Efficacy of imatinib mesylate in the treatment of idiopathic hypereosinophilic syndrome.

Idiopathic hypereosinophilic syndrome (HES) is a myeloproliferative disorder characterized by persistent eosinophilia and organ involvement. Different treatments have been investigated in HES with modest success. It has been suggested that imatinib is active in HES. We treated 9 patients with HES with 100 mg imatinib daily. Doses for patients without response after 4 weeks were increased to 400 mg daily. Prior therapy had failed for 7 patients. Five patients responded: 4 achieved sustained complete remission lasting a median of 12+ weeks (range, 9+ to 36+ weeks), and 1 had a transient response. One patient died in complete remission. Responses occurred within 4 weeks of therapy; only 1 responder required an increase in dose to 400 mg daily. Three of 4 nonresponders failed to respond to an increase in dose. Toxicity was minimal. We conclude that imatinib therapy is effective for HES.