Therapies targeted to human epidermal growth factor receptor 2 (HER2) have 
proven effective against tumors positive for HER2 amplification, but there is an 
unmet clinical need for the treatment of tumors that express HER2 protein in the 
absence of HER2 amplification. [fam-] trastuzumab deruxtecan (DS-8201a) is a 
novel antibody-drug conjugate composed of the anti-HER2 antibody and the 
topoisomerase I inhibitor, an exatecan derivative. It has shown efficacy against 
tumors that express HER2 and is currently under evaluation in clinical trials. 
We here show that the antitumor activity of [fam-] trastuzumab deruxtecan is 
dependent on the expression level of HER2 protein in colorectal cancer (CRC) 
cell lines negative for HER2 amplification. We established isogenic CRC cell 
lines that express various levels of HER2 protein in the absence of HER2 
amplification, and we found that cells that express HER2 at a high level were 
sensitive to [fam-] trastuzumab deruxtecan but not to conventional HER2-targeted 
therapies. Furthermore, [fam-] trastuzumab deruxtecan manifested a bystander 
killing effect both in vitro and in vivo, with cells essentially negative for 
HER2 expression also being killed in the presence of HER2-expressing cells, an 
effect that has the potential to overcome heterogeneity of HER2 expression in 
CRC tumors. Our results thus suggest that [fam-] trastuzumab deruxtecan warrants 
further study as a potential treatment for CRC tumors that express HER2 protein 
in the absence of HER2 amplification.