Abstract 7333: PD-L1-directed ISACs target host immune cells to drive powerful antitumor immune responses in a manner distinct from conventional PD-1/PD-L1 blockade

Abstract Background: Monoclonal antibodies targeting PD-1/PD-L1 have demonstrated clinical activity in numerous indications, but a large proportion of patients do not respond to anti-PD-(L)1 therapy, underscoring the need for alternative strategies. PD-L1 can be expressed by tumor cells as well as immune cells in the tumor microenvironment (TME) and tumor-draining lymph nodes. In particular, antigen-presenting cells (APCs) including macrophages and dendritic cells (DCs) can express high levels of PD-L1 and play a key role in regulating T cell responses through the PD-1/PD-L1 pathway. We are developing a novel class of molecules called Immune-Stimulating Antibody Conjugates (ISACs) that consist of tumor- or TME-targeting antibodies coupled to innate immune stimuli such as TLR7/8 agonists. Here, we present preclinical data demonstrating that PD-L1-targeted ISACs can engage both PD-L1-expressing tumor cells and APCs to drive robust immune responses and antitumor activity. Our results indicate a critical role for immune cell-expressed PD-L1 in antitumor activity and suggest a mechanism of action uncoupled from PD-1/PD-L1 blockade. Methods: ISACs were prepared using proprietary anti-human PD-L1 antibodies and TLR7/8 agonist payloads. Anti-mouse PD-L1 ISACs were made using antibodies from Bioxcell. In vitro activity was assessed in human whole blood, tumor/DC cocultures, and purified PD-L1+ myeloid cell populations. In vivo efficacy studies were performed in syngeneic and xenograft tumor models. PD-L1 knockout tumor cells were generated using CRISPR/Cas9. Results & Conclusions: PD-L1 ISACs induced robust, target-dependent activation of human myeloid cells cocultured with PD-L1-expressing tumor cells, as measured by secretion of cytokines such as IL-12 and TNFα. PD-L1 ISACs also strongly activated macrophages and DCs polarized to express high levels of PD-L1, demonstrating the ability to directly activate PD-L1-expressing APCs. Systemically administered PD-L1 ISACs were well tolerated in mice and showed improved efficacy over anti-PD-L1 antibodies, with tumor regression including complete responses observed in syngeneic and xenograft models. Mice with complete responses in syngeneic models rejected tumors upon rechallenge, indicating induction of immunological memory. Knockout of tumor PD-L1 expression in a syngeneic model had no impact on efficacy, suggesting that targeting of PD-L1-expressing host immune cells is critical for antitumor activity. PD-L1 ISACs exhibited similar efficacy whether or not the anti-PD-L1 antibody used blocked binding to PD-1, indicating that checkpoint blockade is not a major mechanism of action. These data demonstrate that PD-L1 ISACs represent a compelling new approach to treat cancer, leveraging mechanisms that are distinct from and potentially complementary to conventional PD-1/PD-L1 blockade. Citation Format: Po Y. Ho, Rishali K. Gadkari, Shahid Khan, David T. Omstead, Jennifer E. Melrose, Matthew N. Zhou, Katelynn A. McEachin, Karla A. Henning, William G. Mallet, Steven J. Chapin, Romas A. Kudirka, Shelley E. Ackerman, Michael N. Alonso, Justin A. Kenkel. PD-L1-directed ISACs target host immune cells to drive powerful antitumor immune responses in a manner distinct from conventional PD-1/PD-L1 blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 7333.