Lenvatinib in Combination With Nivolumab Plus Chemotherapy in Metastatic Gastric Cancer Patients With Malignant Ascites

Inclusion Criteria: 1. Newly diagnosed pathologically proven metastatic, unresectable or recurrent gastric or gastroesophageal junction (GEJ) adenocarcinoma 2. Positive for peritoneal metastasis and grade ≥ 2 malignant ascites as confirmed by computed tomography (CT) 3. PD-L1 combined positive score of ≥5 based on the 28-8 assay 4. Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol. Written informed consent and any locally required authorization obtained from the patient/legal representative prior to performing any protocol-related procedures, including screening evaluations 5. Age \> 19 years at time of study entry 6. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 7. Life expectancy of \> 4 months 8. Body weight \> 30kg 9. No existing neuropathy 10. Adequate normal organ and marrow function as defined below: * Hemoglobin ≥9.0 g/dL * Absolute neutrophil count (ANC) 1.5 x (\> 1500 per mm3) * Platelet count ≥100 (or 75) x 109/L (\>75,000 per mm3) * Serum bilirubin ≤1.5 x institutional upper limit of normal (ULN) * AST (SGOT)/ALT (SGPT) ≤2.5 x institutional ULN * Measured creatinine clearance (CL) \> 40 mL/min or Calculated creatinine CL\>40 mL/min by the Cockcroft-Gault formula or by 24-hour urine collection for determination of creatinine clearance: 11. Evidence of post-menopausal status or negative urinary or serum pregnancy test for female pre- menopausal patients. Women will be considered post-menopausal if they have been amenorrheic for 12 months without an alternative medical cause. 12. Patient is willing and able to comply with the protocol for the duration of the study including undergoing treatment and scheduled visits and examinations including follow up Exclusion Criteria: 1. Disease progression within 6 months after completion of adjuvant chemotherapy. 2. Participation in another clinical study with an investigational product during the last 2 weeks 3. Concurrent enrollment in another clinical study, unless it is an observational (non-interventional) clinical study or during the follow-up period of an interventional study 4. Any concurrent chemotherapy, immunotherapy, biologic, or hormonal therapy for cancer treatment. Concurrent use of hormonal therapy for non-cancer-related conditions (e.g., hormone replacement therapy) is acceptable 5. Major surgical procedure within 28 days prior to the first dose 6. Unable to take medication orally 7. Gastrointestinal bleeding 8. Impaired bowel absorption 9. History of allogenic organ transplantation 10. Uncontrolled intercurrent illness, including but not limited to, ongoing or active infection, symptomatic congestive heart failure, uncontrolled hypertension, unstable angina pectoris, cardiac arrhythmia, interstitial lung disease, serious chronic gastrointestinal conditions associated with diarrhea, or psychiatric illness/social situations that would limit compliance with study requirement, substantially increase risk of incurring adverse events (AEs) or compromise the ability of the patient to give written informed consent 11. History of another primary malignancy except for * Malignancy treated with curative intent and with no known active disease ≥ 5 years before the first dose and of low potential risk for recurrence * Adequately treated non-melanoma skin cancer or lentigo maligna without evidence of disease * Adequately treated carcinoma in situ without evidence of disease 12. History of active primary immunodeficiency 13. Active infectious disease 14. Tuberculosis (based on clinical history, physical examination, radiographic findings, and TB testing in line with local practice) 15. Hepatitis B (known positive HBV surface antigen (HBsAg) result) 16. Hepatitis C (patients positive for hepatitis C (HCV) antibody are eligible only if polymerase chain reaction is negative for HCV RNA) 17. Human immunodeficiency virus (positive HIV 1/2 antibodies) 18. Patients with a past or resolved HBV infection (defined as the presence of hepatitis B core antibody \[anti-HBc\] and absence of HBsAg) are eligible. 19. Female patients who are pregnant or breastfeeding, as well as male or female patients of reproductive potential who are not willing to use effective contraception from the time of screening until 5 months following the final dose of lenvatinib. 20. Known allergy or hypersensitivity to the investigational drug or any of its components. 21. Patients with known deficiency of dihydropyrimidine dehydrogenase (DPD). 22. Patients with known hereditary problems such as galactose intolerance, Lapp lactase deficiency, or glucose-galactose malabsorption.