Abstract 4172: Activation of AR signaling by mifepristone enhances prostate cancer growth and impairs enzalutamide response

Abstract

Abstract Androgen receptor (AR) signaling is crucial for normal development and homeostasis of the prostate, and is a key driver of prostate cancer initiation and progression. Hormone therapies that deprive the cancer of androgen have long been a mainstay of prostate cancer treatment. More recently, anti-androgens, such as abiraterone and enzalutamide, have been approved for use in metastatic castration resistant prostate cancer (mCRPC). Evidence also suggests that AR may play an oncogenic role in certain breast cancers. Several recent publications have demonstrated that activation of the Glucocorticoid Receptor (GR) can confer resistance to enzalutamide, and GR has also been shown to provide protection from conventional chemotherapies in other solid tumor indications. Mifepristone, is a synthetic steroidal antagonist of progesterone receptor, and to a lesser extent of GR and AR. It is currently being tested in clinical trials in combination with enzalutamide in mCRPC, and in combination with chemotherapy in triple negative breast cancer (TNBC). We sought to characterize the effect of mifepristone in pre-clinical models of prostate and breast cancer. Here we show that in the absence of androgen, mifepristone acts as a partial AR agonist, and this agonism can only be partially overcome by enzalutamide. We find that in low androgen conditions, mifepristone promotes the growth of prostate cancer cells in vitro and accelerates the growth of prostate tumors in xenograft models. Moreover, when given in combination, mifepristone significantly reduces the efficacy of enzalutamide in the LN-AR xenograft model. We are currently assessing the effects of mifepristone treatment in TNBC. Our findings suggest that partial AR agonist activity of mifepristone may have a negative impact in prostate and other AR positive cancers. We have developed a GR antagonist that is devoid of AR agonism to circumvent undesired effects on proliferation and drug response. Citation Format: Haiying Zhou, Nadine Jachan, Mallika Singh, Chris Tran, Dan McWeeney, Minna Balbas, Emily Schenkein, Tatiana Zovorotinskaya, Erica L. Jackson, Julio Medina, Daqing Sun, Yosup Rew, Xiaohui Du, John Eksterowicz, Xuelei Yan, Liusheng Zhu, Qiuping Ye, Valeria Fantin. Activation of AR signaling by mifepristone enhances prostate cancer growth and impairs enzalutamide response [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 4172. doi:10.1158/1538-7445.AM2017-4172