How do we improve histone deacetylase inhibitor drug discovery?

KEYWORDS:

• Histone deacetylase
• histone deacetylase inhibitors
• cancer
• drugs combination
• multitargeted drugs

1. Introduction

The field of histone acetylation has rapidly evolved over the past few decades. Indeed, a quick literature survey using SciFinder® highlights that there were 18 entries from 1981 to 1991 while there were>850 entries from 1992 to 2002, >11,000 entries for the following 10 years and >8000 entries from 2014 to 2019.

The dynamic status of histone acetylation is under the control of histone deacetylases (HDACs) and histone acetyltransferases (HATs), which play an important role in the regulation of gene expression. While HATs mediate the acetylation of lysine residue associated with gene transcription, HDACs have the opposite effect, and deacetylation leads to a more condensed chromatin structure; this, in turn, leads to transcriptional repression of the gene [1]. Histone deacetylases are often dysregulated and have been recognized as a crucial factor in numerous diseases, including cancer, neurodegenerative and inflammatory diseases [2]. High expression of HDAC8 is correlated with poor survival and advanced disease in neuroblastoma [3]. While high expression levels of HDAC1, 2, and 3 have been shown to be associated with poor patient outcomes in gastric and ovarian cancers [4,5], HDACis have pleiotropic cellular effects including the arrest of cell growth, cell cycle progression, and the induction of apoptosis [6]. Numerous HDACis are nowadays at various stages of clinical trial development for the treatment of cancers. The natural hydroxamate Trichostatin A has served as a lead compound, for the development of the first approved HDACi vorinostat (SAHA). To date, five HDACis have been approved for the treatment of cutaneous T-cell lymphoma. These drugs can be classified into three chemical families: the class of hydroxamic acids such as vorinostat, panobinostat, and belinostat; the class of cyclic peptides such as romidepsin; and the class of o-amino anilides such as chidamide.

Most HDACis have been developed for hematological malignancies. It has also been suggested that HDACis can overcome the resistance of some drugs. In addition, an increasing number of HDACis have been studied as treatment options for central nervous system diseases such as brain cancer, Alzheimer’s disease, and depression [7,8]. Herein, we state our opinion on the discovery and development strategies undertaken to improve the application of these HDACis.

2. Combination therapy with HDACis in cancer therapy

When used as a single therapeutic agent, approved HDACis demonstrated a narrow therapeutic application mostly for the treatment of T cell lymphoma. Additionally, resistance to HDACis was often observed [9], HDACis have shown limited success in the treatment of solid tumors [10]. Combining anticancer drugs with other chemotherapeutic agents usually led to maximize their efficacy whilst reducing toxicity by administering lower drug doses. It also led to synergistic effects and contributed to the reduction in the potential for the development of resistance [10].

Pancreatic ductal adenocarcinoma (PDAC), the third leading cause of cancer death in the United States, is one of the deadliest forms of cancer, with a five-year survival rate of less than 10%. Recent studies have found that the inhibition of HDACs 1, 2, and 6 cooperates with the anticancer effects of gemcitabine both in vitro and in vivo [11]. Oncogenic K-Ras signaling highly depends on the canonical Ras/MEK/ERK pathway to contribute to pancreatic cancer progression. However, numerous efforts of MEK inhibitors have failed to provide an optimal antitumor effect for pancreatic cancer in practice. The co-administration of a MEK inhibitor with a HDACi MPT0E028 yielded synergistic effects on cell viability suppression both in mutated K-Ras and wild-type pancreatic cancer cells and decreased the tumor volume in an AsPC-1 xenograft model compared to each individual treatment alone [12].

Glioblastoma remains a challenge in oncology in part due to tumor heterogeneity. Simultaneous inhibition of HDAC and Bromodomain proteins results in a pronounced synergistic reduction in cellular viability in patient-derived xenograft glioblastoma cells. Also, in a recent study, it was demonstrated that in orthotopic patient-derived GBM xenografts, the combination treatment including panobinostat, birabresib (OTX015), and sorafenib reduces tumor growth, and culminates in a significant regression of tumors in vivo [13].

Intensifying combining HDACi with alternative anticancer agents both in preclinical and clinical settings could, therefore, be a promising avenue to realize their full therapeutic potential.

3. Selective inhibition of HDAC

In addition to their robust anticancer activity, HDACis are involved in diverse neuroactive functions such as neuroprotection, neurogenesis, and neurological disorders. However, many of these HDACis have failed at various levels of preclinical and clinical trials for central nervous system (CNS) disorders, mostly due to limited efficacy and nonspecific toxicity [14]. Therefore, there is a clear mandate for the design and development of novel selective HDACis to overcome these limitations, which ultimately would lead to potential therapeutics for treating numerous neurological and psychiatric disorders. Multiple forms of HDAC exist, and the development of more effective HDAC inhibitors would benefit from knowing the specific HDAC(s) that are involved in the regulation of synaptic plasticity. Tsai and colleagues generated mice that overexpressed either HDAC1 or HDAC2 specifically in neurons [15]. Mice that overexpressed HDAC2 showed impaired associative learning compared with wild-type (WT) mice, whereas HDAC1-overexpressing mice did not differ from WT mice. HDAC2-overexpressing mice also performed worse than WT and HDAC1-overexpressing mice in tests of spatial learning and spatial working memory. This suggests that HDAC2, but not HDAC1, negatively regulates synapse formation and synaptic plasticity and suggests it to be an interesting target for the HDAC inhibitor. This discovery paves the way for the development of selective HDAC2i that might improve memory in patients with neurodegenerative disorders.

Vorinostat inhibits class I HDACs (1, 2, 3, and 8) as well as class IIb HDAC6. It crosses the blood-brain barrier and shows therapeutic effects in the animal models of various neurological disorders but with nontargeted side effects [16]. Tubastatin-A, a selective HDAC6 inhibitor, has demonstrated therapeutic efficacy in rodent models of cognitive and neurodegenerative disorders [17]. In addition, tubastatin-A shows minimal toxic effects, unlike other HDACis, including vorinostat. However, its low BBB permeability limits its potential to become a candidate for the treatment of neurological disorders.

4. Prodrug concept for hydroxamate HDAC

In acute myeloid leukemia, as a monotherapy protocol, HDACsi led to modest or poor clinical outcomes [18]. Many factors are responsible for this nonoptimal response, such as side effects, fast elimination, poor tissue penetration caused by the metabolically labile and polar hydroxamic acid group. Several prodrug approaches, using carbamates [19], quinone-based moieties [20] and boron [21]-masked hydroxamic acid have been proposed. However, this approach is still in its infancy, and many studies are needed to demonstrate its proof of concept in clinical studies.

5. Expert opinion

Altering gene expression profiles through inhibition of HDACs is currently emerging as a powerful technique in therapy. Although several HDAC inhibitors have been approved for hematological malignancies, their efficacy in solid tumors has not been shown. Exploratory clinical studies can open the way toward a greater application of HDACis in this regard. Recently, a phase III trial of tucidinostat (chidamide) plus exemestane improved progression-free survival compared with placebo plus exemestane in patients with advanced, hormone receptor-positive (HR⁺) and HER2 negative breast cancer that progressed after previous endocrine therapy [22]. These results support the use of HDACis as chemosensitizers to increase the effectiveness of other chemotherapeutic compounds and may thus be an avenue to achieve their full therapeutic potential. On the other hand, numerous multi-target inhibitors have demonstrated an advantage in enhancing anti-tumor efficacy and reducing side effects. Therefore, the research of multi-target inhibitors is also an important direction for the development of next-generation HDACis. Additionally, there have recently been many efforts into the integration of HDAC inhibitor warheads with other targeting functionalities into one compound as dual inhibitory agents, and these have been very successful [23–26].

First-generation HDACis are mainly pan-inhibitors with several side effects, such as fatigue, thrombocytopenia, and neutropenia. Considering the distinct tissue distribution and cellular localization of HDACs, as well as the relationship between specific HDAC isoforms and different diseases, isoform selective HDACis may possess a better therapeutic index and fewer adverse effects that can pave the way for fighting new diseases, such as parasitic diseases. However, the therapeutic advantages of isoform-selective HDACis have not yet been proved clinically and are still being studied.

The development of isotype-specific HDACis is challenging in part due to their high sequence homology between catalytic sites. The design of selective HDACis has made great progress by modifying the cap region and linker of their scaffold. rIt should also be noted that the development of X-ray crystallography and computer-aided drug design may provide further contributions to the design of novel selective HDACis.

HDACis, which have hydroxamic acid as a zinc-binding group (ZBG), are not very stable in vivo and are rapidly metabolized. Vorinostat has a half-live of <2 h and is rapidly glucuronidated, so requires a continuous injection of this compound to reach the desired therapeutic level. These problems can be solved by designing adequate prodrugs or by using a new and more stable ZBG.

An increasing number of HDACis are being investigated for the treatment of central nervous system diseases. Since classical HDACis such as vorinostat, tubastatin, and valproic acid have low brain uptake due to poor blood-brain barrier permeability, this limits considerably their clinical applications for CNS diseases. In the next years, the major challenge, therefore, will be to overcome these difficulties and find a new scaffold which might be a viable starting point for the development of CNS-penetrant HDACis.

In addition to the inhibition of HDAC, the inhibition of other HDACs containing a multiprotein complex could also be of interest. Sin3 (switch independent 3) functions in a transcriptional repressive complex that contains numerous core factors and at least one class I histone deacetylase (HDAC). Sin3B is required for hematopoietic stem cell (HSC) quiescence and its inhibition could force quiescent cancer stem cells to cycle and sensitize them to cytotoxic therapies. Molecules that effectively prevent the assembly of Sin3 complexes also represent an interesting strategy for the treatment of cancer stem cells (CSCs) [27].

In conclusion, evidence continues to mount for the role of HDACis in many diseases such as neurological diseases, inflammation, and HIV. However, there is still plenty of work to do to realize the full value of these drugs for human medicine.

Declaration of Interest

The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer Disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This manuscript has not been funded.