Hypereosinophilia in patients with multiple sclerosis treated with natalizumab

Objective: To report asymptomatic hypereosinophilia as a potential side effect in patients treated with natalizumab, an alpha-4 integrin blocking agent. Methods: A case series of 3 patients treated with natalizumab for relapsing-remitting multiple sclerosis including functional and phenotypic characterization of their peripheral blood lymphocytes and eosinophils is presented. Results: Marked peripheral blood eosinophilia with more than 2,000 cells/mm(3) emerged in all 3 patients after the fourth natalizumab infusion and was asymptomatic. Hypereosinophilia was associated with enhanced Th2 activity, ceased with drug discontinuation, and in 2 of 3 patients recurred with drug resumption. Despite persistently high eosinophil counts, there were no signs of end-organ damage. Conclusions: Hypereosinophilia may occur during treatment with natalizumab. It seems to reflect enhanced Th2 activity and recedes with systemic corticosteoids. If the patient is asymptomatic, natalizumab may be continued, provided that other causes of eosinophilia are excluded and the patient is carefully monitored. Neurology (R) 2011;77:1561-1564