2-Chlorodeoxyadenosine therapy for idiopathic hypereosinophilic syndrome. 
Idiopathic hypereosinophilic syndrome (HES) is a rare disease with no established effective therapy.  It has been reported that interleukin-5 (IL-5) produced by helper T cells plays a major role in the proliferation of eosinophils.  The nucleotide analogue 2-chlorodeoxyadenosine (2-CdA), which induces excellent clinical responses in hairy cell leukemia, is known to suppress helper T cells; therefore, we used 2-CdA, alone or in combination with cytarabine, to treat patients with idiopathic HES.  2-CdA alone and combined with cytarabine resulted in a rapid and sustained decrease in circulating eosinophils in two patients with idiopathic HES that was refractory to steroids, hydroxyurea and cytarabine.  The efficacy of 2-CdA alone and combined with cytarabine exceeded by far that of cytarabine alone.  However, reverse transcription-polymerase chain reaction (RT-PCR) did not show production of IL-5 or granulocyte-macrophage colony-stimulating factor mRNA in T cells as previously reported, and multiple cytokine receptors were found on eosinophils in idiopathic HES, suggesting that IL-5 may not be the sole cytokine involved in the regulation of idiopathic HES.  The clinical efficacy of 2-CdA in idiopathic HES needs to be established on a large group of patients.