HDAC inhibitors in liver cancer: which route to take?

KEYWORDS:

• Hepatocellular carcinoma
• epigenetics
• histone acetylation
• histone deacetylase inhibitor
• drug combination

1. Introduction: current trial results on HDACi for HCC

During the last years, our knowledge about epigenetic regulation mechanisms in normal human physiology and homeostasis as well as about the implications of their de-regulation (especially hypermethylation of DNA and hyperacetylation of histones) in human degenerative diseases and cancer has greatly increased. Based on convincing experimental findings regarding mechanistic insights into epigenetic dysbalances, further investigations on therapeutic approaches have been intensified in parallel [1]. Nevertheless, the translation of basic research and clinically relevant application has not yet really been achieved, whereby clinical studies in phase I to II have already been started and are ongoing [2].

As comprehensively summarized and discussed by authors of the review ‘Unmasking the diagnostic and therapeutic potential of histone deacetylases for liver cancer’, epigenetics is essentially involved in hepatic carcinogenesis via aberrant histone acetylation [3]. By this, classical hallmarks of cancer such as proliferation, cell cycle regulation, differentiation, apoptosis, and neo-angiogenesis as well as migration have been shown to be epigenetically influenced through dysregulated histone deacetylases (HDACs) based on various in vitro and in vivo experiments with different human liver cancer cell lines. Consecutively, the in-situ-expression pattern of some of the HDACs in resected liver cancer specimens could be related to clinical endpoints like progression-free survival or overall survival [4]. Therefore, the application of HDAC inhibitors (HDACi) seems to be a very attractive therapeutic option in liver cancer [5], especially as liver cancer is still associated with late-stage presentation, progressive disease, and limited therapeutic strategies – currently resulting in a very low 5-year overall survival rate of 3–11% for regional and distant stage of liver and intrahepatic bile duct cancer at primary diagnosis [6]. While the mentioned clinical trials suggest very optimistic effects at first glance, nevertheless, a detailed and critical analysis reveals a more realistic situation [7–9]: in the study by Yeo et al., Belinostat (inhibitor of all zinc-dependent HDAC enzymes) was applied in phase II clinical trial for unresectable HCC patients as monotherapy without combination with other pharmacological agents showing a PFS and OS of 2.6 and 6.6 months [8]. The SHELTER study (Resminostat (an oral pan-HDACi against HDAC 1, 2, and 3) plus sorafenib as second-line therapy of advanced hepatocellular carcinoma) found a median time to progression and overall survival of 1.8 and 4.1 months for resminostat and 6.5 and 8.0 months for the combination demonstrating a survival benefit of 3.8 months for the combinatory treatment strategy [7]. Finally, the study by Tak et al. with first-line combination, therapy of sorafenib plus resminostat versus sorafenib monotherapy for advanced hepatocellular carcinoma in east Asian patients revealed no clear overall survival benefit [9]. Interestingly, patient stratification of this clinical trial could be related to more favorable outcome parameters with time to progression or overall survival time up to 6.9 or 13.1 months, respectively. The common denominator of these clinical trials is a highly selective patient group with progressive, unresectable, locally advanced or metastatic HCCs indicating a very bad ‘starting position’ for treatment with HDACis combined with low numbers of patients in the following-up. Furthermore, the application dose and timing of resminostat or belinostat were fixed without any real dose escalation. Taken together, the situation for treatment with HDACis is currently rather difficult: these drugs are mainly applied in the patients in a rather poor clinical condition and mostly rather progressive status of their HCC leading to unsatisfactory survival times and outcome.

2. Expert Opinion - HDACi for HCC: which route to take?

Nevertheless, (i) what are the promising aspects and (ii) what are the currently rather frustrating questions of HDAC-based epigenetic treatment strategy in liver cancer?

• (i) First, as already mentioned above, several in vitro, in vivo and in situ analyses showed a significant HDAC expression in human liver cancer indicating that HDACs seem to play a key role in the cancerogenesis of human liver cancer [4,5]. However, it should be kept in mind that some HDACs could promote and some rather inhibit the hallmarks of cancer indicating the double and contradictory faces of HDACs. Furthermore, all of the applied HDAC inhibitors showed plausible anti-cancer profiles with ‘classical’ anti-proliferative and pro-apoptotic effects in HDACi-treated liver cancer cells. Furthermore, as previously shown, HDACis interact with ‘other’ cancer-related mechanisms such as endoplasmatic reticulum stress [10] or autophagy [11] in human liver cancer which could be targetable by pathway-specific drugs. Here, new routes are opened for further combinatory treatment strategies. Finally, HDACis regulate miRNAs [12], which are centrally involved in human liver cancerogenesis and are thus linked to HDAC-based epigenetic regulations [13].

• (ii) However, most of the applied HDACis are not HDAC class-specific, mostly inhibiting one or more HDAC classes. Until now, preceding analysis of the methylation/acetylation status or HDAC class-specific expression pattern in liver cancer specimen prior to HDACi treatment for assessing/predicting the efficiency of HDACis are completely missing in the above mentioned clinical studies [7–9]. Investigations on combinatory treatment strategies are only partially hypothesis- or in silico-based, but mostly rather based on clinically established treatment schedules. Therefore, intensive preclinical (in vitro) research should be combined with in situ analyses for developing predictive biomarkers in order to allow for selection of promising HDACis in human liver cancer.

Nevertheless, the application of HDACis seems to be very promising options in the future, when considering human diseases showing significant therapeutic success after long-term epigenetic treatment, e.g. in patients with myelodysplastic syndrome and acute myeloid leukemia with the ‘hypomethylating’ agent like 5-azacitidine or decitabine [14]. Of note, a second generation DNA methyltransferase inhibitor (guadecitabine, SGI-110) which is resistant to cytidine deaminase showed promising results in preclinical models of HCC. Jueliger et al. [15] demonstrated SGI-110 to efficiently inhibit HCC xenograft growth and vascularization and based on the results of Liu et al. [16], SGI-110 might function for HCC treatment via a three epigenome-based mechanisms: re-repression of tumor suppressor genes by i) promotor demethylation and ii) reversal of PRC2-mediated silencing as well as iii) increasing immunogenicity by up-regulation of endogenous retroviruses [17]. As shown for triple-negative breast cancer cells, a combination of SGI-110 with HDACi’s might further increase the anti-tumor effects – probably by reverting (reprogramming) the EMT phenotype (epithelial-mesenchymal transition) of cancer cells [18].

For HDACis, the following serious questions and challenges regarding the three ‘d’s’, i.e. ‘drug combination’, ‘drug development’, and ‘drug timing’, need to be solved in order to significantly improve the clinical efficiency in the future:

2.1. Drug combination

Currently, sorafenib and regorafin are combined with HDACis in clinical trials [7–9], which however does not reflect the heterogeneous molecular pathways of human liver cancer. Additionally, no specific marker profile gives conclusive hints for the application the classic drugs. A practical approach seems to be the molecular analysis of tumorigenic key drivers and possible druggable mutations in the liver cancer specimen of each patient using cancer-based xenografts in combination with high-throughput molecular platforms with next-generation sequencing before starting a specific HCC treatment. In relation to the increasing impact of the tumor mutation burden, the epigenetic tumor-associated effects should be investigated using whole-genome methylation and acetylation analyses. These large and complex data sets must be integrated to find the predictive and precise combinatory treatment schedule for each HCC [19].

2.2. Drug development

First of all, highly HDAC class-selective HDACi should be developed to target HDACs and not groups or complete classes to reduce pleiotropic and off-target effects of the already established pan-HDACis [20]. Further on, chemical technologies could be applied to combine HDACi with another enzymatic activity in order not to inactivate, but to irreversibly cleave the target unregulated HDAC in the liver cancer tissue based on proteolysis-targeting chimeras [21].

2.3. Drug timing

The efficiency of parallel or serial drug combinations must be proven, since HDACi may reverse or inhibit classical drug resistance. Additionally, the role of drug combinations needs to be investigated to assess the additive or synergistic therapeutic crosstalk of classical drugs and HDACis using bioinformatic platforms [22].

Taken together, HDACis have the potency to become the ‘blockbusters’ in human cancer and especially in human liver cancer in future if our experimental and clinical ‘tasks’ are adequately performed within the next years.

Declaration of interest

This paper was not funded. The authors have 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 paper was not funded.