Varying Response to Imatinib in CML Patients with BCR-ABL^M244V Mutation.

BCR-ABL kinase domain mutation in a CML patient with clinical resistance to imatinib is generally accepted as ‘causal’. Here we report the kinetics of the mutant BCR-ABL transcripts in 6 CML patients with the M244V mutation that are at variance with the accepted view. Prior to starting imatinib, one patient in chronic phase (CP) was previously untreated, 4 patients in CP had previously received alfa-interferon, and the sixth had received an autograft but had subsequently progressed to accelerated phase (AP). Patients were started on 400 mg/day dosage which was escalated to 600, 800 or 1000 mg/day following little or no response. Philadelphia chromosome was the only cytogenetic abnormality detected at the time of diagnosis and at start of imatinib treatment. The median time from diagnosis to starting imatinib was 19 months (range: 3 – 120 months). The M224V mutations were detected initially as a result of routine screening by Sanger’s direct sequencing of cDNA from blood of patients on imatinib, at which time the mutant clone in each patient already constituted >80% of BCR-ABL transcripts, measured by QR-PCR. We then retrospectively measured levels of mutated BCR-ABL transcripts in all the stored samples, median 14.5 samples (5 – 28), by pyrosequencing. Of the 4 patients for whom there were pre-imatinib samples the BCR-ABL^M244V allele was identified in one case (previously treated with IFN-alfa) at 3%, but was not detected in the other three. In the 2 patients for whom there were no pre-imatinib samples, the BCR-ABL^M244V allele was present at 3.0% and 93% respectively in the earliest available sample. In the 5 patients whose earliest samples showed low or absent mutant transcripts the non-mutated BCR-ABL population was progressively replaced by BCR-ABL^M244V-positive cells, eventually becoming the dominant clone with a median time from first sample to maximal level of 35 months (11 – 54 months). In three patients total BCR-ABL transcripts decreased and they achieved complete cytogenetic remission (CCyR). In the other three patients (including the newly diagnosed patient) there was little or no reduction in BCR-ABL transcript numbers and all failed to achieve CCyR. One patient in this latter group acquired trisomy 8, reported 40 months prior to blastic phase; she died following allogeneic SCT. The response to increased imatinib dose in three patients is consistent with the reported IC₅₀ for the M244V mutation, i.e. moderate resistance, but the response in the other three patients is not. In summary, the response to imatinib in 6 patients with the same BCR-ABL mutation is variable. Furthermore, these observations imply the mutation is unlikely to be the sole cause of the resistance in at least in 3 of the patients. The observed heterogeneous phenotype of these 6 CML patients is likely to be due to a myriad of undefined contributory factors.