Study of Pembrolizumab (MK-3475) as Monotherapy in Participants With Previously-Treated Locally Advanced Unresectable or Metastatic Colorectal Cancer (MK-3475-164/KEYNOTE-164)

Inclusion criteria:

- Histologically-proven locally advanced unresectable or metastatic high colorectal
carcinoma

- Locally confirmed MMR deficient or MSI status

- Have been previously treated with standard therapies, which must include, for Cohort
A, fluoropyrimidine, oxaliplatin, and irinotecan, and for Cohort B, at least one line
of systemic standard of care therapy: fluoropyrimidine + oxaliplatin or
fluoropyrimidine + irinotecan +/- anti-VEGF/EGFR monoclonal antibody

- Eastern Cooperative Oncology Group performance status of 0 or 1

- Life expectancy of greater than 3 months

- Provide an archival or newly obtained tumor tissue sample (Cohort B)

- At least one measureable lesion

- Female participants of childbearing potential should be willing to use 2 methods of
birth control or be surgically sterile, or abstain from heterosexual activity for the
course of the study through 120 days after the last dose of study medication

- Male participants should agree to use an adequate method of contraception starting
with the first dose of study therapy through 120 days after the last dose of study
medication

- Adequate organ function

Exclusion criteria:

- Currently participating in another study and receiving trial treatment, participated
in a study of an investigational agent and received trial treatment within 4 weeks of
the first dose of medication in this study, or used an investigational device within
4 weeks of the first dose of medication in this study

- Active autoimmune disease that has required systemic treatment in past 2 years

- Diagnosis of immunodeficiency or receiving systemic steroid therapy or any other form
of immunosuppressive therapy within 7 days prior to the first dose of study
medication

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

- Prior monoclonal antibody (mAb), chemotherapy, targeted small molecule therapy, or
radiation therapy within 2 weeks prior to study Day 1 or not recovered (i.e., ≤ Grade
1 or at baseline) from adverse events due to a previously administered agent

- Prior therapy with an anti-programmed cell death (PD)-1, anti-PD ligand 1
(anti-PD-L1), or anti-PD-L2 agent

- Known additional malignancy that is progressing or requires active treatment with the
exception of basal cell carcinoma of the skin or squamous cell carcinoma of the skin
that has undergone potentially curative therapy, or in situ cervical cancer

- Received a live vaccine within 30 days of planned start of study medication

- Known history of human immunodeficiency virus (HIV)

- Known active Hepatitis B or C

- Known history or any evidence of interstitial lung disease or active, non-infectious
pneumonitis

- Active infection requiring systemic therapy

- Known psychiatric or substance abuse disorders that would interfere with cooperation
with the requirements of the trial

- Pregnant or breastfeeding, or expecting to conceive or father children within the
projected duration of the trial, starting with the screening visit through 120 days
after the last dose of trial medication