The Current State of NAD+‐Dependent Histone Deacetylases (Sirtuins) as Novel Therapeutic Targets

Sirtuins are NAD⁺‐dependent protein deacylases that cleave off acetyl, as well as other acyl groups, from the ε‐amino group of lysines in histones and other substrate proteins. Seven sirtuin isotypes (Sirt1–7) have been identified in mammalian cells. As sirtuins are involved in the regulation of various physiological processes such as cell survival, cell cycle progression, apoptosis, DNA repair, cell metabolism, and caloric restriction, a dysregulation of their enzymatic activity has been associated with the pathogenesis of neoplastic, metabolic, infectious, and neurodegenerative diseases. Thus, sirtuins are promising targets for pharmaceutical intervention. Growing interest in a modulation of sirtuin activity has prompted the discovery of several small molecules, able to inhibit or activate certain sirtuin isotypes. Herein, we give an update to our previous review on the topic in this journal (Schemies, 2010), focusing on recent developments in sirtuin biology, sirtuin modulators, and their potential as novel therapeutic agents.     

Synthesis and biological evaluation of novel quinolone derivatives dual targeting histone deacetylase and tubulin polymerization as antiproliferative agents

A strategy to develop chemotherapy agents by combining two complimentary chemo-active groups into a single molecule may have higher efficacy and fewer side effects than that of single-target drugs. In this article, we describe the synthesis and evaluation of a series of novel dual-acting levofloxacin–HDACi conjugates to target both histone deacetylase (HDAC) and tubulin polymerization. These bifunctional conjugates exhibited potent inhibitory activities against HDACs and tubulin polymerization. In docking analysis provides a structural basis for HDACs inhibition activities. Moreover, these conjugates showed selective anticancer activity that is more potent against MCF-7 compared to other four cancer cells A549, HepG2, PC-3, HeLa, but they had no toxicity toward normal cells.

Synthesis of a series of novel dual-acting levofloxacin–HDACi conjugates, which show potent inhibitory activities against HDACs, tubulin polymerization, and significant antiproliferative effect on MCF-7 cells.

Cancer is a highly complex multifactorial disease involving multiple cross-talking between signaling networks. Almost all single-target-based drugs suffer from severe toxicities or other undesirable side effects. In contrast, combination therapy, which combines multiple anticancer agents working with different mechanisms, might have superior efficacy and fewer side effects compared to single-target treatments.^1,2Histone deacetylases (HDACs) are epigenetic enzymes that are capable of removing acetyl groups from ε-amino groups of lysine residues in histone or other nonhistone proteins.³ Abnormal expression of HDACs has been observed in various types of cancer,^4–6 and these enzymes have emerged as important targets in the development of anticancer drugs. Consequently, inhibition of HDAC activity is now recognized as a powerful strategy for cancer therapy. There are 18 human HDAC isoforms categorized into four major classes: class I (HDACs 1, 2, 3, 8), class IIa (HDACS 4, 5, 7, 9), class IIb (HDACs 6 and 10), and class IV (HDAC 11) are Zn²⁺-dependent metalloenzymes, while class III (SirTs 1–7) are NDA⁺-dependent sirtuins.^7,8 Of these HDAC isoforms, only HDACs 1, 2, 3, and 6 have shown biologically relevant deacetylation ability.⁹ Specially, selective inhibition of HDAC6 may have fewer side effects than pan-HDAC and class I isoform.^10–12To date, more than twenty HDAC inhibitors have been initiated in clinical trials, and four HDAC inhibitors vorinostat (SAHA),¹³ romidepsin (FK-228),¹⁴ belinostat (PXD-101),^15,16 panobinostat (LBH-589),¹⁷ have been approved by FDA for the treatment of T-cell lymphoma, cutaneous T-cell lymphoma and multiple myeloma. However, most of them are pan-HDAC (SAHA, LBH-589) or class I selective (FK-228, PXD-101) inhibitors, which usually lead to several mild to severe side effects.^16,18,19 In addition, most of HDAC inhibitors lack visible efficacy against solid tumor,^14,20 the doses given in clinical are much higher, which severely limit their clinical utility for the treatment of broad spectrum of cancer. Therefore, preclinical evaluation of new HDAC inhibitors will need to focus on improving HDAC isoform selectivity and enhancing potency against solid tumors. One strategy may be able to ameliorate the shortcomings of current inhibitors, which is to develop a dual-acting HDAC inhibitor (HDACi) by incorporation of the surface recognition group of prototypical HDACi into other anticancer drugs, forming a single molecule that can modulate intracellular multiple targets, other than various HDAC isoforms. So far, a few examples of bifunctional HDACi-derived conjugates have been obtained.^21–26 Expansion of the diversity of such bifunctional conjugates could lead to broad acting, therapeutically viable anticancer drugs.In another aspect, fluoroquinolones (FQs) have recently been proven as an excellent class of broad-spectrum anticancer drugs against a variety of cancer cells such as bladder cancer,²⁷ non-small cell lung carcinoma,²⁸ colorectal carcinoma cells,²⁹etc. For instance, it has been demonstrated that levofloxacin (Lv) displays antiproliferative activity against various cancer cells.^30,31 Additionally, many of fluoroquinolones were potent inhibitors of tubulin polymerization and exhibited selective activity against some tumor cell types.^32,33 More importantly, fluoroquinolones have favorable pharmacokinetic profiles and good adsorption, which possess an established record of safety.³⁴ Therefore, on the basis of therapeutic effectives of aforementioned HDACi as well as fluoroquinolone, we conceived that concurrent inhibition of HDAC and tubulin polymerization would be a viable alternative approach for cancer treatment.In this work, we describe the design, synthesis and biological evaluation of novel dual-action levofloxacin–HDACi conjugates, which can be prepared conveniently by direct connection of levofloxacin with a triazole-liked SAHA (Fig. 1). The levofloxacin–SAHA conjugates (compounds 8a–c and 9a–c) of this design not only have HDACi unit but also have a second pharmacologically quinolone scaffold. Thus, they expand the exploration of bifunctional HDACi-derived conjugates. For comparison, the carboxylic acid analogues (by replacing the hydroxamic acid (–CONHOH) group with (–COOH), compounds 6a–c and 7a–c) were also prepared and evaluated for their HDAC and tubulin polymerization inhibition activity, antiproliferative activity and cell-type selectivity, etc.Open in a separate windowFig. 1Design of dual-acting levofloxacin–HDACi conjugates.     

Histone deacetylase inhibitors selectively suppress expression of HDAC7

There are 18 histone deacetylases (HDAC) generally divided into four classes based on homology to yeast HDACs. HDACs have many protein substrates in addition to histones that are involved in regulation of gene expression, cell proliferation, and cell death. Inhibition of HDACs can cause accumulation of acetylated forms of these proteins, thus altering their function. HDAC inhibitors (HDACi), such as the hydroxamic acid-based vorinostat (suberoylanilide hydroxamic acid), inhibit the zinc-containing classes I, II, and IV, but not the NAD(+)-dependent class III, enzymes. HDACis are a group of novel anticancer agents. Vorinostat is the first HDACi approved for clinical use in the treatment of the cancer cutaneous T-cell lymphoma. Factors affecting expression of HDACs are not well understood. This study focuses on the effect of the HDACi vorinostat on the expression of class I and class II HDACs. We found that vorinostat selectively down-regulates HDAC7 with little or no effect on the expression of other class I or class II HDACs. Fourteen cell lines were examined, including normal, immortalized, genetically transformed, and human cancer-derived cell lines. Down-regulation of HDAC7 by vorinostat is more pronounced in transformed cells sensitive to inhibitor-induced cell death than in normal cells or cancer cells resistant to induced cell death. Modulation of HDAC7 levels by small interfering RNA-mediated knockdown or by HDAC7 overexpression is associated with growth arrest but without detectable changes in acetylation of histones or p21 gene expression. Selective down-regulation of HDAC7 protein may serve as a marker of response of tumors to HDACi.     

Cellular and molecular effects of sirtuins in health and disease

Sirtuins are NAD+-dependent protein deacetylases that are broadly conserved from bacteria to humans. Because sirtuins extend the lifespan of yeast, worms and flies, much attention has been paid to their mammalian homologues. Recent studies have revealed diverse physiological functions of sirtuins that are essentially similar to those of their yeast homologue, Sir2 (silent information regulator 2). Sirtuins are implicated in the pathology of many diseases, for which sirtuin activators such as resveratrol have great promise as potential treatments. In the present review, we describe the functions of sirtuins in cell survival, inflammation, energy metabolism, cancer and differentiation, and their impact on diseases. We also discuss the organ-specific functions of sirtuins, focusing on the brain and blood vessels.     

Small molecule antagonists for chemokine CCR3 receptors

The chemokine receptor CCR3 is believed to play a role in the development of allergic diseases such as asthma, atopic dermatitis, and allergic rhinitis. Despite the conflicting results that have been reported regarding the importance of eosinophils and CCR3 in allergic inflammation, inhibition of this receptor with small molecule antagonists is thought to provide a valuable approach for the treatment of these diseases. This review describes the structure–activity relationships (SAR) of small molecule CCR3 antagonists as reported in the scientific and patent literature. Various chemical classes of small molecule CCR3 antagonists have been described so far, including (bi)piperidine and piperazine derivatives, N‐arylalkylpiperidine urea derivatives and (N‐ureidoalkyl)benzylpiperidines, phenylalanine derivatives, morpholinyl derivatives, pyrrolidinohydroquinazolines, arylsulfonamides, amino‐alkyl amides, imidazole‐ and pyrimidine‐based antagonists, and bicyclic diamines. The (N‐ureidoalkyl)benzylpiperidines are the best studied class in view of their generally high affinity and antagonizing potential. For many of these antagonists subnanomolar IC₅₀ values were reported for binding to CCR3 along with the ability to effectively inhibit intracellular calcium mobilization and eosinophil chemotaxis induced by CCR3 agonist ligands in vitro. © 2009 Wiley Periodicals, Inc. Med Res Rev, 30, No. 5, 778–817, 2010     

Insulin gene variable number of tandem repeats is associated with increased fat mass during adolescence in non‐obese girls

Obesity and related disorders have become a major health problem. Understanding the interaction between genetic and environmental factors influencing the susceptibility to develop obesity is important when pinpointing people at risk. In a longitudinal study of 256 non‐obese adolescents, the influence of the insulin gene (INS) variable number of tandem repeats (VNTR) on anthropometric measures and fat mass was investigated. The adolescents were examined at the age of 12.4 (2.3) (mean, SD) and 16.2 (2.3) years, and at follow‐up with dual x‐ray absorptiometry (DXA) for measurement of body composition. INS VNTR classes I and III alleles were investigated using the ?23T/A single nucleotide polymorphism as a surrogate marker. There was a non‐significant trend towards increased body mass index (BMI) and fat mass with the class III allele in girls. Homozygotes for the INS VNTR class III allele had a greater increase in BMI compared with those that were homozygous or heterozygous for the class I allele (3.8 (1.6) versus 2.4 (1.9)?kg/m², p?=?0.03), and they had higher fat mass (36.4 (3.9) versus 31.3 (6.8)%, p?=?0.02) at follow‐up. Our finding that homozygosity of the INS VNTR class III allele seems to predispose to increased weight gain and fat mass raises the possibility that this genotype may be one of the important factors in the gene‐environment interaction that eventually results in overweight and insulin resistance.     

Defective enzyme protein in lipoprotein lipase deficiency

A monoclonal antibody to lipoprotein lipase (LPL) has been used in an enzyme-linked immunosorbent assay (ELISA) for LPL protein mass. Measurement of LPL immunoreactive mass in pre- and postheparin plasma distinguished three classes of abnormalities in patients with classical deficiency of lipoprotein lipase activity. The class I defect consisted of the absence of LPL immunoreactive homodimer in pre- and postheparin plasma compatible with a potential 'null allele'. Patients with a class II defect had almost no LPL immunoreactive mass in preheparin plasma but showed an increase in their LPL mass of 68 +/- 23 ng ml-1 (mean +/- SD) after heparin. Patients with the class III defect had considerable amounts of LPL immunoreactive material in preheparin plasma (159 +/- 190 ng ml-1). Heparin administration, however, caused very little additional release of LPL into the plasma (16 +/- 51 ng ml-1). Thus although both class II and class III patients had an LPL protein with abnormal catalytic activity, class III patients also appeared to have a defect in heparin binding of LPL. To test this hypothesis, postheparin plasma of classes II and III patients was analysed by heparin-Sepharose chromatography. In contrast to class II patients, the LPL immunoreactive mass of class III patients did not show affinity for the heparin and eluted in the column void volume, suggesting the class III defect is also associated with a defect in heparin binding.     

Pulmonary Embolism Severity Index and troponin testing for the selection of low‐risk patients with acute symptomatic pulmonary embolism

Summary. Background: The combination of the Pulmonary Embolism Severity Index (PESI) and troponin testing could help physicians identify appropriate patients with acute pulmonary embolism (PE) for early hospital discharge. Methods: This prospective cohort study included a total of 567 patients from a single center registry with objectively confirmed acute symptomatic PE. On the basis of the PESI, each patient was classified into one of five classes (I–V). At the time of hospital admission, patients had troponin I (cTnI) levels measured. The endpoint of the study was all‐cause mortality within 30 days after diagnosis. We calculated the mortality rates in four patient groups: group 1, PESI class I–II plus cTnI < 0.1 ng mL^?1; group 2, PESI classes III–V plus cTnI < 0.1 ng mL^?1; group 3, PESI classes I–II plus cTnI ≥ 0.1 ng mL^?1; and group 4, PESI classes III–V plus cTnI ≥ 0.1 ng mL^?1. Results: The study cohort had a 30‐day mortality of 10% [95% confidence interval (CI), 7.6–12.5%]. Mortality rates in the four groups were 1.3%, 14.2%, 0% and 15.4%, respectively. Compared with non‐elevated cTnl, the low‐risk PESI had a higher negative predictive value (NPV) (98.9% vs. 90.8%) and negative likelihood ratio (NLR) (0.1 vs. 0.9) for predicting mortality. The addition of non‐elevated cTnI to low‐risk PESI did not improve the NPV or the NLR compared with either test alone. Conclusions: Compared with cTnl testing, PESI classification more accurately identified patients with PE who are at low risk of all‐cause death within 30 days of presentation.     

Morphological parameters in lupus nephritis: their relevance for classification and relationship with clinical and histological findings and outcome

One hundred and sixty-seven renal biopsies from 147 patients with lupus nephritis were studied retrospectively to assess the contribution to morphological classification by features assessed with immunofluorescence and electron microscopy, together with pathological indices obtained by scoring specific histologic changes. The prognostic relevance of the histologic scoring was also evaluated. The biopsies were assigned to the following classes: I, absence of glomerular lesions; II, mesangial proliferation; III, focal segmental proliferation; IVa, diffuse (more than 50 per cent of the glomeruli) but segmentally distributed proliferation; IVb, diffuse and generalised proliferation; IVc, extracapillary proliferation; Va, pure membranous changes; Vb, membranous changes with slight mesangial proliferation; VI, association of class V and class III or IV. The incidence and degree of some glomerular and non-glomerular 'active' and 'sclerotic' changes as assessed by light microscopy were evaluated in the different classes. Both the activity and sclerosis indices obtained by scoring these lesions were found to be significantly higher in classes with glomerular proliferative changes. Eighteen patients had a second biopsy and two of these had a third; more severe changes were observed in nine and improvement in four. In 146 biopsies light microscopy findings were compared with immunofluorescence patterns (negative, mesangial, mesangial and peripheral, peripheral, membranous). The mesangial pattern was mainly present in class II with a few examples in classes I and III; in the last two the mesangial-peripheral pattern was most common; the peripheral pattern was by far the most common in class IV (a, b and c) and frequent in class VI; a membranous pattern was the rule in class V and occasionally found in class VI. Immunoglobulins (Igs) and complement (C) fractions were simultaneously present in most cases, IgG, C3 and C1q being the commonest in all classes. Except for IgM and fibrinogen, the differences in distribution of Igs and C fractions among the various classes were statistically significant. The deposits most commonly found by electron microscopy in all biopsies were mesangial; subendothelial deposits were mainly found in classes with active glomerular changes, frequently associated with deposits at the other sites in the most severe cases. A highly significant correlation was found between the activity index and the sclerosis index and severity of the clinical picture at biopsy. An unfavourable progress was confined mainly to classes with extensive intracapillary proliferation and correlated significantly with the highest activity and sclerosis indices.(ABSTRACT TRUNCATED AT 400 WORDS)     

Development of nociceptin receptor (NOP) agonists and antagonists

The nociceptin opioid (NOP) receptor is the most recently discovered member of the family of the opioid receptors; its endogenous agonist is the peptide nociceptin. Due to the subsequent elucidation of its physiological role in both central and peripheral nervous system and in some non‐neural tissues, there is a rapidly growing interest in the pharmacological application of substances active on this receptor. Despite the current clinical use of a morphinane‐based NOP/MOP mixed ligand (buprenorphine) as an analgesic and in the treatment of drug addictions, so far just a few clinical trials have been made with selective NOP ligands. However, the perspective of their utilization is rapidly growing. Agonists can find applications in the treatment of neuropathic pain, anxiety, cough, drug addition, urinary incontinence, anorexia, congestive heart failure, hypertension; and antagonists for pain, depression, Parkinson's disease, obesity, and as memory enhancers. Besides peptide ligands, which are still subjected to many pharmacological investigations, many different chemical classes of NOP ligands have been discovered: piperidines, nortropanes, spiropiperidines, 4‐amino‐quinolines and quinazolines, and others. The new advances in establishing structure–activity relationships, also with the help of modeling studies, can permit the development of more active and selective molecules. © 2010 Wiley Periodicals, Inc. Med Res Rev 31:605‐648,2011     

Current and future disease‐modifying therapies in multiple sclerosis

The development of disease‐modifying therapies (DMT) in multiple sclerosis (MS) has rapidly evolved over the last few years and continues to do so. Prior to the United States Food and Drug Administration approval of the immunomodulatory agent, interferon‐β1b in 1993, no other drug had been shown to alter the course of the disease in a controlled study of MS. At present, there are five licenced disease‐modifying agents in MS – interferon‐β1b, interferon‐β1a, glatiramer acetate, natalizumab and mitoxantrone. All have shown significant therapeutic efficacy in large controlled trials. However, current therapies are only partially effective and are not free from adverse effects. Moreover, available DMTs are overwhelmingly biased in favour of those with relapsing‐remitting disease. Effective treatment for progressive MS is severely limited, with only interferon‐β1b and mitoxantrone having licenced use in secondary progressive, but not primary progressive disease. Monoclonal antibodies, such as natalizumab selectively target immune pathways involved in the pathogenic process of MS. Alemtuzumab, daclizumab and rituximab are other notable monoclonal antibodies currently undergoing phase II and III trials in MS. Alemtuzumab has so far shown promising therapeutic benefit in relapsing disease, although immunological adverse effects have been a problem. Oral therapies have the benefit of improved tolerability and patient compliance compared with current parenteral treatments. Cladribine and fingolimod (FTY720) have shown encouraging results in their phase III clinical trials. It is also worth noting the evidence for starting DMT in patients with clinically isolated syndrome, whereby early treatment has shown to delay the onset of clinically definite MS in separate phase III studies.     

Potassium channels in vascular smooth muscle: a pathophysiological and pharmacological perspective

Potassium (K⁺) ion channel activity is an important determinant of vascular tone by regulating cell membrane potential (MP). Activation of K⁺ channels leads to membrane hyperpolarization and subsequently vasodilatation, while inhibition of the channels causes membrane depolarization and then vasoconstriction. So far five distinct types of K⁺ channels have been identified in vascular smooth muscle cells (VSMCs): Ca⁺²‐activated K⁺ channels (BK_Ca), voltage‐dependent K⁺ channels (K_V), ATP‐sensitive K⁺ channels (K_ATP), inward rectifier K⁺ channels (K_ir), and tandem two‐pore K⁺ channels (K₂P). The activity and expression of vascular K⁺ channels are changed during major vascular diseases such as hypertension, pulmonary hypertension, hypercholesterolemia, atherosclerosis, and diabetes mellitus. The defective function of K⁺ channels is commonly associated with impaired vascular responses and is likely to become as a result of changes in K⁺ channels during vascular diseases. Increased K⁺ channel function and expression may also help to compensate for increased abnormal vascular tone. There are many pharmacological and genotypic studies which were carried out on the subtypes of K⁺ channels expressed in variable amounts in different vascular beds. Modulation of K⁺ channel activity by molecular approaches and selective drug development may be a novel treatment modality for vascular dysfunction in the future. This review presents the basic properties, physiological functions, pathophysiological, and pharmacological roles of the five major classes of K⁺ channels that have been determined in VSMCs.     

Strategies in developing promising histone deacetylase inhibitors

Histone deacetylases (HDACs) are a family of enzymes that have been of interest in drug discovery for more than 30 years. Inhibitors of HDACs are potential therapeutics for various diseases, such as neurodegenerative diseases, inflammation, viral infection, and especially cancer. Most HDAC inhibitors (HDACi) are designed for cancer therapy. In 2006, suberoylanilide hydroxamic acid was approved by the US Food and Drug Administration for once‐daily oral treatment of advanced cutaneous T‐cell lymphoma. In the meantime, there have been aggressive efforts to bring HDACi to the market for every major tumor type, either as a single therapy or in combination, and a number of compounds are currently undergoing clinical trials. Multiple strategies have been applied to the rational design of drugs targeting HDACs by taking advantage of the new developments in proteomics, chemogenomics, cheminformatics, and computational chemistry/biology. Herein, we review the current methods successfully used in developing novel HDACi. © 2009 Wiley Periodicals, Inc. Med Res Rev, 30, No. 4, 585–602, 2010     

Water-soluble optical sensors: keys to detect aluminium in biological environment

Metal ion plays a critical role from enzyme catalysis to cellular health and functions. The concentration of metal ions in a living system is highly regulated. Among the biologically relevant metal ions, the role and toxicity of aluminium in specific biological functions have been getting significant attention in recent years. The interaction of aluminium and the living system is unavoidable due to its high earth crust abundance, and the long-term exposure to aluminium can be fatal for life. The adverse Al³⁺ toxicity effects in humans result in various diseases ranging from cancers to neurogenetic disorders. Several Al³⁺ ions sensors have been developed over the past decades using the optical responses of synthesized molecules. However, only limited numbers of water-soluble optical sensors have been reported so far. In this review, we have confined our discussion to water-soluble Al³⁺ ions detection using optical methods and their utility for live-cell imaging and real-life application.

Aluminium ion plays a critical role in cellular health. Therefore, its detection in water is highly demanding.     

Next-generation of selective histone deacetylase inhibitors

Histone deacetylases (HDACs) are clinically validated epigenetic drug targets for cancer treatment. HDACs inhibitors (HDACis) have been successfully applied against a series of cancers. First-generation inhibitors are mainly pan-HDACis that target multiple isoforms which might lead to serious side effects. At present, the next-generation HDACis are mainly focused on being class- or isoform-selective which can provide improved risk–benefit profiles compared to non-selective inhibitors. Because of the rapid development in next-generation HDACis, it is necessary to have an updated and state-of-the-art overview. Here, we summarize the strategies and achievements of the selective HDACis.

Histone deacetylases (HDACs) are clinically validated epigenetic drug targets for cancer treatment.     

Bcr-Abl-independent imatinib-resistant K562 cells show aberrant protein acetylation and increased sensitivity to histone deacetylase inhibitors

Bcr-Abl-independent signaling pathways are known to be involved in imatinib resistance in some patients with chronic myelogenous leukemia (CML). In this study, to find new targets for imatinib-resistant CML displaying loss of Bcr-Abl kinase target dependence, we isolated imatinib-resistant variants, K562/R1, K562/R2, and K562/R3, which showed profound declines of Bcr-Abl levels and its tyrosine kinase activity, from K562 cells. Importantly, the imatinib resistance mechanism in these variants also included aberrant acetylation of nonhistone proteins such as p53, Ku70, and Hsp90 that was due to upregulation of histone deacetylases (HDACs) and down-regulation of histone acetyltransferase (HAT). In comparison with K562 cells, the imatinib-resistant variants showed up-regulation of HDAC1, -2, and -3 (class I HDACs) and class III SIRT1 and down-regulation of CBP/p300 and PCAF with HAT activity, and thereby p53 and cytoplasmic Ku70 were aberrantly acetylated. In addition, these were associated with down-regulation of Bax and up-regulation of Bcl-2. In contrast, the class II HDAC6 level was significantly decreased, and this was accompanied by an increase of Hsp90 acetylation in the imatinib-resistant variants, which was closely associated with loss of Bcr-Abl. These results indicate that alteration of the normal balance of HATs and HDACs leads to deregulated acetylation of Hsp90, p53, and Ku70 and thereby leads to imatinib resistance, suggesting the importance of the acetylation status of apoptosis-related nonhistone proteins in Bcr-Abl-independent imatinib resistance. We also revealed that imatinib-resistant K562 cells were more sensitive to suberoylanilide hydroxamic acid, an HDAC inhibitor, than K562 cells. These findings may have implications for HDAC as a molecular target in imatinib-resistant leukemia cells.