This study looks at using a medicine called navtemadlin for patients with a type of cancer called endometrial cancer (EC). This cancer starts in the lining of the uterus. The study is for those who have advanced or came back after treatment and had a good response to chemotherapy (using strong drugs to kill cancer cells). Navtemadlin is tested as a maintenance treatment, which means it helps keep cancer from coming back after initial treatment. The study has two parts. In Part 1, different doses of navtemadlin will be tested to find the best one. Part 2 will compare the best dose of navtemadlin to a placebo (a fake treatment). To join, patients must have a specific type of EC called TP53WT, have good overall health, and have completed certain chemotherapy.

• The study is in two parts and uses different doses of navtemadlin.
• Participants must have specific cancer type and be in good health.
• The study compares navtemadlin to a placebo in Part 2.