Abstract P4-08-01: Targeting CDK7 enhances the antitumor efficacy of enzalutamide in androgen receptor-positive triple-negative breast cancer

Abstract

Abstract Background: Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer. Among TNBC subtypes, the luminal androgen receptor (LAR) subtype expresses high levels of androgen receptor (AR), tends to be less proliferative, and generally responds poorly to neoadjuvant therapy. Previous studies have shown that AR inhibition suppressed proliferation of AR+ or LAR subtype TNBC cells and tumor growth in xenograft models. Thus, AR is a promising therapeutic target for this TNBC subtype. Here, we identified kinase targets for enhancing the antitumor efficacy of the AR inhibitor enzalutamide in preclinical models and investigated the underlying molecular mechanisms. Methods: We performed a nonbiased high-throughput kinome siRNA screening (targeting 709 genes) to identify a synergistic partner for enhancing the antitumor efficacy of the AR inhibitor enzalutamide in AR+ LAR TNBC. The growth inhibitory effects of enzalutamide alone or combined with kinase inhibitors were determined using the sulforhodamine B staining assay and clonogenic assay. The effect of treatments on the expression of target proteins of interest was examined by Western blot analysis. Results: Enzalutamide was not effective as a single agent to inhibit the proliferation of AR+ TNBC cells (IC50 >15 µM in MDA-MB-453, CAL-51, CAL-148, MFM223, HCC2185, SUM149, SUM159, and SUM185 cells). Nonbiased high-throughput kinome siRNA screening identified PI3K/AKT/mTOR, cell cycle, and JNK pathways as potential canonical targets for combination with enzalutamide to enhance its antitumor efficacy in AR+ LAR TNBC. Among these pathways, inhibition of cell cycle progression using the CDK7 inhibitor UD-017 showed the most synergistic anti-proliferation effect with enzalutamide in AR+ LAR MDA-MB-453 (50.61% reduction in proliferation compared with enzalutamide alone, P < 0.001; and 48.98% reduction in proliferation compared with UD-017 alone, P < 0.001) and SUM185 (40.41% reduction in proliferation compared with enzalutamide alone, P < 0.05; and 42.76% reduction in proliferation compared with UD-017 alone, P < 0.05) TNBC cells. Furthermore, CDK7 knockdown using siRNA significantly enhanced the sensitivity of MDA-MB-453 (60.98%, P < 0.01) and SUM185 (30.99%, P < 0.01) cells to enzalutamide, and AR knockdown significantly enhanced the sensitivity of MDA-MB-453 (32.64%, P < 0.01) and SUM185 (43.62%, P < 0.01) cells to UD-017, in both cases with a sensitivity similar to that of enzalutamide plus UD-017. This result suggests that the synergy of enzalutamide and UD-017 results from specific targeting of AR and CDK7. Downstream target analysis revealed that the combination of enzalutamide and UD-017 dramatically reduced the expression of c-MYC. c-MYC knockdown using siRNA dramatically suppressed colony formation in both MDA-MB-453 and SUM185 cells at a degree similar to that of enzalutamide plus UD-017, whereas c-MYC overexpression reversed the synergistic effect of the combination treatment. This result suggests that UD-017 synergizes with enzalutamide through inhibition of c-MYC–mediated tumorigenesis. Conclusion: Our results suggest that enzalutamide synergizes with UD-017 by inhibiting c-MYC–mediated oncogenic activity. These in vitro data warrant future in vivo studies of the antitumor synergy of enzalutamide plus UD-017 in AR+ LAR TNBC models. Citation Format: Xuemei Xie, Marwa Manai, Jon A. Fuson, Troy Pearson, Dileep R. Rampa, Debu Tripathy, Jangsoon Lee, Naoto T. Ueno. Targeting CDK7 enhances the antitumor efficacy of enzalutamide in androgen receptor-positive triple-negative breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-08-01.