Depletion of p185erbB2, Raf-1 and mutant p53 proteins by geldanamycin derivatives correlates with antiproliferative activity

Regulation of Mitogen-Activated Protein Kinase Signaling Pathways by the Ubiquitin-Proteasome System and Its Pharmacological Potential

Mitogen-activated protein kinase (MAPK) cascades are evolutionarily conserved signaling pathways that play essential roles in transducing extracellular environmental signals into diverse cellular responses to maintain homeostasis. These pathways are classically organized into an architecture of three sequentially acting protein kinases: a MAPK kinase kinase that phosphorylates and activates a MAPK kinase, which in turn phosphorylates and activates the effector MAPK. The activity of MAPKs is tightly regulated by phosphorylation of their activation loop, which can be modulated by positive and negative feedback mechanisms to control the amplitude and duration of the signal. The signaling outcomes of MAPK pathways are further regulated by interactions of MAPKs with scaffolding and regulatory proteins. Accumulating evidence indicates that, in addition to these mechanisms, MAPK signaling is commonly regulated by ubiquitin-proteasome system (UPS)-mediated control of the stability and abundance of MAPK pathway components. Notably, the biologic activity of some MAPKs appears to be regulated mainly at the level of protein turnover. Recent studies have started to explore the potential of targeted protein degradation as a powerful strategy to investigate the biologic functions of individual MAPK pathway components and as a new therapeutic approach to overcome resistance to current small-molecule kinase inhibitors. Here, we comprehensively review the mechanisms, physiologic importance, and pharmacological potential of UPS-mediated protein degradation in the control of MAPK signaling. SIGNIFICANCE STATEMENT: Accumulating evidence highlights the importance of targeted protein degradation by the ubiquitin-proteasome system in regulating and fine-tuning the signaling output of mitogen-activated protein kinase (MAPK) pathways. Manipulating protein levels of MAPK cascade components may provide a novel approach for the development of selective pharmacological tools and therapeutics.

Resorcylic acid lactones (RALs) and their structural congeners: recent advances in their biosynthesis, chemical synthesis and biology

Resorcylic acid lactones (RALs) are naturally occurring 14-membered macrolactones that constitute a class of polyketides derived from fungal metabolites and that possess significant and promising biological activity.

Phase II trial of gemcitabine and tanespimycin (17AAG) in metastatic pancreatic cancer: a Mayo Clinic Phase II Consortium study

OBJECTIVES

Heat Shock Protein 90 (HSP90) is a molecular chaperone that stabilizes many oncogenic proteins. HSP90 inhibitors may sensitize tumors to cytotoxic agents by causing client protein degradation. Gemcitabine, which has modest activity in pancreas cancer, activates Chk1, a client protein of HSP90. This phase II trial was designed to determine whether 17AAG could enhance the clinical activity of gemcitabine through degradation of Chk1 in patients with stage IV pancreatic cancer.

METHODS

A multicenter, prospective study combining gemcitabine and 17AAG enrolled patients with stage IV pancreatic adenocarcinoma, adequate liver and kidney function, ECOG performance status 0-2, and no prior chemotherapy for metastatic disease. The primary goal was to achieve a 60 % overall survival at 6 months. Sixty-six patients were planned for accrual, with an interim analysis after 25 patients enrolled.

RESULTS

After a futility analysis to achieve the endpoint, accrual was halted with 21 patients enrolled. No complete or partial responses were seen. Forty percent of patients were alive at 6 months. Median overall survival was 5.4 months. Tolerability was moderate, with 65 % of patients having ≥ grade 3 adverse events (AE), and 15 % having grade 4 events.

CONCLUSIONS

The lack of clinical activity suggests that targeting Chk1 by inhibiting HSP90 is not important in pancreatic cancer sensitivity to gemcitabine alone. Further studies of HSP90 targeted agents with gemcitabine alone are not warranted.

N-acetylcysteine prevents the geldanamycin cytotoxicity by forming geldanamycin-N-acetylcysteine adduct

Geldanamycin (GDN) is a benzoquinone ansamycin antibiotic with anti-proliferative activity on tumor cells. GDN cytotoxicity has been attributed to the disruption of heat shock protein 90 (Hsp90) binding and stabilizing client proteins, and by the induction of oxidative stress with concomitant glutathione (GSH) depletion. The later mechanism of cytotoxicity can be abrogated by N-acetylcysteine (NAC). It was suggested that NAC prevents GDN cytotoxicity mainly by the restoring of glutathione (GSH) level (Clark et al., 2009). Here we argue that NAC does not protect cells from the GDN cytotoxicity by restoring the level of GSH. A detailed LC/MS/MS analysis of cell extracts indicated formation of GDN adducts with GSH. The amount of the GDN-GSH adduct is proportional to the GDN concentration and increases with incubation time. While nanomolar and low micromolar GDN concentrations induce cell death without an apparent GSH decrease, only much higher micromolar GDN concentrations cause a significant GSH decrease. Therefore, only high micromolar GDN concentrations can cause cell death which might be related to GSH depletion. Addition of NAC leads to the formation of adducts with GDN which diminish formation of GDN adducts with GSH. NAC also forms stable adducts with GDN extracellularly. Although NAC induces an increase in the GSH pool, this effect is not crucial for abrogation of GDN cytotoxicity. Indeed, the presence of NAC in the growth medium causes a rapid conversion of GDN into the GDN-NAC adduct, which is the real cause of the abrogated GDN cytotoxicity.

Heat-shock protein 90 inhibitors as antitumor agents: a survey of the literature from 2005 to 2010

INTRODUCTION

Heat-shock protein 90 (Hsp90) is a validated novel anticancer target with unique features. As a molecular chaperone, Hsp90 is implicated in maintaining the conformation, stability, activity and cellular localization of several key oncogenic client proteins that are involved in signal transduction pathways leading to proliferation, cell-cycle progression, apoptosis, angiogenesis and metastasis. As a result, inhibitors of Hsp90 achieve their promising anticancer activity through disruption of the Hsp90 protein function, thereby freezing the chaperone cycle; this in turn decreases the affinity of Hsp90 for client proteins, thus leading to proteasome-mediated degradation of oncogenic client proteins within cancer cells.

AREAS COVERED

This review provides recent background information on Hsp90 inhibitors. It also highlights a panel of compounds of interest reported in patents and discusses the clinical results of the promising drug candidates.

EXPERT OPINION

In the past 5 years, Hsp90 inhibitors have remained the focus of much interest as new potential anticancer agents. A large variety of scaffolds were studied in both academia and industry. Consequently, these significant research efforts have provided several promising drug candidates for further clinical development. Further progress in the development of Hsp90 inhibitors, combined with a deeper understanding of the chaperon characteristics, strengthens their promise in cancer therapy.

Regulation of senescence in cancer and aging

Senescence is regarded as a physiological response of cells to stress, including telomere dysfunction, aberrant oncogenic activation, DNA damage, and oxidative stress. This stress response has an antagonistically pleiotropic effect to organisms: beneficial as a tumor suppressor, but detrimental by contributing to aging. The emergence of senescence as an effective tumor suppression mechanism is highlighted by recent demonstration that senescence prevents proliferation of cells at risk of neoplastic transformation. Consequently, induction of senescence is recognized as a potential treatment of cancer. Substantial evidence also suggests that senescence plays an important role in aging, particularly in aging of stem cells. In this paper, we will discuss the molecular regulation of senescence its role in cancer and aging. The potential utility of senescence in cancer therapeutics will also be discussed.

In vitro metabolism of 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin in human liver microsomes

The objective of this study was to investigate the oxidative metabolism pathways of 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17-DMAG), a geldanamycin (GA) derivative and 90-kDa heat shock protein inhibitor. In vitro metabolic profiles of 17-DMAG were examined by using pooled human liver microsomes (HLMs) and recombinant CYP450 isozymes in the presence or absence of reduced GSH. In addition to 17-DMAG hydroquinone and 19-glutathionyl 17-DMAG, several oxidative metabolites of 17-DMAG were detected and characterized by liquid chromatography-tandem mass spectrometry. Different from previously reported primary biotransformations of GA and GA derivatives, 17-DMAG was not metabolized primarily through the reduction of benzoquinone and GSH conjugation in HLMs. In contrast, the primary biotransformations of 17-DMAG in HLMs were hydroxylation and demethylation on its side chains. The most abundant metabolite was produced by demethylation from the methoxyl at position 12. The reaction phenotyping study showed that CYP3A4 and 3A5 were the major cytochrome P450 isozymes involved in the oxidative metabolism of 17-DMAG, whereas CYP2C8, 2D6, 2A6, 2C19, and 1A2 made minor contributions to the formation of metabolites. On the basis of the identified metabolite profiles, the biotransformation pathways for 17-DMAG in HLMs were proposed.

Antiproliferative and apoptotic activities of tosylcyclonovobiocic acids as potent heat shock protein 90 inhibitors in human cancer cells

We evaluated whether inhibition of heat shock protein 90 (hsp90) function by novobiocin derivatives could induce the degradation of signal transducers that drive cancer cell growth and thereby promote apoptosis. Removal of the noviose moiety in novobiocin and introduction of a tosyl substituent at C-4 or C-7 coumarin nucleus provided derivatives 4TCNA and 7TCNA which compared favourably with novobiocin in MCF-7 breast cancer cells. Here we extend the antiproliferative and apoptotic properties of these analogues to a panel of cancer cell lines. Destabilization of hsp90 client proteins Raf-1, HER2, and cdk4 suggests inhibition of hsp90 chaperoning function. In HT29 colon and IGROV1 ovarian cancer cells, the growth inhibiting effect of 4TCNA and 7TCNA was consistent with the stimulation of cell death as assessed by the processing and activation of caspase 9, 8, 7 and 3 and the subsequent cleavage of poly(ADP-ribose) polymerase (PARP). In Ishikawa endometrial adenocarcinoma cells, 4TCNA also promoted apoptosis and the processing of PARP. These derivatives impacting multiple pathways involved in the neoplastic process may represent promising drugs for cancer therapy.

Oxidative stress plays a critical role in inactivating mutant BRAF by geldanamycin derivatives

The geldanamycin derivatives 17-allylamino-17-demethoxygeldanamycin (17-AAG) and 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG) are promising chemotherapeutic drugs that inhibit heat shock protein 90 (HSP90) function. Previous studies have shown that 17-AAG/DMAG treatment induces the degradation of mutant BRAF (V600E) and inhibits the activation of mitogen-activated protein/extracellular signal-regulated kinase 1/2 (MEK1/2). We have found, however, that HSP90 inhibition alone is not sufficient for efficient BRAF(V600E) degradation in some cells. HSP90 inhibitors structurally unrelated to geldanamycin, radicicol and novobiocin, while inducing the degradation of the HSP90 client protein RAF-1 fail to induce BRAF(V600E) degradation or inhibit MEK1/2 activation in HT29 human colon cancer cells. Moreover, after treatment with 17-DMAG, the kinase activity of residual, undegraded BRAF(V600E) was also lost. Incubation of cells with a reactive oxygen species (ROS) scavenger, N-acetyl cysteine, partially restored kinase activity and also partially prevented BRAF(V600E) degradation due to 17-DMAG treatment. Conversely, treatment with the ROS producing drug menadione clearly inhibited MEK1/2 and reduced BRAF(V600E). These results suggest that in addition to direct inhibition of HSP90, the antitumor effect of geldanamycin and its derivatives is also mediated though the production of ROS, which may directly inactivate tumorigenic mutant BRAF(V600E).

Concerted actions of multiple transcription elements confer differential transactivation of HSP90 isoforms in geldanamycin-treated 9L rat gliosarcoma cells

HSP90 chaperones are transducer proteins of many signaling pathways in cells. Using a highly specific inhibitor, geldanamycin (GA), an increasing number of the HSP90 client proteins have been identified. Nevertheless, there is little information on the differential transactivation of the two isoforms of the hsp90 genes, hsp90alpha and beta, in cells under stress conditions. Here, we demonstrate the differential expression of the HSP90 isoforms, HSP90alpha and beta, in rat gliosarcoma 9L cells using a modified SDS-PAGE system that allowed us to distinguish the isoforms. We subsequently assessed the transcriptional controls involving the transcription elements located in the promoter regions of the hsp90 genes. At the protein level, HSP90alpha is more responsive to GA in terms of rate of de novo synthesis and amount of accumulation, as shown by metabolic-labeling and Western-blotting analyses. Upregulation of the hsp90 genes was demonstrated by real-time qPCR. The promoter elements hsp90alpha-HSE2 and hsp90beta-HSE1 were also identified to be the major transcription elements involved in GA-activated gene expression, as shown by EMSA, whereas the results of supershift showed that the transcription factor HSF1 is also involved. Moreover, EMSA results of analysis of the GC box showed differences in both the initial amounts and inductive response of hsp90s transcripts, whereas analysis of the TATA box showed GA responsiveness in hsp90alpha only. Collectively, these results indicate that GA exerts its regulatory effects through transcription elements including heat-shock elements (HSEs), GC boxes and TATA boxes, resulting in differential transactivation of hsp90alpha and hsp90beta in rat gliosarcoma 9L cells.

Heat shock protein inhibitors increase the efficacy of measles virotherapy

Oncolytic measles virus strains have activity against multiple tumor types and are currently in phase I clinical testing. Induction of the heat shock protein 70 (HSP70) constitutes one of the earliest changes in cellular gene expression following infection with RNA viruses including measles virus, and HSP70 upregulation induced by heat shock has been shown to result in increased measles virus cytotoxicity. HSP90 inhibitors such as geldanamycin (GA) or 17-allylaminogeldanamycin result in pharmacologic upregulation of HSP70 and they are currently in clinical testing as cancer therapeutics. We therefore investigated the hypothesis that heat shock protein inhibitors could augment the measles virus-induced cytopathic effect. We tested the combination of a measles virus derivative expressing soluble human carcinoembryonic antigen (MV-CEA) and GA in MDA-MB-231 (breast), SKOV3.IP (ovarian) and TE671 (rhabdomyosarcoma) cancer cell lines. Optimal synergy was accomplished when GA treatment was initiated 6-24 h following MV infection. Western immunoblotting confirmed HSP70 upregulation in combination-treated cells. Combination treatment resulted in statistically significant increase in syncytia formation as compared to MV-CEA infection alone. Clonogenic assays demonstrated significant decrease in tumor colony formation in MV-CEA/GA combination-treated cells. In addition there was increase in apoptosis by 4,6-diamidino-2-phenylindole staining. Western immunoblotting for caspase-9, caspase-8, caspase-3 and poly(ADP-ribose) polymerase (PARP) demonstrated increase in cleaved caspase-8 and PARP. The pan-caspase inhibitor Z-VAD-FMK and caspase-8 inhibitor Z-IETD-FMK, but not the caspase-9 inhibitor Z-IEHD-FMK, protected tumor cells from MV-CEA/GA-induced PARP activation, indicating that apoptosis in combination-treated cells occurs mainly via the extrinsic caspase pathway. Treatment of normal cells, such as normal human fibroblasts, however, with the MV-CEA/GA combination, did not result in cytopathic effect, indicating that GA did not alter the MV-CEA specificity for tumor cells. One-step viral growth curves, western immunoblotting for MV-N protein expression, QRT-PCR quantitation of MV-genome copy number and CEA levels showed comparable proliferation of MV-CEA in GA-treated vs -untreated tumor cells. Rho activation assays and western blot for total RhoA, a GTPase associated with the actin cytoskeleton, demonstrated decrease in RhoA activation in combination-treated cells, a change previously shown to be associated with increase in paramyxovirus-induced cell-cell fusion. The enhanced cytopathic effect resulting from measles virus/GA combination supports the translational potential of this approach in the treatment of cancer.

Emerging targeted therapies for breast cancer

Breast cancer is the most common neoplasia among women in the world. For the last few years there has been an increasing interest in the development of agents against molecular targets considered to be involved in the process of malignant transformation or tumor progression. Experimental data indicate that various intracellular signaling pathways may be activated or overexpressed in patients who have breast cancer. Targeted therapies against these pathways have recently become one of the most active and promising areas of development in oncology.

Hsp90 inhibitors: small molecules that transform the Hsp90 protein folding machinery into a catalyst for protein degradation

The 90 kDa heat shock proteins (Hsp90) are responsible for the conformational maturation of nascent polypeptides and the renaturation of denatured proteins. In transformed cells, numerous mutated and overexpressed proteins rely on the Hsp90 protein folding machinery for tumor progression. The Hsp90-mediated protein folding process is dependent upon ATP, and when inhibitors of ATP are present, the Hsp90 machinery is unable to fold client proteins into their biologically active form, which results in the degradation of protein substrates via the ubiquitin-proteasome pathway. Consequently, Hsp90 has evolved into a promising anti-cancer target because multiple oncogenic proteins can be simultaneously degraded as a consequence of Hsp90 inhibition. This review serves to explain the Hsp90 protein folding process, the impact of Hsp90 inhibition, the identification of natural product inhibitors, and the development of rationally designed inhibitors of the Hsp90 protein folding machinery.

Biotransformation of geldanamycin and 17-allylamino-17-demethoxygeldanamycin by human liver microsomes: reductive versus oxidative metabolism and implications

Comparative metabolite profiling of geldanamycin and 17-allylamino-17-demethoxygeldanamycin (17AAG) using human liver microsomes in normoxia and hypoxia was conducted to understand their differential metabolic fates. Geldanamycin bearing a 17-methoxy group primarily underwent reductive metabolism, generating the corresponding hydroquinone under both conditions. The formed hydroquinone resists further metabolism and serves as a reservoir. On exposure to oxygen, this hydroquinone slowly reverts to geldanamycin. In the presence of glutathione, geldanamycin was rapidly converted to 19-glutathionyl geldanamycin hydroquinone, suggesting its reactive nature. In contrast, the counterpart (17AAG) preferentially remained as its quinone form, which underwent extensive oxidative metabolism on both the 17-allylamino sidechain and the ansa ring. Only a small amount (<1%) of 19-glutathione conjugate of 17AAG was detected in the incubation of 17AAG with glutathione at 37 degrees C for 60 min. To confirm the differential nature of quinone-hydroquinone conversion between the two compounds, hypoxic incubations with human cytochrome P450 reductase at 37 degrees C and direct injection analysis were performed. Approximately 89% of hydroquinone, 5% of quinone, and 6% of 17-O-demethylgeldanamycin were observed after 1-min incubation of geldanamycin, whereas about 1% of hydroquinone and 99% of quinone were found in the 60-min incubation of 17AAG. The results provide direct evidence for understanding the 17-substituent effects of these benzoquinone ansamycins on their phase I metabolism, reactivity with glutathione, and acute hepatotoxicity.

The Bioreduction of a Series of Benzoquinone Ansamycins by NAD(P)H:Quinone Oxidoreductase 1 to More Potent Heat Shock Protein 90 Inhibitors, the Hydroquinone Ansamycins

We have previously evaluated the role of NAD(P)H:quinone oxidoreductase 1 (NQO1) in the bioreductive metabolism of 17-(allylamino)-demethoxygeldanamycin (17AAG) to the corresponding hydroquinone, a more potent 90-kDa heat shock protein (Hsp90) inhibitor. Here, we report an extensive study with a series of benzoquinone ansamycins, which includes gel-danamycin, 17-(amino)-17-demethoxygeldanamycin, and 17-demethoxy-17-[[2-(dimethylamino)ethyl]amino]-geldanamycin. The reduction of these benzoquinone ansamycins by recombinant human NQO1 to the corresponding hydroquinone ansamycins was monitored by high-performance liquid chromatography (HPLC) and confirmed by liquid chromatography/mass spectrometry. Inhibition of purified yeast Hsp90 ATPase activity was augmented in the presence of NQO1 and abrogated by 5-methoxy-1,2-dimethyl-3-[(4-nitrophenoxy)methyl-]indole-4,7-dione (ES936), a mechanism-based inhibitor of NQO1, showing that the hydroquinone ansamycins were more potent Hsp90 inhibitors than their parent quinones. An isogenic pair of human breast cancer cell lines, MDA468 and MDA468/NQ16, differing in expression of NQO1, was used, and HPLC analysis showed that hydroquinone ansamycins were formed by the MDA468/NQ16 cells, which could be prevented by ES936 pretreatment. The MDA468/NQ16 cells were more sensitive to growth inhibition after treatment with the benzoquinone ansamycins compared with the MDA468 cells; this increased sensitivity could be reduced by ES936 pretreatment. The increased duration of benzoquinone ansamycin exposure showed increased potency and -fold inhibition in MDA468/NQ16 cells relative to the parental MDA468 cells. Computational-based molecular modeling studies displayed additional contacts between yeast Hsp90 and the hydroquinone ansamycins, which translated to greater interaction energies compared with the corresponding benzoquinone ansamycins. In conclusion, these studies show that the reduction of this series of benzoquinone ansamycins by NQO1 generates the corresponding hydroquinone ansamycins, which exhibit enhanced Hsp90 inhibition.

Apoptosis-based therapies and drug targets

The pathogenesis of many diseases is most closely connected with aberrantly regulated apoptotic cell death. The past 15 years have witnessed an explosion in the basic knowledge of mechanisms that regulate apoptosis and the mediators that either trigger or inhibit cell death. Consequently, great interest has emerged in devising therapeutic strategies for modulating the key molecules of life-and-death decisions. Numerous novel approaches are currently being followed employing gene therapy and antisense strategies, recombinant biologics or classical organic and combinatorial chemistry in order to target specific apoptotic regulators. Although drug development is still in its infancy, several therapeutics have progressed to clinical testing or have even been approved in record time. This review outlines the recent advances in the field of apoptosis-based therapies and explores some highlights of a very active field of drug development.

Identification of genes involved in the sensitivity to antitumour drug 17-allylamino,17-demethoxygeldanamycin (17AAG)

In the present study we analysed the gene expression database provided by the National Cancer Institute in an attempt to correlate activity profiles of geldanamycin, 17AAG and 11 other analogues in 60 human tumor cell lines with their gene expression profiles determined by the cDNA microarray technique. On the basis of the activity profiles two classes of geldanamycin analogues could be distinguished, having geldanamycin and 17AAG, respectively, as prototype compounds (denominated as gelda-like and 17AAG-like classes). Application of the "soft" statistical methodology of PLS (partial least squares modelling in latent variables or projections to latent structures) allowed us to evaluate the influence of each gene expression target in determining the therapeutical responses. The transcript encoding the translocating chain-associated membrane protein (TRAM) showed a significant statistical correlation with activity profiles of 17AAG. In order to validate the role of TRAM in determining sensitivity to 17AAG we induced a selective knocking-down of this transcript by the RNA interference methodology in H226 non-small cell lung carcinoma cell line. The efficiency of double-stranded RNA oligonucleotides (short-interfering RNAs, siRNAs) was determined by measuring TRAM mRNA levels by quantitative real-time RT-PCR at different times (24-72 hours) after siRNA lipotransfection. A significant increase in chemosensitivity to 17AAG was observed in siRNA-silenced cells. Although a number of factors may affect tumour sensitivity to 17AAG the present methodology allowed us to dissect out a single parameter which may be partly responsible for its activity.

Development and validation of a liquid chromatography/tandem mass spectrometry method for the determination of the novel anticancer agent 17-DMAG in human plasma

An accurate, sensitive, robust and selective liquid chromatography/tandem mass spectrometry (LC/MS/MS) method for the determination of 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin hydrochloride (17-DMAG) in human plasma has been developed and validated. Plasma samples were prepared by liquid/liquid extraction with ethyl acetate. The chromatographic separation was achieved within 9 min on a Synergy Polar column with a linear gradient and a mobile phase consisting of methanol and 0.1% formic acid in water. Detection of 17-DMAG and the internal standard (IS), olomoucine, was achieved by MS/MS with electrospray ionisation in positive ion mode. The calibration curve, ranging from 1.89 to 1890 nM, was linear r > 0.994 using a 1/y2 weighted linear regression. The assay showed no significant interferences from endogenous compounds. The lower limit of quantitation (LLOQ) was 1.89 nM, using 250 microL of plasma, with inter-assay precision (%RSD) and accuracy (%RE) values of 11.6% and -5.8%, respectively. Intra-assay precision ranged from 7.8-13.6%. The method described here is being used to evaluate the pharmacokinetic profiles of 17-DMAG given as a once weekly infusion in patients with advanced solid tumours.

Overcoming limitations of natural anticancer drugs by combining with artificial agents

During a billion years of evolution, living creatures have perfected cytotoxic agents to kill other organisms without killing themselves, thus providing us with antibiotics to kill bacteria without killing eukaryotic (e.g. human) cells. Some natural agents inhibit specifically most vital cellular structures and functions in cancer cells. However, nature was not creating antibiotics for cancer, and natural agents kill cancer cells precisely because they share targets with normal cells. To discriminate between particular cancer cells and normal cells, we can design or select artificial agents that are not necessarily lethal but are aimed either at cancer-specific targets or at dispensable and even unavailable (in cancer cells) targets. Using rational drug combinations, such selective agents can assist natural agents to eradicate cancer cells selectively.

Pharmacological induction of Hsp70 protects apoptosis-prone cells from doxorubicin: comparison with caspase-inhibitor- and cycle-arrest-mediated cytoprotection

Selective modulation of cell death is important for rational chemotherapy. By depleting Hsp90-client oncoproteins, geldanamycin (GA) and 17-allylamino-17-demethoxy-GA (17-AAG) (heat-shock protein-90-active drugs) render certain oncoprotein-addictive cancer cells sensitive to chemotherapy. Here we investigated effects of GA and 17-AAG in apoptosis-prone cells such as HL60 and U937. In these cells, doxorubicin (DOX) caused rapid apoptosis, whereas GA-induced heat-shock protein-70 (Hsp70) (a potent inhibitor of apoptosis) and G1 arrest without significant apoptosis. GA blocked caspase activation and apoptosis and delayed cell death caused by DOX. Inhibitors of translation and transcription and siRNA Hsp70 abrogated cytoprotective effects of GA. Also GA failed to protect HL60 cells from cytotoxicity of actinomycin D and flavopiridol (FL), inhibitors of transcription. We next compared cytoprotection by GA-induced Hsp70, caspase inhibitors (Z-VAD-fmk) and cell-cycle arrest. Whereas cell-cycle arrest protected HL60 cells from paclitaxel (PTX) but not from FL and DOX, Z-VAD-fmk prevented FL-induced apoptosis but was less effective against DOX and PTX. Thus, by inducing Hsp70, GA protected apoptosis-prone cells in unique and cell-type selective manner. Since GA does not protect apoptosis-reluctant cancer cells, we envision a therapeutic strategy to decrease side effects of chemotherapy without affecting its therapeutic efficacy.

Activated Tyrosine Kinase Ack1 Promotes Prostate Tumorigenesis: Role of Ack1 in Polyubiquitination of Tumor Suppressor Wwox

Aberrant activation of tyrosine kinases is linked causally to human cancers. Activated Cdc42-associated kinase (Ack1), an intracellular tyrosine kinase, has primarily been studied for its signaling properties but has not been linked to specific pathologic conditions. Herein, we report that expression of activated Ack1 in LNCaP cells, while minimally increasing growth in culture, enhanced anchorage-independent growth in vitro and dramatically accelerated tumorigenesis in nude mice. Molecular chaperone heat shock protein 90beta (Hsp90beta)-bound Ack1 and treatment of cells with geldanamycin, a Hsp90 inhibitor, inhibited Ack1 kinase activity and suppressed tumorigenesis. Further, we identify the tumor suppressor WW domain containing oxidoreductase (Wwox) as an Ack1-interacting protein. Activated Ack1 tyrosine phosphorylated Wwox, leading to rapid dissociation of the Ack1-Wwox complex and concomitant Wwox polyubiquitination followed by degradation. Tyrosine phosphorylation of Wwox was critical for its degradation, as splice variant WwoxDelta5-8 that was not phosphorylated by Ack1 failed to undergo polyubiquitination and degradation. It has been reported that phosphorylation of Wwox at Tyr33 stimulated its proapoptotic activity. We observed that Y33F Wwox mutant was still tyrosine phosphorylated and polyubiquitinated by Ack1 action. Site-directed mutagenesis revealed that activated Ack1 primarily phosphorylated Wwox at Tyr287, suggesting that phosphorylation of distinct tyrosine residues activate or degrade Wwox. Primary androgen-independent prostate tumors but not benign prostate showed increased tyrosine-phosphorylated Ack1 and decreased Wwox. Taken together, these data indicate that Ack1 stimulated prostate tumorigenesis in part by negatively regulating the proapoptotic tumor suppressor, Wwox. Further, these findings suggest that Ack1 could be a novel therapeutic target for prostate cancer.

New approaches and therapeutics targeting apoptosis in disease

Apoptosis, the major form of cellular suicide, is central to various physiological processes and the maintenance of homeostasis in multicellular organisms. Presumably, even more important is a causative or contributing role of apoptosis to various human diseases. These include situations with unwanted cell accumulation (cancer) and failure to eradicate aberrant cells (autoimmune diseases) or disorders with an inappropriate loss of cells (heart failure, stroke, AIDS, neurodegenerative diseases, and liver injury). The past decade has witnessed a tremendous progress in the knowledge of the molecular mechanisms that regulate apoptosis and the mediators that either prevent or trigger cell death. Consequently, apoptosis regulators have emerged as key targets for the design of therapeutic strategies aimed at modulating cellular life-and-death decisions. Numerous novel approaches are currently being followed employing gene therapy and antisense strategies, recombinant biologics, or classical organic and combinatorial chemistry to target specific apoptotic regulators. Convincing proof-of-principle evidence obtained in several animal models confirms the validity of strategies targeting apoptosis and revealed an enormous potential for therapeutic intervention in a variety of illnesses. Although numerous apoptotic drugs are currently being developed, several therapeutics have progressed to clinical testing or are already approved and marketed. Here we review the recent progress of apoptosis-based therapies and survey some highlights in a very promising field of drug development.

Phase I pharmacokinetic-pharmacodynamic study of 17-(allylamino)-17-demethoxygeldanamycin (17AAG, NSC 330507), a novel inhibitor of heat shock protein 90, in patients with refractory advanced cancers

PURPOSE

17-(Allylamino)-17-demethoxygeldanamycin (17AAG), a benzoquinone antibiotic, down-regulates oncoproteins by binding specifically to heat shock protein 90 (HSP90). We did a phase I study of 17AAG to establish the dose-limiting toxicity and maximum tolerated dose and to characterize 17AAG pharmacokinetics and pharmacodynamics.

EXPERIMENTAL DESIGN

Escalating doses of 17AAG were given i.v. over 1 or 2 hours on a weekly x 3 schedule every 4 weeks to cohorts of three to six patients. Plasma pharmacokinetics of 17AAG and 17-(amino)-17-demethoxygeldanamycin (17AG) were assessed by high-performance liquid chromatography. Expression of HSP70 and HSP90 in peripheral blood mononuclear cells was measured by Western blot.

RESULTS

Forty-five patients were enrolled to 11 dose levels between 10 and 395 mg/m2. The maximum tolerated dose was 295 mg/m2. Dose-limiting toxicity occurred in both patients (grade 3 pancreatitis and grade 3 fatigue) treated with 395 mg/m2. Common drug-related toxicities (grade 1 and 2) were fatigue, anorexia, diarrhea, nausea, and vomiting. Reversible elevations of liver enzymes occurred in 29.5% of patients. Hematologic toxicity was minimal. No objective responses were observed. 17AAG pharmacokinetics was linear. Peak plasma concentration and area under the curve of 17AG, the active major metabolite of 17AAG, increased with 17AAG dose, but the relationships were more variable than with 17AAG. 17AAG and 17AG in plasma were >90% protein bound. There were no consistent changes in peripheral blood mononuclear cell HSP90 or HSP70 content.

CONCLUSIONS

17AAG doses between 10 and 295 mg/m2 are well tolerated. 17AAG pharmacokinetics is linear. Peripheral blood mononuclear cell HSP90 and HSP70 are uninformative pharmacodynamic markers. The dose recommended for future studies is 295 mg/m2 weekly x 3, repeated every 4 weeks.

Preclinical toxicity of a geldanamycin analog, 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17-DMAG), in rats and dogs: potential clinical relevance

PURPOSE

17-DMAG is a hydrophilic derivative of the molecular chaperone inhibitor 17-(allylamino)-17-demethoxygeldanamycin (17-AAG; NSC-330507), which is currently being evaluated for the treatment of cancer in clinical trials. 17-DMAG offers a potential advantage over 17-AAG because its aqueous solubility eliminates the need for complicated formulations that are currently used for administration of 17-AAG. In addition, 17-DMAG undergoes only limited metabolism compared to 17-AAG. The present results are from preclinical toxicity studies evaluating 17-DMAG in rats and dogs.

METHODS

Doses of 0, 2.4, 12 and 24 mg/m2 per day were administered to rats, while dogs received doses of 0, 8 or 16 mg/m2 per day. In both species, 17-DMAG was administered i.v. (slow bolus for rats; 1-h infusion for dogs) daily for 5 days. An additional cohort of dogs received 16 mg/m2 per day orally for 5 days. Clinical observations were noted, and standard hematology and clinical chemistry parameters were monitored. Selected tissues were evaluated microscopically for drug-related lesions. Tissue and plasma 17-DMAG concentrations were measured by HPLC/MS at selected time-points on days 1 and 5.

RESULTS

Daily i.v. administration of 17-DMAG at doses of 24 mg/m2 per day in rats or 16 mg/m2 per day in dogs produced lethality on day 6, approximately 24 h following the last dose. Body weight loss was common in rats and dogs. Drug-related gastrointestinal, bone marrow and hepatic toxicities were also common in rats and dogs. Dogs also exhibited signs of renal and gallbladder toxicity. Plasma concentrations of 17-DMAG increased proportionately with dose in rats and disproportionately with dose in dogs. In rat tissues, however, only fourfold to sixfold increases in 17-DMAG concentrations were observed with a tenfold increase in dose. The highest concentrations of 17-DMAG were found in the liver of rats, with progressively lower concentrations in the spleen, lung, kidney and plasma. Regardless of the route of administration, higher drug concentrations were present in plasma (rat and dog) and tissue (rat) samples obtained on day 5 compared to those obtained on day 1. Although plasma concentrations decreased with time, 17-DMAG was still detected in dog plasma for at least 24 h after drug administration.

CONCLUSIONS

With the recent approval of 17-DMAG for clinical use, the data generated from these preclinical studies will provide guidance to clinicians as they administer this drug to their patients. The MTD of 17-DMAG was 12 mg/m2 per day in rats and 8 mg/m2 per day in dogs; therefore, the recommended starting dose for phase I trial is 1.3 mg/m2 per day for 5 days. Gastrointestinal and bone marrow toxicity were dose-limiting in rats, and gastrointestinal, renal, gallbladder and bone marrow toxicity were dose-limiting in dogs. All adverse effects were fully reversible in surviving animals after treatment was complete.

Intracellular signal transduction pathway proteins as targets for cancer therapy

Circulating cytokines, hormones, and growth factors control all aspects of cell proliferation, differentiation, angiogenesis, apoptosis, and senescence. These chemical signals are propagated from the cell surface to intracellular processes via sequential kinase signaling, arranged in modules that exhibit redundancy and cross talk. This signal transduction system comprising growth factors, transmembrane receptor proteins, and cytoplasmic secondary messengers is often exploited to optimize tumor growth and metastasis in malignancies. Thus, it represents an attractive target for cancer therapy. This review will summarize current knowledge of selected intracellular signaling networks and their role in cancer therapy. The focus will be on pathways for which inhibitory agents are currently undergoing clinical testing. Original data for inclusion in this review were identified through a MEDLINE search of the literature. All papers from 1966 through March 2005 were identified by the following search terms: "signal transduction," "intracellular signaling," "kinases," "proliferation," "growth factors," and "cancer therapy." All original research and review papers related to the role of intracellular signaling in oncogenesis and therapeutic interventions relating to abnormal cell signaling were identified. This search was supplemented by a manual search of the Proceedings of the Annual Meetings of the American Association for Cancer Research, American Society of Clinical Oncology, and the American Association for Cancer Research (AARC)--European Organisation for Research and Treatment of Cancer (EORTC)--National Cancer Institute (NCI) Symposium on New Anticancer Drugs.

Combined inhibition of extracellular signal-regulated kinases and HSP90 sensitizes human colon carcinoma cells to ionizing radiation

Indomethacin, a common nonsteroidal anti-inflammatory drug, has been shown to enhance radiation-mediated cell-killing effect through the activation of p38 mitogen-activated protein kinase (MAPK). We found that indomethacin strongly reduced the basal level of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in HT-29 human colon carcinoma cells. The inhibition of ERK1/2 by indomethacin was only observed in cells with high basal activities of ERK1/2 such as HT-29 cells, but not in cells with low basal activities, such as HeLa. Cell cycle analysis of HT-29 cells exposed with indomethacin showed a partial G1/S arrest and slow DNA synthesis. However, the treatment with NS398, a specific COX-1/2 inhibitor, failed to show any effect on cell cycle, indicating that the inhibition of COX-1/2 is not responsible for cell cycle arrest. Since U0126, a specific inhibitor for MEK1/2, also induced a partial G1/S arrest, the G1/S arrest induced by indomethacin is, at least in part, caused by the inhibition of ERK1/2. Cell proliferation of HT-29 was inhibited by the treatment of U0126 but not in HeLa cells, and the treatment of HT-29 cells with U0126 enhanced radiation sensitivity possibly due to the accumulation of cells in G1 phase. We found that 17-allylamino-17-demethoxygeldanamycin, a geldanamycin delivative, radiosensitized HT-29 cells at a relatively low dose of irradiation, and indomethacin and U0126 further enhanced this effect. Therefore, tumor cells with elevated ERK1/2 activity can be effectively sensitized to radiation treatment by a combinational inhibition of HSP90 and MAPK activity.

Nm23-H1 metastasis suppressor expression level influences the binding properties, stability, and function of the kinase suppressor of Ras1 (KSR1) Erk scaffold in breast carcinoma cells

Metastatic disease is a significant contributor to cancer patient mortality. We previously reported that the Kinase Suppressor of Ras1 (KSR1) scaffold protein for the Erk mitogen-activated protein kinase pathway coimmunoprecipitated the metastasis suppressor protein Nm23-H1. We now hypothesize that altered expression levels of Nm23-H1 influence the binding properties, stability, and function of the KSR1 scaffold. Increased coimmunoprecipitation of Hsp90 with KSR1 was observed in either stable or transient transfectants of nm23-H1 in MDA-MB-435 human breast carcinoma cells. Similar trends were also observed in the cytoplasmic and nuclear fractions of cells. Cells expressing high levels of Nm23-H1 exhibited increased KSR1 degradation in the presence of either cycloheximide or an Hsp90-directed drug currently in clinical trial, 17-allylamino-17-demethoxygeldanamycin (17-AAG). In agreement with KSR1 degradation data, high-Nm23-H1-expression cells were preferentially inhibited in anchorage-independent colonization assays by 17-AAG. KSR1 scaffold binding patterns are dynamic in both the cytoplasmic and nuclear compartments, modulated by metastasis suppressor expression. Metastasis suppressor expression levels can impact traditional signaling pathways, such as the Erk pathway, resulting in altered tumor cell sensitivity to cancer therapeutics.

Characterization of the combined cellular survival effects of benzoquinone-ansamycins and ionizing radiation

PURPOSE

Benzoquinone-ansamycins were the first compounds characterized with the ability to inhibit the function of heat shock protein 90 and its related family members. We investigated the composite effect of ionizing radiation and of these novel substances on the survival of malignant cells.

METHODS

PC-3M prostate carcinoma cells were treated in vitro with increasing radiation doses in the presence or absence of Hsp90-active and Hsp90-inactive benzoquinone-ansamycins. Cytotoxicity was determined by the crystal violet dissolution assay.

RESULTS

Twenty-four hour treatment with increasing geldanamycin doses (10 nM-1 microM) reduced cellular survival by 1.5 logs for all drug dose levels. Concomitant irradiation with a single fraction of 3 Gy reduced cellular survival by 2 logs, independently of drug dose. The treatment with 100 nM geldanamycin for 24 h combined with ionizing radiation (1-5 Gy) during the first hour of drug exposure reduced cellular survival by 1.5-2 logs depending on radiation-energy dose level, while no changes in cell survival were detectable with equimolar geldampicin, a benzoquinone-ansamycin known not to inhibit Hsp90.

CONCLUSIONS

The inhibition of Hsp90 and the concomitant exposure to ionizing radiation decrease cellular survival of malignant cells. These data contribute to laying the foundation for the translational use of Hsp90 inhibitors in the multimodal therapy of cancer.

Syntheses of photolabile novobiocin analogues

Novobiocin was recently shown to inhibit Hsp90 through a previously unrecognized C-terminal ATP binding site. Although the N-terminal region of Hsp90 has been solved by X-ray crystallography, the C-terminal region has not. In an effort to elucidate the C-terminal binding site of Hsp90, four photolabile analogues of novobiocin were prepared.

Synthesis and biological activities of novel 17-aminogeldanamycin derivatives

Geldanamycin interferes with the action of heat shock protein 90 (Hsp90) by binding to the N-terminal ATP binding site and inhibiting an essential ATPase activity. In a program directed toward finding potent, water soluble inhibitors of Hsp90, we prepared a library of over sixty 17-alkylamino-17-demethoxygeldanamycin analogs, and compared their affinity for Hsp90, ability to inhibit growth of SKBr3 mammalian cells, and in selected cases, water solubility. Over 20 analogs showed cell growth inhibition potencies similar to that of 17-allylamino-17-demethoxygeldanamycin (17-AAG), the front-runner geldanamycin analog that is currently in multiple clinical trials. Many of these analogs showed water solubility properties that were desirable for formulation. One of the most potent and water-soluble analogs in the series was 17-(2-dimethylaminoethyl)amino-17-demethoxygeldanamycin (17-DMAG), which was independently prepared by the NCI and will soon enter clinical trials. Importantly, the binding affinity of these analogs to the molecular target Hsp90 does not correlate well with their cytotoxicity in SKBr3 cells.

Pharmacokinetics and pharmacodynamics of 17-demethoxy 17-[[(2-dimethylamino)ethyl]amino]geldanamycin (17DMAG, NSC 707545) in C.B-17 SCID mice bearing MDA-MB-231 human breast cancer xenografts

PURPOSE

17-demethoxy 17-[[(2-dimethylamino)ethyl]amino]geldanamycin (17DMAG, NSC 707545) is a water-soluble analogue of 17-(allylamino)-17-demethoxygeldanamycin (17AAG), a compound currently in clinical trials. These preclinical studies: (1) characterized 17DMAG concentrations in plasma, normal tissues, and tumor after i.v. delivery to mice; and (2) correlated tumor and normal tissue 17DMAG concentrations with alterations in heat shock protein 90 (HSP90) and selected HSP90-chaperoned proteins.

METHODS

At specified times after i.v. administration of 75 mg/kg 17DMAG, SCID mice bearing s.c. MDA-MB-231 human breast xenografts were killed and plasma and tissues were retained. 17DMAG concentrations were determined by HPLC. Raf-1, heat shock protein 70 (HSP70), and HSP90 in tissues were determined by Western blotting.

RESULTS

Peak plasma 17DMAG concentration was 15.4+/-1.4 microg/ml. The area under the plasma 17DMAG concentration versus time curve was 1072 microg/ml min, corresponding to a total body clearance of 70 ml/kg/min. Peak 17DMAG concentrations in liver (118.8+/-5.7 microg/g), kidney (122.9+/-10.6 microg/g), heart (81.3+/-8.1 microg/g), and lung (110.6+/-25.4 microg/g) occurred at 5-10 min, while peak concentrations in spleen (70.6+/-9.6 microg/g) and tumor (9.0+/-1.0 microg/g) occurred at 30-45 min. At 48 h, 17DMAG was detectable in tumor but not in any normal tissue. Raf-1 in tumors of 17DMAG-treated mice killed at 4, 7, 24 and 48 h was about 20% lower than in tumors from vehicle-treated mice. HSP90 and HSP70 in tumors of 17DMAG-treated animals were significantly lower than in tumors of control animals at 4, 7, and 24 h. Hepatic Raf-1 was decreased by more than 60% at all times after 17DMAG treatment; however, hepatic HSP90 was not affected. HSP70 was undetectable in livers of vehicle-treated mice or mice killed at 2 or 4 h after 17DMAG treatment, but was detected in livers at 7, 24 and 48 h. 17DMAG did not affect renal Raf-1. In contrast, renal HSP70 and HSP90 were decreased by more than 50% at 2 and 4 h after 17DMAG treatment. Renal HSP70 increased approximately twofold above that in kidneys from vehicle-treated control mice at 7 and 24 h, while HSP90 relative protein concentration was no different from that in controls.

CONCLUSIONS

Plasma pharmacokinetics of 17DMAG in tumor-bearing mice were similar to those previously reported in nontumor-bearing mice. 17DMAG was distributed widely to tissues but was retained for longer in tumors than normal tissues. Raf-1, HSP90, and HSP70 were altered to different degrees in tumors, livers, and kidneys of 17DMAG-treated animals. These data illustrate the complex nature of the biological responses to 17DMAG.

Prospective strategies to enforce selectively cell death in cancer cells

Although induction of apoptosis (cell death mediated by caspases) determines response to cancer therapy, this approach is limited by lack of selectivity in available apoptosis-inducing agents. Furthermore, most cancers, almost by definition, are resistant to apoptosis, growth arrest and cell senescence. Then, how can anticancer agents kill cancer cell without unacceptable toxicity to a patient? The potential therapeutic approaches range from selective inhibition of antiapoptotic pathways, antiangiogenic therapy, tissue-selective therapy (including immunotherapy) to exploitation of, for example, drug resistance, oncoprotein addiction, unrestricted cell cycles, hypermitogenic and hypoxic features of cancer cells. These overlapping and complementary approaches rely on rational drug combinations (at mechanism-based doses and sequences) aimed at matching targets. To ensure killing of cancer cells selectively, we may combine apoptosis- and senescence-inducing agents with inhibitors of apoptosis (to protect normal cells), inhibitors of signal transduction with cell cycle-dependent chemotherapy, antiangiogenic agents with hypoxia-inducible factor-1 inhibitors, tissue-selective therapy with differentiating agents and activators of death receptors with chemotherapy. In theory, consecutive use of these drug combinations may control cancer.

Growth factor receptors as targets for lung cancer therapy

Dysregulated signal transduction of growth factor receptors contributes to the process of malignant transformation by promoting cell proliferation, motility, and invasion through extracellular matrix as well as angiogenesis. Epidermal growth factor receptors (EGFR), and to a lesser extent HER2/neu, is overexpressed in the majority of nonsmall cell lung cancer (NSCLC) compared with normal tissue, making them ideal targets for the development of novel therapeutics for this disease. Multiple clinical trials have demonstrated that antireceptor strategies employing antagonistic monoclonal antibodies or low molecular weight tyrosine kinase inhibitors against EGFR are well tolerated and occasionally result in objective clinical responses in patients with advanced NSCLC. This report provides an overview of the molecular basis and the preclinical evidence supporting clinical development of anti-EGFR therapy as well as results of phase I-III clinical trials of these compounds in treating patients with solid tumors including NSCLC.

JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in cell cycle progression and leukemogenesis

The roles of the JAK/STAT, Raf/MEK/ERK and PI3K/Akt signal transduction pathways and the BCR-ABL oncoprotein in leukemogenesis and their importance in the regulation of cell cycle progression and apoptosis are discussed in this review. These pathways have evolved regulatory proteins, which serve to limit their proliferative and antiapoptotic effects. Small molecular weight cell membrane-permeable drugs that target these pathways have been developed for leukemia therapy. One such example is imatinib mesylate, which targets the BCR-ABL kinase as well as a few structurally related kinases. This drug has proven to be effective in the treatment of CML patients. However, leukemic cells have evolved mechanisms to become resistant to this drug. A means to combat drug resistance is to target other prominent signaling components involved in the pathway or to inhibit BCR-ABL by other mechanisms. Treatment of imatinib-resistant leukemia cells with drugs that target Ras (farnysyl transferase inhibitors) or with the protein destabilizer geldanamycin has proven to be a means to inhibit the growth of resistant cells. This review will tie together three important signal transduction pathways involved in the regulation of hematopoietic cell growth and indicate how their expression is dysregulated by the BCR-ABL oncoprotein.

Physiologically-based pharmacokinetics and molecular pharmacodynamics of 17-(allylamino)-17-demethoxygeldanamycin and its active metabolite in tumor-bearing mice

A whole-body physiologically-based model was developed to describe the pharmacokinetics of the ansamycin benzoquinone antibiotic 17-(allylamino)-17-demethoxygeldanamycin (17AAG) and its active metabolite 17-(amino-)-17-demethoxygeldanamycin (17AG) in blood, normal organs (lung, brain, heart, spleen, liver, kidney, skeletal muscle) and implanted human tumor xenograft in nude mice. The distribution of 17 AAG in all organs was described by diffusion-limited exchange models, while that of 17 AG was described by perfusion-limited models. The intrinsic clearances of 17AAG and 17AG in the liver were uniquely identified using local models and were estimated to be 4.93 ml/hr and 3.34 ml/hr. It was also estimated that the formation of 17AG in liver accounted for 40% of the 17AAG intrinsic clearance. The model for the distribution of both 17AAG and 17AG in the human breast cancer tumor xenograft included vascular, interstitial and intracellular compartments, which yielded the predicted cellular concentrations of 17AAG and 17AG two to three times higher than the corresponding whole tissue measurements at steady state. Estimates of the vascular-interstitial permeability surface-area product were similar for 17AAG and 17AG (0.23 ml/hr and 0.26 ml/hr). However, the interstitial to cellular transport rate of 17AG was three-fold greater than that of 17AAG, which resulted in the preferential uptake of 17AG over 17AAG in tumor. Indirect response models were developed to describe the combined action of 17AAG and 17AG on the onco-proteins Raf-1 and p185erbB2 in tumor. The half-life of endogenous protein turnover was estimated to be 22.6 hr for Raf-1 and 8.6 hr for p185erbB2, and both were comparable to corresponding values measured in vitro. A model for the molecular chaperon heat shock proteins HSP70 and HSP90 was developed based on the molecular mechanism of heat shock auto-regulation and the action of 17AAG and 17AG on these proteins. The model provided in vivo estimates of endogenous HSP70 and HSP90 turnover. In modeling pharmacokmetics and pharmacodynamics, Bayesian inference was employed to estimate the kinetic, physiological and molecular parameters when prior information was available.

Synergistic antileukemic interactions between 17-AAG and UCN-01 involve interruption of RAF/MEK- and AKT-related pathways

Interactions between the protein kinase C (PKC) and Chk1 inhibitor UCN-01 and the heat shock protein 90 (Hsp90) antagonist 17-AAG have been examined in human leukemia cells in relation to effects on signal transduction pathways and apoptosis. Simultaneous exposure (30 hours) of U937 monocytic leukemia cells to minimally toxic concentrations of 17-AAG (eg, 400 nM) and UCN-01 (eg, 75 nM) triggered a pronounced increase in mitochondrial injury (ie, loss of mitochondrial membrane potential [Deltapsim]; cytosolic release of cytochrome c), caspase activation, and apoptosis. Synergistic induction of apoptosis was also observed in other human leukemia cell types (eg, Jurkat, NB4). Coexposure of human leukemia cells to 17-AAG and the PKC inhibitor bisindolylmaleimide (GFX) did not result in enhanced lethality, arguing against the possibility that the PKC inhibitory actions of UCN-01 are responsible for synergistic interactions. The enhanced cytotoxicity of this combination was associated with diminished Akt activation and marked down-regulation of Raf-1, MEK1/2, and mitogen-activated protein kinase (MAPK). Coadministration of 17-AAG and UCN-01 did not modify expression of Hsp90, Hsp27, phospho-JNK, or phospho-p38 MAPK, but was associated with further p34cdc2 dephosphorylation and diminished expression of Bcl-2, Mcl-1, and XIAP. In addition, inducible expression of both a constitutively active MEK1/2 or myristolated Akt construct, which overcame inhibition of ERK and Akt activation, respectively, significantly attenuated 17-AAG/UCN-01-mediated lethality. Together, these findings indicate that the Hsp90 antagonist 17-AAG potentiates UCN-01 cytotoxicity in a variety of human leukemia cell types and suggest that interference with both the Akt and Raf-1/MEK/MAP kinase cytoprotective signaling pathways contribute to this phenomenon.

The Hsp90 chaperone complex as a novel target for cancer therapy

BACKGROUND

Heat shock protein 90 (Hsp90) is responsible for chaperoning proteins involved in cell signaling, proliferation and survival. 17-allylamino-17-demethoxygeldanamycin (17-AAG) is an anticancer agent currently in phase I trials in the USA and UK. It represents a class of drugs, the benzoquinone ansamycin antibiotics, capable of binding and disrupting the function of Hsp90, leading to the depletion of multiple oncogenic client proteins.

MATERIALS AND METHODS

Studies were identified through a PubMed search, review of bibliographies of relevant articles and review of abstracts from national meetings.

RESULTS

Preclinical studies have demonstrated that disruption of many client proteins chaperoned by Hsp90 is achievable and associated with significant growth inhibition, both in vitro and in tumor xenografts. Following an overview of the mechanism of action of ansamycin antibiotics and the pathways they disrupt, we review the current clinical status of 17-AAG, and discuss future directions for combinations of traditional antineoplastics with 17-AAG.

CONCLUSIONS

17-AAG represents a class of drugs capable of affecting multiple targets in the signal transduction pathway involved in tumor cell proliferation and survival. Early results from phase I studies indicate that 17-AAG administration results in an acceptable toxicity profile while achieving in vivo disruption of client proteins.

Medulloblastoma sensitivity to 17-allylamino-17-demethoxygeldanamycin requires MEK/ERKM

ERBB2 increases the sensitivity of breast cancer cells to the HSP90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG). This has been attributed to the disruption of ERBB3/ERBB2 heterodimers that maintain a crucial cell survival signal via phosphatidylinositol 3-kinase/AKT. ERBB2 confers a poor clinical outcome in medulloblastoma, the most common malignant pediatric brain tumor. Here, we show that medulloblastoma cell sensitivity to 17-AAG is directly related to ERBB2 expression level. Furthermore, overexpression of exogenous ERBB2 in these cells induces spontaneous homodimerization, further enhancing cell sensitivity to 17-AAG. In contrast to breast cancer cells, this increased sensitivity to 17-AAG does not result from cell dependence on AKT1 activity. Rather, we show that 17-AAG generates a dose- and time-dependent increase in MEK/ERK signaling that is required for the drug to inhibit the proliferation of medulloblastoma cells and that ERBB2 sensitizes medulloblastoma cells to 17-AAG by up-regulating basal MEK/ERK signaling. We further show that down-regulation of MEK1 activity markedly reduces the sensitivity of medulloblastoma, breast, and ovarian cancer cells to 17-AAG, whereas expression of a constitutively active MEK1 potentiates the activity of 17-AAG against these cells. Therefore, intact MEK/ERK signaling may be required for optimal 17AAG activity against a variety of tumor cell types. These data identify a new mechanism by which 17-AAG inhibits the proliferation of cancer cells. Defining the precise mode of action of these agents within specific tumor cell types will be crucial if this class of drugs is to be efficiently developed in the clinic.

Development of a purine-scaffold novel class of Hsp90 binders that inhibit the proliferation of cancer cells and induce the degradation of Her2 tyrosine kinase

The first published synthesis and characterization of a purine-scaffold library of hsp90 inhibitors is presented. The purine-scaffold represents a platform for the creation of easily synthesizable and derivatizable soluble molecules that are amenable for oral administration. The most active compound of the series (71) exhibits binding to hsp90 comparable to the natural product derivative 17AAG that is now in Phase I clinical trial as a cancer therapeutic. Induces the degradation of Her2 tyrosine kinase and arrests the MCF-7 breast cancer cell line at low micromolar concentrations (IC50=2 microM).

Relationship of Mcl-1 isoforms, ratio p21WAF1/cyclin A, and Jun kinase phosphorylation to apoptosis in human breast carcinomas

Full length Mcl-1 is an anti-apoptotic protein consisting of two closely migrating 42/40kDa species. We now investigated the relationship of these isoforms to the expression of cell cycle stimulatory (cyclin A) and inhibitory (p21WAF1) proteins and to the induction of apoptosis in wt p53 MCF-7 and mutant p53 SKBR3 human breast carcinomas. The latter cells exhibited lower 42kDa Mcl-1, higher expression of cyclin A relative to that of p21WAF1, and apoptosis in response to okadaic acid, a phosphatase 1/2A inhibitor. The proteasome inhibitor MG-115 selectively increased expression of the 40kDa Mcl-1 isoform and induced p21WAF1, but also promoted preferential apoptosis in SKBR3 cells. Neither okadaic acid nor MG-115 caused comparable effects in MCF-7 cells. However, vanadate or acetyl furanonaphthoquinone induced the 40kDa Mcl-1 and greater Jun kinase (JNK) phosphorylation without apoptosis-associated PARP fragmentation in MCF-7 cells. Our data suggest that the higher susceptibility of SKBR3 cells to undergo apoptosis may be partly due to their greater proliferative potential (cyclin A), low expression of the anti-apoptotic 42kDa Mcl-1 isoform, and suboptimal JNK activation in response to stress.

Use of radiolabelled choline as a pharmacodynamic marker for the signal transduction inhibitor geldanamycin

There is an urgent need to develop non-invasive pharmacodynamic endpoints for the evaluation of new molecular therapeutics that inhibit signal transduction. We hypothesised that, when labelled appropriately, changes in choline kinetics could be used to assess geldanamycin pharmacodynamics, which involves inhibition of the HSP90 molecular chaperone-->Raf1-->Mitogenic Extracellular Kinase-->Extracellular Signal-Regulated Kinase 1 and 2 signal transduction pathway. Towards identifying a potential pharmacodynamic marker response, we have studied radiolabelled choline metabolism in HT29 human colon carcinoma cells following treatment with geldanamycin. We studied the effects of geldanamycin, on net cellular accumulation of (methyl-(14)C)choline and (methyl-(14)C)phosphocholine production. In parallel experiments, the effects of geldanamycin on extracellular signal-regulated kinase 1 and 2 phosphorylation and cell viability were also assessed. Additional validation studies were carried out with the mitogenic extracellular kinase inhibitor U0126 as a positive control; a cyclin-dependent kinase-2 inhibitor roscovitine and the phosphatidylinositol 3-kinase inhibitor LY294002 as negative controls. Hemicholinium-3, an inhibitor of choline transport and choline kinase activity was included as an additional control. In exponentially growing HT29 cells, geldanamycin inhibited extracellular signal-regulated kinase 1 and 2 phosphorylation in a concentration- and time-dependent manner. These changes were associated with a reduction in (methyl-(14)C)choline uptake, (methyl-(14)C) phosphocholine production and cell viability. Brief exposure to U0126, suppressed phosphocholine production to the same extent as Hemicholinium-3. In contrast to geldanamycin and U0126, which act upstream of extracellular signal-regulated kinase 1 and 2, roscovitine and LY294002 failed to suppress phosphocholine production. Our results suggest that when labelled with carbon-11 isotope, (methyl-(11)C)choline may be a useful pharmacodynamic marker for the non-invasive evaluation of geldanamycin analogues.

Hsp-90-associated oncoproteins: multiple targets of geldanamycin and its analogs

Geldanamycin (GA), herbimycin A and radicicol bind heat-shock protein-90 (Hsp90) and destabilize its client proteins including v-Src, Bcr-Abl, Raf-1, ErbB2, some growth factor receptors and steroid receptors. Thus, Hsp90-active agents induce ubiquitination and proteasomal degradation of numerous oncoproteins. Depending on the cellular context, HSP90-active agents cause growth arrest, differentiation and apoptosis, or can prevent apoptosis. HSP-active agents are undergoing clinical trials. Like targets of most chemotherapeutics, Hsp90 is not a cancer-specific protein. By attacking a nonspecific target, HSP-90-active compounds still may preferentially kill certain tumor cells. How can this be achieved? How can therapeutic potentials be exploited? This article starts the discussion.

P53: an ubiquitous target of anticancer drugs

The p53 tumor suppressor can induce growth arrest, apoptosis and cell senescence. Not surprisingly, p53 is an appealing target for therapeutic intervention. Although current anticancer agents do not directly interact with p53, these agents (including DNA damaging drugs, antimetabolites, microtubule-active drugs and inhibitors of the proteasome) cause accumulation of wt p53. Depending on the p53 status of cancer cells, diverse therapeutic strategies are under development. These include pharmacological rescue of mutant p53 function and reactivation of wt p53 in E6-expressing cells. For protection of normal cells, strategies range from abrogation of wt p53 induction, thereby decreasing the toxicity of DNA damaging agents, to activation of wt p53-dependent checkpoints, thereby protecting cells against cell cycle-dependent therapeutics.