Response to "Letter to the editor on the paper: The majority of natalizumab-treated MS patients have high natalizumab concentrations at time of re-dosing"

Background: Natalizumab is efficacious in the treatment of relapsing-remitting multiple sclerosis. All patients receive the same treatment regimen of 300mg every 4weeks, despite differences in pharmacokinetics between individual patients. Objective: To give neurologists insight into natalizumab concentrations at time of re-dosing, we investigated longitudinal natalizumab concentrations in 80 patients in relation to disease activity, with possible influencing factors. Methods: In a prospective observational cohort study, natalizumab trough serum concentrations were measured in 80 patients. Data on demographics, duration of treatment, Expanded Disability Status Scale, clinical exacerbations, brain magnetic resonance imaging (MRI), and body weight were collected. Results: We measured high (10 mu g/mL) natalizumab trough concentrations in 94% of patients. Intra-individual concentrations were stable. The spread in concentrations was substantial and did not correlate with disease activity. We found a negative association between natalizumab concentration and body weight (=-0.30, p=0.010). Interpretation: The majority of patients showed high natalizumab serum concentrations at time of re-dosing. Alternative treatment regimens could lead to more efficient use of natalizumab, but caution is warranted regarding the possibility of recurrence of disease activity. Prospective clinical trials are needed to establish the safety of extended dose intervals in natalizumab treatment.