Background and Importance We know that natalizumab is an effective treatment in patients with relapsing-remitting multiple sclerosis with high activity. More doubts arise regarding its safety which will lead to having to closely monitor the patient.

Aim and Objectives To evaluate the safety of treatment with natalizumab for relapsing-remitting multiple sclerosis (RRMS), specifically John Cunningham virus (JCV) infection that can cause Progressive Multifocal Leukoencephalopathy (PML). Also evaluate effectiveness by counting outbreaks during treatment and time in treatment.

Material and Methods Retrospective observational study since the approval of the drug. All patients with RRMS under treatment with natalizumab were included and the variables sex, age, previous and subsequent treatments, positive JCV serology at any time, duration of treatment, relapses and number of them, and reason for discontinuing treatment were collected. Data was extracted from FarmaTools® software database and the electronic medical.

Results A total of 75 patients were analysed, 47 (63%) of them women. Mean age at the time of initiation of treatment of 41 years (28–69), median number of previous lines of 1 (0–5), being used as first line in 15 patients (20%), second line in 42 patients (56%). The patients analysed were on treatment for an average of 2.6 years, the reasons for suspension being: Positive JCV serology 39 (52%), adverse effects 11 (15%), outbreaks six (8%), progressive worsening five (7% ), unknown cause three (4%) and 2 (3%) discontinued due to pregnancy. Nine (12%) are still receiving treatment. Sixteen patients (21%) had an outbreak during the time on treatment.

Conclusion and Relevance A large proportion of the patients analysed manage to reach the 2-year treatment period, after which the risk of JCV infection increases. At that point, the majority of patients discontinue treatment. The drug is well tolerated, with little suspension of treatment due to adverse effects, and is usually chronic fatigue (also associated with the disease). Effective drug, with only 16 patients having an outbreak during treatment. With these data, we can conclude that in our patients it has been an effective treatment, used once the patient has high activity to stop it. Regarding safety, JCV would be the main drawback, requiring close monitoring for possible infection.

Conflict of Interest No conflict of interest.