Abstract #LB-207: The antitumor effects of OSU-HDAC42, a histone deacetylase inhibitor, in HER2-positive breast cancer are mediated partly through Hsp90 modulation

Purpose: Elevated HER-2/neu expression in primary breast tumors is associated with frequent relapse and poor prognosis. For this reason, HER-2/neu has been actively pursued as a target for novel therapeutic agents. Histone deacetylase (HDAC) inhibitors, a new class of antitumor drugs, have exhibited the ability to downregulate HER2 through hyperacetylation of heat shock protein 90 (hsp90), an important ATP-dependent chaperone that mediates the stability and maturation of a variety of important oncogenic proteins, including HER2, Akt and ER\#945;. In the present study, we assessed the efficacy of OSU-HDAC42, a novel phenylbutyrate-based HDAC inhibitor, in inducing anti-proliferation and hsp90 down-regulation in both in vitro and in vivo HER2+ breast cancer models.                  Methods: HDAC42 was tested for effects on the human breast cancer cell lines BT474, SKBR3, MDA-MB-231 and MCF-7. Cell viability (MTT) and cell cycle (flow cytometry) changes were determined. Cellular protein levels of HER2, phospho-HER2, PARP cleavage, Akt1/2, ER\#945;, hsp90, hsp70 and \#946;-actin were detected by immunoblotting. Acetylation of hsp90 was assessed via hsp90 immunoprecipitation and acetyl-lysine immunoblotting. The ATP-agarose pull-down assay tested for cellular hsp90 activity. The in vivo efficacy of OSU-HDAC42 was evaluated in a syngeneic orthotopic HER2+ mammary tumor model, using AIN-76A diets ± OSU-HDAC42 at 208 mg/kg of diet. Tumor volume, mass and gene expression changes served as in vivo endpoints.                  Results: SKBR3 (HER2+, ER\#945;-) was the most susceptible to the antiproliferative effects of OSU-HDAC42 after 72 hours of treatment (IC₅₀ = 0.034 \#956;mol/L), followed by BT474 (HER2+, ER\#945;+), MCF-7 (HER2-, ER\#945;+) and MDA-MB-231 (HER2-, ER\#945;-) cells with IC₅₀ values calculated at 0.16, 0.2 and 0.79 \#956;mol/L, respectively, which correlated with the level of inhibition of Hsp90 client protein expression (HER2, Akt). OSU-HDAC42 not only yields 43- or 65- fold more potent cell killing than suberoylanilide hydroxamic acid (SAHA; vorinostat) or MS-275, but exerts a more potent suppressive effect on the expression levels of Hsp90 client proteins (HER2, ER\#945; and Akt) and apoptosis induction in HER2+ breast cancer cells. In vivo administration of OSU-HDAC42 resulted in significant reductions of 76% and 82% in NT5 (HER2+,ER-) tumor mass and volume, respectively, which correlated with tubulin hyperacetylation, increased PARP cleavage, decreased pHER2 and pAkt levels in OSU-HDAC42 treated mouse tumors.  Conclusions: OSU-HDAC42 is a potent inhibitor of HER-2+ breast cancer, mediated in part through hsp90 downregulation. As an orally bioavailable HDAC inhibitor with greater potency than other clinically available HDACi agents, OSU-HDAC42 warrants evaluation in clinical trials as a novel therapy for HER2+ breast cancer.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr LB-207.