Preclinical Evaluation of Bcr/Abl-Directed Tyrphostins

DESCRIPTION (provided by applicant): The death of an estimated 22,000 U.S. patients from acute and chronic leukemias in 2003 underscores the continuing need for new agents to treat this group of diseases. The tyrphostin, AG957, and its adamantyl ester derivative, adaphostin, originally came to attention at the National Cancer Institute (NCI) as Bcr/abl kinase inhibitors that were thought to compete for the peptide substrate binding site of the enzyme. Ex vivo studies completed during the present funding period demonstrated that AG957 and adaphostin selectively kill myeloid progenitors from patients with chronic myelogenous leukemia (CML) compared to normal controls. Additional experiments demonstrated that adaphostin also induces apoptosis in blasts from patients with acute myelogenous leukemia (AML) and lymphocytes from patients with chronic lymphocytic leukemia. These observations suggested the possibility of a different cytotoxic mechanism that does not depend upon the inhibition of Bcr/abl. Consistent with this hypothesis, we observed that adaphostin treatment causes elevated levels of reactive oxygen species (ROS) in a variety of cell lines. Further experiments revealed that buthionine sulfoximine (BSO), which diminishes glutathione (GSH) levels, increases adaphostin toxicity, whereas, the antioxidant N-acetylcysteine diminishes adaphostin toxicity. To complement the ongoing development of adaphostin at NCI, which is currently at the stage of IND toxicology, pharmacokinetics, and drug formulation, we now propose to: 1) Determine the mechanism by which adaphostin elevates ROS in susceptible cells; 2) Evaluate the basis for its selectivity against CML and AML cells as compared to normal marrow progenitors; and 3) Assess the possibility that the therapeutic index of adaphostin can be further enhanced using an agent that reportedly depletes GSH selectively in neoplastic cells. Collectively, these studies will yield new information about the action of a novel class of potential antileukemic agents. In addition, by providing new insight into an apparent defect in the ability of leukemia cells to handle oxidative stress, these experiments might help identify a new pathway that can be targeted for improved therapy of acute and chronic leukemias.