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Resistance to current antibiotics is rapidly
increasing. In its 2014 report of global
antimicrobial resistance, the World Health
Organization (WHO) portrayed high levels
of antibiotic resistance in the bacteria that
cause common infections. A number of
leading authorities have issued passionate
statements urging action, including the
Director-General of the WHO, Margaret
Chan; the Director of the Wellcome Trust,
Jeremy Farrar; and the Director of the US
Center for Disease Control (CDC), Tom
Frieden. The United Kingdom’s Chief
Medical Officer, Sally Davies, warned that
the country could find itself back in the
nineteenth century in terms of its ability to
treat bacterial infections. The seriousness of
the threat has been compared with those of
global warming and terrorism (see Further
information).
The so-called ESKAPE pathogens
(Enterococcus faecium, Staphylococcus
aureus, Klebsiella pneumoniae, Acinetobacter
baumannii, Pseudomonas aeruginosa, and
Enterobacter spp.) are especially important
owing to their role in many infections in
human organs (such as the lung and urinary
tract), the frequency of antibiotic resistance
amongst them and the lack of alternative
antibiotics1. Several of these pathogens
are Gram-negative bacteria, which are of
particular concern as in these organisms
resistance of up to 50% against carbapenems,
the current last line of defence, has been
reported in some developing countries1.
A few new antibiotics against Gram-positive
bacteria have become available in recent
years, but no totally new class of antibiotic
has been introduced for the treatment of
Gram-negative infections for more than
40years.
In South Asia, the Middle East and
the Mediterranean, modern medicine is
already under threat from these multidrug
resistant (MDR) Gram-negative bacteria2
(K.pneumoniae, A.baumannii, P.aeruginosa,
and Enterobacter spp.). European data span-
ning 2005–2010 indicate growing resistance
to cephalosporins, fluoroquinolones and
aminoglycosides, and a 30% mortality rate
for patients with septicaemia due to MDR
Escherichiacoli3. Data from the USA show
a similar pattern. The 2013 report from the
CDC highlighted carbapenem-resistant
Enterobacteriaceae (CREs) as an urgent
threat4. In Asia, substantial resistance has
emerged in both India and China, with
resistance levels reported in the range of
50–80%. This has caused increased use
of carbapenems, which were previously
reserved for extreme cases of infection in
the very sick, the immune-compromized or
as a last resort. Now, bacteria have adapted
and selected for carbapenem-destroying
enzymes, known as carbapenemases, and
few antibiotics remain effective against
these CREs. K.pneumoniae, E.coli,
P.aeruginosa and A.baumannii produce
metallo-β-lactamases such as K.pneumo-
niae carbapenemase (KPC) and New Delhi
metallo-β-lactamase (NDM) — enzymes
that degrade numerous antibiotics contain-
ing a β-lactam ring, such as penicillins,
cephalosporins and carbapenems. Bacteria
carrying the genes that encode these
enzymes are becoming resistant to all
available penicillins, cephalosporins and
β-lactamase inhibitors, including clavulanic
acid and avibactam (FIG.1). These bacteria
are also resistant to virtually all other anti-
biotics, with the exception of colistin, an old
(and somewhat toxic) polymixin class anti-
biotic, although even colistin resistance has
now emerged in South Asia. Both KPC and
NDM, as well as Verona integron-encoded
metallo-β-lactamase (VIM), have been
reported in Pseudomonas spp. In some areas
of the world, including the United States,
Israel, Italy, Greece and China, the emer-
gence of bacteria that produce KPCs, which
render them resistant to carbapenems, is
becoming a seriousthreat.
A few derivatives of older antibiotic
classes or combinations incorporating new
β-lactamase inhibitors such as avibactam
and tazobactam offer some hope in the short
term. Two compounds that have reached
PhaseIII trials — eravacycline5, a next-
generation tetracycline, and plazomicin6,
a next-generation aminoglycoside —
have activity against Gram-negative organ-
isms. The recently approved ceftazidime–
avibactam7 combination is effective against
OPINION
Antibiotic resistance breakers: can
repurposed drugs fill the antibiotic
discovery void?
David Brown
Abstract | Concern over antibiotic resistance is growing, and new classes of
antibiotics, particularly against Gram-negative bacteria, are needed. However,
even if the scientific hurdles can be overcome, it could take decades for sufficient
numbers of such antibiotics to become available. As an interim solution,
antibiotic resistance could be ‘broken’ by co-administering appropriate
non-antibiotic drugs with failing antibiotics. Several marketed drugs that do not
currently have antibacterial indications can either directly kill bacteria, reduce
the antibiotic minimum inhibitory concentration when used in combination with
existing antibiotics and/or modulate host defence through effects on host innate
immunity, in particular by altering inflammation and autophagy. This article
discusses how such ‘antibiotic resistance breakers’ could contribute to reducing
the antibiotic resistance problem, and analyses a priority list of candidates for
further investigation.
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Nature Reviews Drug Discovery
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AOP, published online 23 October 2015; doi:10.1038/nrd4675
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Nature Reviews | Drug Discovery
Folate synthesis
• Sulfonamides
• Trimethoprim
Nucleic acid
synthesis
• DNA gyrase
• Quinolones
Cell membrane
disrupters
• Polymyxins
RNA polymerase
• Rifamycins
Protein synthesis
50S subunit
• Macrolides
• Oxazolidinones
• Chloramphenicol
30S subunit
• Aminoglycosides
• Tetracyclines
Cell wall synthesis
• β-lactams
• Penicillins
• Cephalosporins
• Carbapenems
• Monobactams
• Glycopeptides
LPS and/or
TLR4 blockers
mRNA
50S
30S Peptide PABA DHF THF
several MDR Gram-negative bacteria;
the ceftolozane–tazobactam8 combination
(also recently approved) has good
anti-pseudomonal activity; and the
aztreonam–avibactam combination7, which
is in PhaseIII trials, works against many
organisms that produce mannose-binding
lectin. However, resistance to these combina-
tions will probably soon arise and there
are no totally new chemical classes of anti-
biotic on the horizon for the treatment of
Gram-negative bacteria.
It is therefore crucial that ways of
breaking resistance to current antibiotics are
found as soon as possible. One strategy to
achieve this goal is to co-administer another
drug with the failing antibiotic, which
restores sufficient antibacterial activity. The
use of such antibiotic resistance breakers
(ARBs) to salvage antibiotics is exemplified
by the long-standing, successful and wide-
spread co-administration of β-lactamase
inhibitors, such as clavulanic acid, with
β-lactam antibiotics, such as amoxicillin9,10.
Resistance to amoxicillin and to this com-
bination of drugs has been slow to emerge.
However, mutation of the β-lactamase TEM1
— thought to be the greatest driver of resist-
ance to this class of antibiotics — has now
occurred owing to the selection of organisms
with clavulanate-insensitive β-lactamases.
As noted above, several new β-lactamase
inhibitors offer the hope of counteracting
resistance to β-lactam antibiotics in the near
term, but further exploitation of β-lactamase
inhibitors may be of limited use in the longer
term, as there has been a 100-fold increase
in the number of known β-lactamases in the
past 40years11.
Surprisingly, the success of β-lactamase
inhibitors has not led to substantial clinical
and commercial exploitation of the concept
of ARBs beyond this class. Attempts to
reduce resistance by blocking efflux pumps
on bacterial cells — which can diminish the
effectiveness of antibiotics by lowering their
intracellular concentration — have been
pursued for many years, so far without nota-
ble success. However, efforts continue and
deserve further attention12. Novel combina-
tions of existing classes of antibiotics could
also be investigated; for example, macrolides
may be able to synergize with β-lactams and
fluoroquinolones13–18.
This article, however, focuses on the
identification of broad-spectrum ARBs by
repurposing marketed drugs and nutra-
ceuticals. ARBs selected from marketed
drugs would be particularly useful as their
development could be faster, cheaper and
probably have a higher success rate than that
for new molecules. This could be crucial,
given the pressing need for strategies to
tackle antibiotic resistance, the long develop-
ment timelines for new antibiotics and the
challenging financial environment for new
antibiotic research and development. One
ARB could potentially revitalize several anti-
biotics in a class11, and some ARBs may even
work across classes. Lethal bacterial infec-
tions might be effectively treated with far
fewer compounds than would be required
to replace existing antibiotics. Moreover, the
concept may help to extend the lifespan of
future antibiotic classes. Here, after high-
lighting the priority bacteria, key antibiotics
to be salvaged and the properties of ARBs,
we discuss a list of proposed priority candi-
dates for further investigation and issues for
their development.
Repurposing to provide ARBs
Priority antibiotics and bacteria. ARBs
should be sought to salvage one or more
key members of each mechanistic antibiotic
class, particularly those used against Gram-
negative bacteria. Thus, the antibiotics
that most need ARBs are: cephalosporins
and carbapenems (which disrupt cell wall
synthesis); polymyxins (which disrupt cell
membrane synthesis); fluoroquinolones
(which disrupt DNA synthesis); tetracyclines
and aminoglycosides (which disrupt protein
synthesis by inhibiting the 30S ribosomal
subunit); and macrolides (which disrupt
protein synthesis by inhibiting the 50S
ribosomal subunit).
Acquired carbapenemases have been
highlighted as the greatest immediate threat
to the effectiveness of the antibiotic arsenal2.
Carbapenemases are encoded by genes that
are transferable between bacteria and confer
resistance to many of the most heavily used
antibiotics — carbapenems and β-lactam
antibiotics. The carbapenems are the last
good line of defence against MDR Gram-
negative bacteria, and the consensus is that
extending the useful lifespan of this class of
drugs is a top priority. As such, the intrave-
nous formulation of a broad-spectrum ARB
for use together with a carbapenem (or a
cephalosporin) in intensive-care hospital
settings should be top priority.
ARBs that are effective against
K.pneumoniae, E.coli, P.aeruginosa and
A.baumannii, the four Gram-negative
organisms whose resistance to antibiotics
is of greatest concern (all of which produce
carbapenemases and are thus resistant to
many β-lactam antibiotics), are of utmost
importance. A secondary priority is to
target Gram-positive bacteria, especially
methicillin-resistant S.aureus (MRSA) and
Clostridiumdifficile (which causes C.difficile-
associated disease (CDAD)), as these organ-
isms cause recurring problems that are
associated with substantial deathrates.
Figure 1 | Sites of antibacterial action and mechanisms of resistance. Antibiotics can be classi-
fied by their mechanism of action. Resistance to one antibiotic within a class can confer resistance
to others with the same target. Resistance arises by two main mechanisms: random mutations
during DNA replication and transfer of DNA between bacteria, often as plasmids. The transferred
DNA can contain genes that confer resistance, and natural selection then favours the survival of the
resistant bacteria during antibiotic treatment of a patient. DHF, dihydrofolic acid; LPS, lipopoly-
saccharide; PABA, para-aminobenzoic acid; THF, tetrahydrofolic acid; TLR4, Toll-like receptor 4.
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Properties and identification of potential
ARBs. There are several properties that
potential ARBs could possess. First, ARBs
could have direct antibacterial activity, even
if they are not used clinically as antibiotics.
Second, ARBs could increase the efficacy of
antibiotics and/or combat antibiotic resist-
ance mechanisms. Third, ARBs could help
to clear the infection by interacting with host
targets to activate host defence mechanisms;
for example, by blocking the pro-inflamma-
tory Toll-like receptors (TLRs) or promoting
autophagy (BOX1). Arguably the most inter-
esting potential ARBs are those that display
more than one of these properties.
A literature review was conducted
searching for potential non-antibiotic can-
didate drugs or nutraceuticals that are not
used as antibiotics but have one or more of
these three ARB properties. Drug safety
and the ability to achieve a drug plasma
concentration (by intravenous or oral
routes) that is similar to published mini-
mum inhibitory concentrations (MICs) for
antibacterial action are also important, and
combinations are more often successful if
the combination partner attacks a molecu-
lar target that is different from that of the
antibiotic. Therefore, these aspects were also
used for prioritization. Last, the priorities
above were also considered with regard to
the type of infections.
For the drugs that were short-listed, a
written summary was prepared of relevant
mechanisms and invitro and invivo data,
as well as any available clinical data. Then,
through individual and group discussions
with global experts, the strengths and weak-
nesses of each drug were identified. Based
on these discussions, the priority drugs with
the strongest evidence supporting a potential
role in breaking resistance are presented in
TABLES1,2 and grouped into three categories
in the discussion below: potential ARBs for
Gram-negative bacteria, potential ARBs
for Gram-positive bacteria and potential
ARBs for both classes. Some drugs such as
aspirin19,20, diclofenac21–23, ibuprofen24–26,
ivermectin27,28, lauric acid or monolaurin29,30,
metformin31–33, and vitaminD3 (REFS34,35)
were excluded owing to a lack of compel-
ling evidence, although future research
could identify these drugs as potential ARBs
(TABLE3).
Potential ARBs for Gram-negative bacteria
Ciclopirox. Ciclopirox has been used for
several decades as a topical antifungal agent
without the emergence of resistance. It is a
broad-spectrum agent with activity against
most clinically relevant dermatophytes,
yeasts, and moulds. Moreover, it has antibac-
terial activity, although this has never been
exploited clinically. Ciclopirox kills a wide
range of bacteria including many Gram-
negative and Gram-positive species36.
Recently, it was reported that this drug
has direct antibacterial activity against sev-
eral of the high-priority MDR Gram-negative
bacteria37. When tested against antibiotic
resistant A.baumannii, E.coli and K.pneu-
moniae, ciclopirox inhibited bacterial growth
at concentrations of 5–15 μg per ml, regard-
less of the antibiotic resistance status. The
authors suggested that the compound inhib-
ited the synthesis of lipopolysaccharide (LPS)
in the surface coat of Gram-negative bacteria.
This would be a particularly valuable mecha-
nism, as the LPS coat protects Gram-negative
bacteria from the entry of many antibiotics.
Inhibition of LPS synthesis might render
Gram-negative bacteria susceptible to anti-
biotics that are normally reserved for Gram-
positive organisms.
Ciclopirox also chelates intracellular iron,
which probably results in the inhibition of
metal-dependent enzymes. In Candida
albicans, ciclopirox has also been reported
to alter the regulation of the genes encoding
iron permeases or transporters (FTR1, FTR2
and FTH1), a copper permease (CCC2), an
iron reductase (CFL1) and a siderophore
transporter (SIT1)38. Addition of FeCl3 to
ciclopirox-treated cells reversed the effect of
the drug on gene regulation, indicating that
its antifungal activity may be at least partially
caused by iron limitation38.
Other mechanistic studies have indi-
cated that, in addition to the effects on iron,
ciclopirox also downregulates nucleotide
binding proteins39 and inhibits mammalian
target of rapamycin (mTOR) signalling,
thereby inducing autophagy in mammalian
cells40. It seems likely that ciclopirox would
also activate autophagy in immune cells
(BOX1).
The rapid development of ciclopirox
for use against bacterial infections either
alone or as an ARB could be aided by the
existing pharmacokinetic, metabolic, toxi-
cological and clinical data. Ciclopirox is
usually administered topically, however,
owing to the interest in ciclopirox for treat-
ment of haematological malignancies such
as multiple myeloma41,42, systemic dosing
of this drug has been investigated41. Data
from animal studies and a single human
Box 1 | Host-targeted drugs that induce autophagy may break antibiotic resistance
Autophagy eliminates unwanted constituents from cells, including pathogens, damaged
organelles and aggregated proteins. During fasting or starvation, autophagy recycles cytoplasmic
material to maintain cellular homeostasis. Over the past few years, an increased understanding of
the pathways of autophagy has led to recognition of its role in a broad range of disease processes,
including host defence against pathogens. There are several excellent reviews on the role of
autophagy as a defence against microbial invasion125,126.
Bacteria that are degraded by intracellular autophagy include Group A Streptococcus spp.127,
Salmonella spp.128, Shigella spp.129, Listeria monocytogenes130 and Mycobacterium tuberculosis131–133.
However, some bacteria have evolved to subvert this process134–136.
The best characterized protein involved in autophagy is mammalian target of rapamycin
(mTOR). Autophagy is induced by direct inhibitors of mTOR or by inhibitors of pathways that
activate mTOR — classI phosphoinositide 3‑kinases (PI3Ks) and receptor tyrosine kinases that
activate the AKT pathway — thereby repressing autophagy. Inhibitors of these enzymes may
provide useful therapeutics by inducing autophagy, although none is marketed at the present
time. Similarly, modulators of 5′‑AMP‑activated protein kinase (AMPK)137, mitogen‑activated
protein kinases (MAPKs; including extracellular signal‑regulated kinases (ERKs)) and the WNT
signalling pathway138 may be useful to increase autophagy and promote bacterial clearance.
Drugs that inhibit some of these pathways are in development for the treatment of cancer and
might prove effective in treating some infectious diseases.
Particularly intriguing is the recent observation that activation of autophagy specifically in the
gut leads to systemic effects139. Remarkably, activation of intestinal AMPK induces autophagy in
both the gut and the brain and slows systemic ageing. Activation of autophagy in the gut alone
could therefore be sufficient to aid the clearance of systemic infections. This possibility is very
relevant in assessing the potential of the AMPK activators, some of which have relatively low
bioavailability.
It is currently unclear whether activators of autophagy would be sufficiently powerful to use
without antibiotics, but they could be used as ARBs when co‑administered with antibiotics,
similarly to β‑lactamase inhibitors. There are already autophagy‑activating drugs in clinical use or
under clinical investigation for other diseases. If modulation of autophagy does emerge as a useful
drug approach for the treatment or prevention of bacterial diseases, it is possible that useful
medicines could be repurposed from other indications.
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Table 1 | Priority compounds for repurposing as ARBs
Compound Class Structure Potential mechanisms of action
For Gram-negative bacteria
Ciclopirox Antifungal: used without
the development of
resistance for several
decades N O
OH
Ciclopirox inhibits the synthesis of the LPS coat
of Gram-negative bacteria37, chelates iron and
regulates the genes encoding iron permeases
or transporters (FTR1, FTR2 and FTH1), copper
permease (CCC2), iron reductase (CFL1) and
siderophore transporter (SIT1). It may also
contribute to antimicrobial effects38, and it can
induce autophagy40
Loperamide
(Imodium; Janssen
Pharmaceutica)
Anti-motility: used for the
treatment of diarrhoeal
diseases
NO
N
OH
Cl
Loperamide facilitates tetracycline uptake45.
With cephalosporins, loperamide dissipated
the electrical component (ΔΨ) of the proton
motive force (PMF). In this same assay,
cephalosporins selectively dissipated the
transmembrane chemical component (ΔpH) of
the PMF. The elimination of both ΔΨ and ΔpH
completely abolishes PMF and explains the
observed synergy between loperamide and
cephalosporins45
For Gram-positive bacteria
Berberine A traditional medicine
in Europe, Asia and the
Americas. It is used for the
treatment of diarrhoea
caused by Giardia lamblia
and the Gram-negative
bacteria Escherichiacoli,
Klebsiella spp. and Vibrio
cholerae
N+
O
O
O
O
Direct antibacterial action may be due, in
part at least, to inhibition of Gram-positive
bacterial sortase49. It binds to TLR4–MD2,
thereby antagonizing LPS signalling51.
Berberine inhibits TLR4–NF-κB–MIP2
signalling, thus decreasing neutrophil
infiltration53, downregulates the expression
of pro-inflammatory genes (such as those
encoding TNF, IL-1β, IL-6, MCP1, iNOS and
COX2 (REF.54)) and activates AMPK, thus
inducing autophagy55
For both Gram-negative and Gram-positive bacteria
Curcumin A food flavouring, colouring
and neutraceutical
O O
OH
OO
HO
Curcumin competes with the LPS of
Gram-negative bacteria to block excessive
inflammatory responses and prevent
bacterial invasion. It inhibits TLR2 and
TLR4 signalling72,73, downregulates TLR
expression74,75, prevents the upregulation of
IL-8 expression79 and induces autophagy by
inhibiting the AKT–mTOR pathway71
Epigallocatechin-
3-gallate (EGCG)
EGCG is one of the most
abundant polyphenols in
green tea and is thought to
be responsible for most of
the supposed therapeutic
benefits of green tea
consumption
OHO
OH
O
OH
OH
OH
O
OH
OH
OH
EGCG has a broad range of mechanisms,
including inhibition of DNA gyrase103, blockade
of TLR4 binding106 and signal transmission, and
inhibition of conjugative transfer of the
R plasmid of E.coli107
(+)-Naltrexone
and (+)-Naloxone
These are selective opioid
antagonists used to counter
the effects of opioid
overdose
(+)-Naloxone (+)-Naltrexone
O
OH
HO
O
N
O
OH
HO
O
N
Both compounds block TLR4–MD2 signalling119
AMPK, 5ʹ-AMP-activated protein kinase; ARBs, antibiotic resistance breakers; COX2, cyclooxygenase 2; IL-1β, interleukin-1β; iNOS, inducible nitric oxide
synthase; LPS, lipopolysaccharide; MCP1, monocyte chemotactic protein 1; MIP2, macrophage inflammatory protein 2; mTOR, mammalian target of rapamycin;
NF-κB, nuclear factor κB; TLR4, Toll-like receptor 4; TNF, tumour necrosis factor.
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oral dosing study suggest that drug safety is
satisfactory for human trials of ciclopirox
for the treatment of cancers41. On dosing
radiolabelled ciclopirox to humans, 96%
of the label was recovered from urine;
however, a specific plasma or serum assay
for ciclopirox olamine was not used in
this study43. The authors stated that the
pharmacological drug levels of ciclopirox
required for anti-tumour activity are achiev-
able even though its half-life is short, but
require dosing several times a day. They
noted, however, that the drug seemed to be
well absorbed after oral administration. As a
result of this analysis, this group sponsored
a PhaseI study44, which demonstrated that
oral ciclopirox olamine achieved plasma lev-
els of 50 ng per ml after a single oral dose of
20–40 mg per m2 and had biological activity
in patients with advanced haematological
malignancies. Dose-limiting gastrointestinal
toxicities were observed in patients receiv-
ing the highest oral dose administered four
times a day, but not at lower doses or at a
less frequent dosing schedule. Dosing regi-
mens against bacteria are likely to be shorter
than those for the treatment of cancer, and
gastrointestinal toxicity is less likely if the
drug is administered intravenously as an
ARB against life-threatening bacteria in a
hospital setting, where the need for ARBs is
greatest.
Ciclopirox has at least two of the three
properties suggested for an ARB as it is
directly antibacterial and it induces the host
defence response by causing autophagy.
However, data are required on whether or
not this drug is effective in combination with
antibiotics. Moreover, its activity against a
broad range of bacteria needs investigation.
The appropriate clinical dose can then be
calculated, and safety studies will be required
to assess the therapeutic index, particularly
for intravenous dosing. Concomitant activity
against C.albicans is a bonus, because
Candida spp. are a leading cause of catheter-
associated infections, which have high
mortalityrates.
Loperamide. This μ-opioid receptor agonist
has long been used as an anti-motility agent
in the treatment of diarrhoeal diseases.
Ejim etal.45 recently showed that lopera-
mide, which has no antibacterial activity
perse, acts synergistically with several
classes of antibiotic. They screened off-
patent non-antimicrobial drugs as a source
of molecules that might synergize with
antibiotics at sub-MIC concentrations.
Loperamide, at a concentration of 16 μg per
ml or greater, increased the antibacterial
efficacy of eight tetracycline antibiotics
against Gram-negative pathogens (but not
Gram-positive pathogens) and was active
in combination with the broad-spectrum
tetracycline-class antibiotic minocycline in
a mouse model of salmonellosis. In addition
to tetracyclines, loperamide increased the
efficacy of cephalosporins (but not other
cell-wall-directed antibiotics) and the outer-
membrane-permeating antibiotic polymyxin
B invitro. The authors concluded that it is
improbable that the synergy observed invivo
was the result of the antiperistaltic activity
of loperamide, as the potentiation was
observed at concentrations of minocycline
that do not impair bacterial growth even
upon prolonged exposure.
Loperamide has also been shown to
sensitize Gram-negative bacteria to ‘Gram-
positive antibiotics’ (REF.46). In the presence
of loperamide, the aminocoumarin anti-
biotic novobiocin inhibited the growth of
E.coli. Loperamide may alter the cell shape
and small-molecule permeability of E.coli,
similar to the mechanism through which
colistin boosts the effectiveness of vancomy-
cin and rifampin47. The authors suggested
that the altered cell shape may cause dysreg-
ulation of the influx and efflux machinery of
Gram-negative bacteria and thereby enable
the accumulation of otherwise-excluded
antibiotics. This concept could be further
exploited by screening current drugs and
nutraceuticals for similar effects against a
wide range of Gram-positive antibiotics.
One concern is that repurposing Gram-
positive antibiotics for Gram-negative
pathogens could promote resistance to these
agents by transfer of resistance determi-
nants, but in view of the greater threat of
Gram-negative organisms this may be a
risk worthtaking.
Loperamide is not orally bioavailable,
but it could be used orally as an ARB to treat
gut infections, such as diarrhoeal diseases,
and intravenously for other infections if it is
proven to be safe. Although loperamide is
an opioid, it has no opiate-like effects when
administered orally or intravenously. It does
not cross the blood–brain barrier because it
is subject to efflux by P-glycoprotein; there-
fore, intravenous loperamide could not be
used with inhibitors of P-glycoprotein.
Potential ARBs for Gram-positive bacteria
Berberine. The plant-derived isoquinoline
alkaloid berberine has a long history of use
for the treatment of several conditions. In
particular, it has been used in traditional
medicine in Europe, Asia and the Americas
to treat diarrhoea caused by Giardia lamblia
as well as the Gram-negative bacteria E.coli,
Klebsiella spp. and Vibrio cholerae. It has
broad-spectrum direct antibacterial activity
against staphylococcal, streptococcal
and enterococcal species, including MDR
Table 2 | ARBs for each main antibiotic class
Drug (target) ARBs for Gram-negative
bacteria
ARBs for Gram-positive
bacteria
Carbapenems,
cephalosporins and
penicillins (cell wall
synthesis)
• Ciclopirox
• Loperamide (intravenous)
• Macrolides
• EGCG
• Naloxone, naltrexone and
curcumin (for gut pathogens and
LPS-driven endotoxic shock)
• Curcumin
• EGCG
• Berberine
Polymyxins (cell membrane) Loperamide NA
Aminoglycosides (protein
synthesis)
None identified NA
Fluoroquinolones (nucleic
acid synthesis)
None identified Curcumin
Tetracyclines (protein
synthesis)
Loperamide Curcumin
Glycopeptides (cell wall
synthesis)
NA Naloxone, naltrexone and
curcumin (with vancomycin
or metronidazole for the
treatment of CDAD)
Macrolides (protein
synthesis)
NA None identified
‘Gram-positive antibiotics’ Loperamide NA
ARBs, antibiotic resistance breakers; CDAD, Clostridium difficile-associated diarrhoea; EGCG, epigallocatechin-3
gallate; LPS, lipopolysaccharide; NA, not applicable.
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strains of Mycobacteriumtuberculosis and
MRSA. In vitro, berberine is about ten-
times more potent against Gram-positive
bacteria than Gram-negative. Its ability to
kill Streptococcus pneumoniae and S.aureus
may be particularly relevant as these are
among the most common bacterial causes
of pneumonia.
When tested alone invitro against
clinical isolates of MRSA48, berberine
showed moderate activity against all strains
tested, with MICs of 32–128 μg per ml.
Table 3 | Potential drugs excluded as high priority, based on the current lack of data
Candidate drug
for repurposing
Current approved
indication or
common usage
Summary of evidence Additional information required
Aspirin Pain, inflammation
and anti-platelet
• In vitro: reduces resistance to aminoglycosides in
Klebsiellapneumoniae and enhances the susceptibility
of Helicobacterpylori to antibacterials140; however, it
induces resistance to many other antibiotic classes141.
• Invivo: reduces the incidence of Staphylococcusaureus
in a rabbit model of aortic valve endocarditis142, is
additive or synergistic if combined with vancomycin in a
rabbit model of endocarditis caused by S.aureus143 and
reduced the prevalence of nasal S.aureus19and S.aureus
bacteremia20 in patients receiving haemodialysis
Confirmation of activity alone and in
synergy with antibiotics against antibiotic-
resistant MRSA. A safety assessment is
needed for intravenous dosing
Diclofenac Pain and
inflammation
• In vitro: directly antibacterial against antibiotic-
sensitive and antibiotic-resistant clinical isolates of
S.aureus, Listeria monocytogenes, Escherichiacoli and
Mycobacterium spp., including anti-plasmid activity21,22.
Synergistic in combination with streptomycin against
E.coli and Mycobacterium spp., and with gentamicin
against Listeria spp.144; blocked both cAMP-activated
and Ca2+-activated chloride secretion in intestinal
epithelial cells infected with Vibrio cholerae23
• Invivo: Effective in mice in treating infections of
V.cholerae23, Salmonella spp.145, Listeria spp.144, and
Mycobacterium tuberculosis146
The MIC90 needs to be improved — it
is typically 100 μg per ml, which is two
orders of magnitude above the Cmax
exposure achieved with a 50 mg oral
dose in humans. Direct or synergistic
antibacterial actions against drug-
resistant Gram-negative bacteria should
be investigated at safe doses
Ibuprofen Pain and
inflammation
• Invivo: mice infected with M. tuberculosis that were
treated with ibuprofen lived longer than control
animals24,25. Oleocanthal, the active constituent of
olive oil, affects the same receptor as ibuprofen and
has antibacterial activity26
The in vitro and invivo activity against
the major drug-resistant Gram-negative
bacteria, alone and with antibiotics,
should be explored. A safety assessment
for intravenous dosing is also required
Ivermectin Clinical and
veterinary
nematode infections
• In vitro: inhibits growth of Chlamydia trachomatis in
epithelial cells27
• Invivo: improved survival in mice subjected to ‘endotoxic
shock’ with a lethal dose of LPS28, decreased levels of
inflammatory cytokines28 and activated autophagy147
The effects of ivermectin in combination
with antibiotics should be investigated
against drug-resistant Gram-negative
bacteria. Safety for intravenous dosing
should also be assessed
Lauric acid (active
metabolite is
monolaurin)
Neutraceutical
(coconut oil)
• In vitro: inhibits the synthesis of most staphylococcal
toxins and other exoproteins29. Blocks induction, but
not constitutive synthesis, of β-lactamase30
An exploration of invitro and invivo
activity against the major drug-resistant
Gram-negative bacteria, alone and
with antibiotics, is necessary. A safety
assessment is also required for both oral
and intravenous dosing
Metformin Anti-diabetic • Invivo: enhances phagocytosis by macrophages
in a mouse model of E.coli lung infection148. AMPK
activation with metformin increased the survival
rate in mice challenged with LPS in an endotoxemia
model32, reduced cholera-toxin-mediated increases
in intestinal chloride secretion33 and decreased
disease severity in mice and humans infected with M.
tuberculosis149,150
For intestinal infections, the synergy with
antibiotics against bacteria such as V.
cholerae and E.coli needs to be assessed,
which may be possible at the current
approved dose levels of metformin. For
systemic infections, the plasma levels
required for systemic effects to break
antibiotic resistance should be defined,
and their safety determined. It must be
determined whether AMPK activation in
the gut is sufficient for systemic synergy
with antibiotics139
VitaminD Calcium absorption
and bone health,
and tuberculosis151
• In vitro: inhibits mycobacterial entry and survival
within macrophages through the induction of
autophagy34,152. IL-32 stimulates the immune system
to kill M.tuberculosis, but only in the presence of
sufficient vitaminD3 levels35
Studies on IL‑32 and vitaminD3 should
be extended to other bacteria. The levels
of vitaminD3 required invivo must be
determined and the safety of the levels
required for both oral and intravenous
dosing assessed
AMPK, 5ʹ-AMP-activated protein kinase; cAMP, cyclic AMP; Cmax, maximum concentration; IL-32, interleukin-32; LPS, lipopolysaccharide; MIC90, the minimum
concentration that inhibits 90% of bacterial isolates; MRSA, methicillin-resistant S.aureus.
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Ninety per cent inhibition of MRSA growth
was obtained with a concentration of 64 μg
per ml or less of berberine. These effects
may be due, at least in part, to inhibition of
the Gram-positive bacterial sortase enzyme,
an important anti-virulence target49. In the
Gram-negative E.coli, berberine targets
assembly of the cell division protein FtsZ50.
It inhibits the assembly kinetics of the Z-ring
and perturbs cytokinesis. It also destabilizes
FtsZ protofilaments and inhibits the FtsZ
GTPase activity. Berberine binds to FtsZ
with high affinity (dissociation constant
(KD) = 0.023 μM) and thus halts the first stage
in bacterial cell division.
Invivo, berberine protected mice
challenged with Salmonella typhimurium:
50% of the mice that did not receive berber-
ine treatment died by the end of the eighth
day after infection, whereas, with doses of
10, 20, 30 and 40 mg per kg of berberine, 60,
60, 70 and 90% of the infected mice survived
to the eighth day, respectively51.
Encouragingly, another study reported
that bacteria are poor at generating resist-
ance to berberine52. The MICs of bacterial
cultures (E.coli, S.aureus, Bacillussubtilis,
Proteusvulgaris, S.typhimurium and
P.aeruginosa) did not increase over 200 gen-
erations despite treatment with berberine at
a concentration of 50% of itsMIC.
There is also evidence that berberine
could be an ARB48: berberine markedly
lowered the MICs of ampicillin and oxacillin
against MRSA; an additive effect was found
between berberine and ampicillin; and a syn-
ergistic effect was found between berberine
and oxacillin48. In the presence of 1–50 μg
per ml berberine, levels of MRSA adhe-
sion and intracellular invasion were notably
decreased compared with the vehicle-treated
control group. These results suggest that ber-
berine may have direct antimicrobial activity,
the potential to restore the effectiveness of
β-lactam antibiotics against MRSA, and the
ability to inhibit MRSA adhesion and intra-
cellular invasion. Berberine may also have
ARB activity by increasing the host defence
response. It protects against LPS-induced
intestinal injury in mice by inhibiting the
TLR4–NF-κB–MIP2 (Toll-like receptor
4–nuclear factor κB–macrophage inflam-
matory protein 2 (also known as CXCL2))
pathway in ileal cells and decreasing neu-
trophil infiltration53. Berberine can also act
as an LPS antagonist by binding to TLR4–
MD2 (also known as LY96) complexes and
blocking LPS–TLR4 signalling in murine
macrophage-like cells (RAW 264.7)51. This
may explain its reported effectiveness against
Gram-negative bacteria-induced diarrhoeal
diseases despite its lower invitro activity
against Gram-negative bacteria than against
Gram-positive species.
During infection, berberine drives
suppression of pro-inflammatory responses
through activation of 5ʹ-AMP-activated
protein kinase (AMPK) in macrophages54,
a property that could also lead to antibac-
terial activity via autophagy. Mechanistic
studies have shown that berberine down-
regulates expression of proinflammatory
genes such as tumour necrosis factor
(TNF), interleukin-1β (IL1B), IL6, mono-
cyte chemotactic protein 1 (MCP1; also
known as CCL2), inducible nitric oxide
synthase (iNOS; also known as NOS2), and
cyclooxygenase 2 (COX2; also known as
PTGS2) (REF.54).
Metformin, a commonly used antidiabetic
agent, also activates AMPK, and so ber-
berine has been studied as an antidiabetic
agent55,56. In patients with type 2 diabetes,
berberine has been reported to have an
efficacy equivalent to that of metformin57.
Berberine has also been investigated as a
chemotherapeutic agent, and so its potential
for cell toxicity would need to be accounted
for if it were to be used in any human studies
as an ARB. The bioavailability of berberine
is reportedly less than 5% owing to poor
absorption and rapid clearance. Berberine
seems to be subject to P-glycoprotein-
mediated efflux from the intestine and liver.
Absorption has been enhanced with sodium
caprate, a medium chain fatty acid found in
milk fat and coconut oil. However, bioavail-
ability issues do not seem to have been limit-
ing in the human studies reportedabove.
Potential ARBs for both bacterial classes
Curcumin. Curcumin is a constituent of the
popular spice turmeric, which has been used
for centuries in both cooking and traditional
medicine across the Indian subcontinent.
It is currently being investigated for efficacy
against a number of diseases, including can-
cer, and against mechanisms of ageing58,59.
Curcumin has direct antibiotic activity
at concentrations of 125–1,000 μg per ml
against a broad range of bacteria, includ-
ing some Gram-negative species (including
E.coli, P.aeruginosa, V.cholerae, S.aureus
and B.subtilis)60. Although curcumin
had some antibacterial effects against
Helicobacterpylori infections invitro61
and in animal studies62, human studies
have produced mixed results63–65.
There is also evidence supporting use of
curcumin against the Gram-positive organ-
ism C.difficile. Invitro, curcumin inhibited
the growth of 21 strains of C.difficile at a
concentration of 128 μg per ml, which is
obtainable in the colon through ingestion
of food or by dosage in capsules66. In clini-
cal practice, ingestion of 4 g per day would
achieve this concentration in the gut66. In
regions where curcumin is a regular dietary
ingredient it is typically consumed at 2–4 g
perday.
Curcumin also synergizes with antibiotics.
In combination studies with cefaclor, cefo-
dizime, or cefotaxime, concentrations of
0.1–1.0 μg per ml reduced the MIC values by
three- to eightfold against diarrhoea-causing
bacteria, such as E.coli and V.cholerae, as
well as against another Gram-negative
species, P.aeruginosa, and the Gram-positive
S.aureus67. Against MRSA, curcumin poten-
tiated the antimicrobial action of cefixime,
cefotaxime, vancomycin, tetracycline,
oxacillin, ampicillin, ciprofloxacin and
norfloxacin68,69.
Numerous potential mechanisms of
action have been reported, including inhi-
bition of sortase70. Curcumin also induces
autophagy by inhibiting the AKT–mTOR
pathway71. Its chemical structure (a polyphe-
nol with the ability to bind to many proteins
through ionic and hydrogen bonds) may
explain its promiscuous activity (TABLE1).
In host-defence studies, curcumin
blocks the binding of the LPSs from Gram-
negative bacteria to MD2 in the TLR4–MD2
complex72,73 and downregulates expres-
sion of intestinal TLR2, TLR4 and TLR9
(REFS74,75). It also decreases the production
of TNF, IL-1, IL-2, IL-6, IL-8 and IL-12,
MCP1 and migration and invasion-inhib-
itory protein76–80. The ability of curcumin
to block the interaction between MD2 and
bacterial proteins could also explain its effi-
cacy in treating Gram-positive infections.
C.difficile is a Gram-positive species with
no LPS coat, but its surface layer proteins
are recognized by the MD2 component of
the TLR4–MD2 complex in monocytes and
epithelial cells, stimulating NF-kB activation
and causing apoptotic intestinal epithelial
cell detachment81,82.
Through the inhibition ofNF-κB83,
curcumin prevents the upregulation of IL-8
expression in response to infection84. IL-8
levels are elevated in patients with severe
C.difficile colitis85,86; in these patients, dis-
ease severity correlates with increased levels
of IL-8, IL-6 and eotaxin, and IL-8 expres-
sion correlates with treatment failure after
metronidazole and vancomycin therapy87,88.
Curcumin may be an effective ARB for
patients with C.difficile infections by modu-
lating their gut cytokine response, especially
in those patients with relapsing infection.
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Well-controlled oral intervention trials
studying the use of curcumin to treat C.dif-
ficile infections are lacking. In one trial, the
drug, in the form of turmeric, was dosed by
enema89. In this trial, the turmeric enemas
were as effective as vancomycin enemas for
treating C.difficile colitis — the infection
was eradicated in 76% and 83% of patients,
respectively, compared with 66% in the
placebo group. At 21days post-treatment,
clinical severity was reduced by 60% in the
vancomycin and turmeric groups, compared
to a reduction of 38% in the placebo group.
Recurrence developed in 10% of patients
treated with vancomycin, 9% of those in the
turmeric group and 29% of patients who
received the placebo.
Some authors have reported that cur-
cumin has poor bioavailability90, which is
due to a combination of adverse properties:
poor aqueous solubility, poor absorption and
rapid conjugative clearance. However, neu-
roscientists have reported therapeutic levels
in the brain following oral dosing of either
curcumin or preparations with enhanced
bioavailability91. Brain levels reached 3 μM
for curcumin and 6 μM for tetrahydro-
curcumin92 and positive effects have been
reported in animal models of Alzheimer dis-
ease91,93. Similar efficacy was observed after
intraperitoneal dosing in a model of cerebral
ischaemia in rats73, suggesting that bioavail-
ability may not be limiting.
Further exploration of the ability of cur-
cumin to synergize with antibiotics60 and
potentially reduce antibiotic resistance in
bacterial pathogens of the gastrointestinal
tract is warranted. In view of the diversity
of opinions on bioavailability, it would seem
wise to focus additional oral investigations
on those bacteria that cause diarrhoeal
diseases and/or gain entry through the gut.
These can over-stimulate TLR4 in particu-
lar, causing a ‘cytokine storm’ and excessive
inflammation that aids their entry into
sterile gut wall tissues and the bloodstream.
The ability of curcumin to dampen down
this excessive inflammatory response may
lead to preventative or treatment options for
gut-invading Gram-negative bacteria, such
as Salmonella spp., Shigella spp., and E.coli,
as well as the Gram-positive C.difficile.
Intravenous doses of curcumin may be effec-
tive against systemic infections in which the
cytokine storm has devastating effects, such
as Gram-negative bacteria-mediatedsepsis.
Epigallocatechin‑3‑gallate (EGCG).
Epigallocatechin-3-gallate (EGCG) is one
of the most abundant polyphenols in green
tea and is thought to be responsible for most
of its supposed therapeutic benefits. EGCG
has been extensively studied in many disease
areas and written about in several thousand
scientific publications, with most published
over the past two decades. The anti-infective
effect of green tea was first reported more
than 100years ago by Major J. G. McNaught,
an army surgeon who showed that green
tea killed the Gram-negative organisms that
lead to typhoid fever (Salmonella typhi) and
brucellosis (Brucella melitensis)94.
Two recent comprehensive reviews95,96
have detailed mild antibiotic activity of
EGCG alone invitro and substantial synergy
of EGCG with a broad range of antibiotics to
treat both Gram-positive and Gram-negative
organisms, particularly if antibiotic resistance
is present. EGCG has positive synergistic
effects, although the occasional adverse effect
on resistance invitro has been reported101.
EGCG can sensitize MRSA to all types
of β-lactam antibiotics, including ben-
zylpenicillin, oxacillin, methicillin, ampicil-
lin, carbapenems and cephalexin97–100. The
fractional inhibitory concentration (FIC)
indices of β-lactams tested against 25 iso-
lates of MRSA ranged from 0.126 to 0.625
when used in combination with EGCG at
6.25, 12.5 or 25 μg per ml. When used in
combination with three carbapenems that
do not usually show strong activity against
MRSA, EGCG showed additive and syner-
gistic effects, bringing potency to within a
useful range: the MICs of imipenem in the
presence of EGCG at 3.125, 6.25, 12.5 and
25 μg per ml were restored to the susceptibil-
ity breakpoint (<4 μg per ml) for 8, 38, 46
and 75% of the MRSA isolates, respectively,
thus rendering these bacteria ‘susceptible’
(REF.101 ). EGCG is able to break the resist-
ance of many bacteria to β-lactams and
carbapenems, and it also increased the
efficacy of inhibitors of protein or nucleic
acid synthesis102. However, EGCG may
have adverse effects when combined with
glycopeptide antibiotics (vancomycin or
teicoplanin)101. Importantly, EGCG seems
to have no adverse effects on commensal
bacteria103.
EGCG may also be useful in treating
Gram-negative infections. In vitro, EGCG
killed MDR and extended-spectrum
β-lactamase (ESBL)-producing strains of
E.coli that were isolated from urinary tract
infections104. Invivo, green tea and EGCG
dose-dependently abrogated endotoxin-
induced high mobility group protein B1
(HMGB1) release from murine macrophage-
like RAW 264.7 cells and dose-dependently
protected mice against lethal endotoxemia
and sepsis105. The authors noted that doses
of EGCG given orally to septic mice (4 mg
per kg, which is 10 mM) were comparable
to those achievable in humans after inges-
tion of a few cups of green tea (1 mM).
Importantly, delayed and repeated admin-
istration of EGCG beginning 24hours after
onset of sepsis substantially rescued mice
from lethal sepsis, supporting a therapeutic
potential for EGCG in the clinical manage-
ment of sepsis.
The mechanisms by which EGCG exerts
its effects on bacteria seem to be very broad;
this is probably due to its chemical structure,
which contains phenolic groups capable
of making ionic and hydrogen bonds with
multiple proteins. EGCG binds to the pep-
tidoglycans of the bacterial cell wall and
inhibits penicillinase activity, protecting
penicillins from inactivation106. It also alters
the cell wall of S.aureus107. It has been sug-
gested that the ability of EGCG to reverse
methicillin resistance is mediated by inhibi-
tion of the synthesis of the penicillin-binding
protein 2a (PBP2a) as well as inhibition of
β-lactamase secretion97. In addition, EGCG
inhibits DNA gyrase108, dihydrofolate reduc-
tase102 and specific reductases (FabG and
FabI) in bacterial typeII fatty acid synthe-
sis109. EGCG also blocked H.pylori binding
to TLR4 on gastric epithelial cells110, inhib-
ited conjugative transfer of the R plasmid in
E.coli111 — which could lead to decreased
sharing of antimicrobial genes between
bacteria — and inhibited the activity of the
streptococcal efflux pump Tet(K), which
is involved in resistance to tetracycline112.
However, the effect of EGCG on a range
of bacterial efflux pumps needs further
definition. Additionally, EGCG activates
host defence and, therefore, it may be effec-
tive at lower plasma concentrations than
would be expected by simple extrapolation
from invitro data. EGCG also increases
autophagy113,114, possibly through activation
of AMPK115.
Human trials of EGCG in combination
with antibiotics are required. EGCG has rela-
tively poor bioavailability in animals and it
is unclear whether plasma levels in humans
would be sufficiently high to exert a syner-
gistic effect with antibiotics116. Prodrugs of
EGCG could improve bioavailability117,118.
However, EGCG might be most useful as an
ARB in topical infections, such as MRSA,
and possibly as an oral treatment for gas-
trointestinal infections. In addition, EGCG
could be useful as an intravenous agent com-
bined with carbapenems or other antibiotics
against diseases caused by systemic MRSA
infection, such as pneumonia, septicemia
and urinary tract infections.
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(+)‑naloxone and (+)‑naltrexone. The
opioid antagonist (−)-naloxone and the
non-opioid (+)-naloxone inhibit TLR4 sig-
nalling and block the MD2–TLR4-mediated
inflammatory side effect of opioids. They
may bind preferentially to the LPS bind-
ing pocket of MD2 rather than to TLR4
(REF.11 9 ). (+)-naloxone has no known
off-target effects120, however, its half-life
is short, whereas the structurally related
(+)-naltrexone has a half-life of 4–6hours
in humans121.
These (+)-isomers are devoid of opioid-
like activity, and theoretically they could be
used to treat some bacterial infections, as
blocking TLR4 –MD2 in the gut could pre-
vent bacterial LPS from triggering a cytokine
storm driven by IL-6 and IL-8 and thereby
prevent invasion by gut pathogens. These
or other opioid antagonists could be useful
if co-administered orally with antibiotics to
treat E.coli or Shigella spp. intestinal infec-
tions or to prevent CDAD in the elderly.
They could also be useful if administered
intravenously for the treatment of LPS-
driven systemic endotoxic shock because
of their potential to block the release of
pro-inflammatory cytokines.
Conclusions and next steps
Several potential ARBs are available for
β-lactam antibiotics (carbapenems, cepha-
losporins and penicillins), which is the most
important class of antibiotic for the treat-
ment of antibiotic-resistant Gram-negative
bacteria. Several of these ARBs disrupt the
bacterial cell wall, which contains poly-
anionic LPS and is stabilized by the cross-
bridging of divalent cations122. Drugs that
target these divalent cations destabilize the
membrane, increase its permeability and
allow access of molecules that were partially
or fully excluded. Indeed, several polycation
antibiotics — for example, polymyxins,
aminoglycosides and dibasic macrolides
such as azithromycin — act through this
mechanism123 and these are ARBs them-
selves when co-administered with β-lactam
antibiotics. In addition to potentially
salvaging our best Gram-negative anti-
biotics, this approach may also make
‘Gram-positive antibiotics’ useful against
Gram-negative bacteria. Further careful
screening of polycation molecules in the
drug pharmacopeia may identify new
ARBs of thistype.
No compelling ARBs were identified for
two classes of antibiotics that are particularly
useful, the fluoroquinolones and the amino-
glycosides, although a report of successful
use of EDTA with gentamycin may indicate
that EDTA could be used as an ARB124.
Future research aimed at identifying
ARBs for these classes could be highly
valuable.
In the reviewed literature, most of the
potential ARBs discussed above are shown
to be effective as directly antibacterial and/or
additive to antibiotics at concentrations
of 25 μg per ml or lower, which is the level
required for interest by consortia such as
the Innovative Medicines Initiative (IMI)’s
ENABLE project, a European Union initia-
tive that supports new drug development
of antibacterials against Gram-negative
organisms. Ciclopirox, loperamide, cur-
cumin, EGCG and berberine are potential
ARBs with encouraging data at 25 μg per
ml. Most of the drugs considered in this
Perspective have direct antibiotic activ-
ity: sometimes they exhibit additive effects
with antibiotics, and sometimes they syn-
ergize with co-administered antibiotics
at 0.50–50 μg per ml. For the treatment of
systemic infections, the maximum dose of
drug approved for use in humans must, at
minimum, achieve this level of exposure
at the maximum concentration (Cmax). For
topical (including gastrointestinal) infec-
tions, Cmax may not be relevant because
high local concentrations of both drug
and antibiotic will be present, with a much
higher probability of achieving therapeu-
tic concentrations at the relevant site. The
first application of ARBs to reach the clinic
would almost certainly be in topical and
gastrointestinal infections, although the
most urgent clinical need is for intravenous
agents against Gram-negative bacteria.
The combination of two known drugs with
known pharmacological, pharmacokinetic
and safety profiles is possibly the best-case
scenario for low-risk drug development.
The data on the use of these molecules as
ARBs come from many different laboratories
using diverse methodologies and often give
a range of potencies for each molecule. The
data need to be confirmed against the cur-
rent most lethal strains to enable calculation
of the plasma concentrations of ARBs that
will be required. It will then be necessary
to conduct safety assessments to determine
a therapeutic index for each ARB. The
agents considered in this Perspective are all
ingested by humans today, so they already
have a lengthy safety record; however, their
safety does need to be confirmed for the
particular doses, combinations and routes of
administration that would be used to treat
bacterial infections.
Additionally, there are regulatory consid-
erations that need to be addressed (BOX2).
Will regulatory authorities require three-way
clinical trials, comparing each drug individ-
ually with the combination? Do we have
the time to perform such perfect clinical
trials? The Ebola epidemic in West Africa
has shown that society is now amenable to
fast-track development of new drugs when
a global emergency dictatesit.
Profit margins for combinations of
known drugs may be low even if they are
lifesavers. Pharmaceutical or biotechnol-
ogy companies are unlikely to invest in this
approach, although those that currently sell
antibiotics may be able to preserve sales and
reach new patents by adding an ARB in a
new combination. In particular, as no ARBs
have been identified for fluoroquinolones
Box 2 | Regulatory pathways for repurposed drugs
In the United States, the US Food and Drug Administration (FDA) has in place a regulatory
pathway, 505(b)(2), that applies to a new use or new formulation of an approved drug. It allows
the applicant to use the existing safety, pharmacology and toxicology data for regulatory
purposes, provided that the doses and exposure used are the same or lower. The application can
refer to published literature, product labels and product monographs. Parallel regulations apply
to investigational new drug filings.
Europe and other regions have similar regulations. The intellectual property could not be
protected with a composition of matter claim, but in many countries a ‘method of use’ patent
could be filed for the discovery of a new indication for an old drug that is novel, unexpected and of
value to humanity. Together with a use claim, the development of a new formulation, possibly
incorporating a different dose or route of administration, can further support market exclusivity.
Another possibility is to use repurposed drugs off‑label. The properties of a repurposed drug
could be publicized through scientific literature and conferences. If the appropriate formulation
is the same, the drug could be used without formal regulatory approval for the secondary use.
However, although a physician may prescribe a drug for a use other than the approved
indication, drug companies have been sanctioned severely for marketing products along these
lines. In addition, some payers, under certain conditions, restrict the reimbursement of products
that are used off‑label; the prescribing physician also incurs a greater element of product
liability. Regulatory approval avoids these problems and may also be preferable as it safeguards
standards of quality.
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and aminoglycosides, a screening campaign
to find ARBs for these antibiotics could be
commercially viable if supported by ‘method
of use’ patents (BOX2). However, in general,
unless a new economic model is developed
for antibiotics, the development of ARBs
will have to be pursued by government,
public sector or philanthropic agencies,
or combinations ofthese.
David Brown is at Alchemy Biomedical Consulting,
St Johns Innovation Centre, Cowley Road,
Cambridge CB4 0WS, UK.
e-mail: davidbrown1000@btinternet.com
doi:10.1038/nrd4675
Published online 23 October 2015
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Acknowledgements
The author thanks the following for expert discussions on the
drugs reviewed: A. Coates (clinical antibiotic resistance and
ARB concept); S. Shaunak (clinical antibiotic resistance, TLRs
and innate immune system); N. Ktistakis (autophagy);
D. Cavalla (drug repurposing); and members of the Science and
Technology Advisory Committee of Antibiotics Research UK.
Competing interests statement
The author declares no competing interests.
FURTHER INFORMATION
WHO — antimicrobial resistance: global report on
surveillance 2014: http://www.who.int/drugresistance/
documents/surveillancereport/en/
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