Trastuzumab Has Opposing Effects on SN-38-induced Double-strand Breaks and Cytotoxicity in HER2-positive Gastric Cancer Cells Depending on Administration Sequence

We investigated the effects of trastuzumab, an anti-HER2 humanized monoclonal antibody, on DNA breaks induced by SN-38, a topoisomerase-1 inhibitor, in gastric cancer cell lines positive or negative for HER2 expression. NCI-N87 (HER2+) and MKN74 (HER2-) cells were exposed to SN-38 in the presence or absence of trastuzumab. Trastuzumab was added either prior to or after SN-38. Effects of trastuzumab on the induction of gamma-H2AX, a marker of DNA double-strand breaks, the cytotoxicity of SN-38 and cell cycle progression were determined. When trastuzumab was administered following SN-38, it increased γH2AX levels and cytotoxicity of SN-38 in NCI-N87 cells, but not in MKN74 cells. In contrast, pretreatment with trastuzumab reduced SN-38-induced γH2AX expression and cytotoxicity of SN-38 in NCI-N87 cells, but not in MKN74 cells. Trastuzumab delayed cell cycle progression in NCI-N87 cells only. Trastuzumab has opposing effects on SN-38-induced double-strand breaks and cytotoxicity depending on the order of administration of the two agents.