Abstract C012: Discovery of a Best-in-Class small molecule p53 Y220C reactivator: Breaking through the potency ceiling

Abstract The tumor suppressor p53 is inactivated in approximately 50% of human cancers. In about 1% of solid tumors, a specific Y220C point mutation induces p53 misfolding, leading to its inactivation. This mutation also creates a unique, druggable pocket on the protein's surface, making it a promising target for cancer therapy. First generation small molecule reactivators (PC14586), designed to bind to this pocket, to refold p53 and to restore its tumor-suppressive functions, have shown clinical efficacy in patients harboring the Y220C mutations. However, first-generation compounds are limited by modest potency, necessitating high dosing in patients. This high-dose requirement increases the risk of adverse events and severely limits the therapeutic window, both as monotherapies and in combination regimens. As a result, these compounds fail to deliver the drug pressure needed to fully reactivate this critical tumor suppressor and restrict combinability with other treatments Thus, there is a clear need to develop a highly potent P53 small molecule reactivator to fully unlock the therapeutic potential of this target. At Onco3R Therapeutics, our patient centric approach, integrating deep translational science with rational, structure-based and AI-augmented drug design, has led to the identification of best-in-class series of p53 Y220C reactivators. Our lead series exhibits an increase in potency up to 300-fold compared to the clinical compound PC14586 across diverse preclinical assays, including biochemical p53 refolding, cellular p53 refolding, and cellular p21 induction studies. Importantly, this next-level potency enables robust activation of the pro-apoptotic protein PUMA and induces potent, selective cytotoxicity in p53 Y220C mutant cells and in an isogenic cell model. In vivo, our compounds demonstrate superior target engagement at comparable exposure levels and achieve strong tumor regression in a p53 Y220C cell-derived xenograft (CDX) model. Additionally, we have improved in vitro metabolic stability and have established detailed SAR/SPR, guiding the design towards improved ADME profiles and pharmacokinetic properties. Here, we identified highly optimized p53 Y220C small molecule reactivator series with clear best-in-class potential. Comprehensive characterization and optimization are now ongoing to identify clinical development candidates with the ultimate goal of achieving superior efficacy at substantially lower doses, maximizing safety, and delivering meaningful benefits to patients with p53 Y220C mutations. Citation Format: Ruben Boon, Jo Alen, Nihed Draoui, Koen Vandyck, Elke Behaeghel, Godelieve Lammens, Sandrine Vendeville, Lijs Beke, Pieter Peeters, Stéphane de Cesco, Bart Stoops, Pierre Raboisson, Francois Gonzalvez. Discovery of a Best-in-Class small molecule p53 Y220C reactivator: Breaking through the potency ceiling [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 C012.