A Study of Pembrolizumab (MK-3475) in Combination With Etoposide/Platinum (Cisplatin or Carboplatin) for Participants With Extensive Stage Small Cell Lung Cancer (MK-3475-604/KEYNOTE-604)

Inclusion Criteria:

- Has a documented new diagnosis of SCLC by histology or cytology from brushing,
washing, or needle aspiration of a defined lesion. Participants who do not have
histology samples (defined as core or excisional biopsy, or resections) will need to
undergo a new biopsy to provide a tissue sample.

- Has extensive-stage disease defined as Stage IV (T any, N any, M 1a/b) by the American
Joint Committee on Cancer (AJCC), Seventh Edition

- Has ≥1 lesion that meets the criteria for measurable, as defined by RECIST 1.1, and is
appropriate for selection as a target lesion, as determined by local site
investigator/radiology assessment

- Has provided archival tumor tissue sample or newly obtained core or excisional biopsy
of a tumor lesion not previously irradiated

- Has Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1

- Has a life expectancy of ≥3 months

- Has adequate organ function

- Female and male participants of childbearing potential must be willing to use an
adequate method of contraception, starting with the first dose of study treatment
through 120 days after the last dose of study treatment and up to 180 days after last
dose of chemotherapeutic agents

Exclusion Criteria:

- Has received prior systemic therapy for the treatment of SCLC

- Is currently participating and receiving study treatment or has participated in a
study of an investigational agent and received study treatment or used an
investigational device within 4 weeks of the first dose of treatment for another
health-related problem

- Is expected to require any other form of antineoplastic therapy for SCLC, including
radiation therapy, while on study. (Prophylactic cranial irradiation will be possible
for those participants with stable disease or better at the completion of the 4 cycles
of chemotherapy with or without pembrolizumab.)

- Has known central nervous system (ie, brain and/or spinal cord) metastases and/or
carcinomatous meningitis. Participants with brain metastases may participate only if
they satisfy all of the following:

- Has completed treatment (eg, whole brain radiation treatment [WBRT], stereotactic
radiosurgery, or equivalent) ≥14 days prior to the first dose of study treatment,

- Has no evidence of new or enlarging brain metastases confirmed by post-treatment
repeat brain imaging performed ≥3 weeks after pre-treatment brain imaging, and

- Is neurologically stable without the need for steroids for ≥7 days before first dose
of study treatment.

- Has had major surgery within 3 weeks prior to receiving the first dose of study
treatment or has not recovered adequately from toxicity and/or complications from an
intervention prior to receiving the first dose of study treatment.

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

- Has a known history of interstitial lung disease

- Has a known additional malignancy that is progressing or requires active treatment.
Exceptions include early stage cancers (carcinoma in situ or Stage 1) treated with
curative intent, basal cell carcinoma of the skin, squamous cell carcinoma of the
skin, in situ cervical cancer, or in situ breast cancer that has undergone potentially
curative therapy.

- Has active autoimmune disease that has required systemic treatment in the past 2
years. Replacement therapy is not considered a form of systemic treatment.

- Has a known history of, or active, neurologic paraneoplastic syndrome

- Has clinically active diverticulitis, intra-abdominal abscess, gastrointestinal
obstruction, and/or abdominal carcinomatosis

- Has a history of a severe hypersensitivity reaction to treatment with another
monoclonal antibody

- Is taking chronic systemic steroids (in doses exceeding 10 mg daily of prednisone
equivalent) within 7 days prior to the first dose of study treatment

- Has a diagnosis of immunodeficiency or is receiving any form of immunosuppressive
therapy within 7 days prior to the first dose of study treatment

- Has received a live vaccine within 30 days prior to the first dose of study treatment

- Has received prior therapy with an anti-programmed cell death protein-1 (anti-PD-1),
anti-programmed cell death-ligand 1 (anti-PD-L1), or anti-programmed cell death-ligand
2 (anti-PD-L2) agent or with an agent directed to another co-inhibitory T-cell
receptor (i.e., cytotoxic T-lymphocyte-associated protein 4 [CTLA-4], tumor necrosis
factor receptor superfamily member 9 [TNFRSF9, OX-40, CD137]) or has previously
participated in a Merck pembrolizumab (MK-3475) clinical trial

- Has severe hypersensitivity (Grade ≥3) to pembrolizumab and/or any of its excipients

- Has an active infection requiring systemic therapy

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

- Has a known history of Hepatitis B or known active Hepatitis C virus infection

- Has a known history of active TB (Bacillus Tuberculosis)

- Has symptomatic ascites or pleural effusion. A participant who is clinically stable
following treatment for these conditions (including therapeutic thoraco- or
paracentesis) is eligible.

- 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 study treatment through and up to 180 days after last dose of
chemotherapeutic agents