ABL Mutations in Late-Chronic Phase Chronic Myeloid Leukemia Patients with Cytogenetic Refractoriness to Imatinib Are Associated with a Greater Likelihood of Progression to Blast Crisis and Shorter Survival. on behalf of the GIMEMA Working Party on Chronic Myeloid Leukemia.

Using a novel denaturing-high performance liquid chromatography (D-HPLC)-based screening method, we investigated the frequency and prognostic relevance of ABL mutations in 47 CML patients who never achieved a major cytogenetic response in 12 months of Imatinib therapy. Seven patients were enrolled in the CML/011/STI571 trial (early-CP patients treated with Imatinib 400 mg/d and peghilated-interferon) while the remaining 40 were enrolled in the CML/002/STI571 trial (late-CP patients resistant/refractory to α-interferon, treated with Imatinib 400 mg/d). For each patient, a longitudinal analysis was done on all the available samples collected from Imatinib start up to the twelfth month of therapy. A nested RT-PCR was set up and, for each sample, two fragments of 393 and 482 bp spanning the ABL kinase domain were analysed. Two out of 7 (29%) early-CP patients had mutations, resulting in both cases in novel aminoacid substitutions (F311I, E355D). Mutations were already detectable at 6 and 9 months, respectively, from Imatinib start. Nineteen out of 40 (48%) late-CP patients had mutations. Eleven patients showed mutations falling in close proximity (M244V) or within (G250E, Y253F, Y253H, E255K, E255V) the P-loop. Eight patients showed mutations outside the P-loop (F311L, F317L, M351, E355G, F359V, H396R and a silent mutation at codon 298). Mutations were already detectable after a median of 3 months (range, 1–6) from onset therapy. At the time of mutation detection, all patients but four had sustained hematologic response. Presence of a missense mutation was significantly associated with a greater likelihood of subsequent progression to accelerated phase/blast crisis (AP/BC)(P=0.0002) and shorter survival (P=0.001). Patients with mutations falling in close proximity or within the P-loop had a particularly poor outcome in terms of time to progression to AP/BC with respect to the remaining mutated patients (P=0.03). Our results indicate that: i) ABL kinase domain mutations may be found also in the setting of early-CP CML patients, even though they are not the predominant mechanisms of resistance; ii) as many as 50% of late-CP patients who fail to achieve cytogenetic control of the disease in the first 6 months of therapy already have evidence of a mutation by D-HPLC analysis; iii) within cytogenetic nonresponders, presence of ABL mutations may identify a subset of patients with particularly poor prognosis. Thus, irrespective of the hematologic response, regular monitoring for emerging mutations in the first months of Imatinib administration may play a crucial role in detecting patients for whom a revision of the therapeutic strategy should be considered.