Introduction: Triple-negative breast cancer (TNBC) is a clinically aggressive breast cancer subtype which lacks estrogen receptor expression. Histone deacetylase (HDAC) inhibitors have previously been shown to induce estrogen receptor expression in TNBC models. Paragazole is a selective class I HDAC inhibitor (HDACs 1, 2, and 3) currently in preclinical development. In this project, we explored the antiproliferative and proapoptotic activity of paragazole and investigated the combination of paragazole and chemotherapy in a panel of breast cancer cell lines.

Methods: Breast cancer cell lines were exposed to varying sub-micromolar concentrations of paragazole alone and in combination with carboplatin, paclitaxel, and gemcitabine. Proliferation was assessed using an SRB assay and analyzed using the Calcusyn program, whereby synergy was defined as a Combination Index of less than 1. Cell cycle analysis was performed using flow cytometry and apoptosis was analyzed using a caspase 3/7 assay. Baseline HDAC expression was measured by RT-PCR and RNA-Seq was used pre- and post-treatment in a subset of 2 sensitive and 2 resistant cell lines.

Results: Exposure to paragazole resulted in a decrease in cell proliferation at submicromolar concentrations with more robust antiproliferative activity observed in the TNBC cell lines compared to the estrogen receptor positive cell lines. An increase in apoptosis that was maximal at 24-48 hours was observed with single agent paragazole in a subset of sensitive TNBC cell lines. The combinations of paragazole with either paclitaxel, gemcitabine, or carboplatin all resulted in additive or synergistic growth inhibition. Expression of HDAC 1 was higher in the sensitive TNBC cell lines than in the resistant lines. Exposure to paragazole led to an increase in CARM1 mediated estrogen receptor expression.

Discussion: These in vitro results demonstrate that paragazole has antiproliferative and proapototic activity against TNBC cell lines and may potentiate the activity of chemotherapy. These data support confirmation of the findings using in vivo models and the investigation of paragazole in combination with anti-estrogen agents such as tamoxifen and fulvestrant.

Citation Format: David P. Astling, John J. Tentler, Benjamin R. Cross, Timothy P. Newton, Todd M. Pitts, Eric Gunther, Xuedong Liu, S. Gail Eckhardt, Jennifer R. Diamond. A novel class I histone deacetylase (HDAC) inhibitor, paragazole, demonstrates antiproliferative and proapoptotic effects in triple-negative breast cancer models. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1006. doi:10.1158/1538-7445.AM2013-1006