Pembrolizumab + Epacadostat vs Pembrolizumab + Placebo in Recurrent or Progressive Metastatic Urothelial Carcinoma

Inclusion Criteria: * Histologically-confirmed diagnosis of urothelial carcinoma of the renal pelvis, ureter, bladder, or urethra, that is transitional cell, or mixed transitional/non-transitional (predominantly transitional) cell type. * Progression or recurrence of urothelial carcinoma following one prior platinum containing chemotherapy regimen for metastatic or unresectable locally advanced disease. A participant who receives a neoadjuvant or adjuvant platinum-containing regimen following cystectomy for localized muscle-invasive urothelial carcinoma is acceptable (without further systemic treatment), if recurrence/progression occurs ≤ 12 months following completion of therapy. * Measurable disease based on RECIST v1.1. * Have provided an archival tumor tissue sample or newly obtained core or excisional biopsy of a tumor lesion not previously irradiated for PD-L1 analysis. * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. * Adequate organ function per protocol-defined criteria. Exclusion Criteria: * Urothelial carcinoma that is suitable for local therapy with curative intent. * History or presence of an abnormal electrocardiogram (ECG) that, in the investigator's opinion, is clinically meaningful. * Known additional malignancy that is progressing or has required active treatment within the past 3 years. * Known active central nervous system (CNS) metastases and/or carcinomatous meningitis. Participants with previously treated brain metastases may participate provided they are radiologically stable, ie, without evidence of progression for at least 4 weeks by repeat imaging. * Active autoimmune disease that has required systemic treatment in past 2 years. * Known history of human immunodeficiency virus (HIV) infection. HIV testing is not required unless mandated by local health authority. * 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. * Use of protocol-defined prior/concomitant therapy.