HDAC inhibitors were first identified and isolated from natural sources (reviewed by
Yoshida et al., 2003). In the intervening two decades, the list of HDAC inhibitors has expanded to include hydroxamic acids, short-chain fatty acids, boronic acids, α-keto acids, cyclic tetrapeptides, benzamides, ketones, isothiocyanates, organosulfur compounds, selenium-based compounds and their metabolites, and other miscellaneous agents (
Minucci and Pelicci, 2006;
Delage and Dashwood, 2009a;
Lane and Chabner, 2009;
Nian et al., 2009a,
b;
Suzuki et al., 2009;
Desai et al., 2010;
Noureen et al., 2010). Based on the features of the active site pocket in the presence and absence of bound ligands (
Finnin et al., 1999;
Vannini et al., 2004,
2007;
Somoza et al., 2004;
Bottomley et al., 2008;
Dowling et al., 2008;
Schuetz et al., 2008;
Ficner, 2009), and computational modeling
in silico (
Vannini et al., 2007;
Nian et al., 2008,
2009b;
Ortore et al., 2009;
Suzuki et al., 2009;
Wang, 2009;
Oger et al., 2010), numerous HDAC inhibitor candidates have been identified. These compounds typically have a functional group that interacts with the zinc atom in the enzyme pocket, a spacer “arm” that fits into the channel near the active site, and in many (but not all cases) a cap group that associates with residues near the surface.
Before their mechanisms of action were elucidated, small molecule hydroxamic acids and cyclic tetrapeptides were observed to alter the differentiation status of cancer cells in culture (reviewed by
Myzak and Dashwood, 2006b;
Santini et al., 2007;
Jones and Steinkühler, 2008).
Yoshida et al. (1990) were the first to report on the potent HDAC inhibitory activity of TSA, a natural compound isolated from
Streptomyces platensis. Subsequent studies showed that TSA reversed the morphological transformation of oncogenic
ras-transformed NIH3T3 cells (
Futamura et al., 1995). In addition, TSA increased global histone H3 and H4 acetylation, enhanced the expression of hepatocyte-specific genes, and induced hepatocyte differentiation in human hepatoma cells (
Yamashita et al., 2003). In human embryonic kidney 293 (HEK293) cells, the glutathione
S-transferase (GST) inhibitor ethacrynic acid potentiated the effects of TSA (
Myzak et al., 2004), implicating the mercapturic acid pathway in the metabolism of this prototype HDAC inhibitor. The mercapturic acid pathway is a glutathione-dependent pathway that plays a critical role in the detoxification of a large number of foreign compounds (also known as xenobiotics). This pathway is modulated by many factors, including dietary constituents (
Higdon et al., 2007; see also sulforaphane text below). In principle, therefore, nutrient interactions that induce the mercapturic acid pathway might lower the efficacy of TSA and structurally related HDAC inhibitors
in vivo. This might account for the fact that TSA shows no effect in animal models due to its “metabolic instability” (
Masuoka et al., 2008).
Due to these concerns, alternative hydroxamate-based HDAC inhibitors have been developed. Vorinostat (suberoylanilide hydroxamic acid, SAHA) has been described as hitting “the happy medium … potent enough to be useful and tolerated in patients” (
Marks and Breslow, 2007). Early Phase I studies in humans suggested that vorinostat was well-tolerated (
Kelly et al., 2003), had linear pharmacokinetics and good bioavailability (
Kelly et al., 2005), and was effective in hematologic malignancies, including Hodgkin’s disease and subtypes of non-Hodgkin’s lymphoma (
O’Connor et al., 2006). Phase 2 trials of vorinostat demonstrated activity in patients with cutaneous T-cell lymphoma (
Duvic et al., 2007) and modest single-agent responses in patients with glioblastoma multiforme (
Galanis et al., 2009). Other clinical trials have been conducted with vorinostat, alone and in combination with cancer therapeutic agents (
Fouladi et al., 2010;
Kadia et al., 2010;
Ramalingam et al., 2010;
Wilson et al., 2010). Marked interindividual pharmacokinetic variability has been observed with vorinostat, possibly related to pharmacogenetic influences on glucuronidation (
Kang et al., 2010), or to dietary factors that modulate the mercapturic acid pathway (
Higdon et al., 2007).
Like TSA, trapoxin was shown to induce morphological reversion in transformed NIH3T3 fibroblasts (
Itazaki et al., 1990;
Yoshida and Sugita, 1992). Subsequent work demonstrated that trapoxin was an irreversible HDAC inhibitor, and that chemical reduction of the epoxide group abolished the inhibitory activity (
Kijima et al., 1993). The latter observation hinted at the possibility that reductive metabolism might play a role in lowering the efficacy of trapoxin and structurally related HDAC inhibitors
in vivo. Trapoxin resembles TSA in lacking efficacy in animal models due to the “metabolic instability” of the parent compound (
Masuoka et al., 2008).
On the other hand, cellular reduction of the disulfide bond in depsipeptide (FK228) generates a more active compound, most likely a mercaptobutenyl intermediate that fits into the HDAC pocket (
Desai et al., 2010). This HDAC inhibitor was first isolated as a fermentation product from
Chromobacterium violaceum (reviewed by
Masuoka et al., 2008). FK228 has progressed to clinical trials under the name romidepsin, with evidence for “significant and sustainable single-agent activity and an acceptable safety profile” (
Whittaker et al., 2010). Depsipeptide thus provided one of the earliest examples of metabolism generating an HDAC inhibitor, but other examples are now known, including the various compounds from dietary sources (see below).
HDAC inhibitors were discovered based on their ability to induce differentiation in cancer cells, and this continues to be an active area of research. For example, neuroblastoma cells differentiate in response to HDAC8-selective inhibitors or targeted knockdown of HDAC8 (
Oehme et al., 2009b), and human leukemia differentiate after treatment with HDAC inhibitors FK228 and sodium phenylbutyrate (
Savickiene et al., 2010). Sodium phenyl-butyrate has been used clinically in the treatment of disorders such as maple syrup urine disease (
Brunetti-Pierri et al., 2010), and there is growing interest in the neuroprotective properties of this compound and its metabolites (
Gardian et al., 2005;
Ryu et al., 2005;
Petri et al., 2006;
Hogarth et al., 2007;
Ebbel et al., 2010). A recently completed Phase 2 study in patients with amyotrophic lateral sclerosis (Lou Gehrig’s disease) concluded that blood levels of phenylbutyrate, and of its primary metabolite phenylacetate, increased with dosage, and that 9 g/day was effective for improving histone acetylation status (
Cudkowicz et al., 2009). Phenylbutyrate shares structural features with the antiepileptic agent valproic acid (
Göttlicher, 2004), and with the oldest known dietary HDAC inhibitor, butyrate.