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