Serum HER2 extra-cellular domain, S100ß and CA 15-3 levels are independent prognostic factors in metastatic breast cancer patients.

<bold>Background: </bold>Metastatic breast cancer (MBC) prognosis is highly variable, depending on various factors such as the biological subtype, the performance status, disease extension…. A better evaluation of a patient's prognostic factors could allow for a more accurate choice of treatments. The role of serum tumor markers remains, however, unclear in this population. Considering the recent interest in phenotypic changes and tumor heterogeneity during breast cancer progression, additional tumor markers could be interesting in this setting.<bold>Methods: </bold>Two hundred fifty MBC patients treated at the Montpellier Cancer Institute (2008-2015) were retrospectively selected, based on the availability of frozen serum samples. The usual MBC clinical and pathological variables were collected, altogether with Cancer Antigen 15-3 (CA15-3), Carcinoembryonic Antigen (CEA), HER2 extra-cellular domain (ECD), Neuron Specific Enolase (NSE), S100ß protein and Matrix Metalloproteinase 9 (MMP-9) serum levels in order to determine their prognostic value.<bold>Results: </bold>With a median follow-up of 40.8 months, median overall survival was 16.2 months (95 % CI 12.4-20.6). In multivariate analysis, the performance status, brain or subcutaneous metastases, the number of previous metastatic chemotherapy lines and the tumor biological subtype were independent prognostic factors. Elevated CA 15-3 (HR = 1.95, IC 95 % 1.31-2.93, p = 0.001), HER2 ECD (regardless of tumor HER2 status, HR = 2.51, IC 95 % 1.53-4.12, p < 0.001) and S100ß (HR = 1.93, IC 95 % 1.05-3.54, p = 0.033) serum levels were independently associated with a poor outcome.<bold>Conclusions: </bold>Serum CA 15-3, HER2 ECD and S100ß could represent useful independent prognostic factors in MBC. Of particular interest is the independent value of serum HER2 ECD levels, regardless of the tumor HER2 status, possibly linked to metastatic tumor heterogeneity. [ABSTRACT FROM AUTHOR]