The introduction of Imatinib in the treatment of Chronic Myelogenous Leukemia (CML) leads to the achievement of Complete Cytogenetic Response (CCR) in about 70% of patients: however, in the remaining 30% of patients there is a persistance of Ph+ cells also after standard (400 mg/day) and increased (600 mg/day) dose of Imatinib. These patients are thus cytogenetically resistant to Imatinib alone and their management is at present unclear. From 11/2002 to 11/2003, 10 patients in chronic phase (6 male and 4 female, median age 52.5 years, range 29 – 68 years) with persistance of 100% Ph+ cells (9 patients) or BCR/ABL + cells (1 patient with Ph- BCR/ABL+ CML at onset) after standard (at least 6 months of treatment) followed by increased dose (at least 3 months of treatment) of Imatinib alone, were considered resistant and added Hydroxyurea (HU) to Imatinib. Seven patients have been pretreated with IFN before Imatinib; median times from diagnosis and from Imatinib treatment to HU addition were 51 months (range 23 – 151) and 14 months (range 10 – 31), respectively. HU was given according to WBC count: patients with WBC < 10 x 109/l started HU at the dose of 1 g/day, patients with WBC > 10 x 109/l at the dose of 1.5 g/day. Imatinib was continued at the same previous dosage (600 mg/day in 6 patients and 400 mg/day in 4 patients who did not tolerate increased dosage for hematological toxicity). Three patients achieved a complete response (2 CCR after 3 and 12 months respectively and 1 molecular complete response after 9 months in the patient Ph- BCR/ABL+ at onset) and 1 patient achieved a partial CR (Ph+ < 33%) after 9 months: the remaining 6 patients were resistant with persistance of 100% Ph+ cells. Toxicity was mild and only 1 patient discontinued for 2 weeks the association due to transient thrombocytopenia: no extra-hematological toxicity has been recorded. After a median follow-up of 14 months (range 20 – 10), 2 patients (1 resistant and 1 after 5 months from the achievement of CCR) evolved in Blastic Phase (BP), 5 patients are in stable chronic phase with 100% Ph+ cells and 3 patients are still in response after 4,6 and 7 months respectively. In conclusion, the association of HU with Imatinib seems capable to induce cytogenetic response in at least one third of patients resistant to Imatinib alone, with minimal toxicity: a longer follow-up and a comparison with other associations is needed to evaluate the quality and duration of response in such group of patients.

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