Abstract 3818: A novel multi-specific antibody targeting PD-L1-overexpressing cancers that redirects and stimulates antigen-committed CD8+ T cells through concomitant engagement of a T cell costimulatory receptor (2018)

Abstract Targeting PD-L1-overexpressing cells with therapeutic antibodies is a clinically validated strategy for the treatment of multiple solid tumors. In order to increase efficacy, PD-1/PD-L1 blocking agents are currently being tested in combination with additional immune checkpoint modulators (ICMs). However, such combination therapies are associated with considerable treatment-related adverse events, resulting in a narrow therapeutic window and thereby limiting treatment efficacy. To maximize potency and improve the safety of ICM combination approaches, we sought to design a multi-specific molecule bearing two ICM domains that depletes PD-L1-overexpressing cancer cells via selective recruitment and stimulation of tumor-reactive effector T cells in the tumor microenvironment. The molecule's design consists of three monovalent antibody variable-domain fragments (Fvs) specific for PD-L1, human serum albumin and a T cell costimulatory receptor fused in a single-chain (PD-L1/HSA/costim tri-specific scDb-scFv). The monovalent and Fc-less structure of the molecule ensures that agonism of the costimulatory receptor on effector cells can only arise when the molecule concomitantly binds to PD-L1 on the surface of target cells. The addition of a half-life-extending anti-SA domain, meanwhile, not only enables convenient dosing but also should promote delivery of the molecule to tumor microenvironments. Here, we demonstrate the successful production of a novel multi-specific antibody that potently blocks PD-L1/PD-1 signaling and elicits T cell costimulation solely in the presence of cells that overexpress PD-L1, as confirmed using a transgenic Jurkat reporter T cell-line in co-cultures with HT29 and HCC827 cancer cell-lines. We show that the molecule elicits such costimulation considerably more selectively and potently than a first-generation, clinical-stage, anti-costim IgG. Furthermore, the tri-specific is incapable of stimulating T cells in the absence of primary T cell activation. Finally, in an HCC827 xenograft model in hPBMC-supplemented NOG mice, the tri-specific slowed tumor growth and enhanced intratumoral CD8+ T cell activation to a greater extent than monospecific IgG variants of the anti-PD-L1 and anti-costimulatory receptor domains. These data support the hypothesis that broadening the therapeutic window for a promising, clinically validated ICM-based treatment strategy can be achieved by combining multiple ICM domains in an Fc-less, monovalent multi-specific that selectively forms immunological synapses between cancer cells and activated effector cells. Citation Format: Tea Gunde, Sebastian Meyer, Stefan Warmuth, Alexandre Simonin, Christian Hess, Matthias Brock, Timothy Egan, David M. Urech. A novel multi-specific antibody targeting PD-L1-overexpressing cancers that redirects and stimulates antigen-committed CD8+ T cells through concomitant engagement of a T cell costimulatory receptor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3818.