Background: Single agent or combination chemotherapy regimens, typically including doxorubicin or gemcitabine, represent standard of care options for first- and second-line therapy in patients (pts) with metastatic LMS. However, response rates (ORR) and progression-free (PFS)/overall survival (OS) remain poor. Lurbinectedin (PharmaMar S.A. and Jazz Pharmaceuticals) uniquely binds DNA, inducing DNA double strand breaks leading to apoptosis and delaying progression through phase S/G2 of the cell cycle. Lurbinectedin is a novel structural analog of trabectedin with improved toxicity profile, potency, and pharmacokinetics. In a prior pilot study, we showed that the combination of lurbinectedin and doxorubicin (L+D) was safe, with early signs of clinical activity, particularly in LMS. Thus, we designed this investigator-initiated/investigator-sponsored phase 1b lead-in to optimize doses of L+D, to be followed by a randomized (1:1) phase 2 study of L+D versus doxorubicin monotherapy in anthracycline-naïve LMS. Methods: Pts age > 18 years with locally advanced or metastatic, unresectable LMS (non-GIST soft-tissue sarcoma histologies allowed in Phase 1b), without prior anthracycline or lurbinectedin/trabectedin, ECOG PS < 3, measurable disease by RECIST 1.1, and normal organ function, are eligible. Phase 1b dosing will include a fixed dose of lurbinectedin and two dose levels of doxorubicin. The Phase 1b lead-in follows a standard 3+3 design where dose escalation will occur if 0/3 or 1/6 patients experience a dose-limiting toxicity (DLT). Tumor assessments are conducted every two cycles. Once the recommended phase 2 dose (RP2D) is confirmed, Phase 2 will be initiated. Fifty pts will be randomized 1:1 including doxorubicin +/- lurbinectedin. Randomization will be stratified by uterine v. non-uterine origin of LMS. Pts progressing on single agent doxorubicin will be allowed to cross over to lurbinectedin monotherapy. The Phase 2 primary endpoint is PFS. Secondary endpoints include disease control rate, ORR, OS, PFS2 (for doxorubicin monotherapy patients who cross to lurbinectedin monotherapy). Archival tumor, germline DNA, and ctDNA will be collected for correlative studies exploring genomic markers of sensitivity/resistance. We will provide respective point estimate along with 90% confidence interval for each of the arms. Log-rank test will be performed to test the difference in survival (both PFS and OS) between groups. Regression analyses of survival data will be based on the Cox proportional hazards model. The first pt in dose-level 1 of the Phase 1b lead-in was enrolled in February 2022. Clinical trial information: NCT05099666.