Abstract 1626: PMC309, a highly selective anti-VISTA antibody enhances T cell activation through blocking the interaction of T cells and myeloid derived suppressor cells (MDSC)

TME (tumor microenvironment) consists of blood & lymphatic vessels, stromal cells, and immune cells such as lymphocytes, macrophages, and antigen-presenting cells (APC). Cellular interactions among various immune cell types within TME regulate tumor growth, metastasis, and resistance to cancer therapy. V-domain Ig suppressor of T cell activation (VISTA) is a negative checkpoint regulator highly expressed on tumor-infiltrating myeloid-derived suppressor cells (MDSC) and tumor-associated macrophages (TAMs) and VISTA blockade could enhance antitumor immunity in TME. PMC-309 binding to VISTA expressing cells is highly selective and the selectivity is maintained even in the low pH conditions that mimic TME. Binding of PMC-309 on MDSC cells enhances the secretion of IFN-γ, TNF-α, and IL-2 in Mixed Lymphocyte Reaction (MLR) settings. In in vivo study, PMC-309 suppressed tumor growth, which was further attenuated with an anti-PD1 antibody. The anti-tumor activity was associated with enhanced T cell activation, increased secretion of pro-inflammatory cytokines, and increased penetration of tumor-infiltrating leukocytes (TIL) into TME. Patients who became resistant to anti-CTLA-4 and anti-PD(L)1 antibody showed increased expression of VISTA, suggesting that anti-VISTA therapeutics can help to treat the patients who do not respond well to other immuno-oncology drugs. We are developing PMC-309 as a next generation immuno-oncology (IO) agent, which can convert immunologically cold into hot tumors as a mono- or combo- therapy with other IO agent. Citation Format: Cheon Ho Park, Sang Soon Byun, Jin Yong An, Sun Hyung Ha, Hye Rim Han, Nu Ri Kang, Beom Yong Park, Eun Ah Lee, Hyun Sun Park, Jin-San Yoo, Weon Sup Lee. PMC309, a highly selective anti-VISTA antibody enhances T cell activation through blocking the interaction of T cells and myeloid derived suppressor cells (MDSC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1626.