Antifungal activity of substituted aurones

Article abstract of DOI:10.1016/j.bmcl.2017.01.012

Novel antifungals are in high demand as there is a growing resistance to antifungals currently in use. In particular, opportunistic fungal infections caused by Candida spp. are on the rise with infections by this genus accounting for the most severe funga

Full text of DOI:10.1016/j.bmcl.2017.01.012

Bioorganic & Medicinal Chemistry Letters xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Antifungal activity of substituted aurones
b,c,
Caleb L. Sutton a,c, Zachary E. Taylor b, Mary B. Farone a,c, , Scott T. Handy
⇑
⇑
a Department of Biology, Middle Tennessee State University, Murfreesboro, TN 37132, United States
b Department of Chemistry, Middle Tennessee State University, Murfreesboro, TN 37132, United States
c Tennessee Center for Botanical Medicinal Research, Middle Tennessee State University, Murfreesboro, TN 37132, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel antifungals are in high demand as there is a growing resistance to antifungals currently in use. In
particular, opportunistic fungal infections caused by Candida spp. are on the rise with infections by this
genus accounting for the most severe fungal infections following chemotherapy, implantation proce-
dures, and in patients with HIV/AIDS. A series of simple aurone analogs were synthesized and screened
Received 21 November 2016
Revised 4 January 2017
Accepted 5 January 2017
Available online xxxx
for antifungal activity versus Candida spp. Several compounds displayed activity at 100
having IC50 values below 20 M for three species of Candida. One of the compounds tested here also exhi-
bits anti-biofilm activity for mid-maturation growth.
lM, with two
l
Keywords:
Antifungal
Aurone
Ó 2017 Elsevier Ltd. All rights reserved.
Candida albicans
Candida tropicalis
Candida glabrata
Biofilm
The organism, Candida albicans, is a dimorphic fungus that
causes opportunistic infections of the oral cavity and genitalia in
humans. C. albicans is normally a commensal gut organism carried
by a large proportion of the population with no ill effect. The
organism infects host tissue by switching from the unicellular
yeast form to a multicellular, invasive filamentous form. C. albicans
causes a variety of diseases under the collective term ‘‘candidiasis,”
with C. albicans as the most prevalent cause.1 The term candidiasis
encompasses infections that range from the superficial, such as
oral thrush or the common vaginal yeast infection, to the more
serious infection candidemia, as is found in immunocompromised
patients with diseases such as AIDS, those undergoing chemother-
apy treatments, or patients with implants.2
with a narrow range of targets: the cell wall, cell membrane, and
DNA synthesis. Most of the drugs are also associated with side
effects or toxicity in their hosts and of particular concern is the
emergence of resistance to many of the commonly used antifun-
gals for C. albicans.4,5
Amphotericin B is one of the most commonly utilized treat-
ments for severe fungal infections.6 This drug works by binding
ergosterol in the cell membrane of fungi and causing pore forma-
tion, leading to cell death. However, amphotericin B can be toxic
to patients with common side effects including kidney, liver, and
heart damage due to the similarity of lipids within both fungal
and mammalian cell membranes.7
The azoles are antifungals that inhibit the biosynthesis of ergos-
Although C. albicans comprises 80% of infections caused by Candida
spp., two other species within the genus were chosen for screening due
to their distinct fungal physiology and clinical significance. C. glabrata
and C. tropicalis represent species that readily form biofilms and are a
growing concern as opportunistic pathogens due to the low level resis-
tance to the azole drug class that is mostly commonly used to treat fun-
gal infection.3 These two non-C. albicans species account for the
majority of all other infections not caused by C. albicans.
terol, specifically via lanosterol 14-a-demethylase inhibition,
which is the enzyme that converts lanosterol to ergosterol in
yeasts.8,9 There are many accounts of azole resistance occurring
due to alteration in drug target, alternate sterol biosynthetic path-
ways being utilized, reduction of target enzyme and overexpres-
sion of the antifungal drug target.10,11
The echinocandins such as caspofungin and micafungin inhibit
biosynthesis of 1,3-b-d-glucan, an integral molecule in fungal cell
walls, via the disruption of 1,3-b-d-glucan synthase. Without a
functioning synthase, yeasts are unable to maintain stable cell
walls, leading to cell lysis. Resistance to this class of treatment
can occur due to point mutations that prevent the inhibition of this
enzyme.11
Despite increased incidences of immunocompromised patients
and invasive fungal disease, limited antifungals are available and
⇑
Corresponding authors at: Tennessee Center for Botanical Medicinal Research,
Middle Tennessee State University, Murfreesboro, TN 37132, United States.
E-mail addresses: Mary.Farone@mtsu.edu (M.B. Farone), Scott.Handy@mtsu.edu
(S.T. Handy).
Flucytosine (5FC), one of the oldest treatments for systemic fun-
gal infections, is an antifungal with no inherent antifungal activity.
http://dx.doi.org/10.1016/j.bmcl.2017.01.012
0960-894X/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Sutton C.L., et al. Bioorg. Med. Chem. Lett. (2017), http://dx.doi.org/10.1016/j.bmcl.2017.01.012

2
C.L. Sutton et al. / Bioorganic & Medicinal Chemistry Letters xxx (2017) xxx–xxx
Table 2
Inhibitory (IC50) Values for Candida spp.
Aurone
C. albicans
C. glabrata
C. tropicalis
16
lM
11
lM
10
lM
1009
18
lM
10
lM
12
lM
Scheme 1. Aurone synthesis.
9051
Table 1
Aurone screening results against C. albicans.
Converted to 5-fluorouracil within a fungal cell, 5FC inhibits fungal
cell development by interfering with DNA and RNA synthesis.
Resistance has developed that prevent yeasts from taking up the
drug and prevent the conversion of 5FC to 5-fluorouracil, the com-
pound that prevents yeasts ability to thrive.11
Entrya Compound
Number
Ar
Percent Inhibition (at
100 M)
l
1b
1009
9078
2911
2011
9056
9053
9088
9055
99.4 4.7
45.6 2.6
40.0 6.5
14.2 6.1
12.5 0.4
11.2 2.2
1.0 1.1
Aurones are a naturally occurring type of flavonoid in plants
that lend flowers such as snapdragons and dahlias, a bright yellow
pigment.12 Classified as phytoalexins, or toxins produced by a plant
to fight infection, aurones differ structurally from flavonoids in that
they possess a five membered ring instead of a six membered ring
in which a benzofuranone ring is linked through a carbon-carbon
double bond to a phenyl moiety.13,14 Due to the antifungal activity
of a group of related biosynthetic precursor compounds, flavonoids
and chalconoids, it is suggested that aurones might also possess
antifungal activity.15,16 Through synthetic modification of these
naturally occurring products, novel antifungal compound classes
have been discovered, including 2,2-bisaminomethylated aurone
analogs for which studies showed increased survival of mice
infected with C. albicans, in vivo.13,17
Due to the recent interest of the Handy group in the synthesis of
non-natural aurone derivatives featuring a wide range of func-
tional groups, a number of previously unreported analogs have
been prepared using a variety of methods. (Scheme 1, for yields
and synthetic details, see supporting information) Armed with
these compounds, two main series of interest were identified.
The first was oxygenated (Table 1, entries 1–8). In these cases, both
the location and the degree of oxygenation (and the presence of
free phenols) were all found to be important, with aurone 1009
proving to be the best. Rather interestingly, switching the relative
position of the hydroxyl and methoxy groups on the phenyl ring
(1009 compared to 9053) or with two methoxy groups (2011) or
a reduction from two hydroxyl groups in the benzofuranone por-
tion to one hydroxyl group (2911) all resulted in significant reduc-
tions in activity.
2
3c
4
5
6
7
8
16.7 0.2
9b
1001
9051
26.0 2.3
99.9 3.0
10
11
12
2021
2008
78.8 8.8
53.9 4.2
The second series focused on heteroaromatics, of which the pyr-
idyl system was optimal, with the 2-pyridyl compound 9051 being
the best of the three pyridyl isomers. (Table 1, entries 10, 12, and
13) Other heteroaromatics, including other nitrogen-containing
ones were less active, with only the 2-pyrrolyl compound 2021 dis-
playing even modest activity. Thus, beyond the geometric consid-
erations, it appears that the nucleophilicity of the nitrogen is of
considerable importance as well in this series.
13
9253
1.1 0.1
Two of these compounds – 9051 and 1009 - displayed particu-
larly high levels of activity against C. albicans (see supporting infor-
mation for screening methods) and were selected for further study.
These two aurones were investigated for their inhibitory activ-
ity against three different species of Candidas. (Table 2) In all cases,
14
2026
2906
43.9 2.3
27.5 2.2
15b
IC50 values below 20 lM were observed.
Many microorganisms form structures called biofilms on a vari-
ety of both living and artificial surfaces. C. albicans is the most com-
mon fungus to form biofilms and differs physiologically from
a
b
c
With unsubstituted benzofuranone except as noted.
6,7-Dihydroxybenzofuranone.
6-Hydroxybenzofuranone.
Please cite this article in press as: Sutton C.L., et al. Bioorg. Med. Chem. Lett. (2017), http://dx.doi.org/10.1016/j.bmcl.2017.01.012

C.L. Sutton et al. / Bioorganic & Medicinal Chemistry Letters xxx (2017) xxx–xxx
3
Fig. 1. C. albicans was cultured for 24 h at 37 °C on a chamber slide for intermediate biofilm formation, after which appropriate treatments were added for an additional 24 h.
Panel A shows untreated (medium alone) C. albicans biofilm. Panel B shows biofilm treated with 2 g/mL amphotericin B. Panels C-D show biofilm treated with 16 M and
100 M of aurone 1009, respectively. All panels represent treatment of biofilms with appropriate controls or aurones for 24 h.
l
l
l
planktonic individuals of the same species sometimes becoming up
to 1000 fold more resistant to antifungal treatment.18–21 C. albicans
is known to form biofilms in many instances such as immunosup-
pressive therapy following transplantation procedures and during
the use of indwelling medical devices.18,19 When a Candida biofilm
forms on an indwelling medical device such as a catheter, heart
valve, artificial joint or a variety of other artificial surfaces, candidi-
asis may occur. If infection occurs, treatment is effective only after
the artificial device is removed due to the antifungal resistance of
the biofilm. In cases where the device is not removed and an inva-
sive candidiasis infection occurs, mortality rate has been shown be
as high as 40%.22 C. albicans biofilms are divided into three stages,
early phase (0–11 h), intermediate (12–30 h), and maturation
(31–72 h), with each having distinct properties.23 Inhibitory activ-
ity of aurones 1009 and 9051 against intermediate C. albicans bio-
films was investigated (methods included in supporting
information). Although aurone 9051 was not inhibitory to biofilms
at concentrations tested, results using 1009 are shown in Fig. 1.
Supplementary material
Supplementary data associated with this article (including syn-
thetic procedures, spectral data, and information regarding the bio-
logical methods) can be found in the online version, at http://
dx.doi.org/10.1016/j.bmcl.2017.01.012.
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Acknowledgements
The authors acknowledge support from Middle Tennessee State
University and the Tennessee Center for Botanical Medicinal
Research.
Please cite this article in press as: Sutton C.L., et al. Bioorg. Med. Chem. Lett. (2017), http://dx.doi.org/10.1016/j.bmcl.2017.01.012