Medicinal chemistry of histone deacetylase inhibitors

Today, we are witnessing an explosion of scientific concepts in cancer chemotherapy. It has been considered for a long time that genetic instability in cancer should be treated with drugs that directly damage the DNA. Understanding the molecular basis of malignant diseases shed light on studying phenotypic plasticity. In the era of epigenetics, many efforts are being made to alter the aberrant homeostasis in cancer without modifying the DNA sequence. One such strategy is modulation of the lysine acetylome in human cancers. To remove the acetyl group from the histones, cells use the enzymes that are called histone deacetylases (HDACs). The disturbed equilibrium between acetylation and deacetylation on lysine residues of histones can be manipulated with histone deacetylase inhibitors (HDACi). Throughout the review, an effort will be made to present the mechanistic basis of targeting the HDAC isoforms, discovered selective HDAC inhibitors, and their therapeutical implications and expectations in modern drug discovery.


Introduction
The life of each cell is extremely turbulent. The control of discordant cell responses is mediated through communication between various biomolecules, starting from gene activation or repression, synthesis of mRNA and proteins. Diverse extracellular signals may induce changes in protein synthesis, cellular phenotype and integrity, metabolism and eventually cell survival. Protein biosynthesis in human cells is a costly process and not all cellular stimuli will induce de novo synthesis of proteins. Timewise, posttranslational modifications of certain proteins such as histones present effective control of cellular events via regulation of gene expression (1). Histones are major protein components of chromatin. Octamers consisting of two copies of core histones (H2A, H2B, H3 and H4) wrapped with DNA constitute functional units of chromatinnucleosomes. Being rich in lysine and arginine residues, histone's posttranslational modifications and consequent alteration of net charge of histones provide the basis for epigenetic regulation of gene expression, either by alteration of chromatin compaction or by the recruitment of specific proteins involved in transcriptional regulation (2,3). Numerous histone marks are mapped in human cells and they are considered as epigenetic regulators of gene expression. Small endogenous molecules such as acetyl, methyl, benzoyl, phosphate, crotonyl, succinyl groups can be covalently attached to the amino acid residues (particularly lysine and arginine) (4,5). The modified charged amino acid residues on histones may alter the DNA compaction, thus modifying the gene transcription (6).
The occurrence of histone marks is catalyzed by histone modifying enzymes, which can be classified into three major groups. Histone writers are the enzymes that catalyze the covalent attachment of the aforementioned small molecules, such as histone methyltransferases (HMTs), histone acetyltransferases (HATs). The second group are histone erasers, which modify the histones in the opposite way to writers, and these are histone demethylases (HDMs), histone deacetylases (HDACs). The third group of histone modifying enzymes is referred to as histone readers, which recognize histone modifications and promote the recruiting of numerous chromatin modifiers and transcription factors (7). The unique language of histone modifications (also known as a histone code) changes the function of genes without altering the DNA sequence.
The modification of histones by acetylation plays a key role in the epigenetic regulation of gene expression and is controlled by the balance between histone deacetylases (HDACs) and histone acetyltransferases (HAT). Histone deacetylases are the enzymes that remove the acetyl group from lysine residues in histone proteins on DNA and are widely studied epigenetic erasers due to their involvement in the pathophysiology of numerous malignancies (e.g. breast cancer, pancreatic cancer, hematological cancers) (8,9). Their different tissue distribution, subcellular localization and diverse catalytic activities opened many avenues for drug discovery campaigns of selective HDAC inhibitors. It is worth mentioning that HDACs enzymes are classified in four different classes, according to their homology sequence (7,8). The classes I, II and IV are zinc-dependent metalloenzymes, which use the Zn 2+ ion for catalytic hydrolysis of acetyllysine moieties. Class I is comprised of four isoforms (HDAC1, HDAC2, HDAC3 and HDAC8) (12). Class II is divided in two subclasses -IIa and IIb. The isoforms belonging to subclass IIa are HDAC4, HDAC5, HDAC7 and HDAC9, whereas HDAC6 and HDAC10 belong to subclass IIb (11,13). The third class of HDAC enzymes belongs to nicotinamide adenine dinucleotide (NAD + )-dependent deacetylases, commonly known as sirtuins, and it includes 7 isoforms (SIRT1-7) (14). The class IV HDAC is comprised of the unique HDAC11 isoform (15). It should be noted that HDACs are hydrolyzing other histone acyl-marks, such as N-terminal myristoyllysine and myristoylglycine, crotonyl, octanoyl etc. (16) which suggests their roles in metabolic homeostasis of the cell (17).
HDAC inhibitors (HDACis) are a class of compounds that increase the acetylation of lysine residues on histone proteins, as well as other proteins, by inhibiting the activity of HDACs enzymes. Histone deacetylase inhibitors induce growth arrest, differentiation and apoptosis of cancer cells ex vivo (18), as well as in vivo in tumor-bearing animal models (19), and are now undergoing clinical trials as anti-tumor agents. HDACis comprise structurally diverse compounds that are a group of targeted anticancer agents. The first of these new HDACi, vorinostat (suberoylanilide hydroxamic acid), has received Food and Drug Administration approval for treating patients with cutaneous T-cell lymphoma. In this review, we will outline the medicinal chemistry of selective Histone DeACetylase inhibitors (HDACi), targeting specific isoforms within classes, as well as the rationale for their use in certain therapeutic areas.

Mechanism of acetyl group hydrolysis by HDACs
The mechanism of acetyl group hydrolysis by classical Zn 2+ -dependent HDACs was first studied in bacterial histone deacetylase-like protein (20). There have been numerous mechanistic explanations proposed on the acetyl group cleavage from lysine side chains on histones. The catalytic site of HDACs is described as 14Å long tube-like pocket (Figure 1, sketched with brown tube) in which the catalytic Zn 2+ ion (chelated with two conserved aspartates and one histidine) is located at the bottom of the channel (21). When N-acetyllysine enters the catalytic pocket (stage [A]), the carbonyl oxygen coordinates Zn 2+ ion, whereas the neighboring histidine deprotonates nucleophilic water. In the next stage [B], the generated oxyanion is chelated to the Zn 2+ and the intermediate is hydrogen bonded to the conserved tyrosine. Finally, in the stage [C] the intermediate is decomposed to the acetyl anion and protonated lysine. The active site of HDACs is therefore composed of conserved histidine-aspartate dyads which regulated the pH of the microenvironment (22), tyrosine and Zn 2+ ion, all of which are crucial for the HDAC catalytic activity. Sirtuins contain a highly conserved catalytic core located at the interface of the two domains of enzyme structure -Rossmann fold domain and zinc-binding domain ( Figure  2). In contrast to the other classes of HDACs, the mechanism of deacetylation of sirtuins (class III of HDACs) is Zn 2+ independent and implies the presence of NAD + . Although NAD + is a redox-active metabolite, it is important to note that during deacetylation reaction catalyzed by sirtuins, there is no change of oxidation states of reactants. A simplified mechanism of the reaction is depicted in Figure 2. Once NAD + and N-acetyllysine peptide are bound, the carbonyl oxygen of acetyl group attacks the anomeric position of ribose, cleaving nicotinamide and forming alkylimidate intermediate. After deprotonation of ribose at 2'-OH induced by the presence of conserved histidine residue as a general base, cyclic intermediate is formed which hydrolyze in the last step of reaction to produce 2'-O-acetyl-ADP-ribose. The formation of nicotinamide during catalysis is reversible, and nicotinamide itself is a pan-sirtuin inhibitor (23).

HDAC inhibitors in drug discovery
The story of the first discovered HDAC inhibitor (SAHA, suberoylanilide hydroxamic acid, Vorinostat) remains one of the textbook examples of how serendipity may lead to inspiring breakthroughs in medicinal chemistry. SAHA was discovered after observations that polar solvent DMSO (dimethylsulfoxide) may induce differentiation of murine erythroleukemia cells (MELC) to normoblasts which synthesize hemoglobin (24). Further structure-activity relationship studies led to the conclusion that bishydroxamate derivatives inhibit the growth of MELC and lead to induced erythroid differentiation. (Figure 3A). (25)  From a mechanistic point of view, hydroxamic acid is a well-known zinc binding group (ZBG) which coordinates the catalytic zinc ion (Zn 2+ ) in many human enzymes. Hydroxamic acids are delivered to the bottom of the pocket via aliphatic or aromatic linkers, which mimic the lysine side chain (Figures 3B). The CAP (capping, surface recognition) group of the HDAC inhibitors interacts with the amino acid residues located at the entrance of the catalytic pocket and their presence/bulkiness are considered as important structural features for selective inhibition of certain HDACs.
First sirtuin inhibitors were discovered in 2001 and since then have represented an active area of research (26). Two decades of inhibitor development have resulted in a plethora of chemical scaffolds. Unfortunately, efforts have resulted in only one clinical candidate (27). Structural studies have indicated that many sirtuin inhibitors interact with NAD + -binding site (especially nicotinamide binding sub-pocket) and/or peptide substrate-binding site. High conservation between these two sites across the sirtuin family represents a significant challenge for the optimization of inhibitors' isoform selectivity. Besides selectivity, achieving low nanomolar potency and optimal pharmacokinetics is a long-standing challenge in sirtuin inhibitor drug discovery (27). Sirtuin inhibitors could be roughly divided into two classes: small molecule and mechanism-based inhibitors. Small molecule inhibitors non-covalently interact with amino acid residues from one or both binding sites in the sirtuin catalytic core. On the other hand, mechanism-based inhibitors are usually peptide or peptidomimetic molecules that are designed in such a way to represent substrates for sirtuin catalyzed deacylation. Upon the initiation of the reaction, mechanism-based inhibitors produce longer-lived (or stalled) intermediates that act as bisubstrate (interacting with both binding sites) inhibitory species. The general strategy for the design of mechanism-based inhibitors includes the introduction of warhead moieties such as thioamide or thiocarbamoyl moiety and some derivatives are reviewed in section 8 (28).

Selective class I HDAC inhibitors
Class I of HDAC inhibitors are considered as prototypical epigenetic inhibitors. The nuclear localization of the class I HDACs makes them available for associations in transcriptional complexes (such as NuRD, Sin3A, CoREST) (29). These transcriptional complexes regulate cell survival and death, autophagy, changes in the cell morphology and cell differentiation. In recent years, drugging the HDAC1/HDAC2 isoforms has been validated for the reversal of the HIV latency (30).
The challenge in the development of specific class I HDAC inhibitors lies in the structural biology of these enzymes. The first two isoforms, HDAC1 and HDAC2 share the sequence homology of 86% (31). Owing to this, many reported class I HDAC inhibitors show non-selective profiles. The examples of the class I HDAC inhibitors are presented in Figure 4. Compound 1 (Entinostat) was discovered as a class I HDAC inhibitor, with an inhibitory profile against HDAC1-3 isoforms and low potency to HDAC8. Further crystallography studies with class I HDACs shed light on an additional foot pocket close to the catalytic site. The exploitation of the additional hydrophobic pocket by introducing the aromatic rings in position 5 of 2-aminoanilides (compound 2, Figure 4) improved the selectivity towards HDAC1/HDAC2 isoforms. Merck & Co employed a parallel medicinal chemistry strategy to synthesize, screen and identify selective HDAC3 inhibitors. They identified compound 3 in which the substitution of 2amino group with 2-methylthio group within ZBG clearly demonstrates selective HDAC3 inhibitory profile (32). It is interesting to mention that the HDAC8 isoform is the most studied isoform from a crystallographic point of view. The Protein Data Bank website (https://www.rcsb.org/) has more than 90 deposited structures of HDAC8 in complex with diverse inhibitors. HDAC8 has several features distinct from other isoforms among class I HDACs. The most important structural feature is the flexible surface loop L1 at the N-terminal portion (amino acids from 30 to 36) (33). Moreover, the leucine amino acid residues within the catalytic pocket of HDAC1-3 are substituted by tryptophan in HDAC8, making the Zn 2+ ion at the bottom of the pocket hardly targeted by voluminous 2-aminoanilides (26,27). The outer rim of the catalytic cavity is shaped with F152, F208 and M274, where the F152 residue acts as a gatekeeper. Hence, the most common scaffold used in the design of selective HDAC8 inhibitors is meta-substituted phenylhydroxamate (36). Several selective HDAC8 inhibitors are presented in Figure 5. Interested readers are referred to references (37)(38)(39)(40)(41)(42) where the most recent selective HDAC8 inhibitors are disclosed. Compound 4 (PCI-34051) is developed as a nanomolar HDAC8 inhibitor (IC50 = 10 nM) with more than 200 times lower affinities to other HDACs (43).  Hemijske strukture selektivnih inhibitora HDAC8 izoforme

Selective class IIa HDAC inhibitors
The class IIa HDACs is known to shuttle between the nucleus and the cytoplasm. They exert lower deacetylase activities than class I HDACs, due to the replacement of catalytic tyrosine with histidine (Y976H) (45). The endogenous substrates for these isoforms have recently been disclosed, such as myosin heavy chain (MyHC) isoforms, peroxisome proliferator-activated receptor gamma co-activator 1alpha (PGC-1α), and heat shock cognate 71 kDa protein (Hsc70) (46). Nowadays, there are validated data that support targeting class IIa HDACs in diabetes (47), neurological diseases (48) and heart failure (49).
In 2013, Lobera et al. (50) ran a high-throughput screening of the two-million GlaxoSmithKline's library of compounds and discovered inhibitors with trifluoromethyloxadiazolyl moiety as a nonchelating ZBG (compound 6, Figure 6). It is demonstrated that one fluorine and oxygen from the 1,2,4-oxadiazole interact with Zn 2+ ion via weak electrostatic interactions. By the end of 2019, Novartis had published highly potent class IIa HDACi (compound 7) which presents a valuable chemical tool for studying skeletal muscle atrophy and sarcopenia (46). Hemijske strukture inhibitora HDAC IIa klase

Selective class IIb HDAC inhibitors
Histone deacetylases 6 and 10 differ from other HDAC classes for diverse reasons. Both isoforms contain two domains, HDAC6 has two catalytic deacetylase domains, however, HDAC10 has one deacetylase domain and another one deficient in amino acids required for deacetylase activity (51). Furthermore, HDAC6 and HDAC10 isoforms are mainly located in the cytoplasm. HDAC6 controls the acetylation status of alpha-tubulin and/or microtubules (52) and HDAC10 is deacetylating polyamines, such as N 8 -acetylspermidine (51). HDAC6 is also referred to as tubulin-deacetylase (TDAC), whereas HDAC10 is considered as polyamine deacetylase (PDAC). The architecture of these isoforms is well-studied by Christianson, whose crystallography studies on zebrafish models (PDB codes HDAC6: 5EDU and HDAC10: 5TD7) (53,54) expanded our understanding of the protein structure, function and pharmacomodulation of class IIb HDACs. Studies with peptide constructs revealed that the second catalytic domain (CD2) of HDAC6 is responsible for the tubulin-deacetylase activity and it is considered as a valid target for computer-aided drug discovery (CADD) campaigns.
It is proposed that every pharmacophoric feature (chemistry of the ZBG, linker and CAP group) contributes to the selectivity of the HDAC6 inhibitors (55). Computational (56,57) and experimental (44,45) studies published in our group show that both the alkylhydroxamates (8,9) and phenylhydroxamates (10) may inhibit the HDAC6 isoform selectively (Figure 7). Apart from hydroxamic acid derivatives, 3-hydroxypyridin-2-thiones (11) are reported as ZBG which specifically inhibits the HDAC6 isoform (60). Compounds with significantly voluminous CAP group, such as tubacin (9), were identified as valuable chemical tools to study HDAC6 inhibition. Surprisingly, capless HDAC6 inhibitors (Figure 7) demonstrated nanomolar inhibitory profiles, illustrating the importance of the entropic contribution to HDAC6-inhibitor binding selectivity (61). These fragment-like molecules (12,13) inhibit the HDAC8 isoform in submicromolar concentrations, which should be regarded as an important task while designing novel selective HDAC6 inhibitors. Hemijske stukture selektivnih inhibitora HDAC6 Polyamine deacetylase (HDAC10) was studied in detail by Miller et all (62). This group found that tubastatin A (compound 15, previously shown as a selective HDAC6 inhibitor, (63)) was a more potent HDAC10 inhibitor, due to the presence of basic nitrogen in the CAP group. This tertiary nitrogen establishes a salt bridge with the gatekeeping residue (Glu272) which is required for polyamine deacetylase activity. When tetrahydro-β-carboline scaffold is replaced with the 3,4-dihydropyran (compound 16), the compounds show selectivity towards the HDAC6 isoform (Figure 8). N 8 -acetylspermidine derivatives in complex with zebrafish HDAC10 were studied by Xray crystallography (64) and it was found that the most potent compound is thioacetate prodrug 16 which is hydrolyzed to give thiolate active compound, as an HDAC10 inhibitor (17, Figure 8).

Selective class IV HDAC inhibitors
The single representative in class IV, HDAC11, is the smallest isoform among metal-dependent HDACs. It has recently been shown that HDAC11 has a more efficient glycine demyristoylation activity than deacetylase activity. Its implication in the IFNsignaling pathway has become advantageous for the drug design of novel antiviral and anticancer compounds (51,52).
There is a small number of available selective HDAC11 inhibitors, some of which are presented in Figure 9. It is assumed that the natural product garcinol 18 could chelate Zn 2+ ion via the enol form of the β-diketone moiety or with the catechol moiety (67). An interesting finding was recently published by Chellappan and colleagues, demonstrating that selective HDAC11 inhibitors (compound 19) are potent in preventing the growth of adenocarcinoma stem cells when co-cultured with cancer associated fibroblasts (CAFs) (68). To conclude, compounds 19 and 20 are potent lysine demyristoylases (69), whereas compound 20 is pure and cell-permeable selective HDAC11 inhibitor. Hemijske strukture selektivnih inhibitora HDAC11

Selective class III HDAC inhibitors (sirtuins)
Due to their involvement in many biological processes in the human organism, seven members of the sirtuins family are considered as potential targets in the treatment of several diseases, including cancer, neurodegenerative and cardiovascular diseases (70). In this section, some of the most important achievements and latest breakthroughs in the field of the medicinal chemistry of sirtuin inhibitors are presented.

Non-selective sirtuin inhibitors
According to the sequence alignment of seven sirtuins, the most conserved domains are recorded between SIRT1-3 isoforms, making it particularly difficult to achieve the ultimate goal of selectivity (71). Non-selective sirtuin inhibitors presented in this section of the review are depicted in Figure 10. Back in the early 2000s, using phenotypic yeastbased high-throughput screenings, two groups independently discovered the first SIRT inhibitors -splitomicin (compound 21) and sirtinol (compound 22) (26,72). Both compounds were based on a β-naphtol scaffold. Considering the sensitivity on hydrolysis of the δ-lactone moiety in splitomicin, further efforts resulted in cambinol, 23, and its derivatives as a well-suited compound for in vitro studies. Cambinol was shown to be well tolerated and to inhibit the growth of Burkitt lymphoma xenografts in mice (73). Salermid, 24, reverse amide of sirtinol, 22, was designed utilizing CADD techniques (74). Another group of non-selective inhibitors identified through computational approaches, specifically virtual screening, was the thiobarbiturate group of inhibitors. Among this group, indole derivative, compound 25, exhibited slight selectivity towards SIRT1 (75).
Tenovins are another class of non-selective SIRT1/2 inhibitors, discovered through cell-based screening aimed at discovering tumor suppressor p53 activators. Among tenovins, Tenovin-6, 26, is now commonly used in in vitro studies due to favorable solubility (76). The extensive anticancer activity of tenovin-6 has been thoughtfully studied in the last years (77). Pseudopeptidic mechanism-based inhibitors based on thiourea, 27, were reported as well (78). Some of the most potent small molecule SIRT1/2/3 inhibitors reported so far were thieno[3,2-d]pyrimidine-6-carboxmamides (e.g. compound 28). Crystallographic analysis indicated that common carboxamide of 28 binds in the nicotinamide portion of the NAD + -binding site and the aliphatic portions of the inhibitors extend through the substrate channel (79).

Selective SIRT1 inhibitors
The only sirtuin inhibitor that reached clinical studies was the moderately selective SIRT1 inhibitor EX-527, also known as Selisistat 32 (Figure 11). Namely, SIRT1 inhibitor EX-527 was tested in a clinical trial for Huntington's disease (83). This clinical trial was dropped after phase II, even though EX-527 was found to be safe and well tolerated. EX-527 was discovered in high-throughput screening on SIRT1 and it was described as a low molecular weight, cell-permeable, orally bioavailable, and metabolically stable inhibitor (84). The very first reported co-crystal structure of SIRT1 was in a complex with EX-527, which helped to decipher the structure-activity relationship of EX-527 and its derivatives. Indole moiety of EX-527 interacts with substrate binding site, while amide moiety is responsible for displacing nicotinamide of NAD + and it is crucial for the inhibitory activity of this group of inhibitors (85). Indole appears to be one of the privileged scaffolds in selective SIRT1 inhibitors design (25,27) since the presence of this heterocycle in the structure of SIRT inhibitors shifts selectivity towards SIRT1 (27,86). In the group of mechanism-based inhibitors, one example is compound 33. Although this inhibitor should not be strictly considered as a mechanism-based inhibitor, it represents the most selective SIRT1 inhibitor reported so far in this group of sirtuin inhibitors (87).

Selective SIRT2 inhibitors
The latest breakthrough in the field of medicinal chemistry of selective SIRT2 inhibitors is the discovery of a hydrophobic "selective pocket" in the structure of the SIRT2 (88). This discovery provided a rationale for the design of the selective SIRT2 inhibitors and many inhibitors exploiting the "selectivity pocket" have been described.
Besides 2-((4,6-dimethylpyrimidin-2-yl)thio)acetamide motif, there are several reports on other SIRT2 inhibitors which exploit the "selectivity pocket". One group of highly selective thienopyrimidinones, 37, was discovered using structure-based virtual screening, and characterized in terms of the binding mode through crystallographic studies (92). Another group of anilinobenzamides was also described to bind to a "selectivity pocket" (compound 38) (93). This group was further optimized into the mechanism-based inhibitor, 39, by exploiting substrate binding site which resulted in highly potent and selective inhibitor with antiproliferative activity in breast cancer cells and potent neurite outgrowth activity in neuro-2a (N2a) cells (94). Another example of inhibitor binding to the "selectivity pocket" is 40, identified through the high-throughput screen (95). Among mechanism-based inhibitors which occupy the "selectivity pocket" of SIRT2, thiomyristoyl compound 41 was thoughtfully tested in various human cancer cells and mouse models of breast cancer (96). The results indicated that 41 has broad anticancer effects and limited toxicity, qualifying this group of inhibitors as promising for future development. Related groups of potent, selective, stable in serum and watersoluble inhibitors have recently been reported (97).

Selective SIRT3 inhibitors
A SIRT1 activator SRT1720 (compound 42, Figure 12) was one of the first reported SIRT3 inhibitors with significant potency and selectivity (98). An interesting strategy for developing SIRT3 peptide inhibitor, which implies the introduction of triphenylphosphonium moiety as a mitochondrial targeting moiety, was used to yield nonselective SIRT1-3 inhibitor with SIRT3 specific biological response (compound 43, Figure 12). This compound has shown comparable effects to less potent, but more SIRT3 selective cambinol analog 44 on the induction of autophagy and cell death in diffuse large B cell lymphomas (99).
Thiourea mechanism-based inhibitors, 45, derived from a group of inhibitors represented by 33, with significant selectivity towards SIRT3 were also described (100).

Selective SIRT4 inhibitors
SIRT4 has remained without a selective inhibitor until the moment of writing this review. With crystal structure and activity assays for SIRT4 emerging just recently, it is expected that the discovery of selective inhibitors will happen soon enough. Nicotinamide and suramin, as pan sirtuin inhibitors, were characterized on SIRT4 inhibition using currently developed assays. It was reported that SIRT4 is the most sensitive on nicotinamide inhibition among sirtuins, which could represent a good starting point for future fragment-based optimization campaigns (101,102).

Selective SIRT5 inhibitors
Many selective and potent inhibitors of SIRT5 reported so far are in the class of a mechanism-based peptidomimetic inhibitors. One of the most promising inhibitors reported to date is thiourea-based 46 (Figure 12) (103). However, a recent study, using systematic shortening of available SIRT5 peptidomimetic inhibitors, suggested (S)-3-(2naphthylthio)succinylamide scaffold (47, Figure 12) as a useful starting point for the systematic development of small molecules that potently and selectively target SIRT5 structure (104).

Selective SIRT6 inhibitors
Significant achievements have been made in SIRT6 inhibitors discovery over the past few years. Utilizing the crystal structure of SIRT6 in complex with adenosine diphosphate ribose as a starting point for in silico structure-based screening, Parenti et al. reported the first selective inhibitors of SIRT6, among which salicylate-based compounds was singled out as the most promising for future optimization (105). Further optimization of obtained salicylate-based compound resulted in compound 48 with improved potency and selectivity exhibited to sensitize pancreatic cancer cells to gemcitabine (Figure 12) (106).
Trichostatin A, 49 an inhibitor of HDACs from class I and II, was discovered to inhibit SIRT6, but not other SIRT isoforms. Although 49 inhibits HDACs through a zinc binding mechanism, the inhibition of SIRT6 was unrelated to the coordination of the zinc which was validated by crystallographic studies. This is the first hydroxamic acid among sirtuin inhibitors reported so far (107).
Among natural products, quercetin derivatives catechin gallate (50a) and gallocatechin gallate (50b) were reported to be potent inhibitors of SIRT6. According to the recently reported crystal structure, catechin gallate occupies the same allosteric binding site on SIRT6 as SIRT6 activators (108,109). Potent mechanism-based peptidomimetic inhibitors of SIRT6 were also described. Unfortunately, these inhibitors demonstrated a lack of selectivity (110).

Selective SIRT7 inhibitors
The first potent mechanism-based cyclic peptide SIRT7 inhibitors were reported just recently (51, Figure 12). However, they lack selectivity (111). At the same time, through in vitro enzymatic screening, a potent small molecule inhibitor of SIRT7 was reported (52, Figure 12). Despite reporting inhibition of cancer growth in vivo on a mice model of uterine sarcoma, the authors of the study didn't report selectivity of the inhibitor across sirtuin family (112). The crystal structure of SIRT7 has not yet been discovered, except N-terminal fragment.
The clinically used HDAC inhibitors are licensed for hematological cancers, specifically in the therapy of T-cell lymphoma and multiple myeloma (113).

Conclusions
It is of paramount importance to continue the research of histone deacetylase inhibitors in the medicinal chemistry. So far, the global oncology market has been introduced with five HDAC inhibitors, and none of the SIRT inhibitors. Though the great improvements in our understanding of the chemical biology of HDAC and SIRT inhibitors, the puzzle should be completed with several objectives: a) The in vitro assays for HDAC and SIRT inhibitory profiles should be revalidated to assess other catalytic activities rather than deacetylation, b) Nanomolar selective SIRT inhibitors are highly welcome to further probe NAD +dependent control of acetylome and c) Design and synthesis of novel HDAC and SIRT inhibitors should be conducted with data on HDAC-protein and SIRT-protein crosstalk in cellular signaling.