Use of the PRO Onc Assay (Prometheus) to measure HER2 overexpression/activation in circulating tumor cells (CTCs) in women with HER2-negative metastatic breast cancer (MBC): A Sarah Cannon Research Institute (SCRI) trial.

618 Background: HER2- targeted therapy has markedly improved the treatment of women with HER2- positive breast cancer. FISH testing for HER2 overexpression is the most reliable assay method; treatment decisions are usually based on HER2 expression of the breast primary tumor. Accurate determination of HER2 status by testing CTCs may improve patient management by allowing ongoing assessment of HER2 status during treatment and/or identifying additional patients (pts) for HER2- targeted therapy. Methods: The PRO Onc assay uses a multiplexed immunoassay format to provide expression and activation profiling of HER2 and other signal transduction molecules. The assay can detect overexpression and activation (phosphorylation) of HER2, with sensitivity at a single CTC level. We obtained blood specimens from 57 women with metastatic HER2- negative breast cancer who were receiving standard chemotherapy. In these women, HER2 status had previously been determined by FISH (32) or 0 – 1+ IHC testing (25). All specimens ...