Background: Oncogenic alterations in RET represent important therapeutic targets in thyroid cancer. We aimed to assess the safety and antitumour activity of pralsetinib, a highly potent, selective RET inhibitor, in patients with RET-altered thyroid cancers., Methods: ARROW, a phase 1/2, open-label study done in 13 countries across 71 sites in community and hospital settings, enrolled patients 18 years or older with RET-altered locally advanced or metastatic solid tumours, including RET-mutant medullary thyroid and RET fusion-positive thyroid cancers, and an Eastern Co-operative Oncology Group performance status of 0-2 (later limited to 0-1 in a protocol amendment). Phase 2 primary endpoints assessed for patients who received 400 mg once-daily oral pralsetinib until disease progression, intolerance, withdrawal of consent, or investigator decision, were overall response rate (Response Evaluation Criteria in Solid Tumours version 1.1; masked independent central review) and safety. Tumour response was assessed for patients with RET-mutant medullary thyroid cancer who had received previous cabozantinib or vandetanib, or both, or were ineligible for standard therapy and patients with previously treated RET fusion-positive thyroid cancer; safety was assessed for all patients with RET-altered thyroid cancer. This ongoing study is registered with clinicaltrials.gov, NCT03037385, and enrolment of patients with RET fusion-positive thyroid cancer was ongoing at the time of this interim analysis., Findings: Between Mar 17, 2017, and May 22, 2020, 122 patients with RET-mutant medullary and 20 with RET fusion-positive thyroid cancers were enrolled. Among patients with baseline measurable disease who received pralsetinib by July 11, 2019 (enrolment cutoff for efficacy analysis), overall response rates were 15 (71%) of 21 (95% CI 48-89) in patients with treatment-naive RET-mutant medullary thyroid cancer and 33 (60%) of 55 (95% CI 46-73) in patients who had previously received cabozantinib or vandetanib, or both, and eight (89%) of nine (95% CI 52-100) in patients with RET fusion-positive thyroid cancer (all responses confirmed for each group). Common (≥10%) grade 3 and above treatment-related adverse events among patients with RET-altered thyroid cancer enrolled by May 22, 2020, were hypertension (24 patients [17%] of 142), neutropenia (19 [13%]), lymphopenia (17 [12%]), and anaemia (14 [10%]). Serious treatment-related adverse events were reported in 21 patients (15%), the most frequent (≥2%) of which was pneumonitis (five patients [4%]). Five patients [4%] discontinued owing to treatment-related events. One (1%) patient died owing to a treatment-related adverse event., Interpretation: Pralsetinib is a new, well-tolerated, potent once-daily oral treatment option for patients with RET-altered thyroid cancer., Funding: Blueprint Medicines., Competing Interests: Declaration of interests VS reports grants from Blueprint Medicines and LOXO Oncology–Eli Lilly; research funding or grant support from Roche–Genentech, Novartis, Bayer, GlaxoSmithKline (GSK), Nanocarrier, Vegenics, Northwest Biotherapeutics, Berg Heath, Incyte, Fujifilm, PharmaMar, D3, Pfizer, MultiVir, Amgen, AbbVie, Alfasigma, Agensys, Boston Biomedical, Idera Pharma, Inhibrx, Exelixis, Medimmune, Altum, Dragonfly Therapeutics, Takeda, Immunogen, National Comprehensive Cancer Network (NCCN), NCI-CTEP and UT MD Anderson Cancer Center, Turning Point Therapeutics, and Boston Pharmaceuticals, travel support from Novartis, PharmaMar, ASCO, ESMO, Helsinn, and Incyte, and consultancy or advisory board participation for Helsinn, LOXO Oncology–Eli Lilly, R-Pharma US, Incyte, QED Pharma, Medimmune, and Novartis, and another relationship with Medscape. MIH reports advisory board roles for Eli Lilly, LOXO Oncology, and Blueprint Medicines, and research funding from Eli Lilly. LJW reports personal fees and other support from Blueprint Medicines; personal fees from Bayer, Blueprint Medicines, Exelixis, Genentech, Eli Lilly, and Merck, and non-financial support from Eli Lilly and Merck. MS reports grants from AstraZeneca and Bristol-Myers Squibb (BMS), and personal fees from AstraZeneca, Boehringer Ingelheim, BMS, GSK, Janssen, MorphoSys, Novartis, Roche, Takeda, Amgen, and MSD. ASM reports consulting or advisory board roles (with honoraria to his institution) for Janssen, Genentech, BMS, AbbVie, AstraZeneca, travel or accommodation or expenses from AbbVie and Roche, and research funding from Novartis, NIH, Mark Foundation, and is a non-remunerated board member of the Mesothelioma Applied Research Foundation. GC reports personal fees from Roche, Pfizer, Novartis, Eli Lilly, Foundation Medicine, BMS, Samsung, AstraZeneca, Daiichi Sankyo, Boehringer Ingelheim, GSK, and Seagen, non-financial support from Roche and Pfizer, grants from Merck, and other support from Ellipses Pharma. MSB has nothing to disclose. VWZ reports personal fees from AstraZeneca, Blueprint Medicines, Roche-Foundation Medicine, Roche–Genentech, Takeda, Turning Point Therapeutics, and Xcovery. SL reports personal fees from Eisai and Eli Lilly. DWB reports personal fees from Blueprint Medicines. CSB reports grants from Blueprint Medicines, Daiichi Sankyo, Celgene, Pfizer, Lilly Oncology, AbbVie, Rain Therapeutics, Spectrum Pharmaceuticals, and personal fees from AstraZeneca, Turning Point Therapeutics, NCCN, and Takeda. DA reports grants and personal fees from Blueprint Medicines; grants from Exelixis, Kura Oncology, Merck, Cue, Aduro, Eli Lilly, Pfizer, Celgene–BMS, Novartis, AstraZeneca, Atara Bio, Cellex, Innate, Sensei, Matrix Biomed, Hookipa, CoFactor, Medimmune, and ISA, and personal fees from Exelixis, Merck, Cue, twoXAR, Vaccinex, Zilio, and TargImmune. BK has nothing to disclose. IM reports grants from an ESMO Research Fellowship sponsored by Roche, and personal fees for speaker bureau participation from MSD. EG reports personal fees from Genentech, Hoffman–LaRoche, Ellipses Pharma, Neomed Therapeutics, Boehringer Ingelheim, Janssen, Seattle Genetics, TFS, Alkermes, ThermoFisher, BMS, and MSD, grants from Menarini and Glycotope, and other support from Novartis, Roche, and ThermoFisher. JFG reports personal fees from BMS, Roche–Genentech, Merck, Takeda, LOXO Oncology–Lilly, Blueprint Medicines, Oncorus, Regeneron, Gilead, AstraZeneca, Pfizer, Novartis, and Moderna, grants from Takeda and Novartis, and institutional research support from BMS, LOXO Oncology–Lilly, Blueprint Medicines, Novartis, Moderna, Alexo, Tesaro, Jounce, Adaptimmune Therapeutics, and an immediate family member who is an employee of and holds equity in Ironwood Pharmaceuticals. GL reports other support from Bayer, Blueprint Medicines, Cue Biopharma, Eisai, Exelixis, Genentech, Eli Lilly, LOXO Oncology, Merck, and Rakuten Medical. C-CL reports personal fees from BeiGene, Blueprint Medicines, Boehringer Ingelheim, BMS, Daiichi Sankyo, Eli Lilly, Novartis, Takeda, and Roche. YG has nothing to disclose. DS reports grants from Blueprint Medicines, personal fees from MSD, Eisai, Ipsen, Bayer, AstraZeneca, Surface Oncology, and non-financial support from Eisai, Ipsen, and Monia Therapeutics. SGM, CC, and HZ are employees of and hold equity interest in Blueprint Medicines. CDT is a former employee of and holds equity interest in Blueprint Medicines. MHT reports other support from Bayer, Blueprint Medicines, Novartis, Sanofi–Genzyme, ArQule, BMS, Eisai, Merck, Array BioPharma, LOXO Oncology, Abreos Biosciences, and Arch Oncology., (Copyright © 2021 Elsevier Ltd. All rights reserved.)