Background: Gastric cancer is the 5th most frequently diagnosed cancer and the 3rd leading cause of cancer deaths. HER2/neu is overexpressed in 15% to 25% of patients with gastric cancer and associated with a poor prognosis. Monoclonal antibodies against HER2/neu have been shown to be effective but alternative treatments are needed due to cost and global availability issues. IMU-131 is a B-cell peptide vaccine composed of 3 B cell epitopes derived from the extracellular domain of HER2/neu. Polyclonal antibodies against IMU-131 peptides binding 3 separate regions (DIII, IV) of HER2/neu have been shown to elicit antitumor activity in vitro and a phase I study demonstrated safety and immunogenicity in Her-2 +/++ metastatic breast cancer patients. Fusion of the single peptides into a hybrid peptide conjugated to CRM197 in conjunction with the adjuvant Montanide (P467-CRM-Montanide) improved formulation and stability of the vaccine. With the present Phase 1b/2 trial performed in patients with HER2/neu overexpressing gastric or gastroesophageal junction (GEJ) adenocarcinoma, we hypothesized that administration of IMU-131 in addition to chemotherapy is safe and immunogenic, and will prolong survival and may delay tumor progression and/or reduce tumor burden. Methods: This study is an international open-label multicenter study performed in 16 Asian and Eastern European sites with a maximum of 18 patients enrolled in Phase 1b. This dose escalation study is designed to assess safety, tolerability, immunogenicity and recommended phase 2 dose (RP2D) of IMU-131. Each patient is administered three injections of IMU-131, at a single dose level on Days 0, 14, and 35, accompanied by chemotherapy cycles of cisplatin and 5-fluorouracil or capecitabine every 21 days. The RP2D will be evaluated in the dose expansion Phase 2 study with 68 patients being enrolled. Results: The study is ongoing with the completion of the phase 1b portion in 4Q18. Conclusions: No conclusions can be drawn at this time. Clinical trial information: NCT02795988