Abstract B80: HSP90 inhibitor SNX‐2112 and prodrug SNX‐5422 (PF‐04929113) inhibit key signal pathways Akt, Src and STAT3 and reactivate tumor suppressor p53 in head and neck cancer

Purpose: Heat shock 90 (Hsp90) is a molecular chaperone that interacts with and stabilizes many client substrates involved in modulating cell proliferation, survival, and apoptosis. Natural product Hsp90 inhibitors, such as benzoquinone ansamycins and their derivatives (i.e. geldanamycin, 17AAG), degrade oncogene and signal protein clients of Hsp90 and possess anti‐tumor activity. Due to the limited solubility and hepatotoxicity caused by these agents, other Hsp90 inhibitors with a more favorable profile have been intensely sought after. In this study we investigated the biological activity of a synthetic small molecule inhibitor of Hsp90, SNX‐2112, or the orally available prodrug SNX‐5422 (PF‐04929113), in human head and neck squamous cell carcinomas (HNSCC). Experimental Design: The effects of SNX‐2112 on cellular proliferation and Hsp90 client signal molecules, p53, and p53‐targets were examined in human HNSCC cell lines from the University of Michigan Squamous Cell Carcinoma (UMSCC) series. The effect of SNX‐5422 administered by oral gavage on tumor growth was examined in the UMSCC‐11A murine xenograft model. Protein expression of Hsp90 client signal molecules were measured in tumor specimens harvested at three time points by immunohistochemistry (IHC) during SNX‐5422 treatment, to identify the molecules mediating drug effects on proliferation, apoptosis and angiogenesis. Results: SNX‐2112 exhibited potent growth inhibition in the UMSCC cell lines, with an IC50 range of 25 ‐ 75 nmol/L, and inhibited pro‐survival signal kinase phospho‐Akt at 24, 48, and 72 hours following exposure to SNX‐2112 in UMSCC‐11A. We also observed increased expression of pro‐apoptotic transcription factor p53, and target gene PUMA, at 24, 48, and 72 hours following SNX‐2112 exposure. Pro‐drug SNX‐5422 (PF‐04929113) administered 20 mg/kg 5 days/wk or 50 mg/kg 3 days/wk for 3 wks significantly decreased tumor growth. By IHC, tumor specimens showed increased staining for p53 and apoptosis (TUNEL), and decreased staining for signal molecules Src, p‐STAT3, and p‐Akt together with markers of proliferation (Ki67) and vessel density (CD31). No apparent toxicity was observed in mice with SNX‐5422 administered 20 mg/kg 5 days/wk for three weeks. Conclusion: Synthetic Hsp90 inhibitor SNX‐2112 is effective at degrading Hsp90 clients that play a significant role in proliferation, cell survival, and angiogenesis in HNSCC in vitro. In addition, SNX‐5422 inhibits tumor growth by promoting apoptosis through modulation of several key signaling pathways. SNX‐5422 warrants investigation as a clinical agent for treatment of head and neck cancer. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B80.