SUN-LB23 Translational Feasibility of Steroidogenic Factor-1 Antagonists as a Novel Targeted Therapy for Adrenocortical Cancer

Abstract Adrenocortical carcinoma (ACC) is an aggressive cancer with devastating outcomes. ACC is usually locally advanced or metastatic at diagnosis and, despite tumor resection plus chemotherapy, has a high rate of recurrence. The 5-year survival rate among metastatic ACC patients is less than 15%. ACC responds poorly to the single FDA-approved drug, mitotane, which is non-specifically adrenolytic and highly toxic. Other chemotherapy regimens tested have been unsuccessful in improving overall survival. Steroidogenic factor-1 (SF-1 or NR5A1) is an orphan nuclear receptor essential for growth and development of the adrenal gland and is the major, active transcription factor in ACC (1,2). To address the need for a targeted therapy in ACC, we have identified potent small molecule SF-1 antagonists that block SF-1 transcriptional activity through the ligand-binding domain (IC50 = 15-20 nM in a CHO cell reporter assay). In short-term dissociated cell cultures established from SJ-ACC3 (3), a pediatric ACC patient-derived tumor xenograft (PDX), the SF-1 antagonists OR-907S and OR-070 blocked DNA synthesis as measured by inhibition of EdU incorporation in SF-1+ cells (IC50 = 500-600 nM, >80% efficacy at 10 μM) whereas OR-907R, the 100-fold less potent enantiomer of OR-907S, is nearly inactive. Because the SF-1 antagonist sensitivity of the dissociated SJ-ACC3 cells declines markedly with repeated passage of the PDX in immunocompromised (C.B-17 SCID) mice, we have utilized an alternative model system for evaluating tumor target engagement and growth inhibition: the rat Leydig tumor cell line (R2C), which is growth-inhibited by the SF-1 antagonists OR-907S and OR-070 in vitro (IC50 = 60-100 nM) and as a xenograft in immunocompromised mice (CD-1 nude). The SF-1-responsive gene signature identified by RNAseq in R2C cell cultures by comparison of OR-907S and OR-907R was replicated by OR-070 and other orally-bioavailable lead antagonists in R2C xenografts following 3 days of dosing, indicating engagement of SF-1 by these compounds. Significantly, R2C tumors were growth-inhibited following daily oral dosing for 4 weeks with OR-070 (10-30 mg/kg). These findings suggest that SF-1 antagonists could be a targeted therapy for ACC. OR-070 has >30% oral bioavailability in rat and dog, indicating that this structural class of SF-1 antagonists has potential for clinical development.References: (1) Mohan, et al., Curr. Opin. Endocrinol. Metab. Res., 2019; 8:72; (2) Corces, et al., Science, 2018; 362:eaav1898; (3) Pinto, et al., Clin. Cancer. Res., 2013; 19:1740.