Tumor Selective Immune Responses Of Sta551, A Novel Anti-Cd137 Agonist Antibody Activated By Extracellular Atp

Background Agonistic antibodies targeting CD137 in clinic have failed due to severe hepatotoxicity, leading to the development of bispecific approaches that must rely on high tumor-associated antigen expression to crosslink CD137. Here we report on STA551, a novel anti-CD137 agonist antibody which binds to CD137 only in the presence of ATP. Extracellular ATP concentration is well-known to be elevated in tumor tissue while remaining tightly regulated in non-tumor tissue, suggesting that STA551 can activate immune cells only in tumor tissue and not elsewhere. Thus, STA551 has great potential to overcome the limitations of conventional CD137-targeted antibodies. Methods We evaluated in vitro STA551’s effect on IFN-γ production from human CD8+ T cells. We also evaluated in vivo STA551’s effect on tumor growth, RNAseq-based immune-related gene expression, immunohistochemistry, and T cell activation in tumor and non-tumor tissues in human CD137 knock-in mice treated with mouse surrogate STA551 (Sta-MB) or urelumab (Ure-MB) in combination with anti-PD-L1 antibody. Results In a human T cell assay, STA551 induced IFN-γ only in the presence of ATP. In contrast, urelumab induced IFN-γ regardless of ATP concentration. In mice with Colon 38 tumors, Sta-MB inhibited tumor growth at least as strongly as Ure-MB, but whereas Ure-MB elicited systemic immune responses in draining lymph node, spleen, and liver, Sta-MB appeared to evade such responses. To confirm immune responses in tumors, we evaluated immune-related gene expression and found changes after treatment with Sta-MB or Ure-MB. These results suggest that STA551 works only in tumor tissue. Furthermore, Sta-MB with anti-PD-L1 antibody synergistically inhibited tumor growth and dramatically changed immune-related gene expression, CD8+ T cell infiltration, and PD-L1 expression without systemic immune responses. Also, it was well-tolerated in cynomolgus monkey in a repeated-dose toxicity study*. Conclusions STA551 is a novel anti-CD137 agonist antibody that exerts agonistic activity selectively in tumors without on-target toxicity in non-tumor tissues, regardless of tumor-associated antigen expression. These results strongly support the clinical testing of STA551 for the treatment of solid tumors. STA551 is currently being tested in a phase 1 clinical study. Ethics Approval All animal studies were reviewed and approved by the Institutional Animal Care and Use Committee (IACUC).