Abstract 1136: Heat shock protein 90 inhibitors suppress PDGFRA reactivation and other receptor tyrosine kinase activation important in drug resistant gastrointestinal stromal tumors
Authors: Yamamura Masahiro , Yamauchi Akira , Katase Naoki , Okawaki Makoto , Katata Yousuke , Kuribayashi Futoshi , Kurebayashi Junichi , Yamaguchi Yoshiyuki 
Published: July 2017

Abstract
Background: Inhibition of KIT oncoproteins by imatinib induces clinical responses in most gastrointestinal stromal tumor (GIST) patients. However, many patients develop imatinib resistance due to secondary KIT mutations. The drug resistance mechanism has only been partially elucidated, and the sunitinib resistance mechanism is also unknown. In this study, we elucidate the new imatinib resistant mechanism and find effective inhibitors.
Methods: We have established imatinib-resistant cells GIST-T1R by culturing cells with increasing concentrations of imatinib. We analyzed receptor tyrosine kinase and intracellular signal protein strongly expressed in resistant cells using Western blotting and Phosphorylation Array. Then, the antitumor effect was examined using an agent that inhibits strongly expressed molecules of the resistant cells.
Results: The IC50 of imatinib in the sensitive cells(GIST-T1) and the resistant cells(GIST-T1R) was 10 nM and 20 μM, respectively, which demonstrated 2000-fold difference. These imatinib resistant cells were also cross-resistant to sunitinib. In western blotting, PDGFRA phosphorylation and protein expression were enhanced in GIST-T1R compared to GIST-T1. When GIST-T1 was treated with imatinib, KIT phosphorylation was suppressed but PDGFRA phosphorylation was enhanced. Imatinib treatment of GIST-T1R enhanced PDGFRA phosphorylation and protein expression as compared to before imatinib treatment. Phosphorylation of PDGFRA in these resistant cells could not be suppressed by regorafenib. Furthermore, in the protein phosphorylated array, in addition to phosphorylation of PDGFRA in resistant cells, phosphorylation of multiple RTKs was also enhanced. An HSP90 inhibitor AUY922 capable of simultaneously suppressing a plurality of proteins was used. AUY922 suppressed GIST-T1 and GIST-T1R cell proliferation at low concentrations and was more effective than regorafenib. Furthermore, AUY922 inhibited phosphorylation and protein expression of PDGFRA in drug-resistant cells, as well as phosphorylation of other several receptor tyrosine kinases.
Conclusion: Activation of PDGFRA was considered to be important for the mechanism of drug resistance of GIST. Furthermore, since activation of multiple receptor tyrosine kinases can be observed simultaneously in drug resistance, it is considered that inhibiting a single target is not effective. Since HSP90 inhibition suppresses PDGFRA protein expression and possibly inhibits multiple target proteins at the same time, it is the most effective treatment for drug resistant GIST.
Citation Format: Masahiro Yamamura, Akira Yamauchi, Naoki Katase, Makoto Okawaki, Yousuke Katata, Futoshi Kuribayashi, Junichi Kurebayashi, Yoshiyuki Yamaguchi. Heat shock protein 90 inhibitors suppress PDGFRA reactivation and other receptor tyrosine kinase activation important in drug resistant 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 1136. doi:10.1158/1538-7445.AM2017-1136