The present invention relates to a method of treating HER2
EU overexpressing cancers. The inventors discovered that the heme-mediated formation of dimers and in general oligomers of Trastuzumab is associated with an improved complement-mediated cytotoxicity on breast cancer cells. The present data highlight that the sensitivity to heme of Trastuzumab, may have major repercussion on its therapeutic activity. Thus the invention relates to the combination of an HER2
eu antibody with a heme and/or of its oligomers and its therapeutic composition in the HER2
EU characteristic cancer treatment.