EXTH-43. DIFFERENTIATED PHARMACOLOGY OF THE IMIPRIDONE ONC201, THE FIRST SELECTIVE DRD2/3 ANTAGONIST IN CLINICAL NEURO-ONCOLOGY

Abstract

ONC201, founding member of the imipridone class of compounds, has demonstrated evidence of tumor shrinkage along with exceptional safety in recurrent glioblastoma patients. In this study, we identified and characterized a previously unknown binding target of ONC201. BANDIT – a machine learning-based drug target identification platform – predicted that ONC201 would bind with high selectivity to the G-protein coupled receptors (GPCRs) dopamine receptor D2(DRD2) and D3 (DRD3). DRD2 is overexpressed in glioblastoma, controls pro-survival mechanisms, and its antagonism causes pro-apoptotic effects in malignant cells. PathHunter® β-arrestin and cAMP assays determined that ONC201 selectively antagonizes DRD2 and DRD3. Consistent with BANDIT and in contrast to DRD2 blocking antipsychotics, ONC201 did not antagonize other dopamine receptors or other closely related GPCRs with identified endogenous ligands. Schild analyses and radioligand competition assays revealed DRD2 affinities that were consistent with those identified for ONC201 anticancer activity. In accordance with superior selectivity, ONC201 exhibited a wider therapeutic window compared to other antipsychotics. In support of the hypothesis that selectively targeting D2-like receptors yields superior anti-cancer efficacy, combined DRD2/DRD1 inhibition was found to be inferior to DRD2 inhibition alone. ONC201 exhibited a very slow association rate for DRD2 relative to antipsychotics, whereas the dissociation rate was similar to atypical antipsychotics that are better tolerated clinically. Shotgun mutagenesis across 350 amino acids of DRD2 identified 8 residues critical for ONC201-mediated antagonism of DRD2-induced calcium flux. Several residues were not conserved among other dopamine receptors, suggesting a potential role in conferring ONC201 selectivity. Consistent with competitive inhibition, several mutated residues were within the orthosteric binding site (OBS), however, two distal residues were identified outside of the OBS suggesting a secondary binding pocket. In summary, receptor pharmacology of ONC201, the first selective DRD2/3 antagonist in clinical neuro-oncology, may explain its unique selectivity, safety, and anti-cancer activity in clinical trials.