ABL

Abstract: Clinical studies with the Abl tyrosine kinase inhibitor STI-571 in chronic myeloid leukemia demonstrate that many patients with advanced stage disease respond initially but then relapse. Through biochemical and molecular analysis of clinical material, we find that drug resistance is associated with the reactivation of BCR-ABL signal transduction in all cases examined. In six of nine patients, resistance was associated with a single amino acid substitution in a threonine residue of the Abl kinase domain known to form a critical hydrogen bond with the drug. This substitution of threonine with isoleucine was sufficient to confer STI-571 resistance in a reconstitution experiment. In three patients, resistance was associated with progressive BCR-ABL gene amplification. These studies provide evidence that genetically complex cancers retain dependence on an initial oncogenic event and suggest a strategy for identifying inhibitors of STI-571 resistance.

Abstract: In tumor cells from virtually all patients with chronic myelogenous leukemia, the Philadelphia chromosome, a fusion of chromosomes 9 and 22, directs the synthesis of the P210bcr/abl protein. The protein-tyrosine kinase activity and hybrid structure of P210bcr/abl are similar to the oncogene product of the Abelson murine leukemia virus, P160gag/v-abl, which induces acute lymphomas. To determine whether P210bcr/abl can induce chronic myelogenous leukemia, murine bone marrow was infected with a retrovirus encoding P210bcr/abl and transplanted into irradiated syngeneic recipients. Transplant recipients developed several hematologic malignancies; prominent among them was a myeloproliferative syndrome closely resembling the chronic phase of human chronic myelogenous leukemia. Tumor tissue from diseased mice harbored the provirus encoding P210bcr/abl. These results demonstrate that P210bcr/abl expression can induce chronic myelogenous leukemia. Retrovirus-mediated expression of the protein provides a murine model system for further analysis of the disease.

Abstract: Resistance to the ABL kinase inhibitor imatinib (STI571 or Gleevec) in chronic myeloid leukemia (CML) occurs through selection for tumor cells harboring BCR-ABL kinase domain point mutations that interfere with drug binding. Crystallographic studies predict that most imatinib-resistant mutants should remain sensitive to inhibitors that bind ABL with less stringent conformational requirements. BMS-354825 is an orally bioavailable ABL kinase inhibitor with two-log increased potency relative to imatinib that retains activity against 14 of 15 imatinib-resistant BCR-ABL mutants. BMS-354825 prolongs survival of mice with BCR-ABL-driven disease and inhibits proliferation of BCR-ABL-positive bone marrow progenitor cells from patients with imatinib-sensitive and imatinib-resistant CML. These data illustrate how molecular insight into kinase inhibitor resistance can guide the design of second-generation targeted therapies.

Abstract: The inadvertent activation of the Abelson tyrosine kinase (Abl) causes chronic myelogenous leukemia (CML). A small-molecule inhibitor of Abl (STI-571) is effective in the treatment of CML. We report the crystal structure of the catalytic domain of Abl, complexed to a variant of STI-571. Critical to the binding of STI-571 is the adoption by the kinase of an inactive conformation, in which a centrally located "activation loop" is not phosphorylated. The conformation of this loop is distinct from that in active protein kinases, as well as in the inactive form of the closely related Src kinases. These results suggest that compounds that exploit the distinctive inactivation mechanisms of individual protein kinases can achieve both high affinity and high specificity.