Destabilization of Bcr-Abl/Jak2 Network by a Jak2/Abl Kinase Inhibitor ON044580 Overcomes Drug Resistance in Blast Crisis Chronic Myelogenous Leukemia (CML).

Bcr-Abl is the predominant therapeutic target in chronic myeloid leukemia (CML), and tyrosine kinase inhibitors (TKIs) that inhibit Bcr-Abl have been successful in treating CML. With progression of CML disease especially in blast crisis stage, cells from CML patients become resistant to imatinib mesylate (IM) and other TKIs, resulting in relapse. Because Bcr-Abl is known to drive multiple signaling pathways, the study of the regulation of stability of Bcr-Abl in IM-resistant CML cells is a critical issue as a possible therapeutic strategy. Here, we report that a new dual-kinase chemical inhibitor, ON044580, induced apoptosis of Bcr-Abl+ IM-sensitive, IM-resistant cells, including the gatekeeper Bcr-Abl mutant, T315I, and also cells from blast crisis patients. In addition, IM-resistant K562-R cells, cells from blast crisis CML patients, and all IM-resistant cell lines tested had reduced ability to form colonies in soft agar in the presence of 0.5 µM ON044580. In in vitro kinase assays, ON044580 inhibited the recombinant Jak2 and Abl kinase activities when the respective Jak2 and Abl peptides were used as substrates. Incubation of the Bcr-Abl+ cells with ON044580 rapidly reduced the levels of the Bcr-Abl protein and also reduced the expression of HSP90 and its client protein levels. Lysates of Bcr-Abl+ cell lines were found to contain a large signaling network complex composed of Bcr-Abl, Jak2, HSP90, and its client proteins as detected by a gel filtration column chromatography, which was rapidly disrupted by ON044580. Therefore, targeting Jak2 and Bcr-Abl kinases is an effective way to destabilize Bcr-Abl and its network complex, which leads to the onset of apoptosis in IM-sensitive and IM-resistant Bcr-Abl+ cells. This inhibitory strategy has potential to manage all types of drug-resistant CML cells, especially at the terminal blast crisis stage of CML, where TKIs are not clinically useful.