Pharmacodynamics of Natalizumab Extended Interval Dosing in MS.

Objective To determine if the concentration and saturation of natalizumab (NTZ) administration at extended interval dosing (EID; every 5-8 weeks) over 18 months is able to be maintained in the range considered adequate to sustain the clinical efficacy of NTZ. Methods In a cross-sectional assessment of patients with multiple sclerosis (MS) who received standard interval dosing (every 4 weeks) or EID, serum NTZ concentrations were measured using ELISA, and alpha(4)-integrin receptor saturations were analyzed via cytometry, in blood samples obtained at trough timepoints. Results Trough serum concentration was above the "therapeutic" concentration of 2.0 mu g/mL in 72% of EID patients. Trough saturation was above the "therapeutic" 50% threshold in 79% of EID-treated patients. Our model predicted that at least 9 NTZ infusions/year are required to maintain adequate trough saturation and concentration levels. Higher body mass index (BMI) was a predictor of suboptimal trough saturation on EID NTZ. Conclusions Trough alpha 4-integrin receptor saturation >50% correlated with high clinical efficacy of NTZ in previous studies. A continual treatment with EID maintains receptor saturation and concentration that are in the "therapeutic range" for most patients. This finding provides biological plausibility for the clinical efficacy of NTZ EID. Patients with higher BMI may require closer clinical and MRI follow-up.