The Plateau of BCR-ABL Transcript Level ≥0.1% May Select CML Patients in Complete Cytogenetic Remission for Mutation Analysis.

The major cause of the imatinib resistance is the presence of point mutations within the ABL kinase domain of BCR-ABL that are occurred in approximately 35-70% of patients displaying resistance to imatinib. Identification of mutation also in imatinib responders is associated with disease progression (Khorashad et al. 2008, J Clin Oncol 29:4806). According to ELN (European LeukemiaNet) patients should be screened for mutations in the case of: 1) hematological resistance / relapse, 2) cytogenetic resistance / relapse, 3) 5 – 10-fold increase of BCR-ABL transcript, 4) prior to therapy with 2nd generation TKIs, and 5) at 3-month intervals under therapy with 2nd generation TKIs. The BCR-ABL rise fold threshold for mutation analysis is highly discussed nowadays. Recently Press et al. (Blood 2009, prepublished online) identified a 2.6-fold increase in BCR-ABL RNA as the optimal cutoff for predicting a concomitant KD mutation, with a sensitivity of 77% (94% if including subsequent samples).

In this study, we aimed to detect BCR-ABL mutation in patients responding to imatinib with CCgR expressing the plateau of BCR-ABL transcript level ≥0.1% (IS; International Scale) in a minimum of 3 subsequent samples (minimal duration 6-9 months) and to evaluate the risk of disease progression in this group of patients.

From 140 CML patients in CCgR after imatinib therapy, 32 showed constant BCR-ABL transcript levels ≥0.1% after the initial reduction. Altogether 134 samples of peripheral blood of these 32 patients were tested for mutation in BCR-ABL kinase domain by direct sequencing. Patients with constant BCR-ABL values <0.1% were not included because of the limitation of mutation detection by sequencing as shown our tests of reproducibility and repeatability.

Median follow-up of BCR-ABL transcript level plateau within the range of evaluated variability of RQ-PCR analysis was in 32 patients 12 months (range 6-64); 22/32 patients were treated with imatinib in the first line. Mutation was detected by direct sequencing with the measured sensitivity of 20-100% of mutant BCR-ABL present in the sample with the minimum copy number of total BCR-ABL 100 (corresponding to 0.1% of BCR-ABL measured by our RQ-PCR) in 9/32 patients (28%); 5/9 were in the first line imatinib treatment. Loss of CCgR or 1 log rise of BCR-ABL was observed in 5/9 patients median 5 months (range 4-17) since first detection of mutation. One patient with no mutation relapsed 12 months after the start of the BCR-ABL plateau. In 5/32 patients without mutation the BCR-ABL level significantly decreased after the 1st plateau to the levels that stayed unchanged for a median of 11 months (range 7-28); three of them achieved major molecular response (MMR; <0.1% BCR-ABL).

In our study, patients expressing plateau of BCR-ABL transcript level ≥0.1% did not achieve MMR and were at more risk to lose their responses. We suppose that the fluctuation of BCR-ABL values ∼0.1% within the consecutive samples should not be classified as MMR achievement. Only levels constantly below 0.1% should be defined as MMR. The BCR-ABL constant levels ≥0.1% clearly select patients in CCgR for mutation analysis. This approach highly reduces the number of examinations for mutation in CML responders presenting a cost-effective alternative applicable in clinical practice.

Supported by MZOUHKT2005 and research grant from Bristol-Myers Squibb.

Machova Polakova:Bristol-Myers Squibb: Research Funding.