Clinicopathological features of HER2-low expressed advanced gastric cancers.

459 Background: The DESTINY-Breast04 data demonstrated the antibody-drug conjugate trastuzumab deruxtecan (T-DXd) was also effective for metastatic breast patients (pts) with HER2-low expression. Now that the HER2-low expressed cancer is categorized as an independent subset in breast cancer patients. It is unknown whether patients with the HER2-low expression are distinguishable from the HER2 negative or positive and are potential candidates for the anti-HER2 therapy in advanced gastric cancer (AGC) patients. Methods: We retrospectively reviewed the medical records of AGC pts who received standard chemotherapy with platinum containing regimens as the first-line treatment between Jan, 2011 and Dec, 2018 at the Cancer Institute Hospital of the JFCR. AGC pts were classified according to the HER2 status using immunohistochemistry (IHC) and in situ hybridization (ISH) as follows; HER2 negative (IHC 0), HER2-low (IHC 1+ or 2+/ISH-) and HER2 positive (IHC2+/ISH+ or 3+). This study was conducted to investigate the clinicopathological features and prognosis of HER2-low AGC pts compared with HER2 negative and positive. Results: A total of 734 AGC pts were received HER2 testing and classified into three groups according to HER2 status (HER2 negative; n=410, HER2-low; n=154, HER2 positive; n=170). The proportion of male (negative; 61.5%, low; 63.6% and positive; 69.4%), intestinal histology (negative; 21.0%, low; 44.2% and positive; 59.8%), liver metastases (negative; 18.3%, low; 24.8% and positive; 46.5%), higher serum CEA level (>ULN) (negative; 32.2%, low; 41.6% and positive; 56.5%) and higher serum CA19-9 level (>ULN) (negative; 34.1%, low; 36.7% and positive; 56.5%) were gradually increased along with HER2 expression level. The proportion of peritoneum metastasis (negative; 56.3%, low; 44.8% and positive; 21.8%) was decreased along with the HER2 expression level. One hundred and fifty-two pts (89.4%) received combination chemotherapy with trastuzumab in the first-line treatment. Sixteen pts received trastuzumab in the HER2-low (n=3) and HER2 negative (n=13) in the clinical trial. The median survival time (MST) of pts with HER-low was the same with that of HER2 negative (15.7ms). The statistically significant difference was observed in OS between the HER2-low and HER2 positive (HR 1.43 95% CI: 1.12-1.83, P=0.003). Conclusions: AGC Pts with HER2-low have intermediate clinicopathological features between HER2 negative and positive. A Novel effective anti-HER2 therapy for the HER2-low expressed tumor would be warranted in AGC treatment.