Abstract C120: Antitumor effect of HER3-DXd, an antibody-drug conjugate targeting HER3, in gastric cancer cell lines
Authors: Hwang Hae Min , Kim So Hyeon , Ham Sujin , Lee Minyoung , Noh Youlim , Kim Yu-Jin , Lee Changyun , Kim Jinyoung , Lee Dae-Won , Lee Kyung-Hun , Im Seock-Ah 
Published: December 2023

Abstract
Background: HER3, one of the HER family members, is expressed across various solid tumors and known to be an important oncogenic driver. HER3-DXd is an antibody-drug conjugate (ADC) that combines patritumab, an anti-HER3 antibody, and deruxtecan, a topoisomerase 1 inhibitor payload through peptide cleavable linker system. HER3-DXd has been shown to have anti-tumor effects in breast, non-small cell lung, and colorectal cancers, in vitro and in vivo, and is being evaluated in phase 2 and phase 3 clinical trials. However, the effect of HER3-DXd in gastric cancer and the mechanism of sensitivity are largely unknown.
Methods: Seven gastric cancer cell lines were used (NUGC-4, NCI-N87, SNU-601, SNU-216, SNU-638, SNU-668, and MKN-45). Colony formation assay (CFA) was performed with increasing concentration of HER3-DXd (doses range: 0-100 nM) for 14 days. Cell cycle change and internalization of HER3-DXd bound to HER3 protein in the cell membrane were seen by flow cytometry analysis. Apoptotic cell death was assessed by Annexin-V assay. DNA strand breaks were examined using alkaline comet assay. DNA damage accumulation and TOP1-cleavage complexes (TOP1ccs) were identified using immunofluorescence assay.
Results: Seven gastric cancer cell lines showed heterogeneous mRNA expression of HER2 and HER3, which correlated with protein expression. Three cell lines with different sensitivities to HER3-DXd were selected based on CFA. SNU-601 was sensitive to U3-1402 (IC50 value: 11.01 nM), SNU-638 was less sensitive (IC50 value: 50.04 nM), and SNU-668 was insensitive (IC50 value &gt;100 nM). HER3-DXd increased the Annexin-V positive population and the sub-G1 population in SNU-601 and SNU-638 cells. Moreover, cleavage of PARP and caspase-3 were increased in SNU-601 and SNU-638 cells, but not in SNU-668 cells. HER3-DXd was efficiently internalized within 1 hour in all three cells, regardless of their sensitivities to HER3-DXd. Signal transduction through the HER2 and HER3 receptors, which mediates a PI3K-AKT signaling pathway, was inhibited in SNU-601 cells. In SNU-601 and SNU-638 cells, HER3-DXd increased the expression of γ-H2AX and length of the comet tails, which indicate DNA damage. HER3-DXd also induced topoisomerase 1 cleavage complexes (TOP1ccs) in SNU-601 and SNU-638 cells. In contrast, TOP1cc was not observed in resistant SNU-668 cells.
Conclusion: HER3-DXd induced formation of TOP1ccs, subsequent DNA damage, and apoptotic cell death in sensitive cell lines, but not in an insensitive cell line. These data indicate that internalized HER3-DXd causes accumulation of DNA damage through TOP1cc formation, which leads to apoptotic cell death in gastric cancer.
Citation Format: Hae Min Hwang, So Hyeon Kim, Sujin Ham, Minyoung Lee, Youlim Noh, Yu-Jin Kim, Changyun Lee, Jinyoung Kim, Dae-Won Lee, Kyung-Hun Lee, Seock-Ah Im. Antitumor effect of HER3-DXd, an antibody-drug conjugate targeting HER3, in gastric cancer cell lines [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr C120.