The anti-HER3 antibody in combination with trastuzumab exerts synergistic antitumor activity in HER2-positive gastric cancer

•Targeting both HER2 and HER3 with the combination of anti-HER3 antibody and trastuzumab may result in an improved treatment effects on HER2-positive GC.•The mechanism relies on inhibitory action of phosphorylation of HER3 as well as AKT and ERK signals.•A HER2-negative whereas NRG1-positive PDX model also benefited from combination treatment of anti-HER3 antibody and trastuzumab.