The purpose of this study is to test how well pembrolizumab shrinks Early-Stage NK/T-cell Lymphoma (ENKTL) in participants who have not yet received chemotherapy.

Inclusion Criteria:

• Histologic diagnosis of extranodal NK/T, nasal type cell lymphoma at the enrolling institution
• 18 years of age on day of signing informed consent
• Have a performance status of ≤ 1 on the ECOG Performance Scale
• Have measurable disease by PET/CT
• Demonstrate adequate organ function as defined below:

RENAL:

Serum Creatinine ≤ 1.5 x upper limit of normal (ULN)

OR

Measured or calculated creatinine clearance (GFR can also be used in place of creatinine or CrCL) Serum creatinine ≥ 60 mL/min for subject with creatinine levels > 1.5 x institutional ULN ≤ 1.5 x upper limit of normal (ULN)

OR

Measured or calculated creatinine clearance (GFR can also be used in place of creatinine or CrCL) ≥ 60 mL/min for subject with creatinine levels > 1.5 x institutional ULN

HEPATIC:

Serum total bilirubin ≤ 1.5 x ULN

OR

Serum total bilirubin ≤ 3 x ULN for subjects with liver metastases

AST (SGOT) and ALT (SGPT) ≤ 2.5 x ULN

OR

AST (SGOT) and ALT (SGPT) ≤ 5 x ULN for subjects with liver metastases

CARDIAC:

Ejection fraction ≥ 50%

PULMONARY:

Hemoglobin-adjusted diffusing capacity for carbon monoxide ≥ 50%

- Women of childbearing potential* must have a negative urine or serum pregnancy test (minimum sensitivity 25 IU/L or equivalent units of HCG) within 72 hours prior to receiving the first dose of study medication

• *A woman of childbearing potential is a sexually mature female who has not undergone a hysterectomy or bilateral oophorectomy; or has not been naturally postmenopausal for at least 24 consecutive months (i.e. has had menses at any time in the preceding 24 consecutive months)

  • Women of childbearing potential must be willing to use an adequate method of contraception

• Must agree to practice 2 effective methods of contraception from the time of signing the informed consent form through 120 days after the last dose of study therapy, or agree to completely abstain from heterosexual intercourse

  • Male subjects of childbearing potential must agree to use an adequate method of contraception

• Male subjects, even if surgically sterilized (i.e. statue post vasectomy) must agree to 1 of the following: Practice effective barrier contraception during the entire study therapy, or agree to completely abstain from heterosexual intercourse

Exclusion Criteria:

• Received prior treatment for extranodal NK/T cell lymphoma
• Medical illness unrelated to lymphoma, which, in the opinion of the treating physician and/or institutional principal investigator, makes participation in this study inappropriate.
• Has received a live vaccine or live-attenuated vaccine within 30 days prior to the first dose of study drug. Administration of killed vaccines is allowed.
• Has a known additional malignancy that is progressing or has required active treatment within the past 2 years. Note: Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or carcinoma in situ (e.g. breast carcinoma, cervical cancer in situ) that have undergone potentially curative therapy are not excluded.
• Has known active CNS metastases and/or carcinomatous meningitis.
• Has severe hypersensitivity (≥ Grade 3) to pembrolizumab and/or any of its excipients
• 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 a history of (non-infectious) pneumonitis that required steroids or has current pneumonitis.
• Has an active infection requiring systemic therapy.
• Has a known history of Human Immunodeficiency Virus (HIV).
• Has active Hepatitis B (defined as HBV DNA is detected) or known active Hepatitis C virus (defined as HCV RNA is detected) infection.
• Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study, interfere with the subject's participation for the full duration of the study, 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 study, starting with the screening visit through 120 days after the last dose of trial treatment.
• Has had an allogenic tissue/solid organ transplant.