Evaluation of (4-aminobutyloxy)quinolines as a novel class of antifungal agents

Article abstract of DOI:10.1016/j.bmcl.2013.06.014

Antifungal assessment of eighteen 5-, 6- and 8-(4-aminobutyloxy)quinolines revealed a significant susceptibility of the tested fungi and yeast strains (Candida albicans, Rhodotorula bogoriensis, Aspergillus flavus and Fusarium solani) toward different hal

Full text of DOI:10.1016/j.bmcl.2013.06.014

Bioorganic & Medicinal Chemistry Letters 23 (2013) 4641–4643
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Evaluation of (4-aminobutyloxy)quinolines as a novel class of
antifungal agents
Stéphanie Vandekerckhove a, Hai Giang Tran b, Tom Desmet b, Matthias D’hooghe a,
⇑
a SynBioC Research Group, Department of Sustainable Organic Chemistry and Technology, Faculty of Bioscience Engineering, Ghent University,
Coupure Links 653, B-9000 Ghent, Belgium
b Centre for Industrial Biotechnology and Biocatalysis, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
Antifungal assessment of eighteen 5-, 6- and 8-(4-aminobutyloxy)quinolines revealed a significant sus-
ceptibility of the tested fungi and yeast strains (Candida albicans, Rhodotorula bogoriensis, Aspergillus flavus
and Fusarium solani) toward different halo-substituted 8-(4-aminobutyloxy)quinolines. The six most
potent compounds displayed antifungal activities similar to those of established antifungal agents such
as Amphotericin B, Fluconazole and Itraconazole, and one representative also showed a promising
broad-spectrum antifungal profile. The introduction of an aminoalkoxy side chain at the 8-position of
a halo-substituted quinoline core might thus provide a new class of lead structures in the search for novel
antifungal agents.
Received 9 May 2013
Revised 4 June 2013
Accepted 6 June 2013
Available online 13 June 2013
Keywords:
Quinolines
Antifungal agents
Mycoses
Ó 2013 Elsevier Ltd. All rights reserved.
Until the 1970s, the number of fungal infections (mycoses) was
relatively low and could be held steady for many years. Most of
these infections were easily cured and mainly superficial, resulting
in the fact that there was one ‘golden standard’ in antifungal che-
motherapy: Amphotericin B.1 However, the incidence and severity
of fungal infections has increased considerably over the years, caus-
ing significant morbidity and mortality (particularly in patients
with impaired immunity due to diabetes, AIDS, cancer, etc.).2–5
Nowadays there are many types of mycoses, which can be
caused by opportunistic pathogens (endogenous or acquired from
the environment) or invasive fungal infections, and these mycoses
are often associated with corticosteroid therapy, antibiotic treat-
ments, diabetes, surgery, etc.6,7 Even though mycoses are becom-
ing more frequent, they are still difficult to diagnose clinically.3,8
In addition, another important issue to be addressed concerns
the emergence of (cross)resistance against the most frequently
used antifungal drugs, necessitating a continuous input of novel
antifungal drug candidates with high efficacy rates and low toxic-
ity, especially in the treatment of some invasive fungal infections
such as aspergillosis and fusariosis.3–5,8,9
recognized as privileged structures in medicinal chemistry and re-
side in molecules with various biological activities, such as antima-
larial, antibacterial, antiprotozoic, anti-HIV, anticancer and
antifungal agents.11–16
Chemical derivatization of 8-hydroxyquinoline, a simple fungi-
static compound known since the early 1920s,17 has been evalu-
ated intensively in the chemical literature in order to augment
its antifungal activity. Chlorinated, brominated and bulky (halo-
)substituted derivatives have thus been synthesized and revealed
improved fungitoxicity in comparison with the parent molecule
8-hydroxyquinoline.18–25 These derivatization studies mainly fo-
cused on the introduction of various substitutions on the quinoline
ring. On the other hand, O-functionalization of 8-hydroxyquinoline
has hardly been examined so far, and only a few 8-alkoxyquino-
lines have been described as antifungal agents.26 Elaboration of
O-functionalized quinolines as a potential new class of antifungals
can thus be considered as a relevant challenge within antimicrobial
research. In continuation of our interest in the design of novel
biologically active quinolines,27–29 and knowing that 5-, 6- and
8-(4-aminobutyloxy)quinolines have previously revealed good
antimalarial potency and moderate to low cytotoxicity,27 the
present study focuses on the evaluation of the antifungal proper-
ties of the above-mentioned types of quinolines.
Consequently, a lot of research is being performed in order to
discover novel and potent antifungal agents.1,2,10 Polyenes, azoles,
cyclic lipopeptides, allylamines and thiocarbamates are the most
frequently encountered structural units within antifungal drug re-
search, but also quinoline-based systems have emerged as an
important class of antifungal agents.11 Quinolines are generally
Thus, 5-, 6- and 8-(4-aminobutyloxy)quinolines 4 and 5 were
prepared according to a recently disclosed protocol.27 In brief,
these quinolines were generated through a three-step approach,
starting with O-alkylation of 5-, 6- or 8-hydroxyquinolines 1 with
(2-methyl)allyl bromide. The obtained allyloxyquinolines 2 were
subjected to a rhodium-catalyzed hydroformylation toward the
⇑
Corresponding author. Tel.: +32 9 2649394; fax: +32 9 2646221.
E-mail address: matthias.dhooghe@UGent.be (M. D’hooghe).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.06.014

4642
S. Vandekerckhove et al. / Bioorg. Med. Chem. Lett. 23 (2013) 4641–4643
Table 1
corresponding linear aldehydes 3. Finally, the desired [4-(diethyl-
amino)butyloxy]- and [4-(4-chlorobenzylamino)butyloxy]quino-
lines 4 and 5 were synthesized via reductive amination of the
aldehydes 3 using the appropriate amine and a reducing agent
(Scheme 1, Table 1). The spectral properties of these compounds
4 and 5 have been described elsewhere.27
The antifungal activity of these 18 compounds was then tested
on two yeast strains (Candida albicans (IHEM 374) and Rhodotorula
bogoriensis (MUCL 11796)) and two mould strains (Aspergillus fla-
vus (ATCC 44310) and Fusarium solani (CBS 230.34)). These strains
were subcultured onto potato dextro medium (Oxoid) at 25 °C and
the inocula were prepared by suspending and diluting a fresh and
mature culture of each strain in a 0.85% sterile saline solution for
desired density, equivalent to 0.5 McFarland standard (stock inoc-
ula). In a preliminary screening test, the antifungal activity of these
18 compounds was studied on the afore-mentioned fungi by using
the disk diffusion susceptibility test.30,31 These results are depicted
in Table 2.
Substitution pattern of quinolines 4 and 5
Entry
Compound
Quinolin-5-, 6- or 8-yl
R1
R2
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4f
4g
4h
4i
8-yl
8-yl
8-yl
8-yl
8-yl
8-yl
8-yl
8-yl
8-yl
6-yl
5-yl
8-yl
8-yl
8-yl
8-yl
6-yl
5-yl
5-yl
H
H
H
5-Cl
CH3
H
5,7-di-Cl
5,7-di-I
5,7-di-Br
5-Cl, 7-I
CH3
CH3
CH3
H
5-Cl, 7-I
2-CH3, 5,7-di-Cl
H
CH3
CH3
H
9
10
11
12
13
14
15
16
17
18
4j
4k
5a
5b
5c
5d
5e
5f
H
H
H
5-Cl
CH3
H
5,7-di-I
2-CH3, 5,7-di-Cl
H
H
H
CH3
CH3
H
H
CH3
5g
It is important to note that all tested strains appear to be sus-
ceptible only toward the 8-(4-aminobutyloxy)quinolines 4a–i
and 5a–d. This implies that moving the aminobutyloxy side chain
to the 5- or 6-position of the quinoline ring is detrimental for
antifungal activity. Moreover, in agreement with previous
research,18–20,22,26 these results demonstrate that the antifungal
activity of 8-(4-aminobutyloxy)quinolines increases with the
degree of substitution of the quinoline ring. It also seems that
compounds bearing a 4-(diethylamino)butyloxy side chain display
antifungal activity against yeasts (C. albicans and R. bogoriensis) and
moulds (A. flavus), while compounds with a 4-(chlorobenzylami-
no)butyloxy side chain appear to be active only against yeast
strains. In the end, the six most active compounds of the
8-(4-aminobutyloxy)quinoline class (4d, 4e, 4f, 4h, 4i and 5d) were
subjected to a Minimum Inhibitory Concentration (MIC) determi-
nation test via microdilution.32 The MIC values are reported in
Table 3.
be more selective toward yeasts, while compounds 4h and 5d
show very specific activity toward Rhodotorula bogoriensis. Finally,
molecule 4i exhibits antifungal activity against yeasts as well as
moulds. Furthermore, its antifungal activity in terms of MIC val-
ues is comparable to or lower than that of standard, commercially
available antifungal reagents such as Itraconazole, Fluconazole or
Amphotericin B.2,33–36 Therefore, compound 4i can be considered
as a promising novel broad-spectrum antifungal lead compound
that can significantly suppress growth of pathogenic yeasts and
moulds.
The data displayed in Table
4 indicate the potential
druglikeness of 8-(4-aminobutyloxy)quinolines. As can be seen,
the (calculated) molecular properties molecular weight (MW),
CLogP, number of hydrogen bond (HB) donors and acceptors, and
the sp3 fraction of the most promising compounds 4d–f, 4h,i and
5d correspond very well with those of the established quinoline
antimalarial drugs Chloroquine and Pamaquine. These molecular
properties have been correlated with a number of key parameters
in drug discovery,37 pointing to the conclusion that the data in Ta-
ble 4 further support the potential of halogenated 8-(4-aminobu-
tyloxy)quinolines as a new class of lead compounds.
The MIC values correspond well with the results of the preli-
minary screening test and prove that all six tested compounds ex-
hibit considerable antifungal activity. Compound 4d in particular
displays low MIC values for C. albicans, R. bogoriensis and A. flavus,
indicating that both yeasts and moulds are susceptible to this
compound. Compounds 4e and 4f on the other hand prove to
R1
O
R2
N
N
4a-k (56-95%)
Et2NH
MeOH
AcOH
R2
Br
NaCNBH3
20 bar CO/H2
Rh(acac)(CO)2
DPBO-ligand
K2CO3
R1
R1
OH
O
R2
R1
O
R2
toluene
O
H
N
acetone
N
N
1
2a-n (57-92%)
3a-n (59-98%)
1) 4-ClC6H4CH2NH2
MgSO4, CH2Cl2
2) NaBH4,
MeOH
Cl
R1
O
R2
N
HN
5a-g (48-94%)
Scheme 1. Synthesis of 4-(quinolinyloxy)butyraldehydes 3 and their conversion into (4-aminobutyloxy)quinolines 4 and 5.

S. Vandekerckhove et al. / Bioorg. Med. Chem. Lett. 23 (2013) 4641–4643
4643
Table 2
Results of the disk diffusion susceptibility test (diameter of antifungal zone (mm)—
L of 0.1 mg/ L per disc)
References and notes
2
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30
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4k
5a
5b
5c
5d
5e
5f
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Compound Candida
Rhodotorula
bogoriensis
Aspergilus
flavus
Fusarium
solani
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albicans
4d
4e
4f
4h
4i
0.50
0.50
0.25
4.00
0.25
ND
0.25
0.50
0.50
0.25
0.12
0.25
0.50
4.00
2.00
4.00
0.50
ND
ND
ND
ND
ND
0.50
8.00
5d
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30. The fungal inocula were spread out on a PDA (Oxoid) plate by a sterile glass
hockey. The empty 6 mm disc (BBLTM Sensi-DiscTM) containing 0.2 mg of the
antifungal compound dissolved in DMSO was dispensed on the plate. The
ND = not determined.
Table 4
Comparison of some (calculated) molecular properties of 8-(4-aminobutyloxy)quin-
olines 4d–f, 4h,i and 5d with those of established quinoline drugs Chloroquine and
Pamaquine
control disk was dispensed with 2 lL of the pure solvent (DMSO). All the plates
were incubated at 25 °C and antifungal activity was measured after 24 h of
incubation for yeasts and after 48 h of incubation for filamentous fungi. The
microbial inhibition was determined by measuring the diameter of the clear
zone around microbial colonies. These experiments were repeated three times.
31. NCCLS. Method for Antifungal Disk Diffusion Susceptibility Testing of Yeasts;
Approved Guideline. NCCLS document M44-A [ISBN 1-56238-532-1]. NCCLS, W.
V. R., Suite 1400, Wayne, Pensylvania 19087-1898 USA, 2004.
32. The MIC values of antifungal reagents were determined by a microdilution test
which was performed in 96-well microtiter plates. The double concentration
RPMI-1640 supplemented with glutamine and a pH indicator, but without
bicarbonate (Sigma), was employed to prepare the log2 dilution series (0–
500 mg/L) of the antifungal agents. The inocula prepared in the previous part
Compound
MW (g/
mol)
CLogPa HB
donors
HB
acceptors
sp3
fractionb
4d
4e
4f
4h
4i
355.30
538.20
444.20
446.75
369.33
437.79
5.42
6.19
5.72
5.78
5.92
7.06
5.06
4.34
0
3
3
3
3
3
3
3
4
0.50
0.50
0.50
0.50
0.53
0.32
0.50
0.53
0
0
0
0
1
1
1
5d
Chloroquine 319.87
Pamaquine 315.45
were diluted ten times with distilled water. Each well contains 100
working solution of antifungal dilution and inoculum in a volume of 100
The microdilution plates were incubated without agitation at 37 °C for 24 h,
48 h or 72 h for yeast, A. flavus and F. solani, respectively. The plates were read
with a microtiter plate spectrophotometer (Bio-rad) at 530 nm. The MIC value
evalution was based on the guidelines of EUCAST protocols.
l
L of
L.
a
Calculated using ChemDraw Ultra 7.0.
Ratio of sp3 carbon atoms over total number of carbon atoms.
l
b
33. Rodriguez-Tudela, J. L.; Arendrup, M. C.; Barchiesi, F.; Bille, J.; Chryssanthou, E.;
Cuenca-Estrella, M.; Dannaoui, E.; Denning, D. W.; Donnelly, J. P.; Dromer, F.;
Fegeler, W.; Lass-Florl, C.; Moore, C.; Richardson, M.; Sandven, P.; Velegraki, A.;
Verweij, P.; Susceptibi, S. A.; EUCAST, E. Clin. Microbiol. Infect. 2008, 14, 398.
34. Alastruey-Izquierdo, A.; Cuenca-Estrella, M.; Monzon, A.; Mellado, E.;
Rodriguez-Tudela, J. L. J. Antimicrob. Chemother. 2008, 61, 805.
35. Tudela, J. L. R.; Donnelly, J. P.; Arendrup, M. C.; Arikan, S.; Barchiesi, F.; Bille, J.;
Chryssanthou, E.; Cuenca-Estrella, M.; Dannaoui, E.; Denning, D.; Fegeler, W.;
Gaustad, P.; Lass-Florl, C.; Moore, C.; Richardson, M.; Schmalreck, A.; Velegraki,
J. A.; Verweij, P.; Antimicrobial, E. E. C. Clin. Microbiol. Infect. 2008, 14, 982.
36. Swinne, D.; Watelle, M.; Nolard, N. Rev. Iberoam. Micol. 2005, 22, 24.
37. Walters, W. P.; Green, J.; Weiss, J. R.; Murcko, M. A. J. Med. Chem. 2011, 54,
6405.
In conclusion, antifungal assessment of 5-, 6- and 8-(4-amin-
obutyloxy)quinolines resulted in the identification of six
8-(4-aminobutyloxy)quinolines displaying considerable antifungal
activity, with one compound showing a significant potential as a
novel broad-spectrum antifungal candidate. The introduction of
an aminoalkoxy side chain on a halo-substituted quinoline core
at the 8-position might thus provide a new perspective within
the search for new antifungal agents.
Acknowledgment
The authors are indebted to Ghent University (BOF) for financial
support.