Anti-HER2 treatment in advanced metastatic HER2-negative breast cancer patients with elevated serum levels of the HER2 extracellular domain and/or HER2 overexpressing circulating tumor cells.

Abstract

597Background: Patients (pts) with HER2-negative (HER2-) advanced metastatic breast cancer (MBC) may present with elevated serum levels of the HER2 extracellular domain (sHER2) or HER2 overexpressing circulating tumor cells (CTCs), potentially qualifying for anti-HER2 therapy. This retrospective study sought to clarify the value of HER2-directed Tx in ´occult´ HER2-positive (HER2+) MBC pts in the clinical routine. Methods: 31 pts with HER2- MBC (ER+, n = 27) exhibiting sHER2 values > 15 ng/mL (n = 8), HER2+ CTCs (n = 7), or both (n = 16) were included. Pts had failed 2-16 prior systemic treatments (median: 7). Pts received trastuzumab (T: n = 19), lapatinib (L: n = 4), T+L (n = 2), or T+pertuzumab (TP: n = 6). Anti-HER2 Tx was given alone (n = 5), or in combination with endocrine agents (n = 6), cytotoxics (n = 17), or other targeted drugs (n = 3). Responses were scored according to RECIST 1.1, OS was calculated from the start of Tx until death from any reason or loss to follow-up. Results: Anti-HER2 Tx w...