Heat shock protein 90: a potential therapeutic target in leukemic progenitor and stem cells harboring mutant BCR-ABL resistant to kinase inhibitors.

Development of drug resistance has become a major obstacle for tyrosine kinase inhibitors (TKIs) in the treatment of Philadelphia chromosome-positive (Ph(+)) chronic myelogenous leukemia (CML) and other cancers. The BCR-ABL-T315I mutant does not respond to clinically available TKIs, although some newly developed anti-BCR-ABL-T315I TKIs are now being tested in patients. TKIs transiently inhibit kinase activity of BCR-ABL, but do not reduce the level of the BCR-ABL protein. Elimination of mutant BCR-ABL protein would provide a new therapeutic strategy for treating Ph(+) leukemia. We recently showed that inhibition of heat shock protein 90 (Hsp90) by a novel Hsp90 inhibitor, IPI- 504, causes BCR-ABL protein degradation, decreased numbers of leukemia stem cells, and prolonged survival of mice with CML induced by BCR-ABL-T315I. Here we discuss further the mechanisms and effectiveness of Hsp90 inhibition in suppression of survival and proliferation of leukemic progenitor and stem cells in CML mice, and the potential of this anti-Hsp90 strategy in treating CML patients, including those who have developed resistance to TKIs.