Molecular Analysis of Dasatinib Resistance Mechanisms in CML Patients Identifies Novel BCR-ABL Mutations Predicted To Retain Sensitivity to Imatinib: Rationale for Combination Tyrosine Kinase Inhibitor Therapy.

Point mutations within the BCR-ABL kinase domain represent the most common mechanism of resistance to imatinib in patients with CML. Preclinical studies have shown that dasatinib (BMS-354825) is effective at inhibiting the kinase activity of imatinib-resistant BCR-ABL mutants with the notable exception of the T315I mutation, which remains highly resistant to imatinib, dasatinib, and AMN107 (Gorre et al, Science 2001; Shah et al, Science 2004; Weisberg et al, Cancer Cell, 2005). Clinical data from Phase I and II studies of dasatinib in CML confirms the in vitro findings. Each of three imatinib-resistant patients bearing the T315I mutation (CP=1; AP=2) did not achieve objective hematologic or cytogenetic responses during treatment with dasatanib on a Phase I study. Additionally, each of two phase II patients with the T315I mutation (CP=1; LBC=1) treated at UCLA showed no evidence of objective response. We have also detected the T315I mutation in each of two cases of acquired resistance in a phase II (LBC =2) study, and in seven of nine patients with acquired resistance to dasatinib in phase I and II studies (CP=1; MBC=3; LBC=2; Ph⁺ ALL=1).

Notably, we detected a novel BCR-ABL mutation, T315A, in one of the two patients who relapsed without a detectable T315I mutation. The patient is a 53 year-old female whose chronic phase CML had progressed to myeloid blast phase while being treated with imatinib. The imatinib-resistant mutation M244V was identified prior to dasatinib treatment. The patient achieved a major hematologic response (<5% blasts with partial recovery of peripheral blood counts) on dasatinib 90 mg orally given twice daily, but relapsed with MBC after six months. Sequence analysis of the BCR-ABL kinase domain at the time of relapse revealed the presence of the imatinib-resistant mutation M244V as well as the novel mutation T315A. This finding is of particular interest because T315A and several other novel BCR-ABL mutations were recently recovered in a saturation mutagenesis study designed to define potential mechanisms of dasatinib resistance. Remarkably, many of these mutations retain sensitivity to imatinib in vitro (Burgess et al, PNAS, 2005). Through periodic molecular monitoring of other dasatinib-treated patients, we have identified a second novel BCR-ABL mutant, F317I, that developed in an imatinib-resistant CP patient after 9 months of treatment. Similar to T315A, F317I was isolated in the saturation mutagenesis screen for dasatinib resistance and is predicted to retain sensitivity to imatinib. Taken together, our findings implicate the T315I mutation as the principle mechanism of resistance to dasatinib, but more importantly, strongly support the use for combination kinase inhibitor therapy in CML to prevent emergence of drug resistant clones. A phase I trial to assess the safety of combining imatinib with dasatinib is planned.