The prognostic relevance of HER2-positivity gain in metastatic breast cancer in the ChangeHER trial

Breast cancer (BC) heterogeneity is composite in nature, with a wide variety of factors concurring to define several pathological entities, which differ by clinical presentation, pathologic features, therapy administered, and inherent outcomes1. Additional sources of breast cancer heterogeneity may raise during the disease course. In BC patients whose disease was initially diagnosed in the early stage and subsequently progressed with metastatic involvement of one single or multiple site/s, the molecular characteristics of metastatic lesions do not necessary mimic those of the disease initially diagnosed. A well-depicted molecular landscape is crucial for subtype definition, prognostic evaluation and appropriate therapeutic decisions. Accordingly, current guidelines suggest repeating the immunohistochemical (IHC) assessment in patients with metastatic spread and at least one secondary lesion amenable to biopsy2. Discordance in human epidermal growth factor receptor 2 (HER2) status between the tumor and metastatic lesions is widely acknowledged, and not yet completely unraveled in their biologic meaning and prognostic relevance3–11. The overexpression of HER2 or amplification of the related gene is extensively recognized as a feature associated with more aggressive biological behavior12,13. However, the extent to which changes in HER2 status may affect patients’ prognosis is still a matter of debate14. We herein propose an observational study of HER2-positive metastatic breast cancer (mBC) patients treated with the anti-HER2 targeted agents pertuzumab and/or trastuzumab emtansine (T-DM1). Our research question is whether relevant differences exist in long-term outcomes of patients with concordant HER2 status between the primary tumor and its secondary lesion/s compared to patients whose disease revealed HER2-positivity gain at the IHC assessment of metastatic lesions. In our historical cohorts, we also sought to identify factors associated with HER2-positivity gain at the IHC reassessment, for which an impact on prognosis may be foreseen.