Quantitative Assays for the Measurement of HER1-HER2 Heterodimerization and Phosphorylation in Cell Lines and Breast Tumors: Applications for Diagnostics and Targeted Drug Mechanism of Action

Introduction: Ligand-bound and phosphorylated ErbB/HER heterodimers are potent signaling forms of this receptor family, and quantitative measurements of these active receptors may be predictive of patient response to targeted therapies. Using VeraTag technology, we developed and characterized quantitative assays measuring epidermal growth factor (EGF)-dependent increases in activated HER receptors in tumor cell line lysates and formalin-fixed, paraffin-embedded (FFPE) tumor sections. We demonstrated the ability of the assays to quantitatively measure changes in activated HER1 and HER2 receptor levels in cell lines following treatment with 2C4, erlotinib, and lapatinib. We utilized these assays to determine the prevalence and distribution of activated HER1, HER2, and HER1-HER2 heterodimers in 43 HER2-positive breast tumors.

Methods: Assays for activated HER1 and HER2 receptors in FFPE and cell lysate formats were developed using VeraTag technology, which requires the proximity of an antibody pair for light-dependent release of a fluorescently labeled tag, followed by capillary electrophoresis-based quantitation.

Results: Ligand-dependent and independent HER1-HER2 heterodimer levels measured by lysate and FFPE VeraTag assays trended with HER1 and HER2 expression levels in tumor cell lines, which was confirmed by co-immunoprecipitation. The formation of EGF-dependent HER1-HER2 heterodimers were inhibited by the HER2-targeted monoclonal antibody 2C4 and stabilized by the HER1 tyrosine kinase inhibitor (TKI) erlotinib. EGF-dependent HER1 and HER2 phosphorylation was inhibited by lapatinib and erlotinib. Further, we observed that dominant receptor signaling patterns may switch between HER1-HER1 and HER1-HER2, depending on drug mechanism of action and relative levels of HER receptors. In FFPE breast tumors that expressed both HER1 and HER2, HER1-HER2 heterodimers were detected in 25 to 50% of tumors, depending on detection method. The levels of activated phospho-HER1-HER2 heterodimers correlated with HER1 or HER2 levels in an analysis of 43 HER2-positive breast tumors.

Conclusions: VeraTag lysate assays can be used as a tool for understanding the mechanism of action of targeted HER-family inhibitors in the preclinical setting, while VeraTag FFPE assays of activated HER receptors combined with total HER2 measurements (HERmark) in tumor samples may provide a more accurate prediction of clinical response to both HER1 and HER2 targeted therapies.