An Open-Label, Single-Arm, Multicenter Phase II Clinical Study to Evaluate the Efficacy of Azacitidine, Chidamide, and PD-1 Monoclonal Antibody in the Treatment of Refractory/Relapsed Peripheral T-Cell Lymphoma.

Inclusion Criteria:

1. Age ≥18 years and ≤75 years, male or female.

2. Histologically confirmed peripheral T-cell lymphoma (PTCL) by the investigating center, including: peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS); anaplastic large cell lymphoma (ALK-negative or ALK-positive); angioimmunoblastic T-cell lymphoma; enteropathy-associated T-cell lymphoma; NK/T-cell lymphoma; and other PTCL subtypes deemed eligible by the investigator.

3. Relapsed or refractory disease after at least one prior line of systemic therapy. Relapse is defined as disease recurrence after complete response (CR) or progression after partial response (PR) or stable disease (SD). Refractory disease is defined as progressive disease (PD) after 2 cycles of therapy, or SD after 4 cycles of therapy, or failure to achieve response after adequate last-line therapy (e.g., at least 2-3 cycles of systemic chemotherapy without remission), or progression during treatment.

4. Patients considered ineligible for autologous hematopoietic stem cell transplantation by the investigator, or those who refuse such treatment.

5. At least one measurable or evaluable lesion according to the Lugano 2014 classification. Measurable lesion: nodal lesion with longest diameter >1.5 cm and shortest diameter >1.0 cm on CT/PET-CT or MRI; or extranodal lesion with longest diameter >1.0 cm. Evaluable lesion: nodal or extranodal focal uptake on 18F-FDG/PET higher than liver with PET and/or CT features consistent with lymphoma.

6. ECOG performance status 0-2.

7. Life expectancy ≥3 months.

8. Adequate organ and bone marrow function defined as:

   1. Hematology: WBC ≥2.0×10⁹/L (≥1.0×10⁹/L if with bone marrow involvement), ANC ≥1.0×10⁹/L (≥0.5×10⁹/L if with bone marrow involvement), PLT ≥50×10⁹/L (≥30×10⁹/L if with bone marrow involvement), HGB ≥7.0 g/dL; no granulocyte growth factor support, platelet or RBC transfusion within 7 days prior to testing.
   2. Liver function: TBIL ≤1.5×ULN (≤3.0×ULN with liver involvement); ALT and AST ≤2.5×ULN (≤5.0×ULN with liver involvement).
   3. Renal function: Serum Cr ≤1.5×ULN.
   4. Coagulation: INR ≤1.5×ULN; PT and APTT ≤1.5×ULN (unless on anticoagulant therapy with PT/APTT within therapeutic range at screening).
   5. Thyroid function: TSH, FT4, and FT3 within ±10% of normal range (Note: TSH abnormalities due to non-autoimmune causes are acceptable).
   6. Cardiac function: Left ventricular ejection fraction ≥50%, no organic arrhythmia, no significant abnormalities in cardiac enzymes.

Exclusion Criteria:

1. History of other malignancies within the past 5 years, except for those treated with curative intent (e.g., basal cell carcinoma of the skin, carcinoma in situ).

2. Patients with significant dysfunction of major organs.

3. Known involvement of central nervous system (CNS) lymphoma.

4. History of active bleeding or newly diagnosed thrombotic disease, or those with bleeding tendency receiving anticoagulant therapy.

5. Known history of Human Immunodeficiency Virus (HIV) infection and/or Acquired Immunodeficiency Syndrome (AIDS).

6. Patients with active chronic hepatitis B or active hepatitis C.

7. Systemic corticosteroid therapy or other immunosuppressive therapy required for any condition within 14 days prior to initiation of study treatment.

8. Active autoimmune disease requiring systemic treatment within the past two years. Patients with autoimmune diseases not requiring systemic treatment in the past two years may be enrolled.

9. Major surgery within 28 days prior to enrollment, or less than 6 weeks after major organ surgery.

10. Administration of live attenuated vaccines within 4 weeks prior to enrollment or planned during the study period (influenza vaccines excluded).

11. Pregnant or lactating women, and subjects of childbearing potential unwilling to use effective contraception.

12. Psychiatric disorders or individuals unable to provide informed consent.

13. Active infection, except for tumor-related B-symptom fever.

14. Poorly controlled cardiac symptoms or diseases, including:

    i. NYHA Class III or higher heart failure ii. Unstable angina iii. Myocardial infarction within the past year iv. Clinically significant arrhythmias

15. Any other condition deemed by the investigator to make the subject unsuitable for study participation.