Pembrolizumab in Metastatic Anal Cancer

Inclusion Criteria: * Participants must have metastatic anal cancer that has been histologically confirmed. * There is no limit to the number of prior therapies. * Be willing and able to provide written informed consent/assent for the trial. * Be ≥18 years of age on day of signing informed consent. * Have measurable disease based on RECIST 1.1. * Be willing to provide tissue from a newly obtained core or excisional biopsy of a tumor lesion. Newly-obtained is defined as a specimen obtained up to 6 weeks (42 days) prior to initiation of treatment on Day 1. Subjects for whom newly-obtained samples cannot be provided (e.g. inaccessible or subject safety concern) may submit an archived specimen only upon agreement from the Sponsor. * Have a performance status of 0 or 1 on the ECOG Performance Scale. * Demonstrate adequate organ function, all screening labs must be performed within 10 days of treatment initiation. Adequate Organ Function Laboratory Values System Laboratory Value * Hematological * Absolute neutrophil count (ANC) ≥1,500 /mcL * Platelets ≥80,000 / mcL * Hemoglobin ≥8.5 g/dL or ≥5.6 mmol/L * Renal --Serum creatinine OR Measured or calculated creatinine clearance (GFR can also be used in place of creatinine or CrCl) ≤1.5 X upper limit of normal (ULN) OR ≥60 mL/min for subject with creatinine levels \> 1.5 X institutional ULN * Hepatic * Serum total bilirubin ≤ 1.5 X ULN OR Direct bilirubin ≤ ULN for subjects with total bilirubin levels \> 1.5 ULN * AST (SGOT) and ALT (SGPT) ≤ 2.5 X ULN OR ≤ 5 X ULN for subjects with liver metastases * Albumin \>2.8 mg/dL * Coagulation * International Normalized Ratio (INR) or Prothrombin Time (PT) ≤1.5 X ULN unless subject is receiving anticoagulant therapy as long as PT or PTT is within therapeutic range of intended use of anticoagulants * Activated Partial Thromboplastin Time (aPTT) ≤1.5 X ULN unless subject is receiving anticoagulant therapy as long as PT or PTT is within therapeutic range of intended use of anticoagulants * Creatinine clearance should be calculated per institutional standard. * Female subject of childbearing potential must have a negative urine or serum pregnancy within 72 hours prior to receiving the first dose of study medication. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. * Female subjects of childbearing potential must 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 (Reference Section 7.4). Subjects of childbearing potential are those who have not been surgically sterilized or have not been free from menses for \> 1 year. * Male subjects 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 study therapy. Exclusion Criteria: * Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of trial treatment. Subjects requiring systemic steroids are excluded from the trial. The use of physiologic doses of corticosteroids may be approved after discussion with the sponsor. * Has a known history of active TB (Bacillus Tuberculosis) * Hypersensitivity to pembrolizumab or any of its excipients. * Has had a prior anti-cancer monoclonal antibody (mAb) within 4 weeks prior to study Day 1 or who has not recovered (i.e., ≤ Grade 1 or at baseline) from adverse events due to agents administered more than 4 weeks earlier. * Has had prior chemotherapy, targeted small molecule therapy, or radiation therapy within 2 weeks prior to study Day 1 or who has not recovered (i.e., ≤ Grade 1 or at baseline) from adverse events due to a previously administered agent. * Note: Subjects with ≤ Grade 2 neuropathy and alopecia are an exception to this criterion and may qualify for the study. * Note: If subject received major surgery, they must wait ≥ 3 weeks prior to starting study treatment. They must have recovered adequately from the toxicity and/or complications from the intervention prior to starting therapy. * Has a known additional malignancy that is progressing or requires active treatment. Exceptions include basal cell carcinoma of the skin or squamous cell carcinoma of the skin that has undergone potentially curative therapy or in situ cervical cancer. * Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis. Subjects with previously treated brain metastases may participate provided they are stable (without evidence of progression by imaging for at least four weeks prior to the first dose of trial treatment and any neurologic symptoms have returned to baseline), have no evidence of new or enlarging brain metastases, and are not using steroids for at least 7 days prior to trial treatment. This exception does not include carcinomatous meningitis which is excluded regardless of clinical stability. * Has active autoimmune disease that has required systemic treatment in the past 2 years (i.e. with use of disease modifying agents, corticosteroids or immunosuppressive drugs). Replacement therapy (eg., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a form of systemic treatment. * Has an active infection requiring systemic therapy. * Patients that require supplemental oxygen are excluded. * Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with the subject's participation for the full duration of the trial, or is not in the best interest of the subject to participate, in the opinion of the treating investigator. * Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial. * Is pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the pre-screening or screening visit through 120 days after the last dose of trial treatment. * Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent. * HIV+ positive patients are eligible if their CD4+ count ≥ 300/μL and they have an undetectable viral load. In addition, they must be currently receiving Highly Active Antiretroviral Therapy (HAART) and be under the care of an Infectious Diseases specialist. * Patients with hepatitis B and hepatitis C must be under the care of viral hepatitis expert consultant. Patients with hepatitis B are required to be treated with anti-HBV treatment (e.g., entecavir). Patients with hepatitis C need to have received prior and/or ongoing hepatitis C treatment. * Has received a live vaccine within 30 days of planned start of study therapy. Note: Seasonal influenza vaccines for injection are generally inactivated flu vaccines and are allowed; however intranasal influenza vaccines (e.g., Flu-Mist®) are live attenuated vaccines, and are not allowed. * Has a history of (non-infectious) pneumonitis that required steroids or current pneumonitis.