Souza CA, Pagnano KB, Bendit I, Conchon M, Freitas CM, Coelho AM, Funke VA, Bernardo WM Does the diagnosis of mutations guide the choice of treatment in imatinib-resistant patients? In imatinib-resistant CML patients, mutations can assist in the choice of second-generation TKIs (nilotinib or dasatinib). An evaluation of the sensitivity of mutations to inhibitors in in vitro studies (IC50) defines three groups of sensitivity (low, intermediate and high concentrations) of the mutation to: dasatinib (IC50 ≤ 3 nM, 3-60 nM and > 60 nM) and nilotinib (IC50 ≤ 50 nM, 50-500 nM and > 500 nM) with the worst case scenario (resistance) corresponding to high concentrations(78)(B). Hematologic and cytogenetic responses at one year are significantly lower in patients with mutations and in the chronic phase, particularly for mutations with intermediate IC50 (25% and 25%, respectively) compared to low IC50 (96% and 54%, respectively). In the accelerated phase there is also a reduction in the cytogenetic response for mutations with 10% reduction in intermediate IC50 and 31% reduction in low IC50,(78)(B). In the chronic phase, the disease-free survival and overall survival are lower in patients with mutations with high IC50 (0% and 75%, respectively), when compared with mutations with low IC50 (78% and 100%, respectively)(78)(B). The T315I mutation is associated with high IC50 (resistance) but there is no difference comparing dasatinib and nilotinib(78)(B). Other specific mutations associated with high IC50 (resistance) in the chronic phase of CML treated with dasatinib are: T315I/A, F317L/I/V/C and V299L(81-83)(B), and with nilotinib: T315I, Y253H, E255K/V and F359V/C(82,84)(B). The G250E mutation also has an impact on resistance common in the two forms of treatment(83)(B). Mutations associated with resistance to dasatinib such as V299L, T315A and F317I may be sensitive to nilotinib, while the mutation V299L may be resistant to bosutinib(83-85)(B). Complete cytogenetic response subsequent to treatment using dasatinib or nilotinib is lower in patients with resistant mutations [mutations detected by sequencing that confer resistance to the inhibitor received (0%) - T315I, F359V/C, F317L, Y253H, E255V/K] compared to patients with other mutations (mutations detected by sequencing or spectrometry of masses sensitive to the inhibitor received) or without mutations (41% and 49%, respectively). The survival of chronic phase CML patients when resistant mutations are detected is 0% compared with 51% and 45% in patients with other mutations or without mutations, respectively(86)(B). Recommendation: the identification of mutations, especially resistant mutations, can assist in the choice of the TKI, allowing the definition of which therapeutic option will provide the best response. How should monitoring of CML patients taking tyrosine kinase inhibitors be performed? Reports state that monitoring of chronic-phase CML patients for BCR-ABL mutations during imatinib treatment can 374 be achieved with PCR in peripheral blood (BCR-ABL/ABL ratio) correlating this with the result obtained through the usual cytogenetic study of bone marrow. This identifies responsive, partial responsive and unresponsive patients in respect to BCR- ABL/gene control of up to 0.08%, of 0.08% to 10% and of above 10%, respectively(87)(B). In a randomized trial, 1106 CML patients were treated with interferon or imatinib as first-line treatment. All patients who achieved cytogenetic remission performed qPCR for BCR-ABL mutations. The results were expressed in terms of the logarithmic reduction in relation to the median level of transcripts in 30 newly-diagnosed patients. Patients who achieved complete cytogenetic remission and at least a 3-log reduction in the level of transcripts, had progression-free survival of 100% at 24 months compared to 95% for those with complete cytogenetic remission and less than a 3-log reduction in the level of transcripts and 85% for patients without complete cytogenetic response(88)(B). Thus, this form of monitoring also allows you to identify the 2-year progression-free survival with the low values of transcripts(58,88)(B). Using samples from 38 international centers, one study validated the use of an international scale of BCR-ABL values that established 0.1% as a 3-log reduction(89)(B). It is possible to stratify patients by PCR during the 3 years follow-up as patients whose indexes reflect increases, stability or reduction, or even loss of cytogenetic response(53)(B). Plasma imatinib levels are significantly higher in patients with molecular and cytogenetic responses compared to patients without response. The level that differentiates the difference between molecular response and lack of response with the greatest accuracy (77% sensitivity and 71% specificity) is 1002 ng/mL(90)(B). The use of FISH to monitor CML patients on imatinib treatment enables the use of peripheral blood to identify cytogenetic response. A positive result points to the absence of cytogenetic response and a negative result identifies its presence. The association with PCR allows the molecular response to be monitored. In a study published by Reinhold et al., the estimated cytogenetic and molecular responses at 5 years were 81.7% and 67.1%, respectively(54)(B). However, the comparison between the results of FISH using peripheral blood leukocytes and the cytogenetics of the bone marrow may not establish an appropriate correlation in the measurement of CML activity during imatinib treatment(91)(B). The existence or occurrence of mutations in CML patients under TKI treatment, when identified, enables an estimation of the prognosis and guides treatment. High- performance liquid chromatography (HPLC) is a practical and sensitive method to identify mutations to clinically monitor patients(92)(B). Some mutations can be identified by direct sequencing during the follow-up of patients including: T315T, T315I, F317L, V339L, M351T, E355G, Y253F and F359V; these are associated with different responses to available inhibitors. A 31% reduction in the overall survival of patients with mutations is identified in the 3-year follow-up(74)(B). Rev Bras Hematol Hemoter. 2012;34(5):367-82