Background: Treatment options for malignant pleural mesothelioma are scarce. Tazemetostat, a selective oral enhancer of zeste homolog 2 (EZH2) inhibitor, has shown antitumour activity in several haematological cancers and solid tumours. We aimed to evaluate the anti-tumour activity and safety of tazemetostat in patients with measurable relapsed or refractory malignant pleural mesothelioma., Methods: We conducted an open-label, single-arm phase 2 study at 16 hospitals in France, the UK, and the USA. Eligible patients were aged 18 years or older with malignant pleural mesothelioma of any histology that was relapsed or refractory after treatment with at least one pemetrexed-containing regimen, an Eastern Cooperative Oncology Group performance status of 0 or 1, and a life expectancy of greater than 3 months. In part 1 of the study, participants received oral tazemetostat 800 mg once on day 1 and then twice daily from day 2 onwards. In part 2, participants received oral tazemetostat 800 mg twice daily starting on day 1 of cycle 1, using a two-stage Green-Dahlberg design. Tazemetostat was administered in 21-day cycles for approximately 17 cycles. The primary endpoint of part 1 was the pharmacokinetics of tazemetostat and its metabolite at day 15 after administration of 800 mg tazemetostat, as measured by maximum serum concentration (C max ), time to C max (T max ), area under the concentration-time curve (AUC) to day 15 (AUC 0-t ), area under the curve from time 0 extrapolated to infinity (AUC 0-∞ ), and the half-life (t 1/2 ) of tazemetostat, assessed in all patients enrolled in part 1. The primary endpoint of part 2 was the disease control rate (the proportion of patients with a complete response, partial response, or stable disease) at week 12 in patients with malignant pleural mesothelioma per protocol with BAP1 inactivation determined by immunohistochemistry. The safety population included all the patients who had at least one post-dose safety assessment. This trial is now complete and is registered with ClinicalTrials.gov, NCT02860286., Findings: Between July 29, 2016, and June 2, 2017, 74 patients were enrolled (13 in part 1 and 61 in part 2) and received tazemetostat, 73 (99%) of whom had BAP1-inactivated tumours. In part 1, following repeat dosing of tazemetostat at steady state, on day 15 of cycle 1, the mean C max was 829 ng/mL (coefficient of variation 56·3%), median T max was 2 h (range 1-4), mean AUC 0-t was 3310 h·ng/mL (coefficient of variation 50·4%), mean AUC 0-∞ was 3180 h·ng/mL (46·6%), and the geometric mean t 1/2 was 3·1 h (13·9%). After a median follow-up of 35·9 weeks (IQR 20·6-85·9), the disease control rate in part 2 in patients with BAP1-inactivated malignant pleural mesothelioma was 54% (95% CI 42-67; 33 of 61 patients) at week 12. No patients had a confirmed complete response. Two patients had a confirmed partial response: one had an ongoing partial response with a duration of 18 weeks and the other had a duration of 42 weeks. The most common grade 3-4 treatment-emergent adverse events were hyperglycaemia (five [7%] patients), hyponatraemia (five [7%]), and anaemia (four [5%]); serious adverse events were reported in 25 (34%) of 74 patients. Five (7%) of 74 patients died while on study; no treatment-related deaths occurred., Interpretation: Further refinement of biomarkers for tazemetostat activity in malignant pleural mesothelioma beyond BAP1 inactivation could help identify a subset of tumours that are most likely to derive prolonged benefit or shrinkage from this therapy., Funding: Epizyme., Competing Interests: Declaration of interests MGZ has received support for the present manuscript from Epizyme; grants or contracts to her institution from National Institutes of Health, Department of Defense, Precog, Epizyme, GlaxoSmithKline, Polaris, Sellas Life Sciences, Bristol Myers Squibb, Millennium, Curis, and Atara; consulting fees from Ikena, Takeda, GlaxoSmithKline, Aldeyra Therapeutics, and Novocure; has honoraria from Physicians' Education Resource, Medscape, Research to Practice, Medical Learning Institute, and OncLive; and has served as Chair of the Board of Directors (uncompensated) with Mesothelioma Applied Research Foundation. PWS has received research funding from Polaris Group; has served in an advisory role for Merck, Epizyme, Nestle Health Science, and Paredox Therapeutics; and has received expenses related to travel and accommodation from MSD. SP has received consulting fees from Amgen, AstraZeneca, Bayer, Beigene, Blueprint, Bristol Myers Squibb, Boehringer Ingelheim, Daiichi Sankyo, Guardant Health, Incyte, Janssen, Lilly, Merck Serono, MSD, Novartis, Roche, Takeda, Pfizer, Seattle Genetics, Turning Point Therapeutics, and Xcovery; honoraria from AstraZeneca, Bayer, Guardant Health, Janssen, Merck Serono, Roche, Takeda, and Pfizer; payment for expert testimony from Roche and Merck Serono; and has a leadership or fiduciary role with British Thoracic Oncology Group, ALK Positive UK, Lung Cancer Europe, Ruth Strauss Foundation, Mesothelioma Applied Research Foundation, and ETOP-IBCSG Partners Foundation Board. DP has received consulting fees from AstraZeneca, Bristol Myers Squibb, Celgene, Merck, Novartis, Pfizer, Roche, Janssen, and AbbVie; honoraria from AstraZeneca, Bristol Myers Squibb, Boehringer Ingelheim, Celgene, Daiichi Sankyo, Eli Lilly, Merck, Novartis, Pfizer, prIME Oncology, Peer CME, Roche, Janssen, and AbbVie; support for attending meetings or travel from AstraZeneca, Roche, Novartis, and Pfizer; has participated on a data safety monitoring board or advisory board for AstraZeneca, Roche, Novartis, and Pfizer; and has participated in clinical trial research as principal or co-investigator (institutional financial interests) with AstraZeneca, Bristol Myers Squibb, Boehringer Ingelheim, Eli Lilly, Merck, Novartis, Pfizer, Roche, Medimmun, Sanofi Aventis, Taiho Pharma, Novocure, Daiichi Sankyo, Janssen, and AbbVie. ASc has received other fees from Epizyme; personal fees from AstraZeneca, Bristol Myers Squibb, Sanofi, Janssen, and MSD; and non-financial support from AstraZeneca, Bristol Myers Squibb, Roche, and MSD. TP has received consulting fees from AstraZeneca. ASz, JY, YC, VK, and SA are employees of or own stock in Epizyme. DAF has received grants from Astex Pharmaceuticals, Bayer, Boehringer Ingelheim, Bristol Myers Squibb, Bergen Bio, RS Oncology, and Aldeyra; personal fees from Boehringer Ingelheim and Bristol Myers Squibb; and non-financial support from Bristol Myers Squibb, Clovis Oncology, Lilly Oncology, MSD, Bergen Bio, and Roche. PT has received speaker fees from AstraZeneca. All other authors declare no competing interests., (Crown Copyright © 2022. Published by Elsevier Ltd. All rights reserved.)