Acquired Mutations in the BCR-ABL Kinase Domain in Imatinib Resistant Patients. Multicentric Experience in Argentina and Uruguay.

Imatinib induces complete cytogenetic response (CCyR) in 82% of patients with CML in chronic phase. There is a subset of patients who either failed to achieve or lose hematological/CCyR. Kinase domain mutations have emerged as a potential cause for treatment failure.

Aims: In the current study we have investigated ABL kinase domain mutations, amplifications and quantification of BCR-ABL transcripts by Q-PCR in Imatinib resistant patients.

Patients and Methods: A total of 84 patients (pts) with CML treated with Imatinib were studied and 82 were evaluable. Sixty-one (74%) were in chronic phase (CP), eleven (14%) were in accelerated phase (AP), and ten (12%) were in blast crisis (BC).We analyzed the DNA from blood samples. Mutations were screened by conformation sensitive gel electrophoresis (CSGE). The amplification products displaying an abnormal pattern were sequenced using automatic system. The amplification of BCR-ABL rearrangement was studied in interphase nuclei using Vysis extra-signaling probe. Real time quantitative PCR (RQ-PCR) of BCR-ABL transcripts was performed in a subset of 44 patients to assess molecular response, using Light Cycler (Roche®), Syber Green Method.

Results: Eleven mutations from 82 evaluable patients were detected (13%). Seven were in p- loop: M244R (1), L247A (1), G250E (2), Q252H (1), E255K (2), and four in the Imatinib binding: T315I (3), H318L (1). Two patients had BCR-ABL amplifications with 4–6 signals in interphase nucleous and one of them showed extramedullary involvement in skin nodules. Five patients had clonal evolution with double Ph chromosome. Most of mutations were found in patients in late chronic phase (6/11: 54%). The detection occurred at a median of 49 months (range 12–154) after diagnosis. Eighty four percent of cases studied with RQ-PCR, had null molecular response (<1Log Reduction). Imatinib dose in patients with mutations had been escalated to 600–800 mg/day previously.

Conclusion: This is the first multicentric study in spanish-speaking South America. T315I was the most frequent mutation detected in our study and was associated with poor outcome. We found 13% of resistant cases with point mutations, all of them located in the p-loop or imatinib binding.This is an ongoing study and further recruitment is needed to confirm these findings. Early detection of mutations can have prognostic implication and allow therapeutic intervention such as dose escalation, combination therapy or second generation tyrosine kinase inhibitors.