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Background: Alemtuzumab is a high-efficacy treatment approved for relapsing-remitting multiple sclerosis (RRMS). Although clinical trials and observational studies are consistent in showing its efficacy and manageable safety profile, further studies under clinical practice conditions are needed to further support its clinical use., Objective: The aim of this observational retrospective study was to evaluate the effectiveness and safety of alemtuzumab to add to the current real-world evidence on the drug., Methods: A cohort of 115 adult patients with RRMS treated with alemtuzumab between 2014 and 2020 was retrospectively followed up in five centers in Spain. Analysis included annualized relapse rate (ARR), 6-month confirmed disability worsening (CDW), 6-month confirmed disability improvement (CDI), radiological activity, no evidence of disease activity (NEDA-3), and safety signals. Given the different follow-up periods among participants, ARR was calculated using the person-years method. CDI was defined as a ≥ 1.0-point decrease in Expanded Disability Status Scale (EDSS) score assessed in patients with a baseline EDSS score ≥ 2.0 confirmed 6 months apart. CDW was defined as a ≥ 1.0-point increase in EDSS score assessed in patients with a baseline EDSS score ≥ 1.0 (≥ 1.5 if baseline EDSS = 0), confirmed 6 months apart., Results: ARR decreased from 1.9 (95% confidence interval 1.60-2.33) in the year prior to alemtuzumab initiation to 0.28 (0.17-0.37) after 1 year of treatment (87% reduction), and to 0.22 (0.13-0.35) after the second year. Over the entire follow-up period, ARR was 0.24 (0.18-0.30). At year 1, 75% of patients showed no signs of magnetic resonance imaging (MRI) activity and 70% at year 5. One percent of patients experienced 6-month CDW at year 1, 2.6% at year 2, 7.4% at year 3, and no patients over years 4 and 5. A total of 7.7% of patients achieved 6-month CDI in year 1, 3.6% in year 2, and maintained it at years 3 and 4. Most patients achieved annual NEDA-3: year 1, 72%; year 2, 79%; year 3, 80%; year 4, 89%; year 5, 75%. Infusion-related reactions were observed in 95% of patients and infections in 74%. Thyroid disorders occurred in 30% of patients, and only three patients developed immune thrombocytopenia. No cases of progressive multifocal leukoencephalopathy were reported., Conclusions: This study shows that alemtuzumab reduced the relapse rate and disability worsening in real-world clinical practice, with many patients achieving and sustaining NEDA-3 over time. The safety profile of alemtuzumab was consistent with previous findings, and no new or unexpected safety signals were observed. As this was an observational and retrospective study, the main limitation of not having all variables comprehensively available for all patients should be considered when interpreting results., (© 2024. The Author(s).)