Antitumor effect of the combination of proteasome inhibitor (Pi) and histone deacytelase inhibitor (HDACi) in breast cancer (BC).

Abstract

e23156 Background: Previous studies demonstrated the efficacy of HDACi and Pi as single agents in BC. The synergistic combination of HDACi and Pi has been shown to be effective in clinical studies in other tumor types, including multiple myeloma, non-small cell lung cancer, and pancreatic cancer. In this in vitro study, we explored the hypothesis that combination of these two agents may improve overall antitumor effect in BC. Methods: In the experiment, we used 3 BC cell lines: MCF7 (ER+), MDA-MB-231 (ER-), and BCSV (isolated from metastatic ER+ tumor which lost ER expression with disease progression). The cells were treated with carfilzomib (Pi) and scriptaid (HDACi) as single agents and in combination. Cell viability was determined using MTT assay, and combination indexes were determined using CalcuSyn software. Additionally, cells were cultured in 3D tumor spheroid model, and response to treatment was assessed on day 5, 10, 15 and 20. Results: In comparison with control, single agent HDACi, single agent Pi and combination HDACi/Pi increased cell viability by 6%, decreased cell viability by 60% and by 69% respectively (p < 0.0001) in MCF7 cells; and decreased cell viability by 25%, 59% and 66% respectively (p = 0.0061) in BCSV cells. The combination indexes for most HDACi and Pi combinations were lower than 1.0, indicating synergistic effects in MCF-7 and BCSV cells but not in MDA-MB-231 cells. In 3D cultures, the combination of carfilzomib 1µM and scriptaid 10µM led to significant reduction of tumor spheroid size as compared to single agent. By day 20, BCSV tumor spheroid size increased by 258% in control group and by 50% in Pi, decreased by 17% in HDACi and by 30% in HDACi/Pi combination group (p < 0.0001). Conclusions: Combination of HDACi and Pi exhibited synergistic activity in both MCF7 and BCSV monolayer cultures. Additionally, combination HDACi and Pi significantly decreased 3D tumor spheroid size as compared to either single agent alone. Further study is needed to evaluate the mechanisms of Pi/HDACi combination in estrogen-mediated BC tumorigenesis.