Abstract B143: Discovery of Helicon Peptides for the Selective Degradation of the Agonist-Bound Conformation of Androgen Receptor (ARON) in Prostate Cancer

Abstract Stabilized helical peptides (Helicons™) have been demonstrated to bind to intracellular targets at binding sites on proteins that are generally regarded as undruggable e.g., FOG-001 inhibits the b-catenin-TCF4 interaction and is currently being evaluated in the clinic in advanced solid tumors. Helicons can also be converted to degraders, resulting in targeted ubiquitin-proteosome degradation of proteins that play key driver roles in tumorigenesis. One such driver is the androgen receptor (AR), which remains highly relevant in the initiation and progression of prostate tumorigenesis. Currently approved agents and the majority of those in development bind to the inactive conformation of AR that is not androgen bound (AROFF). This mechanism serves to prevent activation of the protein, rather than targeting it in the active, agonist-bound conformation responsible for driving transcription and tumorigenesis. Tumors that retain AR pathway dependence while treated with these inhibitors respond with adaptive activation of AR signaling, which in turn is associated with relapse and disease progression. To address the limitations of current AROFF agents, we developed Helicon peptide degraders that are designed to selectively target the agonist-bound, transcriptionally active conformation of AR (ARON). ARON degraders are expected to degrade AR regardless of mutations in the androgen binding pocket and work better in AR-amplified cells compared to AROFF agents as only a fraction of total AR is in the ON state. Hence, both major mechanisms of resistance to current AR antagonists are expected to be addressed by ARON degraders. Using a combination of de novo computational and structure-based molecular design, we discovered compounds that bind to the AR ligand-binding domain (LBD) in its agonist-bound conformation, at a site distinct from the ligand binding pocket targeted by AROFF agents. These Helicon binders were subsequently engineered as E3-specific proteasome-mediated degraders exhibiting potent ternary complex formation, and selective degradation of AR in prostate cancer cell lines. In these models, degradation of AR leads to potent suppression of transcription and proliferation. Notably, the ARON degraders remained efficacious in low-androgen conditions where only a minority of AR exists in the agonist-bound state, suggesting that this agonist-bound pool may be critical for maintaining proliferation. Importantly, the distinct binding sites of ARON Helicons versus existing AROFF inhibitors and degraders predicts that combining these agents will allow more complete inhibition of AR. Indeed, combination with standard-of-care inhibitors such as enzalutamide revealed additive anti-proliferative effects, likely stemming from a more comprehensive blockade of both AROFF and ARON pools. These findings demonstrate that selective degradation of the ARON pool has the potential to provide a mechanistically novel and therapeutically promising strategy to suppress AR-dependent tumorigenesis across androgen states, including models of castration resistance. Citation Format: Diwakar R. Pattabiraman, Brandon Nicolay, Pieter Beerepoot, Sascha DeVine, Kelsey Barrasso, Terri Jones, Dakota Hawkins, Archana Iyer, Lee Belding, Shelagh Fluharty, Amelia K. Luciano, Paula Ortet, Anthony Quartararo, Yelena Arnautova, David Terry, Brandon Hriniak, Mousa Jafari, Anandan Palani, Markus Haeberlein, Jonathan Hurov. Discovery of Helicon Peptides for the Selective Degradation of the Agonist-Bound Conformation of Androgen Receptor (ARON) in Prostate Cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr B143.