Abstract 2784: Simultaneous checkpoint-checkpoint or checkpoint-costimulatory receptor targeting with bispecific antibodies promotes enhanced human T cell activation

Abstract Combination checkpoint blockade promotes productive anti-tumor clinical responses often associated with an increase in immune-related adverse events. We developed optimized bispecific antibody candidates that simultaneously engage PD1 and CTLA4 (XmAb20717), CTLA4 and LAG3 (XmAb22841), and PD1 and ICOS (XmAb23104), that preferentially bind to cells co-expressing the targeted receptors. Because tumor infiltrating lymphocytes (TILs) typically express multiple immune checkpoints and costimulatory receptors, we hypothesized that these bispecific antibodies will enable selective targeting of tumor-reactive TILs, leading to safer and more cost-effective combination checkpoint blockade. The PD1 x CTLA4 bispecific antibody XmAb20717 and the PD1 x ICOS bispecific antibody XmAb23104 promoted superior in vitro T cell activation when compared to an anti-PD1 bivalent antibody. The CTLA4 x LAG3 bispecific antibody XmAb22841 combines productively with an anti-PD1 bivalent antibody achieving additive T cell activation through triple checkpoint blockade. Treatment with bispecific antibodies caused superior engraftment and activation of human T cells in NSG mice compared to an anti-PD1 bivalent antibody. Bispecific antibodies also potentiate allogeneic anti-tumor activity against human cancer cells in vivo. Each bispecific antibody promoted a unique RNA gene signature suggesting distinct mechanisms of action. These bispecific antibodies demonstrate compelling immune modulatory activity suggesting clinical development is warranted for the treatment of human malignancies. Citation Format: Michael Hedvat, Christine Bonzon, Matthew J. Bernett, Gregory L. Moore, Kendra Avery, Rumana Rashid, Alex Nisthal, Suzanne Schubert, Rajat Varma, Sung-Hyung Lee, Liz Bogaert, Irene W.L. Leung, Seung Chu, Umesh Muchhal, John Desjarlais. Simultaneous checkpoint-checkpoint or checkpoint-costimulatory receptor targeting with bispecific antibodies promotes enhanced human T cell activation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2784.