Abstract 945: Trastuzumab deruxtecan (T-DXd) sensitivity in various levels of HER2 expressing gastric cancer cells
Authors: Yu Seo Young , Park Juin , Kwon Woo Sun , Jeong Inhye , Kang Sun Kyoung , Bae Hyun Joo , Kim Tae Soo , Chung Hyun Cheol , Rha Sun Young 
Published: July 2021

Abstract
Trastuzumab, a humanized anti-HER2 monoclonal antibody, has shown clinically significant benefits for HER2-positive GC patients. Also, all anti-HER2 agents approved by the FDA are used for HER2-positive. However, since HER2-positive is only 10-20% of GC, targeted therapies for non-HER2-positive GC is needed. To overcome these limitations, antibody-drug conjugates (ADCs), which directly deliver cytotoxic drugs to cancer cells, have been developed. T-DXd (Daiichi Sankyo, Japan), an anti-HER2 ADC, can induce the death of neighboring cells because it has a bystander killing effect due to the high membrane permeability of the payload. However, biomarkers for predicting drug response have not yet been studied other than HER2 overexpression/amplification. In this study, we investigated the correlation between HER2 expression level and anti-tumor effect of T-DXd in a 49 GC cell line panel and the HER2-associated molecules related to T-DXd sensitivity. For HER2 status, we analyzed multiple genetic and molecular characteristics using WES, SISH, IHC, flow cytometry, and immunoblotting. GC cell lines (n=49) were divided into 3 groups according to the HER2 expression level through flow cytometry using 4 breast cancer cell lines (SK-BR-3, ZR-75-1, MCF-7, and HCC1937) for quantitative control. Groups with HER2 over- and moderate/low-expression were characterized with higher expression levels compared to ZR-75-1 (HER2 2+) and HCC1937 (HER2-negative), respectively. HER2 non-expression had lower expression levels compared to HCC1937. The sensitivity of T-DXd was evaluated in 49 GC cell lines by cell viability assay. The inhibition rate of T-DXd was calculated at 10 µg/ml. Also, expression levels of RTKs were determined by immunoblotting, and HER2 extracellular domain (ECD) and NRG1 expression were evaluated by ELISA. GC cell lines were divided into 6 HER2 over-, 26 HER2 moderate/low-, and 17 HER2 non-expression cell lines. As expected, HER2 over-expression cell lines were sensitive to T-DXd (5/6, 83.3%). T-DXd also had anti-tumor effects in the HER2 moderate/low-expression cell lines (12/26, 46.2%). There were correlations between HER2 expression level and anti-tumor effect of T-DXd in both HER2 over and moderate/low-expression cell lines. HER2 over and moderate/low-expression cell lines with high HER2 ECD expression were sensitive to T-DXd. NRG1 had relatively low expression in all GC cell lines and was not correlated with T-DXd. Additionally, even HER2 non-expression cell lines were sensitive to T-DXd (10/17, 58.8%). We found that HER2 non-expression cell lines had an RTK amplification (9/17, 52.9%) and among those cell lines, MET amplified cells showed sensitivity to T-DXd. Currently, the evaluation of the underlying mechanisms is ongoing. Our results indicate that T-DXd shows efficacy in HER2-amplified GC cell lines, HER2 moderate/low-expression, and some cell lines with HER2 non-expression.
Citation Format: Seo Young Yu, Juin Park, Woo Sun Kwon, Inhye Jeong, Sun Kyoung Kang, Hyun Joo Bae, Tae Soo Kim, Hyun Cheol Chung, Sun Young Rha. Trastuzumab deruxtecan (T-DXd) sensitivity in various levels of HER2 expressing gastric cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 945.