Abstract 5186: Discovery of a novel histone deacetylase 6 inhibitor that kills drug-resistant breast cancer

Abstract

Abstract Cytotoxic chemotherapy is the standard of care for patients with triple negative breast cancer (TNBC). Most patients with advanced TNBC progress after chemotherapy and die from metastatic disease. Currently, no established targeted therapeutics or biomarkers of outcome/response have been clinically approved in the context of TNBC. The aggressive nature of TNBC is reflected by an increased likelihood of distant recurrence and death within five years following primary intervention and a shorter survival once diagnosed with metastatic disease. To identify novel therapeutics for the treatment of chemoresistant TNBC, we performed a high-throughput screen to identify small molecules that are cancer selective but can kill drug-resistant TNBC cells. We screened a total of 30,000 compounds in duplicate across the TNBC cell line MDA-MB-321 in which the pro-death proteins BAX/BAK were knocked down, and the non-transformed "normal" MCF10A cell line. There was a hit rate of 0.3% in the screen and 85 compounds were retested in the validation cherry pick screen. From this screen 18 compounds were further validated with low-throughput assessment for mitochondrial independent killing and selectivity for cancer cells. To identify the mechanism of action of the lead compound we used a genetic approach to generate an RNAi signature for the compound, which indicated the lead compound was most likely that of histone deacetylase inhibitor (HDAC). Using an in vitro HDAC inhibitor screen, we identified that the compound specifically inhibited HDAC6. Using a series of cell based assays, we determined that treatment of cells with the compound BAS-2 resulted in the same phenotypical response (inhibition of migration, invasion and 3D spheroid formation) as HDAC6 knockout. To identify substrates of HDAC6 in cells we analyzed the proteome for increased acetylation by mass spectrometry following HDAC6 knockout or inhibition with the specific inhibitor, BAS-2. Remarkably, we identified a series of new substrates for HDAC6 that are common in both the HDAC6 knockdown and BAS-2 treated cells. In conclusion, we have identified a novel HDAC6 specific inhibitor that selectively kills cancer cells independent of mitochondrial apoptosis, alters acetylation of over a 50 common proteins and inhibits spheroid formation of TNBC. Citation Format: Catríona Dowling, Eugene Dillion, Michael Hemann, Gerard Cagney, Anthony Letai, Triona Ni Chonghaile. Discovery of a novel histone deacetylase 6 inhibitor that kills drug-resistant 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 5186.