Pralatrexate vs Observation Following CHOP-based Chemotherapy in Undiagnosed Peripheral T-cell Lymphoma Patients

Inclusion Criteria: * Patient has one of the following peripheral T-cell lymphoma (PTCL) subtypes confirmed by an independent central pathology reviewer, using the Revised European American Lymphoma World Health Organization disease classification: * T/natural killer (NK)-cell leukemia/lymphoma * Adult T-cell lymphoma (TCL)/leukemia (human T-cell leukemia virus 1+) * Angioimmunoblastic TCL * Anaplastic large cell lymphoma (ALCL), primary systemic type, excluding anaplastic lymphoma kinase positive (ALK+) with International Prognostic Index (IPI) score less than 2 at initial diagnosis and complete response (CR) after CHOP-based therapy * PTCL-unspecified * Enteropathy-type intestinal lymphoma * Hepatosplenic TCL * Subcutaneous panniculitis TCL * Transformed mycosis fungoides (tMF) * Extranodal T/NK-cell lymphoma nasal or nasal type * Primary cutaneous gamma-delta TCL * Primary cutaneous CD8+ aggressive epidermic cytotoxic TCL * Documented completion of at least 6 cycles of CHOP-based therapy: * CHOP 21 * CHOP 14 * CHOP + etoposide * Other CHOP variants: substitution allowed for 1 component with a drug of the same mechanism of action. Additional components, except alemtuzumab, are allowed. Rituximab may be added if not given within 3 cycles of randomization. * Patient has achieved CR or partial response (PR) per per investigator's assessment following completion of CHOP-based therapy and has had radiological assessment within 21 days prior to randomization. * Eastern Cooperative Oncology Group performance status less than or equal to 2. * Adequate blood, liver, and kidney function as defined by laboratory tests. * Women of childbearing potential must have a negative serum pregnancy test within 14 days prior to randomization and agree to practice a medically acceptable contraceptive regimen from study treatment initiation until at least 30 days after the last administration of pralatrexate. * Men who are sexually active, including those with a pregnant partner, must agree to practice a medically acceptable barrier method contraceptive regimen (eg, condoms) while receiving pralatrexate and for 90 days after the last administration of pralatrexate. * Has given written informed consent. Exclusion Criteria: * Patient has: * Precursor T/NK neoplasms * ALCL (ALK+) with IPI score less than 2 at initial diagnosis and CR after CHOP-based therapy * T cell prolymphocytic leukemia * T cell large granular lymphocytic leukemia * Mycosis fungoides, except tMF * Sézary syndrome * Primary cutaneous CD30+ disorders: ALCL and lymphomatoid papulosis * If there is a history of prior malignancies other than those below, must be disease free for at least 5 years. Patients with malignancies listed below less than 5 years before study entry may be enrolled if they have received treatment resulting in complete resolution of the cancer and have no clinical, radiologic, or laboratory evidence of active/recurrent disease. * non-melanoma skin cancer * carcinoma in situ of the cervix * localized prostate cancer * localized thyroid cancer * Receipt of prior chemotherapy (CT) or radiation therapy (RT) for PTCL, other than a single allowed CHOP regimen, except: * Patients with nasal NK lymphoma who received local RT less than 4 weeks prior to randomization. * Patients with tMF who received 1 systemic single-agent CT (except methotrexate) prior to transformation. * Prior exposure to pralatrexate. * Receipt of systemic corticosteroids within 3 weeks of study treatment, unless patient has been taking a continuous dose of 10 mg/day or less of oral prednisone or equivalent for at least 4 weeks or as part of a CHOP prednisone taper. * Planned use of any treatment for PTCL during the course of the study. * Patient has: * Human immunodeficiency virus (HIV)-positive diagnosis with a CD4 count of less than 100 mm3 or detectable viral load within past 3 months and receiving anti-retroviral therapy. * Hepatitis B (HBV)-positive serology and is receiving interferon therapy or has liver function test results outside the parameters of study inclusion criteria. Other antiviral therapies are permitted if at a stable dose for at least 4 weeks. * Hepatitis C (HCV) virus with detectable viral load or immunological evidence of chronic active disease or receiving/requiring antiviral therapy. * Symptomatic central nervous system metastases or lesions requiring treatment. * Uncontrolled hypertension or congestive heart failure Class III/IV per the New York Heart Association's Heart Failure Guidelines * Active uncontrolled infection, underlying medical condition including unstable cardiac disease, or other serious illness impairing the ability of the patient to receive protocol treatment. * Major surgery within 2 weeks prior to study entry, except for line placement or biopsy procedure.