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Importance Oligomeric amyloid-β peptide binds to cellular prion protein on the neuronal cell surface, activating intracellular fyn kinase to mediate synaptotoxicity and tauopathy. AZD0530 is an investigational kinase inhibitor specific for the Src family, including fyn, that has been repurposed for the treatment of Alzheimer disease. Objective To determine whether AZD0530 treatment slows the decline in cerebral metabolic rate for glucose (CMRgl) and is safe and well tolerated. Design, Setting, and Participants This multicenter phase 2a randomized clinical trial enrolled participants between December 23, 2014, and November 30, 2016. Participants (n = 159) had mild Alzheimer dementia and positron emission tomography (PET) evidence of elevated levels of amyloid-β peptide. Efficacy analyses of all primary and secondary outcomes were conducted in a modified intention-to-treat population. Final analyses were conducted from February 9, 2018, to July 25, 2018. Interventions AZD0530 (100 mg or 125 mg daily) vs placebo for 52 weeks. Main Outcomes and Measures Primary outcome was the reduction in relative CMRgl, as measured by18F-fluorodeoxyglucose (18F-FDG) PET, at 52 weeks in an Alzheimer disease–associated prespecified statistical region of interest. Secondary end points included change in cognition, function, and other biomarkers. Results Among the 159 participants, 79 were randomized to receive AZD0530 and 80 to receive placebo. Of the 159 participants, 87 (54.7%) were male, with a mean (SD) age of 71.0 (7.7) years. Based on a week-2 plasma drug level (target = 180 ng/mL; 30nM free), 15 participants (19.2%) had their AZD0530 dose escalated from 100 mg to 125 mg. Mean plasma levels from weeks 13 to 52 were 220 ng/mL and 36nM free. More participants discontinued treatment with AZD0530 than with placebo (21 vs 11), most commonly because of adverse events. The most frequent adverse events were gastrointestinal disorders (primarily diarrhea), which occurred in 38 participants (48.1%) who received AZD0530 and in 23 (28.8%) who received placebo. In the primary outcome, the treatment groups did not differ in 52-week decline in relative CMRgl (mean difference: −0.006 units/y; 95% CI, −0.017 to 0.006;P = .34). The treatment groups also did not differ in the rate of change in Alzheimer’s Disease Assessment Scale–Cognitive Subscale, Alzheimer’s Disease Cooperative Study–Activities of Daily Living, Clinical Dementia Rating, Neuropsychiatric Inventory, or Mini-Mental State Examination scores. Secondary volumetric magnetic resonance imaging analyses revealed no treatment effect on total brain or ventricular volume but did show trends for slowing the reduction in hippocampal volume and entorhinal thickness. Conclusions and Relevance Statistically significant effects of AZD0530 treatment were not found on relative CMRgl reduction in an Alzheimer disease–associated region of interest or on secondary clinical or biomarker measures. Trial Registration ClinicalTrials.gov identifier:NCT02167256