Abstract 4784: TAS0286/HM05, a novel highly selective RET inhibitor, prominently inhibits various RET defective tumor growth

The rearranged during transfection (RET) gene is a well-known proto-oncogene and encodes a single-pass transmembrane receptor tyrosine kinase. RET fusions and point mutations are oncogenic drivers in NSCLC, medullary thyroid cancer and other solid tumors, and therefore potential targets for cancer therapy. Multikinase inhibitors targeting the RET mutations have been tested in clinical trials with moderate efficacy in terms of tumor shrinkage and PFS.Moreover, multikinase inhibitors are characterized by poor tolerability due to off-target kinase inhibitory activities. To widen the therapeutic index, a selective RET kinase inhibitor is highly desirable. TAS0286/HM05 is a novel highly selective and potent RET kinase inhibitor.RET kinase assay was performed by homogeneous time-resolved fluorescence (HTRF) method. In-vitro proliferation studies and pharmacodynamics analyses were conducted in cancer cell lines with the RET fusions and RET activating mutations. The antitumor efficacy of TAS0286/HM05 was evaluated using mice xenograft models implanted with cancer cells with various RET gene abnormality. TAS0286/HM05 was orally administered for 14 or 28 consecutive days after grouping. In-vitro and in-vivo RET phosphorylation was detected using western blot analysis. The IC 50 value for RET kinase of TAS0286/HM05 was below 1 nM. TAS0286/HM05 showed highly selective RET inhibitory activity among 283 kinases. In cellular assay, TAS0286/HM05 strongly suppressed phosphorylation of RET expressed in cells with various RET fusions and activating mutations, and inhibited cell proliferation at around 10 nM. At this concentration, apoptosis was also observed. The potency was higher than other multikinase inhibitors with RET inhibitory activity. Furthermore, in in-vivo efficacy studies, TAS0286/HM05 significantly inhibited the growth of tumors harboring various RET fusions and activating mutations at a range of 20 to 100 mg/kg/day without any body weight loss. The antitumor efficacy of TAS0286/HM05 was more potent than pre-existing multikinase inhibitors at their maximum tolerated dose. In particular, TAS0286/HM05 dramatically induced tumor regression of 40% within 15 days in animals implanted with LC-2/ad cells, a human lung adenocarcinoma cell line with CCDC6-RET fusion gene. TAS0286/HM05 is a novel and highly selective RET inhibitor with prominent tolerability. The potency of TAS0286/HM05 against tumors with RET abnormalities was stronger than currently marketed multikinase inhibitors being tested in NSCLC patients with RET fusions. TAS0286/HM05 is a promising agent for future clinical development in patients with RET gene abnormalities. Citation Format: Hidenori Fujita, Isao Miyazaki, Masanori Kato, Yukari Yamada, Keiji Ishida, Tomonori Haruma, Haruka Nagasaki, Kenjiro Ito, Akihiro Hashimoto, Yasuo Kodama, Kaoru Funabashi, Emanuela Lovati, Kazutaka Miyadera, Kenichi Matsuo, Yoshikazu Iwasawa. TAS0286/HM05, a novel highly selective RET inhibitor, prominently inhibits various RET defective tumor growth [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4784.