New molecular and biological mechanism of antitumor activities of KW-2478, a novel nonansamycin heat shock protein 90 inhibitor, in multiple myeloma cells.

PURPOSE The heat shock protein 90 (Hsp90) plays an important role in chaperoning oncogenic client proteins in multiple myeloma (MM) cells, and several Hsp90 inhibitors have shown antitumor activities both in vitro and in vivo. However the precise mechanism of action of Hsp90 inhibitor in MM has not been fully elucidated. EXPERIMENTAL DESIGN We evaluated the antitumor activities of KW-2478, a nonansamycin Hsp90 inhibitor, in MM cells with various chromosomal translocations of immunoglobulin heavy chain (IgH) loci both in vitro and in vivo. RESULTS Our studies revealed that exposure of KW-2478 to MM cells resulted in growth inhibition and apoptosis, which were associated with degradation of well-known client proteins as well as a decrease in IgH translocation products (FGFR3, c-Maf, and cyclin D1), and FGFR3 was shown to be a new client protein of Hsp90 chaperon complex. In addition, KW-2478 depleted the Hsp90 client Cdk9, a transcriptional kinase, and the phosphorylated 4E-BP1, a translational inhibitor. Both inhibitory effects of KW-2478 on such transcriptional and translational pathways were shown to reduce c-Maf and cyclin D1 expression. In NCI-H929 s.c. inoculated model, KW-2478 showed a significant suppression of tumor growth and induced the degradation of client proteins in tumors. Furthermore, in a novel orthotopic MM model of i.v. inoculated OPM-2/green fluorescent protein, KW-2478 showed a significant reduction of both serum M protein and MM tumor burden in the bone marrow. CONCLUSIONS These results suggest that targeting such diverse pathways by KW-2478 could be a promising strategy for the treatment of MM with various cytogenetic abnormalities.