Abstract Triple-negative breast cancer (TNBC) is an aggressive subtype lacking durable targeted therapies, and resistance continues to drive poor clinical outcomes. The clinical success of antibody-drug conjugates (ADCs), which combine antibody specificity with potent cytotoxic payloads, highlights their growing clinical impact. Although the approved anti-TROP2 ADC, sacituzumab govitecan, provides clinical benefit, it causes toxicity in normal tissues, underscoring the need for more selective therapeutic strategies. Adhesion-mediated signaling through the FAK-SRC axis is a major survival pathway under therapeutic stress. Our previous study showed that GPR56, an adhesion G protein-coupled receptor (GPCR), activates the FAK-SRC pathway and that targeting GPR56 with an ADC elicited potent antitumor efficacy in colorectal cancer models. Given these results, we examined GPR56 as a potential therapeutic target in TNBC. While TROP2 is a validated ADC target in TNBC, its expression in normal tissues limits tumor selectivity. GPR56, by contrast, is highly expressed in TNBC and associated with poor prognosis, yet shows limited normal tissue expression, potentially conferring a superior therapeutic index for GPR56-targeted ADCs. Our first-generation anti-GPR56 ADC (10C7-Duo), incorporating the DNA-damaging payload duocarmycin, exhibited target-dependent cytotoxicity in GPR56-positive TNBC cell lines, confirming that GPR56 is a viable therapeutic target. However, the monoclonal antibody (mAb) 10C7 exhibited agonistic activity, which could limit its therapeutic potential. Thus, we developed 9E3, a non-agonist anti-GPR56 mAb that internalizes efficiently and traffics to lysosomes for payload delivery. 9E3 was conjugated to a more potent pyrrolobenzodiazepine (PBD) payload using site-specific chemistry, and analyses confirmed successful conjugation, stability, and preserved antigen binding. Functional studies in breast cancer models showed that GPR56 knockdown suppressed, while overexpression enhanced, FAK-SRC phosphorylation, tumor cell growth, and invasion. Currently, we are evaluating 9E3-PBD for in vitro cytotoxic potency and selectivity, as well as in vivo safety and antitumor efficacy in TNBC cell line xenografts and patient-derived models. These findings help establish GPR56 as a clinically relevant adhesion GPCR and introduce a novel ADC approach to target therapeutic resistance in TNBC. By integrating receptor biology with targeted payload delivery, this work lays the foundation for potentially safer and more selective therapeutic options for TNBC patients. Citation Format: Yueh-Ming Shyu, Joan Jacob, Carla Godoy, Treena Chatterjee, Zhengdong Liang, Kendra S. Carmon. Therapeutic targeting of adhesion receptor GPR56 for the treatment of triple-negative breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 5833.
Shyu et al. (Fri,) studied this question.