Abstract CT075: A Phase 1 trial evaluating AC699, an orally bioavailable chimeric estrogen receptor degrader, in patients with advanced or metastatic breast cancer

Abstract Background The role of estrogen receptor (ER) signaling in driving breast carcinogenesis is well established, and endocrine therapy has been a mainstay in the treatment of patients with ER-positive advanced or metastatic breast cancer for decades. Aromatase inhibitors, selective estrogen receptor modulators, and selective ER degraders (SERDs) such as fulvestrant, and more recently elacestrant, are integral components of the therapeutic strategy for patients with ER-positive breast cancer. However, unmet medical need persists in this patient population, due to suboptimal clinical outcomes and the development of resistance to therapy. E3 ligase-engaged chimeric degraders represent a technological advance with the potential to induce potent and deeper ER degradation. Utilizing Accutar's proprietary Protein-Protein Interaction Targeting Chimeras (PPI-TAC) platform, AC699 was designed as a novel chimeric degrader targeting ERα. By effectively linking an ER ligand to an E3-ligase recruiting ligand, AC699 brings ERα in proximity to an E3 ligase, resulting in ubiquitination and subsequent degradation of ERα. Chimeric ER degraders possess the unique advantage of degrading the ER protein without the inherent risk of activating an ER signal. Moreover, these molecules are not degraded alongside the target protein, allowing for their efficient recycling within the cell. This direct mechanism enables chimeric ER degraders to achieve potent ER degradation with increased specificity, thereby potentially providing a higher therapeutic index compared to SERDs. Study Description The AC699-001 trial is a first in human Phase 1 dose escalation study (NCT05654532) which will enroll up to 60 patients with locally advanced or metastatic ER-positive, human epidermal growth factor receptor 2-negative, breast cancer. Patients must have received at least two prior lines of endocrine treatment, or at least one prior line, if combined with a CDK4/6 inhibitor. Prior chemotherapy is not required but must not exceed three prior lines of cytotoxic treatment. Patients must have at least one measurable lesion or at least one predominantly lytic bone lesion. AC699 is administered orally, once daily, with doses ranging from 100 mg to 600 mg in a standard 3+3 dose escalation design. The primary objective is evaluation of the safety and tolerability of AC699. Secondary objectives include assessment of preliminary anti-tumor activity and characterization of the pharmacokinetic profile of single and multiple doses of AC699. Enrollment began in December 2022 and a total of 5 sites have been activated in the United States. Citation Format: Rachel M. Layman, Manish R. Patel, David Cosgrove, Michael Danso, Nancy Mota, Marjorie E. Zettler, Katherine C. Pehlivan, Erika Hamilton. A Phase 1 trial evaluating AC699, an orally bioavailable chimeric estrogen receptor degrader, in patients with advanced or metastatic breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (7Suppl): Abstract nr CT075.