Occurrence of antibodies against natalizumab in relapsing multiple sclerosis patients treated with natalizumab

Background: In the clinical trials about 9 % of natalizumab treated multiple sclerosis (MS) patients generated anti-natalizumab antibodies, of which 6 % were persistent and 3 % transient. The occurrence of antibodies reduced serum levels of natalizumab, decreased bio-efficacy, and abrogated the therapeutic efficacy. Objective: The objective was to assess the frequency of anti-natalizumab antibodies in an unselected cohort of patients from four different countries. Methods:We measured anti-natalizumab antibodies in a large cohort of 4881 unselected patients from four MS centres that systematically measured antibodies in patients treated with natalizumab. We applied the same ELISA assay developed by Biogen Idec and used in the pivotal trials of natalizumab. Results: Antibodies occurred in 4.5 % (95 % confidence interval, CI: 4.0–5.1%) of the patients, and were persistent in 3.5 % (95 % CI: 3.0–4.0%) and transient in 1.0 % (95 % CI: 0.7–1.3%) of the patients. The frequencies of permanently antibody positive patients did not show statistically significant differences between the four centres, whereas the fre-quencies of transiently antibody positive patients showed some variations. Conclusion: The frequencies of antibodies appeared to be of the same magnitude in the four centres, but might be less than in the pivotal studies of natalizumab