Background: Trastuzumab deruxtecan is a HER2-directed antibody-drug conjugate approved by the US Food and Drug Administration and the European Medicines Agency for HER2-mutant non-small-cell lung cancer. Few treatment options exist for patients with HER2-mutant solid tumours beyond lung cancers. We investigated trastuzumab deruxtecan in metastatic solid tumours with specific activating HER2 mutations., Methods: In this open-label, phase 2, basket study done in 29 centres in Asia, Europe, and North America, we investigated trastuzumab deruxtecan (5·4 mg/kg every 3 weeks by intravenous infusion) in patients aged 18 years or older with unresectable or metastatic solid tumours with specific activating HER2 mutations, an Eastern Cooperative Oncology Group performance status of 0 or 1, and disease progression following previous treatment (previous HER2-targeted therapy was permitted) or with no satisfactory alternative treatment options. The primary endpoint was confirmed objective response rate by independent central review. Anti-tumour activity and safety were analysed in all patients who received at least one dose of trastuzumab deruxtecan. This trial is registered with ClinicalTrials.gov, NCT04639219, and is active but no longer recruiting., Findings: Between Dec 30, 2020, and Jan 25, 2023, 102 patients (62 [61%] female and 40 [39%] male; median age 66·5 years [IQR 58-72]; 51 [50%] White, two [2%] Black or African American, 38 [37%] Asian, and 11 [11%] did not have race information reported) with solid tumours with activating HER2 mutations received trastuzumab deruxtecan and were included in the anti-tumour activity and safety analyses sets. Patients had a median of three (IQR 2-4) previous treatment regimens. The median duration of follow-up was 8·61 months (IQR 3·71-12·68). The objective response rate by independent central review was 29·4% (95% CI 20·8-39·3; 30 of 102 patients). 52 (51%) patients had a treatment-emergent adverse event of grade 3 or worse; the most common events (in ≥5% of patients) were anaemia (16 [16%]) and neutrophil count decreased (eight [8%]). Drug-related treatment-emergent serious adverse events occurred in ten (10%) patients. Adjudicated drug-related interstitial lung disease or pneumonitis of any grade occurred in 11 patients (11%; three grade 1, five grade 2, one grade 3, and two grade 5); there were two (2%) cases of fatal adjudicated drug-related interstitial lung disease or pneumonitis., Interpretation: Trastuzumab deruxtecan showed anti-tumour activity and durable responses in heavily pretreated patients across multiple tumour types with activating HER2 mutations, with no new safety signals. Prespecified HER2 mutations might be targeted by HER2-directed antibody-drug conjugates and our findings support further investigation of trastuzumab deruxtecan in the pan-tumour setting., Funding: AstraZeneca and Daiichi Sankyo., Competing Interests: Declaration of interests BTL has served as an uncompensated advisor and consultant to Amgen, AstraZeneca, Boehringer Ingelheim, Bolt Biotherapeutics, Daiichi Sankyo, Genentech, and Lilly. He has received research grants to his institution from Amgen, AstraZeneca, Bolt Biotherapeutics, Daiichi Sankyo, Genentech, Jiangsu Hengrui Pharmaceuticals, and Lilly. He has received academic travel support from Amgen. He is an inventor on two institutional patents at MSK (US62/685,057 and US62/514,661) and has intellectual property rights as a book author at Karger Publishers and Shanghai Jiao Tong University Press. He is supported by the Memorial Sloan Kettering Cancer Center Support Grant P30 CA008748 and Research Project Grant R01CA249666 from the National Institutes of Health. FM-B reports institutional grants or contracts from Aileron, AstraZeneca, Bayer, Calithera, Curis, CytomX, Daiichi Sankyo, Debiopharm, eFFECTOR, Genentech, Guardant, KLUS, Novartis, Puma Biotechnology, Taiho, and Takeda; personal consulting fees from AbbVie, Aduro, Alkermes, AstraZeneca, Calibr (a division of Scripps Research), Daiichi Sankyo, Debiopharm, EcoR1 Capital, eFFECTOR, F Hoffmann-La Roche, GT Apeiron, Genentech, Harbinger Health, IBM Watson, Infinity, Jackson Laboratory, Kolon Life Science, LegoChem Biosciences, Lengo Therapeutics, Menarini Group, OrigiMed, PACT Pharma, Parexel International, Pfizer, Protai, Samsung Bioepis, Seattle Genetics, Tallac, Tyra Biosciences, Xencor, and Zymeworks; personal meeting or travel support from the Cholangiocarcinoma Foundation, the European Organisation for Research and Treatment of Cancer, and the European Society for Medical Oncology; and personal participation on a data safety monitoring board or advisory board for Black Diamond, Biovica, Eisai, FogPharma, Immunomedics, Inflection Biosciences, Karyopharm, Loxo Oncology, Mersana, OnCusp, Puma Biotechnology, Seattle Genetics, Sanofi, Silverback, Spectrum, Theratechnologies, and Zentalis. AB reports grants or contracts from AstraZeneca/Daiichi Sankyo, Eli Lilly, Genentech/Roche, Immunomedics/Gilead, Merck, Novartis, Pfizer, Radius/Menarini, and Sanofi; consulting fees from BioTheranostics, Daiichi Sankyo/AstraZeneca, Foundation Medicine, Genentech/Roche, Immunomedics/Gilead, Innocrin Pharma, Merck, Novartis, Pfizer, Philips, Puma Biotechnology, Radius Health, Radius/Menarini, Sanofi, and Spectrum Pharmaceuticals; institutional payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from Daiichi Sankyo/AstraZeneca, Genentech, Eli Lilly, Immunomedics/Gilead, Merck, Novartis, Pfizer, Radius Health, and Sanofi; personal participation on a data safety monitoring board or advisory board for AstraZeneca, Daiichi Sankyo, Eisai, Genentech, Gilead, Lilly, Menarini, Merck, Mersana, Novartis, Pfizer, and Sanofi; other personal financial or non-financial interests for AstraZeneca, Daiichi Sankyo, Genentech, Gilead, Lilly, Merck, Mersana, Novartis, Pfizer, and Sanofi. YN reports institutional grants or contracts from AbbVie, AstraZeneca, Bayer, Boehringer Ingelheim, Chugai, Daiichi Sankyo, Eli Lilly, Eisai, Ono, Pfizer, and Taiho; personal payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from AstraZeneca, Bayer, Bristol Myers Squibb, Chugai, Daiichi Sankyo, Eisai, Eli Lilly, Guardant, MSD, Nihon Kayaku, Novartis, Ono, PDR Pharma, Pfizer, Taiho, and Takeda. SS reports personal participation on a data safety monitoring board or advisory board for Agenus, AstraZeneca, Bristol Myers Squibb, CheckmAb, Daiichi Sankyo, GlaxoSmithKline, MSD, Novartis, Seagen, and T-One Therapeutics. PA reports institutional grants or contracts from Roche; personal payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or education events for Amgen and Synthon; support for attending meetings and/or travel from Amgen, Daiichi Sankyo, MSD, Pfizer, and Roche; personal participation on a data safety monitoring board or advisory board for Amcure, Boehringer Ingelheim, Deloitte, Eisai, G1 Therapeutics, Gilead, Lilly, MacroGenics, Menarini, Novartis, Radius, Roche, and Servier. GC reports grants or contracts from Merck; consulting fees from AstraZeneca, Bristol Myers Squibb, Daiichi Sankyo, Ellipsis, Gilead, Lilly, Menarini, Merck, Novartis, Pfizer, Roche, and Seagen; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events for Gilead, Lilly, Novartis, Pfizer, and Relay; support for attending meetings or travel from Daiichi Sankyo. MdM reports institutional payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from Janssen and MSD; and other personal financial or non-financial interests for AbbVie, Achilles, Amunix, Arcus, BioNTech, Catalym, Dizal, Furmo, Genentech, Janssen, Loxo, MSD, Novartis, Numab, PharmaMar, Replimune, Roche, and Seagen. MK reports stock or stock options from Athenex, CRISPR, Eli Lilly, Natera, and Pfizer. D-YO reports personal grants or contracts from Array, AstraZeneca, Basilea, BeiGene, Eli Lilly, Handok, MSD, Novartis, and Servier; and participation on a data safety monitoring board or advisory board for Arcus, ASLAN, AstraZeneca, Bayer, BeiGene, Bristol Myers Squibb/Celgene, Genentech/Roche, Halozyme, IQVIA, Merck Serono, MSD, Novartis, Taiho, Turning Point, Yuhan, and Zymeworks. MK's current affiliation is Breast Medical Oncology, LifeBridge Health, Baltimore, MD, USA. JOP reports personal grants or contracts from ABL Bio, Bristol Myers Squibb (Celgene), Eutilex, MedPacto, and Servier; personal support for attending meetings and/or travel from Minneamrita; personal participation on a data safety monitoring board or advisory board for Adicet Bio, AstraZeneca, Bristol Myers Squibb (Celgene), ImmuneOncia, MediRama, MedPacto Merck, Merck Sereno, MedPacto, and Servier. SP-V reports institutional grants or contracts from AstraZeneca and IMCore Hoffman LaRoche; institutional participation on data safety monitoring board or advisory board for Amgen and Daiichi Sankyo; and other personal financial or non-financial interests for Amgen, AstraZeneca, Bristol Myers Squibb, Clovis, GlaxoSmithKline, Novartis, Oxford Biotherapeutics, PEP Therapy, and Roche. SYR reports institutional grants or contracts from ABL Bio, ALX Oncology, Amgen, Arcus Biosciences, Astellas Pharma, AstraZeneca, BeiGene, Bold Therapeutics, Daiichi Sankyo, Eisai, Ipsen, Leap Therapeutics, MacroGenics, MedPacto, Merck KGaA, MSD, Ono Pharmaceutical, Pfizer, Seagen, Taiho Pharmaceutical, and Zymeworks; consulting fees from Individumed and LG Biochem; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events for Daiichi Sankyo, Eisai, Lilly, and MSD; and participation on a data safety monitoring board or advisory board for Amgen. TS reports personal payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or education events for Bristol Myers Squibb, Chugai Pharmaceutical, Daiichi Sankyo, Eli Lilly, Merck Biopharm, Ono Pharmaceutical, Taiho, Takara Bio, Takeda, and Yakult Honsha; institutional payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or education events for Bristol Myers Squibb, Chugai Pharmaceutical, Daiichi Sankyo, Eli Lilly, Gilead, HUTCHMED, Merck Biopharm, Ono Pharmaceutical, Parexel, Taiho, and Yakult Honsha; and personal participation on a data safety monitoring board or advisory board for Takara Bio. IS reports institutional grants or contracts from AstraZeneca, Genentech, Incyte, MSD, Orion, Pfizer, Puma Biotechnology, and Roche; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing, or educational events from AstraZeneca; support for attending meetings or travel from AstraZeneca, Incyte, and Merck/Pfizer; and other financial or non-financial interests for AstraZeneca, Genentech, Incyte, MSD, Orion, Pfizer, Puma Biotechnology, and Roche. FM reports other personal financial or non-financial interests for AstraZeneca. AS reports personal stock or stock options for AstraZeneca and other financial or non-financial interests for AstraZeneca. KKM reports other personal financial or non-financial interests for AstraZeneca. CS reports personal participation in advisory boards or travel grants from AstraZeneca, Ax's Consulting, Byondis, Daiichi Sankyo, Eisai, Exact Sciences, Exeter Pharma, F Hoffmann-La Roche Ltd, Gilead, Lilly, MediTech, MSD, Novartis, Pfizer, Philips, Pierre Fabre, PintPharma, Puma Biotechnology, SeaGen, Synthon Biopharmaceutical, and Zymeworks. IA declares no competing interests., (Copyright © 2024 Elsevier Ltd. All rights reserved, including those for text and data mining, AI training, and similar technologies.)