Major Molecular Responses to Dasatinib (BMS-354825) Are Observed in Imatinib-Resistant Late Stage Chronic and Advanced CML Patients: Impact and Fate of Imatinib-Resistant Clones in Dasatinib-Treated Patients.

Acquired imatinib-resistance in Ph-positive leukemia is frequently associated with mutations in the BCR-ABL kinase domain. Dasatinib is more potent than imatinib for inhibiting BCR-ABL activity and has preclinical efficacy against imatinib resistant mutations tested so far, except for T315I. We report the molecular analysis of imatinib resistant/intolerant patients treated at UCLA in a Phase I dose escalation trial of dasatinib. We sought to determine whether patients achieve significant reductions in the BCR-ABL level as measured by quantitative PCR (RQ-PCR); the effect of dasatinib on mutant BCR-ABL clones in-vivo; and the contribution of mutations to progression. Patients with accelerated phase or blast crisis CML, or Ph+ALL (AP/BC, n=14) were treated for a median of 5 months (range 1 to 11) and late (median disease duration 8 years) chronic phase CML patients (CP, n=19) for a median of 12 months (range 3 to 21). Patients were tested 2 to 17 times by RQ-PCR (267 analyses) and direct sequencing (167 analyses). Prior to commencing dasatinib, mutations were detected in resistant patients only (8 AP/BC and 15 CP). For our molecular analysis a ≥2-log reduction of BCR-ABL below the standardized baseline was considered significant. This level approximates to a complete cytogenetic response. A 3-log reduction defines a major molecular response (MMR), which is associated with a high progression free survival in imatinib treated patients. Overall, 6 of 14 (43%) AP/BC and 7 of 19 (37%) CP patients achieved ≥2-log reductions (MMR in 4 (29%) and 4 (21%) patients respectively). The response was maintained in 2 of 6 AP/BC and 6 of 7 CP patients achieving ≥2 log reductions. The table classifies the molecular response according to the detection of a baseline mutation.

Mutations were detected at last analysis in all 23 patients with baseline mutations. The sensitivity of the direct sequencing technique is approximately 20%. The same mutation that was present at baseline was present in 21 patients and 5 of these patients had an additional mutation. The remaining 2 patients had different mutations, one being F317I (CP). This mutation has not been reported in imatinib resistant patients to our knowledge. Mutations were present in all patients who progressed (1 CP, 7 AP/BC). Six of these patients had T315I that was detected at baseline (3) or evolved during therapy (3). T315I evolved in 3 other patients who have not progressed (1CP, 2 AP/BC). Overall, the T315I mutation evolved in 6 patients and was accompanied by significant rises in BCR-ABL in all patients of 2.5 to 185-fold. In conclusion 39% of all patients achieved significant molecular responses, with major molecular responses in 24% overall. The majority of CP patients maintained the molecular response. Baseline mutations remained detectable in almost all patients including those with significant BCR-ABL reductions. The mutation that became detectable most frequently during dasatinib therapy was T315I and non-mutation relapse was rare. A focus of future trials may be on managing the remaining resistant mutations.