Abstract 3413: Discovery of GRC 65327, a best-in-class, selective and potent Cbl-b inhibitor for the treatment of advanced solid cancers

Abstract Casitas B-lineage lymphoma-b (Cbl-b), a key member of RING finger E3 ligases, regulates both innate and adaptive immunity by mediating the degradation of signaling transducers. Cbl-b is a master regulator downstream of TAM receptors and CD28, promoting an immunosuppressive tumor microenvironment (TME) that limits the anti-tumor functions of T and NK cells. The absence of CD28 co-stimulation has been associated with T cell exhaustion and resistance to anti-PD-1 therapy. Inhibition of Cbl-b activates T cells even without CD28 co-stimulation, offering a potential therapeutic strategy to overcome immune suppression in the TME. GRC 65327 is a potential best in class, highly active and selective Cbl-b inhibitor. It significantly potentiates IL-2 and IFN-γ production in assays across human and preclinical species, with sub-nanomolar potency. Importantly, GRC 65327 restores function in anti-PD1 resistant exhausted T cells, mimicking the immunosuppressive conditions of the TME. The compound also engages its target in vivo, modulating biomarkers such as Notch1, enhancing its expression on T cells. Additionally, GRC 65327 demonstrated potent tumor cell killing in vitro, further highlighting its therapeutic potential. Efficacy studies in CT26 and MC38 colon tumor models demonstrated significant inhibition at tested doses. When combined with anti-PD1 therapy, GRC 65327 enhanced tumor growth inhibition and immune responses. Complete tumor regression was achieved in combination treatment in approximately 50-65% of animals, suggesting a synergistic effect between GRC 65327 and anti-PD1. Importantly, no treatment-related adverse effects, including changes in body weight, clinical signs, or mortality, were observed in these efficacy models. In vitro safety receptor profiling revealed minimal off-target activity, and hERG inhibition at an IC50 > 10 μM. In a one-month repeat-dose study in mice, no adverse effects were observed at the maximum tested dose of 300 mpk/day. However, liver toxicity was noticed in dose-range finding studies in dogs and monkeys. Despite this, a robust pharmacological response in the tissues was observed around AUC0-24 of 1500 ng·h/mL and an increased number of circulating immune cells were seen at the highest dose tested compared to controls and baseline levels, with no adverse findings up to AUC0-24 of ∼13, 400 ng·h/mL in a one-month repeat-dose study in monkeys. Importantly, no cytokine storm and myeloproliferative effects attributed to c-cbl in the literature were observed in the same study. Additionally, unlike a previous less-selective lead which exhibited RTK-mediated toxicities in various organs during exploratory tox study in rodents, GRC 65327 did not show adverse effects, highlighting its improved selectivity and safety profile. Based on these findings, a first-in-human clinical trial of GRC 65327 is scheduled to begin in advanced solid cancers. Citation Format: Jiju Mani, Atul Akarte, Venkatesha Udupa, Nilanjana Biswas, Vikas Karande, Murugan Chinnapattu, Megha Marathe, Akshata Virdikar, Sneha Pusadkar, Shruti Talekar, Anuj Singh, Pandurang Lambade, Gopal Rajput, Akshaya Wagh, Pramod Pawar, Ajit Jagadale, Dayanidhi Behera, Balasaheb Gavhane, Manoj Pawar, Prashant Ingle, Pankaj Jain, Amol Walunjkar, Pavan Payghan, Sachin Chaudhari, Vidya Kattige, Sravan Mandadi, Madhavi Mulay, Ramesh Senaiar, Mario Perro, Pratima Deshpande, Nagaraj Gowda. Discovery of GRC 65327, a best-in-class, selective and potent Cbl-b inhibitor for the treatment of advanced solid cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3413.