This study is testing a new drug called **INCB123667** for people with advanced solid tumors, which are types of cancer that grow in organs. The study has two parts. First, they will find out the best dose of the drug that can be safely given to patients. Second, they will use that dose to see how well the drug works on different types of cancer, like ovarian or breast cancer. Participants in the study will take the drug alone, without other treatments. **Eligibility**: Adults in France, aged 18-99, can join if they have certain cancers and no other treatment options. They should be able to perform daily activities and have a life expectancy of at least 12 weeks. **Not Eligible**: People with serious heart problems, untreated brain issues, or active infections cannot join. Also, if you have had certain treatments or surgeries recently, you might not qualify. **Key Points**: - The study involves regular visits to the research site to monitor safety and the drug's effects. - You might be asked to provide tumor samples before and during the study. - Pregnant women or those planning to be pregnant cannot participate. This study aims to find a safe and effective dose of the drug for treating these cancers.