Background: The optimal dosing strategy for infliximab in steroid-refractory acute severe ulcerative colitis (ASUC) is unknown. We compared intensified and standard dose infliximab rescue strategies and explored maintenance therapies following infliximab induction in ASUC., Methods: In this open-label, multicentre, randomised controlled trial, patients aged 18 years or older from 13 Australian tertiary hospitals with intravenous steroid-refractory ASUC were randomly assigned (1:2) to receive a first dose of 10 mg/kg infliximab or 5 mg/kg infliximab (randomisation 1). Block randomisation was used and stratified by history of thiopurine exposure and study site, with allocation concealment maintained via computer-generated randomisation. Patients in the 10 mg/kg group (intensified induction strategy [IIS]) received a second dose at day 7 or earlier at the time of non-response; all patients in the 5 mg/kg group were re-randomised between day 3 and day 7 (1:1; randomisation 2) to a standard induction strategy (SIS) or accelerated induction strategy (AIS), resulting in three induction groups. Patients in the SIS group received 5 mg/kg infliximab at weeks 0, 2, and 6, with an extra 5 mg/kg dose between day 3 and day 7 if no response. Patients in the AIS group received 5 mg/kg infliximab at weeks 0, 1, and 3, with the week 1 dose increased to 10 mg/kg and given between day 3 and day 7 if no response. The primary outcome was clinical response by day 7 (reduction in Lichtiger score to <10 with a decrease of ≥3 points from baseline, improvement in rectal bleeding, and decreased stool frequency to ≤4 per day). Secondary endpoints assessed outcomes to day 7 and exploratory outcomes compared induction regimens until month 3. From month 3, maintenance therapy was selected based on treatment experience, with use of thiopurine monotherapy, combination infliximab and thiopurine, or infliximab monotherapy, with follow-up as a cohort study up to month 12. Analysis was by intention to treat. This trial is registered with ClinicalTrials.gov, NCT02770040, and is completed., Findings: Between July 20, 2016, and Sept 24, 2021, 138 patients were randomly assigned (63 [46%] female and 75 [54%] male); 46 received a first dose of 10 mg/kg infliximab and 92 received 5 mg/kg infliximab. After randomisation 1, we observed no significant difference in the proportion of patients who had a clinical response by day 7 between the 10 mg/kg and 5 mg/kg groups (30 [65%] of 46 vs 56 [61%] of 92, p=0·62; risk ratio adjusted for thiopurine treatment history, 1·06 [95% CI 0·94-1·20], p=0·32). We found no significant differences in secondary endpoints including time to clinical response or change in Lichtiger score from baseline to day 7. Two patients who received 10 mg/kg infliximab underwent colectomy in the first 7 days compared with no patients in the 5 mg/kg group (p=0·21). Three serious adverse events occurred in three patients in both the 10 mg/kg group and 5 mg/kg group. After randomisation 2, the proportions of patients with clinical response at day 14 (34 [74%] of 46 in the IIS group, 35 [73%] of 48 in the AIS group, and 30 [68%] of 44 in the SIS group, p=0·81), clinical remission at month 3 (23 [50%], 25 [52%], 21 [48%], p=0·92), steroid-free remission at month 3 (19 [41%], 20 [42%], 18 [41%], p=1·0), endoscopic remission at month 3 (21 [46%], 22 [46%], 21 [48%], p=0·98), and colectomy at month 3 (three [7%] of 45, nine [19%] of 47, five [12%] of 43, p=0·20) were not significantly different between groups. Between day 8 and month 3, the proportion of patients with at least one infectious adverse event possibly related to infliximab was two (4%) of 46 in the IIS group, eight (17%) of 48 in the AIS group, and eight (18%) of 44 in the SIS group (p=0·082). No deaths occurred in the study., Interpretation: Infliximab is a safe and effective rescue therapy in ASUC. In steroid-refractory ASUC, a first dose of 10 mg/kg infliximab was not superior to 5 mg/kg infliximab in achieving clinical response by day 7. Intensified, accelerated, and standard induction regimens did not result in a significant difference in clinical response by day 14 or in remission or colectomy rates by month 3., Funding: Australian National Health and Medical Research Council, Gastroenterology Society of Australia, Gandel Philanthropy, Australian Postgraduate Award, Janssen-Cilag., Competing Interests: Declaration of interests MCC is supported by a Gandel Major Philanthropy Grant and an Australian Postgraduate Award, and has received research funding from Janssen-Cilag. CFDLWS has served as a speaker for DiaSorin, has received educational support from Pfizer, Shire, and Ferring, has received research funding from the Robert C Bulley Charitable Foundation and St Vincent's Hospital Melbourne Research Endowment Fund, and is supported by an Australian National Health and Medical Research Council (NHMRC) Postgraduate Scholarship. DC has received educational support from Viatris and is supported by an NHMRC Postgraduate Scholarship. EKW has served as a consultant, advisory board member, or speaker for AbbVie, Bristol Myers Squibb, Ferring, Janssen, Celltrion, Falk, and Takeda, is supported by an NHMRC Emerging Leadership Level 1 Fellowship, and has received research support from Ferring and Janssen. AP has served as an advisory board member or received speaker fees from AbbVie, Ferring, Janssen, Pfizer, Takeda, and Dr Falk Pharma. BC has served as a consultant, advisory board member, or speaker for AbbVie, Celltrion, Chiesi, Ferring, Janssen, Eli Lilly, MSD, Pfizer, Takeda, and Shire, and has received research support from AbbVie, Celltrion, The Gastroenterological Society of Australia, the Helmsley Charitable Trust, the NHMRC, and Takeda. WN has received educational and research support from Pfizer, AbbVie, Janssen, Shire and Ferring. JB has served as a consultant, advisory board member, or speaker for AbbVie, Ferring, Janssen, Celltrion, Chiesi, Janssen, Pfizer, Amgen, GlaxoSmithKline, Bristol Myers Squibb, Microba, Anatara, and Takeda, and has received research support from the NHMRC, United States Department of Defense, AbbVie, Janssen, Ferring, Pfizer, and Takeda. MG has served on the advisory board of Pfizer and AbbVie and has received speaker fees and research grants from Abbvie, Celltrion, Janssen, Pfizer and travel grants from Pfizer. NM has received speaker fees from Takeda and Baxter, and educational support from Baxter. DRVL has received consulting fees from AbbVie and speaker fees for Takeda. NSD reports serving as an advisory board member or speaker for AbbVie, Celltrion, Chiesi, Ferring, Janssen, Eli Lilly, MSD, Pfizer, Takeda, and Shire, and has received research support from AbbVie, Celltrion, The Gastroenterological Society of Australia, and the NHMRC. RWL reports advisory board membership for AbbVie, Aspen, Bristol Myers Squibb, Celgene, Celltrion, Chiesi, Ferring, Glutagen, Hospira, Janssen, Eli Lilly, MSD, Novartis, Pfizer, Prometheus Biosciences, and Takeda, and research grants from the University of Sydney, the McCusker Charitable Foundation, Celltrion, Shire, Janssen, Takeda, Joanna Tiddy grant, the Gastroenterological Society of Australia, the NHMRC, The Gutsy Group, and Pfizer. MPS has received educational grants or research support from Gilead and Celltrion, speaker fees from Janssen, AbbVie, Ferring, Takeda, Pfizer, Shire, Celltrion, Eli Lilly, and Dr Falk Pharma, and has served on advisory boards or received consultancy fees for Janssen, Takeda, Pfizer, Celgene, AbbVie, MSD, Emerge Health, Gilead, Bristol Myers Squibb, Celltrion, and Eli Lilly, and has received travel grants from Pfizer and Dr Falk Pharma. PDC has served as a consultant, advisory board member, or speaker for AbbVie, Baxter, Ferring, Janssen, Celltrion and Emerge Health, and is supported by a NHMRC Emerging Leadership Level 2 Fellowship, and has received research support from AbbVie, Ferring, Janssen and Pfizer. All other authors declare no competing interests., (Crown Copyright © 2024. Published by Elsevier Ltd. All rights reserved, including those for text and data mining, AI training, and similar technologies.)