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Background Natalizumab/Tyruko ® is a biosimilar to reference Natalizumab (NTZ)/Tysabri® (Biogen). It is a newer medication compared to reference NTZ, with less post marketing clinical data.Method This was a retrospective study analysis of data from clinical reviews at Basildon Hospital, aiming to assess efficacy and tolerability of Tyruko in patients treated by biosimilar as a single disease-modifying therapy of highly active relapsing-remitting multiple sclerosis (RRMS).Results 19 females and 4 males (age 40.96±9.91 years) were treated with Tyruko in 2024. 20 patients were switched off from the on-going Tysabri treatment, 3 patients started Tyruko as their first NTZ treatment. 14 (60.9%) patients, including 3 patients started naïve on Tyruko did not report side effects, 9 (39.1%) patients complained of fatigue and enhancement of residual symptoms, 4 (17.4%) patients due to significant intolerance to biosimilar were switched back to reference drug. JCV test was positive in 9 (39.1%) patients, of which 6 patients levels noted negative in Tysabri JCV titre assay previously, seroconverted to positive low levels with the Tyruko new JCV titre assay. In 2 (8.7%) patients, Tyruko was discontinued due to new radiological findings.Conclusion Further research and consideration of a national audit is needed to assess clinical efficacy and tolerability of biosimilars.elina.melikyan2@nhs.net