Introduction The Tysabri^® Observational Program (TOP) is a 10-year, ongoing, global open-label study in clinical practice settings enrolling natalizumab-treated patients with relapsing-remitting multiple sclerosis (MS).

Methods Annualized relapse rates (ARRs) pre- and post-natalizumab were compared.

Results As of 1 May 2015, 134 UK TOP patients had received a median (range) of 43 (1 64) doses. Median disease duration was 5.3 years, with 3.15 mean relapses in the 2 years pre-natalizumab. Among all patients, 63.4% had prior MS therapy (mean duration 1.8 years). Prior treatments included interferon betas (IFN; n=66), glatiramer acetate (GA; n=34), fingolimod (n=1), and immunosuppressants (n=3). Mean time between stopping prior treatment and starting natalizumab was 4–6 months. ARR decreased from 2.21 pre-natalizumab to 0.23 on treatment (89.6% reduction; p<0.0001). Similarly, in natalizumab patients previously treated with IFN and GA, ARR decreased from 2.03 to 0.21 (89.7% reduction) and from 2.00 to 0.23 (88.5% reduction), respectively (p<0.0001). No progressive multifocal leukoencephalopathy cases or new safety signals were identified in this cohort.

Conclusions Notwithstanding the inherent limitations of this small-cohort analysis, natalizumab reduces relapse rate post IFN or GA.

Supported by Biogen. Disclosures: RN: grant and conference travel support from Biogen; JH: employee of and holds stock/stock options in Biogen.