Trial of Topotecan With VX-970 (M6620), an ATR Kinase Inhibitor, in Small Cell Cancers and Extrapulmonary Small Cell Cancers

Background: Chemotherapy damages cancer cell deoxyribonucleic acid (DNA) so the cells die, and the tumor shrinks. But it may stop working in some people over time. This is partly due to efficient DNA damage repair mechanisms used by tumor cells. VX-970 (M6620) may stop cancer cells from preventing the repair of DNA damaged by chemotherapy. The purpose of this study is to see if using the chemotherapy drug topotecan along with the drug VX-970 (M6620) will improve the response to chemotherapy. Objective: To study the safety and efficacy of VX-970 (M6620) and topotecan in treating small cell lung cancer. Eligibility: Adults at least 18 years old with small cell lung cancer. Design: Participants will be screened with medical history, physical exam, blood and heart tests, and scans. Most of these tests are part of their routine care. Most of these tests will be repeated throughout the study. The study is set in 21-day cycles. Participants will get topotecan intravenous (IV) on days 1 through 5. They will get VX-970 (M6620) IV on day 5 alone or on day 5 and day 2. Participants doctors will monitor them weekly for the first cycle, every 3 weeks after that. For Part 1 of this Study the doses of topotecan and VX-970 (M6620) will be increased (according to the Protocol) to determine the maximum safe dose of the combination. The maximum safe dose of the combination is the dose at which no more than 1 in 6 people have an intolerable side effect. More participants will join in Phase 2. They will take the drugs at the maximum safe dose, on the same schedule as the drugs were taken in Phase 1. Participants will give samples of blood, hair, and tumor tissue (optional) at different times. They will discuss side effects at every visit. A month after stopping taking the drugs, participants will have a physical exam and blood drawn. They will have follow-up phone calls every 3 months.