Pembrolizumab Plus Epacadostat vs Pembrolizumab Plus Placebo in Metastatic Non-Small Cell Lung Cancer (KEYNOTE-654-05/ECHO-305-05)

Inclusion Criteria: * Histologically or cytologically confirmed diagnosis of stage IV NSCLC without epidermal growth factor receptor (EGFR)-sensitizing mutation, ROS1 and/or anaplastic lymphoma kinase (ALK) translocation. * Measurable disease based on RECIST 1.1. * Tumor tissue that demonstrates programmed cell death ligand 1 (PD-L1) expression in ≥ 50% of tumor cells (tumor proportion score \[TPS\] ≥ 50%) as assessed by immunohistochemistry at a central laboratory. * Life expectancy of at least 3 months. * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. * Adequate organ function per protocol-defined criteria. Exclusion Criteria: * Known untreated central nervous system metastases and/or carcinomatous meningitis. * History of (noninfectious) pneumonitis that required systemic steroids or current pneumonitis/interstitial lung disease. * Symptomatic ascites or pleural effusion. * Known history of an additional malignancy, except if the participant has undergone potentially curative therapy with no evidence of that disease recurrence for 5 years since initiation of that therapy. * Active autoimmune disease that has required systemic treatment in past 2 years. * Has had an allogeneic tissue/solid organ transplant. * Has a known history of human immunodeficiency virus (HIV) infection. HIV testing is not required unless mandated by the local health authority. * Has known history of or is positive for active Hepatitis B (HBsAg reactive) or has active Hepatitis C (HCV RNA). Note: Testing must be performed to determine eligibility. * History or presence of an abnormal electrocardiogram (ECG) that, in the Investigator's opinion, is clinically meaningful. * Use of protocol-defined prior/concomitant therapy.