A Novel Steroidogenic Factor-1 Antagonist, OR-449, as a Targeted Therapy for Adrenocortical Cancer

Abstract Adrenocortical carcinoma (ACC) is a rare cancer with an annual incidence of 0.7-2.0 cases per million in the US and EU. Surgical resection combined with adjuvant chemotherapy remains the primary treatment for patients with advanced disease. Although surgery is an option for most patients, oftentimes cases presenting with locally advanced or metastatic disease are not candidates for surgical resection. Furthermore, the non-specific adrenolytic and highly toxic FDA-approved chemotherapy drug, mitotane, is only marginally effective in adult and pediatric ACC. Steroidogenic factor-1 (SF-1 or NR5A1), an orphan nuclear receptor that is essential for the growth and development of the adrenal gland, is highly expressed in adult and pediatric ACC. Moreover, SF- is the major, active transcription factor in ACC (1,2). To address the unmet need for a targeted therapy in ACC, we identified potent small molecule SF-1 antagonists that block SF-1 transcriptional activity through the SF-1 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), OR-449 and a related SF-1 antagonist, OR-907S, 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. OR-449, which has >20% oral bioavailability and a long plasma half-life (> 9 h) in mouse, rat and dog, inhibited growth of SJ-ACC3 tumors grown in immunocompromised mice following daily oral dosing (30 mg/kg) for 4 weeks. SF-1-responsive genes, both down- and up-regulated, identified by RNAseq (by comparison of OR-907S and OR-907R in SJ-ACC3 dissociated cell culture), also responded to OR-449 in SJ-ACC3 xenografts following 7 days of dosing, indicating transcriptional regulation of SF-1 by OR-449. Importantly, in an exploratory 2-week mouse safety study, OR-449 showed no adverse effects when dosed up to 100 mg/kg. Taken together, these findings suggest that SF-1 antagonists could provide a safe and effective targeted therapy for ACC. 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.