This study is being done to learn more about a new medicine called PF-08634404 and how well it works when given with chemotherapy to adults with extensive-stage small cell lung cancer (ES-SCLC), a fast-growing type of lung cancer that has spread widely in the body.

To join the study, participants must meet the following conditions:

• Be 18 years or older.
• Have extensive-stage small cell lung cancer confirmed by lab tests.
• Have not received chemotherapy or radiation for this type of lung cancer.
• Be in good physical condition and have healthy organs based on medical tests.

The study has two parts:

• In the first part, researchers will check how safe the study medicine is and how well people tolerate it when given with chemotherapy.
• In the second part, they will compare study medicine plus chemotherapy to another approved treatment (atezolizumab plus chemotherapy) to see which works better.

Participants will receive the treatment through IV infusions (medicine given directly into a vein). The treatment will be given in repeated time periods called cycles. Some participants will continue receiving the study medicine alone after the initial treatment.

Inclusion Criteria:

• Histologically or cytologically confirmed extensive-stage small cell lung cancer (ES-SCLC).
• Participants have not received systemic therapy (chemotherapy, radiotherapy, chemoradiation) for ES-SCLC.
• Treatment-free for at least 6 months since last chemo/radiotherapy, among those treated (with curative intent) with prior chemo/radiotherapy for limited-stage SCLC
• Have at least one measurable lesion as the targeted lesion based on RECIST V1.1.
• Eastern Cooperative Oncology Group performance status of 0 or 1.
• Adequate organ function

Exclusion Criteria:

• known active CNS lesions, including brainstem, meningeal, or spinal cord metastases or compression
• Leptomeningeal disease
• Clinically significant risk of hemorrhage or fistula
• history of another malignancy within 3 years
• active autoimmune diseases requiring systemic treatment within the past 2 years

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.

Phase 2 is open-label, whereas Phase 3 is double-blind, randomized design.