Abstract 1000: Targeting androgen receptors and cyclin-dependent kinases 4 and 6 in breast cancer

Abstract

Abstract Androgen receptor (AR) is a transcription factor that plays dual roles in breast cancer: promoting or inhibiting proliferation depending on expression and activity of estrogen receptor-alpha. Expression of AR is detected in up to 90% of all breast cancers. The duration of treatment with aromatase inhibitor and expression of AR are correlated in circulating-tumor cells (CTCs) from breast cancer patients with bone metastases. Constitutively active splice variants of AR (AR-Vs) are expressed in primary breast cancer specimens, cell lines, and CTCs from breast cancer patients. Detection of AR-Vs in CTCs from breast cancer patients is associated with bone metastases. Hence, targeting AR is a potential therapeutic strategy for AR-positive breast cancers. Anti-androgens such as enzalutamide and bicalutamide are in clinical trials for breast cancer patients. However, these approaches target the AR C-terminal ligand-binding domain, which is not present on AR-Vs. Expression of AR-Vs can drive cancer cell growth and resistance to antiandrogens. Ralaniten is an effective AR antagonist that targets AR N-terminal domain (NTD) to inhibit the transcriptional activities of both AR and AR-Vs. Cyclin-dependent kinases 4 and 6 (CDK4/6) are activated by cyclin D1 to phosphorylate Rb and are critical in the transition from G1 to S phase in cell cycle. Luminal AR subtype of triple negative breast cancer is particularly sensitive to CDK4/6 inhibitors and this sensitivity is associated with AR expression. Our objective is to evaluate efficacy of ralaniten in combination with palbociclib on the growth of AR-expressing breast cancers. AR-expressing breast cancer cell line, SUM159PT, and AR-negative cell line, MDA-MB-468 (negative control) were used. To assess the effects of ralaniten on cell growth, we examined cell proliferation by BrdU incorporation, cell viability by alamarBlue staining, and cell cycle by BrdU and 7-aminoactinomycin D staining. Cells were treated with ralaniten with or without palbociclib, or control vehicle. Each analysis was performed after 1 doubling time. Ralaniten significantly inhibited proliferation of SUM159PT, but not AR-negative MDA-MB-468 cells. Enzalutamide was less effective than ralaniten to block proliferation. Cell viability assays showed an additive effect on SUM159PT cells in combination treatment of ralaniten and palbociclib. Ralaniten disrupted cell cycle in SUM159PT cells and caused cell accumulation in late S phase. Interestingly, combination treatment caused cell accumulation in G1 phase and cell reduction in late S phase. In summary, ralaniten effectively inhibited the growth of AR-expressing breast cancer cells. Additional cell lines are being tested. In vivo efficacy of ralaniten and combinations to inhibit the growth of AR-expressing breast cancer xenografts are under investigation. Targeting AR by ralaniten or an analogue may benefit breast cancer patients whose cancers express AR. Citation Format: Amy H. Tien, Nasrin R. Mawji, Jun Wang, Marianne D. Sadar. Targeting androgen receptors and cyclin-dependent kinases 4 and 6 in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1000.