The primary purpose of this study is to evaluate the anti-tumor activity of TLN-254 monotherapy in participants with relapsed or refractory T-cell lymphoma.

Inclusion Criteria:

Age:

1. At least 18 years of age at the time of signing the informed consent form (ICF).

   Type of Participant and Disease Characteristics:

2. Cohort 1: Peripheral T-cell lymphoma (PTCL) that has relapsed after, or not responded to at least one prior systemic treatment regimen. Participants with Anaplastic large cell lymphoma (ALCL) should have received prior brentuximab vedotin treatment.

   • Nodal T-follicular helper (TFH) cell lymphoma angioimmunoblastic; Follicular helper T-cell lymphoma, angioimmunoblastic type (angioimmunoblastic T-cell lymphoma).
   • Nodal TFH cell lymphoma, follicular type; Follicular helper T-cell lymphoma, follicular type.
   • Nodal TFH cell lymphoma, not otherwise specified (NOS); Follicular helper T-cell lymphoma, NOS.
   • PTCL, NOS.
   • Anaplastic large-cell lymphoma, Alkaline phosphatase (ALK) positive.
   • Anaplastic large-cell lymphoma, ALK negative.

   Cohort 2: Relapsed/refractory CTCL which has relapsed after, or not responded to at least two prior systemic treatments.

   • Sezary syndrome
   • Mycosis fungoides

3. Participant must have measurable disease at study entry.

4. Freshly biopsied or archival tissue available.

   Diagnostic Assessments:

5. Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2.

6. Adequate organ function.

   Contraception:

7. Participants must accept and follow the pregnancy prevention plan.

Exclusion Criteria:

Medical Conditions:

1. Participants must not have prior systemic anti-cancer treatment less than or equal to (≤) 5 half-lives or 4 weeks, allogeneic SCT≤90 days or autologous SCT ≤60 days prior to study intervention initiation.
2. Participants must not have any condition, including significant acute or chronic medical illness, active or uncontrolled infection, or the presence of laboratory abnormalities, that places participants at unacceptable risk if participating in this study.
3. Current or past history of central nervous system (CNS) involvement.

Other Exclusions:

• Pregnant or lactating women.
• Unable to swallow tablets.