Combination of the histone deacetylase inhibitor LBH589 and the hsp90 inhibitor 17-AAG is highly active against human CML-BC cells and AML cells with activating mutation of FLT-3

Heat-shock protein 90 inhibitors in cancer therapy: 17AAG and beyond

Heat-shock protein 90 (HSP90) has diverse functions in mammalian cells. It acts as molecular chaperone, together with several co-chaperone molecules (such as Hop, Hip, p23, cdc37, Aha, and immunophilins). HSP90 binds to its client proteins (such as steroid receptors, AKT, Bcr-Abl, Apaf-1, survivin, cyclin dependent kinases which are involved in signal transduction that regulate cell cycle, survival, and death, and promote their proper protein folding, assembly, and transportation across different cellular compartments. Failure of Hsp90 chaperone activity leads to misfolding of client proteins, which leads to ubiquitination and proteasome degradation, and this deregulating cellular homeostasis. Since tumor cells frequently overexpress the active form of HSP90, which is more susceptible to inhibition by small molecules such as geldanamycin and its analogs, HSP90 became an attractive target for cancer therapy. This paper will review the recent advances in HSP90-biology and will discuss the emerging role of the HSP90 inhibitors such as 17-allylamino-17 demethoxy-geldanamycin and other HSP-90-directed small molecules in cancer therapy.

RNA interference targeting of Bcr-Abl increases chronic myeloid leukemia cell killing by 17-allylamino-17-demethoxygeldanamycin

17-Allylamino-17-demethoxygeldanamycin (17-AAG) induces degradation of Hsp90 client proteins, including Bcr-Abl, however, its clinical use as an anti-tumor agent may be limited by toxicity and modest efficacy. We reasoned that Bcr-Abl targeting by RNA interference (RNAi) might selectively increase the activity of 17-AAG against Bcr-Abl+ leukemia cells. 17-AAG in combination with targeting small interfering RNAs (siRNAs) reduced Bcr-Abl protein levels, triggered increases in markers of apoptosis and decreased cell viability more effectively than did control siRNA and 17-AAG together, or Bcr-Abl targeting siRNA alone. Combination targeting strategies such as this may therefore achieve enhanced therapeutic potency.

Combined effects of novel tyrosine kinase inhibitor AMN107 and histone deacetylase inhibitor LBH589 against Bcr-Abl-expressing human leukemia cells

AMN107 (Novartis Pharmaceuticals, Basel, Switzerland) has potent in vitro and in vivo activity against the unmutated and most common mutant forms of Bcr-Abl. Treatment with the histone deacetylase inhibitor LBH589 (Novartis) depletes Bcr-Abl levels. We determined the effects of AMN107 and/or LBH589 in Bcr-Abl-expressing human K562 and LAMA-84 cells, as well as in primary chronic myelogenous leukemia (CML) cells. AMN107 was more potent than imatinib mesylate (IM) in inhibiting Bcr-Abl tyrosine kinase (TK) activity and attenuating p-STAT5, p-AKT, Bcl-x(L), and c-Myc levels in K562 and LAMA-84 cells. Cotreatment with LBH589 and AMN107 exerted synergistic apoptotic effects with more attenuation of p-STAT5, p-ERK1/2, c-Myc, and Bcl-x(L) and increases in p27 and Bim levels. LBH589 attenuated Bcr-Abl levels and induced apoptosis of mouse pro-B BaF3 cells containing ectopic expression of Bcr-Abl or the IM-resistant, point-mutant Bcr-AblT315I and Bcr-AblE255K. Treatment with LBH589 also depleted Bcr-Abl levels and induced apoptosis of IM-resistant primary human CML cells, including those with expression of Bcr-AblT315I. As compared with either agent alone, cotreatment with AMN107 and LBH589 induced more loss of cell viability of primary IM-resistant CML cells. Thus, cotreatment with LBH589 and AMN107 is active against cultured or primary IM-resistant CML cells, including those with expression of Bcr-AblT315I.

P210 Bcr-abl tyrosine kinase interaction with histone deacetylase 1 modifies histone H4 acetylation and chromatin structure of chronic myeloid leukaemia haematopoietic progenitors

The BCR-ABL fusion gene, originating from the balanced (9;22) translocation, is the molecular hallmark and the causative event of chronic myeloid leukaemia (CML). The interactions of its p210 protein constitutively activated and improperly confined to the cytoplasm with multiple regulatory signals of cell cycle progression, apoptosis and self-renewal, induce the illegitimate enlargement of clonal haematopoiesis and genetic instability that drives its progression towards the fully transformed phenotype of blast crisis. However, its effects on the basic transcription machinery and chromatin remodelling are unknown. Our study underscored histone H4 hyperacetylation associated with p210 tyrosine kinase in vitro and in vivo and its role in BCR-ABL transcription. Histone H4 hyperacetylation proceeds, at least partly, from the 'loss of function' of histone deacetylase 1 protein, a critical component of Rb-mediated transcriptional repression, in consequence of its cytoplasmatic compartmentalisation.

Targeting tumor angiogenesis with histone deacetylase inhibitors: the hydroxamic acid derivative LBH589

PURPOSE

Angiogenesis is required for tumor progression and represents a rational target for therapeutic intervention. Histone deacetylase (HDAC) inhibitors have been shown to have activity against various tumor cell types by inhibiting proliferation and inducing apoptosis both in vitro and in vivo. HDAC inhibitors have also been reported to inhibit angiogenesis. The goal of this study was to characterize the antiangiogenic and antitumor activity of a recently developed HDAC inhibitor, the hydroxamic derivative LBH589.

MATERIALS AND METHODS

To evaluate the antiangiogenesis activity of LBH589, we did cell cycle analysis, cell proliferation, tube formation, invasion assays in vitro, and Matrigel plug assay in vivo. To determine the antitumor activity of LBH589, we established human prostate carcinoma cell PC-3 xenografts in vivo. To evaluate the effect of LBH589 on endothelial signaling pathways, gene expression, and protein acetylation, we did Western blots and reverse transcription-PCR in human umbilical vein endothelial cells (HUVEC). Immunohistochemical analysis was done to evaluate new blood vessel formation in vivo.

RESULTS

LBH589 induced acetylation of histone H3 and alpha-tubulin protein in HUVECs. Histone and nonhistone protein acetylation correlated with induction of G(2)-M cell cycle arrest, inhibition of HUVEC proliferation, and viability. Noncytotoxic concentrations of LBH589 inhibited endothelial tube formation, Matrigel invasion, AKT, extracellular signal-regulated kinase 1/2 phosphorylation, and chemokine receptor CXCR4 expression. In vivo dosing of mice with LBH589 (10 mg/kg/d) reduced angiogenesis and PC-3 tumor growth.

CONCLUSION

This study provides evidence that LBH589 induces a wide range of effects on endothelial cells that lead to inhibition of tumor angiogenesis. These results support the role of HDAC inhibitors as a therapeutic strategy to target both the tumor and endothelial compartment and warrant the clinical development of these agents in combination with angiogenesis inhibitors.

Targeting chaperones in transformed systems – a focus on Hsp90 and cancer

The molecular chaperone Hsp90 is a protein with important roles in maintaining the functional stability and viability of cells under a transforming pressure. Cancer cells harbour mutated oncogenic proteins or proteins with dysregulated function and the chaperone is required to maintain their folded and functionally active conformation. In addition, by chaperoning key proteins such as Raf-1, Akt, survivin and hTERT, Hsp90 regulates signalling pathways necessary for the growth, survival and limitless replicative potential of most tumours. Important elements of the apoptotic pathways are also regulated by Hsp90. Overall, these characteristics propose Hsp90 as an important target of whose inhibition may aim at a wide-range of oncogenic transformations. Several years into Hsp90 research have shed light into the feasibility, but also the limitations, of such an approach. In this review, the authors present the current understanding on the relevance and possibility of translating Hsp90 inhibitors into therapeutic agents in cancer therapy.

17-Allylamino-17-Demethoxygeldanamycin Synergistically Potentiates Tumor Necrosis Factor–Induced Lung Cancer Cell Death by Blocking the Nuclear Factor-κB Pathway

Nuclear factor-κB (NF-κB), a survival signal induced by tumor necrosis factor (TNF), contributes substantially to the resistance to TNF-induced cell death. Previous studies suggest that heat shock protein 90 (Hsp90) regulates the stability and function of receptor-interaction proteins (RIP) and IκB kinase β (IKKβ), the key components of the TNF-induced NF-κB activation pathway. In this study, we showed that the Hsp90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17AAG) was synergistic with TNF to induce apoptotic cell death in a panel of lung tumor-derived cell lines. Treatment with 17AAG caused degradation of RIP and IKKβ that, in turn, blocked TNF-induced NF-κB activation and antiapoptotic gene expression. The synergistic cytotoxicity was detected only when TNF treatment followed 17AAG preexposure. Importantly, the potentiation of cell death was abolished in NF-κB-disabled cells that express a nondegradable IκBα mutant (IκBαAA). These results suggest that the cytotoxicity seen with 17AAG and TNF treatment results from blocking TNF-induced NF-κB activation. The other components of the TNF receptor I signaling cascade were not altered, whereas TNF-induced c-Jun NH2-terminal kinase activation and apoptosis were potentiated. A similar synergism for inducing apoptosis was also observed in 17AAG-treated and TNF-related apoptosis-inducing ligand (TRAIL)–treated cancer cells. Our results suggest that NF-κB plays a key role in the resistance of lung cancer cells to TNF and TRAIL and that disabling this survival signal with 17AAG followed by TNF or TRAIL treatment could be an effective new therapeutic strategy for lung cancer. (Cancer Res 2006; 66(2): 1089-95)

Inhibitors of signal transduction protein kinases as targets for cancer therapy

Cancer development requires that tumour cells attain several capabilities, including increased replicative potentials, anchorage and growth-factor independency, evasion of apoptosis, angiogenesis and metastasis. Many of these processes involve the actions of protein kinases, which have emerged as key regulators of all aspects of neoplasia. Perturbed protein kinase activity is repeatedly found to be associated with human malignancies, making these proteins attractive targets for anti-cancer therapy. The last decade has witnessed an exponential increase in the development of specific small protein kinase inhibitors. Many of them are in clinical trials in patients with different types of cancer and some are successfully used in clinic. This review describes different approaches that are currently applied to develop such specific protein kinase inhibitors and provides an overview of protein kinase inhibitors that are currently in clinical trials or are administered in the clinic. Focus is directed on inhibitors against receptor tyrosine kinases and protein kinases participating in the signalling cascades.

Flavopiridol and histone deacetylase inhibitors promote mitochondrial injury and cell death in human leukemia cells that overexpress Bcl-2

Interactions between the cyclin-dependent kinase (CDK) inhibitor flavopiridol and histone deacetylase (HDAC) inhibitors (suberoylanilide hydroxamide and sodium butyrate) were examined in human leukemia cells (U937 and HL-60) ectopically expressing Bcl-2/Bcl-x(L) and in primary AML cells. Coadministration of flavopiridol with HDAC inhibitors synergistically potentiated mitochondrial damage (cytochrome c, second mitochondria-derived activator of caspases/direct IAP binding protein with low pI, and apoptosis-inducing factor release), caspase activation, poly(ADP-ribose) polymerase degradation, and cell death in both wild type and Bcl-2- or Bcl-x(L)-overexpressing cells and induced a pronounced loss of clonogenicity. In contrast, Bcl-2 and Bcl-x(L) largely blocked these events in cells exposed to the cytotoxic agent 1-beta-d-arabinofuranosylcytosine (ara-C). Enforced expression of dominant-negative Fas-associated death domain failed to protect cells from the flavopiridol/histone deacetylase inhibitor (HDACI) regimen, arguing against the involvement of the receptor pathway in lethality. Ectopic expression of a phosphorylation loop-deleted Bcl-2 or Bcl-2 lacking the serine(70) phosphorylation site, which dramatically protected cells from ara-C lethality, delayed but did not prevent flavopiridol/HDAC inhibitor-induced mitochondrial injury, cell death, or loss of clonogenicity. Ectopic expression of Bcl-2 or Bcl-x(L) was also unable to prevent the flavopiridol/HDACI regimen from inducing a conformational change in and mitochondrial translocation of Bax, and it did not attenuate Bax dimerization. As a whole, these findings indicate that in contrast to certain conventional cytotoxic agents such as ara-C, overexpression of Bcl-2 or Bcl-x(L) are largely ineffective in preventing perturbations in Bax, mitochondrial injury, and cell death in human leukemia cells subjected to simultaneous CDK and HDAC inhibition. They also raise the possibility that a strategy combining CDK and HDAC inhibitors may be effective against drug-resistant leukemia cells overexpressing Bcl-2 or Bcl-x(L).

Hsp90 inhibitors in the clinic

Specific inhibitors of Hsp90 have recently entered human clinical trials. At the time of writing, trials have been initiated only in metastatic cancer, although a rationale exists for using these agents in a variety of human diseases where protein (mis)folding is involved in the disease pathophysiology. Hsp90 inhibitors offer a unique anti-cancer opportunity because they provide simultaneous combinatorial blockade of multiple oncogenic pathways. The first compound in this class, 17-AAG, has completed phase I trials and phase II trials are in progress. The toxicity has been manageable and evidence of possible clinical activity has been seen in metastatic melanoma, prostate cancer and multiple myeloma. Other inhibitors with improved properties are approaching clinical trials. This chapter presents an update of the current clinical trials using Hsp90 inhibitors, focussing on the areas that will be increasingly relevant in the next 5 years.

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.

Activity of suberoylanilide hydroxamic Acid against human breast cancer cells with amplification of her-2

PURPOSE

We determined the effects of suberoylanilide hydroxamic acid (SAHA), a histone deacetylase inhibitor, on hsp90 and its client proteins Her-2, AKT, and c-Raf, as well as evaluated the cytotoxic effects of co-treatment of SAHA with trastuzumab or docetaxel in human breast cancer BT-474 and SKBR-3 cells containing amplification of Her-2.

EXPERIMENTAL DESIGN

The cells were treated with SAHA (1.0-5.0 micromol/L) and/or trastuzumab (5-40 microg/mL) or docetaxel (5-20 nmol/L). Following this, apoptosis and the levels of p21(WAF1), p27(KIP1), AKT, c-Raf, and Her-2, as well as of the key regulators of apoptosis were determined. Synergistic interaction between drugs was evaluated by median dose-effect analysis.

RESULTS

Treatment with SAHA up-regulated p21(WAF1) and p27(KIP1) levels, increased the percentage of cells in G2-M phase of the cell cycle, as well as induced apoptosis in a dose-dependent manner. This was associated with up-regulation of the pro-death Bak and Bim, as well as with attenuation of the levels of Her-2 and XIAP, survivin, Bcl-2, and Bcl-x(L) proteins. SAHA treatment induced acetylation of hsp90. This reduced the chaperone association of Her-2 with hsp90, promoting polyubiquitylation and degradation of Her-2. SAHA also attenuated the levels of c-Raf and AKT. Co-treatment with SAHA significantly increased trastuzumab or docetaxel-induced apoptosis of BT-474 and SKBR-3 cells. Additionally, median dose-effect analysis revealed that co-treatment with SAHA and trastuzumab or docetaxel induced synergistic cytotoxic effects against the breast cancer cells.

CONCLUSIONS

These preclinical findings support the development of SAHA in combination with docetaxel and/or trastuzumab against Her-2-amplified breast cancer.

Histone acetylation-independent effect of histone deacetylase inhibitors on Akt through the reshuffling of protein phosphatase 1 complexes

Despite advances in understanding the role of histone deacetylases (HDACs) in tumorigenesis, the mechanism by which HDAC inhibitors mediate antineoplastic effects remains elusive. Modifications of the histone code alone are not sufficient to account for the antitumor effect of HDAC inhibitors. The present study demonstrates a novel histone acetylation-independent mechanism by which HDAC inhibitors cause Akt dephosphorylation in U87MG glioblastoma and PC-3 prostate cancer cells by disrupting HDAC-protein phosphatase 1 (PP1) complexes. Of four HDAC inhibitors examined, trichostatin A (TSA) and HDAC42 exhibit the highest activity in down-regulating phospho-Akt, followed by suberoylanilide hydroxamic acid, whereas MS-275 shows only a marginal effect at 5 microm. This differential potency parallels the respective activities in inducing tubulin acetylation, a non-histone substrate for HDAC6. Evidence indicates that this Akt dephosphorylation is not mediated through deactivation of upstream kinases or activation of downstream phosphatases. However, the effect of TSA on phospho-Akt can be rescued by PP1 inhibition but not that of protein phosphatase 2A. Immunochemical analyses reveal that TSA blocks specific interactions of PP1 with HDACs 1 and 6, resulting in increased PP1-Akt association. Moreover, we used isozyme-specific small interfering RNAs to confirm the role of HDACs 1 and 6 as key mediators in facilitating Akt dephosphorylation. The selective action of HDAC inhibitors on HDAC-PP1 complexes represents the first example of modulating specific PP1 interactions by small molecule agents. From a clinical perspective, identification of this PP1-facilitated dephosphorylation mechanism underscores the potential use of HDAC inhibitors in lowering the apoptosis threshold for other therapeutic agents through Akt down-regulation.

Synergistic interactions between MEK1/2 and histone deacetylase inhibitors in BCR/ABL+ human leukemia cells

Interactions between the histone deacetylase inhibitor SAHA and the pharmacologic MEK1/2 inhibitor PD184352 were examined in Bcr/Abl+ human leukemia cells. Coadministration of minimally toxic concentrations of SAHA (or sodium butyrate) and PD184352 (or U0126) resulted in a synergistic increase in mitochondrial damage, caspase activation, and apoptosis in K562 and LAMA 84 cells. Similar interactions were observed in CD34+ cells from two patients with CML and in imatinib mesylate-resistant K562 cells but not in normal human CD34+ bone marrow cells. These events were associated with a marked increase in ROS generation, inactivation of ERK and Akt, downregulation of p21CIP1, Bcr/Abl, and cyclin D1, and activation of JNK. Of these events, ROS generation, ERK inactivation, and cytochrome c/AIF release were largely caspase-independent, whereas the other phenomena displayed varying degrees of caspase-dependence. Using pharmacologic and genetic approaches, generation of ROS, p21CIP1 downregulation, and inactivation of Akt and MEK were found to play significant functional roles in SAHA/PD184352-mediated lethality, whereas JNK activation and Raf-1 downregulation were determined to represent secondary events. These findings indicate that interruption of the MEK/ERK pathway substantially lowers the threshold for HDAC inhibitor-mediated oxidative injury, mitochondrial dysfunction, and apoptosis, suggesting that this approach warrants further examination in Bcr/Abl+-related malignancies.

Synergistic antileukemic interactions between 2-medroxyestradiol (2-ME) and histone deacetylase inhibitors involve Akt down-regulation and oxidative stress

Interactions between the endogenous estradiol metabolite 2-medroxyestradiol (2-ME) and histone deacetylase inhibitors (HDACIs) have been investigated in human leukemia cells. Coadministration of subtoxic or marginally toxic concentrations of 2-ME and SAHA or sodium butyrate in diverse human leukemia-cell types resulted in a marked increase in oxidative damage (eg, generation of reactive oxygen species [ROSs]), mitochondrial injury (eg, cytochrome c release and Bax translocation), caspase activation, and apoptosis. These interactions were also noted in primary human leukemia cells but not in normal bone marrow CD34+ cells. Synergistic interactions between these agents were associated with inactivation of Akt and activation of c-Jun N-terminal kinase (JNK). Essentially all of these events were reversed by free radical scavengers such as the manganese superoxide dismutase (MnSOD) mimetic TBAP and catalase. Notably, treatment with 2-ME/HDACIs resulted in down-regulation of thioredoxin, MnSOD, and glutathione peroxidase. Enforced activation of Akt blocked 2-ME/HDACI-mediated mitochondrial injury, caspase activation, and JNK up-regulation, but not generation of ROSs. Pharmacologic or genetic (siRNA) interruption of the JNK pathway also significantly attenuated the lethality of this regimen. Together, these findings support a model in which antileukemic synergism between 2-ME and HDACIs stems primarily from induction of oxidative damage, leading in turn to Akt inactivation and JNK activation, culminating in mitochondrial injury and apoptosis. They also raise the possibility that these events may preferentially occur in leukemic versus normal hematopoietic cells.

Signal Transduction of Oncogenic Flt3

Activating mutations of Fms-like tyrosine kinase 3 (Flt3) are the most common genetic lesions in acute myeloid leukemia (AML) and are present in approximately one third of AML patients. The 2 classes of Flt3 mutations are internal tandem duplications in the juxtamembrane domain and point mutations in the tyrosine kinase domain. In normal hematopoietic progenitor cells, Flt3 ligand induces the activation of several downstream signal-transduction mediators, including phosphoinositol 3-kinases, Src kinases, mitogen-activated protein kinases, and the phosphorylation of several adaptor proteins. Oncogenic mutations in Flt3 result in ligand-independent constitutive and deregulated activation of these signaling pathways. In addition, however, oncogenic mutations of Flt3 also result in the activation of aberrant signaling pathways, including strong activation of STAT5, induction of STAT target genes, and repression of myeloid transcription factors c/EBP-3 and Pu.1. Aberrant activation of these signaling pathways by oncogenic Flt3 may play a critical role in mutant Flt3-mediated leukemic transformation.

Heat-shock protein 90 inhibitors in antineoplastic therapy: is it all wrapped up?

Heat-shock protein (Hsp)-90 belongs to the class of molecular chaperone proteins that are capable of sensing cellular stress. Although Hsp90 is essential for viability, the pharmacological inhibition of this chaperone has emerged as an attractive means to inhibit tumorigenesis. This phenomenon is due to a unique property of Hsp90; its 'client proteins' are universally involved in signal transduction pathways commonly dysregulated in, and contributing to, cancer. The natural product geldanamycin, a potent ansamycin Hsp90 inhibitor, has served as a lead compound for the development of several derivatives that are currently undergoing clinical trials. Inhibition of Hsp90 with geldanamycin simultaneously depletes Hsp90-associated clients and impairs numerous signalling cascades that depend on chaperone function. Importantly, tumour cells are exquisitely sensitive to Hsp90 inhibition, lending credence to the feasibility of selectively targeting cancer tissue via the pharmacological modulation of Hsp90 function. Even more remarkably, Hsp90 inhibitors sensitise tumour cells to the cytotoxic effects of a variety of standard therapeutics, and thus, they are likely to have broad utility in combination therapy. Although these are promising developments, much remains to be discovered about client-chaperone biology and the tumour-specific effects of Hsp90 blockade. This information is required to fully grasp the multi-faceted roles of Hsp90 in cancer biology towards the goal of optimising the use of these agents in the clinic. Elucidation of these nuances will undoubtedly lead to better targeting of relevant oncogenic pathways and translate into the development of more effective anticancer regimens.

Phase I and Pharmacokinetic Study of MS-275, a Histone Deacetylase Inhibitor, in Patients With Advanced and Refractory Solid Tumors or Lymphoma

Purpose

The objective of this study was to define the maximum-tolerated dose (MTD), the recommended phase II dose, the dose-limiting toxicity, and determine the pharmacokinetic (PK) and pharmacodynamic profiles of MS-275.

Patients and Methods

Patients with advanced solid tumors or lymphoma were treated with MS-275 orally initially on a once daily × 28 every 6 weeks (daily) and later on once every-14-days (q14-day) schedules. The starting dose was 2 mg/m2and the dose was escalated in three- to six-patient cohorts based on toxicity assessments.

Results

With the daily schedule, the MTD was exceeded at the first dose level. Preliminary PK analysis suggested the half-life of MS-275 in humans was 39 to 80 hours, substantially longer than predicted by preclinical studies. With the q14-day schedule, 28 patients were treated. The MTD was 10 mg/m2and dose-limiting toxicities were nausea, vomiting, anorexia, and fatigue. Exposure to MS-275 was dose dependent, suggesting linear PK. Increased histone H3 acetylation in peripheral-blood mononuclear-cells was apparent at all dose levels by immunofluorescence analysis. Ten of 29 patients remained on treatment for ≥ 3 months.

Conclusion

The MS-275 oral formulation on the daily schedule was intolerable at a dose and schedule explored. The q14-day schedule is reasonably well tolerated. Histone deacetylase inhibition was observed in peripheral-blood mononuclear-cells. Based on PK data from the q14-day schedule, a more frequent dosing schedule, weekly × 4, repeated every 6 weeks is presently being evaluated.

Inhibition of histone deacetylase 6 acetylates and disrupts the chaperone function of heat shock protein 90: a novel basis for antileukemia activity of histone deacetylase inhibitors

The hydroxamic acid (HAA) analogue pan-histone deacetylase (HDAC) inhibitors (HDIs) LAQ824 and LBH589 have been shown to induce acetylation and inhibit the ATP binding and chaperone function of heat shock protein (HSP) 90. This promotes the polyubiquitylation and degradation of the pro-growth and pro-survival client proteins Bcr-Abl, mutant FLT-3, c-Raf, and AKT in human leukemia cells. HDAC6 is a member of the class IIB HDACs. It is predominantly cytosolic, microtubule-associated alpha-tubulin deacetylase that is also known to promote aggresome inclusion of the misfolded polyubiquitylated proteins. Here we demonstrate that in the Bcr-abl oncogene expressing human leukemia K562 cells, HDAC6 can be co-immunoprecipitated with HSP90, and the knock-down of HDAC6 by its siRNA induced the acetylation of HSP90 and alpha-tubulin. Depletion of HDAC6 levels also inhibited the binding of HSP90 to ATP, reduced the chaperone association of HSP90 with its client proteins, e.g. Bcr-Abl, and induced polyubiquitylation and partial depletion of Bcr-Abl. Conversely, the ectopic overexpression of HDAC6 inhibited LAQ824-induced acetylation of HSP90 and alpha-tubulin and reduced LAQ824-mediated depletion of Bcr-Abl, AKT, and c-Raf. Collectively, these findings indicate that HDAC6 is also an HSP90 deacetylase. Targeted inhibition of HDAC6 leads to acetylation of HSP90 and disruption of its chaperone function, resulting in polyubiquitylation and depletion of pro-growth and pro-survival HSP90 client proteins including Bcr-Abl. Depletion of HDAC6 sensitized human leukemia cells to HAA-HDIs and proteasome inhibitors.

Epigenetic and chromatin modifiers as targeted therapy of hematologic malignancies

Epigenetic regulation of gene expression is mediated through alterations in the DNA methylation status, covalent modifications of core nucleosomal histones, rearrangement of histones, and by RNA interference. It is now abundantly clear that deregulation of epigenetic mechanisms cooperates with genetic alterations in the development and progression of cancer and leukemia. Epigenetic deregulation affects several aspects of tumor cell biology, including cell growth, cell cycle control, differentiation, DNA repair, and cell death. This raises the strong possibility that reversing deregulated epigenetic mechanisms may be an effective treatment strategy for leukemia and cancer. This treatment strategy may either be designed to separately or collectively target the specific perturbations in the epigenetic mechanisms found in human hematologic malignancies. The following review describes our current understanding of the important deregulated epigenetic mechanisms and the preclinical and clinical development of epigenetic and chromatin modifiers in the therapy of these disorders.