Abstract 1330: HSP90 inhibitor has a possibility to overcome imatinib resistance in gastrointestinal stromal tumors
Authors: Saito Yurina , Takahashi Tsuyoshi , Serada Satoshi , Fujimoto Minoru , Tanaka Koji , Miyazaki Yasuhiro , Makino Tomoki , Kurokawa Yukinori , Yamasaki Makoto , Nakajima Kiyokazu , Takiguchi Shuji , Mori Masaki , Doki Yuichiro , Naka Tetsuji 
Published: July 2017

Abstract
[Background] Gastrointestinal stromal tumors (GIST) are the most common mesenchymal tumors of the digestive tract and are reported to harbor gain-of-function mutations in the KIT gene, which contribute to the development of sporadic GISTs. This knowledge has facilitated the development of targeted therapies with tyrosine kinase inhibitors and the revolutionary chemotherapeutic drug imatinib mesylate (IM). In clinical trials, the disease control rate was nearly 85%, and corresponding 2-year overall survival rates ranged from 70-80%, indicating markedly improved patient outcomes compared with anecdotal data in the pre-IM era. Despite its effectiveness, half of GISTs treated with IM develop resistance within 2 years, largely due to the accumulation of additional kinase domain mutations accompanied by concomitant re-activation of the KIT tyrosine kinase, even in the presence of IM. Heat shock protein 90 (HSP90) is one of chaperon molecules required for the proper folding, function, and stability of various client proteins such as KIT. The aim of this study is to clarify the efficacy of HSP90 inhibitor against IM resistant GIST.
[Material and Method] We used the established human GIST cell line GIST-T1, and two IM-resistant cell lines (GIST-T1R8, GIST-T1R9), which had additional kinase domain mutations accompanied by concomitant re-activation of the KIT tyrosine kinase same as clinical samples, by exposure to IM. These resistant cell lines exhibited imatinib IC50 values (&gt;10 μM) that were &gt;1000-fold higher than the parental cell line. We investigated the cytotoxicity and signaling inhibition by HSP90 inhibitor using by the WST-8 assay, caspase3/7 apoptosis assay and western blotting. Immunobolts for KIT and for KIT-depending signaling pathways were observed at 12 hours of treatment. Caspase 3/7 apoptosis assay was measured after 24h.
[Result] HSP90 inhibitor showed growth inhibition not only for GIST T1 but also GIST-T1R8 and GIST-T1R9. Apoptosis induction of these cell lines were also confirmed by the exposure to HSP90 inhibitor in a dose-dependent manner. To find the mechanism of apoptosis induced by HSP90 inhibitor, we investigated the signaling pathway depending on KIT. HSP90 inhibitor showed inhibition of phosphorylated-KIT, a client for HSP90, in both IM sensitive and resistant GIST cell lines. And, it also inhibited downstream of KIT signaling, e.g. p-ERK and p-AKT.
[Conclusions]HSP90 inhibitor showed the anti-tumor efficacy for imatinib resistant GIST in vitro. It might have a possibility to apply for clinical use.
Citation Format: Yurina Saito, Tsuyoshi Takahashi, Satoshi Serada, Minoru Fujimoto, Koji Tanaka, Yasuhiro Miyazaki, Tomoki Makino, Yukinori Kurokawa, Makoto Yamasaki, Kiyokazu Nakajima, Shuji Takiguchi, Masaki Mori, Yuichiro Doki, Tetsuji Naka. HSP90 inhibitor has a possibility to overcome imatinib resistance in gastrointestinal stromal tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1330. doi:10.1158/1538-7445.AM2017-1330