Insights into ErbB signaling from the structure of the ErbB2-pertuzumab complex.

Proc Amer Assoc Cancer Res, Volume 45, 2004 2475 The inappropriate activation or overexpression of members of the epidermal growth factor family of receptors (ErbB) is associated with the development and severity of many cancers, particularly breast cancer, where overexpression correlates strongly with a more aggressive disease and poor survival rate. Antibodies against ErbB1 and ErbB2 have proven to be effective therapeutically for the treatment of several cancers. The anti-ErbB2 monoclonal antibody trastuzumab (Herceptin) is indicated for patients whose metastatic tumors overexpress ErbB2. In contrast, pertuzumab (Omnitarg) is currently being evaluated in Phase II clinical trials in patients whose breast, non-small cell lung, prostate, or oviarian tumors do not contain the amplified ErbB2 gene. In order to provide a molecular basis for understanding the mechanism of action of pertuzumab, we have determined the X-ray crystal structure at 3.2 angstrom resolution of the soluble extracellular domain of ErbB2 in a complex with the antigen binding fragment of pertuzumab. Pertuzumab binds near the center of the second cysteine-rich domain, a region of the ErbB receptor family that is highly conserved and previously shown to be necessary for ErbB1 receptor homodimerization. Alanine substitutions of ErbB2 residues that contact pertuzumab, while effecting pertuzumab binding affinity for ErbB2, do not detectably influence ErbB2 heterodimerization with ErbB3, indicating a steric mechanism for the ability of pertuzumab to inhibit ErbB2 receptor dimerization and signaling. Additional mutagenesis of this region of ErbB2 suggests that residues that have previously been shown to be required for ErbB1 homodimerization are dispensable for ErbB2 heterodimerization. These findings suggest that certain aspects of ErbB2 heterodimerization with other ErbB family members may be fundamentally different than ErbB1 homodimerization. In conclusion, Pertuzumab inhibits the ability of ErbB2 to function as a co-receptor in ligand-mediated ErbB signaling by sterically blocking a binding pocket necessary for receptor dimerization.