Background: This study was initiated to investigate the prognostic significance of circulating tumor cell (CTC)\r\nenumeration and the predictive value of CTC HER2 expression for efficient anti-HER2 therapy in HER2-positive\r\nmetastatic breast cancer (MBC) patients.\r\nMethods: Sixty HER2-positive MBC patients were enrolled in the present study. Before the initiation of systemic\r\ntreatment, CTCs from 7.5 ml of blood were analyzed using the CellSearch system. The progression-free survival (PFS)\r\nof the patients was estimated using Kaplan-Meier survival curves.\r\nResults: CTCs were detected in 45% (27/60) of the patients, who had shorter median PFS than those without CTCs\r\n(2.5 vs. 7.5 months, P = 0.0125). Furthermore, referring to the standard HER2 testing that uses immunohistochemistry\r\n(IHC), we proposed a CTC HER2-positive criterion, defined as >30% of CTCs over-expressing HER2. Among patients\r\nundergoing anti-HER2 therapy, those with HER2-positive CTCs had longer PFS (8.8 vs. 2.5 months, P = 0.002). Among\r\npatients with HER2-positive CTCs, the median PFS for those receiving anti-HER2 therapy was significantly longer\r\nthan those who were not (8.8 vs. 1.5 months, P = 0.001). Notably, up to 52% (14/27) of the HER2-positive patients\r\nwere CTC HER2-negative, and anti-HER2 therapy did not significantly improve the median PFS in these patients\r\n(2.5 vs. 0.9 months, P = 0.499).\r\nConclusions: Our findings underscore the necessity of a comprehensive CTC analysis, which may provide valuable\r\nprognostic and predictive information for optimizing individually tailored therapies in HER2-positive MBC patients.\r\nTo test this idea, additional large cohort, multi-center and prospective clinical trials are needed.
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