Abstract 497: Human CD73 knock-in mice facilitate evaluation of in vivo efficacy of anti-human CD73 cancer immunotherapies

Abstract

Abstract In recent years, immune checkpoint inhibitors (ICIs) have revolutionized cancer treatment. However, most patients with solid cancers do not respond to the current ICIs against PD-1/PD-L1 or CTLA-4 alone. Stimulating an effective anti-tumor immune response may require countering additional immunosuppressive pathways in the tumor microenvironment (TME). Adenosinergic pathways are critical to maintain normal immune system homeostasis, and tumors could exploit them to avoid immune attack. CD73 (NT5E) is a surface enzyme that catalyzes the final hydrolysis of AMP to adenosine, which impairs immune function through adenosine receptors like A2AR (see our poster on humanized A2AR mice) on immune cells. Some tumors overexpress CD73 driven by hypoxia in the TME, which associates with poor prognosis in some cases. It is also expressed on the surface of multiple immune cells that infiltrate tumors, including T-cells, B-cells, NK cells, macrophages, and myeloid-derived suppressor cells. Several anti-CD73 antibodies have started early stage clinical trials as a treatment for advanced cancers. Combined anti-CD73, anti-PD-1, and anti-CTLA4 therapies synergize to reduce tumor burden and extend survival. Thus, there is a need for pre-clinical models to evaluate the safety and efficacy of CD73-targeted therapeutics before they enter human trials. Biocytogen has generated a human CD73 knock-in (KI) mouse (B-hCD73) and a human CD73-expressing MC38 cell line (MC38-hCD73) for both in vitro functional validation and in vivo efficacy evaluation of CD73-specific antibodies. Human CD73 (hCD73) whole coding sequence was inserted following exon 2 of the mouse Cd73 gene. B-hCD73 mice did not exhibit any immune or developmental abnormalities compared to wild-type mice, but splenic Tregs of B-hCD73 mice and not wild-type expressed hCD73. Anti-hCD73 antibodies bound well only to splenocytes of hCD73 KI mice and also blocked the enzyme activity of CD73. Using our MC38-hCD73 tumor model, we showed robust efficacy of anti-hCD73 antibodies in inhibiting tumor growth in vivo. Furthermore, in PD-1/PD-L1/CD73 triple KI humanized mice implanted with MC38 tumors, combined CD73 and PD-1/PD-L1 blockade showed greater tumor inhibition than monotherapies. Taken together, CD73 humanized mice and their variants are useful tools for in vivo efficacy evaluation of candidate human CD73 antibodies and combination therapies being considered for clinical trials. Citation Format: Chengzhang Shang, Huilin Li, Jing Zhang, Eugene Lin, Luke (Zhaoxue) Yu, Yuelei Shen. Human CD73 knock-in mice facilitate evaluation of in vivo efficacy of anti-human CD73 cancer immunotherapies [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 497.