Therapeutic Cancer Vaccine (AST-301, pNGVL3-hICD) in Gastric Cancer

Participants will provide informed consent and will undergo Pre-Screening/Screening procedures before taking part in the study. Participants will be in Arm 1 and Arm 2 will be conducted in parallel. AST-301 will be administrated every 3 weeks for a total of 3 immunizations in Arm 1 and a total of 6 immunizations in Arm 2.

• Arm 1: 3 immunizations of AST-301 admixed with immunoadjuvant recombinant human granulocyte-macrophage colony stimulating factor (rhuGM-CSF) administered at 3-week intervals. (Total 300 μg of AST-301)
• Arm 2: 6 immunizations of AST-301 admixed with immunoadjuvant recombinant human granulocyte-macrophage colony stimulating factor (rhuGM-CSF) administered at 3-week intervals. (Total 600 μg of AST-301) Randomization will be stratified according to HER2 expression (HER2 low expression or HER2 overexpression).

For both Arm 1 and Arm 2 of the study there will be a Pre-screen period, followed by study periods: a Screening Period (Day -28 to Day -1), a Treatment Period (3 cycles/Arm 1 and 6 cycles/Arm 2), an end of treatment (EOT) visit and follow-up visits.

Key Inclusion Criteria:

• Underwent a curative surgery with standard lymph node dissection (confirmed with no residual tumor, R0 resection) and have completed standard adjuvant treatment
• Has stages II or III according to the 8th edition of the American Joint Committee on Cancer (AJCC)
• HER2 low expression and HER2 overexpression diagnosed according to the 2016 College of American Pathologists (CAP)/American Society for Clinical Pathology (ASCP)/American Society of Clinical Oncology (ASCO) guidelines
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1
• Demonstrates adequate organ function.

Key Exclusion Criteria:

• Has a history of hypersensitivity or other contraindications to rhuGM-CSF
• Has a history of other malignancies ≤5 years prior to first administration of Investigational Product (IP) except for adequately treated non-melanoma skin cancer or epithelial carcinoma without evidence of disease.
• Has received systemic immunosuppressants or were treated with systemic immunosuppressants ≤4 weeks prior to the first administration of Investigational Product (IP).
• Has a history of autoimmune disease or inflammatory disease
• Has active infection including tuberculosis, hepatitis B, hepatitis C or human immunodeficiency virus (HIV) infection
• Is pregnant or breastfeeding or expecting to conceive children