This is a phase Ib/II, multicenter, open-label study of YL201 combined with atezolizumab. The study will include 2 parts. Part 1 of the study is a dose escalation in participants with previously untreated ES-SCLC to determine the safety and tolerability of YL201 in combination with fixed dose of atezolizumab. The planned dose levels of YL201 are 1.2 mg/kg, 1.6 mg/kg and 2.0 mg/kg. Part 2 consists of a dose optimization stage followed by a dose expansion stage. During the dose optimization stage, participants will be randomized 1:1:1 to receive either YL201 at 1.2 mg/kg,1.6 mg/kg or 2.0 mg/kg Q3W in combination with fixed dose of atezolizumab. The decision to initiate the dose expansion stage in Part 2 and choose one or two of the YL201 dose level(s) will be based on the review of safety, PK, and efficacy from the dose optimization stage. Treatment will continue until disease progression, unacceptable toxicity, or withdraw of consent.

Inclusion Criteria:

1.18 years of age or older. 2. Histologically or cytologically confirmed diagnosis of ES-SCLC 3. Eastern Cooperative Oncology Group performance status of 0 or 1 4. Adequate hematologic and end-organ function

Exclusion Criteria:

1. No prior systemic anti-cancer treatment for ES-SCLC
2. No prior treatment targeting B7H3 or topoisomerase I inhibitor
3. No clinically active brain metastases or spinal cord compression
4. No current or history of interstitial lung disease (ILD)/ pneumonitis
5. No clinically significant cardiovascular disease (eg, New York Heart Association class II to IV congestive heart failure)

Other protocol-defined inclusion/ exclusion criteria may apply.