Phase I and pharmacodynamic study of an orally administered novel inhibitor of histone deacetylases, SB939, in patients with refractory solid malignancies

HDAC inhibitors activate lipid peroxidation and ferroptosis in gastric cancer

Gastric cancer remains among the deadliest neoplasms worldwide, with limited therapeutic options. Since efficacies of targeted therapies are unsatisfactory, drugs with broader mechanisms of action rather than a single oncogene inhibition are needed. Preclinical studies have identified histone deacetylases (HDAC) as potential therapeutic targets in gastric cancer. However, the mechanism(s) of action of HDAC inhibitors (HDACi) are only partially understood. This is particularly true with regard to ferroptosis as an emerging concept of cell death. In a panel of gastric cancer cell lines with different molecular characteristics, tumor cell inhibitory effects of different HDACi were studied. Lipid peroxidation levels were measured and proteome analysis was performed for the in-depth characterization of molecular alterations upon HDAC inhibition. HDACi effects on important ferroptosis genes were validated on the mRNA and protein level. Upon HDACi treatment, lipid peroxidation was found increased in all cell lines. Class I HDACi (VK1, entinostat) showed the same toxicity profile as the pan-HDACi vorinostat. Proteome analysis revealed significant and concordant alterations in the expression of proteins related to ferroptosis induction. Key enzymes like ACSL4, POR or SLC7A11 showed distinct alterations in their expression patterns, providing an explanation for the increased lipid peroxidation. Results were also confirmed in primary human gastric cancer tissue cultures as a relevant ex vivo model. We identify the induction of ferroptosis as new mechanism of action of class I HDACi in gastric cancer. Notably, these findings were independent of the genetic background of the cell lines, thus introducing HDAC inhibition as a more general therapeutic principle.

Tissue-Engineered Disease Modeling of Lymphangioleiomyomatosis Exposes a Therapeutic Vulnerability to HDAC Inhibition

Lymphangioleiomyomatosis (LAM) is a rare disease involving cystic lung destruction by invasive LAM cells. These cells harbor loss-of-function mutations in TSC2, conferring hyperactive mTORC1 signaling. Here, tissue engineering tools are employed to model LAM and identify new therapeutic candidates. Biomimetic hydrogel culture of LAM cells is found to recapitulate the molecular and phenotypic characteristics of human disease more faithfully than culture on plastic. A 3D drug screen is conducted, identifying histone deacetylase (HDAC) inhibitors as anti-invasive agents that are also selectively cytotoxic toward TSC2-/- cells. The anti-invasive effects of HDAC inhibitors are independent of genotype, while selective cell death is mTORC1-dependent and mediated by apoptosis. Genotype-selective cytotoxicity is seen exclusively in hydrogel culture due to potentiated differential mTORC1 signaling, a feature that is abrogated in cell culture on plastic. Importantly, HDAC inhibitors block invasion and selectively eradicate LAM cells in vivo in zebrafish xenografts. These findings demonstrate that tissue-engineered disease modeling exposes a physiologically relevant therapeutic vulnerability that would be otherwise missed by conventional culture on plastic. This work substantiates HDAC inhibitors as possible therapeutic candidates for the treatment of patients with LAM and requires further study.

A phase II study of addition of pracinostat to a hypomethylating agent in patients with myelodysplastic syndromes who have not responded to previous hypomethylating agent therapy

Hypomethylating agents (HMAs) are standard of care for higher-risk myelodysplastic syndromes (MDS). However, less than half of patients achieve objective responses and most eventually lose their response. Pracinostat is a pan-histone deacetylase inhibitor with demonstrated activity in advanced myeloid malignancies. This phase II study explored the benefit of adding pracinostat to HMAs in MDS patients who did not respond to single-agent HMA treatment. The goal was to estimate the clinical improvement rate [complete remission (CR), marrow CR, partial response (PR) and haematological improvement]. Group 1 included patients with primary/secondary HMA failures; Group 2 included those who did not achieve response but had stable disease (SD) after single-agent HMAs. Forty-five patients (39 Group 1, 6 Group 2) received a median of 3 cycles. Among all patients, 1 (2%) had CR, 7 (16%) had marrow CR and 18 (40%) had SD; disease progression occurred in 3 (7%). Median overall survival was 5·7/5·6 months for Group 1/2. Grade ≥3 adverse events occurred in 38 patients (84%) leading to treatment discontinuation in 12 (33%). Adding pracinostat to HMAs did not improve outcomes in patients previously treated with HMAs. Frequent dose modifications/early discontinuation resulted in suboptimal drug exposure. A reduced pracinostat dose may improve tolerability and efficacy.

Non-coding DNA in IBD: from sequence variation in DNA regulatory elements to novel therapeutic potential

Genome-wide association studies have identified over 200 loci associated with IBD. We and others have recently shown that, in addition to variants in protein-coding genes, the majority of the associated loci are related to DNA regulatory elements (DREs). These findings add a dimension to the already complex genetic background of IBD. In this review we summarise the existing evidence on the role of DREs in IBD. We discuss how epigenetic research can be used in candidate gene approaches that take non-coding variants into account and can help to pinpoint the essential pathways and cell types in the pathogenesis of IBD. Despite the increased level of genetic complexity, these findings can contribute to novel therapeutic options that target transcription factor binding and enhancer activity. Finally, we summarise the future directions and challenges of this emerging field.

Identification of brain metastasis genes and therapeutic evaluation of histone deacetylase inhibitors in a clinically relevant model of breast cancer brain metastasis

Breast cancer brain metastases remain largely incurable. Although several mouse models have been developed to investigate the genes and mechanisms regulating breast cancer brain metastasis, these models often lack clinical relevance since they require the use of immunocompromised mice and/or are poorly metastatic to brain from the mammary gland. We describe the development and characterisation of an aggressive brain metastatic variant of the 4T1 syngeneic model (4T1Br4) that spontaneously metastasises to multiple organs, but is selectively more metastatic to the brain from the mammary gland than parental 4T1 tumours. As seen by immunohistochemistry, 4T1Br4 tumours and brain metastases display a triple-negative phenotype, consistent with the high propensity of this breast cancer subtype to spread to brain. In vitro assays indicate that 4T1Br4 cells have an enhanced ability to adhere to or migrate across a brain-derived endothelial monolayer and greater invasive response to brain-derived soluble factors compared to 4T1 cells. These properties are likely to contribute to the brain selectivity of 4T1Br4 tumours. Expression profiling and gene set enrichment analyses demonstrate the clinical relevance of the 4T1Br4 model at the transcriptomic level. Pathway analyses implicate tumour-intrinsic immune regulation and vascular interactions in successful brain colonisation, revealing potential therapeutic targets. Evaluation of two histone deacetylase inhibitors, SB939 and 1179.4b, shows partial efficacy against 4T1Br4 metastasis to brain and other sites in vivo, and potent radio-sensitising properties in vitro. The 4T1Br4 model provides a clinically relevant tool for mechanistic studies and to evaluate novel therapies against brain metastasis.

This article has an associated First Person interview with Soo-Hyun Kim, joint first author of the paper.

Summary: The authors introduce a new syngeneic mouse model of spontaneous breast cancer brain metastasis, demonstrate its phenotypic, functional and transcriptomic relevance to human TNBC brain metastasis, and test novel therapies.

Phase I study of CKD-581, a pan-histone deacetylase inhibitor, in patients with lymphoma or multiple myeloma refractory to standard therapy

Background The objective of this study was to assess the safety, dose-limiting toxicities (DLTs), maximum tolerated dose (MTD), pharmacokinetics, and anti-tumor efficacy of CKD-581, a novel pan-histone deacetylase inhibitor, in patients with lymphoma or multiple myeloma (MM) refractory to standard therapy. Methods In this phase I study, CKD-581 was intravenously administered on days 1, 8, and 15 of a 28-day cycle. A standard 3 + 3 cohort design was used to determine the MTD. Acetylated histones H3 and H4 in peripheral blood mononuclear cells were measured for pharmacodynamic assessment in a subpopulation of patients. Results Thirty-nine patients were treated with CKD-581 at 9 dose levels from 10 mg/m² to 210 mg/m². The DLTs were grade 3 neutropenia that delayed the treatment for >2 weeks (one patient at a dose of 50 mg/m²) and grade 4 thrombocytopenia (two patients at a dose of 210 mg/m²). The MTD of CKD-581 was 160 mg/m². The most common grade 3/4 treatment-related adverse events were thrombocytopenia (n = 5, 12.8%) and neutropenia (n = 2, 5.1%). The peak concentration and area under the curve values for CKD-581 increased in proportion to the dose, indicating linear pharmacokinetics. A partial response was observed in 2 patients (5.6%), and stable disease was observed in 16 (44.4%) patients. In the pharmacodynamic evaluation, acetylation of H3 and H4 was observed at all doses of ≥50 mg/m². Conclusion CKD-581 was well tolerated by the patients with lymphoma or MM refractory to standard therapy. It exhibited dose-proportional pharmacokinetics and modest anti-tumor efficacy.

Phase 1 dose escalation multicenter trial of pracinostat alone and in combination with azacitidine in patients with advanced hematologic malignancies

BACKGROUND

Pracinostat is a potent histone deacetylase inhibitor with antitumor activity in both solid tumor and acute myeloid leukemia (AML) cell lines. Pracinostat is reported to have modest clinical activity in patients with advanced solid tumors. Given the higher preclinical sensitivity of hematologic malignancies to pracinostat, the authors conducted a phase 1 study to assess the safety, maximum tolerated dose, recommended phase 2 dose, efficacy, pharmacokinetics, and pharmacodynamics of pracinostat in patients with advanced hematological malignancies.

METHODS

Pracinostat was administered orally 3 times a week for 3 weeks on a 28-day cycle. Patients were assigned to 7 dose levels using a 3 + 3 dose escalation design.

RESULTS

A total of 44 patients were enrolled, 25 of whom had AML and 14 of whom had myelodysplastic syndrome. The maximum tolerated dose was 120 mg and the recommended phase 2 dose was 60 mg. Two patients with AML achieved a response: 1 complete remission (CR) and 1 complete cytogenetic response. Despite a dose-dependent increase in the plasma concentration of pracinostat, a similar increase in histone acetylation was not observed. As an extension, 10 additional patients with myelodysplastic syndrome were enrolled to assess the safety and efficacy of pracinostat in combination with azacitidine. Six patients achieved a CR and 3 achieved a CR without platelet recovery with no added toxicity.

CONCLUSIONS

The results of the current study demonstrate that pracinostat is safe, with modest single-agent activity in patients with hematological malignancies. Cancer 2017;123:4851-9. © 2017 American Cancer Society.

Histone deacetylase inhibitor pracinostat in doublet therapy: a unique strategy to improve therapeutic efficacy and to tackle herculean cancer chemoresistance

Context Histone deacetylase inhibitors (HDACi) have shown promising results in neurodegeneration and cancer. Hydroxamate HDACi, including vorinostat, have shown encouraging results in haematological malignancies, but the poor pharmacokinetic of these inhibitors leads to insufficient tumour concentration limiting their application against solid malignancies. Objective This article deals with novel HDAC inhibitor pracinostat (SB939) and delineates its therapeutic role in solid and haematological malignancies. The article provides rigorous details about the underlying molecular mechanisms modulated by pracinostat to exert cytotoxic effect. The article further highlights the doublet therapy that may be used to tackle monotonous cancer chemoresistance. Methods Both old and the latest literature on pracinostat was retrieved from diverse sources, such as PubMed, Science Direct, Springer Link, general Google search using both pracinostat and SB939 keywords in various ways: after thorough evaluation the topic which can fulfil the current gap was chosen. Results Pracinostat shows potent anticancer activity against both solid and haematological malignancies compared to the FDA-approved drug vorinostat. This marvellous inhibitor has better physicochemical, pharmaceutical and pharmacokinetic properties than the defined inhibitor vorinostat. Pracinostat has  >100-fold more affinity towards HDACs compared to other zinc-dependent metalloenzymes and shows maximum efficacy when used in doublet therapy. Conclusion Pracinostat shows potent anticancer activity even against therapeutically challenging cancers when used in doublet therapy. However, the triplet combination studies of the defined inhibitor that may prove even more beneficial are still undone, emphasizing the desperate need of further research in the defined gap.

Epigenetic treatment of solid tumours: a review of clinical trials

Epigenetic treatment has been approved by regulatory agencies for haematological malignancies. The success observed in cutaneous lymphomas represents a proof of principle that similar results may be obtained in solid tumours. Several agents that interfere with DNA methylation-demethylation and histones acetylation/deacetylation have been studied, and some (such as azacytidine, decitabine, valproic acid and vorinostat) are already in clinical use.

The aim of this review is to provide a brief overview of the molecular events underlying the antitumour effects of epigenetic treatments and to summarise data available on clinical trials that tested the use of epigenetic agents against solid tumours. We not only list results but also try to indicate how the proper evaluation of this treatment might result in a better selection of effective agents and in a more rapid development.

We divided compounds in demethylating agents and HDAC inhibitors. For each class, we report the antitumour activity and the toxic side effects. When available, we describe plasma pharmacokinetics and pharmacodynamic evaluation in tumours and in surrogate tissues (generally white blood cells).

Epigenetic treatment is a reality in haematological malignancies and deserves adequate attention in solid tumours. A careful consideration of available clinical data however is required for faster drug development and possibly to re-evaluate some molecules that were perhaps discarded too early.

A Phase I/II Multicenter, Open-Label Study of the Oral Histone Deacetylase Inhibitor Abexinostat in Relapsed/Refractory Lymphoma

PURPOSE

Additional targeted therapeutics are needed for the treatment of lymphoma. Abexinostat is an oral pan-histone deacetylase inhibitor (HDACi) displaying potent activity in preclinical models. We conducted a multicenter phase I/II study (N = 55) with single-agent abexinostat in relapsed/refractory lymphoma.

EXPERIMENTAL DESIGN

In phase I, 25 heavily pretreated patients with any lymphoma subtype received oral abexinostat ranging from 30 to 60 mg/m(2) twice daily 5 days/week for 3 weeks or 7 days/week given every other week. Phase II evaluated abexinostat at the maximum tolerated dose in 30 patients with relapsed/refractory follicular lymphoma or mantle cell lymphoma.

RESULTS

The recommended phase II dose was 45 mg/m(2) twice daily (90 mg/m(2) total), 7 days/week given every other week. Of the 30 follicular lymphoma and mantle cell lymphoma patients enrolled in phase II, 25 (14 follicular lymphoma, 11 mantle cell lymphoma) were response-evaluable. Tumor size was reduced in 86% of follicular lymphoma patients with an investigator-assessed ORR of 64.3% for evaluable patients [intent-to-treat (ITT) ORR 56.3%]. Median duration of response was not reached, and median progression-free survival (PFS) was 20.5 months (1.2-22.3+). Of responding follicular lymphoma patients, 89% were on study/drug >8 months. In mantle cell lymphoma, the ORR was 27.3% for evaluable patients (ITT ORR 21.4%), and median PFS was 3.9 months (range, 0.1-11.5). Grade 3-4 treatment-related adverse events (phase II) with ≥ 10% incidence were thrombocytopenia (20%), fatigue (16.7%), and neutropenia (13.3%) with rare QTc prolongation and no deaths.

CONCLUSIONS

The pan-HDACi, abexinostat, was overall well tolerated and had significant clinical activity in follicular lymphoma, including highly durable responses in this multiply relapsed patient population.

A phase II study of the HDAC inhibitor SB939 in patients with castration resistant prostate cancer: NCIC clinical trials group study IND195

BACKGROUND

SB939 is a potent oral inhibitor of class 1, 2, and 4 histone deacetylases (HDACs). These three HDAC classes are highly expressed in castration resistant prostate cancer (CRPC) and associated with poor clinical outcomes. We designed a phase II study of SB939 in men with metastatic CRPC.

METHODS

Patients received SB939 60 mg on alternate days three times per week for 3 weeks on a 4-week cycle. Primary endpoints were PSA response rate (RR) and progression-free survival (PFS). Secondary endpoints included objective response rate and duration; overall survival; circulating tumor cell (CTC) enumeration and safety. Exploratory correlative studies of the TMPRSS2-ERG fusion and PTEN biomarkers were also performed.

RESULTS

Thirty-two patients were enrolled of whom 88 % had received no prior chemotherapy. The median number of SB939 cycles administered was three (range 1-8). Adverse events were generally grade 1-2, with five pts experiencing one or more grade three event. One patient died due to myocardial infarction. A confirmed PSA response was noted in two pts (6 %), lasting 3.0 and 21.6 months. In patients with measurable disease there were no objective responses. Six patients had stable disease lasting 1.7 to 8.0 months. CTC response (from ≥5 at baseline to <5 at 6 or 12 weeks) occurred in 9/14 evaluable patients (64 %).

CONCLUSION

Although SB939 was tolerable at the dose/schedule given, and showed declines in CTC in the majority of evaluable patients, it did not show sufficient activity based on PSA RR to warrant further study as a single agent in unselected patients with CRPC.

A phase II study of SB939, a novel pan-histone deacetylase inhibitor, in patients with translocation-associated recurrent/metastatic sarcomas-NCIC-CTG IND 200†

BACKGROUND

A subgroup of sarcomas is characterized by defining chromosomal translocations, creating fusion transcription factor oncogenes. Resultant fusion oncoproteins associate with chromatin-modifying complexes containing histone deacetylases (HDAC), and lead to epigenetic transcriptional dysregulation. HDAC inhibitors were shown to be effective in vitro, reversing gene repression by these complexes, restoring PTEN expression and apoptosis via the PI3K/Akt/mTOR pathway.

PATIENTS AND METHODS

SB939 is an oral inhibitor of classes 1 and 2 HDAC. Eligible patients with recurrent or metastatic translocation-associated sarcoma (TAS) by local pathology were treated with 60 mg/day every other day for 3 of 4 weeks. Central pathology review was conducted with fusion oncogenes characterized, and HDAC2 expression correlated with efficacy in pre-specified methods.

RESULTS

Twenty-two patients were treated with a median of 2 cycles. Fourteen patients were assessable for response with confirmed specific chromosomal translocations; 8 had a best response of stable disease (SD) (median duration 5.4 months) with no confirmed objective responses. The 3-month progression-free survival (PFS) rate was 49%. Among those with HDAC2 score ≥5, 7/10 had SD, versus 0/3 with HDAC2 score <5. SB939 was considered as well tolerated with <10% patients experienced ≥grade 3 toxicity.

CONCLUSION

This study was stopped prematurely due to prolonged unavailability of SB939. No objective responses were seen. Although the observed SD in HDAC2 high patients was interesting, due to the small sample size, no definitive conclusion can be drawn about the efficacy of SB939 in this patient population.

CLINICAL TRIAL

NCT01112384.

Review article: selective histone deacetylase isoforms as potential therapeutic targets in inflammatory bowel diseases

BACKGROUND

A link between histone deacetylases (HDACs) and intestinal inflammation has been established. HDAC inhibitors that target gut-selective inflammatory pathways represent a potential new therapeutic strategy in patients with refractory inflammatory bowel diseases (IBD).

AIMS

To review the use of selective HDAC inhibitors to treat gut inflammation and to highlight potential improvements in selectivity/sensitivity by additional targeting of HDAC-regulating microRNAs (miRNAs).

METHODS

Original articles and reviews have been identified using PubMed search terms: 'histone deacetylase', 'HDAC inhibitor', 'inflammatory bowel disease', 'gut inflammation,' and 'microRNA and HDAC'.

RESULTS

The use of butyrate in distal colitis provided the first evidence that inhibition of HDACs decreases intestinal inflammation in IBD. HDAC inhibitors, such as valproic acid, vorinostat and givinostat, reduce inflammation and tissue damage in experimental murine colitis. Potential mechanisms of action for HDAC inhibitors include increased apoptosis, reduction of pro-inflammatory cytokine release, regulation of transcription factors and modulation of HDAC-regulatory miRNAs. HDAC2, HDAC3, HDAC6, HDAC9 and HDAC10 isoforms seem to be specifically involved in chronic intestinal inflammation, justifying the use of selective inhibitors as new therapeutic strategies in IBD. Controlling miRNAs for these isoforms can be identified.

CONCLUSIONS

The pro-inflammatory influence of HDACs in the gut has been confirmed, but mostly in murine studies. Considerably more human data are required to permit development of selective HDAC inhibitors for IBD treatment. Inhibition of key HDAC isoforms in combination with modulation of HDAC-regulatory miRNAs has potential as a novel therapeutic approach.

New and emerging HDAC inhibitors for cancer treatment

Epigenetic enzymes are often dysregulated in human tumors through mutation, altered expression, or inappropriate recruitment to certain loci. The identification of these enzymes and their partner proteins has driven the rapid development of small-molecule inhibitors that target the cancer epigenome. Herein, we discuss the influence of aberrantly regulated histone deacetylases (HDACs) in tumorigenesis. We examine HDAC inhibitors (HDACis) targeting class I, II, and IV HDACs that are currently under development for use as anticancer agents following the FDA approval of two HDACis, vorinostat and romidepsin.

A phase I study of histone deacetylase inhibitor, pracinostat (SB939), in pediatric patients with refractory solid tumors: IND203 a trial of the NCIC IND program/C17 pediatric phase I consortium

BACKGROUND

Pracinostat (SB939) is a potent oral inhibitor of class 1, 2, and 4 histone deacetylases (HDAC). The adult recommended phase II dose (RP2D) is 60 mg po three times per week (t.i.w.) for 3 weeks every 4 weeks. This study assessed the toxicities and pharmacokinetics of pracinostat and determined the RP2D in children with refractory solid tumors.

METHODS

Pediatric patients with refractory solid tumors were treated with oral pracinostat t.i.w. for 3 consecutive weeks, followed by 1 week off dosing. Three dose levels-25, 35, and 45 mg/m(2) were evaluated using a standard 3 + 3 cohort design. Pharmacokinetic (PK) studies were optional.

RESULTS

Twelve patients were enrolled. The most common diagnosis was Ewing sarcoma. Most adverse events (AEs) were hematological with five (40%) patients experiencing grade 3 neutropenia. Non-hematological AEs were generally grade 1. No dose limiting toxicities occurred. More hematological and non-hematological AEs occurred at 45 mg/m(2) : Two of five patients experienced Grade 3 neutropenia and one each Grade 3 thrombocytopenia and leucopenia, Grade 1 fatigue and anorexia occurred in three. The RP2D was declared to be 45 mg/m(2) (comparable to an adult dose of 80 mg). One patient had a best response of stable disease (duration of 2.9 months). Three patients on 25 mg/m(2) and one each on 35 and 45 mg/m(2) participated in the PK study. No dose related changes in Cmax or AUC occurred.

CONCLUSIONS

Pracinostat is reasonably well tolerated in children with refractory solid tumors. The RP2D is 45 mg/m(2) .

Evolution of clinical trial design in early drug development: systematic review of expansion cohort use in single-agent phase I cancer trials

PURPOSE

To evaluate the use and objectives of expansion cohorts in phase I cancer trials and to explore trial characteristics associated with their use.

METHODS

We performed a systematic review of MEDLINE and EMBASE, limiting studies to single-agent phase I trials recruiting adults and published after 2006. Eligibility assessment and data extraction were performed by two reviewers. Data were assessed descriptively, and associations were tested by univariable and multivariable logistic regression.

RESULTS

We identified 611 unique phase I cancer trials, of which 149 (24%) included an expansion cohort. The trials were significantly more likely to use an expansion cohort if they were published more recently, were multicenter, or evaluated a noncytotoxic agent. Objectives of the expansion cohort were reported in 74% of trials. In these trials, safety (80%), efficacy (45%), pharmacokinetics (28%), pharmacodynamics (23%), and patient enrichment (14%) were cited as objectives. Among expansion cohorts with safety objectives, the recommended phase II dose was modified in 13% and new toxicities were described in 54% of trials. Among trials aimed at assessing efficacy, only 11% demonstrated antitumor activity assessed by response criteria that was not previously observed during dose escalation.

CONCLUSION

The utilization of expansion cohorts has increased with time. Safety and efficacy are common objectives, but 26% fail to report explicit aims. Expansion cohorts may provide useful supplementary data for phase I trials, particularly with regard to toxicity and definition of recommended dose for phase II studies.

Role of Hydroxamate-Based Histone Deacetylase Inhibitors (Hb-HDACIs) in the Treatment of Solid Malignancies

Hydroxamate-based histone deacetylase inhibitors (Hb-HDACIs), such as vorinostat, belinostat and panobinostat, have been previously shown to have a wide range of activity in hematologic malignancies such as cutaneous T-cell lymphoma and multiple myeloma. Recent data show that they synergize with a variety of cytotoxic and molecular targeted agents in many different solid tumors, including breast, prostate, pancreatic, lung and ovarian cancer. Hb-HDACIs have a quite good toxicity profile and are now being tested in phase I and II clinical trials in solid tumors with promising results in selected neoplasms, such as hepatocarcinoma. This review will focus on their clinical activity and safety in patients with advanced solid neoplasms.

Activity of histone deacetylase inhibitors and an Aurora kinase inhibitor in BCR-ABL-expressing leukemia cells: Combination of HDAC and Aurora inhibitors in BCR-ABL-expressing cells

Background

The use of imatinib, an ABL tyrosine kinase inhibitor, has led to a dramatic change in the management of BCR-ABL-positive leukemia patients. However, resistance to imatinib mediated by mutations in the BCR-ABL domain has become a major problem in the treatment of these patients.

Methods

In the present study, we examined the activity of histone deacetylase (HDAC) inhibitors in combination with an Aurora kinase inhibitor in BCR-ABL-expressing cells.

Results

We found the HDAC inhibitors vorinostat and/or pracinostat (SB939) induced apoptosis in BCR-ABL-expressing cells. Additionally, HDAC inhibitors reduced levels of Aurora A and B protein. An Aurora kinase inhibitor, tozasertib (VX-680), inhibited growth, promoted pro-apoptotic activity, reduced the phosphorylation of BCR-ABL and Crk-L, and activated caspase-3 and poly (ADP-ribose) polymerase (PARP) in BCR-ABL-positive cells. Moreover, after treatment with tozasertib, HDAC protein expression was decreased. Combination of vorinostat or pracinostat with tozasertib had a synergistic inhibitory effect on the proliferation of T315I cells. Phosphorylation of Crk-L decreased, and PARP activation increased after treatment with vorinostat or pracinostat and tozasertib. Moreover, combination of vorinostat or pracinostat and tozasertib significantly increased the extent of apoptosis in primary chronic myeloid leukemia cells.

Conclusions

This study demonstrated that combination of HDAC and Aurora inhibitors was highly effective against BCR-ABL-expressing cells.

The prostate cancer blocking potential of the histone deacetylase inhibitor LBH589 is not enhanced by the multi receptor tyrosine kinase inhibitor TKI258

Pharmacologic options for patients with castration-resistant prostate cancer are limited. It has been suggested that targeting intracellular molecules, which have been altered during neoplastic development, may slow tumor growth. Therefore, the growth-blocking potential of the histone deacetylase-inhibitor LBH589 and the multiple tyrosine kinase-inhibitor TKI258, applied alone or in combination, was investigated in a panel of prostate cancer cell lines. PC-3, DU-145 or LNCaP cells were treated with various concentrations of LBH589 and/or TKI258. Tumor cell growth, cell cycle regulating proteins, HDAC3- and HDAC4-expression and histone H3 and H4 acetylation were then evaluated by MTT assay and Western blotting. LBH589 dose-dependently blocked prostate cancer cell growth. In contrast, TKI258 did not down-regulate tumor cell growth up to a 1,000 nM dosage. LBH589 elevated histone H3 and H4 acetylation. The cell cycle regulators cyclin B, cyclin D1, cdk1 and cdk4 were down-regulated in PC-3, whereas the suppressor proteins p21 and p27 were up-regulated in LNCaP by LBH589. TKI258 up-regulated p27 in PC-3 or p21 in LNCaP and additionally elevated cyclin B, cyclin D1, cdk1 and cdk4 in both cell lines. Presumably, the increase in cyclin and cdk caused by TKI258 counteracts the benefit of p21 or p27 up-regulation, resulting in TKI258 non-responsiveness. The LBH589/TKI258-combination was not superior to the LBH589 single-drug use in terms of growth reduction. Obviously, TKI258 did not enhance the sensitivity of prostate cancer cells towards an HDAC based regimen. Therefore, the LBH589/TKI258-combination probably does not provide an optimum strategy in fighting advanced prostate cancer.

Antimalarial activity of the anticancer histone deacetylase inhibitor SB939

Histone deacetylase (HDAC) enzymes posttranslationally modify lysines on histone and nonhistone proteins and play crucial roles in epigenetic regulation and other important cellular processes. HDAC inhibitors (e.g., suberoylanilide hydroxamic acid [SAHA; also known as vorinostat]) are used clinically to treat some cancers and are under investigation for use against many other diseases. Development of new HDAC inhibitors for noncancer indications has the potential to be accelerated by piggybacking onto cancer studies, as several HDAC inhibitors have undergone or are undergoing clinical trials. One such compound, SB939, is a new orally active hydroxamate-based HDAC inhibitor with an improved pharmacokinetic profile compared to that of SAHA. In this study, the in vitro and in vivo antiplasmodial activities of SB939 were investigated. SB939 was found to be a potent inhibitor of the growth of Plasmodium falciparum asexual-stage parasites in vitro (50% inhibitory concentration [IC(50)], 100 to 200 nM), causing hyperacetylation of parasite histone and nonhistone proteins. In combination with the aspartic protease inhibitor lopinavir, SB939 displayed additive activity. SB939 also potently inhibited the in vitro growth of exoerythrocytic-stage Plasmodium parasites in liver cells (IC(50), ~150 nM), suggesting that inhibitor targeting to multiple malaria parasite life cycle stages may be possible. In an experimental in vivo murine model of cerebral malaria, orally administered SB939 significantly inhibited P. berghei ANKA parasite growth, preventing development of cerebral malaria-like symptoms. These results identify SB939 as a potent new antimalarial HDAC inhibitor and underscore the potential of investigating next-generation anticancer HDAC inhibitors as prospective new drug leads for treatment of malaria.

Mechanisms of resistance to histone deacetylase inhibitors

Histone deacetylase (HDAC) inhibitors are a new class of anticancer agents. HDAC inhibitors induce acetylation of histones and nonhistone proteins which are involved in regulation of gene expression and in various cellular pathways including cell growth arrest, differentiation, DNA damage and repair, redox signaling, and apoptosis (Marks, 2010). The U.S. Food and Drug Administration has approved two HDAC inhibitors, vorinostat and romidepsin, for the treatment of cutaneous T-cell lymphoma (Duvic & Vu, 2007; Grant et al., 2010; Marks & Breslow, 2007). Over 20 chemically different HDAC inhibitors are in clinical trials for hematological malignancies and solid tumors. This review considers the mechanisms of resistance to HDAC inhibitors that have been identified which account for the selective effects of these agents in inducing cancer but not normal cell death. These mechanisms, such as functioning Chk1, high levels of thioredoxin, or the prosurvival BCL-2, may also contribute to resistance of cancer cells to HDAC inhibitors.

Optimization of the in vitro cardiac safety of hydroxamate-based histone deacetylase inhibitors

Histone deacetylase (HDAC) inhibitors have shown promise in treating various forms of cancer. However, many HDAC inhibitors from diverse structural classes have been associated with QT prolongation in humans. Inhibition of the human ether a-go-go related gene (hERG) channel has been associated with QT prolongation and fatal arrhythmias. To determine if the observed cardiac effects of HDAC inhibitors in humans is due to hERG blockade, a highly potent HDAC inhibitor devoid of hERG activity was required. Starting with dacinostat (LAQ824), a highly potent HDAC inhibitor, we explored the SAR to determine the pharmacophores required for HDAC and hERG inhibition. We disclose here the results of these efforts where a high degree of pharmacophore homology between these two targets was discovered. This similarity prevented traditional strategies for mitigating hERG binding/modulation from being successful and novel approaches for reducing hERG inhibition were required. Using a hERG homology model, two compounds, 11r and 25i, were discovered to be highly efficacious with weak affinity for the hERG and other ion channels.

Discovery of (2E)-3-{2-butyl-1-[2-(diethylamino)ethyl]-1H-benzimidazol-5-yl}-N-hydroxyacrylamide (SB939), an orally active histone deacetylase inhibitor with a superior preclinical profile

A series of 3-(1,2-disubstituted-1H-benzimidazol-5-yl)-N-hydroxyacrylamides (1) were designed and synthesized as HDAC inhibitors. Extensive SARs have been established for in vitro potency (HDAC1 enzyme and COLO 205 cellular IC(50)), liver microsomal stability (t(1/2)), cytochrome P450 inhibitory (3A4 IC(50)), and clogP, among others. These parameters were fine-tuned by carefully adjusting the substituents at positions 1 and 2 of the benzimidazole ring. After comprehensive in vitro and in vivo profiling of the selected compounds, SB939 (3) was identified as a preclinical development candidate. 3 is a potent pan-HDAC inhibitor with excellent druglike properties, is highly efficacious in in vivo tumor models (HCT-116, PC-3, A2780, MV4-11, Ramos), and has high and dose-proportional oral exposures and very good ADME, safety, and pharmaceutical properties. When orally dosed to tumor-bearing mice, 3 is enriched in tumor tissue which may contribute to its potent antitumor activity and prolonged duration of action. 3 is currently being tested in phase I and phase II clinical trials.