Trial of EP0057, a Nanoparticle Camptothecin With Olaparib in People With Relapsed/Refractory Small Cell Lung Cancer

Background: EP0057 (formerly CRLX101) consists of a sugar molecule cyclodextrin linked to a chemotherapy drug called camptothecin. The combined molecule or "nanoparticle drug conjugate" travels through the blood. Once inside cancer cells, the chemotherapy drug is released from the molecule. Olaparib is a drug that may stop cancer cells from repairing the deoxyribonucleic acid (DNA) damage caused by chemotherapy. Researchers want to see how safe it is to give EP0057 and olaparib together and to see how well the combination treats a specific type of lung cancer called small cell lung cancer (SCLC). Objectives: To test the safety and maximum dose of EP0057 and olaparib together. To test how well they treat small cell lung cancer. Eligibility: Adults 18 and older with small cell lung cancer. Design: Participants will be screened with standard cancer care tests. Participants will get the 2 study drugs in 28-day cycles. EP0057 will be given every 2 weeks, through a small plastic tube in an arm vein. Olaparib will be taken by mouth twice a day most days. Participants will keep a pill diary. For Cycle 1, participants will have 3 visits. All other cycles will have 2 visits. At study visits, participants may have: * Blood and hair samples taken * History and Physical exam * Questions about health and side effects * Pregnancy test * Optional tumor biopsy where a piece of tumor is removed by needle after numbing the skin. * Computed tomography (CT) scan * Injection of EP0057 (twice per cycle) * Olaparib prescription Participants will have a follow-up visit 4 weeks after finish taking the drugs. They will have a physical exam and blood tests. They may have a tumor biopsy. The study team will call the patient every 3 months for follow up after completing the study treatment.