Multiple BCR-ABL kinase domain mutations confer polyclonal resistance to the tyrosine kinase inhibitor imatinib (STI571) in chronic phase and blast crisis chronic myeloid leukemia.

Through sequencing analysis of blood or bone marrow samples from patients with chronic myeloid leukemia, we identified BCR-ABL kinase domain mutations in 29 of 32 patients whose disease relapsed after an initial response to the tyrosine kinase inhibitor imatinib. Fifteen different amino acid substitutions affecting 13 residues in the kinase domain were found. Mutations fell into two groups-those that alter amino acids that directly contact imatinib and those postulated to prevent BCR-ABL from achieving the inactive conformational state required for imatinib binding. Distinct mutations conferred varying degrees of imatinib resistance. Mutations detected in a subset of patients with stable chronic phase disease correlated with subsequent disease progression. Multiple independent mutant clones were detected in a subset of relapsed cases. Our data support a clonal selection model of preexisting BCR-ABL mutations that confer imatinib resistance.

Sequential ABL kinase inhibitor therapy selects for compound drug-resistant BCR-ABL mutations with altered oncogenic potency.

Molecularly targeted kinase inhibitor cancer therapies are currently administered sequentially rather than simultaneously. We addressed the potential long-term impact of this strategy in patients with chronic myelogenous leukemia (CML), which is driven by the fusion oncogene BCR-ABL. Analysis of BCR-ABL genotypes in CML patients who relapsed after sequential treatment with the ABL inhibitors imatinib and dasatinib revealed evolving resistant BCR-ABL kinase domain mutations in all cases. Twelve patients relapsed with the pan-resistant T315I mutation, whereas 6 patients developed novel BCR-ABL mutations predicted to retain sensitivity to imatinib based on in vitro studies. Three of these patients were retreated with imatinib (or the chemically related compound nilotinib) and responded; however, selection for compound mutants (2 or 3 BCR-ABL mutations in the same molecule) can substantially limit the potential effectiveness of retreating patients with inhibitors that have previously failed. Furthermore, drug-resistant mutations, when compounded, can increase oncogenic potency relative to the component mutants in transformation assays. The Aurora kinase inhibitor VX-680, currently under clinical evaluation based on its activity against the T315I mutation, is also effective against the other commonly detected dasatinib-resistant mutation in our analysis, V299L. Our findings demonstrate the potential hazards of sequential kinase inhibitor therapy and suggest a role for a combination of ABL kinase inhibitors, perhaps including VX-680, to prevent the outgrowth of cells harboring drug-resistant BCR-ABL mutations.

Transient potent BCR-ABL inhibition is sufficient to commit chronic myeloid leukemia cells irreversibly to apoptosis.

The BCR-ABL inhibitor dasatinib achieves clinical remissions in chronic myeloid leukemia (CML) patients using a dosing schedule that achieves potent but transient BCR-ABL inhibition. In vitro, transient potent BCR-ABL inhibition with either dasatinib or imatinib is cytotoxic to CML cell lines, as is transient potent EGFR inhibition with erlotinib in a lung cancer cell line. Cytotoxicity correlates with the magnitude as well as the duration of kinase inhibition. Moreover, cytotoxicity with transient potent target inhibition is equivalent to prolonged target inhibition and in both cases is associated with BIM activation and rescued by BCL-2 overexpression. In CML patients receiving dasatinib once daily, response correlates with the magnitude of BCR-ABL kinase inhibition, thereby demonstrating the potential clinical utility of intermittent potent kinase inhibitor therapy.