∙ Screening Cohort (non-MSK patients only)

• Age ≥18 years at time of consent

• Has freely given written informed consent to participate in the study

∙ Treatment Cohort:

• Pathologically-confirmed AITL at the enrolling institution, with confirmed IDH2 mutation (by MSK ddPCR). For R/R patients, disease must have relapsed or progressed after at least one systemic therapy, diagnostic tumor samples have at least 5% tumor.

• Age ≥18 years at time of enrollment

• Previous systemic anti-cancer therapy for AITL must have been discontinued at least 2 weeks or 5 half-lives (whichever is longer) prior to treatment.

• i) See section 6.2 Subject Exclusion Criteria for guidelines regarding adjuvant and maintenance therapy for prior malignancy.

• ii) Patients who have received localized RT as part of their immediate prior therapy may be allowed to enroll with shorter washout period after discussion with the MSKCC Principal Investigator iii) Systemic corticosteroids must be tapered to 25 mg/day prednisone (or equivalent) upon start of investigational treatment iv) Topical steroids for treating cutaneous involvement of AITL is permitted

• Performance status, as assessed in the ECOG grading system, ≤2

• Laboratory criteria (use of GCSF and/or blood product transfusions to reach eligibility criteria must be discussed with the MSK PI on a case-by-case basis): i) Absolute neutrophil count ≥1.0 K/μL or ≥0.5 K/μL if due to lymphoma ii) Platelet count ≥80 K/μl or ≥50 K/μl if due to lymphoma iii) Calculated creatinine clearance ≥30 mL/min by the Modification of Diet in Renal Disease (MDRD) glomerular filtration rate iv) Total bilirubin ≤1.5x upper limit of normal (ULN) or ≤3x ULN if documented hepatic involvement with lymphoma, or ≤5x ULN if history of Gilbert's syndrome; AST and ALT ≤ 3x ULN; ≤ 5x ULN if due to lymphoma involvement

• Measurable disease, defined by either of:

‣ Revised International Working Group Classification for systemic lymphoma(13)

⁃ Atypical T lymphocytes quantifiable by flow cytometry or morphology in the peripheral blood or bone marrow

• Women of reproductive potential\* must have a negative serum or urine β human chorionic gonadotropin (β-hCG) pregnancy test. All women of reproductive potential must agree to use adequate methods of birth control throughout the study and for 30 days after the last dose of study drug. \*A woman of reproductive potential is a sexually mature woman 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 must agree to not breastfeed during the study period

• Male subjects must agree to practice true abstinence from sexual intercourse or to the use of highly effective contraceptive methods with non-pregnant female partners of childbearing potential at screening and throughout the course of the study, and should avoid conception with their partners during the course of the study and for 4 months following the last study treatment

⁃ Subject is willing and able to adhere to the study visit schedule and other protocol requirements