Novel synthesis of nitro-quinoxalinone derivatives as aldose reductase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2014.03.053

A novel, non-acid series of nitroquinoxalinone derivatives was synthesized and tested for their inhibitory activity against aldose reductase as targeting enzyme. All active compounds displayed an 8-nitro group, and showed significant activity in IC₅₀ values ranging from 1.54 to 18.17 μM. Among them 6,7-dichloro-5,8-dinitro-3-phenoxyquinoxalin-2(1H)-one (7e), exhibited the strongest aldose reductase activity with an IC₅₀ value of 1.54 μM and a good SAR (structure-activity relationship) profile.

Full text of DOI:10.1016/j.bmcl.2014.03.053

Accepted Manuscript
Novel synthesis of Nitro-Quinoxalinone derivatives as Aldose reductase inhib-
itors
Saghir Hussain, Shagufta Parveen, Xiangyu Qin, Xin Hao, Shuzhen Zhang, Xin
Chen, Changjin Zhu, Bing Ma
PII:
S0960-894X(14)00280-7
DOI:
http://dx.doi.org/10.1016/j.bmcl.2014.03.053
BMCL 21446
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
31 January 2014
14 March 2014
17 March 2014
Please cite this article as: Hussain, S., Parveen, S., Qin, X., Hao, X., Zhang, S., Chen, X., Zhu, C., Ma, B., Novel
synthesis of Nitro-Quinoxalinone derivatives as Aldose reductase inhibitors, Bioorganic & Medicinal Chemistry
Letters (2014), doi: http://dx.doi.org/10.1016/j.bmcl.2014.03.053
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Novel synthesis of Nitro-Quinoxalinone derivatives as Aldose reductase inhibitors
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Saghir Hussain, Shagufta Parveen, Xiangyu Qin, Xin Hao, Shuzhen Zhang, Xin Chen, Changjin
Zhu* and Bing Ma*
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aDepartment of Applied Chemistry, Beijing institute of Technology, No.5, Zhongguancun South Street,
100081, Beijing, China
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To whom correspondence should be addressed. For Changjin Zhu: Tel.: +86-10-68918506; Fax:
+86-10-68918506; E-mail: zcj@bit.edu.cn.
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aAbbreviations: ALR2, Aldose reductase; AKR, aldo-keto reductase; NADPH, ß-nicotinamide adenine
dinucleotide phosphate reduced form; ARIs, Aldose reductase inhibitors; SAR, structure-activity
relationship
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Abstract
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A novel, non-acid series of nitroquinoxalinone derivatives was synthesized and tested for their
inhibitory activity against aldose reductase as targeting enzyme. All active compounds displayed an
8-nitro group, and showed significant activity in IC50 values ranging from 1.54 to 18.17 µM. Among
them 6,7-dichloro-5,8-dinitro-3-phenoxyquinoxalin-2(1H)-one (7e), exhibited the strongest aldose
reductase activity with an IC50 value of 1.54 µM and a good SAR (structure- activity relationship)
profile.
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Keywords
Quinoxalinone derivatives, Aldose Reductase inhibitors, Structure-Activity relationship
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Diabetes Mellitus (DM) is a complex metabolic disorder characterized by an elevated level of blood
glucose called hyperglycemia. The polyol pathway of glucose metabolism is activated in the
hyperglycemia, which subsequently results in the development of long-term diabetic complications
including retinopathy, nephropathy, neuropathy and cataract.1,2,3,4 Aldose reductase (ALR2, EC I.I.I.21),
a cytosolic enzyme and a member of aldo-keto reductase (AKR) superfamily, is the first and rate
limiting enzyme of the polyol pathway in which ALR2 catalyzes the NADPH-dependent reduction of
glucose to sorbitol, which then is converted into fructose by NAD+-dependent sorbitol dehydrogenase
(Fig. 1) Several lines of evidence suggest that the primary cause of long-term diabetic complications is
due to the activation of ALR2 and consequent imbalance of NADPH/ NADP+ and NAD+/ NADH
coenzymes resulting in oxidative stress inside the cell along with overproduction of fructose.5,6,7,8
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Figure 1. The polyol pathway of glucose metabolism
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Therefore, ALR2 has been considered as target enzyme for suitable drug candidates to check the delay
in onset, progression and further development of long-term diabetic complications.
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In past years, a number of structurally different aldose reductase inhibitors (ARIs) comprising two
chemical classes have been developed but the clinical efficacy and potency of these compounds still
pose a challenge, and most of them have deleterious side effects. The first class comprises carboxylic
acid ARIs such as, tolrestat9, zenarestat10 and ponalrestat11, and the second involves cyclic imides like
sorbinil12, fidarestat13, minalrestat14 and ranirestat (AS-3201)15 (Fig. 2). Currently, only epalrestat, a
carboxylic acid drug is available on the market and used for the treatment of neuropathy in Japan, India
and China. 16 Indeed, whereas the carboxylic acids ARIs show potent in vitro activity as aldose reductase
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inhibitors, their effectiveness decreases in vivo. In their turn, the cyclic imide ARIs often develop
toxicity and exhibit some side effects.12 Therefore, development of different types of ARIs is still
needed. A preferred approach to pursue the desired therapeutic and pharmacokinetic properties is to
design and synthesize specific non-carboxylic acid and non-cyclic imide ARIs. Compounds with
scaffolds containing nitro substituents find a wide range of applications in biological and pharmaceutical
areas, and often are involved as drug candidates. 17 To this aim, we designed quinoxalinone-based nitro
derivatives and evaluated them for their properties as ARIs (Aldose reductase inhibitors). The present
article focuses on the syntheses of quinoxalinones containing one, two or three nitro groups and on the
role of those nitro groups in the inhibition of ALR2.
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The preparation of starting materials dichloroquinoxalin-2(1H)-one 5 and 6- or 7-bromo
quinoxalinone 8a,b has been reported previously.18 Syntheses of target compounds 7a-f and 9a-b were
accomplished as outlined in Scheme 1 and 2. Compound 5 reacted with various phenols produced
3-phenoxyquinoxalinone intermediates 6a-d. Further, aromatic nitration of 6a-c using a mixture of Cu
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(NO3)2-3H2O and Ac2O proceeded first at the C8, then at the C5 position to afford compounds 7a-e.
Finally, aromatic and N1-nitration of 6d yielded compound 7f as shown in scheme 1.19 In a similar way,
aromatic nitration of 6- and 7- bromoquinoxalinone compounds (8a, b) yielded the corresponding two
products 9a, b as shown in scheme 2.
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Scheme 1
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o
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Scheme 1. Reagents and conditions: (a) Phenols, dimethylformamide (DMF), K2CO3, 24 h, 75 C; (b)
Cu (NO3)2-3H2O, Ac2O, CH2Cl2, 12 h, room temperature (rt)
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Scheme 2
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Scheme 2. Reagents and conditions: (a) Cu (NO3)2-3H2O, Ac2O, CH2Cl2, 12 h, rt
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The synthetic work led to a series of compounds containing one, two or three nitro groups located
on the benzene ring of the quinoxalinone core and the N1 position as shown in table 1. All of the
quinoxalinone derivatives were evaluated for their potential inhibitory effect on ALR2 isolated from dog
lenses. The IC50 values were determined by linear regression analysis of the log of the
concentration-response curve.18 The in vitro assay of these derivatives showed an interesting inhibition
profile with significant inhibitory activity in an IC50 range of 1.54-18.17 µM.
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Compounds 7a-f that had one nitro group at the C8 position, or two to three nitro groups at C5, C8 and
N1 positions of the quinoxalinone framework showed significant activity in the low micromolar range.
Among these, compound 7e bearing two nitro groups at C5 and C8 positions was the most active with
the IC50 value of 1.54 µM, whereas compound 7a bearing a single 8-nitro group had a much lower
activity. The decrease in the activity of 7a could be attributed to the 4-fluoro substituent on the phenoxy
ring as compared with 7d.18 However, addition of one more nitro group on the benzene ring of 7a
resulted in an increase in the activity for 5,8-dinitroquinoxalinone 7b. This synergy reappeared in the
most active compound 7e which equally showed an enhanced activity by adding a 5-nitro group to 7d.
Replacing the 6-chloro with a bromo substituent in compound 7a to form 6-Br-8-nitro compound 9a
also increased the activity. Compound 7f having three nitro groups at C5, C8, and N1 showed a
moderate activity with IC50 = 7.1 µM. The 7-Br-5,6-dinitro compound 9b displayed the least activity. In
addition, it should be noted that although both 7b and 9b have two nitro groups on the benzene ring of
the quinoxalinone skeleton, there was a significant difference in activity between the two dinitro
compounds indicating that the 8-nitro group has a positive impact on the activity. Indeed, while active
compounds 7a-f and 9a all possess this structural feature, the second most active compound 7d displays
a single 8-nitro group (Table 1).
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Table 1. Biological activity data of quinoxalinone derivatives
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Comp. No.
R1
R2
R3
ALR2 IC50 [µM]a
26.67 % b
11.81
4.46
6c
7a
H
4-F
H
H
4-F
NO2
H
7b
H
4-Br
4-H
4-H
7.46
7c
H
H
3.14
7d
H
NO2
1.54
7e
NO2
3,5-diMe NO2
4F
7.1
7f
6-Br and 8-NO2
5.8
9a
7-Br and 5,6-diNO2 4F
18.17
0.12
9b
epalrestat
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aIC50 (95% CL) values represent the concentration required to effect 50% enzyme inhibition
bRepresents the percent inhibition of compound determined at concentration of 10 µM
To understand the mechanistic details of biological activity and the importance of nitro group at C8
position, the docking of 7d and 7a compounds was performed. The more active compound, 7d was
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docked into the binding pocket of the human ALR2/NADP+/lidorestat complex (PDB code: 1Z3N). As
shown in Figure 3a-b, the results of 7d revealed that ligand fits into the active site of the enzyme. The
nitro group orientate itself towards the anion-binding pocket by making two hydrogen bonds one with
indole N-H bond of Trp 20: one through the N-atom (3.52 Å) and the second through an O-atom of the
nitro group (2.68 Å). Additionally, the oxygen of the 3- phenoxy group interacts via hydrogen bonding
with Cys 298 (2.77 Å). The phenoxy ring is stabilized by stacking with the phenolic ring of Tyr 298 and
fits into the specificity pocket formed by Leu 300, Cys 303, Cys 298, Thr 113, Phe 122, and Trp 111
while the quinoxalinone core structure penetrates into the hydrophobic pocket formed by the side chains
of Trp 20, Phe 122, Trp 79 and Trp 219. Actually, both the 8-nitro group and the 3-phenoxy moiety
without para-substituent provide a favored conformation for interaction of 7d with the active site,
resulting in an enhanced activity for 7d compared with 7a. In contrast, docking of 7a (Figure 3c, d)
revealed the formation of a single hydrogen bond only between an O-atom of the nitro group and the NH
group of Trp 20 (3.27 Å). The nitro group is oriented far away from the anion-binding pocket, and
similarly the phenoxy group is located slightly farther from the specificity pocket, 18 these findings
indicate different interaction patterns for the two compounds. The docking results provide support for
the observation that 7d is a more potent inhibitor than 7a.
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Figure 3. Docking of inhibitors into active site of ALR2; a) Docking of 7d into active site of ALR2 in
ribbon diagram; b) in surface representation. The docked position of 7d is shown in cyan (C), red (O)
and blue (N); hydrogen bonds are shown as light yellow dashed lines; c) Docking of 7a into active site
of ALR2 in ribbon diagram; d) in surface representation. The docked position of 7a is shown in cyan (C),
red (O) and blue (N); hydrogen bonds are shown as light yellow dashed lines.
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In conclusion, we have developed a novel, non-acid series of ARIs consisting of nitroquinoxalinone
derivatives 7a-f and 9a, b. These compounds showed significant activity with IC50 values of 1.54-18.17
µM. The most potent inhibitor was compound 7e with two nitro groups at C5 and C8 and a 3-phenoxy
substituent. Our SAR studies revealed the 8-nitro and 3-phenoxy groups as the more important features
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for the activity, which is further enhanced by adding a 5-nitro substituent. However, substitution at the
para-position of the 3-phenoxy moiety decreased the activity.
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Acknowledgments
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This work was supported by the National Natural Science Foundation of China (grant no. 21272025),
the Research Fund for the Doctoral Program of Higher Education of China (grant no. 20111101110042),
and the Science and Technology Commission of Beijing (China) (grant no. Z131100004013003)
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Graphical Abstract
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