Background: Rituximab is a chimeric monoclonal anti-CD20 B-cell depleting antibody increasingly used off-label in multiple sclerosis (MS). The clinical relevance of anti-drug antibodies (ADA) against rituximab in MS is unknown.

Objective: To determine frequency of ADA in relation to B-cell counts, allergic reactions and clinical efficacy in a large cohort of MS treated patients.

Methods: Cross-sectional study with collection of serum samples from 339 MS patients immediately before a scheduled rituximab infusion. ADA were detected using an in-house validated electrochemiluminescent immunoassay and a commercial ELISA to compare methods. Data on patient demographics and clinical outcomes were retrieved from the Swedish MS Registry and patient records.

Results: ADA were detected in 37% of relapsing-remitting MS and 26.5% in progressive forms of MS. Presence of ADA decreased with increasing number of rituximab infusions. There was a significant association between both presence and titres of ADA, and incomplete B-cell depletion, but not with infusion/adverse reactions or clinical outcomes at the group level. Only five patients terminated rituximab during follow up, four of which were ADA positive.

Conclusions: Rituximab treatment is associated with a high degree of ADA, which correlates with efficacy of B cell depletion, however, the clinical relevance of ADA remains uncertain.