Trastuzumab is a humanized monoclonal antibody against HER2 approved by FDA for breast and gastric cancer therapy. However, only a quarter of patients have the potential to benefit from it, and most of them develop resistance within therapy. The main purpose of this study is to broaden trastuzumab's therapeutic window by conjugating trastuzumab with recombinant cucurmosin to form an immunotoxin called T-CUS245C. T-CUS245C was chemically conjugated and the purification of T-CUS245C was evaluated by SDS-PAGE. SRB tests showed a remarkable cytotoxicity of T-CUS245C with IC50 values in picomolar range on HER2 positive cancer cells without significantly proliferation inhibition on HER2 negative cells (P < 0.01). Confocal microscopy verified the time-dependent internalization effects of T-CUS245C and revealed that the lethal efficacy can be increased by provoking the internalization. These results indicate the therapeutic potential of T-CUS245C for the HER-2 targeted therapy. (C) 2019 The Authors. Published by Elsevier Inc.