Abstract 3877: IND-enabling characterization of DRD2/3 imipridone antagonist ONC206 for oncology

Dopamine receptor D2 (DRD2) is a G protein-coupled receptor that is overexpressed and critical for survival in several cancers. ONC201, an imipridone small molecule, is a DRD2/3 antagonist in Phase II advanced cancer clinical trials with a compelling safety and efficacy profile. We evaluated the binding target, anti-tumor activity, biodistribution and safety of ONC206, a chemical derivative of ONC201 with the same imipridone core structure. GPCR profiling with β-Arrestin recruitment revealed that ONC206 selectively antagonizes dopamine receptors DRD2 and DRD3. ONC206 exhibited a Ki of ~320nM for DRD2 with complete specificity across human GPCRs and complete DRD2 antagonism. Schild analyses of ONC206 in cAMP and β-Arrestin recruitment assays revealed hallmarks of non-competitive DRD2 antagonism, unlike antipsychotics but similar to ONC201. Shotgun mutagenesis across DRD2 identified 7 residues critical for ONC206-mediated antagonism at orthosteric and allosteric sites. While 6 mutated residues were also critical for ONC201-mediated antagonism, the impact and magnitude of different mutants varied between the two compounds and one of the allosteric residues was unique to ONC206. In vitro profiling of ONC206 in >1000 GDSC cancer cell lines demonstrated broad nanomolar efficacy (GI50 5µM), suggesting a wide therapeutic window. Robust inhibition of tumor growth without body weight loss was observed in HuCCT1 cholangiocarcinoma and MHH-ES-1 Ewing’s sarcoma subcutaneous xenografts with 50-100 mg/kg oral ONC206 weekly or every 2 weeks. Biodistribution studies in Sprague-Dawley rats revealed a ~12 µM plasma Cmax with a half-life of ~6 hours upon a single oral dose of 50 mg/kg. Additionally, 5-10 fold higher ONC206 concentrations were observed in adrenal gland, bile duct, brain and bone marrow relative to plasma concentrations. GLP toxicology studies with weekly oral ONC206 in Sprague-Dawley rats and beagle dogs at doses above or equivalent to efficacious doses revealed no dose-limiting toxicities. In both species, observations at the highest dose were mild and reversible. The no observed adverse event level (NOAEL) was ≥ 16.7 mg/kg in dogs and ≥ 50 mg/kg in rats, which both correspond to a human dose of approximately 500 mg assuming standard allometric scaling. These results provide rationale for a 50 mg starting ONC206 dose in dose escalation clinical trials in patients with DRD2-dysregulated tumors. Citation Format: Varun V. Prabhu, Abed Rahman Kawakibi, Neel Madhukar, Mathew J. Garnett, Ultan McDermott, Cyril H. Benes, Lakshmi Anantharaman, Neil Charter, Sean Deacon, Alexander VanEngelenburg, Joseph B. Rucker, Benjamin J. Doranz, Jessica Rusert, Robert Wechsler-Reya, Olivier Elemento, Martin Stogniew, Wolfgang Oster, Sharon DeMorrow, R. Benjamin Free, David R. Sibley, Joshua E. Allen. IND-enabling characterization of DRD2/3 imipridone antagonist ONC206 for oncology [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3877.