Abstract 2114: Tumor cell-targeted STING-agonist antibody-drug conjugates achieve potent anti-tumor activity by delivering STING agonist specifically to tumor cells andFcγRI-expressing subset of myeloid cells

STING pathway agonism has emerged as a potential therapeutic strategy to stimulate anti-tumor immune responses. We have previously shown that tumor cell-targeted antibody-drug conjugates (ADCs) carrying a novel STING agonist induce anti-tumor activity without causing substantial elevations in systemic cytokine levels, thus suggesting a therapeutic advantage of STING agonist ADCs relative to unconjugated agonists. ADCs constitute a proven therapeutic modality that is ideally suited to enable systemic administration and delivery of the conjugated drug to desired cell types within the tumor microenvironment. In addition to delivering STING agonist into the antigen-expressing tumor cells, antigen-bound ADCs deliver STING agonist to tumor-resident myeloid cells through Fcγ receptor (FcγR)-mediated internalization. In this study we investigated the mechanism of FcγR-mediated internalization of the tumor cell-targeted STING-agonist ADCs into myeloid cells and the nature of the subsequent STING pathway activation. We developed flow cytometry-based assays to determine the changes in FcγRI, FcγRII, and FcγRIII cell surface detection levels in the presence of ADCs specifically designed to be either proficient or deficient in FcγR-binding. Combined with functional assays based on co-culture of cancer cells and FcγRI knock out myeloid cells, we identified FcγRI as the major Fcγ receptor that mediates target-bound ADC internalization into myeloid cells in vitro. Even though FcγRI is expressed only by a subset of CD11b+ myeloid cells, tumor cell-targeted ADCs induce greater production of interferons and other cytokines and more potent cancer cell killing than CD11b-targeted-ADCs, which deliver STING agonist into FcγRI- (non-productive) as well as FcγRI+ (productive) myeloid cells. Finally, we demonstrate that myeloid cells within dissociated primary human tumors from multiple donors express FcγRI and are capable of tumor cell killing in response to tumor cell-targeted STING agonist ADCs in vitro. In summary, our data indicate that the ADC-mediated delivery of a STING agonist specifically into FcγRI-expressing myeloid cells efficiently activates innate immune responses in the most relevant immune cell types within the tumor microenvironment. Citation Format: Naniye Malli Cetinbas, Kalli C. Catcott, Travis Monnell, Jahna Soomer-James, Keith Bentley, Susan Clardy, Bingfan Du, Eoin Kelleher, Marina Protopopova, Cheri Stevenson, Joshua D. Thomas, Alex Uttard, Dorin Toader, Je