KEYNOTE-859: a Phase III study of pembrolizumab plus chemotherapy in gastric/gastroesophageal junction adenocarcinoma.

Current guidelines recommend two-drug cytotoxic chemotherapy with a fluoropyrimidine (fluorouracil or capecitabine) and a platinum-based agent (oxaliplatin or cisplatin) as first-line treatment for advanced gastric cancer. Pembrolizumab monotherapy has demonstrated durable antitumor activity in patients with advanced programmed death ligand 1-positive (combined positive score ≥1) gastric/gastroesophageal junction adenocarcinoma. Accumulating evidence indicates that combining pembrolizumab with standard-of-care chemotherapy for the treatment of advanced or metastatic cancer improves clinical outcomes. We describe the rationale for and the design of the randomized, double-blind, placebo-controlled, Phase III KEYNOTE-859 study, which is investigating pembrolizumab in combination with chemotherapy as first-line treatment for patients with human epidermal growth factor receptor 2-negative advanced unresectable or metastatic gastric/gastroesophageal junction adenocarcinoma. The planned sample size is 1542 patients, and the primary end point is overall survival. Clinical trial registration: NCT03675737 (ClinicalTrials.gov).