INTRODUCTION: New therapeutic approaches in unresectable malignant pleural mesothelioma (MPM) are eagerly awaited. Trabectedin is an antitumor agent with an effect on cancer cell proliferation and a modulating action on tumor microenvironment. The ATREUS study explored the activity and safety of trabectedin in patients with unresectable MPM.METHODS: Epithelioid patients with MPM received trabectedin as second-line while biphasic/sarcomatoid patients with MPM as first- or second-line therapy. Treatment was given intravenously at an initially planned dose of 1.3 mg/m2 every 3 weeks, until progression or unacceptable toxicity. The primary endpoint was progression-free survival rate at 12 weeks (PFS12wks).RESULTS: Overall, 78 patients (54%) had epithelioid and 67 (46%) nonepithelioid MPM. PFS12wks in 62 evaluable patients with epithelioid MPM was 43.5% (80% confidence interval 34.9%-52.5%); median progression-free and overall survival were 2.4 and 9.0 months, respectively. PFS12wks in 52 evaluable patients with nonepithelioid MPM was 30.8% (90% confidence interval 20.3%-42.9%); median progression-free and overall survival were 1.7 and 5.4 months. Trabectedin starting dose was amended due to excess of liver toxicity. Eighty-four (64%) and 48 (36%) patients received 1.3 mg/m2 and 1.1. mg/m2, respectively. The most common grade 3-4 toxicities were hepatotoxicity, leukopenia/neutropenia, and fatigue. Grade 3-4 hepatotoxicity was reported in 59 (70%) patients treated at 1.3 mg/m2, and in 19 (40%) treated at 1.1 mg/m2.CONCLUSIONS: Trabectedin showed modest clinical activity, at the expense of relevant liver toxicity. Further development of this drug in MPM at full doses is not warranted.