Human epidermal growth factor receptor-2 (HER-2)/neu (ErbB2), a member of the epidermal growth factor family of receptors, is overexpressed in 20-30% of breast cancers. It is an attractive target for receptor-directed antitumor therapy using mAbs. Unlike other epidermal growth factor receptor family members, HER-2/neu does not bind a high-affinity ligand, but rather functions as the preferred dimerization partner. Pertuzumab (Omnitarg) is a humanized mAb directed against the HER-2/neu dimerization domain that inhibits receptor signaling. The recent definition of the crystal structure of the HER-2/neu-pertuzumab complex demonstrated that the receptor dimerization region encompassed residues 266-333. Based on the three-dimensional structure of the complex, we have designed three conformational peptide constructs (sequences 266-296, 298-333, and 315-333) to mimic regions of the dimerization loop of the receptor and to characterize their in vitro and in vivo antitumor efficacy. All the constructs elicited high-affinity peptide Abs that inhibited multiple signaling pathways including HER-2/neu-specific inhibition of cellular proliferation and cytoplasmic receptor domain phosphorylation. All the peptide Abs showed Ab-dependent cellular cytotoxicity to varying degrees with the 266-296 constructs being equally effective as compared with Herceptin. The 266-296 peptide vaccine had statistically reduced tumor onset in both transplantable tumor models (FVB/n and BALB/c) and significant reduction in tumor development in two transgenic mouse tumor models (BALB-neuT and VEGF(+/-)Neu2-5(+/-)). The 266-296 construct represents the most promising candidate for antitumor vaccination and could also be used to treat a variety of cancers with either normal or elevated expression of HER-2 including breast, lung, ovarian, and prostate.