanized IgG-1 monoclonal antibody against B7-H3 conjugated to a topoisomerase 1 (TOP1) inhibitor. GSK5764227 is predicted to selectively target the tumor cells by delivering potent payload, however the role of direct killing vs effects on bystander tumor cells is unknown. Methods: In this study by using spatial imaging endpoints, we evaluate the expression of B7-H3 and associated genes/ proteins in surgically resected NSCLC tumors samples (Stage IB-IIIB). We then establish a NSCLC patient derived tumor explants (PDE) model, which re-capitulates the pathological tumor microenvironment (TME), and treat the PDE fragments with anti-B7-H3 ADC (GSK5764227), naked anti-B7-H3mAb (GSK5764224), and human-IgG1 control mAb in culture for 72hrs. Using multiplex imaging endpoints, we assess the spatial distribution and binding of GSK5764227 and payload (TOP1 inhibitor) in key cellular subsets and evaluate cell killing (Cleaved Caspase-3) within the TME. Results: In NSCLC tumor tissues, the B7-H3 protein is abundantly expressed in proliferative tumor epithelial and cancer associated fibroblast (CAFs) cells, but at low-levels in adjacent normal tissue. TOP1 and TOP1 inhibitor sensitivity genes are abundantly expressed in proliferative tumor epithelial and fibroblasts, suggesting that these cells maybe be sensitive to ADC. In PDE models, GSK5764227 and payload showed preferential binding to B7-H3 positive epithelial (EpCam+), fibroblast (aSMA+) and macrophage (CD68+) subsets. In a subset of B7-H3 negative cells, the payload was also detected, suggestive of either downregulation/degradation via internalization of B7-H3 receptor after binding to the ADC, or payload diffusion in B7-H3 negative cells (bystander-effect). This correlated with significant increase in cleaved caspase-3+ staining in payload+ tumor epithelial, macrophages and fibroblast, suggestive that the cell killing could be driven by direct payload deposition in the target population and that all 3-cell types are sensitive to killing by the TOP1 inhibitor payload. Conclusions: Our results potentially aid in mechanistic understanding of the inter-play between ADC binding and payload deposition, bystander effect and heterogeneity of tumor response, which will support the future design of clinical trials with GSK5764227. Citation Format: Debayan Mukherjee, Jenny Wade, Morgan Heycock, Mint Htun, Benjamin Miller, Zeinab Mokhtari, Diana Munera, Anna Pasto, Paul Barber, Tony NG, Sue Griffin. Exploring both targeted and untargeted (bystander) effects of GSK5764227 antibody-drug conjugate in patient-derived tumor explants (PDEs) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3524.