Abstract C128: TAS-116, an orally available HSP90α/β selective inhibitor, has potent antitumor activity in small cell lung cancer as a single agent and in combination with an anthracycline derivative, amrubicin.
Authors: Muraoka Hiromi , Yoshimura Chihoko , Hashimoto Akihiro , Ito Kenjiro , Kitade Makoto , Yonekura Kazuhiko , Ohkubo Shuichi , Utsugi Teruhiro 
Published: August 2014

Background: The molecular chaperone HSP90 has been considered a promising target for cancer therapy. We have previously reported the discovery of an orally available HSP90α/β selective inhibitor, TAS-116. TAS-116 led to tumor shrinkage in various human tumor xenograft models, while not inducing retinal toxicity in rats including at doses given above the maximum tolerated dose. Extensive small cell lung cancer (SCLC) is one of the most aggressive types of cancer, and has an unmet medical need for more effective treatments. An anthracycline, amrubicin (AMR), has been used to treat platinum resistant SCLC in Japan. Here we report on the possible application of TAS-116 to treat SCLC both as a single agent and in combination with AMR.

Materials and Methods: In vitro drug combination effects were evaluated with calculation of the combination index and cell cycle analysis by fluorescence-activated cell sorting. To clarify the mechanism involved in TAS-116’s enhancement of anthracycline-induced apoptosis the extent of apoptosis was measured in combination with several kinase inhibitors including SB 218078 (CHK1) and MK-2206 (AKT). Antitumor activities of TAS-116 alone and in combination with AMR were evaluated in human SCLC xenograft models.

Results: TAS-116 induced cell growth arrest and apoptosis in SCLC cell lines by inhibiting both PI3K/AKT and MAPK signaling through downregulation of HSP90 clients such as AKT and RAF1. TAS-116 also showed antitumor activity against human SCLC tumor xenograft models. In in vitro explorative combination studies in SCLC cell lines, structurally-related anthracyclines, AMR and doxorubicin (DOX), both showed synergistic effect upon apoptosis induction in combination with TAS-116. Moreover, TAS-116 enhanced the antitumor activity of AMR in human SCLC tumor xenograft models. Since anthracyclines exert antitumor activity based on their DNA damaging potential through topoisomerase II inhibition, we evaluated whether DNA damage signaling proteins and apoptosis-related proteins were affected by TAS-116 when given in combination with DOX. TAS-116 significantly reduced phosphorylation of CHK1, AKT and ERK1/2. In combination with these kinase inhibitors, SB 218078 significantly and MK-2206 moderately enhanced DOX-mediated apoptosis. Comparatively, TAS-116 enhanced apoptosis to a greater extent than SB 218078.

Conclusion: TAS-116 showed antitumor activity against human SCLC tumor xenograft models alone and in combination with AMR. Our data suggest that TAS-116 enhanced the antitumor activity of anthracyclines through the combined actions of abrogation of the DNA damage-induced G2/M checkpoint and the inhibition of a survival signal by CHK1 and AKT downregulation, respectively. These results suggest that TAS-116 could be a promising agent to treat extensive SCLC.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C128.

Citation Format: Hiromi Muraoka, Chihoko Yoshimura, Akihiro Hashimoto, Kenjiro Ito, Makoto Kitade, Kazuhiko Yonekura, Shuichi Ohkubo, Teruhiro Utsugi. TAS-116, an orally available HSP90α/β selective inhibitor, has potent antitumor activity in small cell lung cancer as a single agent and in combination with an anthracycline derivative, amrubicin. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C128.