Abstract 5674: Anti-CCR8-based bispecific antibodies engineered to preferentially eliminate tumor-infiltrating regulatory T cells: leaving effectors unharmed

Abstract Background: Regulatory T cells (Tregs) play a critical role in maintaining homeostasis and self-tolerance and can hamper anti-tumor immunity through multiple mechanisms. Therefore, the targeted depletion of tumor-infiltrating Tregs is sought to promote effective anti-tumor immunity while preserving peripheral homeostasis. CCR8 has been identified as a potential specific target for tumor-infiltrating Tregs. However, CCR8 is also expressed on memory T cell subpopulations and the consequences of their depletion are still unclear. In an attempt to rationally design highly-specific Treg-depleting agents, TIGIT and CTLA-4 were selected as binding targets for engineering CCR8-based IgG-like bispecifics for highly effective depletion of CCR8/TIGIT or TIGIT/CTLA-4 double-positive Tregs that are enriched in tumors. Here, we detail our results for an anti-CCR8 x TIGIT bispecific as a best-in-class Treg-targeting agent. Methods: Anti-CCR8/TIGIT bispecific antibody candidates were generated in a 1+1 format and screened using several anti-CCR8 and anti-TIGIT binding arms with a wide range of affinities. CCR8 and TIGIT single-positive cells and CCR8/TIGIT double-positive cells were generated to screen for bispecifics with preferential binding and killing activity towards double-positive cells over single-positive cells in vitro. An anti-CCR8/TIGIT surrogate antibody was generated and used to evaluate anti-tumor efficacy as a proof-of-concept study in human-CCR8 knock-in mice, inoculated with CT-26 tumor cells. Results: An anti-CCR8/TIGIT bispecific antibody, named PM1024, was selected from several candidate pairs that satisfied our screening criteria. PM1024 showed strong binding to CCR8/TIGIT double-positive cells but minimal binding to CCR8 and TIGIT single-positive cells. As such, PM1024 induced strong ADCC activity towards double positive cells but much weaker activity towards CCR8 or TIGIT single positive cells. An anti-CCR8/TIGIT surrogate antibody demonstrated significant anti-tumor efficacy in vivo, with similar or better tumor growth inhibition compared to the corresponding monospecific agents. Conclusion: We successfully generated an anti-CCR8/TIGIT bispecific antibody (PM1024) that preferentially eliminates CCR8/TIGIT double-positive cells over single-positive cells. PM1024 bears the attributes to selectively eliminate tumor-infiltrating Treg cells over CCR8 or TIGIT single-positive lymphocytes, such as effector T cells and NK cells. PM1024 may thus function as a safer, more specific, and highly effective tumor-infiltrating Treg-depleting agent. Citation Format: Tianhang Zhai, Shuang Dai, Weifeng Huang, Yuanhong Chen, Shaogang Peng, Shihui Zhou, Liandi Chen, Jijui Zhang, Chao Wang, Chao Wang, Zhiyuan Li, Andy Tsun. Anti-CCR8-based bispecific antibodies engineered to preferentially eliminate tumor-infiltrating regulatory T cells: leaving effectors unharmed. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5674.