Quinolines: a new hope against inflammation

Reviews�POSTSCREEN

Drug Discovery Today � Volume 18, Numbers 7/8 �April 2013 REVIEWS

Quinolines: a new hope againstinflammationSoumita Mukherjee and Manojit Pal

Organic and Medicinal Chemistry, Institute of Life Sciences, University of Hyderabad Campus, Gachibowli, Hyderabad 500046, India

Although a number of anti-inflammatory drugs have been discovered and developed to treat diseases

associated with acute and chronic inflammation, many anti-inflammatories cause adverse side effects.

The quinoline framework has emerged as a new template for the design and identification of novel anti-

inflammatory agents. These agents are classified based on the number of substituents present on the

quinoline ring or compounds containing a quinoline ring fused to other heterocycles. This review

focuses on the discovery of various quinoline derivatives as inhibitors of cyclooxygenase (COX),

phosphodiesterase 4 (PDE4) and tumour necrosis factor-a converting enzyme (TACE), along with

transient receptor potential vanilloid 1 (TRPV1) antagonists.

IntroductionInflammation, the first response of the immune system to harmful

stimuli such as infection or irritation, consists of a cascade of

biochemical events that propagates and matures the inflammatory

response. It is a protective attempt by the organism to remove the

injurious stimuli and initiate the healing process. However, if

uncontrolled, inflammation can lead to a diverse array of acute,

chronic and systemic inflammatory disorders [1–3]. Some of the

diseases related to chronic inflammation include cardiovascular

disease, autoimmune disease, periodontal disease and Alzheimer’s

disease, along with asthma, diabetes, COPD, among others.

Quinolines as anti-inflammatory agentsQuinolines have attracted particular attention owing to their

diverse array of pharmacological properties including the ability

to target several causes of inflammation. These include inhibitors

of cyclooxygenase-2 (COX-2), phosphodiesterase 4 (PDE4) and

tumour necrosis factor (TNF)-a converting enzyme (TACE), as

well as transient receptor potential vanilloid 1 (TRPV1) antago-

nists. Several reviews have appeared on the anticancer or anti-

tumor [4,5], antimalarial [6] and antimicrobial activities [7] of

quinolines. However, to our knowledge, no review is available on

their anti-inflammatory potential. This review will focus on a

Corresponding author:. Pal, M. ([email protected]), ([email protected])

1359-6446/06/$ - see front matter � 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.drudis.

brief description of the pharmacological targets of inflammation

followed by the quinoline-based modulators that have been

identified during the past 10 years.

Targeting COX-2: prostaglandin inhibitionNonsteroidal anti-inflammatory drugs (NSAIDs) inhibit the

enzyme activity that converts polyunsaturated fatty acids to pros-

taglandins during the inflammatory process. The prostaglandin

endoperoxide synthase or fatty acid COX catalyses the two-step

conversion of arachidonate to prostaglandin (PG) H2, the common

intermediate in all prostaglandin syntheses. Both COX isoforms

(i.e. COX-1 and COX-2) are bifunctional and membrane-bound

enzymes that catalyse two sequential reactions: the double dioxy-

genation of arachidonic acid to PGG2 and the reduction of PGG2 to

PGH2, which are then transformed to prostaglandins (i.e. PGE2,

PGD2, PGF2a, PGI2) and thromboxane A2 (TxA2) by different

tissue-specific isomerases [8,9].

The inhibition of COX-2 by NSAIDs results in their anti-inflam-

matory and analgesic activities, whereas simultaneous inhibition

of COX-1 leads to ulcerogenic side effects. The developed COX-2-

specific NSAIDs of today have reduced the adverse effects to a great

extent compared with COX-1 inhibitors – having similar anti-

inflammatory, antipyretic and analgesic activities. However, the

long-term use of some COX-2 inhibitors has been reported to be

associated with an increased risk of cardiovascular and cerebro-

vascular problems [10].

2012.11.003 www.drugdiscoverytoday.com 389

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REVIEWS Drug Discovery Today � Volume 18, Numbers 7/8 �April 2013

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Targeting TRPV1: vanilloid receptor antagonistsThe vanilloid receptor antagonists have emerged as a new class of

drugs for the treatment of chronic pain. The TRPV1 receptors,

found in the peripheral and central nervous systems, are involved

in the transmission and modulation of pain, as well as the

integration of diverse painful stimuli. This receptor channel is

activated by protons (pH 5, chemical stimuli), heat (>428C,

physical stimuli), endogeneous substances (such as endocanna-

binoid anandamide, lipoxygenase and arachidonic acid metabo-

lites) and also by some natural ligands (such as capsaicin, other

vanilloids and resiniferatoxin). This results in Ca2+ influx into the

cells through the channel pore, causing cell membrane depolar-

isation and excitation of primary sensory neurons. It then trans-

mits noxious nerve impulses to the spinal cord and finally

delivers the perception of pain [11]. Upon continued stimulation

from any external stimuli the activity of neurotransmitters is

depleted causing selective damage of the nerves and, thereby,

results in desensitisation to further stimuli. As a result, TRPV1

receptors lose sensitivity to painful stimuli. Various TRPV1 ago-

nists were used to treat pains (e.g. capsaicin, resiniferatoxin, etc.).

However, several adverse side effects like burning sensation,

irritation and neurotoxicity that were associated with this

approach shifted the focus toward the discovery of TRPV1

antagonists. These antagonists block the pain signalling pathway

with potentially fewer side effects. Capsazepine, the first reported

TRPV1 antagonist, has shown antihyperalgesic effects not only

against capsaicin but also against other inflammatory stimuli

[12–14]. However, inability of capsazepine to block acid- or

heat-induced activation of TRPV1 prompted researchers to search

for better TRPV1 antagonists.

Targeting PDE4: cytokine inhibitionPDEs are a superfamily of enzymes that degrade cAMP and cGMP.

The cAMP and cGMP levels are known to maintain many biolo-

gical responses (e.g. secretion, contraction, metabolism and

growth) [15]. The cAMP and cGMP synthesised by adenylyl

cyclases (ACs) and guanylate cyclases (GCs) are degraded (hydro-

lysed) by a variety of PDEs present in the cells to the inactive

products 50-AMP and 50-GMP, thus maintaining the intracellular

levels of cAMP and cGMP. According to their specificity for cAMP

or cGMP, PDEs can be subdivided into 11 different groups or

isozymes (PDE1–PDE11). The cAMP-specific PDE4 isozymes [16]

are encoded by four genes (A–D) that give rise to four isoforms (i.e.

PDE4A–D) [17]. The increased level of cAMP is vital in inflamma-

tory cells because it acts as a negative regulator of the primary

activating pathways such as cytokine release by T cells and the

level is regulated by the cAMP-specific PDE isozymes (predomi-

nantly PDE4) [18]. Hence, targeting and inhibiting the PDE4

enzymes in inflammatory cells can effectively enhance the intra-

cellular cAMP level thereby inhibiting the release of inflammatory

mediators such as cytokines (Fig. 1). Factors that cause increase of

cAMP levels also include inhibition of mast cell mediator release,

suppression of neutrophil degranulation, inhibition of basophil

degranulation and inhibition of monocyte and macrophage acti-

vation. Thus, PDE4 inhibitors have been investigated for the

potential treatment of asthma and COPD. Although roflumilast

has been launched recently in Europe and the USA for the treat-

ment of COPD, development of better PDE4 inhibitors with

390 www.drugdiscoverytoday.com

reduced side effects such as nausea and emesis has become an

active area of research.

Targeting TACE: TNF-a inhibitionTACE is a member of the ADAM (a disintegrin- and metallopro-

teinase-containing enzyme) branch of the zinc metalloproteinase

family and is responsible for cleaving membrane-bound 26 kDa

proTNF-a to its soluble 17 kDa form [19–22]. TNF-a induces pro-

inflammatory cytokines [including interleukin (IL)-1 and IL-6],

activates leukocytes, induces acute-phase reactants and metallo-

proteinases, and inhibits apoptosis of inflammatory cells. It also

has a pivotal role in the progression of diseases like rheumatoid

arthritisis (RA), psoriasis and Crohn’s disease. Among the many

strategies available to inhibit TNF-a production, TACE inhibition

is a promising and potential therapeutic target. TACE is a type I

transmembrane protein, synthesised as a zymogen. It contains a

pro-domain, a catalytic domain, a disintegrin and cysteine-rich

region, a transmembrane segment and a cytoplasmic tail. The free

cysteine residue present in the pro-domain coordinates with the

zinc in the active site of TACE and thus prevents its activity. The

catalytic site of TACE is analogous to matrix metalloproteinase

(MMP). As a result, several early MMP inhibitors were found to

inhibit TACE. But they failed in clinical trials owing to their dose-

limiting adverse musculoskeletal side effects [23–25], thought to

be caused by their specificity toward MMPs [26,27]. Hence, it is

desirable to develop selective TACE inhibitors devoid of MMP

activity [28,29].

Monosubstituted quinolines: novel vanilloid receptorantagonistsMany monosubstituted quinolines have been reported as TRPV1

antagonists. One such report identified quinoline derivatives 1

and 2 [30] (Fig. 2) possessing oral bioavailability in rats (F = 39%

and 17%, respectively) as promising agents in the capsaicin (CAP)

mediated functional assay (IC50 = 1.9 and 0.42 nM, respectively)

and the pH-mediated assay (IC50 = 1.3 and 1.0 nM, respectively).

The maximum plasma concentrations for 1 and 2 were

Cmax = 540 ng/ml and 320 ng/ml at 5 mg/kg p.o., respectively.

SAR on a series of conformationally constrained analogues of

the cis conformer of compounds 1 and 2 identified 7-oxo and

8-oxoquinoline derivatives 3 and 4 as TRPV1 antagonists (Fig. 2).

Compound 3 was orally bioavailable (F = 31% in rats) and showed

potent antagonism against rat TRPV1 [rTRPV1 (CAP) IC50 = 7.4 nM

and rTRPV1 (acid) IC50 = 8.0 nM] and human TRPV1 [hTRPV1

(CAP) IC50 = 3.7 nM and hTRPV1 (acid) IC50 = 4.2 nM] in CAP-

and acid-mediated assays. It also blocked TRPV1-mediated phy-

siological response in the CAP-induced hypothermia model in

rats. However, it was inefficacious at preventing thermal hyper-

algesia generated by complete Freund’s adjuvant in rats [31].

Therefore, to improve the potency and pharmacokinetic (PK)

properties of 3, the 8-oxoquinoline derivative 4 [rTRPV1 (CAP)

IC50 = 15 nM] was designed with an encouraging microsomal sta-

bility [32]. Compound 4 was cleared at the rate of 44 and 135 ml/

min/mg in rat and human, respectively, compared with 111 and

250 ml/min/mg for compound 3.

A quinoline carboxamide derivative 5 (Fig. 2) with N-methyl

substitution showed moderate activities against hTRPV1

(pKb = 6.5) with capsaicin as the agonist [33]. Optimisation of 5


NH2 NH2

NN

N N N N

NNNH2

N N

NN

OH

OH

OHOH

OH

OH

O

O O

O

OO

O

O

O

O

O

O

O

O

OO−

Adenylate

O− O−

O−

O−

P

P

PP

P

cyclase

cAMP-specific

PDEs M+2

Regulatory effects oncytokine release e.g.TNF-α, IL-2, IL-12,LTB4, IFN-γ

ATP AMPcAMP

Inhibit mastcell release

Suppressneutrophildegranulation

Inhibit basophildegranulation

Inhibit monocyteactivation

Inhibit macrophageactivation

Drug Discovery Today

FIGURE 1

Synthesis and degradation of cAMP and its role in cytokine inhibition.

N

CF3

CF3

CF3

3

5 6

1, X = O2, X = CH2

4

N

N

N N

Ph

N N

N

N

N N

N

X

H

HN

N

HN

O

O

O

O

O


FIGURE 2

Monosubstituted quinolines as TRPV1 antagonists.

www.drugdiscoverytoday.com 391



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led to the development of compound 6 (Fig. 2), with a carbox-

amide at the 7-position and no substitution on the quinoline

nitrogen. Compound 6 exhibited good levels of in vitro metabolic

stability (CLi < 5 ml/min/g liver) in human, rat, guinea pig and

dog liver microsomes and also P450 inhibition with IC50 values

>18 mM at five major human isoforms (1A2, 2C9, 2C19, 2D6 and

3A4). It also showed excellent potency against hTRPV1 [pIC50

(acid) = 8.1] and rTRPV1 receptors [pIC50 (acid) = 7.6] [33]. Overall,

quinolines with carboxamide substitution and 8-oxo-substituted

quinolines showed promising TRPV1 antagonism.

Disubstituted quinolines: a pharmacophore for PDE4and TACEA disubstituted potent and selective PDE4 inhibitor, 8-methoxy-

quinoline-4-carboxylic acid (3,5-dichloropyridyl-4-yl)amide (7;

D4418; Fig. 3) (PDE4 IC50 = 0.17 mM; rolipram binding assay

(RBA) IC50 = 0.53 mM) [34] showed good PK profile (Cmax =

473 ng/ml at 5 mg/kg p.o. and F = 62%) in guinea pig along with

reduced liability for emetic and central nervous system (CNS) side

effects. Compound 8 (L454560) containing an unsaturated link at

the 30-position (Fig. 3) showed promising PDE4 activity (PDE4A

IC50 = 1.4 nM and human whole blood assay (HWB) IC50 = 0.16 mM)

[35,36] but triggered an emetic response in squirrel monkeys at a

plasma concentration of 3.8 mM at 10 mg/kg (p.o.). However, the

liabilities associated with 8 were overcome by compounds 9 and 10

[37] (Fig. 3), which were found to be less emetic in squirrel monkeys.

The acid 10 exhibited 62% inhibition of ovalbumin-induced

bronchoconstriction in guinea pig at 30 mg/kg (i.p.). Replacing

the olefin moiety of 8 by a suitable amide linker provided compound

11 (Fig. 3) which showed improved activity in the same assay with

74% inhibition of PDE4 at 30 mg/kg (i.p.) [38].

A disubstituted quinoline 12 (IK682) with g-lactam scaffold

(Fig. 3) was found to be a potent inhibitor of porcine TACE (pTACE

IC50 = 1.0 nM, Ki = 0.56 nM) and exhibited >2000-fold selectivity

for pTACE relative to MMP-1, -2, -9, -13, -14, -15 and -16 [39].

Replacing the hydroxamic acid group with a pyrimidine-2,4,6-

trione moiety resulted in compound 13 (Fig. 3) (pTACE

IC50 = 1.03 mM) [40] which was inactive in the MMP-1, -2, -9

and -13 assays (at 10 mM) and the aggrecanase assay (at 1 mM).

Further SAR studies on different linkers between the pyrimidine-

trione and 4-(2-methylquinolin-4-ylmethoxy)phenyl group

afforded a non-hydroxamate TACE inhibitor 14 (pTACE

IC50 = 2 nM) [41] (Fig. 3).

A tetrahydropyran-substituted-b-aminohydroxamic acid 15

[42] (Fig. 3) with a quinoline core exhibited a Ki of 0.35 nM in

pTACE assays and HWB IC50 value of 150 nM (F = 79% in dog and

15% in rat). Further exploration of hydroxamate-based TACE

inhibitors resulted in b-benzamido hydroxamic acid 16 [43]

(Fig. 3) with LPS-stimulated HWB IC50 = 130 nM and pTACE

Ki = 0.15 nM. It showed selectivity for pTACE >2000-fold relative

to 16 MMPs with oral bioavailability of 58% in rats and 96% in

dogs. Replacing the pyran moiety with (3R,4S)-tetrahydrofuranyl

group of b-benzamido hydroxamic acid resulted in compound 17

[44] (Fig. 3), which showed activity against pTACE (IC50 = 1 nM),

good potency in the cellular assay (HWB IC50 = 33 nM) and oral

bioavailability (F = 21% in rats). To optimise the TACE activity,

oxaspiro[4.4]nonane b-benzamido hydroxamic derivative 18 [45]

(Fig. 3) was synthesised which showed pTACE inhibition


(IC50 = 1.0 nM), with >1000-fold selectivity for pTACE over

MMP-1, -2 and -9 and suppression of TNF-a in a rodent model

of endotoxemia (ED50 = 1–3 mg/kg). Studies on non-hydroxamate

TACE inhibitors resulted a 1,3,4-triazole-2-thione derivative 19

[46] (Fig. 3) which showed pTACE inhibition (IC50 = 1.5 nM) com-

parable to hydroxamates. A completely different class of hydro-

xamate TACE inhibitor [47], for example 20, showed TACE Ki of

6 nM. An increased level of TACE activity (TACE Ki = 3 nM) was

observed when the 2-methyl group of the quinoline ring was

replaced with a trifluoromethyl group (compound 21; Fig. 3).

Although a larger substitution at C-2 [X = Ph: 22 (TACE

Ki = 12 nM)] decreased the activity, replacement of an ester moiety

with a cyano group (e.g. 23; Fig. 3) with the same substitution at C-

2 improved the in vitro profile (TACE Ki = 4 nM). A disubstitution at

C-4 and C-7 of quinoline, for example 24 (Fig. 3), showed good

TRPV1 antagonism (IC50 = 1.65 nM) [48].

A number of 2,3-disubstituted quinolines, for example 25–28

[49] (Fig. 3), showed PDE4 inhibiting properties via increase of the

cAMP level at 10 mM over forskolin control in a cell-based cAMP

reporter assay. A methoxy group present on the indole ring (e.g.

compound 27) showed better results than Cl (25) and Br (26). A

change in the activity was noticed when OMe was shifted from C-5

to C-6 on the indole ring (28). Compound 28 showed a dose-

dependent increase of the cAMP level with EC50 = 0.89 mM com-

parable to rolipram.

It is therefore evident that linking carboxamide to the quinoline

ring through its carbon atom (instead of the nitrogen as detailed in

the previous section) and presence of one more substituent at C-8

shifted the selectivity toward PDE4. PDE4 selectivity was also

noticed in the case of quinolines with different substitution at

the 30-position of an 8-phenyl quinoline derivative. By contrast, 2-

alkyl-4-(phenoxymethyl)quinoline was found to be a suitable

template for the identification of TACE inhibitors.

Trisubstituted quinolines: agents of promiseThe introduction of a third substituent, for example a CF3 group at

C-2 of the quinoline ring (e.g. compound 29 or SCH365351;

Fig. 4), improved the pharmacological profile of PDE4 inhibitor

7 D4418 [50]. Compared with D4418, compound 29 (PDE4

IC50 = 0.051 mM) showed an improved PK in guinea pigs

(Cmax = 380 ng/ml at 5 mg/kg orally, F = 78%) and no emetic side

effects in ferrets when dosed orally at 6 mg/kg. However, it showed

the presence of a major metabolite (i.e. the pyridine N-oxide 30;

Fig. 4) [51] in a rat PK study, levels of which were higher than the

parent compound at time points greater than 3 h. Indeed, com-

pound 30 (SCH351591) was identified as a potent and selective

inhibitor of PDE4 (PDE4 IC50 = 0.06 mM; RBA IC50 = 0.15 mM) with

an improved PK profile in rats and guinea pigs compared with 29

and showed no emetic side effects in ferret at 5 mg/kg oral dose. To

improve the therapeutic index, the dichloropyridine N-oxide por-

tion of 30 was replaced with a five-membered oxazole ring:

compound 31, which was found to be a novel pharmacophore

[52]. SAR studies on the polar end-group of 31 reported the lead

molecule 32 [53] (Fig. 4) which showed high selectivity toward

PDE4B (PDE4B IC50 = 0.06 nM; PDE10 IC50 = 950 nM; PDE11

IC50 = 2300 nM) and efficacy in the rat LPS-induced pulmonary

inflammation model (ED50 = 0.1 mg/kg p.o.) with high plasma

level (Cmax = 960 nM at 10 mg/kg orally) in the rat PK study.


7 (D4418)

12 (IK682)

15

OMe SO2Me

SO2Me

COC6H4F-p

R1

R2

MeO2S

SO2Me

CF3CN

CF3

CO2H

OH

N

N

N

N

N

N

N

N

N

NN

NN

R

R

R

R

N

3′ 3′NH

NH

NH

HN

HNHN

HN

HN

N

HN H

N

HN

HN

N X

N

NNH

NHN

HN

Ms

ClCl

HOHN

O

OO

O

O

O

O

O

OO

O O

O

O

O

O

O

O

O

O

OH

HO

HO

HO

HO

O

O

O

O

O

O

O

O

8 (L454560)11, R =

13, R=

14, R=

18, R=

20, R = CO2Et X = CH3

22, R = CO2EtX = Ph

21, R = CO2Et X = CF3

23, R = CN X = Ph

16, R=

17, R=

2425, R1 = Cl, R2 = H26, R1 = Br, R2 = H27, R1 = OCH3, R2 = H28, R1 = H, R2 = OCH3

10, R =

9, R =


N19, R =

HN

HN N

S

Ac

FIGURE 3

Disubstituted quinolines as PDE4 and TACE inhibitors.


Quinoline containing a carboxyl group at C-4 (e.g. compound 33;

Fig. 4) has been reported as a COX-2 inhibitor with IC50 = 0.07 mM

and SI = 687.1, comparable to celecoxib [54].

Trisubstituted tetrahydroquinoline urea 34 (Fig. 4) has been

reported as a TRPV1 antagonist in human TRPV1 calcium influx

assay and an inhibitor of CYP3A4 enzyme (hTRPV1 IC50 = 7 nM

and 47% CYP3A4 inhibition at 10 mM) [55]. Another compound,

35 (Fig. 4) with 5,5-diphenylpentadienamide moiety at the 4-

position was also reported as a TRPV1 antagonist [56]. The (R)-

enantiomer of 35 (hTRPV1 IC50 = 0.14 and rTRPV1 0.35 nM) was

more potent than the (S)-35 in the capsaicin-based assay and

showed good PK profile in rats, dogs and monkeys. The (R)-35



N

OCH3

CF3

NH

N

O

29 (SCH365351)

ClCl

N

OCH3

CF3

NH

N

O

O

30 (SCH351591)

ClCl

Polar end group

Linker element

Nucleotidebinding site

N

NO

R1

CF3

OCH3

31

H2N

N

NO

H2N NHO

CF3

OCH3

32

N

CO2H

SO2Me33

NN

HN NH

N

CF3

H

O

34

O

NH

F3C O

NH

O

OH

35

F

F

−

+


FIGURE 4

Trisubstituted quinolines as promising agents.

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was effective at preventing mechanical allodynia in rats in a dose-

dependent manner and reversed thermal hyperalgesia in a model

of neuropathic pain induced by sciatic nerve injury.

All these studies revealed that potency of disubstituted quino-

line 7 can be improved by introducing an additional substituent

(compound 29). Further modifications at the 5-position of the

trisubstituted quinoline 29 showed improved PDE4 activity indi-

cating the importance of the nature of a substituent at the 5-

position. The 2,3,4-trisubstituted quinoline with a carboxylic acid

group at the 4-position displayed COX-2-inhibiting properties.

This section exemplifies the importance of carboxamide groups

in TRPV1 antagonism.

Polysubstituted quinolines: substituents do matterBased on promising PDE4B inhibitory properties of a polysubsti-

tuted quinoline 36 (Fig. 5) (pIC50 = 8.4, F = 10% in rats) further

modifications on the anilino benzene ring gave the quinoline

derivatives 37 and 38 [57] (Fig. 5) (PDE4B pIC50 = 9.5, HWB


pIC50 = 8.1, F = 82%; and PDE4B pIC50 = 9.4, HWB pIC50 = 7.6,

F = 27% in rats, respectively) with improved potency and oral

PK profile in rats. For the direct delivery of PDE4 inhibitor to

the site of action in the lungs compound 39 (or GSK256066; Fig. 5)

was developed, which was suitable for inhaled administration [58].

GSK256066 is a potent PDE4 inhibitor with PDE4B IC50 = 3.2 pM

and LPS-stimulated TNF-a inhibition in human peripheral blood

mononuclear cells (PBMC) IC50 = 0.01 nM, >1000-fold selective for

PDEs 1–3 and 5–7 with no emesis in ferrets.

An N-substituted quinoline [e.g. 1-aroyl derivative of kynurenic

acid (KYNA or 4-hydroxy quinoline-2-carboxylic acid) methyl

ester 40; Fig. 5] has been reported to have anti-inflammatory

activity in the carrageenan-induced rat paw edema model and

affinity for COX-1 and COX-2 in docking studies [59]. KYNA, a

metabolite of tryptophan, is synthesised in brain astrocytes. Its

increased level in brain was thought to be formed by the induction

of NSAIDs, thus contributing to their analgesic efficacy probably

through an inhibitory action on COX-1 [60].


N

R3

SOO

NH

R1

R2

NH2

O

36, R1 = OMe, R2 = H, R3 = H37, R1 = OMe, R2 = F, R3 = Me38, R1 = CN, R2 = H, R3 = Me

N

NH

CONH2S

OMe

CONMe 2

O O

39 (GSK256066)

N

O

CO2Me

O

I

H3C

40

N

NHSO2Me

OPh

41

N

OH

Cl

42

N

CO2H

CO2H

R2

R1SO2Me

43, R1 = Me, R2 = Me44, R1 = Ph, R2 = H45, R1 and R2 = Ph46, R1 and R2 = Cyclohexyl

CO2Me


FIGURE 5

Examples of polysubstituted quinoline derivatives.


A 1-alkynyl-substituted 1,2-dihydroquinoline derivative 41 [61]

(Fig. 5) exhibited dose-dependent inhibition of PDE4B (23%, 16%,

11% and 10% at 10, 3, 1 and 0.3 mM) and significant inhibition of

TNF-a (47% and 35% at 30 and 10 mM) in vitro.

Polysubstituted quinolines (e.g. 4-carboxyl quinoline derivative

42; Fig. 5) have been reported as P-selectin antagonists

(IC50 = 4.5 mM in cell-based flow activity) [62]. P-selectin found

in the inflamed joints of RA patients is an important target for the

prevention and treatment of inflammatory diseases [63]. Com-

pound 42 demonstrated good PK properties in rats (F = 24%). The

4-carboxyl quinoline framework has also been reported as a sui-

table template for the design of COX-1 and COX-2 inhibitors and

the activity was dependent on the lipophilic nature of the C-7 and

C-8 substituents (e.g. compounds 43–46; Fig. 5) [64]. Indeed, the

activity was depleted on moving from 46 to 43 (COX-2

IC50 = 0.043, 0.054, 0.071 and 0.075 mM for 46, 45, 44 and 43,

respectively). Compound 46 showed better inhibition and selec-

tivity (SI > 513) than celecoxib. Its superior activity over 45 was

attributed to the cyclohexyl ring being fused with the quinoline

moiety.

Overall, these studies revealed that an anilino moiety at C-4

and an amide group at C-3, along with other substituents at C-6

and C-8, were important for PDE4 inhibition. Notably, although

compounds 42–46 possess a common functional group at the 4-

position of the quinoline, the variation of substituents at other

positions changed their pharmacological properties. Thus, 42

showed P-selectin antagonism, whereas 43–46 were COX

inhibitors.

Heterocycles fused with a quinoline: structurallydiverse small moleculesQuinolines fused with other heterocycles, for example 4-anili-

nofuro[2,3-b]quinoline 47 and 4-phenoxyfuro[2,3-b]quinoline

48 [65] (Fig. 6), have been reported as anti-inflammatory

agents. With respect to mast cell degranulation performed by

measuring the content of b-glucuronidase in supernatant, com-

pounds 47 (IC50 = 6.5 mM) and 48 (IC50 = 16.4 mM) displayed

superior activities to the reference inhibitor mepacrine

(IC50 = 20.6 mM). Although compound 47 showed promising

inhibition of neutrophil degranulation (IC50 = 7–12 mM), none

of them displayed a similar IC50 value to dexamethasone in the

inhibition of TNF-a formation in macrophage-like cell line RAW

264.7 and microglial cell line N9 (the brain resident macro-

phages).

The pyrazolo[4,3-c]quinoline-4-one 49 (IC50 = 4.7 mM for COX-

1 and 0.24 mM for COX-2) and 50 (IC50 = 5.0 mM for COX-1 and

0.55 mM for COX-2) (Fig. 6) designed from celecoxib [66] exhibited

potency and selectivity toward COX-2. The presence of the NO2

group on the quinolone skeleton and a sulfonamide on the phenyl

ring was thought to be responsible for COX-2 selectivity. These

compounds also showed activity in the carrageenan-induced rat

paw model (25–45% inhibition at 30 mg/kg).

Triazole-fused quinolines, for example 7-alkoxy-1-amino-4,5-

dihydro[1,2,4]triazole[4,3-a]quinolines 51 and 52 [67] (Fig. 6),

have shown comparable anti-inflammatory activities with ibupro-

fen at oral doses of 200, 100 or 50 mg/kg 2 h before xylene

application.



N O

HN Cl

CH3

NOH

47

N O

O

CH2OH

48

N N

R2

CH3

NH

O

49, R1 = NO2, R2 = SO 2NH 250, R1 = NO2, R2 = H

R1N

NN

R1O

H2N

51, R1 = -CH2C6H552, R1 = -CH2C6H4(p-Cl)

NH

S

N

O

O

OO

CO2CH 3

55

Y

X

N

O

O

OH

O

53, X = SO2, Y = NH54, X = NH, Y = SO2

N N

NH

O

NHZ

Ph

57, Z = NH258, Z = NHCOCH3

N N

N

O

Ar H

Ar

NH

N

OH

OH OHOH

OH

N N

N

O

Ar H

Ar

NH

NOH

OH

OHOH

59 60Ar = 4-C6H4Cl

NH

S

N

O

O

OO

CO2H

56


FIGURE 6

Quinoline fused with various heterocycles as anti-inflammatory agents.

Review

s�P

OSTSCREEN

Among the thiazine-fused qunolines, naturally occurring asci-

diathiazones A (53) and B (54) [68] (Fig. 6) were reported to

suppress MSU (monosodium urate) crystal-induced neutrophil

superoxide production at a dose of 25 mM/kg. To improve the

potency further, synthetic analogues 55 and 56 [69] (Fig. 6) were

prepared that exhibited better in vivo activity at 2.5 mM/kg along

with significant inhibition of superoxide production and neutro-

phil infiltration, indicating their potential to target neutrophilic

inflammation by reducing neutrophil recruitment and neutrophil

superoxide production.

Quinolines fused with a pyrimidine ring, for example 57 and 58

[70] (Fig. 6), have shown anti-inflammatory activities in the car-

rageenan-induced paw edema test in rats. The activity was also

TABLE 1

The nature and position of substituents present on the quinoline ri

Biological targets for

treating inflammation

Presence of substituents on the qui

TRPV1 Carboxamide (linked via its N atom)

TACE 2-Alkyl-4-(phenoxymethyl)quinoline (bas

PDE4 Carboxamide (linked via its C atom)

8-Phenyl quinoline with substitution at

COX Carboxyl functionality at 4-position


tested by introducing a glucoside moiety in the pyrimido-quino-

line framework, for example 59 and 60 [71] (Fig. 6), which showed

higher activities than indomethacin at all time points.

Although these data clearly indicate that quinolines fused with

other heterocycles are promising, unlike other quinoline deriva-

tives, these fused structures have a lack of target selectivity. Hence,

efforts should be devoted to improve their efficacy and target

specificity for identification of better anti-inflammatory agents.

Conclusion: the next generation of anti-inflammatorydrugsThe importance of the quinoline class of compounds has been

established in the search for safer and effective anti-inflammatory

ng determine pharmacological activity

noline ring responsible for target specificity

ic skeleton of hydroxamate and non-hydroxamate TACE inhibitors)

30-position; aniline substitution at 4-position and primary amide at 3-position


TABLE 2

Current development status of some quinoline derivatives

Compounds Pharmacological and/or disease targets Company Status

D4418 [72] PDE4 inhibitor for asthma Celltech and Schering-Plough Discontinued (Phase II)

SCH351591 [72] PDE4 inhibitor for asthma Celltech and Schering-Plough Discontinued (Phase I)

GSK256066 PDE4 inhibitor for COPD GlaxoSmithKline Completed Phase II

Compound 16 TACE inhibitor Bristol-Myers Squibb Preclinical candidate

BMS561392 [73] TACE inhibitor for RA and IBD Bristol-Myers Squibb Discontinued (Phase II)

Compound 35 TRPV1 antagonist Kyowa Hakko Kirin Developed as clinical candidate


agents. This is exemplified by the discovery of a variety of quino-

lines as inhibitors of COX, PDE4 and TACE, and TRPV1 receptor

antagonists. The pharmacological activities of these agents mainly

depend on the nature and position of substituents present on the

quinoline ring as summarised in Table 1.

With some of them currently in clinical trials (Table 2),

and many in the early stages of development, these agents

appear to be promising and seem to have potential to become

anti-inflammatory drugs in the future. Overall, the literature

presented here strengthens the claim that a quinoline scaffold

can be a useful pharmacophore for next-generation anti-inflam-

matory drugs to treat a wide range of diseases associated with

inflammation.

AcknowledgmentsThe authors thank Professor J. Iqbal, Director of ILS, Hyderabad,

for encouragement and support. S.M. thanks CSIR, New Delhi,

India, for a Research Associate Fellowship.

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Quinolines: a new hope against inflammation