Abstract 1311: Preclinical assessment of an HDAC inhibitor combined with a retinoid in AI resistant breast cancer.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Treatment with aromatase inhibitors (AI's) is highly effective against breast cancer in ER positive postmenopausal women. However, some patients eventually become resistant to AIs. Tumor initiating cells (TIC's) represent a subpopulation of tumor cells, which show self-renewal capacity. We are focused towards discovering strategies to reduce the growth of breast cancer TICs, which may result in resistance. We have developed a xenograft model that mimics post-menopausal hormone responsive breast cancer. In this model, aromatase transfected human hormone sensitive MCF-7 cells (MCF-7Ca) are inoculated in ovariectomized athymic nude mice and allowed to grow in presence of Δ4A (aromatizable substrate of estrogen). Using this model, we have established that although AI letrozole provides a longer control over tumor growth, tumors eventually began to grow. In the current study, we investigated the effect of ATRA (All-trans Retinoic acid) (125μg/day, ip) and a histone deacetylase (HDAC) inhibitor entinostat (SNDX- 275) (50μg/day, po) with or without letrozole on letrozole resistant tumors in a xenograft model system. Ovariectomized athymic nude mice bearing xenografts of MCF-7Ca cells were treated with letrozole till they became resistant (15 weeks). At this time, the mice were grouped to receive ATRA, entinostat plus ATRA or the combination of ATRA plus entinostat plus letrozole till week 23. The mice treated with entinostat plus ATRA plus letrozole showed a significant decrease in tumor growth rate compared to mice treated with single agents or entinostat plus ATRA (p<0.0001, p=0.02). On week 20, two mice from each treatment group were euthanized and tumors were harvested. The tumors were digested enzymatically with collagenase and hyaluronidase and freed of debris using centrifugation and filtration. Mammosphere forming ability of TICs in the tumor tissue was measured by seeding 10,000 viable cells from each treated tumors under non-adherent conditions to access the self-renewal capacity. The combination of ATRA plus ENT plus letrozole significantly (p<0.01) reduced number of mammospheres formed compared to single agents alone. Quantitative PCR analysis of tumors cells showed a significant downregulation of the known TIC molecular markers, BCRP, ALDH, BMI-1 and Nanog compared to letrozole treated tumors. Similar results were also obtained when LTLT-Ca (long term letrozole treated MCF-7Ca) cells were treated with ATRA and entinostat in combination with letrozole and then seeded (5000 cells) in non-adherent conditions. Combination of ATRA plus entinostat plus letrozole significantly (p<0.0001) reduced mammosphere-forming ability of the LTLT-Ca cells. Overall, these studies indicate that the combination of ATRA, entinostat and letrozole is effective in reducing tumor recurrence in letrozole resistant tumors. Citation Format: Preeti Shah, Gauri J. Sabnis, Olga Goloubeva, Armina Kazi, Amanda Schech, Rabia Gilani, Yael Gau, Saranya Chumsri, Angela Brodie. Preclinical assessment of an HDAC inhibitor combined with a retinoid in AI resistant breast cancer. [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 1311. doi:10.1158/1538-7445.AM2013-1311