1622PHeat shock protein 90 (HSP90) inhibitor as a candidate treatment option for gastrointestinal stromal tumor with acquired resistance for conventional receptor tyrosine kinase inhibitors

Background: Imatinib functions as a specific inhibitor of a number of tyrosine kinase enzymes, such as KIT and PDGFR, by occupying the TK active site, and thus improves the prognosis of gastrointestinal stromal tumor (GIST) patients. However, resistance to the drug appears with prolonged usage. Mechanisms of acquired resistance are still under elucidation. Methods: To evaluate mechanisms of acquired resistance for imatinib, we established a imatinib-resistant GIST cell line, so-called T1R, by culturing the GIST cell line, GIST-T1, with increasing concentrations of imatinib. Next, we analyzed receptor tyrosine kinases (RTKs) and intracellular signals strongly expressed in T1R by western blotting and phosphorylated arrays. Finally, we examined the antitumor effect of an agent which was confirmed to inhibit upregulated RTKs in T1R. Results: Interestingly, imatinib-resistant T1R showed cross resistance to sunitinib, which offers patients with imatinib-resistant GIST a new treatment option to stop further disease progression. By western blotting, T1R showed a suppression of phosphorylation in KIT in contrast to a promotion of phosphorylation in PDGFRA, which never observed before imatinib treatment. A phosphorylation multiplex array also revealed that T1R had additional promotion of phosphorylation in FGFR, Met, Eph, Axl, and Tie2. Therefore we considered activation of PDGFRA owing to one of the candidate machinery of acquired resistance for imatinib as well as sunitinib. HSP90 inhibitors is known to effective against both imatinib-sensitive and resistant GIST models. Therefore, we examined whether HSP90 inhibitors interact with PDGFRA TK kinase activity in T1R. HSP90 inhibitors inhibited the phosphorylation and protein expression of PDGFRA and other RTKs, resulted to inhibit cell proliferation and induce apoptosis in T1R. Conclusions: Activation of multiple RTKs is an essential for acquired resistance for imatinib in GIST. Inhibition of PDGFRA and other RTKs by HSP90 inhibitors has a potential to the next treatment option for GIST which acquired drug resistance for conventional small-molecule, multi-targeted RTK inhibitors. Legal entity responsible for the study: Department of Medical Oncology, Kawasaki Medical School Hospital. Funding: Has not received any funding. Disclosure: All authors have declared no conflicts of interest.