Abstract 4874: Receptor pharmacology and anti-cancer activity of selective DRD2/3 antagonist imipridone ONC206
Authors: Prabhu Varun Vijay , Madhukar Neel , Tarapore Rohinton , Garnett Mathew , McDermott Ultan , Benes Cyril , Charter Neil , Deacon Sean , VanEngelenburg Alexander , Rucker Joseph , Doranz Benjamin , Elemento Olivier , Oster Wolfgang , Stogniew Martin , Allen Joshua 
Published: August 2018

Abstract
Dopamine receptor D2 (DRD2) is a G protein-coupled receptor (GPCR) overexpressed in many cancers and its antagonism causes anti-tumor effects. ONC201, founding member of the imipridone class of small molecules, is a DRD2 antagonist in Phase II advanced cancer clinical trials. We evaluated the binding target and anti-tumor activity of ONC206, an ONC201 analog. An orphan small molecule target prediction algorithm revealed that ONC206, like ONC201, antagonizes DRD2. Experimental GPCR profiling using the PathHunter® β-Arrestin assay, confirmed that ONC206 selectively antagonizes D2-like (DRD2/3) but not D1-like (DRD 1/5) dopamine receptors. In this assay, ONC206 possesses a ~10-fold increased potency for DRD2 compared to ONC201 with a Ki of ~320nM with selectivity that was superior to approved antipsychotics. Shotgun mutagenesis across 350 amino acids of DRD2 identified 7 residues critical for ONC206-mediated antagonism of DRD2-induced calcium flux. Consistent with competitive inhibition, mutated residues were within the orthosteric binding site. While 6 mutated residues were also critical for ONC201 activity, one of the mutated residues was unique to ONC206, suggesting differentiated receptor pharmacology. TCGA analysis and immunohistochemistry of patient-derived tissue microarrays revealed DRD2 was overexpressed in neuroblastoma, sarcoma and pheochromocytoma specimens relative to normal tissues. In vitro profiling of ONC206 in the Genomic of Drug Sensitivity in Cancer collection of cell lines revealed broad nanomolar efficacy across most tumor types (GI50 &lt;78-889nM) and ~5 to 20-fold improved potency relative to ONC201. Bone cancer and neuroblastoma were identified as the most ONC206-responsive solid tumor types that were comparatively less responsive to ONC201. Within bone cancer, Ewing's sarcoma (n=16) was most sensitive to ONC206 with nanomolar sensitivity (GI50 168-303nM) that was superior to ONC201. ONC206 was highly efficacious in neuroblastoma (n=35, GI50 87-589nM) including cell lines derived from metastatic sites and with MYCN amplification associated with poor prognosis. In the PC12 rat pheochromocytoma cell line ONC206 (GI50 200nM) was superior to ONC201. ONC206 time-course experiments revealed anti-cancer effects occurring at 48-72 post-treatment, similar to ONC201. In support of a wide therapeutic window, ONC206 reduced the viability of normal human fibroblasts at relatively high doses (GI50 &gt; 5µM). Efficacy evaluation in the MHH-ES-1 Ewing's sarcoma xenograft model demonstrated that ONC206 (100 mg/kg PO every 10 days) causes significant tumor growth inhibition that was comparable to methotrexate (400 mg/kg, IP) while being better tolerated. Thus, imipridone ONC206 acts as a selective antagonist of DRD2/3 at nanomolar concentrations and may address tumor types where the properties of ONC201 do not permit for complete therapeutic engagement in vivo.
Citation Format: Varun Vijay Prabhu, Neel Madhukar, Rohinton Tarapore, Mathew Garnett, Ultan McDermott, Cyril Benes, Neil Charter, Sean Deacon, Alexander VanEngelenburg, Joseph Rucker, Benjamin Doranz, Olivier Elemento, Wolfgang Oster, Martin Stogniew, Joshua Allen. Receptor pharmacology and anti-cancer activity of selective DRD2/3 antagonist imipridone ONC206 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4874.