Abstract 718: CCR8 antibody drug conjugates: Targeting regulatory T cells for tumor sensitization

Abstract Background: Regulatory T (Treg) cells in the tumor microenvironment have an immune suppressive function, and their presence is associated with poor prognosis. Depletion of Treg cells may be a mechanism to sensitize tumors and represents a novel strategy for cancer immunotherapy. High expression of chemokine receptor CCR8 has been found in Treg cells in microenvironments of numerous tumors but not on Treg cells in systemic lymphoid tissues. The ability to selectively kill Treg cells in tumor environments may provide an opportunity for therapy that leaves Treg function intact elsewhere in the body where they normally mediate tolerance to self-antigen. Methods: We sought to identify CCR8 antibodies for therapeutics and designed an antibody discovery strategy to address the challenges of this target. CCR8 is a member of the G protein-coupled receptor (GPCR) family. Like many GPCRs, CCR8 is a valuable therapeutic antibody target but challenging to access due to its poor expression, membrane-dependent structure, and poor immunogenicity due to sequence conservation. We developed an antibody discovery platform (MPS) that specifically addresses each of these challenges through use of advanced immunization techniques and evolutionarily divergent host species (chickens) for robust immune responses against conserved targets. We extensively engineered CCR8 antigen to enable high expression for immunization. When sufficient antibody diversity was not obtained using our standard immunization techniques, we developed mRNA immunization techniques for this program. Antibody binding was measured using flow cytometry on transfected cells and human Treg cells. The ability of antibodies to inhibit CCR8 function was measured using intracellular calcium influx assays. Antibody internalization was measured using dye uptake assays and flow cytometry. Antibodies were conjugated with cytotoxic payloads and tested for the ability to mediate killing. Results: Using these approaches, we identified a panel of high-affinity antibodies that bound in the nanomolar or subnanomolar range with high specificity, and we selected a lead molecule to progress. We will present data from our antibody discovery campaign including mRNA immunization, binding profiles, GPCR functional data, receptor-mediated internalization, and the ability of antibody-drug conjugates (ADC) to kill CCR8+ cells, including human Treg cells, in vitro. Conclusion: Treg cells are a primary driver of the immunosuppressive microenvironment in many solid tumors. Targeting immune-suppressive CCR8+ Treg cells using ADC has potential to expand the treatment of solid tumors Citation Format: Lewis J. Stafford, Hayley Roth, Sarah Campbell, Samantha Brady, Valerie Fiers, FeiFan Yu, Andrew Tsourkas, Riley Payne, Meghan Pitts, Kyle Guldner, Tim Phillips, Breanna Tyrell, Colleen Ulke, Allison Snyder, Marianne Assogba, Holden Ohl, Ileine Sanchez, Benjamin Doranz, Joseph Rucker, Ross Chambers. CCR8 antibody drug conjugates: Targeting regulatory T cells for tumor sensitization [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 718.