Inclusion Criteria:

• 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 the Lugano Criteria (Cheson et al., 2014).
• Patient must have histologically confirmed relapsed or refractory Peripheral T-cell lymphoma (PTCL) or cutaneous T-cell Lymphoma (CTCL) with tumor stage as per WHO criteria and TNMB classification and staging. 
• There is no upper limit for the number of prior therapies. Patient may have relapsed after prior autologous stem cell transplant.
• Patients who had proir treatment for their disease, as long as there is radiographic evidence of refractory or relapsed disease and the patient meets all other clinical and laboratory criteria for study treatment
• Be willing to provide tissue from a newly obtained core or excisional biopsy of a tumor lesion. Be willing to provide FNA 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 as defined below; all screening labs should be performed within 10 days of treatment initiation.

Hematological

• Absolute neutrophil count (ANC)
  • ≥ 1,000 /mcL.
• Platelets
  • ≥ 75,000 / mcL.
• Hemoglobin
  • ≥ 9 g/dL or ≥5.6 mmol/L without transfusion or EPO dependency (within 7 days of assessment).

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.
• *Creatinine clearance should be calculated per institutional standard.

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
  • > 2.5 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
• Female subject of childbearing potential should 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 an adequate method of contraception – Contraception for the course of the study through 120 days after the last dose of study medication.
  • Note: Abstinence is acceptable if this is the usual lifestyle and preferred contraception for the subject.
• Male subjects of childbearing potential must agree to use an adequate method of contraception - Contraception, starting with the first dose of study therapy through 120 days after the last dose of study therapy.
  • Note: Abstinence is acceptable if this is the usual lifestyle and preferred contraception for the subject.

Exclusion Criteria: 

• Has lack of resolution of adverse events (AE) due to previously administered antineoplastic therapy to grade 1 or less according to National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE), version 4.0.
• Had prior therapy with PD-1 inhibitors.
• Has a diagnosis of immunodeficiency or has been receiving any immunosuppressive therapy within 7 days prior to the first dose of trial treatment. Use of steroid equivalent to prednisone 10 mg/day does not constitute an exclusion criterion.
• Has a known history of active TB (Bacillus Tuberculosis).
• Hypersensitivity to pralatrexate, or decitabine or pembrolizumab or any of its excipients.
• Has received prior allogeneic stem cell transplant.
• 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 (e.g., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a form of systemic treatment. The following are exceptions to this criterion: subjects with vitiligo or alopecia; subjects with hypothyroidism (e.g., following Hashimoto syndrome) stable on hormone replacement; subjects with psoriasis not requiring systemic treatment; patients with autoimmune phenomena secondary to active lymphoma.
• Has known history of, or any evidence of non-infectious pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease.
• Has an active infection requiring systemic therapy.
• 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.
• Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies).
• Has known active Hepatitis B (e.g., HBsAg reactive) or Hepatitis C (e.g., HCV RNA [qualitative] is detected).
• Has received a live vaccine or live-attenuated vaccine within 30 days of planned start of study therapy.  Administration of killed vaccines is allowed.
  • 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.