PMON149 Identification of TR4 Modulators for Treatment of Cushing Disease.

Abstract Cushing Disease (CD) is caused by an adrenocorticotropic hormone (ACTH)-secreting pituitary tumor that causes excess adrenal-derived cortisol. It is a life-threatening "orphan disease" with an annual health care cost >7-fold higher than average patients. Surgical removal is the current first-line therapy but the disease frequently recurs. Repeat surgery, radiation therapy and bilateral adrenalectomy are not always successful and associated with significant morbidity. Currently available drugs or those in clinical trials for CD do not target the pituitary corticotroph tumor itself and escape from control is common with long-term use. A clear unmet need for efficacious and safe therapies that offer biochemical and tumor control exists. We hypothesize that direct targeting of corticotroph tumors to modulate ACTH and in turn, glucocorticoid secretion is the optimal way to treat CD. We recently demonstrated that the orphan testicular receptor 4 (TR4, also known as NR2C2) is a potent regulator of hypothalamic-pituitary-adrenal (HPA) axis function and directly regulates pro-opiomelanocortin (POMC) gene transcription and ACTH secretion. To identify and characterize TR4 small molecule modulators, we used the Flexi Vector system to fuse the TR4 224-615aa LBD to the 3' GAL4 DBD to generate a chimeric fusion plasmid (pBIND-TR4-LBD) that constitutively expresses a Renilla luciferase serving as an internal reporter control. We then transiently transfected our chimeric pBIND-TR4-LBD construct into human embryonic kidney (HEK293) cells which express an exogenous reporter construct pGL4.35 (GloResponse, luc2P/9XGAL4UAS-HEK293 Cell Line, Promega) that is driven by multiple copies of a GAL4 responsive upstream activating sequence (UAS). Following treatment with TR4 ligands or modulators, our TR4-LBD undergoes a conformational change and recruits additional cofactors and RNA polymerase II to the complex to drive GAL4-directed firefly luciferase expression. This dual-luciferase format allows us to detect compound cytotoxicity or off-target change in expression during chemical screening, greatly improving data quality. In a pilot drug screen of a LOPAC library and using robust z-score statistics (z-score>3), representing a luciferase to renilla induction > 3 standard deviations of baseline variability, we have identified several G-protein coupled ion channel regulators as potential "hit" TR4 modulators. These identified GPCRs regulate phosphatidylinositol and cyclic AMP effector systems, and activate several signaling pathway kinases. Mode-of-action and dose-response evaluation to calculate EC50 of these potential TR4 hit compounds is under investigation and lead compounds will be further validated using a POMC transactivation assay in murine and human corticotroph tumor cells. Our studies have begun to identify and rigorously validate compounds that efficiently and specifically abrogate TR4 actions to inhibit ACTH secretion and may lead to the discovery of safe and efficacious lead TR4 inhibitory compounds that can be further advanced as potential drug therapies for CD. Presentation: Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.