Abstract 2691: Role of trastuzumab in the combination treatment for a HER2-positive trastuzumab-resistant gastric cancer xenograft model
Authors: Shu Sei , Yamashita-Kashima Yoriko , Yanagisawa Mieko , Moriya Yoichiro , Harada Naoki 
Published: August 2015

Abstract
Objective: Trastuzumab (Tras) resistance is an important clinical problem to be resolved for HER2-positive gastric cancers (GC). Even after the tumor develops resistance to Tras, continuous anti-HER2 therapies, including Tras with chemotherapy (Tras beyond PD) or trastuzumab emtansine (T-DM1), could be effective treatment options. The Tras-sensitive cell line GLM-1 was established from a liver metastasis of a GC patient (Nakanishi et al. 2005), and the Tras-resistant cell line GLM1-HerR2 (R2) was established from GLM-1 by in vivo selection (Oshima et al. 2014). In this study, we used GLM1 and R2 xenograft models to examine the Tras-resistant mechanism of R2 and the effects of Tras plus paclitaxel (PTX) or T-DM1.
Methods: BALB/c-nu/nu mice subcutaneously inoculated with GLM1 or R2 received Tras (40 mg/kg, qw) intraperitoneally, PTX (15 mg/kg, qw) or T-DM1 (10-40 mg/kg, q3w) intravenously (Day 1). Tumor volume (TV) was measured to evaluate antitumor effects, HercepTest® was used for HER2 status, and flow cytometry to analyze Tras-binding ability. Cell proliferation after knockdown of HER family receptors was measured by BrdU incorporation. Signal transduction was examined by Western blot for tumors excised on Days 2 and 5.
Results: In contrast to GLM-1, R2 showed resistance to Tras treatment in xenograft experiments. There were no clear differences in HER2 score (3+), Tras-binding ability, p-HER2, p-HER3, p-AKT levels, or PIK3CA status (no hot spot mutation). Effects of gene knockdown of EGFR/HER2/HER3 in GLM-1 and R2 on cell proliferation were similar (ca. 10%/50%-60%/10% reduction, respectively). In GLM-1 tumors, Tras treatment decreased p-AKT, increased p27 protein, and decreased p-Rb, resulting in G1 cell cycle arrest, whereas in the R2 tumors changes in those molecules were few. These results suggest that escape from Tras-induced G1 arrest could be a resistant mechanism of R2. Based on the above results, and because in R2 cells HER2 remained a driver of proliferation and mitotic arrest was maintained, we hypothesized that Tras + PTX would show a higher antitumor effect than PTX alone. In the R2 xenograft model on Day 22, the Tras + PTX group showed a significantly higher antitumor activity (TV: 2 ± 3 mm3) than the PTX alone group (TV: 208 ± 209 mm3). In tumor specimens from the PTX and Tras + PTX groups, p-histon H3 (an M phase marker) was increased on Day 2. However, the Tras + PTX group, unlike the PTX group, showed markedly decreased p-histon H3 on Day 5, which suggests that Tras in combination with PTX enhances apoptosis in R2 tumors. Interestingly, T-DM1 showed dose-dependent antitumor activity in both GLM-1 and R2 xenografts.
Conclusion: It was indicated that continuous anti-HER2 treatment, including Tras with chemotherapy or T-DM1 monotherapy, could be efficacious treatments for HER2-positive Tras-resistant GC tumors that emerge during previous Tras treatment.
Citation Format: Sei Shu, Yoriko Yamashita-Kashima, Mieko Yanagisawa, Yoichiro Moriya, Naoki Harada. Role of trastuzumab in the combination treatment for a HER2-positive trastuzumab-resistant gastric cancer xenograft model. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2691. doi:10.1158/1538-7445.AM2015-2691