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Background: Cholinergic urticaria (CholU) is a common subtype of chronic inducible urticaria, where signs and symptoms (e.g. pruritic wheals and angioedema) are triggered by sweating due to physical exercise, passive warming and by other sweat-inducing situations. While guidelines recommend treatment with second-generation H1 antihistamines, approximately 90% of patients report uncontrolled disease. Targeting the histamine 4 receptor (H4R) has shown promise in preclinical/clinical studies of allergic/inflammatory diseases. Izuforant (LEO 152020) is a selective oral H4R antagonist with expected dual antipruritic and anti-inflammatory effects., Objectives: To assess the effects of izuforant in adults with CholU, a common type of chronic urticaria driven by histamine and characterized by high skin levels of H4R expression., Methods: This was a phase IIa randomized double-blind placebo-controlled multicentre crossover trial where patients with CholU with an inadequate response to ≥ 1 standard dose of H1 antihistamine received izuforant 100 mg twice daily or placebo (EUCTR2020-004961-38-DE; NCT04853992). The primary endpoint was change from baseline in Urticaria Activity Score. Exploratory endpoints included CholU activity score over 7 days, urticaria control test, Physician Global Assessment, patient global assessment of severity (PGA-S), provocation tests, Dermatology Life Quality Index and CholU quality of life (CholU-QoL). Pharmacokinetic and pharmacodynamic parameters, and serum biomarkers were assessed, as well as safety and tolerability., Results: Nineteen patients were randomized and included in the full analysis set; 18 completed treatment [mean (SD) age 29.5 (9.8) years; mean (SD) CholU duration 8.0 (6.3) years]. The primary and most of prespecified exploratory endpoints were not met; there were significant improvements in PGA-S for izuforant vs. placebo (P = 0.02), and nonsignificant improvements for other endpoints in quality of life and histamine skin prick test. All adverse events (AEs) experienced with izuforant were considered mild. The most frequently reported (> 1 patient) were nausea (three patients) and upper abdominal pain (two patients), occurring more frequently with izuforant vs. placebo (one patient each). There were no treatment-related serious AEs and no patient receiving izuforant discontinued the study. Treatment with izuforant did not cause downregulation of H4R., Conclusions: This is the first study to explore the role of H4R as a therapeutic target in urticaria. Targeting H4R with izuforant was well tolerated but did not demonstrate significant improvements vs. placebo in the primary endpoint and all but one prespecified exploratory endpoint in CholU., Competing Interests: Conflicts of interest E.G. was an advisor for Novartis. M. Metz is or recently was a speaker and/or advisor for Amgen, AstraZeneca, argenx, Celldex, Escient, Jasper Therapeutics, Novartis, Pharvaris, Sanofi-Aventis and Third Harmonic Bio. S.A. is or recently was a speaker and/or advisor for and/or has received research funding from AstraZeneca, Allakos, Biocryst, CSL Behring, Phavaris, Sanofi, Takeda, ThermoFisher, Moxie and Novartis. A.B. is or has recently been a speaker/advisor/investigator and/or received research funding from AbbVie, Almirall, Amgen, AstraZeneca, Biofrontera, Blueberry Therapeutics, Celldex, Centogene, Galderma, Genentech, Gilead, Incyte, LEO Pharma, Lilly, L’Oréal, Novartis, Sanofi, Regeneron and Shire/Takeda. L.L. has received consulting fees/medical writing/direction strategy fees from Regeneron, Celldex, Gossomer Bio, Kenjockety, Gamida Cell, Arvelle Therapeutics, Sun Pharma, Halozyme, UCARE and Noema Pharma. K.B. is or recently was a speaker and/or advisor for AstraZeneca, Novartis, Biomarin, Synlab, Blueprint, ThermoFisher, Bencard and ALK. G.H. declares grants from Deutsche Forschungsgemeinschaft (DFG) and personal fees from Allergopharma, Biotest, Eli Lilly, Sanofi, AbbVie and Leti outside the submitted work. K.K.S., T.H. and O.E.S. are employees of LEO Pharma. M. Maurer is or recently was a speaker and/or advisor for and/or has received research funding from Allakos, Alvotech, Amgen, Aquestive, Aralez, AstraZeneca, Astria, Bayer, BioCryst, Blueprint, Celldex, Celltrion, Centogene, CSL Behring, Evoemmune, GSK, Ipsen, Kalvista, Kyowa Kirin, LEO Pharma, Lilly, Menarini, Mitsubishi Tanabe Pharma, Moxie, Noucor, Novartis, Orion Biotechnoloy, Pharvaris, Resonance Medicine, Sanofi/Regeneron, Septerna, Takeda, Teva, Trial Form Support International AB, Third HarmonicBio, Valenza Bio, Yuhan Corporation and Zurabio., (© The Author(s) 2024. Published by Oxford University Press on behalf of British Association of Dermatologists. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)