HDAC inhibitors

Histone Acetylation. Figure 1 Histone acetylation is a posttranslational modification of lysine residues of histones. This modification is catalyzed by histone actyl transferases (HATs), which transfer an acetyl group (yellow) from acetyl-Coenzyme A onto the E-amino group of the lysine residue. Histone deacetylation is catalyzed by histone deacetylases (HDACs), which hydrolyze the lysine bound acetyl group. HDAC inhibitors like Trichostatin A (TSA) are known to inhibit the deacetylation reaction in vivo and in vitro. 

In addition to histone deacetylation, histone lysine methylation can also lead to gene silencing which is not blocked by the HDAC inhibitors [6, 51], Several lines of evidence have suggested a connection between cancer and histone lysine methyltrans-ferases (HKMTs) [52], HKMTs catalyze the transfer of methyl group(s) from the cofactor. S -adenosyI-methionine (AdoMet) to some specific lysine residues in the N-terminal histone tails [53, 54], With one exception of Dotl [55], all known HKMTs contain the SET domain which represents a novel structural fold [53, 56], Among SET-domain HKMTs, SET7/9 is one of the best characterized experimentally. It is a... 

Figure 1 Structures of traditional HDAC inhibitors 1-3 and HDAC inhibitors now in clinical trials (4-8).

A series of aryltriazolylhydroxamates were reported as histone deacetylase (HDAC) inhibitors, exemplified by 49 (HDAC IC50 = 9.6 nM) which exhibited activity (L.d. IC50 = 4.5 pg/ mL) in an in vitro antileishmanial assay [48]. 

Other potential approaches involve the use of anti-apoptotic treatments, such as caspase-1 inhibitors or HDAC inhibitors which may interfere with the transcriptional dysregulation seen in Huntington disease. Both approaches have resulted in encouraging results in animal experiments. The use of growth factors has also been suggested as possible treatment for Huntington s disease. 

Crystal structures of a histone deacetylase-like protein (HDLP) and HDAC8 have confirmed a general pharmacophore model for HDAC inhibitors, comprising a cap joined by a hydrophobic linker to a zinc-binding group (ZBG). This model is exemplified by SAHA and the natural product HDACi Trichostatin A (TSA) 2. 

Using a maize HDAC system, aroyl-pyrrolyl-hydroxamides (APHA) had been shown to exhibit 7-78-fold class Ha selectivity when appropriately modified. Continued fine-tuning of the APHA inhibitors, such as meta-fluorine-substituted 20, enabled the generation of an even more selective HDAC inhibitor, with HD1-A IC50 of 0.22 (iM, 176-fold selectivity and which inhibited human HDAC4 but not HDAC1 [53]. 

A large number of patent applications has been filed, most recently describing imidazothiazoles [70], oxazolopyridines [71], benzimidazoles [72], benzothiazoles [73] and imidazopyridines [74] as sirtuin modulators, however it is not yet possible to determine which compound classes will prove most promising. Overall, due to their potential applications as new drug candidates for various indications, the class III HDAC inhibitors are currently a rapidly growing field of interest. 

Like Class I HDACs, all Class II HDACS are inhibited by trichostatin A (TSA). However, unlike other family members, HDAC6 is uniquely resistant to the potent HDAC inhibitors trapoxin-B (Furumai et al, 2001) and sodium butyrate as a... 

Figure 4. Therapeutic strategies to counteract CBP loss of function. CBP loss of function leads to a decrease in histone acetylation levels as well as a decrease in CBP-dependent transcription. Two main approaches can be tested to reverse diis process either resetting HAT functionality or resetting global acetylation levels widi die use of HDAC inhibitors. Whereas both strategies would increase histone acetylation levels, HDAC inhibition would act on a broad range of genes, while CBP activation (overexpression or by a pharmacological approach) would specifically target bodi CBP-dependent histone acetylation and transcription. The structure of some of the HDACi that have been tested in different models, such as small fatty acids and hydroxamic acids, are represented in the boxes...

One of the first HDAC inhibitors to be identified and characterized was sodium butyrate, where it was found to alter the histone acetylation state (Riggs et al, 1977), and further determined to inhibit HDAC activity both in vitro and in vivo (Candido et al, 1978). Almost a decade later trichostatin A (TSA), a fungistatic antibiotic, was found to induce murine erythroleukemia cell differentiation (Yoshida et al, 1987). To date, a wide range of molecules have been described that inhibit the activity of Class I and Class II HDAC enzymes, and with a few exceptions, can be divided into structural classes including (1) small-molecule hydroxamates, such as TSA, suberoylanilide hydroxamic acid (SAHA), scriptaid and oxamflatin (2) short-chain fatty-acids, such as sodium butyrate, sodium phenylbutyrate and valproic acid (VPA) (3) cyclic tetrapeptides, such as apicidin, trapoxin and the depsipeptide FK-228 and (4) benzamides, such as MS-275 and Cl-994 (for reviews see Remiszewski et al, 2002 Miller et al, 2003). Some of these molecules are represented in Fig. 4. 

HDAC inhibitors are also effective in animal models of neurodegeneration demonstrating that these in vitro findings are translatable in vivo. As mentioned above, butyrates are considered to be most effective in vivo in terms of their ability in crossing the blood brain barrier. 

HD In Drosophila models of Huntington s disease, the HDAC inhibitors SAHA and sodium butyrate arrest the progressive neuronal degeneration and lethality (Steffan et al, 2001). SAHA and sodium butyrate have also been demonstrated to extend survival, ameliorate motor deficits and delay characteristic neuropathology in the mouse Huntington s disease model, R6/2 (Ferrante et al, 2003 Hockly et al, 2003). In NaBu-treated animals, animals displayed enhanced acetylation status of histones and pro-survival transcription factors like Spl and reduction in several neuropatho-logical hallmarks like striatal neuronal atrophy (Ferrante et al, 2003). Consistent with the idea that HDAC inhibition relieves transcriptional repression and that protection is downstream of mutant htt, neither SAHA nor sodium butyrate decreased mutant htt expression or aggregates (Ferrante et al, 2003 Hockly et al, 2003). 

Of note, a very recent report shows that SIRTl (sirtuin class of HDAC), that was shown to participate in axonal protection (Araki et al, 2004), could be acting in a neuroprotective way in ALS (Fischer et al, 2005). As noted earlier, this class of HDAC is insensitive to classical HDAC inhibitors, but it should be kept in mind reinstating acetylation homeostasis with classical HDACi might indirectly biased sirtuin s regulations as well. 

Gottlicher M, Minucci S, Zhu P, Kramer OH, Schimpf A, Giavara S, Sleeman JP, Lo Coco F, Nervi C, PeUcci PG, Heinzel T (2001) Valproic acid defines a novel class of HDAC inhibitors inducing differentiation of transformed cells. Embo J 20(24) 6969-6978... 

Interestingly, we have demonstrated a synergistic effect of TNFa and HDAC inhibitor on reactivation of HIV-1 expression in the latently infected U1 cell line (Quivy et al, 2002). Mechanistically, we have demonstrated that HDAC inhibitor prolongs TNFa-induced NF-kB binding to DNA (Quivy et al, 2002 Adam et al, 2003). It is important to note that an array of cytokines, including the proin-flammatory cytokines TNFa and interleukin-1 (inducers of NF-kB), are already copiously expressed in the microenvironment of the lymphoid tissues, which harbor latent viral reservoirs (Navikas et al, 1995) and could therefore, amplify the reactivation potential of HDAC inhibitors used in patients. 

---