A New Abl Kinase Inhibitor (AMN107) Has In Vitro Activity on CML Ph+Blast Cells Resistant to Imatinib.

Imatinib mesylate (STI571), an inhibitor of the bcr/abl tyrosine kinase, is rapidly becoming the first-line therapy for chronic myeloid leukemia (CML). Imatinib has proved remarkably effective at reducing the number of leukemia cells in individual CML patients and promises to prolong life substantially in comparison with earlier treatments. However, the development of resistance to this drug is a frequent setback, particularly in patients in advanced phases of the disease. Therefore, new inhibitors of bcr/abl are needed. Very recently, a new bcr/abl inhibitor, AMN107, has been synthesized. We have tested AMN107 on leukemic cell lines and on blasts isolated from imatinib-resistant CML patients. Western blot analysis with phosphospecific antibodies revealed that in K562 cells AMN107 (10 nM) down-regulated phosphorylation of bcr/abl Tyr177, while the phosphorylation levels of Tyr412 were unaffected. This finding seems particularly important because recent evidence has demonstrated that the signaling pathway emanating from Tyr177 plays a major role in the pathogenesis of CML. Indeed, phosphorylated Tyr 177 forms a high-affinity binding site for the SH2 domain of the adapter protein Grb2. The main effectors of Brb2 are Sos and Ras, however Grb2 also recruits the scaffolding adapter protein Gab2 to bcr/abl via a Grb2,Gab2 complex. Which results in activation of the PI3K/Akt and Erk signalling networks.

In contrast, STI571, even if used at 200 nM, did not diminish phosphorylation of bcr/abl Tyr177. At 10 nM AMN107 blocked K562 cells in the G₁ phase of the cell cycle. To obtain the same effect with imatinib, a 200 nM concentration was required. AMN107 did not affect cell cycle progression of bcr/abl-negative cell lines such as HL60 and NB4, even if the concentration was raised to 200 nM. AMN107 (5 μM for 24 h) significantly increased apoptosis rate in CML blasts isolated from patients resistant to the same concentration of imatinib. Therefore, AMN107 might represent a new bcr/abl selective inhibitor useful for overcoming imatinib resistance.