The purpose of this study is to assess the safety, tolerability and preliminary antitumor activity of SKB500 combinations in patients with small cell lung cancer. The study is divided into two parts. Part 1 will be the safety run-in phase, and Part 2 will be the cohort expansion phase.

Inclusion Criteria:

1. Male or female participants between 18 and 75 years old.

2. Histologically or cytologically confirmed Extensive-stage small cell lung cancer (ES-SCLC):

   • Cohort 1: participant has received or not received prior systemic treatment, with no more than 1 prior systemic therapy in the extensive stage.
   • Cohort 2~3: participant has received no prior systemic treatment.

3. Agree to provide fresh or archival tumor tissue for biomarker analysis.

4. Has at least one measurable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST) Version 1.1.

5. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.

6. Life expectancy >= 12 weeks.

7. Has adequate organ and bone marrow functions.

8. Has recovered to grade ≤ 1 of prior anti-cancer treatment toxicities.

9. Effective contraceptive methods should be used during the study and for 6 months after the end of treatment.

10. Voluntarily join this study, sign the informed consent form, and can comply with the protocol-specified visits and procedures.

Exclusion Criteria:

1. The pathology suggests the presence of both non-small cell carcinoma and small cell carcinoma components.
2. Has previously received medications with the same target or the same toxins.
3. Presence of spinal cord compression or clinically active central nervous system metastases.
4. Presence of clinical symptoms caused by tumor invasion or compression of important organs and blood vessels.
5. Severe infection within 4 weeks or active infection requiring systemic anti-infective treatment within 2 weeks prior to the first dose.
6. With peripheral neuropathy of grade ≥ 2.
7. History of any serious, life threatening, or permanently discontinuing adverse events mediated by immunotherapy, including infusion reactions.
8. Presence of uncontrolled concurrent diseases, including but not limited to decompensated liver cirrhosis, nephrotic syndrome, uncontrolled metabolic disorders, Severe active peptic ulcer, gastrointestinal bleeding, gastrointestinal perforation, intestinal obstruction, etc.
9. Serious or uncontrolled heart disease or clinical symptoms requiring treatment.
10. History of interstitial lung disease (ILD) /noninfectious pneumonitis that require steroid treatment, or currently has ILD/noninfectious pneumonitis.
11. Clinical severe lung damage caused by concurrent lung disease.
12. Uncontrolled hypertension, diabetes, or repeated drainage of pleural effusion/pericardial effusion/ abdominal effusion.
13. Pregnant or lactating women.
14. Rapid disease deterioration in the screening process.