The Synthetic Hest Shock Protein (Hsp) 90 Inhibitor EC141 Induces Degradation of the Bcr-Abl p190 Protein and Apoptosis of Philadelphia Chromosome (Ph)-Positive Acute Lymphoblastic Leukemia (ALL) Cells.

High dose chemotherapy of Ph+ ALL is rarely curative and clinical responses to protein kinase inhibitors have been transient. Although new regimens combining chemotherapy with Bcr-Abl kinase inhibitors improve survival, the long-term prognosis of patients with Ph+ ALL remains guarded. Thus, novel therapeutic strategies are needed. Hsp90 is a ubiquitous molecular chaperone protein required for the folding, activation and assembly of mediators of signal transduction, cell cycle control, and transcription regulation. The Hsp90 inhibitor EC141 (Biogen Idec, Inc.) blocks the chaperone activity of Hsp90 and induces proteasomal degradation of it’s client proteins. Because Hsp90 is a chaperone of Bcr-Abl we investigated the activity of EC141 against the Ph+ ALL B-cell lines Z-119, Z-181 and Z-33 (Estrov et al. J Cell Physiol166: 618, 1996; Leukemia10:1534, 1996). First we studied the effect of EC141 on Hsp levels in Ph+ ALL cells. EC141 (50 nM) down-regulated the protein levels of Hsp90 and upregulated those of Hsp70. Then, the effect of EC141 on the proliferation of Ph+ ALL cells was evaluated using the MTT assay. EC141 inhibited the growth and metabolic activity of Z-119, Z-181 and Z-33 Ph+ ALL cells in a dose-dependent manner at concentrations ranging from 1 to 100 nM. Similar results were obtained with primary bone marrow cells from patients with Ph+ ALL. Using the ALL blast colony culture assay we found that EC141 inhibited the proliferation of marrow-derived ALL colony-forming cells in a dose-dependent fashion. To explore the mechanism of action Z-181 were incubated cells with increasing concentrations of EC141; immunoprecipitation and Western immunoblotting were used to detect changes in cellular protein levels. EC141 degraded the Bcr-Abl p190 protein and inhibited the phosphorylation of CrkL in a dose-dependent manner. Furthermore, exposure of Z-181 cells to EC141 resulted in a time- and dose-dependent activation of procaspase 3, cleavage of poly (adenosine diphosphate-ribose) polymerase and apoptotic cell death as assessed by Annexin V. Taken together, our data suggest that EC141 degrades the Bcr-Abl p190 protein, inhibits proliferation, and induces apoptosis of Ph+ ALL cells. Additional studies aimed at investigating the in vivo activity of EC141 in Ph+ ALL are warranted.