The purpose of this study was to evaluate the efficacy of sugemalimab consolidation therapy versus placebo in patients with LS-SCLC who had not progressed following Concurrent or Sequential Chemoradiotherapy.

This study is planned to enroll approximately 346 eligible patients in China. Patients who fulfill all the inclusion criteria and none of the exclusion criteria will be randomized in a 1:1 ratio to the experimental and control arm. The experimental arm would receive sugemalimab, while the control arm would receive placebo.Tumor assessment will be conducted using computed tomography/magnetic resonance imaging and MRD detection will be performed along with radiologic review during each visit. The above assessment will be conducted every 12 weeks for the first 2 years, every 24 weeks for the third year, and annually thereafter until confirmed disease progression per RECIST 1.1 or death.The primary endpoint is PFS ,which is defined as the time from the date of randomization to the date of disease progression (per RECIST 1.1), any cause of death, loss to follow-up, or initiation of new antitumor therapy, whichever occurred first.

Inclusion Criteria:

1、18 years or older. 2、Histologically or cytologically confirmed small cell lung cancer. 3、ECOG PS=0-1 at enrollment; 4、Limited-stage SCLC (Stage I-III, by AJCC 8th Edition Cancer Staging), and can be safely treated with definitive radiation doses).

5、Inoperable SCLC, or the patient has contraindications to surgery, or the patient refuses surgery.

6、Completion of 4 cycles of chemotherapy (etoposide + carboplatin/cisplatin) concurrent or sequential with radiotherapy during the first two cycles of chemotherapy.

7、After the completion of concurrent or sequential chemoradiotherapy, prophylactic intracranial irradiation (PCI) is allowed based on the common practice of individual sites.

8、The start of concurrent radiotherapy should be no later than the last day of the second course of chemotherapy (the first day of the third cycle of chemotherapy). The interval between the end of chemotherapy cycle and beginning of radiotherapy must not exceed 35 days for sequential chemoradiotherapy.

9、Radiotherapy must be either total 60-66Gy over 6 weeks for the standard qd regimen or total 45Gy over 3 weeks for hyperfractionated bid schedules.

10、Absence of progression after concurrent/sequential chemoradiotherapy (responses should be complete response [CR], partial response [PR] and stable disease [SD]); 11、For patients not receiving PCI, the first dose of sugemalimab shall be administered within 42 days after the completion of chemoradiation therapy. For patients receiving PCI, the first dose of sugemalimab shall be administered within 56 days after the completion of chemoradiation therapy.

12、Life expectancy ≥ 12 weeks. 13、Can provide tumor tissue samples (fresh or archived) for whole exome sequencing; 14、Women of childbearing potential and fertile men must agree to use an effective contraceptive method from signing the master ICF until 180 days after the last dose of investigational product. Women of childbearing potential include premenopausal women and women who became menopausal less than 2 years ago. Women of childbearing potential must have a negative pregnancy test ≤7 days prior to the first dose of investigational product.

15、The subject should have good compliance, who would participate in the research voluntarily, and sign the informed consent.

Exclusion Criteria:

1. Histologically or cytologically diagnosed mixed small cell lung cancer or non-small cell lung cancer.
2. Extensive-stage small cell lung cancer.
3. Has malignant pleural or pericardial effusion.
4. Previously received systemic anti-tumor therapy for SCLC or anti-tumor therapy with immune checkpoint inhibitors.
5. Subjects with active, unstable systemic diseases, such as active infection, uncontrolled hypertension, heart failure (NYHA class >= II), unstable angina pectoris, acute coronary syndrome, severe arrythmia, severe liver, kidney or metabolic diseases, HIV infection.
6. Has history of interstitial lung disease (ILD), drug-induced ILD, or active ILD which required systemic glucocorticoid or immunosuppressive therapy.
7. History of other malignancies within 5 years (excluding basal cell carcinoma of the skin or other carcinoma in situ that has been resected).
8. Pregnant or lactating women.
9. Those who are allergic to the research drug or its components.
10. Subjects who are deemed unable to comply with the study requirements or complete the study.
11. Those with insufficient function of bone marrow or other important organs.