Study of PLK1 Inhibitor, Onvansertib, in Relapsed Small Cell Lung Cancer

This study tests a drug called onvansertib (a PLK1 inhibitor) in people with small cell lung cancer that has come back after previous treatment. PLK1 is a protein that cancer cells depend on to divide; blocking it may slow or stop tumor growth. **You may be eligible if...** - You have been diagnosed with small cell lung cancer - Your cancer has returned or stopped responding to prior treatment (relapsed or refractory) - You have received at least one prior line of therapy - You are 18 or older with adequate organ function **You may NOT be eligible if...** - You have untreated brain metastases (cancer spread to the brain) - You have severe heart, liver, or kidney problems - You are pregnant or breastfeeding - You have had certain other cancers recently - You are taking medications that strongly interact with the study drug Talk to your doctor to see if this trial is right for you.