Effectiveness of natalizumab extended interval dosing in multiple sclerosis patients

Aim: Natalizumab is a highly effective drug used in the treatment of multiple sclerosis, but it is also often associated with a risk of inducing progressive multifocal leukoencephalopathy. In order to min imalize this risk, extended interval dosing has been proposed. The aim of this study was to assess possible consequences of extended interval dosing on the effectivity of the treatment. Patients and methods: We conducted a retrospective analysis of all patients treated with natalizumab using a 6-week interval dosing schedule at the Centre for Diagnostics and Treatment of Demyelinating Disorders at Motol University Hospital. Only patients pre-treated for at least 1 year with a standard 4-week interval schedule, and then treated for at least 1 year with 6-week interval schedule were enrolled. The reasons to switch to an extended interval dosing were either a high risk of progressive multifocal leukoencephalopathy or long-term clinical stabilization on natalizumab. Results:Overall, 25 patients were enrolled in this study. Data were collected for the periods before and after switching to 6-week interval dosing. The average analyzed period was 3.8 +/- 1.9 years (for both dosing regimens altogether). The average annualized relapse rate in 4-week and 6-week interval periods was 0.073 +/- 0.209 and 0.074 +/- 0.178, resp. (P = 1.0000). The average new yearly MRI activity in 4-week and 6-week interval periods was 0.29 +/- 0.40 and 0.16 +/- 0.33, resp. (P =0.1250).The average Expanded Disability Status Scale score in 4-week and 6-week interval periods was 2.29 +/- 0.90 and 2.16 +/- 0.88, resp. (P = 0.0127). There was one case of progressive multifocal leukoencephalopathy recorded in the 6-week interval group; none occurred in the 4-week interval group. Conclusions: In the population of patients treated with natalizumab at our centre, we did not prove that 6-week interval dosing was less effective compared to the standard 4-week interval dosing.