A Phytochemical-Halogenated Quinoline Combination Therapy Strategy for the Treatment of Pathogenic Bacteria

Article abstract of DOI:10.1002/cmdc.201500179

With the continued rise of drug-resistant bacterial infections coupled with the current discouraging state of the antibiotic pipeline, the need for new antibacterial agents that operate through unique mechanisms compared with conventional antibiotics and work in synergy with other agents is at an all-time high. We have discovered that gallic acid, a plant-derived phytochemical, dramatically potentiates the antibacterial activities of several halogenated quinolines (up to 11 800-fold potentiation against Staphylococcus aureus) against pathogenic bacteria, including drug-resistant clinical isolates. S. aureus demonstrated the highest sensitivity towards gallic acid-halogenated quinoline combinations, including one halogenated quinoline that demonstrated potentiation of biofilm eradication activity against a methicillin-resistant S. aureus (MRSA) clinical isolate. During our studies, we also demonstrated that these halogenated quionlines operate through an interesting metal(II) cation-dependent mechanism and display promising mammalian cytotoxicity.

Full text of DOI:10.1002/cmdc.201500179

DOI: 10.1002/cmdc.201500179
Communications
A Phytochemical–Halogenated Quinoline Combination
Therapy Strategy for the Treatment of Pathogenic Bacteria
Yasmeen Abouelhassan,[a] Aaron T. Garrison,[a] Fang Bai,[b, c] Verrill M. Norwood IV,[a]
Minh Thu Nguyen,[a] Shouguang Jin,[b] and Robert W. Huigens, III*[a]
With the continued rise of drug-resistant bacterial infections
coupled with the current discouraging state of the antibiotic
pipeline, the need for new antibacterial agents that operate
through unique mechanisms compared with conventional anti-
biotics and work in synergy with other agents is at an all-time
high. We have discovered that gallic acid, a plant-derived phy-
tochemical, dramatically potentiates the antibacterial activities
of several halogenated quinolines (up to 11 800-fold potentia-
tion against Staphylococcus aureus) against pathogenic bacte-
ria, including drug-resistant clinical isolates. S. aureus demon-
strated the highest sensitivity towards gallic acid–halogenated
quinoline combinations, including one halogenated quinoline
that demonstrated potentiation of biofilm eradication activity
against a methicillin-resistant S. aureus (MRSA) clinical isolate.
During our studies, we also demonstrated that these halogen-
ated quionlines operate through an interesting metal(II)
cation-dependent mechanism and display promising mammali-
an cytotoxicity.
ic) agents that were originally discovered as bacterial growth
inhibitors in whole-cell (phenotypic) screens.[3,4] As a result, the
large majority of our antibiotic arsenal hits relatively few bacte-
rial targets (e.g., cell-wall synthesis machinery, bacterial ribo-
somes, DNA gyrase/topoisomerase, RNA polymerase).[3]
With the rise of drug-resistant bacteria outpacing our ability
to bring new antibacterial entities to market, it is of utmost im-
portance to discover antibacterial agents that operate via new
modes of action and hit new bacterial targets compared with
those of our current antibiotic arsenal. This line of thinking led
to extensive screening campaigns of promising antibacterial
targets in the late 1990s following the first complete sequenc-
ing of a bacterial genome (Haemophilus influenza).[2] Despite
much effort, target-based screens of large compound libraries
have yet to yield a single antibacterial agent in the clinic.
One useful approach to treating resistant bacterial infections
is to administer combination therapies (sets of antibacterial
drugs) to patients rather than administering single agent anti-
biotic therapy.[5–7] Combination therapies can fall under one of
three categories, which include antibacterial combinations
that: 1) inhibit different targets/pathways, 2) inhibit distinct
nodes in the same pathway, or 3) inhibit the same target in dif-
ferent ways.[6] In addition to approved antibiotic combination
treatments, the discovery of new antibacterial combinations
that synergize together against drug-resistant bacteria are of
clinical importance.
The widespread use of antibiotics over the past several de-
cades has accelerated the emergence of drug-resistant bacte-
ria.[1] This global healthcare crisis is amplified by the current
state of our antibiotic pipeline; since the late 1960s, there has
been little success in bringing new antibiotics/antibacterial
agents to the clinic.[2–4] Our current antibiotic arsenal is com-
posed of antibiotic (natural product) and antibacterial (synthet-
Our group recently discovered that brominated quinoline
1 is a potent antistaphylococcal agent, with minimum inhibito-
[a] Y. Abouelhassan,+ A. T. Garrison,+ V. M. Norwood IV,+ M. T. Nguyen,+
Prof. R. W. Huigens, III+
Department of Medicinal Chemistry
College of Pharmacy, University of Florida
1345 Center Drive, Gainesville, FL 32610 (USA)
E-mail: rhuigens@cop.ufl.edu
[b] Dr. F. Bai,+ Prof. S. Jin+
Department of Molecular Genetics & Microbiology
College of Medicine, University of Florida
1600 SW Archer Rd, Gainesville, FL 32610 (USA)
[c] Dr. F. Bai+
ry concentration (MIC) values of 0.39–1.56 mm against Staphylo-
coccus aureus and S. epidermidis strains, including drug-resist-
ant clinical isolates. Quinoline 1 is capable of clearing estab-
lished methicillin-resistant S. aureus, with a half-maximal effec-
tive concentration (EC50) value of 2.6 mm, and S. epidermidis
(EC50 =9.2 mm) biofilms in crystal violet assays.[8] There have
been several reports describing the antibacterial and/or anti-
biofilm activities of plant-derived phytochemicals against
staphylococcus pathogens.[9–12] We were interested to investi-
gate the antibacterial-potentiating activities of several phyto-
College of Pharmacy, Nankai University
94 Weijin Rd, Tianjin 300071 (P. R. China)
[+] The authors declare no competing financial interests.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cmdc.201500179. Synthetic protocols, charac-
terization data (including 1H and 13C NMR spectra, HRMS, m.p. data) for
all new compounds, biological experimental procedures (including sup-
porting images), potentiation results from gallic acid/antibiotic combina-
tions, and potentiation results from various metal(II) cations in combina-
tion with halogenated quinolines can be found in the Supporting Infor-
mation.
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chemicals and related compounds in combination with halo-
genated quinolines (HQs) to determine the effectiveness of
phytochemical–HQ combinations against pathogenic bacteria.
Plant-derived phytochemicals are typically considered safe and
ideal for therapeutic applications as phytochemicals are in
many foods routinely ingested by humans.[10]
(GA), caffeic acid, quercetin and tiron (a related compound
that has known antioxidant activities,[15] but not a plant-de-
rived phytochemical; Scheme 1C). Potentiation assays were set
up in 96-well plates (Figure 1A) and allowed for the rapid de-
termination of potentiation by changes in MIC values for HQ
compounds at a non-growth inhibitory concentration of phy-
tochemical. Using this assay format, three HQ compounds (in
twofold serial dilutions typical of MIC microdilution protocols)
in the presence or absence of a phytochemical at a single con-
centration (dimethyl sulfoxide (DMSO) vehicle lanes with and
without test phytochemicals were included to ensure no
planktonic growth inhibition from phytochemical treatment
alone).
We evaluated our HQ library (Scheme 1A), which includes
quinolines from our previous studies[8] in addition to HQs 8–11
that were synthesized for the purpose of this study
(Scheme 1B). To accomplish the synthesis of 8–11, we used
the Friedländer quinoline synthesis[13] (Scheme 1B), which start-
ed with the nitration of 3-hydroxybenzaldehyde 23 followed
by iron metal reduction with hydrochloric acid to give 2-
amino-3-hydroxybenzaldehyde 24 in 54% yield (over two-
steps). Three diverse b-ketoester/diketones were then con-
densed with 24 by heating in water to afford 8-hydroxyquino-
lines 25–27 in 26–65% yield. Final bromination or iodination
of 25–27 using N-bromosuccinimide (NBS) or N-iodosuccini-
mide (NIS) afforded HQs 8–11 in 62–94% yield. HQ small mole-
cules 13, 15, 16 and 19 were synthesized using previously
reported routes[8,14] (see Supporting Information).
We selected S. aureus ATCC 29213 as a model strain to begin
testing the growth-inhibitory effects of HQ–phytochemical
combinations. From our panel of phytochemicals, GA showed
the most dramatic potentiation (>100-fold for multiple HQs;
Table 1) of HQ growth inhibition against S. aureus 29213 while
caffeic acid and tiron demonstrated good potentiation activi-
ties of HQ small molecules (up to 64-fold potentiation;
Table S1 in the Supporting Information). Interestingly, the phy-
tochemical quercetin showed no potentiation effects of HQ
small molecules against S. aureus 29213.
Following the synthesis of the HQ small molecules, we eval-
uated our library in MIC potentiation assays with gallic acid
Scheme 1. A) Library of halogenated quinolines evaluated during our potentiation studies with phytochemicals. B) Friedländer quinoline synthesis used to
rapidly synthesize new halogenated quinolines 8–11. Reagents and conditions: a) iPrONO, [nBu4N][HSO4], H2SO4, CH2Cl2, RT, 15 min; b) Fe powder, HCl, EtOH/
H2O (4:1), reflux, 5 h, 54% yield (two steps); c) H2O, 708C, 5 h, 26–65% yield; d) N-bromo- or N-iodosuccinimide, PhMe, RT, 2 h, 62–94% yield. C) Structures of
phytochemicals and related potentiating agents used in these studies.
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Figure 1. A) Antibacterial potentiation of HQs 1, 21 and 22 with gallic acid (GA) against Staphylococcus aureus 29213. B) Dose–response plots of HQs 1, 4 and
5 in combination with GA (1 mm). C) Kill curves for 1 and GA as single agents and in combination against S. aureus 29213 are presented showing clear syner-
gy. Data in panels B,C represent the meanÆstandard deviation (SD) of three independent experiments.
We define antibacterial potentiation as the ability of our
non-growth-inhibiting potentiating agent (i.e., GA) to lower
MIC values of our HQs by greater than fourfold, thus demon-
strating increased antibacterial activities with combination
treatments. Following this guideline, GA potentiated 90% (18/
20 halogenated 8-hydroxyquinolines) of our HQ library
(Table 1). In addition, GA potentiated 65% (13/20 HQs) of our
HQ library by more than 32-fold against S. aureus 29213 and
40% (8/20 HQs) of the HQ library was potentiated more than
64-fold. Lead HQ 1 was potentiated by GA 11 800-fold against
S. aureus 29213 (1 (single agent): MIC=0.59 mm; 1+GA
(1 mm): MIC=0.00005 mm; Figure 1A). We generated dose–re-
sponse plots of the antibacterial potentiation of HQs 1, 4 and
5 with GA (Figure 1B) and found that the dramatic potentia-
tion of HQ 1 by GA requires higher concentrations (i.e.,
800 mm) in these experiments; however, GA decreased the MIC
values of HQs 1, 4 and 5 at all tested concentrations down to
100 mm. Interestingly, we found GA to be inactive as an anti-
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biological data in Table S2 in the Supporting Information) in
combination with GA in potentiation assays against four MRSA
clinical isolates (ATCC isolate: BAA-1707; isolates from Shands
Hospital in Gainesville, Florida: MRSA-1, MRSA-2, SA-156). GA
potentiated 1 against 75% (3/4) MRSA strains, including 1000-
fold potentiation against SA-156, which was the highest level
of potentiation observed against our MRSA strains during
these investigations. Interestingly, HQ 5 showed a consistent
and high degree of potentiation with GA across all MRSA
strains (32- to 247-fold potentiation), while other iodinated qui-
nolines also showed good potentiation activities with GA.
After identifying the uniquely potent antibacterial combina-
tion of 1 and GA, we were curious to know whether we could
potentiate biofilm eradication (complete demolition of bacteri-
al cells within a biofilm) against a methicillin-resistant S. aureus
clinical isolate (MRSA-2) with this antibacterial combination
since we have previously described potent staphylococcal bio-
film-clearing activities of 1 in crystal violet reporter assays. We
recently reported that vancomycin is unable to eradicate clini-
cal isolate MRSA-2 biofilms at high concentrations (up to
2 mm) despite possessing potent antibacterial activity against
MRSA-2 planktonic cells (MIC=0.78 mm).[16] This is illustrative of
the drug-resistant phenotype displayed by bacterial biofilms
(surface-attached bacterial communities), which are known to
occur in the majority of bacterial infections. When tested in
combination, GA potentiates the biofilm eradication activity of
1 against MRSA-2 biofilms fourfold (1 (single agent): minimum
biofilm eradication concentration (MBEC)=250 mm; 1+GA
(1 mm): MBEC=62.5 mm; Figure 2).
Table 1. Summary of gallic acid (GA) potentiation activities of halogenat-
ed quinoline (HQ) small molecules and control compounds against Staph-
ylococcus aureus strain ATCC 29213.
HQ
MIC[a]
MIC w/ GA[a]
Potentiation
[mm]
[mm]
1
2
3
4
5
6
7
8
9
0.59[b]
12.5
25
0.00005
0.39
2.35[b]
0.59[b]
0.1
11800
32
11
24
125
32
21
3
12.5
12.5
12.5
1.56
1.17[b]
0.59[b]
0.39
0.78
6.25
25
9.38[b]
12.5
6.25
25
9.38[b]
18.8[b]
12.5
0.59[b]
0.78
–
0.39
0.075[b]
0.39
0.075[b]
0.038[b]
0.009b
0.075[b]
0.39
8
10
11
12
13
14
15
16
17
18
19
20
21
22
10
86
83
64
94
125
3
0.1
0.1
2.35[b]
3.13
50
0.39
0.2
8
À6
48
63
369
124
–
0.0016[b]
0.0063
>5000
[c]
–
[a] MIC data are derived from single replicates of 2–4 independent experi-
ments; values were consistent between experiments unless otherwise
noted. The concentration of GA used in the potentiation assays was
1 mm (ꢁ1/5 MIC); [b] Midpoint MIC value for observed twofold range;
[c] GA tested alone exhibited an MIC value of >5000 mm.
Following our investigations with S. aureus, we wanted to
determine the effectiveness of this HQ–GA combination
against a broad spectrum of human pathogens, including:
S. epidermidis, Acinetobacter baumannii, Pseudomonas aerugino-
sa and Klebsiella pneumoniae (Table 2). The library of 14 HQs
tested against the MRSA clinical isolates (see Table S3 in the
Supporting Information) were further evaluated against these
new pathogens. Based on our previous studies, we found that
HQ compounds have moderate antibacterial activity, at best,
against gram-negative pathogens.[8] We first evaluated our
HQ–GA combination against S. epidermidis. We used the methi-
cillin-susceptible ATCC 12228 strain in addition to a methicillin-
resistant S. epidermidis (MRSE-1) clinical isolate. We noticed
bacterial agent against S. aureus 29213 in MIC (MIC ꢀ5 mm;
highest concentration tested) and kill curve (no decrease in
CFUmLÀ1 at 1 mm) experiments (Figure 1C).
We also investigated GA combinations with conventional an-
tibacterial agents (Tables S2 and S4 in the Supporting Informa-
tion), including ciprofloxacin (targets DNA gyrase/topoisomer-
ase IV; inhibits DNA synthesis), methicillin (targets transpepti-
dase; inhibits cell wall synthesis), erythromycin (targets 50S
subunit of ribosome; inhibits protein synthesis), tobramycin
(targets 30S and 50S subunits of ribosome; inhibits protein
synthesis) and vancomycin (targets pentapeptides; inhibits
cell-wall synthesis); however, none of these antibac-
terial agents were potentiated by GA. This result
points to a unique antibacterial mechanism for HQ
compounds that differs from conventional antibiot-
ics.
We evaluated the combination of 1 and GA with
S. aureus 29213 in kinetic experiments to generate
kill curves (Figure 1C and Supporting Information).
We used these kill kinetics to characterize the mode
of antibacterial activity of 1 as bacteriostatic and not
bactericidal. Using the same kill curve, we demon-
strated that the combination of 1 and GA gives im-
pressive bacteriostatic synergy against S. aureus
Figure 2. Gallic acid (GA) potentiation of MRSA-2 biofilm eradication activities with HQ 1.
Wells that have no bacterial growth result from completely eradicated MRSA-2 biofilms
29213 after 24 hours. We then evaluated a panel of
14 analogues (HQs 1, 4, 5, 8–15, 17, 20 and 21; see that were formed on the bottom of the corresponding microtiter wells.
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tibacterial potency (MIC=6.25 mm), but is not potentiated with
GA. Most HQs demonstrate little or no antibacterial activity
(MIC=25–>100 mm) as single antibacterial agents against
A. baumannii; however, against MDR strain BAA-1794, the HQ–
GA combinations demonstrate good antibacterial activity for
select HQ compounds, including compound 1 (alone: MIC= >
100 mm; 1+GA: MIC=9.38 mm), 4 (alone: MIC=25 mm; 4+GA:
MIC=0.3 mm) and 5 (alone: MIC= >100 mm; 5+GA: MIC=
1.56 mm).
Ester HQs 20 and 21 also demonstrate GA potentiation simi-
lar to their free hydroxy counterparts (20 with 4; 21 with 1),
most likely through a prodrug esterase cleavage mechanism.
These esters demonstrated more potent antibacterial activities
(with and without GA) compared to their corresponding hy-
droxy HQs (1 and 4). We tested our panel of HQs in combina-
tion with GA against gram-negative pathogens P. aeruginosa
(PAO1) and K. pneumoniae (ATCC 13883). We did not observe
any potentiation or antibacterial activity with our HQ–GA com-
binations against PAO1, but we did observe 6- to 64-fold po-
tentiation with select HQs (i.e., 1, 4, 5, 17, 20 and 21) with GA
against K. pneumoniae.
Table 2. Gallic acid (GA)–halogenated quinoline potentiation studies
against MRSA clinical isolates and other pathogenic bacteria.
Strain
HQ
MIC[a]
MIC w/ GA[a] Potentiation
[mm]
[mm]
MRSA BAA-1707[b]
MRSA BAA-1707[b]
MRSA BAA-1707[b]
1
4
5
0.78
12.5
12.5
0.038[d]
0.2
21
63
125
0.1
MRSA-1[b]
MRSA-1[b]
MRSA-1[b]
1
4
5
0.78
12.5
9.38[d]
0.05
0.2
0.038[d]
16
63
247
MRSA-2[c]
MRSA-2[c]
MRSA-2[c]
1
4
5
0.78
12.5
12.5
0.39
0.78
0.39
2
16
32
MRSA SA-156[b]
MRSA SA-156[b]
MRSA SA-156[b]
1
4
5
0.39
12.5
12.5
0.00039
0.15[d]
0.1
1000
83
125
S. epi. MRSE-1[c]
S. epi. MRSE-1[c]
S. epi. MRSE-1[c]
1
4
5
3.13
12.5
12.5
0.39
8
11
21
1.17[d]
0.59[d]
A. baumanii BAA-1794[b]
A. baumanii BAA-1794[b]
A. baumanii BAA-1794[b]
1
4
5
>100[e]
12.5
>100[e]
9.38[d]
0.3[d]
1.56
11
42
>64
8-Hydroxyquinolines contain a metal binding motif that in-
cludes the phenolic oxygen and heterocyclic nitrogen atom of
the quinoline ring, which can chelate various metal(II) cat-
ions.[17] It has been proposed that 8-hydroxyquinolines elicit
their antibacterial properties through mechanisms that involve
metal cation binding.[17–20] Since we are interested in develop-
ing antibacterial agents with modes of action that differ from
conventional antibiotic therapies, we decided to evaluate our
HQ library in potentiation assays with Zn2+, Fe2+ and Cu2+ cat-
ions against S. aureus 29213 to determine whether metal(II)
cations are involved in the antibacterial mechanism of HQ
compounds.
K. pneumoniae 13883[b]
K. pneumoniae 13883[b]
K. pneumoniae 13883[b]
1
4
5
>100
>100[e]
>100
6.25
1.56
18.8c
>16
>64
>5
[a] MIC data are derived from single replicates of 2–4 independent experi-
ments; values were consistent between experiments unless otherwise
noted. The concentration of GA used in the potentiation assays was
[b] 1 mm or [c] 250 mm; [d] Midpoint MIC value for observed twofold
range; [e] Partial growth inhibition observed.
We were interested to find that zinc(II) chloride (at 100 mm;
<1/7 MIC) potentiated the antibacterial activities of HQs 4 and
5 by 21- and 83-fold, respectively (for HQ 5, see Figure 3). The
presence of zinc(II) chloride (100 mm) potentiates the antibacte-
rial activities of 18 out of 20 (90%) of our HQ library (4- to 83-
fold) against S. aureus 29213. Ammonium iron(II) sulfate (at
100 mm; ꢁ1/10 MIC) had the opposite effect by diminishing
antibacterial activities of 18 of 20 (90%) HQs. Finally, we co-
that GA administered alone had some antibacterial activities
against these strains (MICs=469 and 625 mm, respectively), so
we used 250 mm for the potentiation assays (~1/2 MIC). The
overall potentiation of HQs by GA was less dramatic against
S. epidermidis (4- to 63-fold potentiation) compared with
S. aureus, but 9 out of 13 HQs (69%) tested in the panel were
potentiated against both S. epidermidis strains.
We then tested HQ–GA com-
binations against gram-negative
pathogen A. baumannii, includ-
ing both susceptible (ATCC
19606) and multidrug-resistant
(ATCC strain BAA-1794) strains.
The ATCC reports that BAA-1794
is resistant to 30 out of 34 anti-
biotics tested (susceptible only
to tigecycline, amikacin and am-
picillin/sulbactam in their panel),
which include b-lactam, amino-
glycoside, tetracycline and fluo-
roquinolone drugs. In our stud-
Figure 3. Potentiation assays with HQ 5 and ZnII, FeII and CuII against Staphylococcus aureus 29213.The results
ies, quinoline antibacterial agent
nitroxoline (17) shows good an- demonstrate that 5 operates through a metal(II)-dependent mechanism.
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treated our HQ library with copper(II) sulfate (at 100 mm; ꢁ1/
10 MIC) and observed both positive potentiation (7 of 20;
35%) and suppression (6 of 20; 30%) of antibacterial activities
with different HQ compounds. Interestingly, 8-hydroxyquino-
line 18 and nitroxoline (17) do not possess any halogen atoms
and were not potentiated by zinc(II) chloride, but did follow
iron(II) and copper(II) potentiation trends similar to HQs. Con-
ventional antibiotics with diverse antibacterial targets were
also evaluated in combination with metal(II) cations and
showed no potentiation or suppression of antibacterial activi-
ties (Supporting Information). Known metal binding com-
pounds, ethylenediaminetetraacetic acid (EDTA) and N,N,N’,N’-
tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), were also
evaluated in antibacterial potentiation assays. While EDTA
(MIC=500 mm) showed changes in antibacterial activities with
metal(II) cations, TPEN showed only two- to fourfold decrease
in antibacterial activities against S. aureus 29213. The dramatic
metal(II) cation potentiation or suppression of antibacterial ac-
tivities is unique, complex, and in some instances follows no
predictable trends (i.e., mixed copper(II) potentiation results). It
is likely that our HQs bind all metal(II) cations tested in these
studies, resulting in differential antibacterial activities. The ob-
served metal(II) activation of HQ structures plays a primary role
in their antibacterial mechanism/target, which has excellent
potential for antibacterial therapies of biofilm-associated bacte-
ria.
Acknowledgements
The authors would like to acknowledge the Emerging Pathogens
Institute at the University of Florida (USA) for a Seed Grant. A.T.G.
and V.M.N. have been awarded University of Florida Graduate
School Fellowships. M.T.N. is a member of the University of Flori-
da Scholars Program and has received support through this pro-
gram.
Keywords: antibacterial potentiation · biofilm eradication ·
combination antibacterial therapy
·
drug discovery
·
halogenated quinolones · phytochemicals
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Finally, we evaluated HQs 1, 4–6, 10, 12 and 14 for cytotox-
icity against HeLa cells in a standard lactate dehydrogenase
(LDH) release assay[21] at 25, 50 and 100 mm. All HQs were
found to demonstrate excellent toxicity profiles (i.e., IC50 values
>100 mm in LDH release assay) and gave selective indexes (SI)
between >8 to >256 when comparing HeLa IC50 values to
S. aureus 29213 MIC values. Nitroxoline (17), an antibacterial
agent used to treat urinary tract infections,[18] reported an IC50
value of ~100 mm against HeLa cells in LDH release assays and
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In conclusion, we have discovered that select combinations
of various phytochemicals, in particular gallic acid (GA), and
halogenated quinoline (HQ) small molecules demonstrate
potent antibacterial activities against a broad spectrum of
pathogenic bacteria, including multidrug-resistant clinical iso-
lates. This promising combination therapy is also effective in
the eradication of methicillin-resistant S. aureus (MRSA) bio-
films. HQs operate through a metal(II)-dependent mechanism
that differs from conventional antibiotics and demonstrate ex-
cellent cytotoxicity profiles for antibacterial development when
tested in HeLa cells in LDH release assays. Phytochemical–HQ
combinations provide a promising new platform for develop-
ing clinically useful antibacterial combination treatments that
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Received: April 21, 2015
Published online on May 15, 2015
ChemMedChem 2015, 10, 1157 – 1162
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