A series of quinoline analogues as potent inhibitors of C. albicans prolyl tRNA synthetase

Article abstract of DOI:10.1016/S0960-894X(00)00697-1

A series of quinoline inhibitors of C. albicans prolyl tRNA synthetase was identified. The most potent analogue, 2-(4bromo-phenyl)-6-chloro-8-methyl-4-quinolinecarboxylic acid, showed IC₅₀=5 nM (Ca. ProRS) with high selectivity over the human enzyme.

Full text of DOI:10.1016/S0960-894X(00)00697-1

Bioorganic & Medicinal Chemistry Letters 11 (2001) 541±544
A Series of Quinoline Analogues as Potent Inhibitors of
C. albicans Prolyl tRNA Synthetase
Xiang Y. Yu,a,* Jason M. Hill,a Guixue Yu,a Yifeng Yang,a Arthur F. Kluge,a
Dennis Keith,a John Finn,a Paul Gallant,b Jared Silvermanb and Audrey Limb
aDepartment of Medicinal Chemistry, Cubist Pharmaceuticals, Inc., 24 Emily Street, Cambridge, MA 02139, USA
bDepartment of Biology, Cubist Pharmaceuticals, Inc., 24 Emily Street, Cambridge, MA 02139, USA
Received 16 October 2000; accepted 7 December 2000
AbstractÐA series of quinoline inhibitors of C. albicans prolyl tRNA synthetase was identi®ed. The most potent analogue, 2-(4-
bromo-phenyl)-6-chloro-8-methyl-4-quinolinecarboxylic acid, showed IC50=5 nM (Ca. ProRS) with high selectivity over the
human enzyme. # 2001 Elsevier Science Ltd. All rights reserved.
The incidence of serious fungal infection continues to
grow and there is an urgent need for novel antifungal
therapies.1,2 Aminoacyl-tRNA synthetases (aaRSs) are
essential enzymes for biological cell growth.3À7 Because
there are signi®cant structural di€erence between fungal
and human enzymes, selective inhibition of aaRSs o€ers a
potential pathway for discovery of novel antifungal agents.
Previously, we have described novel antibacterials targeting
aminoacyl-tRNA synthetases.8,9 Herein, we report novel
quinoline inhibitors of C. albicans prolyl tRNA synthe-
tase. The quinoline lead, 2-(4-bromo-phenyl)-4-quinoline-
carboxylic acid 1a, was identi®ed from high-throughput
screening of library compounds. Compound 1a inhibits
C. albicans prolyl-tRNA synthetase (IC50=0.5 mM) with
high selectivity over the human enzyme (IC50 >1 0 0 mM).
To examine the structure±activity relationships, we have
prepared and evaluated a series of quinoline analogues.
Scheme 1.
Our synthetic approach for the preparation of quinoline
analogues is shown in Scheme 1. Reaction of a sub-
stituted isatin with a ketone gave 4-quinoline carboxylic
acid derivative 1.10 The 6-alkenyl group (2a) was intro-
duced from coupling of iodoquinoline 1b with alkene
under a typical Heck condition.11 The 6-aryl group (2b)
was introduced from a palladium-catalyzed (Suzuki) cou-
pling of arylboronic acid and iodoquinoline 1b.12
quinoline 1c was esteri®ed and converted to the bromide
quinoline 3.13 Treatment of 3 with aqueous silver nitrate
followed by hydrolysis gave the corresponding alcohol
4.14 Dibromination of 1c followed by a similar treat-
ment with aqueous silver nitrate, depending upon the
conditions, yielded benzaldehyde quinoline 6 or benzoic
acid quinoline 7.
Quinoline analogues with modi®cation at the 8-methyl
position were prepared as shown in Scheme 2. The
Derivatives of the 4-quinolinecarboxylic acid were pre-
pared as shown in Scheme 3. Reduction of 1c gave the
hydroxyl derivative 8, which was converted to the sulfa-
moyl derivative 9.8 Treatment of the primary amide with
thionyl chloride yielded the nitrile derivative 12,15 which
*Corresponding author. Tel.: +1-617-576-4171; fax: +1-617-576-0232;
e-mail: xiang@cubist.com
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00697-1

542
X. Y. Yu et al. / Bioorg. Med. Chem. Lett. 11 (2001) 541±544
was reacted with trimethylsilyl azide in the presence of
dibutyltin oxide to a€ord the tetrazole derivative 13.16
8-methyl analogue 1c. Substitution with a halogen at the
7-position and a methyl at the 8-position (1v) and (1w)
decreased the potency. All of these compounds have
high selectivity over the human enzyme.
2-Aryl analogues were evaluated in assays of the amino-
acylation activity of C. albicans prolyl-tRNA synthetase
and the results are shown in Table 1. The bromophenyl
compound 1a was signi®cantly more potent than the
simple phenyl analogue 1d. Analogue 1e with an electron-
donating -OMe group and analogue 1f with an electron-
withdrawing -CF3 group showed a signi®cant loss in
potency. Replacement of the bromophenyl group by a
heterocyclic group such as the 2-Cl-thiophene group (1i)
decreased the activity. The furyl analogue 1j and the
pyridyl analogue 1k were inactive.
To explore other functional groups at the 4-position, we
replaced the carboxylic acid with sulfamoyl amide,
amide, ester, and tetrazole (Table 3). The replacement
of the carboxylic acid with the methylene sulfomyl
amide (9), the amide (10a) and the phenylbenzyl ester
(11a) decreased the potency. The 3,4-dichlorobenzyl
ester (11b), the furfuryl ester (11c), and the tetrazole
derivative 13 had a moderate inhibition. The results
suggest that a carboxylic acid functionality at this posi-
tion is critical for good enzyme inhibition.
Substitution in the benzene portion of quinoline can
dramatically improve the activity (Table 2). 6-Halogen
substituted analogues (1b, 1l and 1m) were all very
active. However, incorporation of hydrophilic moieties
at the 6-position (1o and 1p) decreased the potency. The
6-chloro-8-methyl analogue 1c was the most potent
compound (IC50=5 nM). The 6-chloro-8-hydroxyl-
methlene analogue 4 (IC50=1500 nM), however, was
considerably less potent. This observation implied the
importance of lipophilicity of the substitution in the 8-
position for good inhibition. A comparison of analogue
1c and analogue 1r reveals that changing the relative
position of the chloro group and the methyl group
resulted in a 5-fold weaker inhibition. The 6,8-dimethyl
analogue 1q was 5000 times less potent than the 6-chloro-
In summary, we have developed a series of potent and
selective C. albicans prolyl-tRNA synthetase inhibitors.
The enzymatic potency observed with several of these
analogues is signi®cantly superior to that of lead com-
pound 1a (IC50=500 nM, C. albicans ProRS). SAR
studies suggest that the potency is greatly in¯uenced by
the substitution pattern on the quinoline. Analogue 1c
possesses excellent activity against C. albicans ProRS
(IC50=5 nM) with high selectivity over the human
enzyme (IC50 >100 mM). Enzymology studies indicate
that compound 1c is a non-competitive inhibitor of the
C. albicans ProRS with respect to proline and a competi-
tive inhibitor with respect to ATP. The Ki's were 15 and
5.1 nM versus proline and ATP, respectively. However, the
Scheme 2.
Scheme 3.

X. Y. Yu et al. / Bioorg. Med. Chem. Lett. 11 (2001) 541±544
Table 1. Inhibition of C. albicans prolyl-tRNA synthetase
543
Y
Compd
1a
1d
1e
1f
1g
1h
1i
1j
1k
IC50 (mM) (Ca. ProRS)
IC50 (mM) (human ProRS)
0.5
>100
>100
>100
>100
>100
>100
>100
19
>100
3.6
>100
32
>100
>100
>100
>100
>100
Table 2. Inhibition of C. albicans prolyl-tRNA synthetase
Compd
R5
R6
R7
R8
IC50 (mM) (Ca. ProRS)
IC50 (mM) (human ProRS)
1a
1b
1l
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
Cl
H
H
H
H
I
Cl
Br
CF3O
HOS2O
NH2
H
H
H
H
H
H
H
H
H
H
H
H
H
Cl
Cl
H
Br
F
H
H
H
H
H
H
H
H
H
0.500
0.019
0.026
0.025
0.053
15
3.3
1.3
0.350
0.005
25
0.025
1.5
1.5
0.140
0.220
0.150
0.120
>100
>100
>20
>20
>100
>100
95
1m
1n
1o
1p
2a
2b
1c
1q
1r
4
1s
1t
1u
1v
1w
HO2CCHˆCH2
80
35
40-OH-PH
Cl
Me
Me
Cl
H
H
Br
H
Me
Me
Cl
CH2OH
H
>20
>100
>20
>100
>100
>100
56
H
CF3
Me
Me
70
60
H
Table 3. Inhibition of C. albicans prolyl-tRNA synthetase
X
Compd
CO2H
CH2OSO2NH2
CO2CH2PhPh
1c
910a
11a
11b
11c
13
IC50 (Ca. ProRS, mM)
IC50 (human ProRS, mM) >100
0.005
2.4
Ð
5.7
>100
8.8
>100
0.02
>100
0.07
>100
0.03
>100
Acknowledgements
most active analogue 1c has no whole cell activity (MIC
>1 0 0 mg/mL, C. albicans). Analogue 1v has cellular activ-
ity (MIC=25 mg/mL, C. albicans). Further studies indicate
that the activity is due to a lytic mechanism.
The authors wish to thank the biological and analytical
sta€ at Cubist for enzyme assay data and spectra.

544
X. Y. Yu et al. / Bioorg. Med. Chem. Lett. 11 (2001) 541±544
9. Yu, X. Y.; Hill, J. M.; Yu, G.; Wang, W.; Kluge, A. F.;
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