Inclusion Criteria:
1. The subjects voluntarily joined the study, signed the informed consent, and the compliance was good;
2. Locally advanced unresectable or metastatic neuroendocrine carcinoma confirmed by histopathology or cytology (excluding small cell lung cancer);
3. Disease progression or toxicity intolerance after previous first-line and above treatment;
4. At least one measurable lesion according to RECIST v1.1;
5. Eastern Cooperative Oncology Group Performance Status（ECOG PS） score is 0-1;
6. Age ≥18 and ≤75 years old;
7. Can provide tumor specimens for biomarker detection；
8. Major organ and bone marrow function levels meet the following requirements within 7 days prior to treatment:
① Blood routine: Absolute neutrophil count (ANC) ≥1.5×10\^9/L; Platelet count (PLT) ≥90×10\^9/L; Hemoglobin (Hb) ≥9.0 g/dL; Have not received transfusions of blood products (including erythrocyte and platelet products, etc.) and have not used supportive therapy of growth factors (including colony-stimulating factor, interleukin and erythropoietin, etc.) within 2 weeks before the examination;
② Liver function: serum total bilirubin (TBIL) ≤1.5× upper limit of normal value (ULN); alanine Aminotransferase（ALT） and aspartate aminotransferase (AST) in subjects without liver metastasis ≤3.0×ULN, and ALT and AST in subjects with liver metastasis ≤5.0×ULN;
③ Renal function: serum creatinine (Cr) ≤1.5×ULN;
④ Coagulation function: International standardized ratio (INR) ≤1.5×ULN, and activated partial thromboplastin time (APTT) ≤1.5×ULN;
⑤ Urine routine results showed that urine protein \<2+; For patients with urine protein ≥2+ in routine urine testing at baseline, a 24-hour urine protein quantity of \<1g is required;
9. Expected survival time ≥3 months;
10. Women of reproductive age should agree to use contraceptives (such as Iuds, contraceptives, or condoms) during the study period and for 6 months after the study ends; Have a negative serum or urine pregnancy test within 7 days prior to study enrollment and must be a non-lactating patient; Men should consent to patients who must use contraception during the study period and for 6 months after the end of the study period.
Exclusion Criteria:
1. Previous exposure to any anti-PD-1 /PD-L1/PD-L2/CTLA-4 antibody, VEGF/VEGFR targeting drugs;
2. Previous treatment with temozolomide;
3. Received chemotherapy, targeted therapy, Chinese herbal medicine with anti-tumor indications, or immunomodulatory drugs within 4 weeks prior to enrollment, or remained within 5 half-lives of such drugs;
4. Known to be allergic to any monoclonal antibody, temozolomide preparations, and solfantinib preparations;
5. Use of immunosuppressive drugs within 4 weeks prior to initial administration, excluding nasal, inhalation, or other routes of local corticosteroids or physiological doses of systemic corticosteroids;
6. There was toxicity caused by previous anti-tumor therapy that did not recover to NCI CTCAE5.0 version ≤ Grade 1 before the first administration;
7. History of other primary malignancies, except the following: non-melanoma skin cancer or malignant lentigo with adequate treatment and no evidence of disease recurrence, carcinoma in situ with adequate treatment and no evidence of disease recurrence;
8. Patients with metastatic central nervous system or cancerous meningitis with clinical symptoms;
9. Hepatic encephalopathy, hepatorenal syndrome or Child-Pugh grade B or more severe cirrhosis;
10. Inability to swallow, intestinal obstruction, or other factors affecting the administration and absorption of the drug;
11. Have serious heart disease or discomfort;
12. The patient has any active autoimmune disease or a history of autoimmune disease within 2 years;
13. Pregnant or nursing female patients;
14. A history of immunodeficiency, or other acquired or congenital immunodeficiency diseases, or a history of organ transplantation;
15. Had undergone major surgery within 4 weeks prior to enrollment or expected to require major surgery during the study treatment;
16. Concurrent participation in another interventional clinical study, unless participating in an observational (non-interventional) clinical study or in the follow-up phase of an interventional study;
17. Has a serious concomitant disease or other comorbidities that interfere with planned treatment;
18. Any other circumstances deemed inappropriate for participation in this study by the investigator.
