Abstract 706: CRB-601, an avβ8 blocking antibody, prevents activation of TGFb and exhibits anti-tumor activity associated with immune cell remodeling of the tumor microenvironment

Abstract Background: Transforming growth factor-beta (TGFβ) is a promising immunotherapeutic target in cancer given the association of increased TGFβ signaling in the tumor microenvironment (TME) with immune cell exclusion, and poor clinical outcomes. TGFβ is expressed as a latent form (L-TGFβ) and presented on cell surfaces by L-TGFβ binding proteins (e.g. GARP and LRRC33) as part of the large latent complex, whereupon it is activated by binding to integrins, including integrin αvβ8. Corbus Pharmaceuticals is developing a humanized monoclonal antibody, CRB-601, that binds with high specificity and affinity to αvβ8 and blocks the critical interaction with L-TGFβ that promotes an immune excluded phenotype. Methods: Tumor growth was evaluated in mice bearing orthotopically implanted murine breast cancer EMT6 or colon cancer MC38 and treated with CRB-601, anti-PD1 antibody, or the combination. αvβ8 receptor occupancy (RO) was measured ex vivo in dissociated tumors by flow cytometry (FC). Tumor-infiltrating immune cell populations were characterized by FC, and pSMAD2/3 was measured by Western blot. Results: CRB-601 exhibited dose dependent tumor growth inhibition (TGI) in the EMT6 tumor model. TGI activity at 0.3mg/kg biw dose was 22.0 ± 3.8 %, and reached maximum antitumor activity at a dose between 3 mg/kg biw (41.3 ± 8.4%) and 10 mg/kg biw (48.4 ± 8.4 %). TGI was significantly augmented in combination with anti-PD1. These effects were associated with changes in TME immune cell populations, as characterized by marked increases in infiltrating T cells, NK cells and M1 polarized macrophages, thereby converting immune excluded EMT6 tumors to immune cell-inflamed tumors. Efficacy correlated with cell surface αvβ8 occupancy by CRB-601 as measured by FC in CD45- EMT6 tumor cells, increasing with CRB-601 dose from 5.5 ± 0.4% at 0.3 mg/kg to a maximum of 75.6 ± 23.8% at 10 mg/kg. CRB-601 treatment downregulated phosphorylation of SMAD proteins pSMAD2 and pSMAD3, consistent with blockade of the canonical TGFβ signaling pathway. When combined with anti-PD1, CRB-601 induced a T-cell memory response. In the MC38 model, mice cured by the combination treatment survived long term (>40 mo). Naïve mice adoptively transferred with splenocytes from these mice gained tumor-specific antitumor activity and rejected MC38 tumor cell inoculation. These splenocytes contained an immune cell population of IFN-γ producing cytotoxic T cells with a repertoire consistent with MC38 antigen recognition. Conclusions: CRB-601 is a potent and selective integrin αvβ8 blocking monoclonal antibody that can overcome tumor immune exclusion and enhance the activity of immune checkpoint inhibitors. CRB-601 antitumor activity correlated to increased αvβ8 RO and downregulation of TGFβ signaling. Investigational New Drug (IND) enabling studies are in progress. Citation Format: Daqing Wang, Vaishali Shinde, Maneesh Singh, Rachael Brake, Andrew Kolodziej. CRB-601, an avβ8 blocking antibody, prevents activation of TGFb and exhibits anti-tumor activity associated with immune cell remodeling of the tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 706.