First identification of boronic species as novel potential inhibitors of the Staphylococcus aureus NorA efflux pump

Article abstract of DOI:10.1021/jm401808n

Overexpression of efflux pumps is an important mechanism of bacterial resistance that results in the extrusion of antimicrobial agents outside the bacterial cell. Inhibition of such pumps appears to be a promising strategy that could restore the potency of existing antibiotics. The NorA efflux pump of Staphylococcus aureus confers resistance to a wide range of unrelated substrates, such as hydrophilic fluoroquinolones, leading to a multidrug-resistance phenotype. In this work, approximately 150 heterocyclic boronic species were evaluated for their activity against susceptible and resistant strains of S. aureus. Twenty-four hit compounds, although inactive when tested alone, were found to potentiate ciprofloxacin activity by a 4-fold increase at concentrations ranging from 0.5 to 8 μg/mL against S. aureus 1199B, which overexpresses NorA. Boron-free analogues showed no biological activity, thus revealing that the boron atom is crucial for biological activity. This work describes the first reported efflux pump inhibitory activity of boronic acid derivatives.

Full text of DOI:10.1021/jm401808n

Article
pubs.acs.org/jmc
First Identification of Boronic Species as Novel Potential Inhibitors
of the Staphylococcus aureus NorA Efflux Pump
Fanny Fontaine,†,‡ Arnaud Hequet,§ Anne-Sophie Voisin-Chiret,*,†,‡ Alexandre Bouillon,∥
Aurelien Lesnard,†,‡ Thierry Cresteil,⊥ Claude Jolivalt,§ and Sylvain Rault†,‡
́
†Normandie Univ, France
‡UNICAEN, CERMN (Centre d’Etudes et de Recherche sur le Med
Boulevard Becquerel, 14032 Caen, France
́
icament de Normandie) − FR CNRS INC3M,
§Chimie ParisTech (Ecole Nationale Super
11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France
∥BoroChem S.A.S., 10 rue du Professeur Zarifian, F-14200 Her
́
ieure de Chimie de Paris), Laboratoire Charles Friedel, UMR-7223, CNRS,
́
ouville Saint Clair, France
⊥Institut de Chimie des Substances Naturelles, CNRS, UPR 2301, avenue de la Terrasse, F-91198 Gif sur Yvette, France
ABSTRACT: Overexpression of efflux pumps is an important
mechanism of bacterial resistance that results in the extrusion
of antimicrobial agents outside the bacterial cell. Inhibition
of such pumps appears to be a promising strategy that could
restore the potency of existing antibiotics. The NorA efflux
pump of Staphylococcus aureus confers resistance to a wide
range of unrelated substrates, such as hydrophilic fluoroqui-
nolones, leading to a multidrug-resistance phenotype. In this
work, approximately 150 heterocyclic boronic species were evaluated for their activity against susceptible and resistant strains of
S. aureus. Twenty-four hit compounds, although inactive when tested alone, were found to potentiate ciprofloxacin activity by a
4-fold increase at concentrations ranging from 0.5 to 8 μg/mL against S. aureus 1199B, which overexpresses NorA. Boron-free
analogues showed no biological activity, thus revealing that the boron atom is crucial for biological activity. This work describes
the first reported efflux pump inhibitory activity of boronic acid derivatives.
environmental toxins as well as allowing them to secrete
endogenous products.16 Most efflux pumps are thus able to
remove a broad spectrum of structurally unrelated substrates.
As a collateral consequence of this self-defense mechanism,
antibiotics that may be incidental substrates of efflux pumps are
actively extruded from the bacterial cell. Intrinsic resistance to
various drugs by active efflux is widespread among bacteria:
transporters are expressed at a basal level, resulting in a low level
of resistance.17,18 However, the intensive use of antibiotics results
in overexpression of efflux pumps for which antimicrobial agents
are good substrates. In addition, while allowing bacteria to
survive in the presence of sublethal drug concentrations, efflux
pumps may in turn predispose the microorganisms to develop
mutations in drug target genes.19,20 Efflux-mediated resistance is
thus a growing cause of concern because of its key role in the
selection of high-level target-based resistance.21
INTRODUCTION
■
From the first therapeutic use of penicillin in 1941 to the
advent of antibacterial drugs, each newly marketed antibiotic
has invariably resulted in the emergence of resistant bacterial
pathogens.1,2 At first limited to one or few structurally related
antibiotics, bacterial resistance then progressed toward cross-
resistance between antibiotics that have unrelated scaffolds and
different mechanisms of action, leading to multidrug-resistant
(MDR) microorganisms.3,4 The spread of those superbugs, which
are resistant to almost all available antibiotics, is thus becoming a
worldwide healthcare problem.5−7 Bacteria have developed three
main mechanisms of resistance: (i) enzymatic degradation and
modification of antibiotics,8 (ii) alteration or modification of the
antibiotic target(s),9 and (iii) reduction of intracellular antibiotic
concentrations by reduced uptake and/or activation of the
efflux.10 These mechanisms can be simultaneously expressed
within one single bacterial strain and are responsible for enhanced
resistance.
Efflux-mediated drug resistance was first described in 1980
following the identification of tetracycline resistance in
Escherichia coli.11 This phenomenon results in the release of
different substances outside the bacterial cell mediated by
membrane transporters called efflux pumps12,13 and is found in
both Gram-positive14 and Gram-negative bacteria.15 The original
physiological role of such pumps is to protect bacteria from
The quest for new potent antibiotics remains the main
strategy against drug resistance. In particular, research focuses
on the structural pharmacomodulation of existing antibacterial
classes to overcome microbial-resistance mechanisms22 and on
the development of antibiotics with novel mechanisms of action.
Nonetheless, over the past 30 years, only two completely new
Received: November 22, 2013
Published: February 5, 2014
© 2014 American Chemical Society
2536
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548

Journal of Medicinal Chemistry
Article
Figure 1. Antibacterial properties of organoboron compounds.
classes of compounds have been launched on the market: the
oxazolidinone linezolid in 2000 and the lipopeptide daptomycin
in 2003,23 and resistance to these has already emerged.24
New strategies are thus needed to fight resistant pathogens.25
Recently, research has been moving toward another path: the
fight against resistance mechanisms, particularly the inhibition of
efflux pumps, which appears to be a promising approach that
could restore the potency of existing antibiotics. This strategy
is based on the combination between an efflux pump inhibitor
(EPI) and the active antibiotic agent, preventing its release from
bacteria and therefore increasing its intrabacterial concentra-
tion.26 This approach is similar to the association between a
β-lactam antibiotic and a β-lactamase inhibitor for which
examples are already used in the clinic or are undergoing clinical
development.27 Therefore, EPIs are expected to increase the
potency of antibiotics and to expand their spectrum of activity.
Moreover, as efflux pump are involved in the selection of high-
level resistant strains, such inhibitors may have enhanced clinical
benefits in reducing the rate of resistance development.28
Methicillin-resistant Staphylococcus aureus (MRSA) strains are
of particular concern among resistant microorganisms because
they are responsible for numerous community- and hospital-
acquired infections.29 Although MRSA strains are characterized
by the presence of the mecA gene that confers resistance to
β-lactam antibiotics, these organisms also have the ability to
acquire resistance to several antimicrobial agents such as fluoro-
quinolones, macrolides, aminoglycosides, and tetracyclines.30
These cross resistances result from different mechanisms, of
which the MsrA pump accounts for a significant part. The MsrA
pump is an ABC transporter that uses energy derived from ATP
hydrolysis to efflux antibacterial agents. A recent study revealed
that the increased expression of at least one MDR efflux pump
gene is frequently observed in S. aureus isolates and that MRSA
predominates among those strains.
identified NorA EPI, is commonly used as a control in pump
inhibitory assays, but it is not clinically relevant because of
neurotoxicity at concentrations required for activity.35 Other
natural-based EPIs include flavonolignan and flavone com-
pounds,36,37 N-caffeoylphenalkylamide derivatives,38 piperine
and piperine analogues,39 and polyacylated oligosaccharides.40
Synthetic inhibitors are mainly based on heterocyclic scaffolds,
such as indoles,41 phenothiazines and thioxanthenes,42,43
quinolines and quinolones,44,45 and tricyclic compounds.46
Considering the structural diversity of these compounds, we
were hopeful to discover new classes of NorA EPIs.
For several years, our laboratory has focused on the synthesis
and physicochemical study of various heterocyclic compounds
bearing boronic acid or boronic ester functions.47 Besides their
key role in cross-coupling reactions, some boronic species have
been described for their antimicrobial,48 antineoplastic,49 and
enzyme-inhibitory activities.50 Boronic acids are well-known as
serine-protease inhibitors that act as transition-state analogues
through nucleophilic attack of the active serine on the boron
open shell. Of particular interest are the antibacterial properties
of various organoboron compounds (Figure 1) such as di-
azaborines (I, enoyl acyl carrier protein reductase inhibitors),51
benzoxaboroles (II (AN3365), leucyl-tRNA synthase inhibitors,52
and VI, β-lactamases inhibitors),53 oxazaborolidines (III),54
borinic (IV)55 and boronic (V) esters,56 and boronic acids (VII
and VIII (RPX7009), β-lactamases inhibitors).57,58 This
prompted us to investigate the potential biological properties
of a series of boronic species prepared in our laboratory over the
past decade.
In this article, we describe the first systematic antibacterial
evaluation of heterocyclic boronic acids against S. aureus strains
susceptible or resistant to quinolone or macrolide antibacterials
by active efflux processes. In particular, these compounds were
found to be a novel class of potential NorA efflux inhibitors
that restores the antibacterial activity of ciprofloxacin against
SA-1199B.
NorA-overexpressing strains have been shown to be the most
common among MRSA strains.31 NorA, a transporter of the
major facilitator superfamily, catalyzes the antiport-coupled
transport of the antibiotic with a proton gradient. Its over-
expression confers resistance to a wide range of unrelated
substrates such as hydrophilic fluoroquinolones (ciprofloxacin
and norfloxacin but not lipophilic ones), various biocides
(acriflavine, cetrimide, and benzalkonium chloride), and dyes
(ethidium bromide).32,33 In recent years, several NorA inhibitors
have been identified.34 The alkaloid reserpine, the first natural
RESULTS AND DISCUSSION
■
Microbiological Assays. Compounds were investigated for
their activity against the susceptible S. aureus ATCC25923 strain
and two resistant S. aureus strains that express active efflux
pumps. Between the latter, the S. aureus 1199B strain shows
reduced susceptibility to fluoroquinolones through overexpres-
sion of the NorA efflux pump and also possesses a mutation in
2537
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548

Journal of Medicinal Chemistry
Article
Table 1. Antibacterial Activity of Pyridine-4-boronic Compounds
a
b
Se, susceptible S. aureus ATCC25923. Rt, resistant S. aureus 1199B (NorA). −, inhibition of growth <90% at a concentration of 100 μg/mL. N.D.,
no data.
the gyrase subunit A. Ciprofloxacin has a MIC of 0.5 μg/mL
against the parent susceptible S. aureus 1199 strain and of
16 μg/mL against the resistant clone. The second resistant
strain was S. aureus RN4220, harboring the multicopy plasmid
pUL 5054 that includes the msr(A) gene, conferring resistance
to 14- and 15-membered macrolides through overexpression of
the MsrA efflux pump. Erythromycin has a MIC of 0.5 and 128
μg/mL against the susceptible and resistant strains, respectively.
In each microbiological assay, all compounds were initially
evaluated at a concentration of 100 μg/mL, and for those that
inhibit more than 90% of the bacterial growth, the MIC was
then determined. In a first screening assay, compounds were
evaluated for their intrinsic antibacterial activity against both
susceptible and resistant strains. Compounds that were inactive
alone were then investigated for their ability to restore the potency
of ciprofloxacin or erythromycin against the resistant SA-1199B
and SA-RN4220 strains to identify potential NorA or MsrA efflux
pump inhibitors. Therefore, compounds were evaluated in com-
bination with a subinhibitory concentration of the antimicrobial
agent: (i) MIC/4 (i.e., 4 μg/mL of ciprofloxacin) and (ii) MIC/8
(i.e., 16 μg/mL of erythromycin). Potentiating activities of
inhibitors are reported as minimum modulatory concentration
MMC4 or MMC8 values, which are the lowest concentration of
the inhibitor required to achieve antibacterial activity in combina-
tion with corresponding subinhibitory concentrations of cipro-
floxacin or erythromycin. Reserpine, which is a known inhibitor of
the NorA efflux pump, was used as an internal standard and
displayed a MMC4 of 4−8 μg/mL.
Structure−Activity Relationships. A laboratory-made
chemical library composed of approximately 150 various
organoboron compounds, mainly heterocyclic boronic species
as well as trifluoroborate salts, was tested. Because both boronic
acids and esters display biological activity (Figure 1), we
decided to evaluate both protected and unprotected boronic
species. All compounds from the screening were originally pre-
pared in our laboratory47 or were a gift from BoroChem S.A.S.,
even if some of these are currently commercially available. The
most significant biological results are reported in Tables 1−5.
None of these compounds were significantly active when used
alone against the susceptible ATCC25923 straing, with MICs ≥
64 μg/mL. Except for 14d and 14o−p, all compounds were
inactive when used alone against the fluoroquinolone-resistant
1199B strain. Phenylboronic acids 14d and 14o−p showed
moderate intrinsic antibacterial activity against the 1199B
strain, with MICs of 32 μg/mL. In contrast, all compounds, in
combination with 4 μg/mL of ciprofloxacin, were found to
inhibit more than 90% of the bacterial growth of the 1199B
strain at the initial 100 μg/mL concentration. On the contrary,
only compounds 14d and 14o−p, showing a moderate intrinsic
antibacterial activity, were found to be active to the same extent
(MMC8 = 32 μg/mL, data not shown) against the macrolide-
resistant RN4220 strain in combination with erythromycin.
Differences in activity of the compounds between both efflux
pumps may be due to their structural specificity: the NorA
pump belongs to the MFS family and catalyzes the antiport-
coupled transport of the antibiotic with a proton gradient,59
2538
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548

Journal of Medicinal Chemistry
Article
and the MsrA pump is an ABC transporter which uses energy
derived from ATP hydrolysis.60
Biological results for five-membered heterocyclic and indazole
boronic derivatives (Tables 4 and 5) did not allow for SAR to be
clearly established. One thiophene (15b), one pyrrole (15f),
and two indazole (16b and 16g) compounds showed promising
results, with MMC4 of 8−16 μg/mL.
Determination of the activity of compounds in combination
with ciprofloxacin revealed the presence of 24 promising
molecules with MMC4 ranging from 0.5 to 8 μg/mL, half of
which were pyridine-3-boronic derivatives (3d, 5a−b, 5d−g,
5i−j, 7b, 7d, and 11a) and seven were benzene boronic
compounds (14f−g, 14i, 14n, and 14r−t). Other active
compounds included the pyridine-4-boronic acid pinacol ester
2c, the thiophene-3-boronic acid 15b, the pyrazole-5-boronic
acid pinacol ester 15f, and 2 indazole boronic derivatives 16b
and 16g. The following discussion will refer to the MMC4 of
compounds against the SA-1199B strain with 4 μg/mL of
ciprofloxacin.
Results are shown in Table 1 for pyridine-4-boronic
compounds and in Table 2 for pyridine-3-boronic ones. The
first structure−activity relationship (SAR) studies revealed that
(i) pyridine-3-boronic acids displayed the best potentiating
activity of ciprofloxacin with 11 compounds (3d, 5a−b, 5d−g,
5i−j, 7b, and 7d) having MMC4 ranging from 1 to 8 μg/mL;
(ii) except for compounds 5a and 11a that had the same
MMC4, pyridine-3-boronic esters (9a−b, 11b−f, 12a−b, and
13a) showed lower activities than their corresponding boronic
acids (3d−e, 5b−c, 5e−g, 6a−b, and 7b), and inactive acid
derivatives (3h−i, 6c, and 7c) led to inactive ester analogues
(9c−d, 12c, and 13b); (iii) pyridine-4-boronic derivatives were
mostly inactive except the fluorinated ester 2c, which showed a
surprisingly low MMC4 of 2 μg/mL, and pyridine-4-boronic
acids (1d−e and 1g−h) and pyridine-4-boronic esters (2i−j
and 2l−m) displayed equivalent moderate activity; (iv) for
active boronic acids (5b−c and 6a), the shift of the boronic
moiety from the C-3 to the C-4 position (1a−b and 1h)
resulted in a loss of activity; and (v) the trigonal boronic acid
function of compound 3a gave better results than the negatively
charged tetrahedric potassium trifluoroborate (8a) or sodium
boronate (8b) salts.
Considering the pyridine-3-boronic acids, the 6-substituted
and 5,6-disubstituted derivatives appeared to be the most
promising compounds except for compounds 5h and 6c, which
were inactive. 6-Benzyloxypyridine-3-boronic acid 5f displayed
the highest activity and was able to potentiate ciprofloxacin
by a 4-fold increase at a concentration as low as 1 μg/mL. SAR
studies of the monosubstituted pyridine-3-boronic acids revealed
that (i) apart from compound 3d, 2-substituted-pyridine-3-
boronic acids (3b−c and 3e−h) showed a significative loss of
activity compared to their corresponding 6-substituted deriva-
tives (5a−b and 5d−g); (ii) the 5-substituted 4b was slightly less
potent that its 6-substituted analogue 5j but still exhibited
interesting activity; and (iii) cyclisation at the C-5/C-6 posi-
tions (7d) or introduction of a methyl group at the C-5 position
(7a−b) was well-tolerated.
The data reported in Table 3 reveal similar SAR for benzene
boronic derivatives. Except for 14h, which is inactive, benzene
analogues of the active pyridine-3-boronic acids (compounds
14f−g, 14i, 14n, and 14s vs 5a−b, 5f, 7b, and 7d) displayed
the best activities with MMC4 ranging from 0.5 to 8 μg/mL.
Along with 14r, this revealed the importance of a substituent in
the para position with respect to the boronic moiety, as was
observed for pyridine-3-boronic derivatives. Cyclisation at the
C-3/C-4 positions (14s−t) or introduction of a methyl group
in the meta position of the benzene ring (14n) was also well-
tolerated.
Chemistry. Once we identified pyridine-3- and benzene-
boronic acids as promising NorA inhibitors, we sought to
confirm the importance of the boronic acid moiety for biological
activity on the resistant 1199B strain by testing analogues devoid
of the boron atom. Biological activity of boronic acids relies
on their ability to shift from a neutral and trigonal planar
sp2 configuration to an anionic tetrahedral sp3 one through
nucleophilic attack on the empty p-type orbital of boron. In this
case, electrophilic functions, such as carboxylic acids and esters,
aldehydes, silicate, phosphate, and α-keto acid derivatives are
also able to act as protease inhibitors.61 We chose to limit our
study to the carboxylic acid analogues 19a−k that are bioisosters
of boronic acids and to their aldehyde counterparts, 20a−k.
Because some bacteria can perform hydroxydeboronation
reactions that transform boronic acids into corresponding
alcohols,62 we were also interested in hydroxylated analogues
21a−k and 22. Chosen compounds for this investigation were
pyridine-3-boronic acid (3a,), 6-bromo- (5a), 6-chloro- (5b),
6-benzyloxy- (5f), and 6-chloro-5-methylpyridine-3-boronic
acids (7b), quinoline-3-boronic acid (7d), and their benzenic
counterparts (14a, 14f−g, 14n, and 14s) that gave the best
results from the primary screening (Tables 2 and 3) and whose
desired analogues were easily available either commercially
or synthetically. In particular, all benzene derivatives were
purchased from commercial sources.
We prepared carboxylic acids 19d−e and aldehyde
compounds 20d−e from halogeno-pyridines 18d−e using the
general halogen−metal-exchange procedure developed in our
laboratory for the pyridine series.47a The halogen−lithium
exchange was carried out with n-BuLi in anhydrous ether at
−78 °C, and the lithiated intermediate species were trapped
either by carbon dioxide or dimethylformamide followed
by appropriate hydrolysis. Specific attention must be given to
the carboxylic acid workup because they tend to adopt a
zwitterionic form because of the concomitant presence of the
carboxylic acid and the pyridine functions. Thus, desired
product with protonated carboxylic acid and free amine is only
predominant in the aqueous phase in a narrow pH range that
could be theoretically determined using the MarvinSketch pro-
tonation calculator. Optimized workup involved slow addition
of a 4% aqueous NaOH solution to the reaction mixture, which
allowed solubilization of the carboxylate salt in the aqueous
layer while retaining organic byproducts in the ethereal phase.
The resulting aqueous phase was then carefully acidified by 3 N
HCl until complete precipitation of expected carboxylic acids
19d−e were obtained in good yields (58−87%). Such a
procedure was not necessary for aldehyde derivatives that are
predominant at neutral pH, and hydrolysis followed by classical
workup led to compounds 20d−e in 55−78% yields (Scheme 1).
Then, we obtained hydroxypyridines 21d−f in good yields
(71−93%) from corresponding boronic acids using the hydrogen
peroxide-mediated hydroxydeboronation described in our
laboratory (Scheme 2).63
Finally, we prepared 6-benzyloxypyridine-3-methanol 22
in moderate yield (45%) through reduction of carboxylic acid
19d in the presence of lithium aluminum hydride (LAH) in
anhydrous THF (Scheme 3).
2539
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548

Journal of Medicinal Chemistry
Article
Table 2. Antibacterial Activity of Pyridine-3-boronic Compounds
2540
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548

Journal of Medicinal Chemistry
Article
Table 2. continued
a
b
Se, susceptible S. aureus ATCC25923. Rt, resistant S. aureus 1199B (NorA). −, inhibition of growth <90% at a concentration of 100 μg/mL. N.D.,
no data.
Biological evaluation of these compounds is reported in
as well as the presence of a substituent in the para position with
respect to the boronic moiety. 6-Benzyloxypyridine-3-boronic
acid 5f and 4-benzyloxybenzene boronic acid 14i were found to
be the most promising NorA inhibitors: they potentiate the
activity of ciprofloxacin with respective MMC4 values of 1 and
0.5 μg/mL, they have no significant intrinsic antibacterial
activity, and they show reduced cytotoxicity against human cell
lines. Chemical pharmacomodulation of compound 5f has been
undertaken to design NorA inhibitors with improved potency
and will be the subject of a future publication.
Table 6. The MIC of compound 22 is >128 μg/mL against SA-
1199B, and its MMC4 is of 64−128 μg/mL with ciprofloxacin.
No derivatives showed any intrinsic antibacterial activity against
the SA-1199B strain, with MICs ≥ 128 μg/mL, as well as in
combination with 4 μg/mL of the fluoroquinolone (MMC4 ≥
32 μg/mL). These values are significantly higher than those
displayed by the corresponding boronic acid derivatives
(MMC4 ≤ 8 μg/mL). These results highlight the requirement
of the boron atom for biological activity, which cannot be
simply replaced by carbon.
A survey of the required features for biological activity of
pyridine-3- and benzene boronic acids is summarized in Figure 2.
Cytotoxic Assays. After the evaluation of the active mole-
cules as antibacterial agents, their cytotoxicity was ascertained
on KB cells at a concentration of 10 μM. KB cells are epithelial
cells originated from an epidermal carcinoma of the mouth
sensitive to a variety of structurally different agents and lacking
the efflux pump P-gp (ABC B1). Docetaxel was used as positive
control at its IC50 concentration (0.15 nM). The results are
shown in Table 7 as the percent of cellular growth inhibition.
Most molecules, including compounds 5f, 14i, and 14d,
inhibited less than 80% of the growth of KB cells and were
thus considered as noncytotoxic. 6-Benzyloxypyridine-3-
boronic acid 5f and 4-benzyloxybenzene boronic acid 14i were
thus confirmed as hit compounds for their activity as potential
inhibitors of the NorA efflux pump, and 2-phenoxybenzene
boronic acid 14d, for its antibacterial activity against the SA-
1199B strain. 4-Formylbenzene boronic acids 14o and 14p were
the most cytotoxic at 10 μM, and this may be correlated with the
presence of the aldehyde moiety.
EXPERIMENTAL SECTION
■
Synthesis. All chemical products, reagents, and solvents were
purchased from commercial sources and used without further
purification except THF, which was distilled from Na/benzophenone.
Chromatography was carried out on a column using flash silica gel
60 Merck (0.063−0.200 mm) as the stationary phase. The eluting
solvent indicated for each purification was determined by thin-layer
chromatography (TLC) performed on 0.2 mm precoated plates of
silica gel 60F254 (Merck), and spots were visualized using a UV lamp.
Melting points were determined on a Kofler melting-point apparatus.
IR spectra were recorded on KBr disks using a PerkinElmer BX FT-IR
spectrophotometer. The band positions are given in reciprocal centi-
1
meters (cm−1). H (400 MHz) and 13C (100 MHz) NMR spectra
1
were recorded on a JEOL Lambda 400 spectrometer. H (500 MHz)
and 13C (125 MHz) NMR spectra were recorded on a Bruker 500
Avance III spectrometer. Chemical shifts (δ) are expressed in parts
per million downfield from tetramethylsilane as an internal standard,
and coupling constants (J) are in hertz. The abbreviations used are as
follows: s, singlet; bs, broad singlet; d, doublet; dd, double of doublets;
t, triplet; and m, multiplet. The purities of all tested compounds were
analyzed by LC−MS, with the purity of all compounds being higher
than 95%. Analyses were performed using a Waters alliance 2695, and
MS detection was performed with a SQDetector.
All compounds from the screening were originally prepared in our
laboratory47 or were a gift from BoroChem S.A.S., even if some of these
are currently commercially available. All carbonylated and hydroxylated
benzene analogues and non described pyridinyl ones (Table 6) were
purchased from commercial sources and used as provided by the
manufacturers without additional verification.
6-Benzyloxypyridine-3-carboxylic Acid (19d). To a slurry of
2.5 M of n-BuLi (1.89 mL, 4.73 mmol) in anhydrous diethyl ether
(80 mL) cooled at −78 °C was added a solution of 2-benzyloxy-5-
bromopyridine (1.0 g, 3.79 mmol) in anhydrous THF (10 mL).
The mixture was allowed to react at this temperature over 60 min.
A CO2(g)-inflated balloon was bubbled in the reaction medium and
CONCLUSIONS
■
In this study, we conducted the first investigation on the
antibacterial properties of a series of boronic derivatives against
susceptible and resistant S. aureus strains. Starting from a
chemical library of approximately 150 compounds, we
identified 24 hit compounds for their ability to restore the
activity of ciprofloxacin by a 4-fold increase at concentrations
ranging from 0.5 to 8 μg/mL against the NorA-overexpressing
SA-1199B strain. The first structure−activity relationship
studies pointed out the necessity of the boron atom for activity
2541
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548

Journal of Medicinal Chemistry
Article
Table 3. Antibacterial Activity of Benzene Boronic Acid Compounds
a
b
Se, susceptible S. aureus ATCC25923. Rt, resistant S. aureus 1199B (NorA).−, inhibition of growth <90% at a concentration of 100 μg/mL. N.D.,
no data.
left to react for 60 min. The mixture was allowed to warm to room
temperature and was quenched by slow addition of a 4% aqueous
NaOH solution (50 mL). The resulting aqueous layer was collected
and acidified to pH 2.5 by dropwise addition of 3 N HCl. The
resulting precipitate was filtered on a sintered-glass filter, washed
with ether, and dried to give 19d as a white solid. Yield 87%. mp
6-Chloro-5-methylpyridin-3-ylcarboxylic Acid (19e). Follow-
ing the procedure for carboxylic acid 19d synthesis, 2-chloro-5-iodo-3-
methylpyridine (1.0 g, 3.95 mmol) was reacted with n-BuLi (1.97 mL,
4.93 mmol). The mixture was then treated as described in the
representative procedure (pH 2) to give 19e as a white solid. Yield
68%. mp 171 °C. IR (KBr): ν (cm−1) 3054, 1688, 1592, 1435, 1392,
1
174 °C. IR (KBr): ν (cm−1) 3028, 1701, 1606, 1289, 1132, 725. H
1
1071. H NMR (400 MHz, DMSO-d6): δ 2.38 (3H, s, CH3), 8.24
NMR (400 MHz, CDCl3): δ 5.47 (2H, s, OCH2), 6.86 (1H, d, J =
8.8 Hz, H-5), 7.32−7.48 (5H, m, H-Ph), 8.23 (1H, dd, J = 8.8
and 1.9 Hz, H-4), 8.94 (1H, d, J = 1.9 Hz, H-2). 13C NMR (125
MHz, DMSO-d6): δ 166.1, 165.5, 149.3, 140.1, 136.6, 128.4, 128.0,
127.9, 120.7, 110.8, 67.6. LC−MS (ESI): tR = 6.39 min; [M + H]+
230.13.
(1H, d, J = 2.4 Hz, H-4), 8.70 (1H, d, J = 2.4 Hz, H-2), 13.55 (1H, sl,
COOH). 13C NMR (100 MHz, DMSO-d6): δ 165.6, 154.3, 147.9,
140.4, 132.5, 126.2, 18.9. LC-MS (ESI): tR = 5.02 min; [M + H]+
172.37, 174.38.
6-Benzyloxypyridine-3-carboxaldehyde (20d). To a slurry of
2.5 M of n-BuLi (1.89 mL, 4.73 mmol) in anhydrous diethyl ether
2542
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548

Journal of Medicinal Chemistry
Article
Table 4. Antibacterial Activity of Other Heterocyclic Compounds
a
b
Se, susceptible S. aureus ATCC25923. Rt, resistant S. aureus 1199B (NorA). −, inhibition of growth <90% at a concentration of 100 μg/mL.
Table 5. Antibacterial Activity of Indazole Boronic Compounds
a
b
Se, susceptible S. aureus ATCC25923. Rt, resistant S. aureus 1199B (NorA). −, inhibition of growth <90% at a concentration of 100 μg/mL.
2543
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548

Journal of Medicinal Chemistry
Article
temperature for 12 h and then concentrated to dryness. The residue
was diluted with water (50 mL) and extracted with ethyl acetate. The
organic phase was washed with brine, dried on MgSO4, filtered, and
evaporated. The crude product was purified by silica gel chromatog-
raphy (cyclohexane/EtOAc 8:2), affording 21d as a white solid. Yield
78%. mp 91 °C. IR (KBr): ν (cm−1) 3060, 2870, 1496, 1265, 1043,
Scheme 1. Synthesis of Carbocylic Acids 19d−e and
a
Aldehydes 20d−e
1
755. H NMR (400 MHz, CDCl3): δ 5.27 (2H, s, OCH2), 5.55 (1H,
sl, OH), 6.71 (1H, d, J = 8.8 Hz, H-5), 7.18 (1H, dd, J = 8.8 and 3.2
Hz, H-4), 7.27−7.43 (5H, m, H-Ph), 7.76 (1H, d, J = 3.2 Hz, H-2).
13C NMR (100 MHz, CDCl3): δ 158.1, 147.4, 137.3, 132.5, 128.4,
127.84, 127.8, 127.78, 111.6, 68.0. LC−MS (ESI): tR = 5.95 min;
[M + H]+ 202.45.
a
Reagents and conditions: (i) n-BuLi (1.25 equiv), anhyd. Ether,
−78 °C, 1 h; (ii) CO2(g), −78 °C to rt, 1 h; (iii) hydrolysis; (iv) DMF
(1.25 equiv), −78 °C to rt, 1 h. Isolated yields.
6-Chloro-3-hydroxy-5-methylpyridine (21e). Following the
procedure for hydroxypyridine 21d synthesis, 6-chloro-5-methylpyr-
idine-3-boronic acid 7b (0.3 g, 1.75 mmol) was reacted with a solution
of 35% hydrogen peroxide (0.45 mL, 5.25 mmol). The crude product
was purified by silica gel chromatography (cyclohexane/EtOAc 8:2),
affording 21e as a white solid. Yield 93%. mp 125 °C. IR (KBr): ν
(cm−1) 3012, 2884, 2634, 1581, 1452, 1320, 1236, 743, 644. 1H NMR
(400 MHz, DMSO-d6): δ 2.23 (3H, s, CH3), 7.17 (1H, d, J = 2.9 Hz,
H-4), 7.75 (1H, d, J = 2.9 Hz, H-2), 10.09 (1H, s, OH). 13C NMR
(100 MHz, DMSO-d6): δ 153.4, 139.8, 134.6, 132.5, 126.5, 19.1. LC−
MS (ESI): tR = 4.85 min; [M + H]+ 144.37, 146.39.
a
Scheme 2. Synthesis of Hydroxypyridines 21d−f
3-Hydroxyquinoline (21f). Following the procedure for hydrox-
ypyridine 21d synthesis, quinoline-3-boronic acid 7d (0.4 g, 2.31 mmol)
was reacted with a solution of 35% hydrogen peroxide (0.60 mL,
6.94 mmol). The crude product was purified by silica gel chromato-
graphy (cyclohexane/EtOAc 7:3), affording 21e as a beige solid. Yield
71%. mp 190 °C. IR (KBr): ν (cm−1) 2580, 1602, 1349, 1213, 747.
1H NMR (400 MHz, DMSO-d6): δ 7.47−7.50 (3H, m, H-4, H-6, H-7),
7.77−7.79 (1H, m, H-5), 7.87−7.89 (1H, m, H-8), 8.56 (1H, d, J =
2.9 Hz, H-2), 10.30 (1H, s, OH). 13C NMR (100 MHz, DMSO-d6): δ
150.9, 143.9, 142.4, 129.0, 128.6, 126.7, 126.5, 125.9, 115.3. LC−MS
(ESI): tR = 1.63 min; [M + H]+ 146.42.
a
Reagents and conditions: H2O2 (35%, 3 equiv), CH2Cl2, rt, 12 h.
Isolated yields.
a
Scheme 3. Synthesis of 6-Benzyloxypyridine-3-methanol 22
a
6-Benzyloxypyridin-3-ylmethanol (22). To a solution of
6-benzyloxypyridine-3-carboxylic acid 19d (1.0 g, 4.36 mmol) in
anhydrous THF (50 mL) cooled at 0 °C was slowly added lithium
aluminum hydride (0.41 g, 10.91 mmol). The mixture was allowed to
warm to room temperature and to react at this temperature for 4 h.
Then, it was quenched with water and extracted with ethyl acetate.
The organic phase was washed with brine, dried on MgSO4, filtered,
and evaporated. The crude product was purified by silica gel chromato-
graphy (cyclohexane/EtOAc 6:4), affording 22 as a beige solid. Yield
45%. mp 70 °C. IR (KBr): ν (cm−1) 3327, 1609, 1489, 1289, 1261,
Reagents and conditions: LAH (2.5 equiv), anhyd. THF, 0 °C to rt,
4 h. Isolated yield.
(80 mL) cooled at −78 °C was added a solution of 2-benzyloxy-5-
bromopyridine (1.0 g, 3.79 mmol) in anhydrous THF (10 mL).
The mixture was allowed to react at this temperature over 60 min.
A solution of anhydrous DMF (0.37 mL, 4.73 mmol) was then added
and left to react for 60 min. The mixture was allowed to warm to room
temperature, quenched with water, and extracted with ethyl acetate.
The organic phase was washed with brine, dried on MgSO4, filtered,
and evaporated. The crude product was purified by silica gel
chromatography (cyclohexane/EtOAc 9:1), affording 20d as a beige
solid. Yield 78%. mp < 50 °C. IR (KBr): ν (cm−1) 3033, 2956, 2886,
1
1003, 830, 699. H NMR (400 MHz, CDCl3): δ 1.87 (1H, t, J =
5.1 Hz, OH), 4.62 (2H, d, J = 5.1 Hz, CH2OH), 5.37 (2H, s, OCH2),
6.81 (1H, d, J = 8.8 Hz, H-5), 7.30−7.46 (5H, m, H-Ph), 7.63 (1H, dd,
J = 8.8 and 2.9 Hz, H-4), 8.12 (1H, d, J = 2.9 Hz, H-2). 13C NMR (100
MHz, CDCl3): δ 163.4, 145.7, 138.5, 137.1, 129.1, 128.4, 127.9, 127.8,
111.3, 67.7, 62.5. LC−MS (ESI): tR = 5.63 min; [M + H]+ 217,13.
Microbiological Assays. Bacterial Strains. S. aureus ATCC 25923
was purchased from the Institut Pasteur (CRBIP, Paris, France). The
sensitive SA-1199 strain and the overproducing NorA mutant SA-
1199B were obtained from G.W. Kaatz (University of Michigan,
USA).64 S. aureus RN4220 pUL5054 possessing the multicopy plasmid
pUL5054 including the msr(A) gene, thus overproducing MsrA, was a
gift from C. Bebear (University of Bordeaux, France).65 All strains were
grown at 37 °C in Mueller−Hinton broth (Bio-Rad, Mitry Mory,
France) or spread on Mueller−Hinton agar plates for counting. Colony
forming unit (CFU) monitoring was carried out by counting colonies
present in 2 × 10 μL of serial log dilutions of bacteria inoculum spotted
on MH agar plates. Plates were examined for growth after overnight
incubation at 37 °C.
1
1686, 1603, 1568, 1491, 1349, 1262, 1222, 991, 841, 744, 696. H
NMR (400 MHz, CDCl3): δ 5.48 (2H, s, OCH2), 6.90 (1H, d, J = 8.8
Hz, H-5), 7.33−7.48 (5H, m, H-Ph), 8.08 (1H, dd, J = 8.8 and 2.4 Hz,
H-4), 8.65 (1H, d, J = 2.4 Hz, H-2), 9.96 (1H, s, CHO). 13C NMR
(100 MHz, CDCl3): δ 189.5, 167.1, 152.8, 137.6, 136.3, 128.6, 128.2,
128.1, 126.8, 112.3, 68.6. LC−MS (ESI): tR = 6.81 min; [M + H]+
214.48.
6-Chloro-5-methylpyridine-3-carboxaldehyde (20e). Follow-
ing the procedure for carboxaldehyde 20d synthesis, 2-chloro-5-iodo-
3-methylpyridine (1.0 g, 3.95 mmol) was reacted with n-BuLi (1.97
mL, 4.93 mmol) and DMF (0.38 mL, 4.93 mmol). The crude product
was purified by silica gel chromatography (cyclohexane/EtOAc 9:1),
affording 20e as a white solid. Yield 66%. mp 59 °C. IR (KBr): ν
(cm−1) 3034, 2965, 2879, 1685, 1589, 1388, 1067, 732. 1H NMR (400
MHz, CDCl3): δ 2.49 (3H, s, CH3), 8.03 (1H, d, J = 2.2 Hz, H-4),
8.70 (1H, d, J = 2.2 Hz, H-2), 10.08 (1H, s, CHO). 13C NMR (125
MHz, DMSO-d6): δ 191.5, 155.4, 138.9, 133.3, 130.9, 19.0. LC−MS
(ESI): tR = 5.16 min; [M + H]+ 156.39, 158.40.
Media, Antibiotics, and Culture Conditions. Mueller−Hinton
broth (MH, Bio Rad) was used for all bacteria overnight cultures and
susceptibility testing. Ciprofloxacin was obtained from Sigma-Aldrich
(Saint Quentin Fallavier, France). Reserpine was purchased from Alfa
Aesar (Schiltigheim, France). Stock solutions (ciprofloxacin) were
prepared in sterile water except for reserpine, which was dissolved in
dimethyl sulfoxide (DMSO).
6-Benzyloxy-3-hydroxypyridine (21d). To a stirred solution of
6-benzyloxypyridine-3-boronic acid 5f (0.3 g, 1.31 mmol) in dichloro-
methane (20 mL) was slowly added a solution of 35% hydrogen
peroxide (0.34 mL, 3.93 mmol). The reaction was continued at room
2544
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548

Journal of Medicinal Chemistry
Article
a
Table 6. Evaluation of Antibacterial Activity of Carbonylated and Hydroxylated Analogues
a
N.D., no data.
Figure 2. SAR studies for pyridine and benzene boronic species.
MIC Determination. The inhibitory potential of the boronic
derivatives was tested through the determination of the minimum
inhibitory concentration (MIC) of these complexes against S. aureus
ATCC25923 and S. aureus 1199B overexpressing the efflux pump. The
bacterial inoculum was prepared from a frozen culture incubated in
5 mL of MH broth medium at 37 °C for 18 h. Cells were inoculated
into MH broth and dispensed at 200 μL/well in 96-well microtiter
plates (bacterial concentration in each well = 106 CFU/mL).
Tested compounds, solubilized at 10 mg/mL in DMSO, were then
added using a Biomek 2000 handling robot using 2-fold serial dilutions,
with the highest being 128 mg/L for fully soluble compounds. The
MIC was defined as the concentration that completely inhibited cell
growth during a 24 h incubation at 37 °C. Growth was assayed with a
microplate reader by monitoring absorption at 620 nm. In addition, the
plates were read visually. All experiments were performed in duplicate.
The highest final DMSO concentration used (2.56% v/v) induced
no significant bacterial growth inhibition. Controls were performed on
each microplate with antibiotics known to inhibit the strain’s growth:
ampicillin for S. aureus ATCC25923 (0.25 mg/L), ciprofloxacin (MIC
16 mg/L) for S. aureus 1199B (4 and 32 mg/L), and erythromycin
(MIC 128 mg/L) for S. aureus RN4220 pUL5054 (16 and 256 mg/L).
The plates were incubated at 37 °C, and optical densities were read
over a 24 h period. The accepted variance of the MIC value is
estimated to be a 2-fold difference because of the microdilution
method used.
Cytotoxic Assays. KB cells were provided by the NCI and grown
in DMEM supplemented with 10% FCS, penicillin-streptomycin,
fungizone, and glutamine at 37 °C under 5% CO2. Cells were plated in
96-well tissue culture plates in 200 μL of medium and treated 24 h
later with 2 μL stock solution of compounds dissolved in DMSO using
2545
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548

Journal of Medicinal Chemistry
Article
resistance determinants in Escherichia coli. Proc. Natl. Acad. Sci.
U.S.A. 1980, 77, 3974−3977.
Table 7. Evaluation of the Cytotoxicity
a
a
a
Id.
cytotoxicity
Id.
5j
cytotoxicity
Id.
cytotoxicity
(12) Li, X. Z.; Nikaido, H. Efflux-mediated drug resistance in
bacteria: An update. Drugs 2009, 69, 1555−1623.
2c
3d
5a
5b
5d
5e
5f
0
4
1
1
0
0
2
0
0
0
14o
14p
14r
14s
14t
15b
15f
100
100
6
(13) Cattoir, V. Efflux-mediated antibiotics resistance in bacteria.
Pathol. Biol. 2004, 52, 607−616.
(14) Markham, P. N.; Neyfakh, A. A. Efflux-mediated drug resistance
in Gram-positive bacteria. Curr. Opin. Microbiol. 2001, 4, 509−514.
(15) Blair, J. M.; Piddock, L. J. Structure, function and inhibition of
RND efflux pumps in Gram-negative bacteria: An update. Curr. Opin.
Microbiol. 2009, 12, 512−519.
(16) Piddock, L. J. V. Multidrug-resistance efflux pumps − not just
for resistance. Nat. Rev. Microbiol. 2006, 4, 629−636.
(17) Li, X. Z.; Livermore, D. M.; Nikaido, H. Role of efflux pump(s)
in intrinsic resistance of Pseudomonas aeruginosa: Resistance to
tetracycline, chloramphenicol, and norfloxacin. Antimicrob. Agents
Chemother. 1994, 38, 1732−1741.
(18) Mazzariol, A.; Cornaglia, G.; Nikaido, H. Contributions of the
AmpC beta-lactamase and the AcrAB multidrug efflux system in
intrinsic resistance of Escherichia coli K-12 to beta-lactams. Antimicrob.
Agents Chemother. 2000, 44, 1387−1390.
7b
0
7d
0
11a
14d
14f
14g
14i
14n
0
3
10
0
0
0
0
0
5g
5i
6
16b
16g
0
0
0
a
Cytotoxicity is given as the percent inhibition of cellular proliferation
of KB cells.
a Biomek 3000 (Beckman-Coulter). Controls received the same
volume of DMSO (1% final volume). After a 72 h exposure, MTS
reagent (Promega) was added, and plates were incubated for 3 h at
37 °C; the absorbance was monitored at 490 nm, and results are
expressed as the inhibition of cell proliferation calculated as the ratio
(1 − (OD490 treated/OD490 control))100 in triplicate experiments.
For IC50 determination (50% inhibition of cell proliferation), cells
were incubated for 72 h following the same protocol with compound
concentrations ranging from 5 nM to 100 μM in separate duplicate
experiments.
(19) Sato, T.; Yokota, S. I.; Okubo, T.; Ishihara, K.; Ueno, H.;
Muramatsu, Y.; Fujii, N.; Tamura, Y. Contribution of the AcrAB-TolC
eflux pump to high-level Fluoroquinolone resistance in Escherichia coli
isolated from dogs and humans. J. Vet. Med. Sci. 2013, 75, 407−414.
́
(20) Le Thomas, I.; Couetdic, G.; Clermont, O.; Brahimi, I.; Plesiat,
P.; Bingen, E. In vivo selection of a target/efflux double mutant of
Pseudomonas aeruginosa by ciprofloxacin therapy. J. Antimicrob.
Chemother. 2001, 48, 553−555.
AUTHOR INFORMATION
Corresponding Author
*Phone: +33(0)231566804. E-mail: anne-sophie.voisin@
unicaen.fr.
■
(21) Webber, M. A.; Piddock, L. J. V. The importance of efflux
pumps in bacterial antibiotic resistance. J. Antimicrob. Chemother. 2003,
51, 9−11.
(22) Jabes, D. The antibiotic R & D pipeline: An update. Curr. Opin.
Microbiol. 2011, 14, 564−569.
Notes
The authors declare no competing financial interest.
(23) Hamad, B. The antibiotics market. Nat. Rev. Drug Discovery
2010, 9, 675−676.
ABBREVIATIONS USED
■
(24) Sader, H. S.; Flamm, R. K.; Jones, R. N. Antimicrobial activity of
ceftaroline tested against Staphylococci with reduced susceptibility to
linezolid, daptomycin, or vancomycin from U.S. hospitals, 2008 to
2011. Antimicrob. Agents Chemother. 2013, 57, 3178−3181.
(25) Fernebro, J. Fighting bacterial infections − future treatment
options. Drug Resist. Updates 2011, 14, 125−139.
EPI, efflux pump inhibitor; MMC, minimum modulatory
concentration; MFS, major facilitator superfamily; ABC, ATP
binding cassette; n-BuLi, n-butyllithium
REFERENCES
■
(26) Lomovskaya, O.; Bostian, K. A. Practical applications and
feasibility of efflux pump inhibitors in the clinic − a vision for applied
use. Biochem. Pharmacol. 2006, 71, 910−918.
(27) Shlaes, D. M. New β-lactam−β-lactamase inhibitor combina-
tions in clinical development. Ann. N.Y. Acad. Sci. 2013, 1277, 105−
114.
(1) Levy, S. B.; Marshall, B. Antibacterial resistance worldwide:
Causes, challenges and responses. Nat. Med. 2004, 10, S122−S129.
(2) Prabhavathi, F. Antibacterial discovery and development − the
failure of success? Nat. Biotechnol. 2006, 24, 1497−1503.
(3) Alekshun, M. N.; Levy, S. B. Molecular mechanisms of
antibacterial multidrug resistance. Cell 2007, 128, 1037−1050.
(4) Nikaido, H. Multidrug resistance in bacteria. Annu. Rev. Biochem.
2009, 78, 119−146.
(28) Lynch, A. S. Efflux systems in bacterial pathogens: An
opportunity for therapeutic intervention? An industry view. Biochem.
Pharmacol. 2006, 71, 949−956.
(5) Nordmann, P.; Naas, T.; Fortineau, N.; Poirel, L. Superbugs in
the coming new decade; multidrug resistance and prospects for
treatment of Staphylococcus aureus, Enterococcus spp. and Pseudomonas
aeruginosa in 2010. Curr. Opin. Microbiol. 2007, 10, 436−440.
(6) Shon, A. S.; Russo, T. A. Hypervirulent Klebsiella pneumonia: The
next superbug? Future Microbiol. 2012, 7, 669−671.
(7) Carlet, J.; Jarlier, V.; Harbarth, S.; Voss, A.; Goossens, H.; Pittet, D.
̀
Ready for a world without antibiotics? The pensieres antibiotic resistance
call to action. Antimicrob. Resist. Infect. Control 2012, 1, 11−24.
(8) Wright, G. D. Bacterial resistance to antibiotics: Enzymatic
degradation and modification. Adv. Drug Delivery Rev. 2005, 57, 1451−
1470.
(29) Ippolito, G.; Leone, S.; Lauria, F. N.; Nicastri, E.; Wenzel, R. P.
Methicillin-resistant Staphylococcus aureus: The superbug. Int. J. Infect.
Dis. 2010, 14S4, S7−S11.
́
(30) Doleans-Jordheim, A.; Michalet, S.; Bergeron, E.; Boisset, S.;
Souard, F.; Dumontet, C.; Dijoux-Franca, M. G.; Freney, J. Efflux
pumps: Their role in Staphylococcus aureus antibiotic resistance. Ann.
Biol. Clin. 2008, 66, 499−508.
(31) Kosmidis, C.; Schindler, B. D.; Jacinto, P. L.; Patel, D.; Bains, K.;
Seo, S. M.; Kaatz, G. W. Expression of multidrug resistance efflux
pump genes in clinical and environmental isolates of Staphylococcus
aureus. Int. J. Antimicrob. Agents 2012, 40, 204−209.
(32) Yoshida, H.; Bogaki, M.; Nakamura, S.; Ubukata, K.; Konno, M.
Nucleotide sequence and characterization of the Staphylococcus aureus
norA gene, which confers resistance to quinolones. J. Bacteriol. 1990,
172, 6942−6949.
(33) Neyfakh, A. A.; Borsch, C. M.; Kaatz, G. W. Fluoroquinolone
resistance protein NorA of Staphylococcus aureus is a multidrug efflux
transporter. Antimicrob. Agents Chemother. 1993, 37, 128−129.
(9) Lambert, P. A. Bacterial resistance to antibiotics: Modified target
sites. Adv. Drug Delivery Rev. 2005, 57, 1471−1485.
(10) Kumar, A.; Schweizer, H. P. Bacterial resistance to antibiotics:
Active efflux and reduced uptake. Adv. Drug Delivery Rev. 2005, 57,
1486−1513.
(11) McMurry, L.; Petrucci, R. E., Jr.; Levy, S. B. Active efflux of
tetracycline encoded by four genetically different tetracycline
2546
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548

Journal of Medicinal Chemistry
Article
(34) Handzlik, J.; Matys, A.; Kiec-
in multi-drug resistance (MDR) efflux pump inhibitors of Gram-
́
Kononowicz, K. Recent advances
halopyridinylboronic acids and esters. Part 4: Halopyridin-2-yl boronic
acids and esters are stable, crystalline partners for classical Suzuki
cross-coupling. Tetrahedron 2003, 59, 10043−10049. (e) Primas, N.;
Bouillon, A.; Rault, S. Recent progress in the synthesis of five-
membered heterocycle boronic acids and esters. Tetrahedron 2010, 66,
8121−8136.
(48) Dembitsky, V. M.; Al Quntar, A. A.; Srebnik, M. Natural and
synthetic small boron-containing molecules as potential inhibitors of
bacterial and fungal quorum sensing. Chem. Rev. 2011, 111, 209−237.
(49) Berkers, C. R.; Leestemaker, Y.; Schuurman, K. G.; Ruggeri, B.;
Jones-Bolin, S.; Williams, M.; Ovaa, H. Probing the specificity and
activity profiles of the proteasome inhibitors bortezomib and
delanzomib. Mol. Pharmaceutics 2012, 9, 1126−1135.
positive bacteria S. aureus. Antibiotics 2013, 2, 28−45.
(35) Schmitz, F. J.; Fluit, A. C.; Luckefahr, M.; Engler, B.; Hofmann,
̈
B.; Verhoef, J.; Heinz, H. P.; Hadding, U.; Jones, M. E. The effect of
reserpine, an inhibitor of multidrug efflux pumps, on the in vitro
activities of ciprofloxacin, sparfloxacin and moxifloxacin against clinical
isolates of Staphylococcus aureus. J. Antimicrob. Chemother. 1998, 42,
807−810.
(36) Guz, N. R.; Stermitz, F. R.; Johnson, J. B.; Beeson, T. D.; Willen,
S.; Hsiang, J. F.; Lewis, K. Flavonolignan and flavones inhibitors of
Staphylococcus aureus multidrug resistance pump: Structure−activity
relationships. J. Med. Chem. 2001, 44, 261−268.
(37) Chan, B. C.; Ip, M.; Lau, C. B.; Lui, S. L.; Jolivalt, C.; Ganem-
Elbaz, C.; Litaudon, M.; Reiner, N. E.; Gong, H.; See, R. H.; Fung, K.
P.; Leung, P. C. Synergistic effects of baicalein with ciprofloxacin
against NorA over-expressed methicillin-resistant Staphylococcus aureus
(MRSA) and inhibition of MRSA pyruvate kinase. J. Ethnopharmacol.
2011, 137, 767−773.
(38) Michalet, S.; Cartier, G.; David, B.; Mariotte, A. M.; Dijoux-
Franca, M. G.; Kaatz, G. W.; Stavri, M.; Gibbons, S. N-
Caffeoylphenalkylamide derivatives as bacterial efflux pump inhibitors.
Bioorg. Med. Chem. Lett. 2007, 17, 1755−1758.
(39) Kumar, A.; Khan, I. A.; Koul, S.; Koul, J. L.; Taneja, S. C.; Ali, I.;
Ali, F.; Sharma, S.; Mirza, Z. M.; Kumar, M.; Sangwan, P. L.; Gupta, P.;
Thota, N.; Qazi, G. N. Novel structural analogues of piperine as
inhibitors of the NorA efflux pump of Staphylococcus aureus. J.
Antimicrob. Chemother. 2008, 61, 1270−1276.
(40) Pereda-Miranda, R.; Kaatz, G. W.; Gibbons, S. Polyacylated
oligosaccharides from medicinal Mexican Morning Glory species as
antibacterials and inhibitors of multidrug resistance in Staphylococcus
aureus. J. Nat. Prod. 2006, 69, 406−409.
(41) Samosorn, S.; Bremner, J. B.; Ball, A.; Lewis, K. Synthesis of
functionalized 2-aryl-5-nitro-1H-indoles and their activity as bacterial
NorA efflux pump inhibitors. Bioorg. Med. Chem. 2006, 14, 857−865.
(42) Kaatz, G. W.; Moudgal, V. V.; Seo, S. M.; Kristiansen, J. E.
Phenothiazines and thioxanthenes inhibit multidrug efflux pump
activity in Staphylococcus aureus. Antimicrob. Agents Chemother. 2003,
47, 719−726.
(43) Sabatini, S.; Kaatz, G. W.; Rossolini, G. M.; Brandini, D.;
Fravolini, A. From phenothiazine to 3-phenyl-1,4-benzothiazine
derivatives as inhibitors of the Staphylococcus aureus NorA multidrug
efflux pump. J. Med. Chem. 2008, 51, 4321−4330.
(44) Sabatini, S.; Gosetto, F.; Manfroni, G.; Tabarrini, O.; Kaatz, G.
W.; Patel, D.; Cecchetti, V. Evolution from a natural flavones nucleus
to obtain 2-(4-propoxyphenyl)quinoline derivatives as potent
inhibitors of the S. aureus NorA efflux pump. J. Med. Chem. 2011,
54, 5722−5736.
(45) Pieroni, M.; Dimovska, M.; Brincat, J. P.; Sabatini, S.; Carosati,
E.; Massari, S.; Kaatz, G. W.; Fravolini, A. From 6-aminoquinolone
antibacterials to 6-amino-7-thiopyranopyridinylquinolone ethyl esters
as inhibitors of Staphylococcus aureus multidrug efflux pumps. J. Med.
Chem. 2010, 53, 4466−4480.
(50) Smoum, R.; Rubinstein, A.; Dembitsky, V. M.; Srebnik, M.
Boron containing compounds as protease inhibitors. Chem. Rev. 2012,
112, 4156−4220.
(51) Grassberger, M. A.; Turnowsky, F.; Hildebrandt, J. Preparation
and antibacterial activities of new 1,2,3-diazaborine derivatives and
analogues. J. Med. Chem. 1984, 27, 947−953.
́
(52) Hernandez, V.; Crepin, T.; Palencia, A.; Cusack, S.; Akama, T.;
Baker, S. J.; Bu, W.; Feng, L.; Freund, Y. R.; Liu, L.; Meewan, M.;
Mohan, M.; Mao, W.; Rock, F. L.; Sexton, H.; Sheoran, A.; Zhang, Y.;
Zhang, Y. K.; Zhou, Y.; Nieman, J. A.; Anugula, M. R.; Keramane, E.
M.; Savariraj, K.; Reddy, D. S.; Sharma, R.; Subedi, R.; Singh, R.;
O’Leary, A.; Simon, N. L.; De Marsh, P. L.; Mushtaq, S.; Warner, M.;
Livermore, D. M.; Alley, M. R. K.; Plattner, J. J. Discovery of a novel
class of boron-based antibacterials with activity against Gram-negative
bacteria. Antimicrob. Agents Chemother. 2013, 57, 1394−1403.
(53) Xia, Y.; Cao, K.; Zhou, Y.; Alley, M. R. K.; Rock, F.; Mohan, M.;
Meewan, M.; Baker, S. J.; Lux, S.; Ding, C. Z.; Jia, G.; Kully, M.;
Plattner, J. J. Synthesis and SAR of novel benzoxaboroles as a new class
of β-lactamase inhibitors. Bioorg. Med. Chem. Lett. 2011, 21, 2533−
2536.
(54) Jabbour, A.; Steinberg, D.; Dembitsky, V. M.; Moussaieff, A.;
Zaks, B.; Srebnik, M. Synthesis and evaluation of oxazaborolidines for
antibacterial activity against Streptococcus mutans. J. Med. Chem. 2004,
47, 2409−2410.
(55) Benkovic, S. J.; Baker, S. J.; Alley, M. R. K.; Woo, Y. H.; Zhang,
Y. K.; Akama, T.; Mao, W.; Baboval, J.; Ravi Rajagopalan, P. T.; Wall,
M.; Kahng, L. S.; Tavassoli, A.; Shapiro, L. Identification of borinic
esters as inhibitors of bacterial cell growth and bacterial methyl-
transferases CcrM and MenH. J. Med. Chem. 2005, 48, 7468−7476.
(56) Irving, A. M.; Vogels, C. M.; Nikolcheva, L. G.; Edwards, J. P.;
He, X. F.; Hamilton, M. G.; Baerlocher, M. O.; Baerlocher, F. J.;
Decken, A.; Westcott, S. A. Synthesis and antifungal and antibacterial
bioactivity of cyclic diamines containing boronate esters. New J. Chem.
2003, 27, 1419−1424.
(57) Morandi, F.; Caselli, E.; Morandi, S.; Focia, P. J.; Blazquez, J.;
Shoichet, B. K.; Prati, F. Nanomolar inhibitors of AmpC β-lactamase. J.
Am. Chem. Soc. 2003, 125, 685−695.
(58) Livermore, D. M.; Mushtaq, S. Activity of biapenem (RPX2003)
combined with the boronate β-lactamase inhibitor RPX7009 against
carbapenem-resistant Enterobacteriaceae. J. Antimicrob. Chemother.
2013, 68, 1825−1831.
(59) Paulsen, I. T.; Brown, M. H.; Skurray, R. A. Proton-dependent
multidrug efflux systems. Microbiol. Rev. 1996, 60, 575−608.
(60) Lage, H. ABC-transporters: Implications on drug resistance
from microorganisms to human cancers. Int. J. Antimicrob. Agents 2003,
22, 188−199.
(61) Curley, K.; Pratt, R. F. Effectiveness of tetrahedral adducts as
transition-state analogs and inhibitors of the class C β-lactamase of
Enterobacter cloacae P99. J. Am. Chem. Soc. 1997, 119, 1529−1538.
(62) Negrete-Raymond, A. C.; Weder, B.; Wackett, L. P. Catabolism
of arylboronic acids by Arthrobacter nicotinovorans strain PBA. Appl.
Environ. Microbiol. 2003, 69, 4263−4267.
(46) Sabatini, S.; Gosetto, F.; Serritella, S.; Manfroni, G.; Tabarrini,
O.; Iraci, N.; Brincat, J. P.; Carosati, E.; Villarini, M.; Kaatz, G. W.;
Cecchetti, V. Pyrazolo[4,3-c][1,2]benzothiazines 5,5-dioxide: A
promising new class of Staphylococcus aureus NorA efflux pump
inhibitors. J. Med. Chem. 2012, 55, 3568−3572.
(47) (a) Bouillon, A.; Lancelot, J. C.; Collot, V.; Bovy, P. R.; Rault, S.
Synthesis of halopyridinylboronic acids an esters. Part 1: 6-
Halopyridin-3-yl boronic acids and esters. Tetrahedron 2002, 58,
2885−2890. (b) Bouillon, A.; Lancelot, J. C.; Collot, V.; Bovy, P. R.;
Rault, S. Synthesis of halopyridinylboronic acids an esters. Part 2: 2, 4
or 5-Halopyridin-3-yl boronic acids and esters. Tetrahedron 2002, 58,
3323−3328. (c) Bouillon, A.; Lancelot, J. C.; Collot, V.; Bovy, P. R.;
Rault, S. Synthesis of halopyridinylboronic acids and esters. Part 3: 2,
or 3-Halopyridin-4-yl boronic acids and esters. Tetrahedron 2002, 58,
4369−4373. (d) Bouillon, A.; Lancelot, J. C.; Sopkova de Oliveira
Santos, J.; Collot, V.; Bovy, P. R.; Rault, S. Synthesis of
(63) (a) Voisin, A. S.; Bouillon, A.; Lancelot, J. C.; Rault, S. Efficient
synthesis of halohydroxypyridines by hydroxydeboronation. Tetrahe-
dron 2005, 61, 1417−1421. (b) Matondo, H.; Ouhaja, N.; Souirti, S.;
Baboulene, M. Oxidation and Suzuki cross-coupling reaction from
2547
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548

Journal of Medicinal Chemistry
Article
azaheteroarylboronic acids. Main Group Met. Chem. 2002, 25, 163−
167.
(64) Kaatz, G. W.; Seo, S. M.; Ruble, C. A. Efflux-mediated
fluoroquinolone resistance in Staphylococcus aureus. Antimicrob. Agents
Chemother. 1993, 37, 1086−1094.
(65) Ross, J. I.; Eady, E. A.; Cove, J. H.; Cunliffe, W. J.; Baumberg, S.;
Wootton, J. C. Inducible erythromycin resistance in staphlyococci is
encoded by a member of the ATP-binding transport super-gene family.
Mol. Microbiol. 1990, 4, 1207−1214.
2548
dx.doi.org/10.1021/jm401808n | J. Med. Chem. 2014, 57, 2536−2548