Background: Few treatment options exist for second-line treatment of malignant pleural mesothelioma. We aimed to assess the antibody-drug conjugate anetumab ravtansine versus vinorelbine in patients with unresectable locally advanced or metastatic disease overexpressing mesothelin who had progressed on first-line platinum-pemetrexed chemotherapy with or without bevacizumab., Methods: In this phase 2, randomised, open-label study, done at 76 hospitals in 14 countries, we enrolled adults (aged ≥18 years) with unresectable locally advanced or metastatic malignant pleural mesothelioma, an Eastern Cooperative Oncology Group performance status of 0-1, and who had progressed on first-line platinum-pemetrexed chemotherapy with or without bevacizumab. Participants were prospectively screened for mesothelin overexpression (defined as 2+ or 3+ mesothelin membrane staining intensity on at least 30% of viable tumour cells by immunohistochemistry) and were randomly assigned (2:1), using an interactive voice and web response system provided by the sponsor, to receive intravenous anetumab ravtansine (6·5 mg/kg on day 1 of each 21-day cycle) or intravenous vinorelbine (30 mg/m 2 once every week) until progression, toxicity, or death. The primary endpoint was progression-free survival according to blinded central radiology review, assessed in the intention-to-treat population, with safety assessed in all participants who received any study treatment. This study is registered with ClinicalTrials.gov, NCT02610140, and is now completed., Findings: Between Dec 3, 2015, and May 31, 2017, 589 patients were enrolled and 248 mesothelin-overexpressing patients were randomly allocated to the two treatment groups (166 patients were randomly assigned to receive anetumab ravtansine and 82 patients were randomly assigned to receive vinorelbine). 105 (63%) of 166 patients treated with anetumab ravtansine (median follow-up 4·0 months [IQR 1·4-5·5]) versus 43 (52%) of 82 patients treated with vinorelbine (3·9 months [1·4-5·4]) had disease progression or died (median progression-free survival 4·3 months [95% CI 4·1-5·2] vs 4·5 months [4·1-5·8]; hazard ratio 1·22 [0·85-1·74]; log-rank p=0·86). The most common grade 3 or worse adverse events were neutropenia (one [1%] of 163 patients for anetumab ravtansine vs 28 [39%] of 72 patients for vinorelbine), pneumonia (seven [4%] vs five [7%]), neutrophil count decrease (two [1%] vs 12 [17%]), and dyspnoea (nine [6%] vs three [4%]). Serious drug-related treatment-emergent adverse events occurred in 12 (7%) patients treated with anetumab ravtansine and 11 (15%) patients treated with vinorelbine. Ten (6%) treatment-emergent deaths occurred with anetumab ravtansine: pneumonia (three [2%]), dyspnoea (two [1%]), sepsis (two [1%]), atrial fibrillation (one [1%]), physical deterioration (one [1%]), hepatic failure (one [1%]), mesothelioma (one [1%]), and renal failure (one [1%]; one patient had 3 events). One (1%) treatment-emergent death occurred in the vinorelbine group (pneumonia)., Interpretation: Anetumab ravtansine showed a manageable safety profile and was not superior to vinorelbine. Further studies are needed to define active treatments in relapsed mesothelin-expressing malignant pleural mesothelioma., Funding: Bayer Healthcare Pharmaceuticals., Competing Interests: Declarations of interests HLK reports grants paid to the University of Chicago to support clinical trials for Aduro, AstraZeneca, Bayer, Blueprint, Bristol Myers Squibb, Deciphera, GlaxoSmithKline, Harpoon, Inhibrx, MacroGenics, Merck, Polaris, Seattle Genetics, and Vivace; consulting fees from Bristol Myers Squibb, Deciphera, Inventiva, Novocure, and Seattle Genetics; payment or honoraria for lectures, presentations, speaker bureaus, manuscript writing, or educational events for AstraZeneca; support for attending meetings, travel, or both from AstraZeneca and Inventiva; participation on a data safety monitoring board or advisory board for Bristol Myers Squibb, Inventiva, and Seattle Genetics; and leadership or fiduciary role on board of directors for the International Mesothelioma Interest Group. SN reports personal payment or honoraria for lectures, presentations, speaker's bureaus, manuscript writing, or educational events from Abbvie, AstraZeneca, BeiGene, Boehringer Ingelheim, Bristol Myers Squibb, Eli Lilly, Pfizer, Pharmamar, Roche, and Takeda; and personal fees for participation on a data safety monitoring board or advisory board from AstraZeneca, Bayer, Daiichi Sanko, Eli Lilly, Pfizer, Roche, Sanofi, and Takeda. AB reports payment or honoraria for lectures, presentations, speaker's bureaus, manuscript writing, or educational events from AstraZeneca, Boehringer Ingelheim, Eli-Lilly, Pfizer, Roche, and Takeda; and support for attending meetings, travel, or both for Boehringer Ingelheim, Merck Sharp and Dohme, and Roche. GLC reports personal consulting fees for their advisory role for Novocure and Zai Lab; personal speaker engagements for Astellas, AstraZeneca, Merck Sharp and Dohme, Novocure, and Zai Lab; and support for attending meetings, travel, or both for Astellas, Novocure, and Merck Sharp and Dohme. JGJVA reports consulting fees for the advisory boards for Amphera, Bayer, Bristol Myers Squibb, Eli-Lilly, and Merck Sharp and Dohme; patents issued on allogenic tumour cell lysate and on combination immuno-oncology, owned by Erasmus MC Cancer Centre; participation on a data safety monitoring board or advisory board for Biocad; unpaid leadership role for the International Association for the Study of Lung Cancer; and stock or stock options for Amphera. JSp reports clinical trial reimbursement to Guy's & St Thomas’ NHS Foundation Trust from Bayer, BergenBio, Boehringer Ingelheim, Bristol Myers Squibb, Genmab, IO Biotech, Lytix, Seattle Genetics, and Starpharma; consulting fees paid to King's College London from Apobec, AVACTA, Bristol Myers Squibb, IO Biotech, and Seattle Genetics; support for attending meetings, travel, or both from Amgen and Janssen; participation on a data safety monitoring board or advisory board for AstraZeneca and Merck; and stock or stock options (co-founder) for Epsilogen. PT reports fees for a symposium presentation from AstraZeneca; and a conference registration fee from AstraZeneca. KN reports personal fees for advisory boards from AbbVie, Amgen, AstraZeneca, Bristol Myers Squibb Belgium, and Roche Belgium; personal fees for lectures from Roche Belgium; and support for attending meetings, travel, or both from AstraZeneca, Merck Sharpe and Dohme, and Pfizer. AG reports payment to Newcastle upon Tyne Hospitals NHS Foundation Trust for the care of patients on a study from Bayer (support for this manuscript). JAB reports institutional payment to the Netherlands Cancer Institute, Antoni van Leeuwenhoek Hospital for support of investigator-initiated study from Merck Sharp and Dohme, consulting fees paid to the Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital from Bristol Myers Squibb, and payment to the Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital for participation on a data safety monitoring board or advisory board for Roche. AS reports consulting fees from ArQule and Sanofi; payment or honoraria for lectures, presentations, speaker's bureaus, manuscript writing, or educational events from Abbvie, Amgen, ArQule, AstraZeneca, Bayer, Bristol Myers Squibb, Celgene, Eisai, Eli-Lilly, Gilead, Merck Sharp and Dohme, Novartis, Pfizer, Roche, Sandoz, Servier, and Takeda; and participation on a data safety monitoring board or advisory board for Bayer, Bristol Myers Squibb, Eisai, Gilead, Merck Sharp and Dohme, Pfizer, and Servier. SC reports payment or honoraria for lectures, presentations, speaker's bureaus, manuscript writing, or educational events from Amphera, Boehringer Ingelheim, Bristol Myers Squibb, Hoffmann La Roche, Merck Sharp and Dohme Oncology, and Pfizer; and support for attending meetings, travel, or both from Amphera, Boehringer Ingelheim, Bristol Myers Squibb, Hoffmann La Roche, Merck Sharp and Dohme Oncology, and Pfizer. JPVM reports consultancy fees from Amgen, AstraZeneca, Boehringer Ingelheim, Pfizer, and Roche; payment and reimbursement of expenses for services and consultancy from Amgen; registration fees from AstraZeneca, Merck Sharp and Dohme Belgium, Bristol Myers Squibb, GlaxoSmithKline, and Roche; and travel and accommodation support from AstraZeneca, Bristol Myers Squibb, Merck Sharp and Dohme Belgium, and Roche. AKN reports personal fees for consulting for clinical trials, quality of life research, and tumour measurement from Bayer (support for this manuscript); grants or contracts to institution from AstraZeneca and Douglas Pharmaceuticals; consulting fees from Atara Biotherapeutics (personal), Pharmabcine (institutional), and Seagen (personal); personal payment or honoraria for lectures, presentations, speaker's bureaus, manuscript writing, or educational events from Bristol Myers Squibb; travel support from AstraZeneca; and participation on a data safety monitoring board or advisory board for Bristol Myers Squibb (personal). GBJ reports personal grants or contracts from Adaptimmune, Amgen, AstraZeneca, Bayer, BeiGene, Bristol Myers Squibb, Celgene, Daiichi Sankyo, Elelixis, Genentech, GlaxoSmithKline, Immatics, Immunocore, Incyte, Kite Pharma, Macrogenics, MedImmune, Merck, Novartis, Regeneron, Repertoire Immune Medicines, Roche, Tmunity Therapeutics, Torque, Verastem, and Xcovery; personal consulting fees for AbbVie, Adicet, Amgen, Ariad, AstraZeneca, Bayer, Bristol Myers Squibb, Celgene, Clovis Oncology, Daiichi Sankyo, Genentech, Gilead, Instil Bio, Janssen, Lilly, Maverick Therapeutics, MedImmune, Merck, Novartis, Roche, Tyme Oncology, Viogin Biotech, and Xcovery; participation on a data safety monitoring board or advisory board for Maverick Therapeutics and Virogin Biotech (personal); and stock or stock options for Virogin Biotech. GBJ also has an immediate family member employed by Johnson and Johnson and Janssen. JSi is an employee of Bayer Healthcare Pharmaceuticals with ownership of stock; and reports unpaid leadership roles for the American Statistical Association, International Society for Clinical Biostatistics, and the Pharmaceutical Industry Working Group on Estimands in Oncology. LK is an external employee of Bayer Healthcare Pharmaceuticals. KK is an employee of Bayer AG Pharma with ownership of stock. AOW is an employee of Bayer AG Pharma. BHC and CE are employees of Bayer Healthcare Pharmaceuticals. RH reports institutional support for the conduct of this study via a Cooperative Research and Development Agreement (CRADA) between Bayer and the Center for Cancer Research, National Cancer Institute, Bethesda, MD, USA. He also has a CRADA with TCR(2) for conduct of clinical studies unrelated to this manuscript. DAF reports grants from Astex Therapeutics, Bayer, Boehringer Ingelheim, Bristol Myers Squibb, and Merck Sharp and Dohme; personal fees from Aldeyra, Atara, Bristol Myers Squibb, Inventiva, Lab21, Roche, RS Oncology, and Targovax; and non-financial support from Bristol Myers Squibb, Clovis, Eli-Lilly, and Roche. All other authors declare no competing interests., (Copyright © 2022 Elsevier Ltd. All rights reserved.)