Daclizumab, a humanised monoclonal antibody, reduced multiple sclerosis disease activity in previous non-randomised studies. We aimed to assess whether daclizumab reduces disease activity in patients with active relapsing multiple sclerosis who are receiving interferon beta treatment.

We did a phase 2, randomised, double-blind, placebo-controlled study at 51 centres in the USA, Canada, Germany, Italy, and Spain. Patients with active relapsing multiple sclerosis who were taking interferon beta were randomly assigned to receive add-on subcutaneous daclizumab 2 mg/kg every 2 weeks (interferon beta and high-dose daclizumab group), daclizumab 1 mg/kg every 4 weeks (interferon beta and low-dose daclizumab group), or interferon beta and placebo for 24 weeks. The randomisation scheme was generated by Facet Biotech. All patients and assessors were masked to treatment with the exception of Facet Biotech bioanalysts who prepared data for the data safety monitoring board or generated pharmacokinetic or pharmacodynamic data, a drug accountability auditor, and the site pharmacist. The primary endpoint was total number of new or enlarged gadolinium contrast-enhancing lesions measured on brain MRI scans every 4 weeks between weeks 8 and 24. Effects of daclizumab on prespecified subsets of lymphocytes and quantitative T-cell proliferative response were assessed in an exploratory pharmacodynamic substudy. Analysis was by intention to treat. This trial is registered with ClinicalTrials.gov, number NCT00109161.

From May, 2005, to March, 2006, 288 patients were assessed for eligibility, and 230 were randomly assigned to receive interferon beta and high-dose daclizumab (n=75), interferon beta and low-dose daclizumab (n=78), or interferon beta and placebo (n=77). The adjusted mean number of new or enlarged gadolinium contrast-enhancing lesions was 4·75 in the interferon beta and placebo group compared with 1·32 in the interferon beta and high-dose daclizumab group (difference 72%, 95% CI 34% to 88%; p=0·004) and 3·58 in the interferon beta and low-dose daclizumab group (25%, −76% to 68%; p=0·51). In the pharmacodynamic substudy, daclizumab was not associated with significant changes in absolute numbers of T cells, B cells, or natural killer cells, or T-cell proliferative response compared with interferon beta alone. The number of CD56bright natural killer cells was seven to eight times higher in both daclizumab groups than in the interferon beta and placebo group (interferon beta and low-dose daclizumab group p=0·002; interferon beta and high-dose daclizumab group p<0·0001). Common adverse events were equally distributed across groups.

Add-on daclizumab treatment reduced the number of new or enlarged gadolinium contrast-enhancing lesions compared with interferon beta alone and might reduce multiple sclerosis disease activity to a greater extent than interferon beta alone.

Facet Biotech and Biogen Idec.