The most common site for gastric cancer distant metastases is the peritoneum. Median survival for this group of patients is short and systemic cytotoxic treatment response is poor, partly due to the low uptake of the treatment compounds to the peritoneum during systemic chemotherapy. Infusion of cytotoxic drugs directly into the abdominal cavity has been shown to have a high objective response rate and low toxicity. The IPa-Gastric trial is an open-label, multicentre, randomised, phase-III study in the first line setting in gastric cancer patients with peritoneal metastases. Patients will receive the study treatments until disease progression, unacceptable side effects, the investigator's decision to end treatment for other reasons, death, or end of study. After discontinuing study treatments, each patient will be followed up for all study endpoints that are clinically feasible, until death or end of study. The primary objective is to compare overall survival (OS) for patients randomised to intraperitoneal (IP) paclitaxel and standard ST versus those randomised to standard ST only.

Inclusion Criteria:

• Gastric or gastro-oesophageal junction Siewert type II or III adenocarcinoma verified by biopsy or cytology from the primary tumour
• Peritoneal metastasis verified by biopsy, or cytology from ascites or peritoneal wash fluid
• Staging laparoscopy with assessment of peritoneal cancer index (PCI) performed less than four weeks before enrolment.
• Patients with tumour positive cytology (CYT+) without clinically manifest peritoneal metastases at baseline (PCI 0) staging laparoscopy can be included if they persist to be CYT+ after at least four cycles of systemic chemotherapy.
• Adequate hematology assessment and serum chemistry
• Eastern Cooperative Oncology Group (ECOG) performance status 0-1.
• Age of at least 18 years
• Life expectancy of at least three months
• Assurance that adequate anti-reproductive measures will be taken during study interventions when applicable

Exclusion Criteria:

• Comorbidity that does not allow treatment with ST or IP paclitaxel
• Confirmed or suspected severe abdominal adhesions
• Severe coagulation disorder which precludes surgical interventions
• Distant metastases (including M1 lymph node metastases) other than peritoneal, with the specific exception of ovarian metastases
• Symptomatic ascites already requiring drainage for palliation, or expected to require drainage in the short term
• Peritoneal recurrence of gastric cancer diagnosed within 6 months after curative intent surgery
• Previously received more than 2 cycles of palliative-intent systemic chemotherapy (with the specific exception of cytology positive patients without manifest peritoneal metastases) for the current gastric cancer (up to 2 cycles of first line chemotherapy is allowed). Perioperative chemotherapy within previous curative context is allowed
• Another malignancy that can affect survival within the next three years
• Known or suspected allergies against any product included in the trial interventions
• DYPD deficiency
• Pregnancy or recent delivery within 28 days postpartum or ongoing breastfeeding
• Active sex-life without use of secure contraceptive method.
• If the investigator considers the patient inappropriate for participation in the study for any other reason
• Ongoing or recent participation (within 30 days) in a clinical trial with an investigational medicinal product

A plan for IPD sharing will be added later