Background: Peripheral T-cell lymphomas are aggressive non-Hodgkin lymphomas with few treatment options for relapsed or refractory disease. Valemetostat tosylate (valemetostat) is a potent, novel, dual inhibitor of EZH2 and EZH1. We investigated the clinical activity and safety of valemetostat in patients with relapsed or refractory peripheral T-cell lymphoma, and its safety in patients with relapsed or refractory adult T-cell leukaemia/lymphoma., Methods: VALENTINE-PTCL01 was a multicentre, open-label, single-arm, phase 2 trial performed at 47 hospitals in 12 countries across Asia, Europe, North America, and Oceania. Patients with either peripheral T-cell lymphoma or adult T-cell leukaemia/lymphoma, aged 18 years or older with an Eastern Cooperative Oncology Group performance status of 0-2 received oral valemetostat at 200 mg per day in continuous 28-day cycles until disease progression or unacceptable toxicity. The primary endpoint for patients with peripheral T-cell lymphoma was the CT-based objective response rate by blinded independent central review (BICR) using 2014 Lugano response criteria. Patients who received valemetostat and had a confirmed eligible peripheral T-cell lymphoma subtype on central review were included in the efficacy analysis. The primary endpoint for patients with adult T-cell leukaemia/lymphoma was the safety and tolerability of valemetostat. Safety in both cohorts was assessed in all patients who received at least one dose of valemetostat. The trial is registered with ClinicalTrials.gov, NCT04703192, and EudraCT, 2020-004954-31, and is closed to enrolment., Findings: Between June 16, 2021, and Aug 10, 2022, 133 patients with relapsed or refractory peripheral T-cell lymphoma (median age 69·0 years [IQR 58·0-74·0]; 91 [68%] were male, and 42 [32%] were female) and 22 patients with adult T-cell leukaemia/lymphoma (66·5 years [54·0-73·0]; 15 [68%] were male, and seven [32%] were female) were enrolled. The median follow-up time was 12·3 months (95% CI 11·8-13·8). 52 (44%; 95% CI 35-53) of 119 efficacy-evaluable patients with relapsed or refractory peripheral T-cell lymphoma had an objective response. The most common grade 3-4 adverse events were thrombocytopenia (31 [23%] of 133 patients in the peripheral T-cell lymphoma group and 11 [50%] of 22 patients in the adult T-cell leukaemia/lymphoma group), anaemia (25 [19%] and ten [46%]), and neutropenia (23 [17%] and four [18%]). Serious treatment-emergent adverse events were reported in 53 (40%) patients with peripheral T-cell lymphoma and 15 (68%) patients with adult T-cell leukaemia/lymphoma; nine (7%) patients and one (5%) patient had a serious treatment-emergent adverse event considered to be treatment related, respectively. No treatment-related deaths were reported., Interpretation: These data show that treatment with valemetostat leads to durable responses in patients with relapsed or refractory peripheral T-cell lymphoma, with a manageable safety profile., Funding: Daiichi Sankyo., Competing Interests: Declaration of interests PLZ has received consulting fees from lncyte, Novartis, BeiGene, and SOBI, and honoraria from Takeda, AstraZeneca, MSD, Bristol Myers Squibb (BMS), lncyte, Roche, Gilead, Recordati, Kyowa Kirin, Novartis, BeiGene, Janssen, and SOBI, unrelated to this study. KIz has received manuscript support from Daiichi Sankyo to their institution, related to this work; research funding Chugai, BMS, Incyte, Genmab, LOXO Oncology, Daiichi Sankyo, BeiGene, AbbVie, AstraZeneca, Regeneron, Yakult, Chugai, Otsuka, Novartis, Pfizer, MSD, Bayer, Kyowa Kirin, Eisai, Janssen, Ono Pharmaceutical, Gilead, Astellas, and Amgen to their institution, unrelated to this work; consulting fees from AstraZeneca, Ono Pharmaceutical, Mitsubishi Tanabe, Eisai, Chugai, BMS, AbbVie, Takeda, Zenyaku, Genmab, Kyowa Kirin, MSD, Carna Biosicences, Novartis, Yakult, Nihon Shinyaku, Novartis, and BeiGene, unrelated to this work; and honoraria from AstraZeneca, Ono Pharmaceutical, Eisai, Chugai, Janssen, Symbio, BMS, Daiichi Sankyo, Otsuka, AbbVie, Takeda, Eli Lilly, Genmab, Kyowa Kirin, MSD, Astellas, Pfizer, Meiji Seika Pharma, Novartis, Nihon Kayaku, and Gilead, unrelated to this work. NM-S has received research funding from AstraZeneca, BMS, C4 Therapeutics, Celgene, Corvus Pharmaceuticals, Daiichi Sankyo, Dizal Pharmaceuticals, Genetech/Roche, Incyte, Innate Pharmaceuticals, Secura Bio, Verastem, and Yingli Pharmaceuticals, to their institutions, unrelated to this work; consulting fees from AstraZeneca, Kyowa Hakka Kirin, Karyopharm, Ono Pharmaceuticals, Secura Bios, Daiichi Sankyo, Genentech, and Janssen, unrelated to this work; and participated on a data safety monitoring board and an advisory board for Daiichi Sankyo, unrelated to this work. SKB received manuscript support from Daiichi Sankyo, related to this work; consulting fees and honoraria from Acrotech, Kyowa Kirin, and Seagen, unrelated to this work; and participated on a data safety monitoring board or advisory board for Janssen, unrelated to this work. KIs has received manuscript support from Daiichi Sankyo, related to this work; research funding from Ono Pharmaceutical and Kyowa Kirin to their institutions, unrelated to this work; honoraria from Kyowa Kirin, Takeda, Chugai Pharmaceutical, Celgene, BMS, Daiichi Sankyo, Ono Pharmaceutical, Astellas, Eisai, Pfizer, Otsuka, Sanofi, CSL Behring, AbbVie, Yakult, Janssen Pharmaceutical, and Nippon Shinyaku, unrelated to this work; participated on data safety monitoring board or advisory board for Meiji Seika Pharma and Daiichi Sankyo, unrelated to this work; and received materials from Ono Pharmaceutical to their institution, unrelated to this work. SK has received support from Daiichi Sankyo, related to this work; research funding from Chugai, Kyowa Kirin, and Janssen, unrelated to this work; and honoraria from Daiichi Sankyo, Chugai, Kyowa Kirin, and Janssen, unrelated to this work. EB has received research funding from Amgen and BMS, to their institution, unrelated to this work; honoraria from Novartis, Kite/Gilead, Roche, Takeda, Janssen, and AbbVie, unrelated to this work; support for attending meetings or travel from Roche and Kite/Gilead, unrelated to this work; and participated on data safety monitoring board or advisory board from Novartis, Kite/Gilead, Roche, Incyte, ADC Therapeutics, and AbbVie, unrelated to this work. KC received consulting fees from Roche, Takeda, Celgene, and AbbVie, unrelated to this work; honoraria from Roche and Takeda, unrelated to this work; support for attending meetings or travel from Roche, Takeda, and BMS, to their institution; and participated on data safety monitoring board or advisory board for Daiichi Sankyo, unrelated to this work. GG received honoraria from Ideogen and Takeda, unrelated to this work; support for attending meetings or travel from Roche, Kite-Gilead, Sandoz, BeiGene, and Janssen, unrelated to this work; and participated on data safety monitoring board or advisory board for AbbVie, Roche, Takeda, Kite-Gilead, Italfarmaco, Ideogen, and Genmab, unrelated to this work. EJ received research funding from Celgene, Merck, Pharmacyclics, and F Hoffman-LaRoche, unrelated to this work; honoraria from Merck, Daiichi, BMS, and Bayer, unrelated to this work; and has patents planned, issued, or pending with UpToDate, unrelated to this work. TF has received research funding from ADCT, AstraZeneca, BMS, Corvus, Daiichi, Genmab, Kymera, Merck, Seagen, TESSA, Trillium, Alexion, and Portola, unrelated to this work; consulting fees from ADCT, AstraZeneca, BMS, Epizyme, Genmab, Seagen, Pharmacyclics, and Celgene, unrelated to this work; honoraria from Takeda, Seagen, Genmab, Epizyme, ADCT, AstraZeneca, BMS, Pharmacyclics, AbbVie, and Pfizer, unrelated to this work; and owns stock in OMI and Genomic Testing Cooperative, unrelated to this work. DE received research funding from Nippon Shinyaku Pharmaceutical, Chugai Pharmaceutical, and Eisai Pharmaceutical, to their institution, unrelated to this work; and honoraria from Eisai Pharmaceutical, Chugai Pharmaceutical, SymBio Pharmaceuticals, BMS, Kyowa Kirin Pharmaceutical, and Nippon Shinyaku Pharmaceutical, unrelated to this work. EDD received consulting fees from Takeda, unrelated to this work; honoraria from Takeda and BeiGene, unrelated to this work; and funding for attending meetings or travel from Takeda, unrelated to this work. JZ received research funding from Secura Bio, Astex, CRSPR, Myeloid Therapeutics, and Daichi Sankyo, unrelated to this work; consulting fees from Seattle genetics, Secura Bio, Kyowa Kirin, and Myeloid Therapeutics, unrelated to this work; and honoraria from Kyowa Kirin, unrelated to this work. JJ, SW, TM, AI, and KN are employed by Daiichi Sankyo. TM has received support for attending meetings or travel from Daiichi Sankyo, unrelated to this work. AI, TM, and YC own stock options in Daiichi Sankyo, unrelated to this work. KN owns stock in Daiichi Sankyo, unrelated to this work. SH received research funding from the US National Institutes of Health/National Cancer Institute Cancer Center, (support grant P30 CA008748); research funding from ADC Therapeutics, Affimed, Aileron, Celgene, Crispr Therapeutics, Daiichi Sankyo, Forty Seven, Kyowa Hakko Kirin, Takeda, Seattle Genetics, Trillium Therapeutics, and SecuraBio; and honoraria from Abcuro, Autolus, Auxilius Pharma, Corvus, Cimeio Therapeutics, Daiichi Sankyo, Dren Bio, Kyowa Hakko Kirin, March, Bio, Ono Pharmaceuticals, SecuraBio, Shore line, Biosciences, Takeda, Tubulis and Yingli Pharma. All other authors declare no competing interests., (Copyright © 2024 Elsevier Ltd. All rights reserved, including those for text and data mining, AI training, and similar technologies.)