Chidamide Plus Sintilimab for Chemotherapy-refractory Advanced High-grade Neuroendocrine Neoplasm

Inclusion Criteria: 1. Age ≥ 18 years; 2. Histologically confirmed advanced and metastatic high-grade neuroendocrine neoplasm; 3. Receive ≤ 2 types of chemotherapy for high-grade neuroendocrine tumors and had tumor progression; 4. ECOG ≤ 2; 5. Have at least one measurable lesion according to RECIST version 1.1; 6. Absolute neutrophil count ≥ 1.5×109 / L, platelet count ≥ 100×109 / L, hemoglobin ≥ 90 g/L; 7. Provide tumor samples for pathological diagnosis and PD-L1 biomarker detection; 8. Have ability to sign a written informed consent. Exclusion Criteria: 1. Small cell lung cancer; 2. Have surgery or trauma within 4 weeks before enrollment, or are expected to receive surgical treatment; 3. Previous use of HDAC inhibitors; 4. Previous use of PD-1/PD-L1/PD-L2/CTLA-4 inhibitors; 5. Allergy to related drug components; 6. Have a medical history of immune deficiency diseases, or organ transplantation; 7. Have active autoimmune diseases requiring treatment or a medical history of autoimmune diseases in the past 2 years; 8. Have uncontrolled or significant cardiovascular disease; 9. Abnormal liver function (total bilirubin \> 1.5×upper limit of normal); Transaminases (ALT/AST) \>2.5×upper limit of normal (\>5x upper limit of normal for patients with liver metastases), abnormal renal function (serum creatinine \> 1.5×upper limit of normal); 10. Pregnancy ; 11. Receive any live or live attenuated vaccine within 4 weeks before enrollment; 12. Have serious diseases that may endanger the safety of patients, or affect patients to complete the research; 13. Any serious mental or cognitive disorder; 14. Patients are currently enrolled in another drug clinical trial within 4 weeks prior to enrollment; 15. Any other condition which is inappropriate for the study in the opinion of the investigators.