Abstract: Background and purpose: Human epidermal growth factor receptor-2 (HER2) is a protein expressed by an oncogene, and HER2 positivity tends to indicate rapid tumor progression, susceptibility to lymph node or organ metastasis, resistance to neoadjuvant chemotherapy and poor prognosis. In this study, we investigated the role of serum extracellular domain (ECD) of HER2 in the evaluation of the outcome of patients with breast cancer after neoadjuvant chemotherapy. Methods: A total of 507 serum samples from patients with breast cancer were collected, and Chi-square test was used to compare the age, stage and tissue estrogen receptor, progesterone receptor, Ki-67 and HER2 status with serum HER2-ECD. Forty-eight patients with HER2-positive breast cancer who underwent neoadjuvant chemotherapy were followed up. The serum samples before treatment, after 2 cycles, 4 cycles and 6 cycles were collected. The relationship between the change of serum HER2-ECD levels before and after the treatment and the efficacy of neoadjuvant chemotherapy was analyzed. Results: By comparing serum HER2-ECD and clinic-pathological features of 507 patients, elevated serum HER2-ECD was found in patients ages over 50 years who had stage Ⅲ-Ⅳ breast cancer tissue estrogen receptor (-), progesterone receptor (-) and Ki-67>20%. A follow-up of 48 patients with HER2-positive tissue showed a significant decrease in serum HER2-ECD after 2 cycles of neoadjuvant chemotherapy, from 18.10 ng/mL (13.20-28.95 ng/mL) to 11.20 ng/mL (9.80-12.75 ng/mL) (P<0.01). By evaluating the efficacy of neoadjuvant chemotherapy, we found that the objective response rate in patients with negative serum HER2-ECD (94.7%, 36/38) was significantly higher than that in patients with positive serum HER2-ECD (60.0%, 6/10) (P<0.05). Conclusion: The increase of serum HER2-ECD in breast cancer patients is related to age, stage and tissue estrogen receptor, progesterone receptor and Ki-67, and the level of serum HER2-ECD after 2 cycles of neoadjuvant chemotherapy plays an important role in evaluating the response to neoadjuvant chemotherapy in patients with breast cancer.

Key words: Breast cancer, Neoadjuvant chemotherapy, Response evaluation