Natalizumab concentrations during pregnancy in three patients with multiple sclerosis.

In women with very active multiple sclerosis (MS), natalizumab can be continued during pregnancy to prevent rebound disease activity. Our aim was to evaluate changes in serum natalizumab trough concentrations during pregnancy. Blood samples of 3 patients were collected before, during, and after pregnancy. Natalizumab trough concentrations gradually decreased during pregnancy. The patient with the lowest trough concentrations during the third trimester was treated with extended interval dosing (EID). After delivery, natalizumab concentrations increased to similar levels as before pregnancy. All patients remained clinically and radiologically stable. MS neurologists should be aware of decreasing natalizumab concentrations during pregnancy, especially in patients with low initial trough concentrations and patients with EID.