Abstract 4201: Combination of palbociclib with enzalutamide shows in vitro activity in RB-proficient and androgen receptor-positive triple-negative breast cancer cells

Abstract

Abstract Background Cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors have in vitro synergistic effects and significant clinical efficacy in combination with hormone therapy for estrogen receptor (ER)-positive advanced breast cancer. However, the effects of CDK4/6 inhibitors in triple negative breast cancer (TNBC) are not well-elucidated. Retinoblastoma (RB, a known substrate of CDK4/6) protein pathway deregulation is a frequent occurrence in TNBC and in vitro studies have revealed that pharmacological CDK4/6 inhibition yields a cooperative cytostatic effect with doxorubicin in RB-proficient TNBC models, suggesting that RB may be a biomarker for selecting patients receiving CDK4/6 inhibitors. In addition, anti-androgen therapy have been shown preclinical efficacy in androgen-receptor (AR) positive TNBC cells. Here we tested the combination effect of a CDK4/6 inhibitor palbociclib with an AR antagonist enzalutamide in TNBC cells. Methods MDA-MB-231, MDA-MB-468 and MDA-MB-453 TNBC cell lines were used for in vitro studies. Cell viability was examined by MTT assay. Apoptotic effects were examined by flow cytometry. Signal transduction pathways in cells were assessed by Western blot. Results We correlated the expressions of AR and pRB status in these TNBC cells and identified that MDA-MB-453 were AR-positive/pRB-positive, MDA-MB-231 were AR-negative/pRB-positive, and MDA-MB-468 were AR-negative/pRB-negative cells. Since CDK4/6 inhibition leads to dephosphorylation of pRB, we examined effects of palbociclib and found that palbociclib significantly inhibited the cell growth of pRB-positive cell lines MDA-MB-453 and MDA-MB-231, but did not affect pRB-negative MDA-MB-468 cells. Further combining palbociclib with enzalutamide showed that the growth inhibitory effects of combination therapy were additive in AR-positive/pRB-positive MDA-MB-453 cells, but not in AR-negative/pRB-positive MDA-MB-231 cells, nor AR/pRB double negative MDA-MB-468 cells. Moreover, palbociclib-induced G1/S arrest only exhibited in pRB-proficient MDA-MB-453 and MDA-MB-231 cells. In contrast, enzalutamide did not affect the cell cycle on any of these TNBC cell lines but inhibited phosphorylation of AR signaling in AR-positive MDA-MB-453 cells. In addition, both palbociclib and enzalutamide induced little apoptosis, suggesting cytostatic effects of these agents. Conclusions In summary, both pRB and AR status are important for response of TNBC cells to palbociclib. Combination of palbociclib with enzalutamide shows more prominent cytostatic effects in RB proficient and AR positive TNBC cells, comparing to AR-negative/pRB-positive or AR/pRB double negative TNBC cells. Our data provide a preclinical rationale in selecting patients who might have therapeutic benefit from combining CDK4/6 inhibitors with AR antagonists. Citation Format: Ling-Ming Tseng, Tzu-Ting Huang, Chun-Teng Huang, Ka-Yi Lau, Chun-Yu Liu, Chung-Wai Shiau, Kuen-Feng Chen. Combination of palbociclib with enzalutamide shows in vitro activity in RB-proficient and androgen receptor-positive triple-negative breast cancer cells [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 4201. doi:10.1158/1538-7445.AM2017-4201