Signaling Profile and Anti-Tumor Activity of the Novel HSP90 Inhibitor NVP-AUY922 in Multiple Myeloma.

We have recently shown that Hsp90 is overexpressed in multiple myeloma (MM) and critically contributes to tumor cell survival. Pharmacologic blockade of Hsp90 has consistently been shown to induce MM cell death. However, most data have been obtained with MM cell lines whereas knowledge about the molecular effects of pharmacologic Hsp90 blockade in primary tumor cells is lacking. Furthermore, these investigations have so far exclusively relied on geldanamycin derivatives. Here, we analyzed the anti-tumor effects of a novel diarylisoxazole-based Hsp90 inhibitor (NVP-AUY922) in a large set of primary MM samples and in MM cell lines. The majority of cell lines (n = 8), as well as most primary samples (n = 20), displayed profound apoptotic responses and steep dose-effect curves with EC50 values in the range of 5–15 nM and EC90 values between 8 and 25 nM. This effect was not attenuated in coculture with cells from the bone marrow microenvironment. Some cell lines and about a quarter of primary MM samples displayed greater resilience to drug treatment, with EC50 values but not EC90 values reached at concentrations up to 50 nM. Sensitivity of MM cells to the Hsp90 inhibitor was not correlated with TP53 mutation or Hsp70 induction levels. Western analyses of MM cell lines and flow cytometric analyses of antibody-stained Hsp90 client proteins in primary tumor cells showed that NVP-AUY922-treatment entailed molecular effects and pharmacodynamic properties consistent with abrogation of Hsp90 function. Consequently, downregulation of multiple signaling and survival pathways was detectable through, for example, decreases in the phosphorylated (activated) forms of extracellular signal-regulated kinase (ERK) 1 and 2, signal transducer and activator of transcription (STAT) 3 and glycogen synthase kinase-3beta. All samples treated displayed strong upregulation of Hsp70. Importantly, peripheral blood mononuclear cells as well as primary bone marrow stromal cells were much less affected by high (50–100 nM) concentrations of NVP-AUY922, showing that a therapeutic window might be established for the treatment of multiple myeloma. Taken together, NVP-AUY922 could be a promising new drug for the treatment of a majority of myeloma patients.