OBJECTIVETo examine the correlation between serum human epithelial growth factor receptor 2 extracellular domain (HER2 ECD) and circulating tumor cells (CTC), as well as the dynamic variation of HER2 ECD and its correlation to the therapeutic efficacy.

METHODSFifty-three advanced gastric cancer (AGC) patients who treated in Peking University Cancer Hospital and ever enrolled into CTC study (ClinicalTrial gov. ID: NCT01625702) were retrospectively included in this study.

INCLUSION CRITERIAthe patients were histologically confirmed as locally advanced or recurrent and/or metastatic adencarcinoma; they received two or more cycles of fluorouracil-based chemotherapy or combination targeted therapy; serum CTC was counted before and after therapy; the clinical response was evaluated every 2 cycles of treatment by the presence of at least one measurable lesion according to RECIST version 1.1 criteria. This study was approved by Ethics Committee of Peking University Cancer Hospital, and informed consents were signed by patients. The sera before and after two cycles of treatment were collected for CTC enumeration and HER2 ECD detection, in which the levels of HER2 ECD were measured by chemiluminescence immunoassays method. The positive threshold value of HER2 ECD and CTC number were ≥15 μg/L and ≥3 CTCs/7.5 ml respectively. The progression-free survival (PFS) and overall survival (OS) were compared among different groups using Log-rank tests.

RESULTSIn 53 enrolled patients, 39 were histologically identified as negative HER2, 9 as positive HER2 and another 5 cases were unknown. All the patients received fluorouracil-based chemotherapy, and 9 positive HER2 patients received combined anti-HER2 targeted therapy. Before therapy, the median HER2 ECD concentration of 53 cases was 10.45 (8.0 to 83.2) μg/L. Seven patients exhibited positive HER2 ECD levels, in whom 4 were histologically HER2 positive, but 3 were histologically HER2 negative. The median CTC number of 53 cases was 2 (0 to 668) CTCs/7.5 ml, and the positive rate of CTC was 47.2%(25/53). Following 2 cycles of therapy, a total of 10 histologically HER2 negative patients exhibited positive HER2 ECD levels, in whom 2 also possessed positive HER2 ECD levels, 83.3 μg/L and 46.9 μg/L before therapy, and 22.4 μg/L and 20.4 μg/L after therapy respectively, whereas another 8 patients (10.3 to 14.5 μg/L before therapy) acquired the elevated expression of HER2 ECD following therapy (15.1 to 19.5 μg/L). It seems that the increased level of HER2 ECD after therapy was, though not statistically significant, correlated to low number of CTCs. In histologically HER2 negative patients, pretherapeutic HER2 ECD level (positive vs. negative) was not significantly correlated to PFS (7.6 months vs. 4.4 months, P=0.328) and OS (13.6 months vs. 10.9 months, P=0.679). However, in histologically HER2 positive patients, patients with positive HER2 ECD level before therapy exhibited longer PFS (10.7 months vs. 4.2 months, P=0.025) and OS (16.5 months vs. 8.9 months, P=0.015) compared to those with negative HER2 ECD level. Additionally, CTC number was significantly correlated to prognosis in histologically HER2 negative patients. Patients with positive pretherapeutic CTC number showed longer PFS (5.3 months vs. 3.3 months, P=0.049) and OS (14.3 months vs. 7.6 months, P=0.001) as well. While in histologically HER2 positive patients, CTC number was not obviously correlated to the PFS and OS. In above 8 negative HER2 patients acquiring elevated expression of HER2 ECD following therapy, the increased HER2 ECD level was not correlated to PFS and OS (all P>0.05). In 9 histologically HER2 positive patients, 4 patients who exhibited decreased HER2 ECD level and reduced or constant CTC number had longer PFS (7.5 to 15.3 months) and OS (11.0 to 26.3 months) compared with those 2 patients who suffered from acquired HER2 ECD level following therapy (PFS 3.0 to 4.8 months and OS 7.3 to 8.6 months).

CONCLUSIONSIn histologically HER2 positive patients, increased pretherapeutic HER2 ECD level predicts better prognosis. The acquired elevated HER2 ECD level following therapy is correlated to inefficient therapeutic response. The acquirement of elevated HER2 ECD level can also be found in histologically HER2 negative patients, which may be correlated to the corresponding variation of CTC number.