SNX-5422 significantly reduced MM tumor development in the treatment group (n = 7) compared with control mice (n = 7) (Figure 6A). prolongs survival in a xenograft murine model. Our results indicate that blockade of Hsp90 by SNX-2112 not only inhibits MM cell growth but also acts in the bone marrow microenvironment to block angiogenesis and osteoclastogenesis. Taken together, our data provide the framework for clinical studies of SNX-2112 to improve patient outcome in MM and other hematologic malignancies. == Introduction == Multiple 6-Thio-dG myeloma (MM) is a B-cell malignancy characterized by excess abnormal plasma cells in the bone marrow (BM), bone lesions, and immunodeficiency. Despite treatment with high-dose chemotherapy and stem Rabbit Polyclonal to EGFR (phospho-Ser695) 6-Thio-dG cell transplantation as well as novel agents including bortezomib, thalidomide, and lenalidomide, MM remains incurable.1,2 Heat shock protein 90 (Hsp90) is an important chaperone required for protein folding as well as assembly and maintenance of conformational stability for a suite of proteins (clients) involved in intracellular signaling.3These client proteins and Hsp90-dependent pathways include Akt, Raf, and Her2/neu, with downstream molecules, such as extracellular signal-related kinase (ERK), pS6, and nuclear factor-B (NF-B), which regulates cell survival and proliferation.35Because Hsp90 inhibition induces degradation of its client proteins, it is considered an attractive target for anticancer drugs.6Geldanamycin and its analog 17-allylamino-17-demethoxy-geldanamycin (17-AAG) inhibit the protein function of Hsp90 and induce apoptosis in various tumor cells.4,71017-AAG also shows antitumor activity in an array of human tumor xenograft models11,12and is now undergoing clinical trials.8,10Importantly, previous reports have demonstrated that 17-AAG inhibits proliferation and survival of MM cells, associated with down-regulation of insulin-like growth factor 1 receptor (IGF-1R) and interleukin-6 receptor (IL-6R) signaling 6-Thio-dG (eg, IKK/NF-B, PI-3K/Akt, and Raf/MAPK) as well as downstream molecules (eg, proteasome, telomerase, and HIF-1- activities).13Phase 1 clinical trials using 17-AAG in patients with relapsed or refractory MM and other advanced malignancies showed that its toxicity was clinically manageable.1315Moreover, we have shown that combined Hsp90 inhibitor and proteasome inhibitor treatment induces synergistic MM cell death in preclinical studies,13and clinical trials show that the combination of Hsp90 inhibitor tanespimysin and bortezomib can achieve responses, even in patients resistant to bortezomib alone.16 Although efficacious, these natural productderived Hsp90 inhibitors are limited in dosing frequency by lack of oral availability and concerns surrounding the chemical reactivity of the quinone moiety at the core of the geldanamycin analogs.17Recently, a novel true small molecule class of Hsp90 inhibitor was reported, exemplified by SNX-2112 (Figure 1A).1820SNX-2112 competitively binds to the N-terminal adenosine triphosphate binding site of Hsp90, is highly orally bioavailable when delivered via its prodrug SNX-5422, and is highly potent against various cancers in vitro and in vivo.1820Three phase 1 clinical studies of SNX-5422 are currently recruiting participants in refractory hematologic and solid tumor malignancies (National Institutes of Health Clinical Trials website,http://www.cancer.gov/clinicaltrials). Here we demonstrate that SNX-2112 exhibits more potent activity than 17-AAG against MM as well as other hematologic tumor lines and evaluate the mechanism of this enhanced activity. We further characterize the role of Hsp90 in promoting growth and survival of MM as well as effects on angiogenesis and osteoclastogenesis in the BM microenvironment, and also evaluate the molecular consequences of targeting Hsp90 function. We demonstrate that SNX-2112 induces cytotoxicity, associated with inhibition of Akt and ERK pathways, in MM cell lines as well as patient MM cells. MM cell apoptosis triggered by SNX-2112 is mediated via caspase-8, -9, -3, and poly (ADP-ribose) polymerase (PARP) cleavage. In addition, SNX-2112 overcomes the growth stimulatory effects of exogenous cytokines, such as IL-6 and IGF-1, as well as inhibits growth of MM cells adherent to bone marrow stromal cells (BMSCs). Importantly, Hsp90 inhibition by SNX-2112 targets not only MM cells but also inhibits tubule formation 6-Thio-dG by human umbilical vein endothelial cells (HUVECs) and osteoclast (OCL) formation, associated with down-regulation of Akt and ERK.