Potent antimicrobial activity of 3-(4,5-diaryl-1H-imidazol-2-yl)-1H-indole derivatives against methicillin-resistant Staphylococcus aureus

Article abstract of DOI:10.1016/j.bmcl.2010.04.137

A new series of antimicrobial derivatives [3-(4,5-diaryl-1H-imidazol-2-yl)- 1H-indole)] have been synthesized with potent activity against strains of Staphylococcus aureus, including methicillin-resistant strains (MRSA). Compound 17 [3-(4,5-bis(4-fluoroph

Full text of DOI:10.1016/j.bmcl.2010.04.137

Bioorganic & Medicinal Chemistry Letters 20 (2010) 3518–3520
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Bioorganic & Medicinal Chemistry Letters
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Potent antimicrobial activity of 3-(4,5-diaryl-1H-imidazol-2-yl)-1H-indole
derivatives against methicillin-resistant Staphylococcus aureus
b
b
b
b
Raed A. Al-Qawasmeh a, Mario Huesca b, , Venkata Nedunuri , Robert Peralta , Jim Wright , Yoon Lee ,
*
Aiping Young b
a Department of Chemistry, The University of Jordan, Amman 11942, Jordan
b Lorus Therapeutics Inc., 2 Meridian Road, Toronto, Canada M9W 4Z7
a r t i c l e i n f o
a b s t r a c t
Article history:
A new series of antimicrobial derivatives [3-(4,5-diaryl-1H-imidazol-2-yl)-1H-indole)] have been synthe-
sized with potent activity against strains of Staphylococcus aureus, including methicillin-resistant strains
(MRSA). Compound 17 [3-(4,5-bis(4-fluorophenyl)-1H-imidazol-2-yl)-5-bromo-1H-indole], the most
active derivative was shown to inhibit the growth of all Gram-positive strains tested, including vancomy-
cin resistant Enterococcus faecalis and Enterococcus faecium with no activity against Gram-negative
bacteria.
Received 11 March 2010
Revised 27 April 2010
Accepted 28 April 2010
Available online 17 May 2010
Keywords:
Ó 2010 Elsevier Ltd. All rights reserved.
Novel antibacterials
Gram positive bacteria
MRSA
Infections caused by multi-drug resistant bacteria are of major
health concern worldwide. Particularly important are infections
caused by the Gram-positive bacteria Staphylococcus aureus and spe-
cies of the genus Enterococcus, due to increasing incidence of infec-
tions caused by these microorganisms in hospitals and
communities, and their ability of developing antibiotic resistance
to multiple antibiotics.1 New antibacterial agents to treat infections
caused by these Gram-positive bacteria have recently been intro-
duced, including the semi-synthetic streptogramins quinupristin/
dalfopristin, daptomycin, the synthetic oxazolidinone linezolid
and tigecycline.2 These new treatment options are welcome addi-
tions to the armamentarium against infections caused by Gram-po-
sitive bacteria. However, the identification of intrinsically resistant
strains, reports of emergence of resistance and serious side effects
for some of these new agents, make the development of a diversified
pipeline of antimicrobials a necessity.1
trum of activity of one of the most active compounds identified
through these studies, compound 17 (3-(4,5-bis (4-fluorophenyl)-
1H-imidazol-2-yl)-5-bromo-1H-indole), against bacterial strains
that belong to several Gram-positive and Gram-negative bacterial
species. This series of derivatives was obtained via classical con-
densation of an aldehyde with a dicarbonyl compound to produce
an imidazole ring system,4 with 1H-indole-3-carboxyldehyde as
our choice of aldehyde, along with symmetrical and unsymmetri-
cal benzil derivatives.5
Scheme 1 shows the general synthetic strategy used to synthe-
size these compounds, and Table 1 shows their structural features.6
The antimicrobial activity was evaluated by determining the mini-
mal inhibitory concentration (MIC) using the microdilution suscep-
tibility test.7,8 MIC values were defined as the lowest concentration
at which no visible growth is observed. The results showed a wide
range of antimicrobial activities among the different derivatives
Previously, we reported the synthesis and preliminary biologi-
cal characterization of new 2,4,5 tri-substituted imidazoles, includ-
ing 3-(4,5-diaryl-1H-imidazol-2-yl)-1H-indoles, some of which
exhibited antibacterial, antifungal or anticancer activities.3 We re-
port herein the synthesis and structure-antibacterial activity rela-
tionship of a series of thirty 3-(4,5-diaryl-1H-imidazol-2-yl)-1H-
indole derivatives against bacterial strains of S. aureus, including
methicillin-resistant strains (MRSA). We also determined the spec-
testedwithMICvaluesrangingfrom>64 lg/mLto 1 lg/mL(Table2).
The most active compounds belonged to compounds having the 5-
bromoindole moiety. Derivatives 17 and 22 exhibited activities
against MRSA comparable to that of vancomycin (MIC 1–2
and higher activity than oxacillin (MIC 16 to >64 g/mL) and linezo-
lid (2–4 g/mL).
lg/mL)
l
l
Table 2, shows the superiority of compounds having bromine on
the indole moiety at position 5 over compounds with a methyl
group at position 2 in the same moiety, with the exception of com-
pounds 19 and 23. For example, MICs of compounds 16, 17, 20, 21,
22, 24, 27, and 30 are lower than those of the corresponding com-
pounds with the methyl group at position 2 namely 1, 2, 5, 6, 7, 9,
* Corresponding author. Tel.: +1 4167981200/311; fax: +1 4167982200.
E-mail address: mhuesca@lorusthera.com (M. Huesca).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.04.137

R. A. Al-Qawasmeh et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3518–3520
3519
Ar1
N
Ar2
NH
O
O O
R2
R2
a
+
R1
R1
Ar1
Ar2
N
H
N
H
A
B
C
Scheme 1. Synthesis of the target compounds of general formula C from indole derivatives A and benzil derivatives B. Reagents: (a) NH4Ac, AcOH, reflux 4–8 h.
Table 1
Structures of synthetic 3-(4,5-diaryl-1H-imidazol-2-yl)-1H-indole compounds
R4
R3
N
NH
R1
R2
N
H
Compound
R1
R2
R3
R4
Compound
R1
R2
R3
R4
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
F
H
F
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
H
F
H
F
N(CH3)2
Br
Cl
CH3
H
H
H
O-Ph
Ph
H
H
N(CH3)2
Br
Cl
CH3
Cl
NO2
F
O-Ph
Ph
Br
Ph
NO2
CH3
N(CH3)2
Br
Cl
CH3
H
H
H
O-Ph
Ph
H
H
N(CH3)2
Br
Cl
CH3
Cl
NO2
F
O-Ph
Ph
Br
Ph
NO2
CH3
NO2
H
NO2
H
Table 3
Spectrum of in vitro activity (MIC;
species
lg/mL) of compound 17 against several bacterial
Strain
ATCC #
17
V
B. subtillis
B. subtillis
E. faecalis
E. faecalis
E. faecium
M luteus
S. epidermidis
S epidermidis
S. epidermidis
S .saprophyticus
S. pneumoniae
S. dysgalactiae
S. dysgalactiae
S. pyogenes
S. sanguinis
S. agalactiae
S. agalactiae
E. coli
14579
6633
2
2
2
2
2
1
2
2
2
2
2
2
2
2
1
2
1
0.25
4
16
>64
0.25
0.5
2
4
2
4
0.5
1
1
1
1
2
>64
>64
>64
>64
>64
>64
Table 2
In vitro antimicrobial activity (MIC
lg/mL) of synthetic compounds tested against
29212
51299
51559
10240
13518
12228
35983
15305
49150
12388
12394
19615
10556
13813
12386
12435
35333
19142
39324
14028
13311
strains of Staphylococcus aureus7,8
Compound
MRSA
MSSA
Compound
MRSA
MSSA
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
L
4–8
4–32
2–4
2–4
4
8
8
8
4–8
>64
>64
4
2–4
>64
4–8
2–4
16–>64
8
4
2
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
V
4
1
2–4
>64
2–4
4
1
>64
2
>64
>64
2
4
1
2
2–4
4
>64
2–4
2–4
1
>64
2
>64
>64
2
8
8
8
4
>64
>64
4
2
>64
>64
>64
>64
>64
>64
4
4
>64
2
4
>64
2
E. coli
>64
4
1–2
0.5–4
P. aeruginosa
P. aeruginosa
S. typhimurium
S. typhimurium
1–2
1
Ox
L = linezolid; V = vancomycin; Ox = oxacillin.
V = vancomycin.

3520
R. A. Al-Qawasmeh et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3518–3520
3. (a) Huesca, M.; Al-Qawasmeh, R.; Young, A. H.; Lee, Y. PCT Int. Appl. WO 2004/
016086, 2004.; (b) Huesca, M.; Al-Qawasmeh, R.; Young, A. H.; Lee, Y. PCT Int.
Appl. WO 2005/047266, 2005.
4. (a) Grimmett, M. R. In Comprehensive Heterocyclic Chemistry: The Structure,
Reaction, Synthesis and Uses of Heterocyclic Compounds; Katrizky, A. R., Rees, C.
W., Eds.; Pergamon: Oxford, 1984; Vol. 5, pp 457–498; (b) Grimmett, M. R.
Imidazole and Benzimidazole Synthesis; Academic Press: San Diego Calif, 1997; (c)
Sarshar, S.; Siev, D.; Mjalli, M. M. Tetrahedron Lett. 1996, 37, 835; (d) Agarwal, A.;
Porwal, S.; Chauhan, P. M. S. Lett. Org. Chem. 2006, 3, 712.
12, and 13. Substitutions in the phenyl ring at positions 4 and 5 of
the imidazole moiety with either nitro, phenoxy or phenyl groups
abrogated the activity.
Compounds with halogenated phenyl groups in the imidazole
moiety exhibited higher activity than compounds with other sub-
stituents at the same positions. Compound 17 was the most active
from this series of derivatives. The spectrum of activity of com-
pound 17 against different microorganisms, including Gram-posi-
tive and Gram-negative bacteria, was investigated. In addition, its
activity was compared with that of vancomycin.
5. In this study two indole-3-carboxaldehydes were used, both commercially
available. The benzyl compounds either symmetrical or unsymmetrical were
synthesized based on the following literature: (a) Ogata, Y.; Takagi, K.; Fujii, Y. J.
Org. Chem. 1972, 37, 4026; (b) Adams, R.; Marvel, C. S. In Organic Synthesis;
Whiley J. and Sons: New York, 1941; Coll Vol. 1. p 94; (c) Fisher, A.; Grigor, B. A.;
Packer, J.; Vaughan, J. J. Am. Chem. Soc. 1961, 83, 4208; (d) Chi, K.-W.; Yusubov,
M. S.; Filimonov, V. D. Synth. Commun. 1994, 24, 2119; (e) Armesto, D.; Horspool,
W. M.; Ortiz, M. J.; Perez-Ossorio, R. Synthesis 1988, 10, 799; f Baach, H. C. U.S.
Patent 3,551385, 1970.; (g) Lau, K.; Arnold, F. Org. Prep. Proced. Int. 1980, 12, 327.
6. All the synthetic compounds were fully characterized and show satisfactory
physicochemical properties. In a typical experiment 1 mmol (1 equiv) of the
indole-3-carboxaldehyde was combined with 1.05 equiv of the benzyl
derivative and 20 equiv of ammonium acetate in 5 ml acetic acid. The reaction
mixture was refluxed for 3–4 h and the reaction monitored by TLC. The work up
of the reaction was done by adding the reaction to a stirred ice-water in which a
precipitate formed. The precipitated product was air dried and recrystallized
from ethanol. All the compounds were produced in a quantitative yield. 1H NMR
for selected compounds: 4, mp = 240–245 °C: 1H NMR (CDCl3): d = 7.47 (d, 4H),
7.30–7.34 (m, 1H, 7.14–7.19 (m, 3H), 2.68 (s, 3H). Compound 5: mp = 165–
167 °C; 1H NMR (DMSO-d6): d = 12.13 (s, 1H), 11.33 (s, 1H), 7.94 (d, 2H), 7.57 (d,
2H), 7.39 (br d, 2H), 7.35 (d(1H), 7.05–7.12 (m, 3H), 2.5 (s, 3H). Compound 9:
mp = 247–250 °C; 1H NMR (CDCl3): d = 7.78 (br s, 1H), 7.59 (d, 2H), 7.54 (d, 2H),
7.35–7.39 (m, 2H), 7.28–7.34 (m, 2H), 7.13–7.18 (2H), 7.01–7.05 (m, 2H), 2.72
(br s, 3H). Compound 26: mp = 155.158 °C; 1H NMR (CDCl3): d = 8.08 (d, 4H),
8.07 (br s, 1H), 7.75 (d, 4H), 7.28–7.50 (m, 10H), 7.12 (br d, 2H), 6.97 (br s, 1H).
7. National Council of Clinical Laboratory Services (NCCLS). Methods for Dilution
Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically M7-A5,
National Committee on Clinical Laboratory Standards, 2000; Vol. 20, pp 2.
Compound 17 exhibited consistent activity toward all Gram-po-
sitive strains tested (Table 3), with MIC values ranging from 1 to
2
l
g/mL, including Enterococcus faecium (ATCC # 51559) and
Enterococcus faecalis (ATCC # 51299), both of which are resistant
to vancomycin (MIC >64 g/mL and 16 g/mL, respectively). Com-
l
l
pound 17, however, showed no activity against Gram-negative
bacteria such as Escherichia coli, Pseudomonas aeruginosa and Sal-
monella typhimurium. These results suggest the selectivity of this
new synthetic compound for Gram-positive bacteria.9
In conclusion, the structure-antibacterial activity relationship of
a new series of 3-(4,5-diaryl-1H-imidazol-2-yl)-1H-indole deriva-
tives has been described, with some derivatives exhibiting potent
in vitro antimicrobial activity against various strains of MRSA
and other Gram-positive bacteria. Several synthesized compounds
exhibited comparable activity to that of vancomycin and higher
activity than oxacillin and linezolid.
The characterization of the antimicrobial spectrum of com-
pound 17 indicates a selective activity of this derivative for
Gram-positive bacteria. Studies are currently underway to charac-
terize the mode of action of this series of compounds, including the
investigation of possible common mechanisms involved in the
growth inhibition of Gram-positive bacteria and cancer cells, since
several derivatives were also capable of inhibiting human colon
carcinoma cell proliferation (data not shown).3b Additional studies
are also underway to determine the in vivo efficacy of selected
compounds in several animal models of infection.
Briefly, 10
added to microculture wells containing 90
bacterial cultures in Mueller–Hinton broth medium, to final concentrations from
64 g/mL to 0.06 g/mL.
l
L stock dilutions of each derivative carried out in 50% DMSO were
lL volumes of the corresponding
l
l
8. A collection of two methicillin susceptible strains (MSSA); ATCC-6538 and
ATCC-29213, and six methicillin-resistant strains (MRSA); 1A-218, 1A-318, 1B-
374, 1B-315, 1B-185 and 1B-387 were included in the study representing two
different epidemic isolates (CMRSA-1A and CMRSA-1B). They were identified by
molecular typing by sentinel hospitals as part of the Canadian Nosocomial
Infection Surveillance program: Simor, A. E.; Ofner-Agostini, M.; Bryce, E.;
Green, K.; McGeer, A.; Mulvey, M.; Paton, S. C.M.A.J. 2001, 165, 21.
9. The selectivity of this series of compound for Gram-positive bacteria might be
related to an antimicrobial mechanism specific for Gram-positive bacteria, or to
their inability to penetrate the outer membrane present in Gram-negative
bacteria. The characterization of the mechanism of action of these novel
compounds is underway.
References and notes
1. Leclercq, R. Clin. Microbiol. Infect. 2009, 15, 224.
2. Cornaglia, G.; Rossolini, G. M. Clin. Microbiol. Infect. 2009, 15, 218.