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206 Background: The ’’Neo RAS’’ phenomenon, in which tissue rat sarcoma viral oncogene homolog ( RAS) status converts from mutant (MT) to wild-type (WT) after treatment in metastatic colorectal cancer (mCRC), is gaining attention because epidermal growth factor receptor (EGFR) inhibitors, which were originally considered to be ineffective, may converted to be effective. This multi-center study investigated its incidence and clinicopathological characteristics that are still unclear. Methods: 107 mCRC patients with tissue RAS MT, confirmed using MEBGEN RASKET-B, who were refractory or intolerant to previous chemotherapies, including fluoropyrimidines, oxaliplatin, or irinotecan were enrolled in 4 institutions from June 2021 to August 2022. The RAS status in ctDNA was investigated after prior chemotherapy using ONCOBEAMTM RAC CRC. Clinicopathological characteristics were compared between patients with RAS MT and RAS WT (Neo RAS) in ctDNA. Results: The incidence of Neo RAS WT mCRC was 21.5% (23/107). The frequency of Neo RAS in KRAS exon 2 was significantly lower than that in other alleles such as exon 3 and 4 or NRAS (18.2% [18/99] vs 62.5% [5/8], P = 0.011). There were significant differences in frequency of Neo RAS between male vs female (30.6% [19/62] vs 8.9% [4/45], P = 0.008), absence vs presence of liver metastasis (38.6% [17/44] vs 9.5% [6/63], P < 0.001), and between two groups divided at the median: tumor diameter (> 60.9 mm vs ≤, 3.8% [2/53] vs 38.9% [21/54], P < 0.001), carcinoembryonic antigen level (> 38.2 ng/ml vs ≤, 11.3% [6/53] vs 31.5% [17/54], P = 0.018), carbohydrate antigen 19-9 level (> 158.0 U/ml vs ≤, 9.4% [5/53] vs 33.3% [18/54], P = 0.004). Logistic regression multivariate analysis, absence of liver metastasis (Odds ratio [OR], 4.62; P = 0.019), smaller tumor diameter (OR, 7.92; P = 0.012) and tissue RAS MT in other than KRAS exon 2 (OR, 9.04; P = 0.026) were significantly related to the appearance of Neo RAS WT mCRC. Conclusions: Original RAS status in tissue, tumor diameter and liver metastasis are related to conversion to Neo RAS WT mCRC.

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.