This non-systematic review identified four randomised trials that have tested the efficacy of interferon beta in secondary progressive multiple sclerosis (SPMS). Two were trials of interferon beta 1a (IFNb1a) and two of interferon beta 1b (IFNb1b). All have shown significant reductions in relapse rates and accumulation of new magnetic resonance imaging (MRI) lesions, but only one trial (of IFNb1b) showed significant slowing of disability progression. Post hoc analyses of these trials suggest that the differences in outcomes might be partly explained by the differences between the trials in the proportions of patients with relapsing disease. In one of the trials of IFNb1a (the SPECTRIMS trial), the hazard ratio for progression in the treated relapsing patients with relapses in the two pre-study years was 0.74 compared to placebo patients with pre-study relapses and 1.01 in the treated patients compared to the placebo patients without pre-study relapses. In the same trial, the treatment effects on MRI parameters were more marked in the patients who had recent pre-study relapses compared with those who had not. These observations have led to the recommendation in national guidelines that prescribing of IFNb in SPMS be limited to those patients who have had disabling relapses in the last 2 years. These conclusions should be reviewed when the full results of all four trials have been published.
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