Symbiotic-Lung-14: A Study to Learn About the Study Medicine Called PF08634404 in Combination With Chemotherapy in Adult Participants With Transformed Small Cell Lung Cancer

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