Discovery of 4-hydroxy-2-oxo-1,2-dihydroquinolines as potential inhibitors of Streptococcus pneumoniae, including drug-resistant strains

Article abstract of DOI:10.1016/j.bmcl.2020.127071

New therapies for treating drug-resistant pneumococcal infections are urgently needed. The novel scaffold 6-hydroxy-4-oxo-1,2-dihydro-4H-quinoline was shown to have similar efficacies against all three different serotypes of S. pneumoniae, ATCC 49617 (19F), ATCC BAA-1663 (15B), and ATCC 700904 (19A), in a resazurin-based high-throughput screen using the Korea Chemical Bank library. Further studies to identify a new lead with this scaffold, including tricyclic pyrrolo[3,2,1-ij]quinolone and pyrido[3,2,1-ij]quinolone derivatives, led to the identification of 6d, 7d and 12a. Compound 6d (IC₅₀ = 0.92, 0.75, and 0.77 μM), 7d (IC₅₀ = 0.57, 0.66, and 0.38 μM) and 12a (IC₅₀ = 0.27, 1.03, and 0.62 μM) showed submicromolar IC₅₀ values against 19F, 15B, and 19A, respectively, and thus serve as a starting point for further optimization. While some of compounds in this series exhibited acceptable pharmacokinetic profiles in preliminary in vivo rat experiments, the most active compound 12a showed poor solubility and high plasma protein binding. Our current research efforts are focused on optimizing compounds to improve physicochemical properties as well as potency.

Full text of DOI:10.1016/j.bmcl.2020.127071

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Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of 4-hydroxy-2-oxo-1,2-dihydroquinolines as potential inhibitors
of Streptococcus pneumoniae, including drug-resistant strains
⁎
Srigouri Huddara,b, Chul Min Parkc, Hyung Jun Kimd, Soojin Jangd, Sunkyung Leea,b,
a Drug Information Platform Center, Korea Research Institute of Chemical Technology, Daejeon 34114, Republic of Korea
b Korea University of Science and Technology, Daejeon 34114, Republic of Korea
c Center for Convergent Emerging Virus Infection, Korea Research Institute of Chemical Technology, Daejeon 34114, Republic of Korea
d Discovery Biology Department, Antibacterial Resistance Research Laboratory, Institut Pasteur Korea, Seongnam 13488, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
New therapies for treating drug-resistant pneumococcal infections are urgently needed. The novel scaffold 6-
hydroxy-4-oxo-1,2-dihydro-4H-quinoline was shown to have similar efficacies against all three different ser-
otypes of S. pneumoniae, ATCC 49617™ (19F), ATCC BAA-1663™ (15B), and ATCC 700904™ (19A), in a re-
sazurin-based high-throughput screen using the Korea Chemical Bank library. Further studies to identify a new
lead with this scaffold, including tricyclic pyrrolo[3,2,1-ij]quinolone and pyrido[3,2,1-ij]quinolone derivatives,
led to the identification of 6d, 7d and 12a. Compound 6d (IC50 = 0.92, 0.75, and 0.77 µM), 7d (IC50 = 0.57,
0.66, and 0.38 µM) and 12a (IC50 = 0.27, 1.03, and 0.62 µM) showed submicromolar IC50 values against 19F,
15B, and 19A, respectively, and thus serve as a starting point for further optimization. While some of compounds
in this series exhibited acceptable pharmacokinetic profiles in preliminary in vivo rat experiments, the most
active compound 12a showed poor solubility and high plasma protein binding. Our current research efforts are
focused on optimizing compounds to improve physicochemical properties as well as potency.
Streptococcus pneumonia
Pneumococcal infection
Multidrug resistance
19A strain
1,2-Dihydroquinoline
Pneumococcal disease caused by Streptococcus pneumoniae is a major
public health problem worldwide.1,2 Pneumococcal vaccines have been
used to overcome severe pneumococcal diseases, but due to poor im-
mune response against certain vaccine serotypes, this continues to be a
major global health issue.3–9 Because the primary method of treating
pneumococcal infection is antibiotics, the rapid spread of antibiotic
resistance should be controlled. The resistance of S. pneumoniae to
commonly used antibiotics has continuously increased. Penicillin and
macrolide resistance have been observed at a rate of > 30% in Asian
and some European countries, and this is associated with the emergence
of multidrug resistance (MDR) in S. pneumoniae.10–14 It has been re-
ported that 64% of penicillin-nonsusceptible isolates causing invasive
pneumococcal disease were 19A strain, ATCC 700904™, which is highly
resistant to numerous antibiotics such as penicillin, erythromycin,
clindamycin, chloramphenicol, and tetracycline.7 Exceptionally high
prevalences of MDR in 19A have been observed in certain Asian
countries, including Hong Kong (100%), China (94.7%), and South
Korea (90.6%), which has significantly contributed to an increase in
antibiotic resistance.15,16
based high-throughput screening (HTS) system.17 Through resazurin-
based HTS using the Korea Chemical Bank (KCB), we found several
novel compounds that can inhibit the growth of S. pneumonia, including
a pandrug-susceptible reference strain ATCC 49617™ (19F), ATCC BAA-
1663™ (15B), and a multidrug-resistant strain ATCC 700904™ (19A).
Among those novel compounds, 1-ethyl-4-hydroxy-2-oxo-N-(thiazol-2-
yl)-1,2-dihydroxyquinoline-3-carboxamide (1) showed similarly high
efficacies against all three tested strains, with IC50 values of 6.4, 6.6,
and 6.2 µM against WT, 15B, and 19A, respectively (Fig. 1).
We evaluated two derivatives (2 and 3) of compound 1 that pos-
sessed 4-hydroxy-2-oxo-1,2-dihydroxyquinoline-3-carboxamide moi-
eties in the KCB library and found that compound 3 was also active on
all three S. pneumonia serotypes (Table 1). Compounds 1 and 3, with
novel scaffolds containing 1,2-dihydroquinoline, showed similar activ-
ities against all three tested strains, including the notorious 19A strain.
Then, we explored the feasibility of using this series of compounds for
the discovery of new antibiotics for treating pneumococcal infection.
Here, we describe the design, synthesis, biological evaluation, and
structure-activity relationships of compounds in this series.
Due to the urgent need for new antibiotics to treat pneumococcal
infections, we previously demonstrated the utility of the resazurin-
Compounds with hydrogen, methyl, ethyl, allyl, and n-butyl sub-
stituents at the nitrogen in the 1-position were prepared18,19 (Scheme 1)
⁎ Corresponding author at: Drug Information Platform Center, Korea Research Institute of Chemical Technology, Daejeon 34114, Republic of Korea.
E-mail address: leesk@krict.re.kr (S. Lee).
https://doi.org/10.1016/j.bmcl.2020.127071
Received 28 November 2019; Received in revised form 27 February 2020; Accepted 28 February 2020
0960-894X/©2020ElsevierLtd.Allrightsreserved.
Pleasecitethisarticleas:SrigouriHuddar,etal.,Bioorganic&MedicinalChemistryLetters,https://doi.org/10.1016/j.bmcl.2020.127071

S. Huddar, et al.
Bioorganic&MedicinalChemistryLettersxxx(xxxx)xxxx
hydroxyquinoline-3-carboxylates (5a–5d) were obtained from the cor-
responding anilines (4a–4d) with triethyl methanetricarboxylate
through an N-acylation and cyclization by heating at high temperature
(215 °C) for 15 min without solvent, and moderate yields were achieved
(40–51%). Ethyl esters (5a–5d) were converted to the 3-carboxamides
(6a, 6c, 6d, 7b, 7d, and 8a–8d) by a one-pot amide formation with the
appropriate arylamines including 2-aminothiazole and 2-amino-6-
chlorobenzo[d]thiazole in variable 26 ~ 63% yields.
Fig. 1. Hit against Streptococcus pneumoniae from a KCB library screen.
As shown in Table 1, vancomycin showed similar efficacies against
the three tested strains, but erythromycin was not active on 19A re-
gardless of its high potency against 19F and 15B. The 1-n-butyl deri-
vative (6d) was the most active (IC50 = 0.92, 0.75, and 0.77 μM on
19F, 15B, and 19A, respectively) among the N-(thyiazol-2yl)-3carb-
oxides (6a, 1, 6c, and 6d), but it was 5 times less active than vanco-
mycin. The activities decreased in the order allyl (6c), ethyl (1), and
methyl (6a). Although 8c did not follow this trend, the activities of
other analogues showed similar trends in which the activity increased
with the length of the 1-N-substituent. Among the three different 3-
carboximide analogues, N-(6-chlorobenzo[d]thiazol)2-yl-3-carbox-
amide seemed to be the most active but the difference was not sig-
nificant.
Table 1
Antibiotic activity of 1,2-dihydroxyquinolines.
Cmpd
R1
R2
IC50 (μM)*
19F
15B
19A
Vancomycin
0.15–0.18
0.051–0.1
0.13–0.19
0.12–0.19
Because the length of the 1-N-substituents seemed to dramatically
influence the activity, we designed and synthesized tricyclic compounds
in which the N-alkyl group was connected to the 7-position of the
quinoline ring to form a restricted 5-membered pyrrolidine or 6-mem-
bered piperidine ring, as shown in Scheme 2.20–22 Except when using
cyclic anilines such as indoline (9a) and 1,2,3,4-tetrahydroquinoline
(9b) instead of simple anilines (4a–4d) as the starting material, tricyclic
[3,2,1-ij]quinolines (11a–15a) and pyrido[2,2,1-ij]quinolines (11b,
12b, 14b) were prepared by the same procedures as the bicyclic spe-
cies, 1,2-dihydroxyquinolines, in Scheme 1. The same 3-carboxamides
used to prepare the bicyclic derivatives were introduced here as well.
Additionally, unsubstituted benzothiazole- and 5-methyl-1,3,4-thiadia-
zole-substituted compounds (13a and 15a, respectively) were prepared
in the case of pyrroloquinoline.
Erythromycin
0.037–0.08
No activity
6a
Me
> 20
> 20
> 20
1
Et
6.4
6.6
6.2
6c
6d
Allyl
n-Bu
4.9
4.5
4.2
0.92
0.75
0.77
2
H
> 20
> 20
4.1
> 20
> 20
3.1
> 20
> 20
3.1
7b
3
Et
Allyl
n-Bu
7d
0.57
0.66
0.38
8a
8b
8c
8d
Me
13.1
> 20
> 20
Et
5.9
5.4
3.8
Allyl
n-Bu
> 20
1.0
> 20
1.8
> 20
0.8
* The reported IC50 values for the synthesized compounds are means of tri-
plicate experiments.
Except for the thiazole-substituted 3-carboxamide (11b), which
showed moderate IC50 values of 6.5 (19F), 5.2 (15B), and 6.4 (19A) μM,
pyridoquinolines (12b and 14b) showed poor activities (Table 2). On
the other hand, while the thiazole-substituted pyrroloquinoline (11a)
was not active, 6-chlorobenzothiazole-substituted 12a showed good
potency with IC50 values of 0.27 (19F), 1.03 (15B), and 0.62 (19A) µM.
Unsubstituted [5-6]bicycles, including benzothiazole (13a) and benzi-
midazole (14a), were less potent than 12a but were still active. 5-
to confirm the optimum size of the N-alkyl group; 1-ethyl (1) and 1-allyl
(3) compounds were active, but the 1-H derivative (2) was not active.1
The N-thiazol-2-yl substituent on the 3-carboxamide of 1 was main-
tained (6a, 6c, and 6d), as was the N-(6-chlorobenzo[d]thiazol)2-yl (7b
and 7d) substituent in 3. The efficacy of an N-benzoimidazol-2-yl group
(8a–8d)
was
also
explored.
Ethyl
4-hydroxy-2-oxo-1,2-
Scheme 1. Preparation of 4-hydroxy-2-oxo-1,2-dihydroxyquinoline-3carboxamides. Reagents and conditions: (a) triethyl methanetricarboxylate, 215 °C, 15 min,
40–51% yields; (b) DMF, 160 °C, 30 min, 26–63% yield.
2

S. Huddar, et al.
Bioorganic&MedicinalChemistryLettersxxx(xxxx)xxxx
Scheme 2. Preparation of pyrrolo[3,2,1-ij]quinoline and pyrido[2,2,1-ij]quinoline derivatives. Reagents and conditions: (a) triethyl methanetricarboxylate, 215 °C,
15 min; (b) DMF, 160 °C, 30 min, 16 ~ 74% yield.
Table 2
Table 3
Antibiotic activities of tricyclic compounds containing a 1,2-dihydroxyquino-
line moiety.
In vivo pharmacokinetic profiles in rat.
Parameters*
8d
14a
Cmpd
R
IC50 (μM)*
iv 2 mg/kg
po 10 mg/kg
iv 2 mg/kg
po 10 mg/kg
WT
15B
19A
™
(h)
NA**
NA
6.0
1.2
4.0
12.4
NA
NA
35
NA
2.7
2.7
7.2
21.4
NA
NA
17
ax
max
C
T
(μg/h)
(h)
NA
11a
17.6
16.0
> 20
18.8
6.99
0.18
4.76
NA
7.7
1/2
AUC (μg⋅h/ml)
CL (L/h/kg)
Vss (L/kg)
Ft (%)
25.7
0.07
0.84
NA
12a
13a
14a
15a
0.27
3.6
1.03
8.8
0.62
7.9
*All results are the mean of experiments using three rats.
**NA: not applicable.
2.0
2.1
3.3
carboximide, because the PK profiles of antibiotics are known to be
more important than compounds used for treating other disease classes.
Both compounds showed good PK profiles with high exposure and
reasonable oral bioavailability (Table 3). Tricycle 14a showed lower
oral bioavailability (17%), but its exposure by oral administration was
higher than that of 8d due to its much lower clearance. The PK profile
of the bicyclic dihydroquinoline and the tricyclic pyrroloquinoline
seemed to be acceptable for the discovery of antibiotics.
2.2
2.7
1.7
11b
6.5
5.2
6.4
12b
14b
> 20
> 20
> 20
> 20
> 20
> 20
We determined the physicochemical properties of the most active
compound 12a. Compound 12a showed poor equilibrium solubility as
0.195 ng/mL, determined by LC–MS/MS analysis after shaking for 24 h
and filtration of an aqueous solution (pH 7) of 12a (1 mg/mL).
Compound 12a also showed 98.4% and 99.4% of protein binding on rat
and human plasma, respectively. To overcome these weakness, the
compounds must be modified to improve their physicochemical prop-
erties.
* The reported IC50 values of the synthesized compounds are means of tri-
plicate experiments.
Methyl-1,3,4-thiadiazole-substituted 15a also showed good activity,
with IC50 values of 2.0 (19F), 2.7 (15B), and 1.7 (19A) μM, which in-
dicated that the introduction of substituents at the 5-position of the 5-
membered monocycle and the 6-position of the [5,6]bicycle on the 3-
carboxamide increase the activities.
In this study, we identified 1,2-dihydroquinoline bearing a tricyclic
pyrroloquinoline as a novel antipneumococcal scaffold that is active
against pneumococcal strains of various serotypes, including 19F, 15B,
and 19A. Compounds 6d, 7d and 12a showed submicromolar IC50 va-
lues against all three of the tested serotypes, including an MDR S.
pneumoniae, demonstrating the potential of these compounds to
Additionally, using rats, we preliminarily investigated the in vivo
pharmacokinetic (PK) properties of 1,2-dihydroquinoline 8d and pyr-
roloquinoline 14a, which have the same benzimidazole group on the 3-
3

S. Huddar, et al.
Bioorganic&MedicinalChemistryLettersxxx(xxxx)xxxx
overcome current antibiotic resistance. While some of compounds with
this novel scaffold showed adequate in vivo pharmacokinetic profiles in
rat, the most active compound 12a represented poor equilibrium so-
lubility and high plasma protein binding. Further studies on the opti-
mization of these compounds, especially to improve physicochemical
properties as well as potency, are currently underway.
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Acknowledgments
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