Background: OR2805 is a fully human IgG1 antibody that binds to CD163, an immune-suppressive receptor highly expressed on tumor associated macrophages (TAMs). High numbers of CD163-expressing TAMs generally predict an unfavorable prognosis in solid tumors. CD163-expressing TAMs contribute to an immune-suppressive tumor microenvironment (TME) and inhibit an anti-tumor T-cell response by engaging immune checkpoints, producing immune-suppressive cytokines, and promoting T-cell skewing towards a pro-cancer Th2 phenotype. Relieving the immune suppression of CD163-expressing TAMs in the TME to improve T-cell-mediated responses is a rational adjunct to immune checkpoint inhibitor (CPI) therapy.

Methods: OR2805 was discovered using OncoResponse's discovery platform by cell-based functional and phenotypic assays. B cells derived from CPI elite responders were cultured at clonal density, and IgG antibodies in supernatants were evaluated for binding to myeloid-derived suppressor cells. Variable-regions from positive hits were sequenced, cloned, and expressed as recombinant IgG1. OR2805 was identified as one of the top hits that relieved myeloid cell-mediated immune suppression. Co-cultures of immunosuppressive primary human M2 macrophages and autologous T cells were used to interrogate OR2805-dependent immunomodulatory responses in vitro. The anti-tumor activity of OR2805 in vivo was evaluated in a humanized mouse model.

Results: OR2805 binds to CD163 expressed on TAMs and M2-like macrophages. OR2805 does not bind to other hematopoietic cells nor to a panel of human primary non-immune cells. OR2805 treatment reduces expression of cell-surface markers associated with tumor-promoting M2c-like macrophages. In co-culture assays, OR2805 relieves the suppressive effect of M2 macrophages and results in increased T-cell activation and proliferation, upregulation of T-cell activation markers, and enhanced T-cell-mediated tumor cell killing. Administration of OR2805 in humanized NSG-SGM3 mouse tumor models resulted in approximately 55% and 75% reduction in A549 tumor growth and NCI-H1975 tumor growth, respectively. In this model, OR2805 treatment significantly increased the proportions of human CD8+ T cells and human CD11b+ myeloid cells, as well as significantly enhanced expression of activation markers by human CD8+ T cells.

Conclusions: OR2805 reduces M2 macrophage-mediated immunosuppression and enhances anti-tumor immune responses. OR2805 treatment induces anti-tumor activity in lung cancer xenograft models in humanized mice. These data support further development of OR2805 as an anti-cancer therapy, both as a monotherapy and an addition to current CPI therapy.

Citation Format: Peter Probst, Randi Simmons, Valerie Wall, Meghan Zuck, Myriam Bouchlaka, Sam Lam, Raymond Fox, Darbie Whitman, Tom Graddis, Kamal D. Puri. OR2805, an anti-CD163 antibody derived from an elite responder to checkpoint inhibitor therapy relieves immunosuppression caused by tumor associated macrophages [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 1719.