The efficacy of imatinib mesylate in patients with FIP1L1-PDGFR alpha-positive hypereosinophilic syndrome. Results of a multicenter prospective study

Background and Objectives The hypereosinophilic syndrome (HES) may be associated with the fusion of the platelet derived growth factor receptor alpha (PDGFR alpha) gene with the FlP1L1 gene in chromosome 4 coding for a constitutively activated PDGFRa tyrosine kinase. These cases with FlP1L1-PDGFR alpha rearrangement have been reported to be very sensitive to the tyrosine kinase inhibitor imatinib mesylate. Design and Methods A prospective multicenter study of idiopathic or primary HES was established in 2001 (Study Protocol Registration no. NCT 0027 6929). One hundred and ninety-six patients were screened, of whom 72 where identified as having idiopathic or primary HES and 63 were treated with imatinib 100 to 400 mg daily. Results Twenty-seven male patients carried the FlP1L1-PDGFR alpha rearrangement. All 27 achieved a complete hematologic remission (CHR) and became negative for the fusion transcripts according to reverse transcriptase polymerase chain reaction (RTPCR) analysis. With a median follow-up of 25 months (15-60 months) all 27 patients remain in CHR and RT-PCR negative, and continue treatment at a dose of 100 to 400 mg daily. In three patients imatinib treatment was discontinued for few months, the fusion transcript became rapidly detectable, and then again undetectable upon treatment reassumption. Thirty-six patients did not carry the rearrangement; of these, five (14%) achieved a CHR, which was lost in all cases after 1 to 15 months. Interpretation and Conclusions All patients meeting the criteria for idiopathic or primary HES should be screened for the FlP1L1-PDGFR alpha rearrangement. For all patients with this rearrangement, chronic imatinib treatment at doses as low as 100 mg daily ensures complete and durable responses.