A Phase I/II Study of Sacituzumab Govitecan Plus Berzosertib in Small Cell Lung Cancer, Extra-Pulmonary Small Cell Neuroendocrine Cancer and Homologous Recombination-Deficient Cancers Resistant to PARP Inhibitors

Background: Small cell lung cancer and PARP inhibitor resistant tumors are aggressive cancers. Current treatments for people with these tumors yield little benefit. Researchers want to see if a combination of drugs can help. Objective: To find a safe combination of sacituzumab govitecan and berzosertib and to see if this will cause small cell lung cancer and PARP inhibitor resistant tumors to shrink. Eligibility: People ages 18 and older with a solid tumor, small cell lung cancer, or a homologous recombination-deficient cancer that is resistant to PARP inhibitors Design: Participants will be screened with: Standard clinical exams and tests EKG to test the heart Medical documentation to confirm cancer diagnosis Participants will get sacituzumab govitecan by vein on days 1 and 8 of each 21-day cycle. They will get berzosertib by vein on days 2 and 9. Treatment will continue as long as they can tolerate the drugs and their tumors are either stable or getting better. Before treatment and at least once per cycle, participants will have a physical exam and blood tests. Before treatment and every 2 or 3 cycles, they will have a CT scan. They will have a contrast agent injected into a vein for the scan. Participants will give blood and hair samples and tumor biopsies for research. Biopsies will be taken with a small needle under imaging guidance. After they stop treatment, participants will have a visit 1 month later. They will then be contacted by phone or email every 3 months for the rest of their lives.