V-domain immunoglobulin (Ig)-containing suppressor of T-cell activation (VISTA) also known as PD1 homolog (PD1H) is a co-inhibitory immune checkpoint receptor. VISTA is predominantly expressed on hematopoietic cells, particularly myeloid derived suppressor cells and antigen presenting cells, and at lower levels on CD4+ and CD8+ T cells and Foxp3 Tregs. Multiple studies have demonstrated VISTA can strongly suppress human T-cell activation, and the presence of high VISTA expressing cells in the tumor microenvironment has been postulated as playing a critical role in tumorigenesis and resistance to immunotherapy. VISTA neutralizing antibodies have been previously observed in pre-clinical models to increase the abundance of tumor infiltrating effector T cells as well as their effector functions, resulting in enhanced control of tumor growth, even in the absence of detectable expression of VISTA on the tumor cells, a potential advantage over PD1 or PDL1 blockade. There is an urgent need to develop the most clinically effective anti-VISTA antibodies however the limited characterization of the VISTA pathway and the uncertainty around the cognate VISTA ligand has limited the extent to which in vitro studies can support the selection of optimal antibodies for development. Hummingbird Bioscience’s proprietary Rational Antibody Development platform to the design of neutralizing antibodies against specific epitopes on VISTA. Extensive in silico analyses of the VISTA structure and comparative structural modeling against other B7 protein family members has enabled the prediction of key binding sites where antibody binding will inhibit function, while preserving necessary safety and developability profiles. Subsequently, a directed evolution strategy was used to isolate monoclonal antibodies specifically targeting these key predicted functional domains on the native folded VISTA protein. Hummingbird’s anti-VISTA antibodies bind human and murine VISTA protein with high affinity (kD <5nM) and with high specificity. Antibody HMBD-002, targeting a region on VISTA predicted to be critical to ligand binding, was given as single dose monotherapy to wildtype BALB/C mice, 3 days after inoculation with CT26 syngeneic colon cancer cells. Induction of cytokine release was observed in the mouse sera (up to 4 fold increase in IL-2, IL17 and IFNγ measured at 24 and 72 hrs after dosing), confirming robust activation of the immune response. This was associated with significant effects on tumor progression including delay in progression free survival and > 60% inhibition of tumor growth (after 15 days) with no observable toxicity. Further in vitro and in vivo validation experiments in other cancer models are ongoing, both as monotherapy and in combination with other immune therapies. The first in-human trial of HMBD-002 is planned for 2018.

Citation Format: Piers J. Ingram, Dipti Thakkar, Jerome D. Boyd-Kirkup. HMBD002, a novel neutralizing antibody targeting a specific epitope on the co-inhibitory immune checkpoint receptor VISTA, displays potent anti-tumor effects in pre-clinical models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 587. doi:10.1158/1538-7445.AM2017-587