Degrasyn (a novel tyrophostin) Impacts BCR-ABL Protein Level and Is Cytotoxic to Chronic Myeloid Leukemia Early Progenitors

Despite the potent anti-proliferative and apoptotic effects of tyrosine kinase inhibitors (imatinib, dasatinib) on mature CML cells these compounds have limited activity on Chronic Myeloid Leukemia (CML) early progenitors (Lin-/CD38−/CD34+). Mechanisms of resistance to these agents in the early progenitor population are poorly understood but are distinct from BCR-ABL mutation induced resistance. Degrasyn (WP1130) was previously shown by our laboratory to inhibit CML cell growth and colony formation. Further structure-activity relationship studies led to the development of a more potent compound, CP2005 (CP). CP displays enhanced activity in suppressing growth and activating apoptosis in various CML and ALL cell lines (IC50 ~1 microM). Here we show that total CD34+ from CML patients are more effectively growth inhibited by CP2005 compared to WP1130. An anti-leukemic effect was also noted in primitive progenitors and was associated with the induction of apoptosis. BaF3 cells expressing an EGFP-tagged BCR-ABL protein treated with CP or WP1130 show that both facilitate translocation of BCR-ABL from the cytoplasm to a detergent insoluble compartment which subsequently leads to degradation of BCR-ABL. We observed that the Akt/mTOR pathway is activated by CP which could be partially suppressed by mTOR or PI3K/Akt inhibition (rapamycin and wortmannin, respectively). CP-induced BCR-ABL translocation was associated with a reduction in the level of tyrosine-phosphorylated Stat3 and Stat5. However, although there is activation of survival signals associated with Akt-signaling 2 microM CP is able to promote significant apoptosis (67%) as a single agent in CML early progenitors. Although imatinib was minimally effective in reducing the early progenitor cell survival, co-incubation of imatinib with CP2005 resulted in near complete induction of apoptosis in early progenitor cells (~ 93%), suggesting that the combined effects of both CP and imatinib on BCR-ABL protein degradation and kinase inhibition markedly impair the survival of CML early progenitors. The combination of BCR-ABL degradation and kinase inhibition may be effective in engaging pro-apoptotic signaling in CML early progenitor cells.