Background: Stereotactic body radiotherapy (SBRT) has the potential to ablate localised pancreatic ductal adenocarcinoma. Selective dismutase mimetics sensitise tumours while reducing normal tissue toxicity. This trial was designed to establish the efficacy and toxicity afforded by the selective dismutase mimetic avasopasem manganese when combined with ablative SBRT for localised pancreatic ductal adenocarcinoma., Methods: In this adaptive, randomised, double-blind, placebo-controlled, phase 1b/2 trial, patients aged 18 years or older with borderline resectable or locally advanced pancreatic cancer who had received at least 3 months of chemotherapy and had an Eastern Cooperative Oncology Group performance status of 0-2 were enrolled at six academic sites in the USA. Eligible patients were randomly assigned (1:1), with block randomisation (block sizes of 6-12) with a maximum of 24 patients per group, to receive daily avasopasem (90 mg) or placebo intravenously directly before (ie, within 180 min) SBRT (50, 55, or 60 Gy in five fractions, adaptively assigned in real time by Bayesian estimates of 90-day safety and efficacy). Patients and physicians were masked to treatment group allocation, but not to SBRT dose. The primary objective was to find the optimal dose of SBRT with avasopasem or placebo as determined by the late onset EffTox method. All analyses were done on an intention-to-treat basis. This study is registered with ClinicalTrials.gov, NCT03340974, and is complete., Findings: Between Jan 25, 2018, and April 29, 2020, 47 patients were screened, of whom 42 were enrolled (median age was 71 years [IQR 63-75], 23 [55%] were male, 19 [45%] were female, 37 [88%] were White, three [7%] were Black, and one [2%] each were unknown or other races) and randomly assigned to avasopasem (n=24) or placebo (n=18); the placebo group was terminated early after failing to meet prespecified efficacy parameters. At data cutoff (June 28, 2021), the avasopasem group satisfied boundaries for both efficacy and toxicity. Late onset EffTox efficacy response was observed in 16 (89%) of 18 patients at 50 Gy and six (100%) of six patients at 55 Gy in the avasopasem group, and was observed in three (50%) of six patients at 50 Gy and nine (75%) of 12 patients at 55 Gy in the placebo group, and the Bayesian model recommended 50 Gy or 55 Gy in five fractions with avasopasem for further study. Serious adverse events of any cause were reported in three (17%) of 18 patients in the placebo group and six (25%) of 24 in the avasopasem group. In the placebo group, grade 3 adverse events within 90 days of SBRT were abdominal pain, acute cholangitis, pyrexia, increased blood lactic acid, and increased lipase (one [6%] each); no grade 4 events occurred. In the avasopasem group, grade 3-4 adverse events within 90 days of SBRT were acute kidney injury, increased blood alkaline phosphatase, haematoma, colitis, gastric obstruction, lung infection, abdominal abscess, post-surgical atrial fibrillation, and pneumonia leading to respiratory failure (one [4%] each).There were no treatment-related deaths but one late death in the avasopasem group due to sepsis in the setting of duodenal obstruction after off-study treatment was reported as potentially related to SBRT., Interpretation: SBRT that uses 50 or 55 Gy in five fractions can be considered for patients with localised pancreatic ductal adenocarcinoma. The addition of avasopasem might further enhance disease outcomes. A larger phase 2 trial (GRECO-2, NCT04698915) is underway to validate these results., Funding: Galera Therapeutics., Competing Interests: Declaration of interests CMT reports grant or contracts with US National Institutes of Health (NIH and CPRIT), licenses with Xerient Pharmaceuticals for licensed or individual patent for radioprotection of the upper gastrointestinal tract, and consulting fees with Phebry. JMF reports consultant and advisory board roles for Viewray, and payment for presentations for Boston Scientific. TAA reports an unfunded study steering committee role for Galera Therapeutics (Galera). MP reports research support to their institution from Merck and Varian, royalties to serve as section editor for cholangiocarcinoma, consulting fees from Voxelmetric and Syntactx, a speakers honorarium for Oakstone CME, advisory board participation for Varian, and an unpaid ASTRO Education Committee role. BC reports royalties from Springer, honoraria for Varian-Educational and Oakstone, and a leadership or fiduciary role with the National Comprehensive Cancer Network. MSB reports funding from Galera, institution grants or contracts from Nanobiotix, Silenseed, Trisalis, Artidis, SUC2, CPRIT, AstraZeneca, Merck, Siemens Medical, and Boston Scientific (Augmenix) and consulting fees from Oncosil and Strapax. SP reports consulting or advisory roles for Zymeworks, Ipsen, Novartis, Janssen, Boehringer Ingelheim, and AskGene Pharma, and institutional research funding from Mirati Therapeutics, Lilly, Xencor, Novartis, Rgenix, Bristol-Myers Squibb, Astellas Pharma, Framewave, 4D Pharma, Boehringer Ingelheim, NGM Biopharmaceuticals, Janssen, Arcus Biosciences, Elicio Therapeutics, Bionte, Ipsen, Zymeworks, Pfizer, ImmunoMET, Immuneering, and Amal Therapeutics. C-WDT reports an institution-sponsored research agreement with Sirtex, and individual and institutional honoraria from PanTher. DWK reports grants or contracts from Bold Therapeutics, and payment or honoraria from AstraZeneca. JC reports travel expense money from Galera. EJK reports research grants and contracts from NIH, US Department of Defense, Cancer Prevention and Research Institute of Texas, and Artidis, royalties or licenses from Taylor and Francis, and Kallisio, consulting fees from Kallisio, AstraZeneca, RenovoRx, MD Anderson Physician Network, MJH Life Sciences, and Quantum Aurea Capital, honoraria from Northwestern University and Amplity, a patent pending for 3D oral stents, scientific and medical advisory board roles for Cholangiocarcinoma Foundation, and stock or stock options with Quantum Aurea Capital. PD reports consulting fees from the American Society for Radiation Oncology, payment or honoraria from American Society for Clinical Oncology, Conveners, Physicians Education Resource, Imedex, and Bayer. RAW reports royalties from McGraw-Hill. GOS reports a research agreement with Artios Pharma. RSST reports institutional funding from Galera and NIH National Cancer Institute (NCI). YY reports personal fees from AbbVie, Amgen, Bexion Pharmaceuticals, BeyondSpring Pharmaceuticals, Boehringer Ingelheim Pharmaceuticals, Bristol Myers Squibb, Century Therapeutics, Enliven Therapeutics, Repare Therapeutics, Servier Pharmaceuticals, Starpax Pharmaceuticals, and Vertex Pharmaceuticals during the conduct of the study. PFT reports research funding from NIH NCI. RAB is an employee and officer of, reports receiving compensation from, and holds equity in the sponsor, Galera, residual royalties resulting from the acquisition of Confluence from Aclaris Therapeutics, stock or stock options with Galera, Euclises, Aclaris Therapeutics, and Novocure, and patents for methods for treatment of diseases, (US Pat 10,493,081, 2019 issued to Galera); a patent methods for treatment of diseases (US Pat 9,149,483, 2015 issued to Galera). JH was an employee and officer of, reports receiving compensation from, and holds equity in the sponsor, Galera. JMH reports research funding to their institution and support for attending a meeting from Canopy Cancer Collective, royalties or licenses from Springer, stock or stock options with Histosonics, and consulting fees from Histosonics and Boston Scientific. SEH is an employee of MyCareGorithm and has her own limited liability company, Beyond the White Coat. She has received travel assistance from ViewRay, stock options from Rittenhouse, and her institution received research funding from ViewRay and Galera. LEC, JAJ, VB, MM, GM, NR, EBL, MHGK, SB, and SM declare no competing interests., (Copyright © 2023 Elsevier Ltd. All rights reserved.)