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.