The main purpose of this study is to evaluate the safety and tolerability of IBI3009 and to determine the maximum tolerated dose (MTD) and/or the recommended dose for expansion (RDE) of IBI3009.

Inclusion Criteria:

1. Participants have the ability to understand and give written informed consent for participation in this trial, including all evaluations and procedures as specified by this protocol;
2. Male or female subjects ≥ 18 years old. For Part 1, age ≥18 years and ≤75 years;
3. Eastern Cooperative Oncology Group (ECOG) Performance Status score of 0-1;
4. Minimum life expectancy of ≥ 12 weeks;
5. Adequate organ function confirmed at screening period;
6. Histologically or cytologically confirmed unresectable,metastatic or Extensive-Stage small cell lung cancer (SCLC).

Exclusion Criteria

1. Participating in any other interventional clinical research except observational (non-interventional) study or in the follow-up phase of an interventional study;
2. Has adverse reactions resulting from previous anti-tumor therapies, which have not resolved to Grade 0 or 1 toxicity according to NCI CTCAE v5.0 (except for alopecia, fatigue, pigmentation and other conditions with no safety risk according to investigator's discretion) prior to the first dose of the study drug;
3. Known allergies, hypersensitivity, or intolerance to IBI3009 or its excipients;
4. Undergone major surgery (Craniotomy, thoracotomy or laparotomy, and other surgery according to investigator's discretion, excluding needle biopsy) within 4 weeks prior to the first dose of the study drug, or who are expected to undergo major surgery during the study period, or who have severe unhealed wounds, trauma, ulcers, etc.;
5. Women who are pregnant, have positive results in pregnancy test or are lactating;
6. Not eligible to participate in this study at the discretion of the investigator.