Discovery of novel xanthone derivatives as xanthine oxidase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2011.04.140

Xanthine oxidase is the key enzyme that catalyzes the oxidation of hypoxanthine to xanthine and then to uric acid. In this study, a series of xanthone derivatives were synthesized as effective and a new class of xanthine oxidase inhibitor. Compounds 8a, 8c, 8i, 8g and 8r showed good inhibition against xanthine oxidase. The presence of a cyano group at the para position of benzyl moiety turned out to be the preferred substitution pattern. Molecular modeling studies were performed to gain an insight into its binding mode with xanthine oxidase, and to provide the basis for further structure-guided design of new non-purine xanthine oxidase inhibitors associated with the xanthone framework.

Full text of DOI:10.1016/j.bmcl.2011.04.140

Bioorganic & Medicinal Chemistry Letters 21 (2011) 4013–4015
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of novel xanthone derivatives as xanthine oxidase inhibitors
Lina Hu a, Honggang Hu a, , Weifeng Wu b, Xiaoyun Chai a, Jianfei Luo a, Qiuye Wu a,
⇑
⇑
a Department of Organic Chemistry, School of Pharmacy, Second Military Medical University, Shanghai 200433, China
b Fengxian District Central Hospital, Branch Hospital of Shanghai Sixth People’s Hospital, Shanghai 201400, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Xanthine oxidase is the key enzyme that catalyzes the oxidation of hypoxanthine to xanthine and then to
uric acid. In this study, a series of xanthone derivatives were synthesized as effective and a new class of
xanthine oxidase inhibitor. Compounds 8a, 8c, 8i, 8g and 8r showed good inhibition against xanthine oxi-
dase. The presence of a cyano group at the para position of benzyl moiety turned out to be the preferred
substitution pattern. Molecular modeling studies were performed to gain an insight into its binding mode
with xanthine oxidase, and to provide the basis for further structure-guided design of new non-purine
xanthine oxidase inhibitors associated with the xanthone framework.
Received 22 February 2011
Revised 18 April 2011
Accepted 29 April 2011
Available online 6 May 2011
Keywords:
Xanthone
Derivatives
Ó 2011 Elsevier Ltd. All rights reserved.
Synthesis
Xanthine oxidase
Molecular modeling
Xanthine oxidase (XO) is a key enzyme in catabolic sequence of
the purine nucleotide metabolism in humans and a few other uri-
cotelic species.1 It catalyzes the last two steps in the purine degra-
dation pathway prior to formation of uric acid, that is,
hydroxylation of hypoxanthine to xanthine and then to uric acid
(UA).2 The increasing of UA concentration eventually leads to the
deposition of micro and macroscopic deposits of sodium hydrogen
urate monohydrate crystals in joints and kidneys, which in turn
causes gouty arthritis and UA nephrolithiasis.3,4 Since reducing
plasma levels of UA can prevent gout, therapy is possible with
XO inhibitors that block the production of UA from purine.5 Allopu-
rinol (Fig. 1), a known XO inhibitor and an analogue of hypoxan-
property. Furthermore, it is an effective approach in the search of
novel compounds with potent activity by simple chemical-modifi-
cation on the basis of natural leading compounds. In this Letter, we
described the synthesis and preliminary structure–activity rela-
tionships of xanthone derivatives as potent XO inhibitors.
The synthesis of the hitherto unreported title compounds 8a–t
was as outlined in Scheme 1. 2,4,5-Trimethoxybenzoyl chloride 2
was obtained by treating 2,4,5-trimethoxybenzoic acid with sulfu-
ryl dichloride, which was then alkylated via Friedel–Crafts reacting
with 1,3,5-trimethoxybenzene in absolute ether to give benzophe-
none 3. The free hydroxyl group of 3 was formed by demethylation
in extra amount of aluminium chloride.11 Intramolecular cycliza-
tion of 3 was carried out with tetrabutylammonium hydroxide in
pyridine and water under reflux to provid 4 in high yield,12 which
was demethylated in freshly fused pyridine hydrochloride to form
the key intermediate 5.13 Chloro(methoxy)methane was added
dropwisely into the flask containing 5 and N,N-diisopropylethyl-
amine at 0 °C to give 6 as light-green powder under the condition
of argon protection,14 and subsequently 6 was benzylated with cor-
responding benzyl bromine to obtain 7 in quantitative yield. Finally,
thine, has been widely prescribed as
a
treatment for
hyperuricemia and gout. In some cases, however, severe life-
threatening side effects have been reported. These include a toxic-
ity syndrome dramatized by eosinophilia, vasculitis, rash hepatitis,
and progressive renal failure.6 Hence, the identification of novel,
efficient and less toxic xanthine oxidase inhibitors remains an
important and challenging task.
Xanthone is a class of oxygen-containing heterocyclic com-
pounds widely distributed in nature,7 and exhibits a variety of bio-
logical activities.8–10 In our previous study, compound 5 (Fig. 1)
displayed good inhibitory activity aganist xanthine oxidase. How-
ever, the solubility of 5 is very poor in most solvents, which in-
spired us to explore new derivatives with better physicochemical
OH
O
O
OH
HO
HO
N
N
N
H
N
OH
⇑
Corresponding authors. Tel./fax: +86 21 8187 1228 (H.H.); tel./fax: +86 21 8187
1225 (Q.W.).
Allopurinol
5
E-mail addresses: huhonggang_fox@msn.com (H. Hu), wuqy6439@sohu.com (Q.
Wu).
Figure 1. Chemical structures of allopurinol, compound 5.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.04.140

4014
L. Hu et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4013–4015
O
O
O
O
O
O
COCl
O
c
O
O
COOH
O
a
b
O
OH
O
1
2
3
O
O
OH
O
O
OH
O
O
MOMO
MOMO
e
HO
HO
O
O
d
OMOM
OH
O
O
6
5
4
O
O
O
O
R
R
MOMO
MOMO
HO
HO
f
g
O
OMOM
O
OH
8a-t
7a-t
R=Alkyl, Halogen, NO2, CN.
Scheme 1. Synthesis of compounds 8a–t. Reagents and conditions: (a) SOCl2, reflux, 4–5 h; (b) 1,3,5-trimethoxybenzene, AlCl3, Et2O, rt, 48 h, 72%; (c) TBAOH (25% in H2O),
Py-H2O (1:1), reflux, 4 h, 95%; (d) chloro(methoxy)methane, DIEA, dichloromethane, 0 °C–rt, 8 h; (f) benzyl bromides, K2CO3, DMF, 50 °C, 6 h; (g) Concentrated hydrocloride,
absolute methnol, 55 °C.
the title compounds 8a–8t were achieved by way of deprotection of
7 using concentrated hydrochloric acid in methanol with yield of
50–65%.14 The structures of the synthesized target compounds
were elucidated by 1H NMR, 13C NMR and MS.15 All spectral data
were in accordance with assumed structures.
The inhibitory activity of the newly synthesized compounds
in vitro against XO was measured spectrophotometrically by fol-
lowing uric acid levels at 293 nm.16,17 Compounds presenting an
inhibitory effect higher than 60% at the concentration of 10 lg/
mL were further tested at a wide range of concentrations to deter-
mine their IC50 values, the results were shown in Table 1.
Structure–activity relationships (SARs) were inferred from Ta-
ble 1. In general, there was no significant influence on biological
activity by introducing electron donor or electron withdrawing
substituents on benzyl group. In addition, compound 8r with cyano
group at para position showed significant inhibition on XO, fol-
lowed by 8i with chloride group at para position of benzyl, 8c with
methyl group at para position, 8g with chloride group at ortho po-
sition and 8a with methyl group at ortho position, which suggested
that para substituted benzyl groups are of benefit to the inhibition
rate than that compounds with the same substitutes at ortho or
meta position.
The known X-ray structures of xanthine oxidoreductase with
bound molecules show a highly specific binding pocket, presenting
a long, narrow cavity leading toward the Mo(IV) complex.6,19
Molybdenum-pterin sites of both xanthine oxidase and xanthine
dehydrogenase are structurally equivalent.20 To further under-
stand the binding mode of newly synthesized compounds with
XO, molecular docking studies of compounds 8i and 8r were per-
formed. The crystal structure of xanthine oxidoreductase (PDB en-
try code 1FIQ) was obtained from the Protein Data Bank (http://
www.rcsb.org). The protein was prepared by removing all water
molecules and adding all hydrogen atoms using LibDock protocol
in Disocovery Studio 2.5 software package,21 LibDock, developed
by Diller and Merz,22 is based on matching polar and apolar bind-
ing site features of the protein-ligand complex. It can get a balance
between accuracy and speed.
Table 1
In vitro xanthine oxidase inhibitory activity of xanthone derivatives
According to our docking studies, the novel xanthone deriva-
tives 8i and 8r indeed inserted into the narrow tunnel towards
the Mo(IV) complex. As shown in Figure 2, compounds 8i and 8r
overlapped properly with Molybdenum-pterin active site with
the configuration of ‘L’(showed by Fig. 2A and B respectively). Be-
sides, all of them binded within amino residues: Phe798, Gln1194
and Gln112, which were believed to promote the stabilization of
the binding positions of the xanthone substrate and might be
important for substrate recognition. The presence of these residues
made the region closer to the Mo(IV) complex. Both compounds 8i
and 8r interacted very closely with original ligand docking pose by
hydrogen bonds that involved Gln1040, Ser1082, Gln1261, Gly797,
Gln767 and Cys150. Moreover, additional hydrogen binding be-
tween the carboxyl group of Lys1045 and the nitrogen atom of
compound 8r occurred.
O
O
R
HO
HO
O
OH
Compounds
21.73 1.52 8k
a,b
a,b
Compounds
R
IC50
(l
M)
R
IC50
(lM)
5
—
3-Br
4-Br
n.a
8a
8b
8c
8d
8e
8f
8g
8h
8i
2-CH3 7.08 0.65
3-CH3 13.56 0.28 8m
4-CH3 5.73 0.10
8l
20.06 1.37
2-NO2
3-NO2
4-NO2
2-CN
3-CN
4- CN
n.a
n.a
n.a
n.a
8n
8o
8p
8q
8r
8s
8t
2-F
3-F
4-F
2-Cl
3-Cl
4-Cl
2-Br
n.ac
n.a
n.a
n.a
6.41 0.15
n.a
4.70 0.12
n.a
4.67 0.35
2,4-di-Cl n.a
2,6-di-Cl n.a
8j
In conclusion, a new class of xanthone derivatives as XO inhib-
itor were synthesized and evaluated for their inhibitory activities
against XO. Compounds 8a, 8c, 8g, 8i and 8r demonstrated potent
inhibitory activities against XO with IC50 values lower than
a
IC50 values: the concentration of the inhibitor required to produce 50% inhibi-
tion of xanthine oxidase.
b
Each results was performed in triplicate. Allopurinol was used as positive
control (IC50 = 24.40 0.50
l
M) Ref. 18.
c
n.a.: not active (less than 60% inhibition at 10
lg/mL).
10 lM, which were much better than allopurinol. Molecular

L. Hu et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4013–4015
4015
the active site of XO. Further research in this area is in progress in
our laboratory.
Acknowledgments
This work was supported by grants from Science & Technology
Commission of Shanghai Municipality (Nos. 054319909 and
08JC1405500) and Shanghai Leading Academic Discipline Project
(No. B906).
References and notes
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Figure 2. Superposition models of xanthone derivatives and original ligand docking
pose (the Molybdenum-pterin active site). (A) Compound 8i (pink) with original
ligand (blue); (B) compound 8r (yellow) with original ligand (blue). Hydrogen
bonds are represented by the dashed red lines.
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docking studies provided the molecular basis for rationalizing the
activity of the new xanthone derivatives. In particular, both com-
pounds adopted similar binding poses and interacted closely with