Introduction The Tysabri^® Observational Program (TOP) is a 10-year, ongoing, global, open-label study of long-term outcomes in natalizumab-treated patients with relapsing-remitting multiple sclerosis.

Methods Baseline characteristics were summarised. On-treatment annualised relapse rate (ARR) was compared with baseline.

Results As of 1 May 2014, the UK cohort included 134 patients. Median disease duration was 5.3 years and 84 patients (62.7%) had prior treatment with ≥1 other therapy. ARR decreased from 2.21 at baseline to 0.29 (86.9% reduction; p<0.0001); on-treatment ARR was low with ≥6 months of treatment (0.29; 87.2% reduction; p<0.0001; n=120) and remained low after ≥3.5 years (0.33; 85.7% reduction; p<0.0001; n=20). In patients previously treated with interferon or glatiramer acetate, ARR decreased by 83.5%–87.7% (p≤0.0009). Relapses resulting in hospitalisation or steroid treatment decreased by 93.3% (p<0.0001) and 86.5% (p<0.0001), respectively.

Conclusions UK TOP patients exhibited ≥83.5% reduction in ARR with ≥6 months of natalizumab treatment regardless of prior therapy, a reduction maintained over ≥3.5 years with no waning of effect. Relapses resulting in hospitalisation or steroid treatment were significantly reduced post natalizumab therapy initiation.

Sponsored by Biogen Idec Ltd. Disclosures: RN: grant and conference travel support from Biogen Idec Ltd. RB: employee of Biogen Idec Ltd.