The incidence and significance of anti-natalizumab antibodies -: Results from AFFIRM and SENTINEL

Objective: To determine the incidence and clinical effects of antibodies that develop during treatment with natalizumab. Methods: In two randomized, double- blind, placebo- controlled studies ( natalizumab safety and efficacy in relapsing remitting multiple sclerosis [ MS, AFFIRM] and safety and efficacy of natalizumab in combination with interferon beta- 1a [ INF beta 1a] in patients with relapsing remitting MS [ SENTINEL]) of patients with relapsing multiple sclerosis, blood samples were obtained at baseline and every 12 weeks to determine the presence of antibodies against natalizumab. Antibodies to natalizumab were measured using an ELISA. Patients were categorized as transiently positive if they had detectable antibodies ( >= 0.5 mu g/ mL) at a single time point or persistently positive if they had antibodies at two or more time points >= 6 weeks apart. Results: In the AFFIRM study, antibodies were detected in 57 of 625 ( 9%) of natalizumab- treated patients: Twenty ( 3%) were transiently positive and 37 ( 6%) were persistently positive. Persistently positive patients showed a loss of clinical efficacy as measured by disability progression ( p <= 0.05), relapse rate ( p= 0.009), and MRI ( p <= 0.05) compared with antibody- negative patients. In transiently positive patients, full efficacy was achieved after approximately 6 months of treatment, the time when patients were becoming antibody negative. The incidence of infusionrelated adverse events was significantly higher in persistently positive patients. Results of SENTINEL were similar to AFFIRM, except with regard to sustained disability progression; differences between persistently positive and antibody- negative patients were not statistically significant. Conclusions: The incidence of persistent antibody positivity associated with natalizumab is 6%. Reduced clinical efficacy is apparent in persistently positive patients. Patients with a suboptimal clinical response or persistent infusion- related adverse events should be considered for antibody testing.