Key Inclusion Criteria:

          -  Patients with histologically or cytologically documented, unresectable, locally
             advanced or metastatic transitional cell carcinoma (transitional cell and mixed
             transitional/non-transitional cell histologies) of the urothelium (including renal
             pelvis, ureters, urinary bladder, and urethra)

          -  Patients who have not been previously treated with first-line chemotherapy. Patients
             who have received prior definitive chemoradiation, adjuvant or neoadjuvant treatment
             for locally advanced disease are eligible provided that progression to locally
             advanced or metastatic disease has occurred >12 months from the last therapy [for
             chemoradiation and adjuvant treatment] or >12 months from the last surgery [for
             neoadjuvant treatment].

          -  At least 1 lesion, not previously irradiated, that qualifies as a RECIST 1.1 target
             lesion at baseline.

          -  World Health Organization (WHO)/Eastern Cooperative Oncology Group (ECOG) performance
             status (PS) of 0 or 1 at enrolment

          -  Adequate organ and marrow function as defined in the protocol

          -  Life expectancy ≥12 weeks in the opinion of the investigator

          -  Evidence of post-menopausal status, or negative urinary or serum pregnancy test for
             female pre-menopausal patients.

        Key Exclusion Criteria:

          -  Prior exposure to immune-mediated therapy (with exclusion of Bacillus Calmette
             Guerin), including but not limited to, other anti-CTLA-4, anti-PD-1, anti-PD L1, or
             anti-PD-L2 antibodies, except therapeutic anticancer vaccines, which are permitted.
             Prior local intervesical chemotherapy or immunotherapy is allowed if completed at
             least 28 days prior to the initiation of study treatment.

          -  No severe concomitant condition that requires immunosuppression medication

          -  Untreated central nervous system (CNS) metastases and/or carcinomatous meningitis

          -  Patients who may be eligible for or are being considered for radical resection during
             the course of the study.

          -  Any medical contraindications to platinum (cisplatin or carboplatin) based doublet
             chemotherapy and/or known allergy or hypersensitivity to any of the study drugs or any
             of the study drug excipients