Lurbinectedin + Doxorubicin In Leiomyosarcoma

This is an open label phase 1b/2 study exploring the safety and efficacy of lurbinectedin with doxorubicin. \- The phase 1b trial will follow a standard 3+3 design. Upon determination of the maximum tolerated dose (MTD) and recommended phase 2 dose (RP2D) of lurbinectedin plus doxorubicin, the randomized phase 2 trial in participants with advanced leiomyosarcoma (LMS) will commence. Participants will be randomized 1:1 for enrollment to one of two treatment arms: * Arm 1: Lurbinectedin with Doxorubicin * Arm 2: Doxorubicin Monotherapy This is a Phase Ib/II clinical trial. A Phase Ib clinical trial tests the safety of an investigational drug combination and also tries to define the appropriate dose of the investigational drugs to use for further studies. "Investigational" means that the drugs are being studied. The U.S. Food and Drug Administration (FDA) has not approved lurbinectedin for this specific disease but it has been approved for the treatment of small cell lung cancer. The U.S. FDA has approved doxorubicin as a treatment option for soft-tissue sarcoma. Lurbinectedin is a synthetically derived chemical that binds to DNA. The binding to DNA likely induces cell death. Doxorubicin is a cytotoxic chemotherapy drug that kills cancer cells by several mechanisms including binding to DNA.