Additional Study Information: The purpose of this study is to evaluate the effectiveness and safety of an investigational drug, unesbulin. This investigational drug is being studied in unresectable or metastatic, relapsed, or refractory leiomyosarcoma along with Dacarbazine. Dacarbazine is a drug that is FDA-approved for the treatment of other types of cancer and recommended by the cancer treatment guidelines for sarcomas, but is not FDA-approved to treat leiomyosarcoma.