A prospective clinical trial with the aim of maintaining drug efficacy of natalizumab while extending dose intervals guided by drug concentrations in patients with relapsing remitting multiple sclerosis.

Our main objective is to prove that extending dose intervals guided by natalizumab serum concentrations, will not result in radiological or clinical disease activity in completely stable RRMS patients treated with natalizumab. Adult patients with RRMS with no evident disease activity during the last 12 months of natalizumab infusions with a minimum treatment of 12 months wil be included. Before subsequent natalizumab infusions, serum concentrations will be evaluated. If the concentration exceeds 15μg/ml the dose interval will be extended with a week to a maximum of eight weeks. Patients will get regular brain MRI scans and clinical follow-up.

Inclusion Criteria:

• Relapsing-remitting multiple sclerosis (RRMS) according to the McDonald criteria, revised by Polman 2010
• Natalizumab treatment for 12 months or longer at inclusion.
• An expanded disability status scale (EDSS) score of 0.0-6.0 at baseline.
• Natalizumab level of ≥15 μg/ml
• Written informed consent.

Exclusion Criteria:

• Any MS disease activity (radiologically or clinically) during the last 12 months of natalizumab treatment.
• Unable to undergo frequent MRI.
• The use of other immunomodulatory medication other than natalizumab.