This is an open-label, multi-center Phase II study designed to evaluate the efficacy and safety of Surufatinib plus anti-PD-1/L1 as maintenance therapy after first-line standard of care in patients with ES-SCLC.

This study consists of two parts, i.e., a single-arm Phase IIa study and a randomized, controlled Phase IIb study. All the enrolled patients will be patients with ES-SCLC who did not have PD (determined as per the RECIST v1.1) after prior platinum-based chemotherapy in combination with an anti-PD-1/L1. Approximately 20 patients are planned to be enrolled in the single-arm Phase IIa study to preliminarily observe the efficacy and safety. Approximately 40 patients are planned to be enrolled in the randomized and controlled Phase IIb study with 20 patients per group.

Inclusion Criteria:

1. Patients who sufficiently understand this study and is willing to sign the ICF;
2. Aged from 18 to 75 years (inclusive);
3. Patients with histologically or cytologically confirmed ES-SCLC ;
4. Patients who did not have PD (assessed as per the RECIST v1.1) after prior first-line platinum-based chemotherapy plus an anti-PD-1/L1;
5. Patients with measurable lesions as defined in the RECIST v1.1 ;
6. Life expectancy ≥ 12 weeks;
7. Eastern Cooperative Oncology Group (ECOG) Performance Status score of 0 to 1;
8. Adequate organ function:
9. Females of childbearing potential must have a negative serum pregnancy test within 7 days prior to the first dose. Male or female patients of childbearing potential should be willing to use effective contraceptive methods during the study and within 90 days after last dose of the study drug.

Exclusion Criteria:

1. Previous use of anti-vascular endothelial growth factor (VEGF) drugs/anti-vascular endothelial growth factor receptor (VEGFR) agents;
2. Use of systematic anti-tumor therapies other than first-line platinum-based chemotherapy in combination with anti-PD-1/L1 within 4 weeks prior to the first dose;
3. Presence of central nervous system (CNS) metastasis and/or cancerous meningitis (metastases to meninges);
4. Toxicities associated with previous anti-tumor treatment that has not been resolved to CTCAE Grade ≤ 1, except for alopecia and CTCAE Grade ≤ 2 peripheral neurotoxicity;
5. Uncontrollable malignant hydrothorax, ascites or pericardial effusion;
6. Patients with active autoimmune disorder or immunodeficiency or history of autoimmune disorder or immunodeficiency;
7. Patients with evidence or history of obvious bleeding tendency within 2 months prior to the first dose;
8. Clinically significant cardiovascular disorders;
9. Severe infection within 4 weeks prior to the start of study treatment, including but not limited to hospitalization for infection, bacteraemia or severe pneumonia;
10. Patients who are unable to take oral medications, or with previous surgical history or severe gastrointestinal;
11. Pregnant (with a positive pregnancy test) or lactating women within 5 years before screening.
12. Patients with a history of malignant tumors excluding small cell lung cancer (SCLC) within 5 years before screening.