ATP Citrate Lyase Inhibitors as Novel Cancer Therapeutic Agents

14 Pages·2012·1.07 MB·English

154 Recent Patents on Anti-Cancer Drug Discovery, 2012, 7, 154-167
ATP Citrate Lyase Inhibitors as Novel Cancer Therapeutic Agents
Xu-Yu Zu1,#, Qing-Hai Zhang1,#, Jiang-Hua Liu1, Ren-Xian Cao1, Jing Zhong1, Guang-Hui Yi2,
Zhi-Hua Quan1 and Giuseppe Pizzorno3,*
1Clinical Research Institution, the First Affiliated Hospital, University of South China, Hengyang, Hunan 421001, P.R.
China; 2Institute of Cardiovascular Disease, School of Medicine, University of South China, Hengyang, Hunan 421001,
P.R. China; 3Director, Human Health and Environment Program, Desert Research Institute Summerlin, 10530 Discov-
ery Drive, Las Vegas, NV 89135, USA
Received: November 1, 2011; Accepted: January 12, 2012; Revised: February 6, 2012
Abstract: ATP citrate lyase (ACL or ACLY) is an extra-mitochondrial enzyme widely distributed in various human and
animal tissues. ACL links glucose and lipid metabolism by catalyzing the formation of acetyl-CoA and oxaloacetate from
citrate produced by glycolysis in the presence of ATP and CoA. ACL is aberrantly expressed in many immortalized cells
and tumors, such as breast, liver, colon, lung and prostate cancers, and is correlated reversely with tumor stage and differ-
entiation, serving as a negative prognostic marker. ACL is an upstream enzyme of the long chain fatty acid synthesis, pro-
viding acetyl-CoA as an essential component of the fatty acid synthesis. Therefore, ACL is a key enzyme of cellular lipo-
genesis and potent target for cancer therapy. As a hypolipidemic strategy of metabolic syndrome and cancer treatment,
many small chemicals targeting ACL have been designed and developed. This review article provides an update for the
research and development of ACL inhibitors with a focus on their patent status, offering a new insight into their potential
application.
Keywords: ACL inhibitors, ATP citrate lyase, cancer therapy, citrate, lipogenesis, small chemicals.
1. INTRODUCTION enzymology, mechanism-based and active site-directed in-
hibitors have been developed. This review article updates the
Cancer treatment has been improved considerably during
current research and development of ACL small molecule
the past 50 years. This malignant disease, however, still
inhibitors, with a focus on their patent status.
holds its frightening impact and only about 50% of patients
could be cured [1]. Increasing evidence has highlighted the
2. ATP CITRATE LYASE
importance of cellular metabolism in carcinogenesis [2-4].
Metabolic changes, such as increased lipogenesis, frequently 2.1. Structure of ATP Citrate Lyase
occurs in malignant cells and the impact of metabolic dys-
ATP citrate lyase [ATP citrate (pro-3S)-lyase, ACL or
regulation on tumor development and progression has long
ACLY; EC 4.1.3.8] is an extra-mitochondrial enzyme [18].
been recognized [3, 5]. ATP citrate lyase is a key enzyme
In prokaryotic and lower-rank eukaryotic (e.g. fungi) cells,
linking glucose and lipid metabolism, which converts citrate
ACL is composed of two subunits, but in mammalian cells,
produced by glycolysis to acetyl-CoA and oxaloacetate in
ACL is a 110kDa polypeptide forming a functional homo-
the presence of ATP and coenzyme A. The acetyl-CoA is in
meric tetramer [19]. In humans, there are two ACL isoforms.
turn used for cholesterol and long-chain fatty acid biosynthe-
The ACL isoform-I consists of 1101 amino acids, and iso-
sis [2, 6-10]. Hereby, ACL acts as a mediator between in-
form-II is 10 residues shorter [20]. ACL protein contains five
creased lipogenesis and so-called Warburg effect in cancer
functional domains, named from N-terminus domains 3, 4, 5,
cells. Up to date, numerous reports have shown marked ele-
1 and 2 Fig. (1). Domains 1 and 2 make up the (cid:1)-subunit
vation of ACL expression and enzyme activity in immortal-
(residues 487-820) and domains 3-5 compose the (cid:2)-subunit
ized cells and tumors, including urinary bladder, breast,
(residues 2-425). Domain 1 binds CoA and domain 2 con-
liver, stomach, colon, lung, brain and prostate cancers [11-
tains a phosphorylation site His765, regulating enzyme activ-
17]. Reduction of ACL activity with genetic or pharma-
ity [20]. Domains 3 and 4 adopt an ATP-grasp fold and bind
cologic strategies significantly inhibits cancer cell prolifera-
nucleotide [6, 21, 22]. Domain 5 interacts with domain 2,
tion in a dose-dependent manner and suppresses tumor
producing one of the two "power helices" to whose amino-
growth in animals. As increased understanding of ACL
terminus the phosphohistidine residue bind to in the phos-

phorylated ACL [23-25]. Between domains 5 and 1 lies a
*Address correspondence to this author at the Director, Human Health and stretch of residues that can be phosphorylated on three serine
Environment Program, Desert Research Institute Summerlin, 10530 Discov- or threonine residues [26-28]. When citrate binds to ACL, a
ery Drive, Las Vegas, NV 89135, USA; Tel: [phone];
loop formed by residues 343-348 interacts through hydrogen
Fax: [phone]; E-mail: [email protected]
#These authors contributed equally to this work. bonds with hydroxyl and carboxyl groups on the prochiral
2212-3970/12 $100.00+.00 © 2012 Bentham Science Publishers
ACL as Novel Cancer Therapeutic Target Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 2 155
(cid:2) (cid:2)(cid:3)(cid:4)
(cid:5)(cid:3)(cid:6)

(cid:5)(cid:2)(cid:7) (cid:7)(cid:6)(cid:4)
(cid:3)
(cid:7)(cid:8)(cid:7)

(cid:3)

(cid:9) (cid:5) (cid:6) (cid:10) (cid:3) (cid:11)(cid:12)

(cid:2)(cid:3)(cid:13)(cid:14)(cid:15)(cid:16)(cid:15)(cid:17)(cid:18)(cid:19) (cid:4)(cid:3)(cid:13)(cid:14)(cid:15)(cid:16)(cid:15)(cid:17)(cid:18)(cid:19)
Fig. (1). Human ATP-citrate lyase structure. A. Complex with citrate. The protein is shown as a ribbon diagram, while citrate is shown as
a stick model in magenta. Residues 2-31 and 108-243 form domain 4, which is colored green. Domain 3 includes residues 32-107 and is red.
Domain 5 includes residues 244-425 and is yellow. Domain 1 includes residues 487-624 and is cyan. Domain 2 includes residues 625-820
and is blue. The terminal residues seen in the electron density are labeled with their residue numbers. The ATP-grasp fold, formed by do-
mains 3 and 4, is at the bottom of the diagram and ATP/ADP would be expected to bind to the back, as oriented here [32]. B. Arrangement of
five domains.
center of citrate, and Arg379 forms a salt bridge with the Cytosolic acetyl-CoA serves three important biosynthe-
pro-R carboxylate of citrate. sis: 1) lipogenesis in the liver, adipose tissue, and mammary
gland [7, 9]; 2) acetylcholine biosynthesis in the nervous
2.2. Function of ATP Citrate Lyase tissue [34]; and histone acetylation in nuclei to control DNA
accessibility and gene transcription. More recently, ACL is
ACL links glycolysis to fatty acid/lipid synthesis. Much
found to play a critical role in beta-cell survival [10, 35],
of the bioenergetic supply is produced by the glycolysis of
platelet activation [36], and tumorigenesis [37]. Therefore,
glucose, in cancer cells in particular, and glycolytically de-
ACL has been attractive and its protein chemistry, enzyme
rived pyruvate enters the truncated tricarboxylic acid cycle
kinetics and substrate specificity, and transcriptional and
(TCA cycle, also known as citric acid cycle), where citrate is
post-translational regulations have been extensively studied
preferentially exported to cytosol via the tricarboxylate
[18, 38-44].
transporter [29-31]. In the cytosol, citrate is cleaved by ACL
to produce cytosolic acetyl-CoA and oxaloacetate. The en-
2.3. Regulation of ATP Citrate Lyase
zyme reaction catalyzed by ACL includes four steps [32,
33]: 2.3.1. Transcriptional Regulation
ATP +E (cid:1) E-P + ADP (1) At transcriptional level, ACL is regulated by sterol regu-
latory element-binding protein-1 (SREBP-1) [44], and insu-
E-P + citrate (cid:1) E•citryl-P (2)
lin and glucose metabolites are important stimulatory factors
E•citryl-P + CoA (cid:1) E•citryl-CoA + Pi (3) [45-47]. In vitro, insulin/glucose stimulates ACL expression
in primary hepatocytes through the SREBP-1 pathway, but
E•citryl-CoA (cid:1) E + acetyl-CoA + oxaloacetate (4)
transcription factor Sp1 acts as a repressor [48]. Decreased
Where E represents enzyme (i.e., ACL). ACL is phos- binding affinity of Sp1 to the G/C-rich region at -64 to -41bp
phorylated by ATP on histidine residue at the active site to
in ACL promoter is observed in response to insulin and glu-
form E-P in step 1. The phosphoryl group is transferred to
cose [48-50]. The PI3K/Akt signaling pathway is involved in
citrate in step 2. Citryl-phosphate binds with the enzyme,
insulin-stimulated ACL expression, and tissue-specific
symbolized by E•citryl-P. Phosphate is released in the attack growth factors that activate the PI3K/Akt pathway appear to
by CoA to form a citryl-CoA thioester bond in step 3. In the play a similar role [51]. But nateglinide as an insulin se-
last step, citryl-CoA is cleaved into acetyl-CoA and ox- cretagogue has recently been shown to suppress the expres-
aloacetate.
sion of ATP citrate lyase [52]. Glucose plays a synergistic
(cid:1)
156 Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 2 Zu et al.
role in ACL expression through the PI3K/Akt signaling, in- migration and invasion [16, 60, 61]. Genetic and pharma-
dicating its importance in regulating ACL activity [13, 53, cologic abrogation of ACL activity in cancer cells results in
54]. dose-dependent inhibition of cell proliferation and tumor
growth, and the effectiveness of this treatment rely on the
2.3.2. Posttranslational Regulation
glycolytic phenotype of tumor cells. Cancer cells with a high
ACL activity is also regulated by phosphorylation and rate of glucose aerobic glycolysis are much more sensitive to
dephosphorylation. ACL has a catalytic autophosphorylation ACL inhibition than those with a low glycolytic rate [63]. In
site, His760 that is phosphorylated by ATP as the first step in glucose-dependent cancer cells, selective ACL inhibition or
citrate cleavage [55]. This His760 can also be phosphory- knockdown promotes cell differentiation, elevates mitochon-
lated by nucleoside diphosphate kinase (NDPK) from rat drial membrane potential, decreases cell viability, and stimu-
liver and PC12 cells [56]. ACL is also phosphorylated at lates apoptosis [54, 64]. Hence, therapeutic strategies target-
other sites, including Ser3, Ser450 and Thr446. Evidence ing ACL need taking the glycolytic status of cells into con-
shows that the phosphorylation of these sites in ACL is in- sideration. ACL inhibition may also down-regulate lactate
creased during cell differentiation [55] and in response to dehydrogenase, allowing pyruvate to be imported into mito-
biologically active agents, such as glucagon, insulin, (cid:1)- chondria and enter Krebs cycle [63]. Furthermore, it is note-
adrenergic agonists, vasopressin, and transforming growth worthy that ACL inhibition may affect certain oncogenic
factor (cid:1)1 [57]. In response to insulin, ACL is phosphorylated gene expression by suppressing histone acetylation due to
on Ser450 by cAMP-dependent protein kinase and insulin- reduction of cytosolic acetyl-CoA [65].
stimulated kinase [58, 59]. Ser450 can also be phosphory-
In summary, although it is controversial whether in-
lated by Akt in primary adipocytes [28], indicating that the
creased ACL expression and activity in cancer cells is a
PI3K/Akt pathway is involved not only in ACL transcrip-
cause of tumorigenic process, it is clear that ACL as an en-
tion, but also in ACL activity regulation through phosphory-
zyme linking glucose and lipid metabolism is a promising
lation. In addition, Ser450 and Thr446 as well are also phos-
target for cancer therapy. The research and development of
phorylated by glycogen synthase kinase-3 [10, 55]; and Ser3
ACL inhibitors represent a novel strategy of cancer treat-
of ACL is phosphorylated by cAMP-dependent protein
ment.
kinase. The later abolishes homotropic allosteric regulation
of ACL by citrate and increases enzyme activity [41, 56].
4. ATP CITRATE LYASE INHIBITORS
3. ATP CITRATE LYASE AND CANCER In view of the importance of ACL in glucose and lipid
metabolism, investigators have done much work for design-
It is well known that bioenergetics of cancer cells is al-
ing and developing ACL inhibitors for hypolipidemic treat-
tered, including increased glucose uptake and glycolysis,
ment of metabolic and malignant diseases [66]. In particular,
lactic acid production and lipogenesis [54]. The increased
mechanism-based, active-site directed, or tight-binding small
lipogenesis is a hallmark of cancer and an early event in tu-
chemicals have been developed, having an irreversible, spe-
morigenesis, providing lipids essential for cell growth and
cific inhibition on ACL. Followed is a summary of some
division [60]. It is understood that cancer cells are highly
representative ACL inhibitors.
dependent on the de novo fatty acid synthesis for cellular
lipids [61]. On the other hand, cancer cells typically depend
4.1. Hydroxycitrate
more on glycolysis, the anaerobic breakdown of glucose for
ATP, even in the presence of available oxygen. This phe- Hydroxycitric acid (HCA) [67] is derived from fruit rinds
nomenon is known as Warburg effect [62]. The elevated of Garcinia species, including G. cambogia, G. indica, and
glucose catabolism produces excessive glycolytic end- G. atroviridis that grow prolifically on the Indian subconti-
product, pyruvate that is then converted either to lactate or nent and in Western Sri Lanka [68]. Hydroxycitric acid is
acetyl-CoA for de novo fatty acid synthesis [61]. Glycolysis also produced in calyxes of Hibiscus subdariffa and H. rosa-
may suppress tumor cell differentiation by increasing cytoso- sinensis that are cultivated in several tropical and semitropi-
lic acetyl-CoA and resultant lipid synthesis [13]. cal countries. Nowadays, the extracts of Garcinia fruits or
flowers are popularly used as an ingredient in diet supple-
Cytosolic acetyl-CoA is an essential component of
ments or drinks. Studies with Garcinia extracts have showed
de novo fatty acid synthesis. ACL integrates glucose glyco-
that ingested HCA is absorbed in the gastrointestinal tract
lysis with lipid synthesis by converting citrate to acetyl-CoA
and enters the systemic circulation [69]. However, bioavail-
Fig. (2). Citrate is a metabolite inhibitor of glycolysis and a
ability of HCA is low and its competitive competence with
precursor of acetyl-CoA for de novo fatty acid synthesis.
endogenous citric acid is weak. Administration of HCA at
Therefore, ACL activity may affect not only lipid synthesis
250mg/kg twice a day for three weeks is innocuous with no
but also glucose glycolysis for ATP production, and ACL
effects on weight, behavior, and survival of animals [63].
expression is pervasively up-regulated in immortalized cells
The safety dose to humans is up to 13.5g of hydroxycitrate
and various tumors, including in urinary bladder, breast,
per day [16].
liver, stomach, colon, lung, brain and prostate tumors [11-13,
16]. The phosphorylated ACL (active form) is correlated HCA has two diastereomers due to the existence of two
with tumor stage, differentiation grade, and prognosis. In chiral centers in the molecule. Therefore, there are four
cultured cancer cells, increased ACL expression promotes stereoisomers of HCA [70], comprising two pairs of enanti-
cancer cell proliferation and survival [13, 59] and enhances omers which are (2S,3S)-2-hydroxy citrate and (2R,3R)-2-
aggressive biological behaviors, such as clonogenic growth, hydroxycitrate, (2S,3R)-2-hydroxycitrate and (2R,3S)-2- hy-
(cid:1)
ACL as Novel Cancer Therapeutic Target Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 2 157
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Fig. (2). De novo synthesis of fatty acids from carbohydrate precursors and possible anticancer mechanisms of ACL inhibition. Upon
cellular uptake by glucose transporters, glucose is phosphorylated by hexokinases (HKs) to glucose-6-phosphate, most of which enters the
glycolytic pathway generating pyruvate and ATP. In the mitochondria, pyruvate is converted to acetyl-CoA, entering the citric acid cycle.
Depending on the oxygen tension, citrate can be oxidized to carbon dioxide and oxaloacetate, generating ATP via oxidative phosphorylation,
or can be transported into cytosol, converted by ATP citrate lyase (ACL) to acetyl-CoA and oxaloacetate. Acetyl-CoA is the requisite build-
ing block for fatty acid and cholesterol synthesis. NADPH required for fatty acid synthesis is produced by malic enzyme or via the pentose
phosphate pathway. Under anaerobic conditions, pyruvate may also be used as electron acceptor, producing lactate by lactate dehydrogenase
(LDH), which is excreted from the cell [54]. When genetic and pharmacologic downregulation of ACL is applied, there are four possible
approaches against cancer, labeled with blue-colored numbers. They are inhibition of fatty acid (1) and cholesterol (2) synthesis, glycolytic
pathway (3) by cytosolic citrate, and histone acetylation, affecting oncogenic gene expression (4).
droxycitrate, respectively Fig. (3). Each of the stereoisomers Inhibitory activity of (-)-hydroxycitrate on ACL has been
can form a (cid:2)-lactone ring and, in general, HCA is a mixture extensively investigated. It was reported that ACL activity in
of nonlactone and lactone forms. Nonlactone can be con- colonocyte was decreased by 86.6% at 7mM of HCA [76].
verted to lactone in 1N HCl with a high yield [71]. Under glycolytic condition, hydroxycitrate suppressed mi-
gration of human glioblastoma cells U87 and Glioblastoma
It was found forty years ago that Garcinia’s constituent
cells LN229 with IC at 16.7 and 12.1mM, respectively
(2S,3S)-2-hydroxycitrate, also named 4S-hydroxycitrate or (- 50
[16]. Hydroxycitrate at 10-500(cid:1)M could inhibit growth of
)-hydroxycitrate, is a strong competitive ACL inhibitor [72].
human bladder cancer cells T-24 or colon tumor cells HT-29
Later studies showed that among the four hydroxycitrate
by 5-60%. HCA also inhibits cell invasion and increases 3-
stereoisomers, only (2S,3S)-2-hydroxycitrate possesses in-
hydroxy-3-methylglutaryl-CoA reductase and low-density
hibitory effect on rat and human ACLs [72-74], and its lac-
lipoprotein (LDL) receptor activity in HepG2 cells [77].Used
tone form is less effective [73]. The other three HCA stereoi-
together with lipoic acid, HCA induces significant tumor
somers are not potent inhibitors of ACL with low affinity (30
growth retardation and enhances survival in MBT-2 bladder
- 132(cid:1)M) [75]. Therefore, inhibitory potency of (-)-hydroxy-
transitional cell carcinoma, B16-F10 melanoma and LL/2
citrate depends on its stereochemistry. It is noteworthy that
Lewis lung carcinoma mouse syngenic cancer models [63].
ACL inhibitory (2S,3S)-2-hydroxycitrate does not have sub-
A combination of lipoic acid,hydroxycitrate and cisplatin/
strate activity for this enzyme, but the other three do, in spite
methotrexate shows more tumor suppression efficacy than
of lack of inhibitory activity.
(cid:1)
158 Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 2 Zu et al.
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(cid:2) (cid:11)(cid:20)(cid:20)(cid:21) (cid:3) (cid:20)(cid:21) (cid:3) (cid:20)(cid:21) (cid:2) (cid:11)(cid:20)(cid:20)(cid:21)(cid:20)(cid:21)

(cid:20) (cid:21)(cid:21) (cid:20) (cid:21) (cid:21) (cid:20) (cid:21)(cid:21) (cid:20) (cid:21) (cid:21)
(cid:22)(cid:3)(cid:12)(cid:23)(cid:9)(cid:12)(cid:24)(cid:13)(cid:21)(cid:11)(cid:25) (cid:22)(cid:3)(cid:26)(cid:23)(cid:9)(cid:26)(cid:24)(cid:13)(cid:21)(cid:11)(cid:25) (cid:22)(cid:3)(cid:12)(cid:23)(cid:9)(cid:26)(cid:24)(cid:13)(cid:21)(cid:11)(cid:25) (cid:22)(cid:3)(cid:26)(cid:23)(cid:9)(cid:12)(cid:24)(cid:13)(cid:21)(cid:11)(cid:25)
Fig. (3). Structures of HCA stereoisomers. Upper panels show the nonlactone forms, and lower panels demonstrate the lactone forms [71].
cisplatin or methotrexate alone [78, 79]. In rat, (-)-hydroxy- Earlier studies reported enantiomers of 2-monofluoro-
citrate reduces the synthesis of both cholesterol and fatty citrate Fig. (4C) as substrates and competitive inhibitors of
acids and thus decrease plasma triglyceride level by inhibit- ACL, but their inhibitory activity was relatively weak [73μM
ing ACL [69, 80, 81]. Nevertheless, HCA inhibits lipogene- for the (2S,3S) and 192μM for the (2R,3R) forms] [88]. A
sis in brown adipose tissue in response to glucose feeding, similar enzymatic kinetics emerges from 2-vinylcitrates
but not in starved rat [82]. Therefore, (-)-hydroxycitrate is a which, despites having very high Ki values, are cleaved by
potential agent used for hypolipidaemic treatment of meta- ACL [89]. Therefore, enantiomers (+) and (-)-2,2-difluoro-
bolic or malignant diseases [81]. citrate Fig. (4C) were designed [90], and these difluoro-
citrate isomers have significant stronger competitive inhibi-
4.2. Non-hydroxycitrate Citric Acid Analogues tion to ACL than any compounds published thus far. These
compounds have competitive inhibition against citrate at Ki
Methionine sulfoximine [83] is a well-known irreversible
of 0.7μM for (+)-2,2-difluorocitrate and 3.2μM for (-)-2,2-
inhibitor of glutamine synthetase [6, 83, 84]. Since ACL
difluorocitrate. Their inhibitory patterns with either ATP or
shares the similar reaction mechanism with glutamine syn-
CoA as a substrate are uncompetitive or mixed and the inhi-
thetase, Dolle et al. [84] designed and synthesized a few cit-
bition constants are much weaker. Neither isomer undergoes
ric acid analogs derived from this glutamine synthetase in-
carbon-carbon bond cleavage as a substrate, nor is there evi-
hibitor.
dence of irreversible time-dependent inactivation. When
Diastereomers (+)-12a, b are citric acid analogs by re- ACL is incubated with CoA and difluorocitrate, the maximal
placing the primary carboxylate groups with a sulfoximinoyl intrinsic ATPase rate is 10% of the citrate-induced for (+)-
Fig. (4A), leading to occurrence of enzyme-mediated phos- enantiomers and 2% for (-)-enantiomers.
phorylation at 14a, b rather than 12a, b. These compounds
mimic the enzyme-bound intermediate (citrate phosphate 4.3. Radicicol
anhydride) and thus are potential tight-binding inhibitors of
Radicicol [91] Fig. (5), a 14-membered macrolide, is a
ACL. Unfortunately, testing results showed that only the (+)-
potent tranquilizer, inducing morphological changes of vari-
12a had a weak, reversible inhibition on rat ACL activity
ous transformed cells and cell cycle arrest in G1 and G2
with a Ki at 250(cid:1)M, 10-fold greater than the Km for citrate,
phases, as well as angiogenesis inhibition [92]. Radicicol
but the relative V was only 25% of the normal substrate.
max targets Hsp90, showing strong binding in a manner competi-
This data suggest that the sulfoximinoyl functional group can
tive with ATP [93, 94]. Recent studies have shown that radi-
mimic, to some extent, the carboxylate group in the active
cicol is also a noncompetitive inhibitor of ACL. Kinetic
site of the enzyme, probably due to its hydrogen-bonding
analysis demonstrated that radicicol and its derivative BR-1
ability [84].
Fig. (5) inhibited the activity of rat liver ACL with no appar-
Via covalently modifying the active-site nucleophiles, a ent effect on K but decreased V . As an inhibitor of ACL,
m max
novel class of citrate analogues has been developed [85]. The radicicol has a Ki at 13(cid:1)M for citrate and 7(cid:1)M for ATP.
(-)-5 and (+)-6 are the representative compounds in which Guay et al. [95] reproted that 50(cid:1)M radicicol inhibited ACL
the epoxides are potentially active-site-directed. Other com- activity along with decrease in insulin secretion by about
pounds include chlorocitrates (+)-10, (+)-11 and thiocitrates 50% in the presence of 5 and 10mM glucose or 0.3mM
12-16 Fig. (4B) [86]. Among these agents, epoxyaconitic palmitate in vitro.
acid (-)-5 [87] showed modest reversible inhibition on rat

ACL with Ki at 18(cid:1)M, 22 fold higher than (+) -6 (Ki =
400(cid:1)M).
(cid:1)
ACL as Novel Cancer Therapeutic Target Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 2 159
(cid:2)
(cid:20) (cid:20)
(cid:27)(cid:26) (cid:27)(cid:26)
(cid:21)(cid:20)(cid:20)(cid:11) (cid:12) (cid:21)(cid:20)(cid:20)(cid:11) (cid:12)
(cid:21)(cid:20)(cid:20)(cid:11) (cid:20)(cid:21) (cid:21)(cid:20)(cid:20)(cid:11) (cid:20)(cid:21)

(cid:22)(cid:28)(cid:28)(cid:28)(cid:24)(cid:13)(cid:10)(cid:3)(cid:29)(cid:30)(cid:26)(cid:31)(cid:21) (cid:22)(cid:28)(cid:28)(cid:28)(cid:24)(cid:13)(cid:10)(cid:3)(cid:16)(cid:30)(cid:26)(cid:31)(cid:21)
(cid:22)(cid:28)(cid:28)(cid:28)(cid:24)(cid:13)(cid:10)(cid:5)(cid:29)(cid:30)(cid:26)(cid:31) (cid:22)(cid:20)(cid:24)(cid:22)(cid:20)(cid:13)(cid:24)(cid:3) (cid:22)(cid:28)(cid:28)(cid:28)(cid:24)(cid:13)(cid:10)(cid:5)(cid:16)(cid:30)(cid:26)(cid:31) (cid:22)(cid:20)(cid:24)(cid:22)(cid:20)(cid:13)(cid:24)(cid:3)
(cid:3)
(cid:20) (cid:20)

(cid:21)(cid:20)(cid:20)(cid:11)
(cid:11)(cid:20)(cid:20)(cid:21)
(cid:21)(cid:20)(cid:20)(cid:11) (cid:11)(cid:20)(cid:20)(cid:21)
(cid:21)(cid:20)(cid:20)(cid:11) (cid:21)(cid:20)(cid:20)(cid:11)
(cid:22)(cid:13)(cid:24)(cid:13)(cid:6) (cid:22)!(cid:24)(cid:13)(cid:8)
(cid:11)" (cid:11)"
(cid:21)(cid:20)(cid:20)(cid:11) (cid:11)(cid:20)(cid:20)(cid:21) (cid:20) (cid:11)(cid:20)(cid:20)(cid:21)
(cid:21)(cid:20) (cid:11)(cid:20)(cid:20)(cid:21) (cid:20) (cid:11)(cid:20)(cid:20)(cid:21)

(cid:22)(cid:28)(cid:28)(cid:28)(cid:24)(cid:13)(cid:10)(cid:4) (cid:22)(cid:28)(cid:28)(cid:28)(cid:24)(cid:13)(cid:10)(cid:10)

(cid:12)(cid:26) (cid:12)(cid:21) (cid:26)

(cid:21)(cid:20)(cid:20)(cid:11) (cid:11)(cid:20)(cid:20)(cid:21) (cid:21)(cid:20)(cid:20)(cid:11) (cid:11)(cid:20)(cid:20)(cid:21) (cid:21)(cid:20)(cid:20)(cid:11) (cid:11)(cid:20)(cid:20)(cid:21)
(cid:21)(cid:20) (cid:11)(cid:20)(cid:20)(cid:21) (cid:21)(cid:20) (cid:11)(cid:20)(cid:20)(cid:21) (cid:11)(cid:20)(cid:20)(cid:21)
(cid:22)(cid:13)(cid:24)(cid:13)(cid:10)(cid:3)(cid:30)(cid:26)(cid:31)(cid:21) (cid:10)(cid:6)(cid:30)(cid:28)(cid:26)(cid:31)(cid:12)(cid:21)
(cid:22)(cid:13)(cid:24)(cid:13)(cid:10)(cid:9)(cid:30)(cid:26)(cid:31)(cid:12)#$ (cid:22)(cid:13)(cid:24)(cid:13)(cid:10)(cid:5) (cid:10)(cid:8)(cid:30)(cid:28)(cid:26)(cid:31)(cid:12)(cid:12)#$

(cid:4)

% %
(cid:11)(cid:20)(cid:20)(cid:21) (cid:11)(cid:20)(cid:20)(cid:21) (cid:11)(cid:20)(cid:20)(cid:21) (cid:11)(cid:20)(cid:20)(cid:21)
(cid:21)(cid:20) (cid:11)(cid:20)(cid:20)(cid:21) (cid:21)(cid:20) (cid:11)(cid:20)(cid:20)(cid:21)

,(cid:31)-./-0(cid:23)(cid:31)(cid:23)1(cid:10)(cid:11)(cid:12)(cid:19)(cid:12)(cid:6)(cid:28)(cid:7)(cid:19)(cid:5)(cid:7)(cid:8) ,(cid:31)*./*0(cid:23)(cid:31)(cid:23)1(cid:10)(cid:11)(cid:12)(cid:19)(cid:12)(cid:6)(cid:28)(cid:7)(cid:19)(cid:5)(cid:7)(cid:8)

%(cid:28)(cid:28)%
%(cid:28)(cid:28)%

(cid:11)(cid:20)(cid:20)(cid:21) (cid:11)(cid:20)(cid:20)(cid:21) (cid:11)(cid:20)(cid:20)(cid:21) (cid:11)(cid:20)(cid:20)(cid:21)

(cid:21)(cid:20) (cid:11)(cid:20)(cid:20)(cid:21) (cid:21)(cid:20) (cid:11)(cid:20)(cid:20)(cid:21)

,/*0(cid:31).(cid:31)(cid:23))(cid:28)1(cid:10)(cid:11)(cid:12)(cid:19)(cid:12)(cid:6)(cid:28)(cid:7)(cid:19)(cid:5)(cid:7)(cid:8) ,/-0(cid:31).(cid:31)(cid:23))(cid:28)1(cid:10)(cid:11)(cid:12)(cid:19)(cid:12)(cid:6)(cid:28)(cid:7)(cid:19)(cid:5)(cid:7)(cid:8)
Fig. (4). Representatives of non-hydroxycitrate citric acid analogues. A. Sulfoximine-containing citric acids. B. Epoxide-containing citric
acids, chloro-containing citric acids and thiol-containing citric acid. C. Fluoro-containing citric acid.
ganisms due to their inhibition on electron flow in the mito-
4.4. Antimycins
chondrial respiratory chain between cytochromes b and c1
Antimycins [96] Fig. (6) were antibiotics isolated from [97]. Lately, Barrow et al. reported that antimycins are in-
Streptomyces sp [97]. More recently, novel derivatives were hibitors of ACL against the substrate citrate with promising
identified and named antimycins A2 to A6 and each of them Ki values: 29.5(cid:1)M for antimycin Al, 4.2(cid:1)M for antimycin
is a mixture of two closely related isomers [97]. Antimycins A2, 60.1(cid:1)M for antimycin A3, 64.8(cid:1)M for antimycin A4,
are characterized with a carboxy phenol amido unit, a nine- 55.0(cid:1)M for antimycin A7, and 4.0(cid:1)M for antimycin A8 [97].
memberedcyclic bis-lactone, and two alkyl side chains with
variations of carbon length. Although, well identified as an- 4.5. Butanedioic Acid Derivatives
tibiotics and fungicides, more antimycins have been exten-
A series of 2-substituted butanedioic acids have been
sively investigated in energy metabolism in eukaryotic or-
developed as inhibitors of ACL based on the concept of link-
(cid:1)
160 Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 2 Zu et al.
(cid:11)(cid:21) (cid:21)
(cid:9) (cid:20)
(cid:20)
(cid:20)
(cid:21)
(cid:21)(cid:20) -(cid:5))(cid:28)(cid:6)(cid:28)(cid:6)(cid:12)(cid:10)
(cid:20)
(cid:11)"
(cid:20)(cid:21)
(cid:11)(cid:21) (cid:21)
(cid:9) (cid:20)
(cid:20)
(cid:20)
(cid:20) (cid:21)
(cid:20)

(cid:27)(cid:21) (cid:3)
(cid:27)(cid:21) (cid:12) (cid:20)
(cid:20) (cid:11)"
(cid:20)
(cid:20)(cid:21)
(cid:27)(cid:21)

(cid:3)-(cid:23)’
Fig. (5). Radicicol and representative of radicicol derivatives.

(cid:20)

(cid:27)(cid:21) (cid:20)(cid:21)
(cid:20)
(cid:21) (cid:20) (cid:20)

(cid:27)(cid:21) (cid:20) (cid:26)(cid:10)
(cid:20)

(cid:26)(cid:3)
(cid:20)

(cid:2)(cid:20)(cid:7)(cid:28)&(cid:26)(cid:6)(cid:28)(cid:20)(cid:18)(cid:2)’(cid:5)(cid:18)(cid:18)-’(cid:18)2(cid:18)!(cid:17),!(cid:17)/0!(cid:17)(cid:31)!(cid:17)/.(cid:18)-(cid:31)(cid:18)2(cid:18),!(cid:17)(cid:31)03!(cid:17)/

(cid:2)(cid:20)(cid:7)(cid:28)&(cid:26)(cid:6)(cid:28)(cid:20)(cid:18)(cid:2)’4(cid:18)(cid:18)-’(cid:18)2(cid:18)!(cid:17)(cid:31)!(cid:17),!(cid:17)/0(cid:31).(cid:18)-(cid:31)(cid:18)2(cid:18),!(cid:17)(cid:31)03!(cid:17)/

(cid:2)(cid:20)(cid:7)(cid:28)&(cid:26)(cid:6)(cid:28)(cid:20)(cid:18)(cid:2)(cid:31)(cid:5)(cid:18)(cid:18)-’(cid:18)2(cid:18)!(cid:17),!(cid:17)/0(cid:31).(cid:18)-(cid:31)(cid:18)2(cid:18),!(cid:17)(cid:31)03!(cid:17)/

(cid:2)(cid:20)(cid:7)(cid:28)&(cid:26)(cid:6)(cid:28)(cid:20)(cid:18)(cid:2)(cid:31)4(cid:18)(cid:18)-’(cid:18)2(cid:18)!(cid:17)(cid:31)!(cid:17)(cid:31)!(cid:17)/.(cid:18)-(cid:31)(cid:18)2(cid:18),!(cid:17)(cid:31)03!(cid:17)/

(cid:2)(cid:20)(cid:7)(cid:28)&(cid:26)(cid:6)(cid:28)(cid:20)(cid:18)(cid:2)/(cid:5)(cid:18)(cid:18)-’(cid:18)2(cid:18)!(cid:17),!(cid:17)/0!(cid:17)(cid:31)!(cid:17)/.(cid:18)-/(cid:18)2,!(cid:17)(cid:31)0/!(cid:17)/

(cid:2)(cid:20)(cid:7)(cid:28)&(cid:26)(cid:6)(cid:28)(cid:20)(cid:18)(cid:2)/4(cid:18)(cid:18)-’(cid:18)2(cid:18)!(cid:17)(cid:31)!(cid:17),!(cid:17)/0(cid:31).(cid:18)-(cid:31)(cid:18)2,!(cid:17)(cid:31)0/!(cid:17)/

(cid:2)(cid:20)(cid:7)(cid:28)&(cid:26)(cid:6)(cid:28)(cid:20)(cid:18)(cid:2)5(cid:5)(cid:18)(cid:18)-’(cid:18)2(cid:18)!(cid:17),!(cid:17)/0(cid:31).(cid:18)-(cid:31)(cid:18)2,!(cid:17)(cid:31)0/!(cid:17)/

(cid:2)(cid:20)(cid:7)(cid:28)&(cid:26)(cid:6)(cid:28)(cid:20)(cid:18)(cid:2)54(cid:18)(cid:18)-’(cid:18)2(cid:18)!(cid:17)(cid:31)!(cid:17)(cid:31)!(cid:17)/.(cid:18)(cid:18)-(cid:31)(cid:18)2,!(cid:17)(cid:31)0/!(cid:17)/

(cid:2)(cid:20)(cid:7)(cid:28)&(cid:26)(cid:6)(cid:28)(cid:20)(cid:18)(cid:2)6(cid:5)(cid:18)(cid:18)-’(cid:18)2(cid:18)!(cid:17),!(cid:17)/0(cid:31).(cid:18)(cid:18)-(cid:31)(cid:18)2,!(cid:17)(cid:31)0(cid:31)!(cid:17),!(cid:17)/0(cid:31)

(cid:2)(cid:20)(cid:7)(cid:28)&(cid:26)(cid:6)(cid:28)(cid:20)(cid:18)(cid:2)64(cid:18)(cid:18)-’(cid:18)2(cid:18)!(cid:17)(cid:31)!(cid:17)(cid:31)!(cid:17)/.(cid:18)(cid:18)-(cid:31)(cid:18)2,!(cid:17)(cid:31)0(cid:31)!(cid:17),!(cid:17)/0(cid:31)

(cid:2)(cid:20)(cid:7)(cid:28)&(cid:26)(cid:6)(cid:28)(cid:20)(cid:18)(cid:2)7(cid:5)(cid:18)(cid:18)-’(cid:18)2(cid:18)!(cid:17),!(cid:17)/0!(cid:17)(cid:31)!(cid:17)/.(cid:18)(cid:18)-(cid:31)(cid:18)2,!(cid:17)(cid:31)0(cid:31)!(cid:17),!(cid:17)/0(cid:31)

(cid:2)(cid:20)(cid:7)(cid:28)&(cid:26)(cid:6)(cid:28)(cid:20)(cid:18)(cid:2)74(cid:18)(cid:18)-’(cid:18)2(cid:18)!(cid:17)(cid:31)!(cid:17),!(cid:17)/0(cid:31).(cid:18)(cid:18)-(cid:31)(cid:18)2,!(cid:17)(cid:31)0(cid:31)!(cid:17),!(cid:17)/0(cid:31)

Fig. (6). Basic structure of antimycins.
ing a lipophilic group to a “citrate-like” head. The most by binding to butanedioic acid moiety to the citrate site, sup-
promising compounds are butanedioic acid derivatives with a porting the competitive inhibition mechanism against citrate.
substitute in the second position with an appropriate length Unfortunately, butanedioic acid derivatives could not sup-
(8 atom) spacer for a 2, 4-dichlorophenyl group. The com- press cholesterol or fatty acid synthesis in cultured cells
pounds 1a-c Fig. (7) have reversible Ki’s at 1-3(cid:1)M against (HepG2) due to the poor cell-permeability of these polar
rat ACL and show a competitive inhibitory mechanism chemicals [6].
against citrate and noncompetitive with respect to CoA. Fur-
Novel 2-substituted butanedioic acid derivatives have
ther studies suggest that these compounds interact with ACL
demonstrated better promise as ACL inhibitors [2, 98]. SB-
(cid:1)
ACL as Novel Cancer Therapeutic Target Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 2 161
(cid:11)"
(cid:10)(cid:29)(cid:30)(cid:28)&(cid:31)(cid:12)
(cid:22)(cid:11)(cid:21) (cid:24)
(cid:3)(cid:8) 8 (cid:11)(cid:20)(cid:20)(cid:21) (cid:10)(cid:16)(cid:30)(cid:28)&(cid:31)(cid:11)(cid:20)
(cid:21)(cid:20) (cid:11)(cid:20)(cid:20)(cid:21)
(cid:10)’(cid:30)(cid:28)&(cid:31)(cid:11)(cid:21)
(cid:11)" (cid:3)
(cid:10)

(cid:11)" (cid:11)" (cid:20)(cid:21)
(cid:22)(cid:11)(cid:21) (cid:24) (cid:22)(cid:11)(cid:21) (cid:24)
(cid:3)(cid:8) (cid:11)(cid:20)(cid:20)(cid:21) (cid:3)(cid:8) (cid:11)(cid:20)(cid:20)(cid:21)
(cid:21)(cid:20)(cid:28)(cid:21)(cid:20) (cid:11)(cid:20)(cid:20)(cid:21) (cid:11)" (cid:20) (cid:20)
(cid:11)"
*(cid:3)(cid:23)(cid:31)9’96(cid:24) *(cid:3)(cid:23)(cid:31)95::9
Fig. (7). Representatives of butanedioic acid derivatives.
201076 [66] Fig. (7) was found to be equally potent in inhib- competitive to citrate at Ki of 16(cid:1)M [101] and to CoA at Ki
iting rat (Ki = 1±0.05(cid:1)M) or human (Ki = 1±0.1(cid:1)M) ACL. of 3 (cid:1)M [8].
SB-204990 [66] Fig. (7), the cell-penetrant (cid:4)-lactone prodrug
Sulfur-substituted fatty acid analogue 3-thiadicarboxylic
of SB-201076, was found to be inactive to ACL in the assay
acid Fig. (8) is a novel dicarboxylic acid derivative that can
without cellular context, because SB-201076 is formed in-
inhibit ACL and fatty acid synthase activity [102]. Treatment
side the cells by hydrolysis of lactone. Pre-incubation of SB-
with 3-thiadicarboxylic acid reduced plasma levels of trigly-
204990 with either diabetic or control platelet suspension for
cerides from 5.8 to 2.7mmol/L and cholesterol from 11.0 to
30 minutes at 37°C did not affect ACL activity, but a longer
7.7mmol/L in rats and suppressed the activity of the rate-
incubation led to 61% inhibition for diabetic and 37% for
limiting enzyme in cholesterol biosynthesis, 3-hydroxy-3-
control platelets [36]. Oral administration of SB-201076
methylglutaryl-CoA reductase by 58%. It is believed that
dose-dependently reduces plasma cholesterol (up to 46%)
hyperlipidem in experimental nephrosis could be ameliorated
and triglyceride levels (up to 80%) in rats and plasma choles-
by 3-thiadicarboxylic acid via decreasing the overproduction
terol (up to 23%) and triglyceride levels (up to 38%) in dogs
of very-low-density lipoprotein [102].
[2].
Other biological effects of SB-204990 are observed. 4.7. 2-Hydroxy-N-arylbenzenesulfonamide
In vitro studies showed SB-204490 at 0.1mmol/L decreases
With attempts to identify a cell-permeable ACL inhibi-
acetyl-CoA content in platelet cytoplasm along with suppres-
tors, Li et al. [103] identified 2-hydroxy-N-arylbenzene-
sion of MDA synthesis and platelet aggregation [36]. Inter-
sulfonamide [104] as a modest inhibitor of ACL with 50%
estingly, SB-204990 can reduce D-[6-14C] glucose-depen-
inhibition of ACL activity (IC ) at 1.1(cid:1)M) [103]. Among
dent lipid synthesis in a dose-dependent manner, resulting in 50
approximately 50 analogs synthesized by Li et al. [103], 11
proliferation inhibition and death of tumor cells with active
showed greater than 50% inhibition at 10(cid:1)M Fig. (9) and the
aerobic glycolysis [13]. Recently, SB-204990 has been
compound 9 showed an IC of 0.13μM and had no cytotox-
shown to be effective to induce apoptosis in greater than 50
icity up to 50(cid:1)M. Long-term oral dosing of compound 9 in
50% of cancer cells in an in vitro apoptosis assay at a con-
high-fat-fed mice lowered down plasma cholesterol, triglyc-
centration of less than 50(cid:1)M [66].
eride, and glucose and blocked weight gain. Those data sug-
gest that 2-hydroxy-N-arylbenzenesulfo-namide 9 is the most
4.6. Dicarboxylic Acid Derivatives
potent ACL inhibitor with appreciable cell permeability.
MEDICA compounds are a class of (cid:3),(cid:3)'-methyl-
substituted (cid:2), (cid:5)-dicarboxylic acids, defined as MEDICA 6- 4.8. Purpurone
16 upon the number of carbon atoms. MEDICA 16 [99] Fig.
Purpurone is a purple, non-crystalline glassy solid.
(8) is a (cid:3),(cid:3)'-dimethyl hexadecanedioic acid with hypolipi-
HRFAB mass spectrum ([M + H]+, m/z = 698.1673) estab-
demic effect, leading to 60 and 45% reduction of cholesterol
lished its molecular formula of C40H27NO11 Fig. (10).
and high density lipoprotein in rats, respectively [100]. Re-

cently, MEDICA 16 was reported to be an ACL inhibitor
(cid:22)(cid:11)(cid:21) (cid:24)
(cid:22)(cid:11)(cid:21) (cid:24)
(cid:21)(cid:20)(cid:20)(cid:11) (cid:3) (cid:10)(cid:4) (cid:11)(cid:20)(cid:20)(cid:21)
(cid:21)(cid:20)(cid:20)(cid:11) * (cid:3) (cid:10)(cid:4) * (cid:11)(cid:20)(cid:20)(cid:21)

#()*(cid:11)(cid:25)(cid:28)(cid:10)(cid:8) (cid:9)(cid:13)(cid:19)+(cid:18)(cid:29),(cid:18)’(cid:29)-(cid:16)./0"(cid:18)’(cid:28)(cid:29)’(cid:18),
Fig. (8). Representatives of dicarboxylic acid derivatives.
(cid:1)
162 Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 2 Zu et al.
(cid:20)(cid:21) (cid:20)(cid:21)

(cid:11)" (cid:11)"
(cid:21)(cid:20) (cid:20)(cid:21)

(cid:20)(cid:21)
(cid:20) (cid:12)
(cid:27)(cid:21)(cid:26)
(cid:20)

(cid:13)(cid:14)(cid:15)(cid:16)(cid:14)(cid:17)(cid:18)(cid:19) (cid:20) (cid:21)(cid:13) (cid:22) (cid:23)(cid:5)(cid:24)(cid:25)
(cid:11)(cid:4)
(cid:20) (cid:27) (cid:20)

(cid:6) (cid:9)(cid:5)(cid:9)
(cid:19)(cid:13)1(cid:15) (cid:19)(cid:13)1(cid:15)
(cid:11)(cid:21)(cid:3) (cid:20)(cid:21)
(cid:21)(cid:20)
(cid:11)(cid:21)(cid:3) (cid:20)(cid:21)
(cid:5) (cid:12)(cid:5)(cid:2) (cid:20)(cid:21)
(cid:11)" (cid:11)"

(cid:6) (cid:7)(cid:5)(cid:3)
% %
(cid:20)(cid:21)

(cid:19)(cid:13)1(cid:15)
(cid:7) (cid:11)(cid:5)(cid:3)

(cid:15)-2(cid:15)-.(cid:17)$
(cid:19)(cid:13)1(cid:15)

+ +
Fig. (10). Structure of purpurone.
(cid:8) (cid:4)(cid:5)(cid:9)(cid:10)

viability and cellular ATP level of HepG2 cells at 100μg/ml.
+
Recently, a new rapid synthesis of purpurone has been de-
(cid:20)#$ veloped, which will promote investigation and development
(cid:9) (cid:4)(cid:5)(cid:9)(cid:2)
of new purpurone derivatives as inhibitors of ACL [106].
+

4.9. Other Agents

(cid:10)(cid:11) (cid:3)(cid:5)(cid:10) Apart from ACL inhibitors discussed above, a few other

chemicals have partial or indirect inhibitory activity to ACL
(cid:17)(cid:13)(cid:21)$/
and are discussed below.
4.9.1. 2, 3-Butanedione and Phenylglyoxal
(cid:10)(cid:10) (cid:9)(cid:4)(cid:5)(cid:10)

As arginine-targeted agents, both 2, 3-butanedione and
(cid:27)
#$ (cid:11)(cid:20)#$ phenylglyoxal could inactivate ACL in a concentration-
dependent manner. Phenylglyoxal induces rapid inactivation

of ACL by 85%, but butanedione does to a lesser degree
(cid:10)(cid:12) (cid:8)(cid:5)(cid:9)
(35%). Inhibition to ACL by these two compounds is at least
(cid:20) +
in part due to their modifications of arginine residues at the
#$
CoA binding site. Phenylglyoxal-modified ACL shows a
(cid:20) decrease in V , but its K for substrates does not alter sig-
(cid:10)(cid:13) (cid:4)(cid:5)(cid:2)(cid:7) max m
(cid:20) nificantly. However, ACL substrates, CoA or CoA plus cit-

(cid:18)(cid:13) - rate, could protect the enzyme against inactivation by 2, 3-
butanedione and phenylglyoxal in rats [38].
(cid:20)
(cid:10)(cid:14) (cid:20) (cid:4)(cid:5)(cid:2)(cid:6) 4.9.2. L-Glutamate
+ Glutamate, as the most abundant excitatory neurotrans-
(cid:2)(cid:3) mitter in the vertebrate nervous system, is found to be a spe-
cific inhibitor of ACL from adult rat brain and liver [34,

107]. Glutamate, especially L-glutamate, has a time-depen-
Fig. (9). In vitro SAR summary. In vitro data are at least two sepa- dent inhibitory effect on ACL activity in the presence of both
rate measurements using recombinant hACL [103]. ATP and MgCI2 and a high concentration of ATP could re-
verse this process [107]. Szutowicz et al. found that L-
Purpurone was isolated from the sponge Iotrochota sp. and glutamate is a competitive inhibitor of ACL (Ki = 0.3mM)
has antioxidant activity. Purpurone exhibits its inhibitory with no effect on K and V at a low concentration of
m max
activity on ACL in a dose-dependent manner, and has an MgCL , suggesting excessive Mg2+ ions is indispensable for
2
IC50 of 7μ(cid:2) [105]. Purpurone reduced fatty acid synthesis, glutamate inhibition [107]. The inhibitory activity of gluta-
but not cholesterol [105]. Purpurone had no effect on the mate may arise from the production by glutamine synthetase
of ADP, a known product inhibitor of ACL [34].
(cid:1)
ACL as Novel Cancer Therapeutic Target Recent Patents on Anti-Cancer Drug Discovery, 2012, Vol. 7, No. 2 163
4.9.3. Deoxycholate phosphorylation at histidine by 67%, inactivating ACL
[111]. Vanadate has also inhibitory activity to succinyl-COA
Deoxycholic acid is one of the secondary bile acids that
synthease (SCS) [111].
are metabolic byproducts of intestinal bacteria. Eriyamremu
et al. reported that sodium deoxycholatecould can signifi-
5. CURRENT & FUTURE DEVELOPMENTS
cantly reduce ACL activity in the experimental rat liver
[108]. Cancer is a potentially fatal disease with poor responses
to current therapies. Recent new insights on tumor metabo-
4.9.4. Polychlorinated Biphenyls
lism alterations contribute to the concept of targeting tumor
Polychlorinated biphenyls (PCBs), a molecule composed bioenergetics as an anticancer strategy. Based on the marked
of two benzene rings, are a class of organic compounds with elevation of expression and activity of ACL in many immor-
1 to 10 chlorine atoms attached to biphenyl. It is well known talized cells and tumors and the effect on cancer cell prolif-
that PCBs have great toxicity due to its structural similarities eration or tumor growth induced by ACL downregulation,
to dioxin and certain hormones, such as thyroxine and estra- various ACL inhibitors with different structures and target-
diol. Recently, Kling et al. [109] found that PCBs adminis- ing sites have been designed, synthesized and evaluated on
tered in diet (0.01%, w/v) inhibits citrate cleavage independ- anticancer activity. Of these discussed compounds Table 1,
ent of citrate concentrations. Hereby, PCBs are probably most inhibitors, such as hydroxycitrate, vinylcitrates, thio-
noncompetitive inhibitors of ACL. citrates, fluorocitrates, play a competitive inhibition against
citrate, which are characterized by a common citrate-featured
4.9.5. Vanadate
head. Others are competitive inhibitors to CoA, such as
Vanadium is a trace element essential for the growth and MEDICA 16. Some of them exert a multiple inhibition si-
normal existence of living cells. Vanadium affects many multaneously, like SB-201076 which inhibits ACL activity
biochemical processes, such as protein phosphorylation by through competitive mechanism with respect to citrate and
its biologically active forms, pcntavalenr vanadate (VO3-, noncompetitive manner to CoA. This class of inhibitors may
H2VO4-) or tetravalent vanadyl cation (VO2+) [110]. How- hold stronger promises due to their dual targets.
ever, investigators reported that vanadate at 1 mM inhibits

Table 1. Common Inhibitors of ATP Citrate Lyase.
Study
Inhibotor Type and
Name Feature Tumor Type Reference
In In Clinical
Ki /IC50 Value
Vitro Vivo Trial
1) Decreased plasma choles- Glioblastoma;
Competitive inhibitor with Ki terol and triglyceride levels bladder carcinoma;
of 0.15 and 50μM for rat 2) Suppressed platelet aggre- colon carcinoma; [67, 72-75,
Hydroxycitrate Yes Yes Yes
enzyme and Ki of 300μM for gation melanoma; lung 78, 81]
human enzyme. 3) Suppressed cancer growth, carcinoma; adeno-
carcinoma
migration and invasion
Sulfoximine-
Reversible inhibitior with Ki
containing citric No evidence Yes No No No evidence [6, 83-84]
of 250μM for rat enzyme
acid
Epoxide- Reversible inhibitior with Ki
containing citric of 18μM to 400μM for rat No evidence Yes No No No evidence [85, 87]
acid enzyme
Chloro- Reversible inhibitior with Ki
containing citric of 29μM to 340μM for rat No evidence Yes No No No evidence [85-86]
acid enzyme
Reversible inhibitior with Ki
Thiol-containing
of 58μM to 480μM for rat No evidence Yes No No No evidence [85]
citric acid
enzyme
Fluoro- Competitive inhibitor with Ki
containing citric of 0.7μM to 192μM for rat No evidence Yes No No No evidence [88-90]
acid enzyme
(cid:1)
(cid:1)