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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.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 (CBetween 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 CFurther 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.Epizyme.