Human epidermal growth factor receptor 2 (HER2)-targeted therapy by trastuzumab 
has become increasingly important for treating HER2-positive cancers, and 
trastuzumab emtansine (T-DM1) is expected to serve as an effective alternative 
to trastuzumab. Pertuzumab, a HER2 dimerization inhibitor, showed prolonged 
progression-free survival when used with trastuzumab for HER2-positive breast 
cancer. In this study, we investigated the effect of combining T-DM1 and 
pertuzumab on xenografted gastric tumors. T-DM1 as a single agent showed 
significant antitumor activity in all the three HER2-high expression tumor 
models tested (NCI-N87, SCH and 4-1ST) but was ineffective against two HER2-low 
expression tumors (SNU-16 and MKN-28). Using the T-DM1-sensitive NCI-N87 model, 
the combination efficacy of T-DM1 and pertuzumab was elucidated. The combination 
induced significant tumor regression, whereas T-DM1 or pertuzumab alone did not. 
In cultured NCI-N87 cells stimulated with epidermal growth factor (EGF) or 
heregulin-α, concomitant treatment of T-DM1 and pertuzumab significantly 
inhibited proliferation and increased caspase 3/7 activity compared to either 
agent alone. Only the combination significantly inhibited the phosphorylation of 
EGFR or HER3, and its downstream factor AKT. Suppressed HER3 phosphorylation by 
the combination was also seen in the NCI-N87 xenografted tumors. Compared to 
single agent treatments, the combination treatment significantly enhanced 
antibody-dependent cellular cytotoxicity (ADCC) against NCI-N87 cells. These 
findings suggest that T-DM1 in combination with pertuzumab shows significant 
antitumor activity by increasing AKT signal inhibition and ADCC in HER2-positive 
gastric cancers.