The most frequent causes of imatinib resistance are BCR-ABL mutations, usually selected during treatment. Most of them destabilize the inactive conformation necessary for imatinib binding. The T315I mutation, located at the ATP binding pocket is resistant to imatinib and other available kinase inhibitors and is associated with a poor outcome. The aim of this study was to evaluate the frequency of T315I mutation in patients with CML resistant to imatinib and to describe their outcome. Mutation analysis was performed in two institutions, through direct automated sequencing. Total RNA was extracted from peripheral blood or bone marrow and amplification of the kinase domain of ABL from BCR/ABL was performed, using a semi-nested RT-PCR, to cover amino acids 244–486. PCR product was submitted to direct automated sequencing and compared with normal sequences of BCR-ABL gene (M14752, GenBank). A total of 173 resistant imatinib resistant patients were analyzed. BCR-ABL mutations were detected in 47 out of 173 (27%) patients. Among these patients, 14 (30%) presented T315I mutation. The mutation was found in ten patients during imatinib treatment, in one before start of imatinib, in three after dasatinib (2) and nilotinib (1) failure. Three patients have more than one mutation analysis, performed during different time-points of treatment: imatinib resistance, dasatinib resistance (3) and bone marrow transplantation relapse (1). Overall survival from diagnosis was 39,5% and 17,5% from mutation detection, in a median time of 69 and 13 months respectively. After imatinib resistance, eight patients were treated with dasatinib, one with nilotinib and had no hematological response; tree were treated with Hydrea and two were submitted to allogeneic bone marrow transplantation. Five patients died due to disease progression in median time of 19 months after mutation detection. One patient died after bone marrow transplantation, due to GVHD and infection. Eight patients are alive, three in CP, two in BC, two in hematological response and one in complete cytogenetic response after bone marrow transplantation. In conclusion, in this study we confirmed the high frequency of T315I mutation in imatinib resistant patients and their poor prognosis. Most of them started imatinib in a late chronic or advanced phase, with exception of two patients which started imatinib 800mg in early chronic phase, suggesting that there was inhibition of Ph positive sensitive cells and selection of resistant clones.

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