High-sensitivity detection of BCR-ABL kinase domain mutations in imatinib-naive patients: correlation with clonal cytogenetic evolution but not response to therapy.

Mutations in the kinase domain (KD) of BCR-ABL are the leading cause of acquired imatinib resistance. In some cases, identical mutations were detected at relapse and in pretherapeutic specimens, consistent with selection of resistant clones in the presence of drug. However, the incidence of KD mutations in imatinibnaive patients, irrespective of response to therapy, is unknown. We studied mutation frequency in 66 patients with chronic myelogenous leukemia (CML), using cDNA sequencing and allele-specific oligonucleotide-polymerase chain reaction (ASO-PCR) assays for 8 common mutations. Thirteen patients were positive by ASO-PCR only, 1 by ASO-PCR and sequencing, and 1 by sequencing only (overall frequency, 22.7%). T315I was most frequent (12% of patients). Eleven of the 14 patients with positive ASO-PCR had follow-up samples available for sequencing. Wild-type sequence was detected in 6 of 11, 2 different mutations in 1 of 11, and identical mutations in 4 of 11 patients, 2 of whom had achieved major cytogenetic response. In multivariate analysis mutation detection was associated with clonal cytogenetic evolution, exposure to 6-Thioguanine, and a low platelet count, but not with response to imatinib, event-free survival, and overall survival. KD mutants present at low levels do not invariably lead to relapse, and additional factors are required to induce a fully drug-resistant phenotype.

Quantitative reverse transcription polymerase chain reaction should not replace conventional cytogenetics for monitoring patients with chronic myeloid leukemia during early phase of imatinib therapy.

BACKGROUND AND OBJECTIVES: Imatinib is the new standard drug treatment for patients with chronic myelogenous leukemia (CML). Quantitative reverse transcription-polymerase chain reaction (qPCR) for detection of BCR-ABL transcripts is frequently used for monitoring patients in addition to or instead of conventional cytogenetics, although its place in the overall diagnostic framework is not yet clear. In this study, we compared qPCR and conventional cytogenetics for monitoring of patients during the early phases of imatinib therapy. DESIGN AND METHODS: One hundred and seventeen patients treated with imatinib for CML in chronic or accelerated phase were prospectively followed with qPCR and karyotyping. Comparisons were made between both methods and between qPCR results from bone marrow and peripheral blood. To determine the prognostic impact of qPCR and cytogenetics during the early phase of imatinib treatment on subsequent cytogenetic response and progression-free survival (PFS), a multivariate model was generated that included established prognostic baseline variables. RESULTS: We found a significant correlation between the proportion of Philadelphia (Ph) chromosome-positive metaphases and qPCR in the bone marrow and peripheral blood. Low qPCR values after 3 months of therapy were correlated with major cytogenetic response (MCyR) at 6 months and PFS at 2 years. However, in multivariate analysis, the cytogenetic response at 3 months emerged as the only independent parameter predictive of MCyR at 6 months and PFS at 2 years. INTERPRETATION AND CONCLUSIONS: Our data suggest that conventional karyotyping should remain the standard method for following patients on imatinib during the early phases of therapy.