A Study of IMU-131(HER-Vaxx) and Chemotherapy Compared to Chemotherapy Only in Patients With HER2 Positive Advanced Gastric Cancer

Inclusion Criteria: 1. Patient has been informed of the investigational nature of this study and has given written informed consent in accordance with institutional, local, and national guidelines; 2. Age ≥ 20 years old; 3. Life expectancy of at least 12 weeks; 4. Phase 1b: No prior chemotherapy or radiotherapy for advanced gastric or GEJ cancer within 6 months prior to Day 0; Phase 2: No prior chemotherapy or radiotherapy for advanced gastric or GEJ cancer within 3 months prior to Day 0; 5. Metastatic gastric or GEJ adenocarcinoma, or locally advanced disease not amenable to surgical resection; 6. HER2/neu overexpression (3+ by immunohistochemistry (IHC) or if IHC 2+ confirmed by fluorescent in situ hybridization \[FISH\] or chromogenic in situ hybridization \[CISH\]). Patients with IHC 2+ expression without confirmation of overexpression by fluorescent in situ hybridization \[FISH\] or chromogenic in situ hybridization \[CISH\]) may be included in Phase 1b with agreement of Imugene Limited; 7. Phase 1b: ECOG performance status 0-1; Phase 2: ECOG performance status 0-2; 8. At least one measurable lesion as defined by RECIST 1.1 criteria. Patients with non-measurable lesions may be included in Phase1b with agreement of Imugene Limited; 9. Adequate left ventricular ejection function at baseline, defined as LVEF \> 50% by echocardiogram or MUGA scan (Multi Gated Acquisition Scan); 10. Adequate hematologic function: absolute neutrophil count (ANC) ≥ 1.5 x 109/L, platelet count ≥ 100 x 109/L, and hemoglobin ≥ 9 g/dL; 11. Adequate liver function evidenced by bilirubin ≤ 1.5 x laboratory upper limit of normal \[ULN\], and ALT and AST ≤ 3 x laboratory ULN if no liver involvement or ALT and AST ≤ 5 times laboratory ULN with liver involvement; 12. Adequate renal function (creatinine ≤ 1.5 x laboratory ULN); 13. Willing and able to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures. 14. Male and female patients of childbearing potential must agree to use a highly effective method of contraception throughout the study and for at least 28 days after the last dose of assigned treatment (see section 4.3 for details). A patient is of childbearing potential if, in the opinion of the investigator, he/she is biologically capable of having children and is sexually active. Exclusion Criteria: 1. Previous treatment with trastuzumab or any other HER2/neu targeting antibody or agent; 2. Continuous systemic treatment with either corticosteroids (\>10 mg daily prednisone equivalents) or other immunosuppressive medications within 4 weeks prior to first dose of study treatment. Inhaled or topical steroids and physiological replacement doses of up to 10 mg daily prednisone equivalents are permitted in the absence of active auto-immune disease; 3. Prior organ transplant; 4. Phase 1b: Patient not considered a candidate for 5-FU, capecitabine, or cisplatin chemotherapy; Phase 2: Patient not considered a candidate for 5-FU, capecitabine, cisplatin or oxaliplatin chemotherapy; 5. History of documented congestive heart failure; angina pectoris requiring antianginal medication; evidence of transmural infarction on ECG; poorly controlled hypertension; clinically significant valvular heart disease; high risk uncontrolled arrhythmias; or New York Heart Association (NYHA) class II heart disease; 6. If on warfarin (Coumadin®) or other vitamin K antagonists; 7. Concurrent active malignancy except for adequately controlled limited basal cell carcinoma of the skin; 8. Peripheral neuropathy or hearing loss of NCI CTCAE Grade \> 2; 9. History of uncontrolled seizures, central nervous disorders or psychiatric disability judged by the investigator to be clinically significant and precluding informed consent, participation in the study, or adversely affecting compliance to study drugs; 10. Active infection requiring IV antibiotics; 11. Positive for human immunodeficiency virus (HIV) (HIV 1/2 antibodies) or active hepatitis B (HBsAg reactive) or active hepatitis C (HCV ribonucleic acid \[RNA\] qualitative) infection; 12. Pregnant or lactating females; 13. Major surgery within 4 weeks prior to study entry. Minor surgery (excluding diagnostic biopsy) within 1 week prior to study entry; 14. Has received a live-virus vaccination within 4 weeks of first study vaccination. Seasonal flu vaccines that do not contain live virus are permitted; 15. Current or recent (within 4 weeks of first IMU-131 vaccination) treatment with another investigational drug or participation in another investigational study. 16. Phase 2: Patients with a known diphtheria toxoid hypersensitivity.