Introduction In people with multiple sclerosis treated with interferon-beta or glatiramer acetate, new MRI lesions and relapses during the first year of treatment predict poor prognosis. However, this association has not been studied in those receiving natalizumab.

Methodology Data were collected on early relapses and new MRI activity from an observational cohort of 161 patients on natalizumab, with long-term follow-up. Two outcome measures were used. The first – ‘Clinical Activity Years 1–3’ – defined non-responders as those with relapses or EDSS progression at years 1–3 post-treatment, versus baseline. The second – ‘Disability Progression Years 3+’ – defined non-responders as those with EDSS progression at 3–7 years post-treatment, versus baseline.

Results ‘Clinical Activity Years 1–3’ was strongly predicted by early relapses (Odds Ratio (OR) 8.4, p<0.0001), and by a combined score of early relapses and MRI activity (Modified Rio Score 1: OR 4.9, p<0.001. Modified Rio Score 2: OR 19.0, P<0.01). In contrast, ‘Disability Progression Years 3+’ was not predicted by early relapses or MRI activity.

Conclusions Relapses and new MRI activity within the first year of starting natalizumab predict short-term outcomes, but not long-term disability outcomes. This requires further validation in other cohorts.