Abstract 976: Inhibition of BCL-2 (by APG-2575) and CDK4/6 synergistically induces cell cycle arrest and apoptosis in ER+ breast cancer
Authors: Deng Jing , Tao Ran , Fang Douglas D. , Tang Qiuqiong , Yin Yan , Zhang Kaixiang , Li Yuanbao , Zhai Guoqin , Wang Qixin , Yang Dajun , Zhai Yifan 
Published: July 2021

Abstract
Estrogen receptor positive (ER+)/human epidermal growth factor receptor 2 negative (HER2-) tumors represent the most common subset (about 75%) of all breast cancer cases. Combination treatment with endocrine therapy and a CDK4/6 inhibitor (CDK4/6i) is now the standard of care in early metastatic ER+ breast cancer. However, relapse or resistance to such combination therapy nearly inevitably occurs. Antiapoptotic protein BCL-2 is overexpressed in most cases of primary and metastatic ER+ breast cancer. We have shown that clinical-stage BCL-2 selective inhibitor APG-2575 enhances antitumor activity when combined with palbociclib in ER+ breast cancer xenograft models, including malignancies that are resistant to tamoxifen or progress after CDK4/6i treatment. We investigated mechanisms of action (MOAs) for synergistic effects of this combination.
In MCF-7 cells sensitive to treatment with tamoxifen/fulvestrant, the CDK4/6i palbociclib inhibited expression of both ER and phospho-Rb (pRb) proteins, resulting in cell cycle arrest and senescence as demonstrated by cell cycle analysis and beta-galactosidase staining, respectively. Palbociclib did not initiate apoptosis as measured by TUNEL assays. In contrast, APG-2575 not only disrupted BCL-2:BIM complexes in MCF-7 cells but also blocked cell cycle (G1 to S) progression by suppressing expression of pRB, ER, and cyclin D1 proteins. APG-2575 also increased expression of proapoptotic proteins BIM and Noxa, as well as antiapoptotic protein MCL-1. APG-2575 also led to an increase in complexes of BIM with MCL-1, which is known to confer resistance against BCL-2 inhibitors. Interestingly, the combination treatment weakened induction of MCL-1 protein and reduced MCL-1:BIM complexes. Thus, the combination treatment synergistically triggered breast cancer cell apoptosis, as evidenced by increased TUNEL signal and cleavage of poly (ADP-ribose) polymerase 1 protein (a hallmark of apoptosis).
Taken together, palbociclib drives breast cancer cells into a senescent state, and APG-2575 potentiates this antitumor effect by inducing cellular apoptosis. When combined with APG-2575, palbociclib inhibits MCL-1 expression and MCL-1:BIM complex formation as results of BCL-2 inhibition. Therefore, palbociclib reduces the threshold for apoptosis, while APG-2575 induces apoptosis by disrupting BCL-2:BIM complexes as well as enhancing expression of BIM and Noxa. Similar to palbociclib, APG-2575 also causes cell cycle arrest, and both agents collaboratively induce apoptosis in the combination setting. Our data reveal a viable MOA for synergistic effects, strengthening the scientific rationale for clinical development of the combination of the BCL-2 selective inhibitor APG-2575 and CDK4/6i palbociclib-based therapy in patients with ER+/HER2− breast cancer.
Citation Format: Jing Deng, Ran Tao, Douglas D. Fang, Qiuqiong Tang, Yan Yin, Kaixiang Zhang, Yuanbao Li, Guoqin Zhai, Qixin Wang, Dajun Yang, Yifan Zhai. Inhibition of BCL-2 (by APG-2575) and CDK4/6 synergistically induces cell cycle arrest and apoptosis in ER+ breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 976.