Abstract LB144: Antagonism of SF-1 as a potential targeted therapy for malignant Leydig cell tumors
Authors: Tao Haiyan , Eastwood Emily , Fox Raymond G. , Raheja Neil , Crowe Paul D. , Thacher Scott M. 
Published: June 2022

Abstract
Leydig cell tumors (LCT), originating from androgen-producing interstitial Leydig cells of the testis, represent about 3% of all testicular cancers. LCT belong to the family of sex cord stromal tumors (SCST), a collection of tumors formed in the supporting tissue within the ovaries or testes. Although 90% of LCT are considered benign and can be cured by orchiectomy, in adult patients about 10% of LCT are malignant and metastasis is common. Non-resectable metastatic disease is poorly responsive to radiation and chemotherapy and patients are advised to seek clinical trials. Steroidogenic Factor 1 (SF-1, or NR5A1) is a transcription factor that is essential for the development of the adrenal gland and gonads. SF-1 mutations result in disorders of sexual development, ovarian failure, and adrenal insufficiency. SF-1 is necessary for development of fetal and adult Leydig cells and is strongly expressed in LCT. Significant data support the role of SF-1 in adrenocortical cancer (ACC). To address the need for a targeted therapy in ACC and other SF-1-dependent malignancies, Orphagen has identified potent small molecule antagonists to SF-1*. Here, using R2C, a rat Leydig tumor cell line, we demonstrate that small molecule antagonists of SF-1 inhibit Leydig tumor cell proliferation in vitro and in vivo. OR-449, an orally available inhibitor of SF-1 transcriptional activity (SF-1 Luc IC50 = 16 nM) and OR-907S, a probe SF-1 antagonist (SF-1 Luc IC50 = 22 nM), exhibit striking anti-proliferative activity in R2C cell cultures, inhibiting DNA synthesis by &gt;90% at 1 μM with estimated IC50’s of 0.068 μM and 0.074 μM, respectively. OR-907R, the ~100-fold less active stereoisomer of OR-907S (SF-1 Luc IC50 = 2 μM) was significantly less active in the R2C proliferation assay (estimated IC50 &gt;10 μM). Moreover, in cell lines lacking SF-1 expression, such as HEK293, SF-1 antagonists have no anti-proliferative activity up to 20 μM, suggesting that the anti-proliferative effect on R2C is SF-1-mediated and not due to cytotoxicity. Furthermore, OR-449 completely blocks R2C xenograft tumor growth in immunocompromised mice at an oral dose of 30 mg/kg/day. OR-449 also dose-dependently regulates expression of SF-1 responsive genes, a mRNA signature first identified in R2C culture by comparison of the activity of OR-907S and OR-907R at 1 μM. OR-449 has excellent pharmacokinetic properties and is well-tolerated in repeat dosing toxicity studies in rodents and non-rodents*. These results highlight SF-1 antagonism as a novel targeted therapeutic approach with potential utility in the treatment of LCT and other SCST. OR-449 is currently in IND-enabling studies in order to enter the clinic by the end of 2022. * P. Crowe, et al. A novel steroidogenic factor-1 antagonist, OR-449, as a targeted therapy for adrenocortical cancer. ENDO 2021: J Endocr Soc, Vol5, Supplement_1, A1010
Citation Format: Haiyan Tao, Emily Eastwood, Raymond G. Fox, Neil Raheja, Paul D. Crowe, Scott M. Thacher. Antagonism of SF-1 as a potential targeted therapy for malignant Leydig cell tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB144.