Recent studies have shown that the androgen receptor (AR) is widely expressed across all subtypes of breast cancer, with AR detected in 75-95% of estrogen receptor (ER) positive tumors. Prior to the development of ER-targeted therapy, androgens were used to treat breast cancers with favorable clinical responses. The lack of tissue-selectivity and understanding of mechanism of action of classic androgens led to its declined use in this disease. Selective androgen receptor modulators (SARMs) are a class of molecules developed as tissue selective non-steroidal androgens that modulate AR signaling. However, the effect of SARMs on AR signaling in breast cancer cells is not well understood. RAD140 is an orally available SARM with potent anabolic activity in muscle and bone but a highly attenuated effect in seminal vesicles and prostate. Its efficacy is being evaluated in in vivo xenograft models of AR/ER+ breast cancer. To further our understanding of the mechanism of action of RAD140 in breast cancer cells, we examined the effects of RAD140 on AR and ER signaling pathways. The specificity and selectivity of RAD140 were examined using competitive binding assays for AR, ER, glucocorticoid receptor (GR) and progesterone receptor (PR), and using an AR reporter assay in AR/ER+ ZR-75 breast cancer cells and LNCaP prostate cancer cells. RAD140 was found to bind AR with high affinity and specificity, with 250-fold target selectivity over the next nuclear receptor PR. The AR reporter assay results demonstrated a potent AR agonist activity of RAD140 in ZR-75 breast cancer cells but not in LNCaP prostate cancer cells, in contrast to the full agonist activity of DHT seen in both cell types. RAD140 treatment led to substantial tumor growth inhibition in AR/ER+ breast cancer xenografts. Samples of RAD140-treated xenografts were analyzed by immunocytochemistry, western blotting and quantitative real-time PCR to evaluate the modulation of AR and ER pathways. It was found that the expression of known AR target genes including KLK2, FKBP5 and the tumor suppressor gene, ZBTB16, were potently induced in RAD140-treated tumors. Furthermore, RAD140 led to substantial suppression of known ER target genes including progesterone receptor (PR), TFF1 and GREB1. In conclusion, RAD140 is a potent tissue-selective AR agonist in AR/ER+ breast cancer cells with robust activation of AR targets genes, and a unique mechanism of action that leads to the suppression of ER signaling, and marked anti-tumor activity.

Citation Format: Ziyang Yu, Suqin He, Dannie Wang, Jeffrey Brown, Jamal Saeh, Gary Hattersley. RAD140, a selective androgen receptor modulator, has a differentiated mechanism of action in AR/ER positive breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3609. doi:10.1158/1538-7445.AM2017-3609