This study is being done to learn more about a new medicine called PF-08634404. The study team wants to understand how well PF-08634404 works when given alone or with chemotherapy . Chemotherapy is a type of cancer treatment that uses medicines to destroy cancer cells or stop them from growing. The study is for adults with Transformed Small Cell Lung Cancer (T-SCLC ). T SCLC is a rare lung cancer that happens when one type of lung cancer changes into a more aggressive type after treatment stops working.

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

• Are aged 18 years or older
• Diagnosed with T-SCLC and have not received treatment for this type of lung cancer (a single cycle of chemotherapy may be permitted)
• Prior diagnosis of epidermal growth factor receptor (EGFR)-mutated non-small cell lung cancer treated with tyrosine kinase inhibitors (TKIs)
• Have healthy organs based on medical tests and are in good physical condition

After joining the study, adults will be given chemotherapy in addition to the study medicine. After this combination treatment is finished, the study medicine will be continued alone. Adults will receive the treatment through IV infusions (medicine given directly into a vein). All treatments will be done at clinical study sites, where a trained medical team will monitor adults during and after each visit.

Inclusion Criteria:

• Male or female participants aged ≥18 years at the time of informed consent.
• Histologically or cytologically confirmed T-SCLC. Participant must have had a prior diagnosis of NSCLC with EGFR mutation which transformed to SCLC following the treatment with TKI(s).
• Participants have not received systemic therapy for T-SCLC.
• Have at least one measurable lesion as the target lesion based on RECIST v1.1.
• Have sufficient tumor tissue from the diagnosis of transformed SCLC available.
• Eastern Cooperative Oncology Group performance status of 0 or 1.
• Have a minimum life expectancy of >12 weeks.
• Clinical laboratory values at screening within acceptable limits, as defined in the protocol, including: 1) Hematology, 2) Liver function and 3) Renal function.

Exclusion Criteria:

Participants are excluded from the study if any of the following criteria apply:

• Active or untreated CNS disease, including brain, brainstem, spinal cord, or meningeal metastases. Participants with definitively treated, clinically stable brain metastases may be eligible per protocol criteria. Participants with untreated asymptomatic brain metastases of longest diameter <1 cm are permitted if all of the following criteria are met: absence of neurological symptoms, no need for corticosteroids, and brain metastasis has no evidence of edema or hemorrhagic features.

• Leptomeningeal disease

• Clinically significant risk of hemorrhage or fistula, including tumor necrosis/cavitation, invasion or compression of major blood vessels, airways, or critical organs, or risk of tracheoesophageal or pleuroesophageal fistula

• History of another malignancy (other than NSCLC) within 3 years prior to first dose, except for malignancies with negligible risk of metastasis or death (eg, adequately treated carcinoma in situ, nonmelanoma skin cancer)

• Unresolved toxicity from prior anti-tumor therapy that has not recovered to Grade ≤1 per NCI CTCAE v5.0 (except alopecia or irreversible toxicities deemed stable)

• History of allogeneic organ or hematopoietic stem cell transplantation

• Active autoimmune disease requiring systemic treatment within the past 2 years (Stable replacement therapy and selected low-risk autoimmune conditions are permitted per protocol)

• Interstitial lung disease (ILD), pneumonitis, or significant pulmonary disease, including:

  • Prior or current non-infectious pneumonitis requiring systemic therapy
  • DLCO <50% predicted
  • Severe asthma, COPD, pulmonary embolism, or autoimmune lung involvement

• Uncontrolled or clinically significant cardiovascular, cerebrovascular, metabolic, hepatic, or renal disease within 6 months prior to first dose

• Baseline QTcF >480 msec

• Major surgery or severe trauma within 4 weeks prior to first dose, or planned major surgery during the study

• Clinically significant pleural effusion, pericardial effusion, or ascites requiring repeated drainage

• History of significant bleeding disorders or recent major bleeding events

• Clinically significant gastrointestinal conditions, including recent perforation, fistula, obstruction, or active bleeding

• Active, uncontrolled, or symptomatic infection, including:

  • Active TB
  • Active hepatitis B or C
  • Uncontrolled HIV infection

• History of immunodeficiency

• Severe hypersensitivity or allergic reactions to study intervention components or monoclonal antibodies

• Psychiatric illness or medical condition, including recent suicidal ideation or behavior, that may increase risk or interfere with study participation

• Prior anti-angiogenic therapy or other prohibited anti-tumor or immunomodulatory therapies per protocol-specified washout periods

• Use of prohibited concomitant medications, including high-dose systemic corticosteroids, certain anticoagulants, or live vaccines within protocol-specified timeframes

• Recent participation in another investigational study (within 30 days or 5 half-lives, whichever is longer)

• Pregnant or breastfeeding participants, or unwillingness to comply with contraception requirements

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.