Inclusion Criteria:

          -  Inclusion Criteria for Cohort A: Subjects who meet all of the following criteria will
             be eligible to participate in Cohort A of the study:

             1. Have initiated first-line therapy for NSCLC with pembrolizumab and satisfy the
             following:

               -  Have tumor PD-L1 ≥50% by 22C3 IHC;

               -  Are judged by the Principal Investigator as tolerating pembrolizumab monotherapy;
                  and

               -  Have been on pembrolizumab for at least 3 months (measured from actual first dose
                  date to first dose date of the current study) and the most recent scans are the
                  first scans to objectively demonstrate Progressive Disease per RECIST 1.1.

          -  The first scan conducted a minimum of 21 days after first dose of anti-PD-(L)1 therapy
             must have shown either SD, PR, or CR (ie, not Progressive Disease) per RECIST 1.1; and

          -  The 2 most recent scans (including 1 demonstrating Progressive Disease) are available
             to be reviewed.

          -  Inclusion Criterion for Cohort B

        Subjects who meet the following criterion will be eligible to participate in Cohort B of
        the study:

        1. Are eligible for single-agent pembrolizumab for advanced/metastatic NSCLC in accordance
        with the package insert and have tumor PD-L1 ≥50% by 22C3 IHC.

          -  Inclusion Criteria for Both Cohorts

          -  Subjects must also meet all of the following criteria to be eligible to participate in
             the study:

               1. Have histologically confirmed NSCLC that is inoperable, locally advanced or
                  metastatic (Stage IIIb/IV), and was not treated with chemotherapy in the
                  advanced/metastatic setting; Note: Subjects may have received chemotherapy and/or
                  anti-PD-(L)1 therapy in the neo/adjuvant setting, provided the last dose of
                  therapy was >9 months prior to randomization.

               2. Have available at the site a representative formalin-fixed, paraffin-embedded
                  tumor specimen that enabled diagnosis of NSCLC in a tissue block (preferred) or
                  10 unstained, serial slides, accompanied by an associated pathology report.
                  Cytological or fine-needle aspiration samples are not acceptable; Note: If the
                  archival tissue is neither sufficient nor available, the subject may still be
                  eligible, upon discussion with the Medical Monitor, assuming the subject can
                  provide ≥5 unstained, serial slides. Cytological or fine-needle aspiration
                  samples are not acceptable.

               3. Have provided written informed consent and any authorizations required by local
                  law;

               4. Are ≥18 years of age;

               5. Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1;

        Exclusion Criteria:

          1. Have received platinum-based chemotherapy or initiated anti-PD-(L)1 therapy with
             chemotherapy for locally advanced or metastatic NSCLC; Note: Subjects may have
             received chemotherapy and/or anti-PD-(L)1 therapy in the neo/adjuvant setting,
             provided the last dose of therapy was >9 months prior to randomization.

          2. Have NSCLC with epidermal growth factor receptor or anaplastic lymphoma kinase genomic
             tumor aberrations;

          3. Have gastrointestinal (GI) disease (eg, gastric or intestinal bypass surgery,
             pancreatic enzyme insufficiency, malabsorption syndrome, symptomatic inflammatory
             bowel disease, chronic diarrheal illness, and/or bowel obstruction) that may interfere
             with drug absorption or with interpretation of GI AEs;

          4. Have known symptomatic brain metastases requiring >10 mg/day of prednisone (or its
             equivalent). Subjects with previously diagnosed brain metastases are eligible if they
             have completed their treatment, have recovered from the acute effects of radiation
             therapy or surgery prior to randomization, fulfill the steroid requirement for these
             metastases, the 2 most recent serial magnetic resonance imaging (MRI) scans conducted
             >28 days apart show no central nervous system progression, and are neurologically
             stable and asymptomatic;