Additional Study Information: The purpose of this study is to learn about the safety, tolerability, and effectiveness of an investigational drug called milademetan as compared to the use of trabectedin. In this study, subjects will be randomly assigned (like flipping a coin) to receive one of two study drugs: milademetan or trabectedin. This is an open-label study, meaning you will know which study drug you are assigned to receive. The total duration of study participation will vary by subject and is not known in advance. If you join this study, you could take part in a Screening Period, several 28-day (for milademetan) or 21-day (for trabectedin) treatment cycles, an End of Treatment Visit, and several follow-up visits after you stop treatment.