856 SNS-101, a highly pH-selective VISTA:PSGL-1 inhibitory antibody, potentiates anti-PD-1 sensitivity, expands memory T-cells and enhances tumor infiltration of CD8 T-cells

<h3>Background</h3> VISTA (V-domain Ig suppressor of T-cell activation) is a negative checkpoint regulator (NCR), highly expressed on myeloid cells.¹ PSGL-1 on T-cells has been identified as a novel NCR that limits survival and promotes T-cell exhaustion.² Recently, VISTA was reported to bind PSGL-1 and suppress T-cell activity exclusively under acidic conditions (~pH 6 in lymph nodes or the tumor microenvironment).^{3, 4} Although VISTA inhibition demonstrated excellent therapeutic combinability with other modalities targeting NCRs (e.g. CTLA-4, PD-1/PD-L1),⁵ clinical development of anti-VISTA antibodies has been challenging due to: 1) high clearance via target-mediated drug disposition (TMDD) by VISTA⁺ neutrophils and monocytes at physiologic pH; and 2) cellular activation and cytokine release syndrome (CRS) at sub-therapeutic doses by engagement of VISTA in the blood.⁶ We developed SNS-101, a human monoclonal IgG1 antibody specific for the protonated, active form of VISTA, which is designed to disrupt the immunosuppressive VISTA:PSGL-1 interaction, avoid TMDD and mitigate potential CRS. <h3>Methods</h3> The binding potential of SNS-101 to VISTA⁺ cells was determined in human and non-human primate (NHP) whole-blood by flow cytometry. The effect of SNS-101 on human monocytes and T-cells was evaluated <i>in vivo</i> in human CD34⁺ cord blood cell reconstituted BRGSF mice, which develop both human lymphoid and myeloid compartments. The pharmacokinetic (PK) profile was assessed in NHPs. Anti-tumor efficacy was assessed in VISTA-KI mice implanted with the syngeneic tumor model, MC38, and tumor-infiltrating T-cells were analyzed by flow cytometry. <h3>Results</h3> SNS-101 did not bind to human or NHP VISTA⁺ monocytes, neutrophils and natural killer cells. In humanized BRGSF mice, SNS-101 induced significant expansion of CD4 and CD8 central memory (CCR7⁺CD45RA^-), and naïve (CCR7⁺CD45RA⁺) CD8 T-cells, respectively, but had no significant impact on monocyte activation. PK studies in NHPs showed linear elimination kinetics. Conversely, a non-pH-sensitive antibody bound VISTA⁺immune cells, induced monocyte activation followed by a decrease in cell numbers and was rapidly cleared in NHPs. Anti-tumor efficacy studies in MC-38 demonstrate that SNS-101 enhanced anti-PD-1 response and dose-dependently increased tumor-infiltrating CD8 T-cells. <h3>Conclusions</h3> Our results demonstrate that SNS-101 exhibits linear elimination kinetics in NHPs, overcoming TMDD-induced PK limitations observed with other anti-VISTA antibodies. Importantly, SNS-101 induced expansion of naïve and memory T-cell phenotypes <i>in vivo</i> without activation or depletion of monocytes, differentiating it from non-pH-selective VISTA antibodies. In the MC-38 syngeneic tumor model, SNS-101 demonstrated significant enhancement of anti-tumor effects in combination with anti-PD-1 antibodies through an increase in CD8⁺ T-cells. <h3>References</h3> Yuan L, Tatineni J, Mahoney KM, <i>et al</i>. VISTA: a mediator of quiescence and a promising target in cancer immunotherapy. <i>Trends Immunol</i>. 2021; <b>42</b>:209–227. Tinoco R, Carrette F, Barraza ML, <i>et al</i>. PSGL-1 is an immune checkpoint regulator that promotes T cell exhaustion. <i>Immunity</i>. 2016; <b>44</b>:1190–1203. Johnston RJ, Su LJ, Pinckney J, <i>et al</i>. VISTA is an acidic pH-selective ligand for PSGL-1. <i>Nature</i> 2019; <b>574</b>:565–570. Wu H, Estrella V, Beatty M, <i>et al</i>. T-cells produce acidic niches in lymph nodes to suppress their own effector functions. <i>Nat Commun</i>. 2020; <b>11</b>:4113. Gao J, Ward JF, Pettaway CA, <i>et al</i>. VISTA is an inhibitory immune checkpoint that is increased after ipilimumab therapy in patients with prostate cancer. <i>Nat. Med</i>. 2017; <b>23</b>:551–555. Curis Corporate Presentation Jan 2022 [http://investors.curis.com/events-and-presentations?item=100]