Abstract 1880: PDL1-targeted CD28 costimulatory bispecific antibodies enhance T cell activation in solid tumors

Abstract T cells in the tumor micro-environment require TCR/MHC engagement and co-stimulatory receptor engagement to achieve complete activation. Tumor cells lack expression of CD28 ligands, so we hypothesized that activation of CD28 signaling at the T cell/tumor cell interface could enhance anti-tumor activity. We designed PDL1 x CD28 bispecific antibodies that conditionally costimulate CD28 only in the presence of PDL1 and TCR engagement. As PD(L)1 signaling has been shown to directly inhibit CD28 costimulation, this novel bispecific modality has potential to promote CD28 costimulation while simultaneously preventing the suppression of the same signal. We designed a set of stability-optimized anti-CD28 antibodies that can be paired with anti-PDL1 antibodies to engage both PDL1 and CD28 monovalently using Xencor's XmAb® 1+1 bispecific antibody platform. In vitro T cell activation with these bispecifics was measured by T cell proliferation, cytokine production, and cytotoxicity, in co-cultures of human cancer cell lines mixed with primary human CD3-stimulated T cells. In vitro activity was validated in a CMV recall assay measuring CMV+ T cell proliferation of CMV+ PBMC co-cultured with cancer cell lines ectopically treated with pp65-derived NLV-peptide. In vivo activity was determined with hCD28 humanized mice inoculated with MC38 tumors stably expressing hPDL1-antigen. Finally, safety, tolerability, and pharmacodynamics of PDL1 x CD28 were determined in cynomolgus monkeys. PDL1 x CD28 bispecifics were generated by incorporating an anti-PDL1 mAb capable of blocking PDL1-PD1 interaction and anti-CD28 scFv covering a range of affinities. Multiple PDL1 x CD28 antibodies enhanced T cell degranulation, cytokine secretion, and cancer cell cytotoxicity in concert with CD3 stimulation only in the presence of PDL1. PDL1 x CD28 enhanced proliferation of CMV+ T cells recognizing cancer cells loaded with pp65-derived NLV peptide. In hCD28 mice inoculated with MC38 tumors expressing hPDL1, PDL1 x CD28 inhibited tumor growth significantly greater than an anti-PDL1 antibody alone. PDL1 x CD28 was well tolerated in cynomolgus monkeys. PDL1 x CD28 bispecific antibodies show promising anti-tumor activity and warrant further development. Citation Format: Gregory L. Moore, Veronica Zeng, Juan Diaz, Christine Bonzon, Kendra N. Avery, Ruschelle Love, Matthew Dragovich, Rumana Rashid, Michael Hackett, Irene W. Leung, Jing Qi, Charles G. Bakhit, Umesh S. Muchhal, Norman J. Barlow, John R. Desjarlais, Michael Hedvat. PDL1-targeted CD28 costimulatory bispecific antibodies enhance T cell activation in solid tumors [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 1880.