SNS-101, a highly selective monoclonal antibody against the active form of VISTA, demonstrates significantly reduced cytokine release.

e14504 Background: Active cancer immunotherapeutics frequently give rise to immune-mediated adverse events (AEs), including cytokine release syndrome (CRS) or immune effector cell-associated neurotoxicity syndrome (ICANS). On-target, off-tumor activation of myeloid lineage cells (e.g. monocytes) has been implicated as a factor in the generation of these immune-mediated AEs. VISTA (V-domain Ig suppressor of T-cell activation) is an immune checkpoint, which is highly expressed on myeloid cells, and binds PSGL-1 on T-cells, but only when ‘activated’ by protonation at low pH (̃pH 6). Inhibition of the VISTA:PSGL-1 interaction in the acidic tumor microenvironment has been shown to be efficacious in multiple syngeneic murine tumor models. However, antibodies binding to VISTA at physiological pH 7.4 have significant potential to induce dose-limiting toxicities such as CRS and/or ICANS and be prematurely eliminated from circulation through targeted-mediated drug disposition (TMDD), reducing the likelihood of reaching efficacious drug occupancy levels. Among several non-pH-selective antibodies in clinical development, JNJ-61610588 (now CI-8993) induced dose-limiting on-target CRS at subtherapeutic dose levels and exhibited TMDD. To mitigate potential CRS and prevent TMDD, we developed SNS-101, a highly selective monoclonal IgG1 antibody for “active” VISTA, which inhibits the critical interaction with PSGL-1. Methods: Using flow cytometry at physiological pH, we compared SNS-101 with clinical stage non-pH-selective anti-VISTA antibodies and examined binding to VISTA-positive cell populations in human PBMCs as well as in human CD34+ cord blood cells reconstituted BRGSF-HIS mice (humanized BRGSF-HIS mice), which develop both human lymphoid and myeloid compartments. Furthermore, we performed in vitro and in vivo CRS studies in a HUVEC/PBMC co-culture system and humanized BRGSF-HIS mice, respectively. Results: Under physiological pH, non-pH-selective antibodies bound to monocytes, neutrophils and NK cells whereas SNS-101 did not exhibit any significant interactions. CRS assays indicate that the magnitude of induced cytokine levels was significantly higher with non-pH-selective antibodies compared to SNS-101. Conclusions: We assessed the binding profile of SNS-101 vs. non-pH-selective VISTA antibodies. Our results demonstrate that SNS-101 does not bind to VISTA-positive cells in circulating blood. In addition, in vitro and in vivo CRS studies suggest that SNS-101 has a lower risk of inducing CRS compared to non-pH selective anti-VISTA antibodies, alleviating liabilities previously associated with anti-VISTA antibodies. IND-enabling studies, including pharmacokinetic and toxicology studies, are ongoing.