A Randomized, Double-Blind, International Multicenter, Phase III Study to Evaluate the Anti-Tumor Efficacy and Safety of HLX10 (Recombinant Humanized Anti-PD-1 Monoclonal Antibody Injection) or Placebo in Combination with Chemotherapy (Carboplatin/Cisplatin-Etoposide) and Concurrent Radiotherapy in Patients with Limited-Stage Small Cell Lung Cancer (LS-SCLC)

Patients who voluntarily participate in this clinical study; fully understand and have been informed about the study and have signed the ICF; are willing to follow and able to complete all trial procedures.

Female patients must meet one of the following conditions: a. Menopause (defined as no menstruation for at least 1 year with no confirmed cause other than menopause), or b. Surgically sterilized (removal of the ovaries and/or uterus), or c. Fertile, but must: o be tested negative for serum/urine pregnancy test within 7 days prior to the randomization, and o agree to use contraception methods with an annual failure rate of < 1% or to remain abstinent (avoid heterosexual intercourse from signing the ICF to at least 6 months after the last dose of the study drug) (a contraceptive method with an annual failure rate of <1% includes bilateral tubal ligation, male sterilization, correct use of hormonal contraceptives that can inhibit ovulation, hormone-releasing intrauterine devices and copper-containing intrauterine devices or condoms), and o not breastfeed

Male patients must: agree to remain abstinent (avoid heterosexual intercourse) or take contraception measures as follows: male patients with a pregnant partner or a partner of childbearing potential must remain abstinent or use condoms to prevent drug exposure to the embryo during study treatment and for at least 6 months after the last dose of study drug. Periodic abstinence (e.g., contraception based on calendar day, ovulatory phase, basal body temperature, or postovulatory phase) and external ejaculation are ineligible methods of contraception.

Previous non-systematic anti-tumor treatment should be completed ≥2 weeks prior to the initiation of study medication, and treatment related AEs have returned to ≤grade 1 based on Common Terminology Criteria for Adverse Events (CTCAE) 5.0 (except grade 2 hair loss).

Male or female, aged ≥18 years when signing the ICF.

Histologically diagnosed with SCLC.

Diagnosed with LS-SCLC (stage 1-3 of the AJCC 8th edition of the cancer staging), which can be safely treated with curative radiation doses.

With at least one measurable lesion as assessed by investigator as per RECIST v1.1 within 4 weeks prior to randomization.

Patients must provide tumor tissues that meet the requirements for assay of PDL1 expression level. Patients are assessed for an evaluable PD-L1 expression category (negative: TPS < 1%, positive: TPS ≥ 1%, or not evaluable/not available) by the central laboratory.

ECOG PS of 0 or 1.

Expected survival of at least 6 months.

Laboratory tests verified sufficient organ and marrow function,without serious abnormalities in haematopoietic function or cardiac, hepatic, or renal function, or immunodeficiency within 7 days prior to randomization.

Histologically or cytologically confirmed mixed SCLC.

Patients with peripheral neuropathy ≥ grade 2 by CTCAE.

Patients with human immunodeficiency virus (HIV) infection, and HIV antibody test results are positive.12. Patients with active pulmonary tuberculosis.

Patients with active pulmonary tuberculosis.

Subjects with previous and current interstitial pneumonia, pneumoconiosis, radiation pneumonitis, drug-related pneumonitis, and severe impaired pulmonary function that may interfere with the detection and management of suspected drug-related pulmonary toxicity as judged by the investigator.

With Hepatitis B (positive test for HBsAg or HBcAb and positive test for HBVDNA) or Hepatitis C (positive tests for HCV antibody and HCVRNA). Subjects with a co-infection of hepatitis B and hepatitis C (tested positive for HBsAg or HBcAb, and positive for anti-HCV antibody). Note: Subjects with hepatitis B who are stable on antiviral therapy (HBVDNA≤ 2500 copies/mL or 500IU/mL) can be enrolled.

Subjects with known active or suspected autoimmune diseases. Subjects in a stable state with no need for systemic immunosuppressant therapy are allowed to be enrolled.

Have received treatment with live vaccines within 28 days prior to the first administration. Subjects may receive inactivated viral vaccines for seasonal influenza, but may not receive live attenuated influenza vaccines via intranasal route.

Subjects requiring treatment with systemic corticosteroids (> 10 mg/day therapeutic dose of prednisone) or other immunosuppressive drugs within 14 days prior to the first dose or during the study. However, subjects are allowed to be enrolled under the following conditions: in the absence of active autoimmune disease, subjects are allowed to use topical or inhaled glucocorticoids and ≤ 10 mg/day therapeutic dose of prednisone for adrenal glucocorticoid replacement therapy.

With any active infection requiring systemic anti-infective treatment within 14 days prior to the administration of the investigational product.

Have received any major surgery (defined as surgeries requiring at least 3 weeks of recovery to be able to receive treatment in this study) within 28 days prior to the first dose of the investigational products.

Subjects suitable for surgery. Subjects who are suitable for surgery but refuse surgical treatment can be included.

The subject has previously received other antibodies/drugs against immune checkpoints, such as PD-1, PD-L1, CTLA4, etc.

Participation in any other ongoing interventional clinical studies, or less than 28 days from the end of the previous interventional clinical study treatment to the start of this trial.

Subjects with known history of severe allergy to any monoclonal antibody.

Subjects with known anaphylaxis to carboplatin/cisplatin or etoposide.

Pregnant or lactating women.

Subjects with a known history of psychotropics substance abuse or drug abuse.

In the judgment of the investigator, subjects who have any other factors that may lead to a premature discontinuation.

Subjects expected to require surgical resection during the study.

Primary tumor/lymph node too large for planned radiotherapy.

Patients who have previously received systematic anti-tumor treatments for small cell lung cancer, including but not limited to radiotherapy, chemotherapy, and immunotherapy.

Patients with other active malignancies within 5 years or at the same time. Localized tumors that have been cured such as basal cell carcinoma, squamouscell skin cancer, superficial bladder carcinoma, prostate carcinoma in situ, cervical carcinoma in situ, and breast cancer in situ are acceptable.

Patients who are preparing for or have received an organ or bone marrow transplant.

Patients with pleural, pericardial effusions, or ascites requiring clinical intervention.

Patients with myocardial infarction and poorly controlled arrhythmia (including QTc intervals ≥ 470 ms) (QTc intervals are calculated by Fridericia's formula) within 6 months prior to the first dose of the investigational products.

Class III to IV cardiac insufficiency according to NYHA classification or an left ventricular ejection fraction < 50% by cardiac color Doppler.

Subject has uncontrolled or symptomatic hypercalcemia (> 1.5 mmol/L ionized calcium or calcium > 12 mg/dL or corrected serum calcium > ULN).