Introduction The TYSABRI® Observational Program (TOP) is a global, open-label study of natalizumab for relapsing-remitting multiple sclerosis (RRMS) in clinical practice.

Methods Baseline characteristics were summarised. On-therapy annualised relapse rates (ARRs) were compared with baseline.

Results As of 01 May 2013, 122 patients were enrolled in the United Kingdom (UK) and 5122 patients were enrolled globally. Median duration of RRMS was 5.4 years in the UK and 7.3 years globally. Mean numbers of relapses in the 2 years prior to natalizumab treatment were 3.22 in the UK and 2.85 globally. Mean baseline Expanded Disability Status Scale (EDSS) scores were 4.2 in the UK and 3.5 globally. Median Multiple Sclerosis Severity Scores (MSSS) were 6.2 in the UK versus 4.9 globally (p<0.0001) Compared with the year prior to natalizumab initiation, on-treatment ARRs decreased by 87% in the UK (P<0.0001) and by 88% globally (P<0.0001).

Conclusions The UK Cohort appears to have more aggressive disease, as evidenced by higher MSS score and higher ARR history. Despite this similar significant reductions in ARR were observed in the UK-specific and global cohorts.

Supported by Biogen Idec Ltd.

RN: grant support and support for travel to conferences from Biogen Idec Ltd.

JH: employee of Biogen Idec Ltd.