The purpose of this study is to evaluate the efficacy and safety of Apatinib combined with S-1 as first-line therapy for patients with advanced gastric or gastroesophageal junction adenocarcinoma.

Inclusion Criteria:

1. Adult patients, aged ≥18 years;
2. Histologically confirmed adenocarcinoma of the stomach with inoperable locally advanced or recurrent and/or metastatic disease;
3. Measurable disease, according to the Response Evaluation Criteria in Solid Tumors (RECIST v1.1);
4. Eastern Cooperative Oncology Group (ECOG) performance status of 0 - 3;
5. No previous chemotherapy for advanced/metastatic disease 1) Prior adjuvant/neoadjuvant therapy is allowed if at least 6 months has elapsed between completion of adjuvant/neoadjuvant therapy and enrolment into the study); 2) No previous targeted therapy; 3) For patients who can't tolerate first-line chemotherapy or refuse to accept chemotherapy, if they request to participate in this study, may receive apatinib monotherapy after consultation with the principal investigator in consideration of ethics.
6. Hematological, Biochemical and Organ Functions: HB ≥ 80 g/L, ANC ≥ 1.5×109/L, PLT ≥ 80×109/L, BIL < 1.5×ULN, ALT or AST < 2.5×ULN (or < 5×ULN in patients with liver metastases), Serum Cr ≤ 1×ULN, Cr clearance ≥ 50 mL/min;
7. Life expectancy of more than 3 months;
8. Duration from operation is more than 4 weeks;
9. For women of child-bearing age, the pregnancy test results (serum or urine) within 7 days before enrolment must be negative. They will take appropriate methods for contraception during the study until the 8th week post the last administration of study drug. For men (previous surgical sterilization accepted), will take appropriate methods for contraception during the study until the 8th week post the last administration of study drug.
10. Signed informed consent.

Exclusion Criteria:

1. Patients with poor-controlled arterial hypertension (systolic blood pressure > 140 mmHg or diastolic blood pressure > 90 mm Hg) despite standard medical management; Coronary heart disease, arrhythmia (including QT interval prolongation, for man > 450 ms, for woman > 470 ms), and cardiac dysfunction greater than Class II;
2. Factors affecting the oral medication, such as inability to swallow, chronic diarrhea and intestinal obstruction;
3. Patients with a clear tendency of gastrointestinal bleeding;
4. Abnormal coagulation function (INR > 1.5×ULN, APTT > 1.5×ULN), with tendency of bleed;
5. Pregnant or lactating women;
6. Other conditions regimented at investigators' discretion.