Trastuzumab Beyond Progression in HER2 Positive Metastatic Gastric Cancer

Inclusion Criteria: 1. Over 19 years old 2. Histologically confirmed HER2 overexpressing gastric cancer (HER2 overexpression is defined as IHC 3+ or FISH +) 3. Metastatic gastric cancer 4. Progressive disease has been confirmed after first line treatment including trastuzumab (If the recurrence occurred within 6 months after the completion of postoperative adjuvant therapy, it can be considered as first line treatment.) 5. At least one measurable or evaluable lesion according to RECIST ver 1.1 6. ECOG performance status 0 or 1 7. Appropriate major organ function as defined below, A. Absolute neutrophil count (ANC) ≥ 1,500/mm3 B. Platelet ≥ 100,000/mm3 C. Hemoglobin \> 8.0 g/dL D. Total bilirubin ≤ 1.5 x ULN E. AST and ALT \< 3 x ULN (if there is a live metastasis, AST and ALT ≤ 5 x ULN) F. Serum creatinine ≤ 1.5x ULN or CCr \> 50 mL/min 8. Life expectancy is more than 12 weeks 9. Echocardiography at the time of enrollment showed an ejection fraction ≥ 50% 10. Previous adverse events of chemotherapy or radiotherapy has been resolved to less than grade 1 toxicity (exception: Alopecia, stable peripheral neuropathy, and manageable electrolyte imbalance by replacement therapy are acceptable if they are resolved to less than grade 2) 11. If the urine pregnancy test or serum beta-hCG result is negative in child bearing women 12. If the subject have signed the informed consent form approved by the IRB Exclusion Criteria: 1. Symptomatic or unstable CNS metastasis (exception: appropriately treated brain metastasis without progression of more than 4 weeks after previous CT or MRI, and no steroid treatment for symptom relief is necessary) 2. Active virus, bacteria, or fungal infection (exception: HBV DNA titer is in the normal range for chronic hepatitis B carriers) 3. If previous chemotherapy or radiotherapy was applied within 3 weeks before the administration of study drug 4. If the subject has received major surgery within 4 weeks and the recovery is not complete before the administration of study drug 5. If there is a history of other malignancies within 3 years (exception: cervical intraepithelial cancer, well differentiated thyroid cancer, or skin cancer has been treated completely) 6. QTc interval \> 480 msec, long or short QT syndrome, or Burgada syndrome. In addition, known prolongation of QTc interval or Torsade de Pointes 7. If there is a significant history of cardiovascular disease within 6 months, such as, myocardial infarction, unstable angina, significant cardiac arrhythmia, and uncontrolled hypertension (systolic BP \> 180 mmHg, diastolic BP \> 100 mmHg), congestive heart failure (NYHA class III-IV), pericardial effusion or pericardial tamponade, cardiomyopathy, cerebrovascular disease including transient ischemic stroke, and symptomatic pulmonary embolism. 8. History of symptomatic interstitial pneumonia 9. History of other psychiatric problems, abnormalities of laboratory test which have potential effects on administration of study drugs or participation on the study, or if the subject is inappropriate to be participated according to the investigator's decision (refusal to request and instruction, incooperative attitudes, etc.)