A Phase I Study to Evaluate the Safety and Preliminary Efficacy of [225Ac]Ac-DOTATATE Injection Combined With Tislelizumab in the Maintenance Treatment Period for Patients of Extensive-stage Small Cell Lung Cancer (ES-SCLC) With Somatostatin Receptors (SSTR)+ as First-line Treatment

Inclusion Criteria

• •1. Patients must have the ability to understand and sign an approved informed consent form (ICF).
• •2. Patients must be \>= 18 and \<＝80 years of age.
• •3. Extensive-stage small cell lung cancer that requires histopathological or cytological confirmation.
• •4. Presence of at least 1 measurable site of disease (based on RECIST 1.1).
• •5. ECOG score of 0 or 1.
• •6. SSTR-PET positive.
• •7. Sufficient bone marrow capacity and organ function:
• •Serum creatinine ≤1.5×ULN or creatinine clearance ≥50 ml/min (Cockcroft Gault formula).
• •Hemoglobin≥90g/L, neutrophil count ≥1.5×10\^9/L, platelets≥100×10\^9/L. Serum total bilirubin ≤1.5×ULN. Serum albumin ≥30g/L. Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≤ 2.5×ULN，or ALT/AST≤5×ULN with liver metastases.
• •Partially activated prothrombin time (APTT) ≤1.5 x ULN.
• •8. Subjects of childbearing potential voluntarily use an effective method of contraception, such as condoms, oral or injectable contraceptives, IUDs, etc., during treatment and within 6 months of the last use of the trial drug.