351 Background: Targeted panel next-generation sequencing (NGS) is now actively performed in clinics to guide treatment decisions for advanced cancer patients. However, NGS profilings of advanced gastric cancer (AGC) do not provide relevant genomic biomarkers that could predict response to second-line treatment for AGC patients yet. Methods: Two paclitaxel-based phase Ib/II trials as a second-line treatment for AGC patients were included: GSK2636771 (PI3Kβ selective inhibitor) and paclitaxel for AGC patients with PI3K/Akt pathway alterations (PTEN trial, NCT02615730, presented at GI ESMO 2021) and nivolumab and paclitaxel for EBV-related, dMMR/MSI-H or PD-L1 positive AGC (K-umbrella-003 trial, NCT02951091, presented at AACR 2021). Pre-treatment tumor tissue samples from two trials were subjected to the in-house panel sequencing using CancerMaster Panel V2 (Kwon et al., GI ASCO 2021) that covers 524 genes for single nucleotide variants, 143 for copy-number variations, and 18 for fusions. Baseline genomic alterations were compared with each patient’s response and survival. Oncogenic signaling pathways were annotated manually according to previous reports. Tumor mutation burden (TMB) and Human leukocyte antigen (HLA) profilings were also performed. Results: Baseline tumor tissues from 54 patients (24 from PTEN trial, 30 from K-umbrella-003 trial) were available for NGS. In patients treated with GSK2636771 and paclitaxel, patients with PIK3R1 M326I mutation (n = 3) showed longer PFS (8.1 vs 2.8 months, P= 0.041), and patients without MYC Q48H (n = 13) showed better PFS (4.8 vs 2.6 months, P= 0.013). Genetic alterations in epigenetic pathway were found in 13 patients (54.2%) and showed worse PFS (2.8 vs 5.0 months, P = 0.04). In patients treated with nivolumab and paclitaxel, patients with RTK/RAS pathway alterations excluding deletions (n = 21, 70%) showed significantly longer PFS (4.1 vs 2.6 months, P = 0.0085), and patients with HLA-A02 supertype (n = 11, 36.7%) showed significantly longer PFS (12.4 vs. 2.7 months, P = 0.0068). In both studies, TMB-high was not related to the survival and there were no common predictive genomic alterations. Conclusions: We observed that specific NGS-based genomic alterations such as single gene mutations, altered signaling pathways and HLA genotyping were potential predictors of each targeted agent or immunotherapy as a 2nd line treatment of AGC patients, regardless of paclitaxel. Further correlative studies within AGC trials for exploring predictive biomarkers are warranted. Clinical trial information: NCT02615730; NCT02951091.