Abstract 2935: Preclinical efficacy of the p70S6K/AKT dual inhibitor M2698 in combination with trastuzumab in models of gastric cancer

M2698 is a potent and selective, ATP-competitive dual inhibitor of p70S6K and AKT1/3 that is being evaluated in a phase I clinical trial in cancer patients. Dual pathway inhibition by M2698 may result in improved clinical efficacy by inhibiting downstream pS6 and blocking the increased AKT activity that results from a compensatory feedback loop induced by PI3K/AKT/mTOR (PAM) pathway inhibition. In earlier studies, combination efficacy was observed with M2698 and trastuzumab in HER2+ breast cancer patient derived xenograft (PDX) models, including complete tumor regressions (Huck, BR. et.al. 105 th AACR; 2014: Abstract 4516). To explore the potential combination effects in gastric cancer (GC), we evaluated the effect in in vitro and in vivo GC models. Ten human GC cell lines, which included two HER2+ lines, were treated with M2698 and trastuzumab over various concentrations and cellular proliferation was examined with a WST-8 assay. Combination effects were evaluated by Bliss independence scores and highest single agent (HSA) models. The OE-19 HER2+ cell line had the greatest Bliss and HSA sum values, whereas these measures in the other HER2+ cell line, NCI-N87, were among the lowest in the set of 10 lines, indicating that the combination is synergistic in some HER2+ GC cell lines, such as OE-19. The effects of this combination on downstream markers were analysed by Western blotting. Expression of pS6 was reduced in OE-19 cells by treatment with M2698, but pAKT and pERK were both increased. Upregulated pAKT is a known compensatory feedback mechanism that is inhibited by the dual nature of M2698. Upregulated pERK is a candidate resistance marker for PI3K pathway inhibition. However, the combination of trastuzumab with M2698 blocked this increase in pERK, suggesting that dual inhibition of the MAPK and PAM pathways may contribute to the synergistic anti-proliferative effects. The combination was then tested in the OE-19 xenograft model in vivo and significantly inhibited tumor growth compared to vehicle treatment and monotherapies (p 0.05). In an in vivo pharmacodynamic study, the combination of M2698 and trastuzumab inhibited pS6 and pERK showing effective PI3K and MAPK pathway inhibition by the combination. The agents were also tested alone and in combination (n=3/treatment group) in 27 PDX models of GC in mice (ChemPartner, Shanghai, China). The tumor control rate (tumor stasis or regression) was 11% (3/27) of models treated with M2698, 15% (4/27) with trastuzumab treatment and 22% (6/27) with the combination. The PDX models are currently being evaluated for HER2 status to determine the correlation with the efficacy of the treatments. Based on the data from the OE-19 model, pERK is a candidate marker of combination efficacy which can be further explored in additional GC models. Citation Format: Shota Fukuoka, Takashi Kojima, Yoshikatsu Koga, Mayumi Yamauchi, Masahiro Yasunaga, Yasuhiro Matsumura, Kohei Shitara, Toshihiko Doi, Takayuki Yoshino, Toshio Kuronita, Anderson Clark, Brian Elenbaas, Hong Zhang, Atsushi Ohtsu. Preclinical efficacy of the p70S6K/AKT dual inhibitor M2698 in combination with trastuzumab in models of gastric cancer [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 2935.