Inclusion Criteria:  Applicable to all patients in both Part A and Part B of the study:  Pathological diagnosis (histological) of pancreatic or gastric including esophageal junction cancers. Patient must provide archived tissue block or formalin-fixed paraffin-embedded (FFPE) slides or fresh biopsy prior to start of treatment. Positive claudin 18.2 tumor expression defined as ≥50% of tumor cells demonstrating moderate-to-strong membranous staining (2+/3+) by IHC assay performed on sections of tumor derived from formalin fixed paraffin block. Adequate organ function. Life expectancy >12 weeks. Age ≥18 years. ECOG performance status 0 or 1 at screening. Ability to understand the nature of this study, comply with protocol requirements, and give written informed consent. Patients of reproductive potential: All female study participants of reproductive potential must have a negative serum or urine pregnancy test performed within 48 hours before study entry.  Specific criteria for Part A:  Disease progression or relapse following conventional chemotherapy:  Pancreatic cancer Gastric cancer (including GEJ cancer)  Specific criteria for Part B:  Measurable disease suitable for imaging and efficacy tracking as defined by RECIST 1.1  Disease progression or relapse following conventional chemotherapy:  Pancreatic cancer Gastric cancer (including GEJ cancer)  Exclusion Criteria:  Unresolved toxicities from prior anticancer therapy, defined as having not resolved to baseline or to common terminology criteria for adverse events (CTCAE) grade ≤1, with the exception of alopecia, ≥grade 2 neuropathy, or to the levels dictated in the inclusion/exclusion criteria. Patient has participated in any investigational research study and is being screened for participation within a period of 5 half-lives or 4 weeks, whichever is longer, of the last dose of the investigational therapy. History of severe infusion reaction with monoclonal antibody treatment. Active or latent hepatitis B or active hepatitis C (test within 8 weeks of screening), or any uncontrolled infection at screening. HIV positive test within 8 weeks of screening. Serious active infection at the time of treatment, or another serious underlying medical condition that would impair the ability of the patient to receive protocol treatment. Presence of other active cancers, or history of treatment for invasive cancer ≤3 years. Patients with Stage I cancer who have received definitive local treatment and are considered unlikely to recur are eligible. All patients with previously treated in situ carcinoma (ie, noninvasive) are eligible, as are patients with history of nonmelanoma skin cancer. Psychological, familial, sociological, or geographical conditions that do not permit compliance with the protocol. Active central nervous system (CNS) disease involvement, defined by cerebrospinal fluid (CSF) cytology, magnetic resonance imaging (MRI) or computerized tomography (CT); patients with asymptomatic CNS metastases are eligible if participants have been clinically stable for at least 4 weeks prior to the first dose of study drug and do not require interventions such as surgery, radiation or any corticosteroid therapy for management of symptoms related to CNS disease. Peripheral neuropathy Grades ≥ 2. Active ocular surface disease at baseline (based on ophthalmic evaluation). Pregnant or nursing (lactating) women. Patients who received claudin 18.2 targeting agents previously. Patients who have received or will receive coronavirus disease 2019 (COVID-19) vaccine within 72 hours prior to the first dose of study drug. Prior radiotherapy