Abstract 1314: HDAC inhibitor entinostat reverses letrozole resistance in MCF-7Ca xenografts through modulation of Her-2.

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Abstract Development of aromatase inhibitors (AIs) has significantly improved the treatment outcome of hormone responsive post-menopausal breast cancer. However, not all tumors respond initially and others eventually acquire resistance. Using our hormone sensitive human aromatase expressing MCF-7Ca xenograft model, we also saw that although AI letrozole provides a longer control over tumor growth, tumors eventually began to grow. Furthermore, we observed that Her-2 protein levels within the tumors were upregulated as early as 4 weeks (as tumors were responding). Her-2 levels increased further as the tumors became resistant. A cell line was isolated from these Long-Term Letrozole Treated tumors (LTLT-Ca), which showed increased Her-2 and reduction in ERα and aromatase. The increase in Her-2 protein levels was not accompanied by gene amplification but we observed that Her-2 protein in LTLT-Ca cells was more stable and had a longer half-life than that in MCF-7Ca cells. In addition, when MCF-7Ca cells were deprived of estrogen (E2W) or treated with letrozole, stability of Her-2 protein was immediately increased, suggesting that lower estrogen levels stabilize Her-2. In order to identify novel approaches to restoring sensitivity to AIs, we treated letrozole resistant MCF-7Ca tumors with histone deacetylase (HDAC) inhibitor entinostat (ENT). Addition of ENT to either letrozole or exemestane in letrozole resistant MCF-7Ca tumors led to increased ERα expression, aromatase activity and significantly inhibited further tumor growth, along with downregulation of Her-2, p-Her-2, p-MAPK and p-Akt. Based on the observations made in letrozole resistant tumors treated with entinostat (with or without letrozole), we hypothesized that entinostat reverses letrozole resistance due to modulation of Her-2. Entinostat reduced interaction between HSP-90 and Her-2 in entinostat treated tumors, resulting in degradation of Her-2. Based on the findings that Her-2 is upregulated even when the tumors are responding, we evaluated the effect of adding entinostat to letrozole from the start of treatment in the MCF-7Ca xenografts. Tumors of MCF-7Ca cells were allowed to form in the presence of androstenedione (Δ4A), aromatizable substrate for estrogen. When the tumors reached ∼300mm3, the mice were grouped to receive Δ4A 100μg/day (control), letrozole (1μg/day), entinostat (50μg/day) or combination of entinostat plus letrozole. Mice receiving letrozole treatment had significantly lower tumor growth rate than control (p=0.0001), so did mice treated with the combination of entinostat plus letrozole (p<0.0001). The growth rate of tumors treated with entinostat + letrozole was similar to the growth rate of letrozole alone suggesting that in the absence of acquired or de novo resistance pathways the addition of entinostat did not provide benefit beyond the aromatase inhibitor alone. Citation Format: Gauri J. Sabnis, Armina Kazi, Olga Goloubeva, Saranya Chumsri, Angela Brodie. HDAC inhibitor entinostat reverses letrozole resistance in MCF-7Ca xenografts through modulation of Her-2. [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 1314. doi:10.1158/1538-7445.AM2013-1314