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Purpose: Heat shock protein 90 (HSP90) is required for the proper folding, function, and stability of various client proteins, two of which (KIT and PDGFRα) are critical in the pathogenesis and progression of gastrointestinal stromal tumors (GIST). This phase I study investigated the safety and maximum tolerated dose (MTD) of retaspimycin hydrochloride (IPI-504), a novel potent and selective HSP90 inhibitor, in patients with metastatic and/or unresectable GIST or other soft-tissue sarcomas (STS)., Experimental Design: IPI-504 was administered intravenously at doses ranging from 90 to 500 mg/m(2) twice weekly for 2 weeks on/1 week off. Safety, pharmacokinetic, and pharmacodynamic profiles were determined. Response was assessed by Response Evaluation Criteria for Solid Tumors (RECIST) 1.0 and optionally via 18-fluorodeoxyglucose positron emission tomography (18-FDG-PET) imaging., Results: Fifty-four patients received IPI-504; 37 with GIST and 17 with other STS. The MTD was 400 mg/m(2) twice weekly for 2 weeks on/1 week off. Common related adverse events were fatigue (59%), headache (44%), and nausea (43%). Exposure to IPI-504, 17-AAG, and 17-AG increased with IPI-504 dose. Stable disease (SD) was observed in 70% (26 of 37) of patients with GIST and 59% (10 of 17) of patients with STS. There was one confirmed partial response (PR) in a patient with GIST and one PR in a patient with liposarcoma. Metabolic partial responses occurred in 11 of 29 (38%) patients with GIST., Conclusions: In this study of advanced GIST or other STS, IPI-504 was generally well-tolerated with some evidence of antitumor activity, serving as a clinical proof-of-concept that HSP90 inhibition remains a promising strategy.

Treatment of gastroenteropancreatic neuroendocrine tumors (GEP-NET) is still unsatisfactory and innovative therapeutic approaches are urgently needed. Heat shock protein 90 (Hsp90) is overexpressed in a wide range of tumor types and is an emerging target for the treatment of cancer. However, the potential activity of Hsp90 inhibitors in GEP-NET has not yet been investigated. We studied the antineoplastic activity of the Hsp90 inhibitor IPI-504 on GEP‑NET cells, and characterized its mechanism of action. In human insulinoma (CM) and pancreatic carcinoid (BON) cells IPI-504 induced a dose-dependent growth inhibition by almost 70%. The antiproliferative effect of IPI-504 correlated with a reduction in protein levels of the IGF-1 receptor. Additionally, several proteins of the PI3K/AKT/mTOR pathway, downstream of IGF-1 receptor activation in GEP-NETs, were downregulated as a consequence of Hsp90 inhibition. Combination treatment of IPI-504 with mTOR- or AKT-inhibitors led to additive antiproliferative effects. In addition, effects of IGF-1 receptor tyrosine kinase inhibition were strongly enhanced by IPI-504. Cancer gene expression profiling and FACS analysis revealed that IPI-504 antiproliferative effects were due to both induction of cell cycle arrest and apoptosis. A modified chick chorioallantoic membrane (CAM) assay confirmed the antineoplastic activity of IPI-504 in GEP-NETs in vivo. In conclusion, this study showed that Hsp90 inhibition may be an attractive target for innovative GEP-NET treatment alone or in combination with either IGF-1R or mTOR inhibitors.

Abstract A phase 1 study of IPI-504 (retaspimycin hydrochloride) administered intravenously twice weekly for 2 weeks at 22.5, 45, 90, 150, 225, 300 or 400 mg/m(2) followed by 10 days off-treatment was conducted to determine the safety and maximum tolerated dose (MTD) of IPI-504 in patients with relapsed or relapsed/refractory multiple myeloma (MM). Anti-tumor activity and pharmacokinetics were also evaluated. Eighteen patients (mean age 60.5 years; median 9 prior therapies) were enrolled. No dose-limiting toxicities (DLTs) were reported for IPI-504 doses up to 400 mg/m(2). The most common treatment-related adverse event was grade 1 infusion site pain (four patients). All other treatment-related events were assessed as grade 1 or 2 in severity. The area under the curve (AUC) increased with increasing dose, and the mean half-life was approximately 2-4 h for IPI-504 and its metabolites. Four patients had stable disease, demonstrating modest single-agent activity in relapsed or relapsed/refractory MM.

The activity of the receptor tyrosine kinase KIT is crucial for gastrointestinal stromal tumor (GIST) growth and survival. Imatinib and sunitinib are very effective in advanced GIST, but have no curative potential. The observation that heat shock protein 90 (HSP90) inhibition results in KIT degradation prompted us to assess the efficacy of the HSP90 inhibitor retaspimycin hydrochloride (IPI-504) alone or in combination with imatinib or sunitinib in two GIST xenografts with distinctive KIT mutations. Nude mice were grafted with human GIST carrying KIT exon 13 (GIST-882; n = 59) or exon 11 (GIST-PSW; n = 44) mutations and dosed with imatinib (50 mg/kg twice daily), sunitinib (40 mg/kg once daily), IPI-504 (100 mg/kg 3 times per week), IPI-504 + imatinib, or IPI-504 + sunitinib. We evaluated tumor volume, proliferation and apoptosis, KIT expression and activation, as well as adverse events during treatment. Treatment with IPI-504 alone resulted in tumor regression, proliferation arrest, and induction of tumor necrosis. We documented downregulation of KIT and its signaling cascade in IPI-504-treated animals. Treatment effects were enhanced by combining IPI-504 with imatinib or sunitinib. On histologic examination, liver damage was frequently observed in animals exposed to combination treatments. In conclusion, IPI-504 shows consistent antitumor activity and induces KIT downregulation in GIST, as a single agent, and is more potent in combination with imatinib or sunitinib. The sequence of drug administration in the combination arms warrants further studies.

Ras-driven tumors are often refractory to conventional therapies. Here we identify a promising targeted therapeutic strategy for two Ras-driven cancers: Nf1-deficient malignancies and Kras/p53 mutant lung cancer. We show that agents that enhance proteotoxic stress, including the HSP90 inhibitor IPI-504, induce tumor regression in aggressive mouse models, but only when combined with rapamycin. These agents synergize by promoting irresolvable ER stress, resulting in catastrophic ER and mitochondrial damage. This process is fueled by oxidative stress, which is caused by IPI-504-dependent production of reactive oxygen species, and the rapamycin-dependent suppression of glutathione, an important endogenous antioxidant. Notably, the mechanism by which these agents cooperate reveals a therapeutic paradigm that can be expanded to develop additional combinations., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Purpose: KIT activity is crucial for gastrointestinal stromal tumors (GIST). Imatinib (IMA) and sunitinib (SUN) are very effective KIT-inhibitors in patients with advanced GIST but have no curative potential. We evaluated the efficacy of the novel HSP90 inhibitor IPI-493 alone, or in combination with IMA or SUN in GIST xenografts with KIT mutations., Experimental Design: Nude mice (n = 98) were grafted bilaterally with human GIST carrying KIT exon 11 (GIST-PSW), KIT exon 9 (GIST-BOE), or double, KIT imatinib-sensitive exon 11 and imatinib-resistant exon 17 mutations (GIST-48). Mice were divided into six treatment groups and dosed orally for 15 days as follows: (i) control group, sterile water; (ii) IMA alone; (iii) SUN alone; (iv) IPI-493 alone; (v) IPI-493+IMA; and (vi) IPI-493+SUN., Results: Treatment with IPI-493 resulted in tumor growth stabilization, variable proliferation arrest, induction of apoptosis and necrosis, and downregulation of KIT and its signaling cascade, especially in the GIST-BOE model. Significant reduction of vessel density was observed with IPI-493 treatment, and was equal to SUN treatment in GIST-PSW and GIST-BOE xenografts. IPI-493 treatment effects were enhanced in combination with TKIs, especially with IPI-493+SUN. In our hands, IPI-493 showed dose-dependent liver damages., Conclusions: When administered as a single agent in a xenograft model, the HSP90 inhibitor IPI-493 has consistent antitumor activity and induces KIT downregulation in GISTs with heterogeneous KIT mutations. IPI-493 synergizes with TKIs that are commonly used for the treatment of advanced or IMA-resistant GIST. The antitumor response of IPI-493 is particularly enhanced in combination with SUN., (©2011 AACR.)

Hsp90 facilitates the maturation and stability of numerous oncoproteins, including HER2. The aim of this study was to assess the antitumor activity of the Hsp90 inhibitor IPI-504 in trastuzumab-resistant, HER2-overexpressing breast cancer cells. Therapy with trastuzumab, IPI-504, and the combination of trastuzumab and IPI-504 was evaluated in trastuzumab-sensitive and trastuzumab-resistant cells. Inhibition of protein targets, cell proliferation, and tumor growth was assessed in vitro and in xenograft models. IPI-504 inhibited proliferation of both trastuzumab-sensitive and trastuzumab-resistant cells. Administration of IPI-504 markedly reduced total levels of HER2 and Akt, as well as phosphorylated Akt and mitogen-activated protein kinase (MAPK), to an equal extent in trastuzumab-sensitive and trastuzumab-resistant cells. IPI-504, used as single agent or in combination with trastuzumab, also inhibited in vivo the growth of both trastuzumab-sensitive and -resistant tumor xenografts. As a mechanism for the observed antitumor activity, IPI-504 resulted in a marked decrease in the levels of HER2, Akt, p-Akt, and p-MAPK in trastuzumab-resistant xenografts as early as 12 hours after a single dose of IPI-504. IPI-504-mediated Hsp90 inhibition may represent a novel therapeutic approach in trastuzumab refractory HER2-positive breast cancer.

Despite the promising introduction of the proteasome inhibitor bortezomib in the treatment of mantle cell lymphoma (MCL), not all patients respond, and resistance often appears after initial treatment. By analyzing a set of 18 MCL samples, including cell lines with constitutive or induced resistance to bortezomib, we found a high correlation between loss of sensitivity to the proteasome inhibitor and up-regulation of the prosurvival chaperone BiP/Grp78. BiP/Grp78 stabilization was ensured at a posttranscriptional level by an increase in the chaperoning activity of heat shock protein of 90 kDa (Hsp90). In bortezomib-resistant cells, both BiP/Grp78 knockdown and cell pretreatment with the Hsp90 inhibitor of the ansamycin class, IPI-504, led to synergistic induction of apoptotic cell death when combined with bortezomib. Cell exposure to the IPI-504-bortezomib combination provoked the dissociation of Hsp90/BiP complexes, leading to BiP/Grp78 depletion, inhibition of unfolded protein response, and promotion of NOXA-mediated mitochondrial depolarization. The IPI-504-bortezomib combination also prevented BiP/Grp78 accumulation, thereby promoting apoptosis and inhibiting the growth of bortezomib-resistant tumors in a mouse model of MCL xenotransplantation. These results suggest that targeting unfolded protein response activation by the inhibition of Hsp90 may be an attractive model for the design of a new bortezomib-based combination therapy for MCL.

Several Hsp90 (heat shock protein 90) inhibitors are currently under clinical evaluation as anticancer agents. However, the correlation between the duration and magnitude of Hsp90 inhibition and the downstream effects on client protein degradation and cancer cell growth inhibition has not been thoroughly investigated. To investigate the relationship between Hsp90 inhibition and cellular effects, we developed a method that measures drug occupancy on Hsp90 after treatment with the Hsp90 inhibitor IPI-504 in living cells and in tumor xenografts. In cells, we find the level of Hsp90 occupancy to be directly correlated with cell growth inhibition. At the molecular level, the relationship between Hsp90 occupancy and Hsp90 client protein degradation was examined for different client proteins. For sensitive Hsp90 clients (e.g. HER2 (human epidermal growth factor receptor 2), client protein levels directly mirror Hsp90 occupancy at all time points after IPI-504 administration. For insensitive client proteins, we find that protein abundance matches Hsp90 occupancy only after prolonged incubation with drug. Additionally, we investigate the correlation between plasma pharmacokinetics (PK), tumor PK, pharmacodynamics (PD) (client protein degradation), tumor growth inhibition, and Hsp90 occupancy in a xenograft model of human cancer. Our results indicate Hsp90 occupancy to be a better predictor of PD than either plasma PK or tumor PK. In the nonsmall cell lung cancer xenograft model studied, a linear correlation between Hsp90 occupancy and tumor growth inhibition was found. This novel binding assay was evaluated both in vitro and in vivo and could be used as a pharmacodynamic readout in the clinic.

IPI-504, a water-soluble ansamycin analogue currently being investigated in clinical trials, is a potent inhibitor of the protein chaperone heat shock protein 90 (Hsp90). Inhibition of Hsp90 by IPI-504 triggers the degradation of important oncogenic client proteins. In cells, the free base of IPI-504 hydroquinone exists in a dynamic redox equilibrium with its corresponding quinone (17-AAG); the hydroquinone form binding 50 times more tightly to Hsp90. It has been proposed recently that the NAD(P)H:quinone oxidoreductase NQO1 can produce the active hydroquinone and could be essential for the activity of IPI-504. Here, we have devised a method to directly measure the intracellular ratio of hydroquinone to quinone (HQ/Q) and have applied this measurement to correlate NQO1 enzyme abundance with HQ/Q ratio and cellular activity of IPI-504 in 30 cancer cell lines. Interestingly, the intracellular HQ/Q ratio was correlated with NQO1 levels only in a subset of cell lines and overall was poorly correlated with the growth inhibitory activity of IPI-504. Although artificial overexpression of NQO1 is able to increase the level of hydroquinone and cell sensitivity to IPI-504, it has little effect on the activity of 17-amino-17-demethoxy-geldanamycin, the major active metabolite of IPI-504. This finding could provide an explanation for the biological activity of IPI-504 in xenograft models of cell lines that are not sensitive to IPI-504 in vitro. Our results suggest that NQO1 activity is not a determinant of IPI-504 activity in vivo and, therefore, unlikely to become an important resistance mechanism to IPI-504 in the clinic.

Background: Inhibitors of heat shock protein (Hsp) 90 induce apoptosis in multiple myeloma (MM) cells, but the molecular mechanisms underlying this cytotoxic outcome are not clear. Here, we investigate the effect of IPI-504, a novel and highly soluble inhibitor of the Hsp90 ATPase activity, on the unfolded protein response (UPR) in MM cells. The UPR is a stress response pathway triggered by sensors located at the endoplasmic reticulum (ER) membrane whose function is to reduce an excessive accumulation of misfolded protein in the ER. During normal development of B-lymphocytes to antibody-producing plasma cells, a partial UPR has been described, where IREalpha and ATF-6 are stimulated, whereas the third sensor, PERK, is not induced., Methods: Levels of the activated forms of the three main UPR sensors ATF-6, XBP-1 and PERK/eIF-2 were monitored in two different MM cells lines and one non-MM cell lines under various experimental conditions including incubation with increasing concentration of IPI-504. Also, MM cells were incubated with IPI-504 and several apoptosis markers were monitored., Results: We show here that a partial UPR is constitutively activated in plasma cell-derived MM cells and that IPI-504 can potently inhibit this pathway. IPI-504 achieves this by inactivating the transcription factors XBP1 and ATF6. In addition, IPI-504 also blocks the tunicamycin-induced phosphorylation of eIF2 by PERK. Dose-response and time course experiments reveal that IPI-504's inhibitory effect on the UPR parallels its cytotoxic and pro-apoptotic effects on MM cells., Conclusion: The results presented here suggest that the IPI-504-induced apoptosis might be, in part, mediated by the inhibition of the partial UPR. Other malignancies that rely on intact and efficient UPR to survive could be considered as new indications for Hsp90 inhibitors.

Heat shock protein 90 (Hsp90) is an emerging therapeutic target of interest for the treatment of cancer. Its role in protein homeostasis and the selective chaperoning of key signaling proteins in cancer survival and proliferation pathways has made it an attractive target of small molecule therapeutic intervention. 17-Allylamino-17-demethoxygeldanamycin (17-AAG), the most studied agent directed against Hsp90, suffers from poor physical-chemical properties that limit its clinical potential. Therefore, there exists a need for novel, patient-friendly Hsp90-directed agents for clinical investigation. IPI-504, the highly soluble hydroquinone hydrochloride derivative of 17-AAG, was synthesized as an Hsp90 inhibitor with favorable pharmaceutical properties. Its biochemical and biological activity was profiled in an Hsp90-binding assay, as well as in cancer-cell assays. Furthermore, the metabolic profile of IPI-504 was compared with that of 17-AAG, a geldanamycin analog currently in clinical trials. The anti-tumor activity of IPI-504 was tested as both a single agent as well as in combination with bortezomib in myeloma cell lines and in vivo xenograft models, and the retention of IPI-504 in tumor tissue was determined. In conclusion, IPI-504, a potent inhibitor of Hsp90, is efficacious in cellular and animal models of myeloma. It is synergistically efficacious with the proteasome inhibitor bortezomib and is preferentially retained in tumor tissues relative to plasma. Importantly, it was observed that IPI-504 interconverts with the known agent 17-AAG in vitro and in vivo via an oxidation-reduction equilibrium, and we demonstrate that IPI-504 is the slightly more potent inhibitor of Hsp90.

17-Allylamino-17-demethoxygeldanamycin (17-AAG)1 is a semisynthetic inhibitor of the 90 kDa heat shock protein (Hsp90) currently in clinical trials for the treatment of cancer. However, 17-AAG faces challenging formulation issues due to its poor solubility. Here we report the synthesis and evaluation of a highly soluble hydroquinone hydrochloride derivative of 17-AAG, 1a (IPI-504), and several of the physiological metabolites. These compounds show comparable binding affinity to human Hsp90 and its endoplasmic reticulum (ER) homologue, the 94 kDa glucose regulated protein (Grp94). Furthermore, the compounds inhibit the growth of the human cancer cell lines SKBR3 and SKOV3, which overexpress Hsp90 client protein Her2, and cause down-regulation of Her2 as well as induction of Hsp70 consistent with Hsp90 inhibition. There is a clear correlation between the measured binding affinity of the compounds and their cellular activities. Upon the basis of its potent activity against Hsp90 and a significant improvement in solubility, 1a is currently under evaluation in Phase I clinical trials for cancer.