A Study of IMAB362 in Japanese Subjects With Locally Advanced or Metastatic Gastric or Gastro-esophageal Junction (GEJ) Adenocarcinoma

Inclusion Criteria:

* Subject has histologically or cytologically confirmed diagnosis of gastric or gastro-esophageal junction adenocarcinoma.
* Subject has gastric or gastroesophageal junction (GEJ) adenocarcinoma based on radiographic imaging or endoscopic examination.
* Subject agrees not to participate in another interventional study while on treatment.
* Subject has Eastern Cooperative Oncology Group (ECOG) performance status 0-1.
* Subject has predicted life expectancy ≥ 12 weeks.
* Subject must have an available tumor specimen collected at any time prior to the first dose of study treatment.
* Subject must meet all of the pre-specified criteria on the laboratory tests that will be analyzed locally within 7 days prior to the first dose of study drug.
* Locally advanced or Metastatic gastric or GEJ adenocarcinoma with no standard of care treatment option or subject is ineligible to receive available standard of care treatment option.
* Subject's tumor sample has Claudin (CLDN)18.2 membranous staining with any intensity as determined by central Immunohistochemistry (IHC) testing. (Safety part only)
* Subject has CLDN18.2 high expression in ≥75% of tumor cells demonstrating moderate to strong membranous staining as determined by central IHC testing. (Expansion Part Only)
* Subject is an appropriate candidate for a tumor biopsy and is amenable to undergo a tumor biopsy during the Screening period and on-treatment tumor biopsy. (Expansion Part Only)
* Subject has at least 1 measurable lesion according to Response Evaluation Criteria In Solid Tumors (RECIST) 1.1 within 28 days prior to the first dose of study treatment. For subjects with only 1 measurable lesion and prior radiotherapy, the lesion must be outside the field of prior radiotherapy or must have documented progression following radiation therapy. (Expansion Part Only)

Exclusion Criteria:

* Subject has prior severe allergic reaction or intolerance to a monoclonal antibody, including humanized or chimeric antibodies.
* Subject has had radiotherapy within 2 weeks prior to first dose of study drug. Subject who received palliative radiotherapy to peripheral bone metastases within 2 weeks prior to first dose of study drug and has recovered from all acute toxicities is allowed.
* Subject has received other investigational agents or devices concurrently or within 4 weeks prior first dose of study drug.
* Subject has received systemic immunosuppressive therapy, including systemic corticosteroids 2 weeks prior to first dose of study drug. Subjects using a physiologic replacement dose of hydrocortisone or its equivalent are allowed.
* Subject has gastric outlet syndrome or persistent recurrent vomiting.
* Subject has uncontrolled or significant gastrointestinal hemorrhage.
* Subject has known active central nervous system (CNS) metastases and/or carcinomatous meningitis.
* Subject has a known history of a positive test for human immunodeficiency virus (HIV) infection.
* Subject has a positive test for hepatitis B surface antigen (HBsAg) or hepatitis C virus antibody (anti-HCV). Subjects who are negative for HBsAg, but hepatitis B core antibody (HBcAb) positive, hepatitis B virus deoxyribonucleic acid (DNA) test will be performed and if positive will be excluded. Subjects with positive serology but negative HCV ribonucleic acid (RNA) test results are eligible.
* Subject has had within 6 months prior to first dose of study treatment any of the following: unstable angina, myocardial infarction, ventricular arrhythmia requiring intervention or hospitalization for heart failure
* Subject has active infection requiring systemic therapy.
* Subject has clinically significant other disease or co-morbidity, which may adversely affect the safe delivery of treatment within this trial.
* Subject has psychiatric illness or social situations that would preclude study compliance.
* Subject has active autoimmune disease that has required systemic immunosuppressive treatment in the past 2 years.
* Subject has had a major surgical procedure within 28 days prior to the first dose of study drug.
* Subject has Fridericia-corrected QT interval (QTcF) \> 450 msec for males and \> 470 msec for females on 12-lead electrocardiogram (ECG) at screening based on local testing.
* Subject has any condition which makes the subject unsuitable for study participation.
* Subject has another active malignancy which is likely to require treatment.
* Subjects who find it difficult to adhere to the provisions of treatment and observation specified in the protocol.