Can Trastuzumab Be Effective Against Tumors With Low HER2/Neu (ErbB2) Receptors?

HER2/neu (erbB2) is a member of the erbB family of transmembrane receptor tyrosine kinases, which also includes the epidermal growth factor receptor (EGFR; erbB1), HER3 (erbB3), and HER4 (erbB4). Binding of ligands to the extracellular domain of EGFR, HER3, and HER4 induces the formation of kinase active homoand heterodimers to which HER2 is recruited as a preferred partner. Although HER2 does not bind any of the erbB ligands directly, its catalytic activity can potently amplify signaling by erbB-containing heterodimers via increasing ligand binding affinity and/or receptor recycling and stability. Activation of the erbB network leads to receptor autophosphorylation in C-terminal tyrosines and the recruitment to these sites of cytoplasmic signal transducers that regulate cellular processes such as proliferation, differentiation, motility, adhesion, protection from apoptosis, and transformation. HER2 gene amplification has been reported in approximately 20% of metastatic breast cancers, where it is associated with poor patient outcome. Trastuzumab, a humanized monoclonal immunoglobulin G1 that binds the extracellular domain of HER2, has been shown to induce clinical responses in HER2-overexpressing breast cancers and prolong patient survival when combined with chemotherapy. These studies enrolled either predominantly or exclusively patients with breast cancer that overexpressed HER2 as measured by intense membrane staining with HER2 antibodies in the majority of tumor cells (3 by immunohistochemistry [IHC]) or excess copies of the HER2 gene determined by fluorescent in situ hybridization. Studies with human breast cancer cell lines and primary mammary tumors have shown constitutive phosphorylation of HER2. Thor et al examined the incidence of P-HER2 positivity in 816 primary breast cancers. Only 12% of HER2-positive cancers stained positively with an antibody that recognizes HER2 phosphorylated in Y1248; P-HER2 was not detected in any of the HER2-negative tumors. The molecular basis for this constitutive phosphorylation is not clear but is consistent with the reported ability of wild-type Neu, the mouse/rat homolog of HER2, to multimerize and become activated when present in cells at high density. Thus, it is generally accepted that spontaneous dimerization and activation of HER2 occurs in cancers with HER2 gene amplification. Another potential mechanism of HER2 phosphorylation is transactivation by ligandbound EGFR, HER3, or HER4. Indeed, coexpression of the EGFR ligand transforming growth factor alpha (TGF ) and Neu in the mammary gland of transgenic mice markedly accelerates tumor onset and progression compared with mice expressing Neu or TGF transgenes alone. In this study, TGF Neu bi-transgenic mice exhibited increased tyrosine phosphorylation of both EGFR and Neu. In an analysis of 807 patients with invasive breast cancers, 306 HER2positive tumors also expressed EGFR by IHC. Ninety-seven percent of cancers with phosphorylated HER2 at Y1248 exhibited detectable EGFR and the combination of Y1248 P-HER2 together with cooverexpression of HER2 and EGFR was associated with the shortest patient survival. Finally, HER2 is overexpressed in a cohort of non– small-cell lung cancers and increased HER2 gene copy number has been associated with therapeutic response to EGFR tyrosine kinase inhibitors. These reports provide evidence of EGFR-HER2 cross-talk and of the ability of HER2 to amplify ligand-activated EGFR signals. Immunohistochemical studies have shown HER3 expression and overexpression in over 50% of ductal carcinoma in situ and invasive breast cancers. Although kinase defective, HER3 can be phosphorylated by EGFR or HER2. Phosphorylated HER3 can couple to the phosphatidylinositol-3 kinase (PI3K) pathway directly, whereas EGFR and HER2 cannot. HER2/HER3 heterodimers are potently transforming and both HER2 and HER3 are frequently associated or coexpressed in human breast cancer cell lines, mouse transgenic tumors, and primary breast cancers. In HER2-overexpressing tumor cells, both trastuzumab and EGFR kinase inhibitors inhibit the basal phosphorylation of HER3 and its association with HER2 and with PI3K. Finally, high expression of HER3 predicts early escape from trastuzumab therapy. These data imply that HER3 plays a key role in breast tumor progression mediated by the erbB network, and inhibition of HER3 phosphorylation and/or its cross-talk with HER2 is required for the antitumor effect of HER2 signaling inhibitors. Whether incorporation of markers such as HER3, P-HER2, EGFR, EGFR/HER2 dimers, HER2/HER3 dimers or others that may inform the level of erbB pathway engagement, will allow the identification of patients that respond or escape trastuzumab therapy remains to be investigated. Structural studies using recombinant erbB receptor ectodomains, as well as experiments with cell lines and HER2 monoclonal antibodies that recognize different receptor epitopes, indicate that trastuzumab binds in a region not involved in receptor dimerization and thus is unable to block ligand-induced EGFR/HER2 and HER2/ HER3 heterodimers. These data support the speculation that high levels of HER2-containing heterodimers may be a potential marker of JOURNAL OF CLINICAL ONCOLOGY E D I T O R I A L VOLUME 24 NUMBER 23 AUGUST 1