Early landmark analysis of imatinib treatment in CML chronic phase: less than 10% BCR-ABL by FISH at 3 months associated with improved long-term clinical outcome.

Imatinib has dramatically improved the clinical outcome in chronic myeloid leukemia, chronic phase (CMLcp), but a risk of resistance and serious disease progression still prevails. We have studied 45 newly diagnosed CMLcp patients initiated on imatinib, assessing treatment responses by interphase extral signal (ES)-fluorescence in situ hybridization (FISH), quantitative real-time (q-RT) polymerase chain reaction (PCR), and chromosome banding analysis. In a landmark analysis, an early favorable response, defined as less than 10% BCR-ABL-positive cells by FISH after 3 months of treatment, was identified as a predictive marker of an improved long-term clinical outcome. Of evaluable patients, 51% achieved this response. A large majority, 95% of such responders reached complete cytogenetic responses (CCyR) within 12 months and 100% event-free survival (EFS) at 48 months, when compared with 67 and 65%, respectively, of patients with higher breakpoint cluster region - Abelson (BCR-ABL) positivity at 3 months (P = 0.04; P = 0.006). No similar, significant correlations were noted between early disease assessments with PCR of BCR-ABL mRNA transcripts or of cytogenetics versus a 12-month CCyR or long-term EFS. Our data, based on a limited patient cohort, indicate that (i) FISH can effectively be used in the early assessment of remaining Ph-positive cells to identify patients at risk for a long-term nonoptimal response to imatinib and that (ii) FISH may be more useful than PCR for this purpose.