Study of Pembrolizumab With or Without Platinum-based Combination Chemotherapy Versus Chemotherapy Alone in Urothelial Carcinoma (MK-3475-361/KEYNOTE-361)

Inclusion Criteria:

- Has a histologically or cytologically confirmed diagnosis of advanced/unresectable
(inoperable) or metastatic urothelial carcinoma of the renal pelvis, ureter [upper
urinary tract], bladder, or urethra. Both transitional cell and mixed
transitional/non- transitional cell histologies are allowed, but transitional cell
carcinoma must be the predominant histology.

- Has measurable disease based on RECIST 1.1 as determined by the local site
investigator/radiology assessment.

- Has received no prior systemic chemotherapy for advanced or metastatic urothelial
carcinoma, with the following exceptions:

- Neoadjuvant platinum-based chemotherapy with recurrence >12 months from
completion of therapy is permitted.

- Adjuvant platinum-based chemotherapy following radical cystectomy with recurrence
>12 months from completion of therapy is permitted.

- Has provided tissue for biomarker analysis from an archival tissue sample or newly
obtained core or excisional biopsy of a tumor lesion not previously irradiated from a
muscle invasive urothelial carcinoma or a metastatic biopsy, originally from the
original tumor.

- Has an Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2.

- Demonstrates adequate organ function.

- Female participants of childbearing potential must be willing to use an adequate
method of contraception for the course of the study through 120 days after the last
dose of pembrolizumab or 180 days after chemotherapy treatment.

- Male participants of childbearing potential must agree to use an adequate method of
contraception starting with the first dose of study therapy through 120 days after the
last dose of pembrolizumab or 180 days after chemotherapy treatment.

Exclusion Criteria:

- Has disease that is suitable for local therapy administered with curative intent.

- Is currently participating and receiving study therapy or has participated in a study
of an investigational agent and received study therapy or used an investigation device
within 4 weeks of the first dose of study drug.

- Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any
other form of immunosuppressive therapy within 7 days prior to randomization.

- Has an active autoimmune disease that has required systemic treatment in the past 2
years.

- Has had a prior anti-cancer monoclonal antibody (mAb) for direct anti-neoplastic
treatment within 4 weeks prior to the first dose of study drug (6 weeks for
nitrosoureas or mitomycin C) or who has not recovered (i.e., ≤ Grade 1 or at Baseline)
from adverse events (AEs) due to mAbs administered more than 4 weeks earlier.

- Has not recovered (i.e., AE ≤ Grade 1 or at Baseline) from AEs due to a previously
administered agent.

- Has a known additional malignancy that is progressing or requires active treatment
within the past 5 years.

- Exceptions include basal cell carcinoma of the skin, squamous cell carcinoma of
the skin that has undergone potentially curative therapy or in situ cervical
cancer.

- A history of prostate cancer that was identified incidentally following
cystoprostatectomy for bladder cancer is acceptable, provided that the following
criteria are met: Stage T2N0M0 or lower; Gleason score ≤6; Prostate-specific
Antigen (PSA) level undetectable.

- Has a history of (non-infectious) pneumonitis that required steroids or current
pneumonitis.

- Has a known history of active tuberculosis (TB).

- Has an active infection requiring systemic therapy.

- Has a history of severe hypersensitivity reaction (e.g. generalized rash/erythema,
hypotension, bronchospasm, angioedema or anaphylaxis) to pembrolizumab, gemcitabine,
carboplatin, or cisplatin or their analogs and/or to any of their excipients.

- Has known psychiatric or substance abuse disorders that would interfere with
cooperation with the requirements of the trial. Is a known regular user (including
"recreational use") of any illicit drug(s) or had a recent history (within the last
year) of drug or alcohol abuse.

- Is pregnant or breastfeeding, or expecting to conceive or father children within the
projected duration of the study, starting with the screening visit through 120 days
after the last dose of pembrolizumab or 180 days after the last dose of chemotherapy
treatment.

- Has received prior therapy with an anti-PD-1, or anti-PD-L1, or anti-PD-L2 agent or
with an agent directed to another co-inhibitory T-cell receptor (e.g., cytotoxic
T-lymphocyte-associated protein 4 [CTLA-4], OX-40, CD137).

- Has a known history of human immunodeficiency virus (HIV).

- Has known active hepatitis B or hepatitis C.