Preliminary Clinical Study of Entinostat in Children With Malignant Soft Tissue Sarcoma Harboring PAX-family Gene Abnormalities

Inclusion Criteria: 1. Age \>1 year and ≤18 years. 2. Histologically confirmed malignant soft tissue sarcoma that is PAX3-FOXO1- or PAX7-FOXO1-fusion positive or has another PAX-family gene abnormality. 3. Clinical disease status: (a) monotherapy cohort: completion of standard backbone chemotherapy and/or radiotherapy with a favorable clinical response (CR or PR); (b) combination cohort: inadequate response following standard chemotherapy and/or radiotherapy, or recurrent/progressive disease. 4. Karnofsky Performance Status (KPS) ≥50 or Lansky Performance Status (LPS) ≥50. 5. Adequate bone marrow function: ANC ≥0.5 × 10\^9/L, platelet count ≥50 × 10\^9/L, hemoglobin ≥70 g/L, and WBC ≥1.5 × 10\^9/L, with values not exceeding the upper limit of normal. 6. Life expectancy of at least 6 months. 7. Written informed consent obtained. Exclusion Criteria: 1. Previous hematopoietic stem cell transplantation or organ transplantation. 2. Participation in another drug clinical trial within 2 months before enrollment. 3. Uncontrolled infection or body temperature ≥38°C. 4. Total bilirubin, ALT, and AST \>2.5 × the upper limit of normal. 5. Serum creatinine \>2.0 × the upper limit of normal. 6. Any other condition considered by the investigator to constitute a contraindication to study participation.