This is a study for patients with previously-treated advanced non-small cell lung cancer (NSCLC). The study will evaluate the safety of adding an investigational drug, MK-3475 to standard treatment with gemcitabine. The study will also try to identify the best dose of MK-3475 to give in combination with gemcitabine.

This study is an open-label, non-randomized phase I study, followed by open-label non-randomized phase II study. The first cohort of patients will receive 200 milligrams (mg) of MK-3475 by intravenous infusion over a 21-day period called a cycle along with Gemcitabine 1250 mg/m2 given on Days 1 and 8 of each 21-day cycle for up to 6 cycles. Patients will be seen in the study clinic 12 times over 126 days for an evaluation of signs and symptoms that may represent drug toxicity. Patients may continue to receive MK-3475 (without gemcitabine) for up to 2 years.

Inclusion Criteria:

• Women or men with advanced, histologically proven NSCLC.
• Patients must have received at least one but no more than three prior systemic therapies for advanced disease.
• Any toxicity related to prior therapies that, in the opinion of the investigator, would potentially be worsened with anti-PD1 therapy or gemcitabine should be resolved to less than Grade 1.
• Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1.
• Women of childbearing potential must have a negative pregnancy test
• Ability to give informed consent and comply with the protocol.
• Anticipated survival minimum 3 months.
• Prior therapy with investigational agents must have been completed at least 3 weeks prior to study enrollment.
• Patients must have normal organ and marrow function as seen on protocol-defined blood test results
• Archived tumor tissue (minimum of 8 slides for paraffin-embedded tumor tissue) available
• Measurable disease by RECIST 1.1 criteria.
• Treated brain metastases will be allowed, provided they are asymptomatic.
• Radiation for symptomatic lesions outside the Central nervous system (CNS) must have been completed at least 2 weeks prior to study enrollment.

Exclusion Criteria:

• Prior therapy with any anti-PD-1, anti-PD-L1, or anti-CTLA4 antibody.
• Prior therapy with gemcitabine.
• Prior complications from radiation, such as history of radiation pneumonitis or pulmonary edema that, in the opinion of the investigator, may have risk of increasing toxicity with anti-PD1 therapy.
• Active autoimmune disease except vitiligo or stable hypothyroidism.
• Active and ongoing steroid use, except for non-systemically absorbed treatments (such as inhaled or topical steroid therapy for asthma, cardiopulmonary disease (COPD), allergic rhinitis).
• Active other malignancy, except for controlled basal cell skin carcinoma.
• HIV positive and/or Hepatitis B or C positive.
• Other medical or psychiatric conditions that in the opinion of the Principal Investigator would preclude safe participation in this protocol.

Adverse events were collected over 6 21-day cycles (126 days). All-cause mortality was assessed up to 2 years from the end of treatment.