Activity of New Heat Shock Protein 90 (hsp90) Inhibitor NVP-AUY922 Against Myeloma Cells Sensitive and Resistant to Conventional Agents.

Context: The molecular chaperone hsp90 is a major anti-cancer therapeutic target because it regulates the function of proteins with pivotal roles in tumor cell proliferation, survival and drug resistance, including mutated/chimeric oncoproteins or oncogenic kinases/receptors. Our preclinical studies on the ansamycin hsp90 inhibitor tanespimycin (17-AAG) provided the rationale for clinical trials, either alone or in combination with the proteasome inhibitor bortezomib, for treatment of relapsed/refractory MM. In this study, we report preclinical studies of the new, non-ansamycin, hsp90 inhibitor NVP-AUY922.

Methods/Results: We tested 36 human MM cell lines and observed with MTT colorimetric survival assays potent time- and dose-dependent anti-MM activity of NVP-AUY922. IC₅₀ values were <12.5 nM for 31 of 35 cell lines, which were also more sensitive than all non-malignant cells tested with NVP-AUY922, including bone marrow stromal cells (BMSCs), immunortalized human hepatocytes and normal donor PBMCs, indicating a differential selectivity of NVP-AUY922 against neoplastic tissues vs. non-malignant cells. Importantly, MM cell lines or primary MM tumor cells resistant to dexamethasone (Dex), melphalan, immunomodulatory thalidomide derivatives (IMIDs), bortezomib or TRAIL were sensitive to NVP-AUY922. In addition, MM-1S cells constitutively over-expressing Akt were equally sensitive to NVP-AUY922 compared to their parental cell line. These anti-MM effects were rapid, as MM cells were committed to cell death within 16 hrs of exposure to only 25 nM of NVP-AUY922. NVP-AUY922 overcame the protective effects conferred to MM cells by exogenous IL-6 and IGF-1, while BMSC co-culture attenuated NVP-AUY922 activity only at doses <20 nM, as evidenced by compartment-specific bioluminescence imaging (CS-BLI). Mechanistic studies of NVP-AUY922 treatment of MM-1S cells showed early G2/M arrest followed by increased cell death, accompanied by caspase-3 and -8 cleavage (as early as 16 hrs). These events were preceded by decreased levels of Akt, B-Raf, phospho-MEK, cIAP2, and XIAP; and compensatory upregulation of hsp27 and hsp70. Given the highly single-agent activity of NVP-AUY922 at even low nM concentrations, formal statistical documentation of synergy was not observed in NVP-AUY922 combinations with conventional (e.g. Dex, doxorubicin) or novel (e.g. bortezomib) anti-MM agents. Encouragingly, no evidence of antagonism with any of these combinations was observed, indicating that NVP-AUY922 can be combined with current anti-MM agents in clinical settings.

Conclusion: The new hsp90 inhibitor NVP-AUY922 has potent in vitro activity against MM cells resistant to conventional therapeutics, with selectivity for malignant compared to normal cells. Ongoing in vivo experiments and studies to identify biomarkers of pronounced sensitivity to NVP-AUY922 will help provide a framework for potential clinical trials of NVP-AUY922 in MM and other neoplasias.