Inclusion Criteria:
1. Men or women aged more than or equal to (≥) 18 years old and less than or equal to (≤) 75 years old
2. Histologically or cytologically confirmed ES-SCLC
3. No prior treatment for ES-SCLC
4. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
5. Five white slides (unstained paraffin sections) were available for immunohistochemical SLFN-11 detection and SLFN-11 was positive
6. Extensive clinical stage. American Joint Committee on Cancer (AJCC) 8th edition Stage IV with lesions exceeding one side of the chest and including malignant pleural and pericardial effusions or hematogenous metastases (any T, any N, M1a/b/c); or T3-4 due to multiple nodules in the lung or tumor/nodule size too large to be included in a T3-4 disease within a tolerable radiotherapy schedule
7. The subjects were considered suitable for combining etoposide with cisplatin chemotherapy and low-dose radiotherapy as first-line treatment for extensive -stage small cell lung cancer
8. Have measurable lesions as defined by RECIST1.1, with at least one lesion (never previously treated with radiation) of ≥10 mm longest diameter accurately measured by computed tomography (CT) or magnetic resonance imaging (MRI) at baseline (except for lymph nodes, which must have a short axis of ≥15 mm) and the lesion is suitable for repeat and accurate measurements
9. Patients with brain metastases must be asymptomatic or stable on steroids and anticonvulsants for at least 1 month prior to study treatment. Participants with suspected brain metastases during screening should have a CT/MRI of the brain prior to study
10. No previous treatment with immune checkpoint inhibitors and PARP inhibitors, including but not limited to other anti-PD-1, anti-PD-L1 and anti-programmed cell death ligand 2 (anti-PD-L2) antibodies, with the exception of therapeutic anti-tumor vaccines. No prior chemotherapy or radiation therapy to the chest lesion
11. Weight over 30 Kg
12. Life expectancy ≥ 12 weeks
13. Have adequate organ and bone marrow functional reserve and normal major organ function
14. Patients were compliant, voluntarily enrolled in the study and signed an informed consent form
15. For women or men with childbearing potential: use effective contraception to avoid conception or embryonic drug exposure during treatment and for 5 months after the last dose of sugemalimab and for 6 months after the last dose of cisplatin or etoposide. Female subjects are prohibited from donating eggs during this period and males are prohibited from donating sperm during this period
Exclusion Criteria:
1. Histopathologic or cytopathologic diagnosis of mixed small cell lung cancer or non-small cell lung cancer
2. Limited stage small cell lung cancer
3. Combination of poorly controlled malignant pleural or pericardial effusions requiring continuous drainage
4. Presence of active or symptomatic brain metastases or Leptomeningeal metastases
5. Prior systemic antitumor therapy (chemotherapy, targeted agents such as PARP inhibitors) or immune checkpoint inhibitors for SCLC
6. Presence of active, unstable systemic disease such as active infection, autoimmune disease, inflammatory disease (uncontrolled hypertension, heart failure (NYHA classification \>= Class II), unstable angina, acute coronary syndrome, severe arrhythmia, severe hepatic, renal or metabolic disease, human immunodeficiency virus (HIV) immunodeficiency virus (HIV) infected patients
7. Previous allogeneic stem cell or solid organ transplantation
8. Patients with prior interstitial lung disease, drug-induced interstitial lung disease, or active interstitial pneumonia requiring systemic glucocorticoid or immunosuppressive therapy; Patients with pulmonary interstitial fibrosis or active pulmonary tuberculosis
9. Severe infection within 4 weeks prior to the start of study treatment, including but not limited to hospitalization for infection, bacteremia, or severe pneumonia
10. Received therapeutic oral or intravenous infusion of antibiotics within 2 weeks prior to the start of study treatment
11. Been diagnosed or treated for another malignancy (excluding resected basal cell carcinoma of the skin or other carcinoma in situ) within 5 years prior to randomization to this study
12. For pregnant or lactating females or male or female subjects of reproductive potential who refuse to use effective contraception during treatment and within 5 months of the last dose of sugemalimab and within 6 months of the last dose of cisplatin or etoposide
13. Allergic to the study drug or its components
14. The investigator believes that the patients may not be able to complete the study or comply with the requirements of the study
15. Inadequate bone marrow function and vital organ function reserve
16. Concurrent participation in another clinical study, unless it is an observational (non-interventional) clinical study or a follow-up phase of an interventional study, excluding patients who have received any other experimental drug within 28 days prior to the start of study treatment
17. Patients who are not suitable for etoposide-cisplatin chemotherapy, sugemalimab or olaparib
18. History of thoracic radiotherapy or plan to receive intensive thoracic radiotherapy prior to systemic therapy. External chest radiotherapy for palliative purposes (e.g., bone metastases) is allowed, but must be completed prior to the first administration of study drug
