The Oscillation between Antibacterial and Anticancer Activities

24 Pages·2017·1.52 MB·English

antibiotics
Review
Ubiquitous Nature of Fluoroquinolones: The
Oscillation between Antibacterial and
Anticancer Activities
TemiloluIdowu1,* ID andFrankSchweizer1,2
1 DepartmentofChemistry,UniversityofManitoba,Winnipeg,MBR3T2N2,Canada;
[email protected]
2 DepartmentofMedicalMicrobiologyandInfectiousDiseases,UniversityofManitoba,Winnipeg,
MBR3T1R9,Canada
* Correspondence:[email protected];Tel.:[phone]
Received:30September2017;Accepted:3November2017;Published:7November2017
Abstract: Fluoroquinolonesaresyntheticantibacterialagentsthatstabilizetheternarycomplexof
prokaryotictopoisomeraseIIenzymes(gyraseandTopoIV),leadingtoextensiveDNAfragmentation
and bacteria death. Despite the similar structural folds within the critical regions of prokaryotic
andeukaryotictopoisomerases,clinicallyrelevantfluoroquinolonesdisplayaremarkableselectivity
for prokaryotic topoisomerase II, with excellent safety records in humans. Typical agents that
targethumantopoisomerases(suchasetoposide,doxorubicinandmitoxantrone)areassociatedwith
significanttoxicitiesandsecondarymalignancies,whereasclinicallyrelevantfluoroquinolonesarenot
knowntoexhibitsuchpropensities. Althoughmanyfluoroquinoloneshavebeenshowntodisplay
topoisomerase-independentantiproliferativeeffectsagainstvarioushumancancercells,thosethat
aresignificantlyactiveagainsteukaryotictopoisomeraseshowthesameDNAdamagingproperties
asothertopoisomerasepoisons. Empiricalmodelsalsoshowthatfluoroquinolonesmediatesome
uniqueimmunomodulatoryactivitiesofsuppressingpro-inflammatorycytokinesandsuper-inducing
interleukin-2. Thisarticlereviewstheextendedrolesoffluoroquinolonesandtheirprospectsaslead
fortheunmetneedsof“smallandsafe”multimodal-targetingdrugscaffolds.
Keywords: antibacterial;antiproliferative;antitumor;fluoroquinolone;gyrase;immunomodulation;
topoisomerase
1. Introduction
Antibioticsarenotmade,theyaresimplydiscovered,andthediscoveryprocessisamixedbagof
profoundscientificexplorationand/orfortunatecoincidences. Ourcurrentarsenalismainlymade
upofcompoundsthatwerederivedfromnaturalsources,theverysourceoftheirwoes,andtheir
semi-synthetic derivatives. Microorganisms often secrete trace amount of antibiotics primarily as
warfare agents to kill other bacteria or fungi in the evolutionary struggle to gain an advantage
over other species that are competing for the same ecological niche [1]. Nature’s laboratory is
therefore a good reservoir of antibiotic scaffolds that have been evolutionarily optimized to suit
thephysicochemicalrequirementsforactivityinmicroorganisms,especiallyagainstGram-negative
bacteria. However,giventheneedfortheorganismproducinganantibiotictoprotectitselfagainstthe
harmfuleffectsofsuchanantibioticandthedefensivemechanismevolvedbyotherbacteriatothe
agent,antibiotic-resistancemechanismstonaturalproductsarepresumedtobealreadypresentinthe
bacterialcommunity[2]. Thisphenomenonsignificantlyshortensthetimebetweenwhenanisolated
antibioticisintroducedintotheclinicandwhenfull-fledgedresistanceisobserved. Itistherefore
imperativetodevelopnewmoleculesthathavenotbeenpreviouslyencounteredbymicroorganisms.
Antibiotics2017,6,26;doi:10.3390/antibiotics6040026 www.mdpi.com/journal/antibiotics
Antibiotics 2017, 6, 26 2 of 23
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therefore imperative to develop new molecules that have not been previously encountered by
microorganisms.
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Figure 1. Core structures of the quinolone class of drugs.
Figure1.Corestructuresofthequinoloneclassofdrugs.
On the other hand, cancer is a more devastating chronic disease that claims millions of lives every
Ontheotherhand,cancerisamoredevastatingchronicdiseasethatclaimsmillionsoflivesevery
year worldwide [6]. It is a class of disease in which a group of aberrant cells exhibit uncontrollable
yearworldwide[6]. Itisaclassofdiseaseinwhichagroupofaberrantcellsexhibituncontrollable
growth, invade neighboring tissues or organs, and sometimes metastasize [7]. There are several classes
growth, invade neighboring tissues or organs, and sometimes metastasize [7]. There are several
of drugs currently in use for managing this disease, but of interest are those that target and act on
classes of drugs currently in use for managing this disease, but of interest are those that target
mammalian topoisomerases. Etoposide (Figure 2) is an antitumor agent that displays a mechanism
andactonmammaliantopoisomerases. Etoposide(Figure2)isanantitumoragentthatdisplaysa
of action (in eukaryotic cells) that is similar to that of FQs (in prokaryotic cells). They both act on
mechanism of action(in eukaryotic cells) that issimilar to that of FQs(in prokaryoticcells). They
topoisomerases [8–11]. However, clinically relevant FQs are able to distinguish between bacterial and
bothactontopoisomerases[8–11]. However,clinicallyrelevantFQsareabletodistinguishbetween
mammalian topoisomerases, and avoid cross-reactivity with human type II enzyme even at
bacterial and mammalian topoisomerases, and avoid cross-reactivity with human type II enzyme
concentrations well beyond their therapeutic doses [5]. Interestingly, some clinically relevant FQs
evenatconcentrationswellbeyondtheirtherapeuticdoses[5]. Interestingly,someclinicallyrelevant
have been shown to be potent antiproliferative agents at much lower concentrations than is needed
FQs have been shown to be potent antiproliferative agents at much lower concentrations than is
for their antibacterial activity [12]. It should be noted that FQs that are significantly active against
neededfortheirantibacterialactivity[12]. ItshouldbenotedthatFQsthataresignificantlyactive
eukaryotic enzymes exhibit the same DNA damaging properties as other topoisomerase poisons
against eukaryotic enzymes exhibit the same DNA damaging properties as other topoisomerase
[13,14]. Since clinically relevant FQs are known to be safe in humans, how then do they discriminate
poisons [13,14]. Since clinically relevant FQs are known to be safe in humans, how then do they
cancer cells from healthy cells? What can we learn from their physicochemical properties? Moreover,
discriminatecancercellsfromhealthycells? Whatcanwelearnfromtheirphysicochemicalproperties?
the endowment of quinolones with other “non-classical” biological activities such as anti-HIV-1
Moreover, the endowment of quinolones with other “non-classical” biological activities such as
integrase [15], cannabinoid receptor-2 agonist/antagonist [16,17], anxiolytic agents [18,19], anti-
anti-HIV-1integrase[15],cannabinoidreceptor-2agonist/antagonist[16,17],anxiolyticagents[18,19],
ischemic activities [20], antiviral effects [21], etc. is continually injecting new enthusiasm towards this
anti-ischemicactivities[20],antiviraleffects[21],etc. iscontinuallyinjectingnewenthusiasmtowards
scaffold of drug. This article aims to articulate the biological properties of FQs, how they perform their
thisscaffoldofdrug. ThisarticleaimstoarticulatethebiologicalpropertiesofFQs,howtheyperform
extended roles viz-a-viz their structural basis, and their prospects as a promising drug scaffold.
theirextendedrolesviz-a-viztheirstructuralbasis,andtheirprospectsasapromisingdrugscaffold.
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2. Discovery and Development of Fluoroquinolones
2. DiscoveryandDevelopmentofFluoroquinolones
Before the discovery of quinolones in the 1960s, antibiotics were majorly sourced from natural
Beforethediscoveryofquinolonesinthe1960s,antibioticsweremajorlysourcedfromnatural
products. They were either obtained from plants and animals in the form of host defence peptides
products. Theywereeitherobtainedfromplantsandanimalsintheformofhostdefencepeptides
that were produced as a part of their innate immunity (e.g., cecropins, defensins, magainins,
that were produced as a part of their innate immunity (e.g., cecropins, defensins, magainins,
cathelicidins) [22–25], or isolated directly from microorganism cultures themselves (e.g., penicillin,
cathelicidins)[22–25],orisolateddirectlyfrommicroorganismculturesthemselves(e.g.,penicillin,
aminoglycosides, polymyxins, etc.) [1,26,27]. The accidental discovery of 7-chloroquinolone as an
aminoglycosides, polymyxins, etc.) [1,26,27]. The accidental discovery of 7-chloroquinolone as an
impurity in a distillate during the chemical synthesis of the antimalarial drug, chloroquine, made
impurity in a distillate during the chemical synthesis of the antimalarial drug, chloroquine, made
them the first class of synthetic antibiotics [28]. Although the exact account of how 7-chloroquinolone
themthefirstclassofsyntheticantibiotics[28]. Althoughtheexactaccountofhow7-chloroquinolone
evolved to 1,8-naphthyridone core (Figure 1) (as in nalidixic acid), and back to quinolone core (as in
evolvedto1,8-naphthyridonecore(Figure1)(asinnalidixicacid),andbacktoquinolonecore(asin
ciprofloxacin), has been laced with gaps and counter-arguments, George Lesher (the acknowledged
ciprofloxacin),hasbeenlacedwithgapsandcounter-arguments,GeorgeLesher(theacknowledged
discoverer) and his coworkers at Sterling Drugs (now part of Sanofi) are credited as being the first to
discoverer)andhiscoworkersatSterlingDrugs(nowpartofSanofi)arecreditedasbeingthefirstto
report the antibacterial activities of this class of drugs in the 1960s [28]. A recent article attempts to
reporttheantibacterialactivitiesofthisclassofdrugsinthe1960s[28].Arecentarticleattemptstoshed
shed some lights on the mystery surrounding the discovery process of quinolones, and concluded
somelightsonthemysterysurroundingthediscoveryprocessofquinolones,andconcludedbasedon
based on original documents and Lab notes, that Sterling Drugs and Imperial Chemical Industries
originaldocumentsandLabnotes,thatSterlingDrugsandImperialChemicalIndustries(nowpartof
(now part of AstraZeneca) might have independently discovered this scaffold around the same time
AstraZeneca)mighthaveindependentlydiscoveredthisscaffoldaroundthesametime[29].
[29].
Nonetheless, Lesher and coworkers established the anti-Gram-negative bacteria potency of
Nonetheless, Lesher and coworkers established the anti-Gram-negative bacteria potency of 7-
7-chloroquinoloneduringbiologicalscreening[28],butthecompoundhadlimitedusefulnessduetoits
chloroquinolone during biological screening [28], but the compound had limited usefulness due to
highproteinbinding(approximately90%)andshorthalf-life(about90min)[30]. Theleadcompound
its high protein binding (approximately 90%) and short half-life (about 90 min) [30]. The lead
wasthereforeoptimizedtonalidixicacid(Figure3)in1962,andwasusedextensivelyforover30years
compound was therefore optimized to nalidixic acid (Figure 3) in 1962, and was used extensively for
totreaturinarytractinfections(UTIs)thatarecausedbyGram-negativebacteria,mainlyEscherichiacoli.
over 30 years to treat urinary tract infections (UTIs) that are caused by Gram-negative bacteria,
Thisoptimizationmarkedthebeginningofanactivecampaignofchemicalsynthesistorefinethe
mainly Escherichia coli. This optimization marked the beginning of an active campaign of chemical
structure-activity relationship of FQs, such that biological activity and pharmacokinetics could be
synthesis to refine the structure-activity relationship of FQs, such that biological activity and
improvedandtoxicityanddruginteractionsdiminished[30]. Afurtherintroductionoffluorineatom
pharmacokinetics could be improved and toxicity and drug interactions diminished [30]. A further
atposition-6ofthebicyclicringsystemwasfoundtosignificantlyincreasetissuepenetration,giving
introduction of fluorine atom at position-6 of the bicyclic ring system was found to significantly
risetothefirstFQ,flumequine(Figure3). Quinoloneisagenerictermthatlooselyreferstoaclassof
increase tissue penetration, giving rise to the first FQ, flumequine (Figure 3). Quinolone is a generic
drugsthatincludequinines,naphthyridines,fluoroquinolones,quinazolines,isothiazoloquinolones,
term that loosely refers to a class of drugs that include quinines, naphthyridines, fluoroquinolones,
andrelatedagents. FQs(exceptforenoxacinandgemifloxacin)differfromnalidixicacidinthatthey
quinazolines, isothiazoloquinolones, and related agents. FQs (except for enoxacin and gemifloxacin)
have the same quinolone core as the 7-chloroquine impurity, with the addition of a fluorine atom
differ from nalidixic acid in that they have the same quinolone core as the 7-chloroquine impurity,
atthesixthposition,whilenalidixicacid,gemifloxacin,andenoxacinhave1,8-naphthyridonecore
with the addition of a fluorine atom at the sixth position, while nalidixic acid, gemifloxacin, and
(Figure1). Therationaleforthebackandforthswitchincorescaffoldisunknown,perhapsdueto
enoxacin have 1,8-naphthyridone core (Figure 1). The rationale for the back and forth switch in core
intellectualpropertyconcerns[29],buttheadditionoffluorineconferredmorepotentantibioticaction
scaffold is unknown, perhaps due to intellectual property concerns [29], but the addition of fluorine
andbroaderspectrumofactivityonthisclassofdrugs[4].
conferred more potent antibiotic action and broader spectrum of activity on this class of drugs [4].
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Figure 3. Landmark developmental trends of fluoroquinolones.
Figure3.Landmarkdevelopmentaltrendsoffluoroquinolones.
In a bid to further improve the spectrum of activity against Gram-positive species, lower
InabidtofurtherimprovethespectrumofactivityagainstGram-positivespecies,lowerpotency,
potency, higher frequency of spontaneous bacterial resistance, shorter half-lives, and lower serum
higherfrequencyofspontaneousbacterialresistance,shorterhalf-lives,andlowerserumconcentrations
concentrations of early FQs [31], several modifications, such as side-chain nuclear manipulations and
ofearlyFQs[31],severalmodifications,suchasside-chainnuclearmanipulationsandmono-orbicyclic
mono- or bicyclic ring substitutions, were done. The newer FQs were found to exhibit longer
ringsubstitutions, weredone. ThenewerFQswerefoundtoexhibitlongereliminationhalf-lives,
elimination half-lives, high oral bioavailability, high potency, extensive tissue penetration, and lower
highoralbioavailability,highpotency,extensivetissuepenetration,andlowerincidencesofresistance
incidences of resistance when compared to the earlier ones [32]. Being a fully synthetic class of drug,
whencomparedtotheearlierones[32]. Beingafullysyntheticclassofdrug,thedevelopmentofFQs
the development of FQs has been gradual and systematic (Figure 3). Each generation seems to impart
hasbeengradualandsystematic(Figure3). Eachgenerationseemstoimpartnewpotenciesinatrend
new potencies in a trend that has seen them acquire excellent efficacy towards Gram-positive bacteria
thathasseenthemacquireexcellentefficacytowardsGram-positivebacteriainadditiontotheirnow
in addition to their now optimized potency against Gram-negative bacteria. Remarkably, the newer
optimizedpotencyagainstGram-negativebacteria. Remarkably,thenewerFQsalsodisplayactivity
FQs also display activity against anaerobes [33]. Anaerobes are organisms that do not require oxygen
against anaerobes [33]. Anaerobes are organisms that do not require oxygen for survival, thereby
for survival, thereby making drugs that require an oxygen transport into ribosomes (e.g.,
makingdrugsthatrequireanoxygentransportintoribosomes(e.g.,aminoglycosides)ineffectiveand
aminoglycosides) ineffective and never taken up by these organisms [34,35].
nevertakenupbytheseorganisms[34,35].
Based on their systematic structure optimization process and developmental trends, FQs can be
Basedontheirsystematicstructureoptimizationprocessanddevelopmentaltrends,FQscanbe
distinctly classified into four generations (Table 1).
distinctlyclassifiedintofourgenerations(Table1).
Table 1. Comparison of fluoroquinolone generations (Adapted from refs [4,36,37]).
Table1.Comparisonoffluoroquinolonegenerations(Adaptedfromrefs[4,36,37]).
Generations Microbiologic Activity Administration and Characteristics Indications
First generation Administrationand
Generations MicrobiologicActivity Indications
Nalidixic acid, Oral admiCnihsatrraatcitoenri.s Ltiocws serum and Uncomplicated urinary tract
Cinoxacin Enterobacteriaceae tissue drug concentrations. Narrow infections Not for use in
Firstgeneration
(NDaislicdoinxtiicnauceidd),, Cinoxacin gOrarmal-andemgaitniivsetr caotivoenr.aLgoew serum Uncosymsptelimcaicte idnfuercintiaornyst ract
Enterobacteriaceae andtissuedrugconcentrations. infectionsNotforuseinsystemic
F(Dluimscoenqtuininueed ),
Narrowgram-negativecoverage infections
Flumequine Uncomplicated urinary tract
OOraral laaddmmininiissttrraattiioonn.. LLooww sseerruumm and infections. Not for use in
Second generation Uncomplicatedurinarytract
Enterobacteriaceae. tiasnsudet idssruueg dcroungcecnontrcaetniotrnast.i ons. systemic infections.
CSleacsosn Id Lgoemneerflaotixoancin Enterobacteriaceae. infections.Notforuseinsystemic
(CDlaisscsoInLtionmueedfl)o, xacin EntEenrotebraocbtaecrtiearcieaaceea, e, ImIcmopvpreorrovavgeeded, glgirmraamimte--dnnegeggraaamttiivv-pee ocsoivtievreage, infecCtioomnsp.lCicoamtepdl iucaritnedaruyr itnraacrty
(Discontinued), atyaptiycpailc palapthaothgoegnesn; s; limited gram-positive coverage. Oralt ractaanndd ccaatthheetteerr--rreellaatteedd
Norfloxacin, coverage.Oralandintravenous
Norfloxacin,Enoxacin PsePusdeoumdoomnaosn aaserauerguingoinsoas a and intravenous administration. infecintifoencst.ioGnass.t Groaesnttreoreitnistewriittihs
Enoxacin Class II administration.Higherserum,
ClassIIOfloxacin (cip(criopfrlooflxoacxianc ionnolyn)ly, ), Higher serum, tissue, and sevewreitdhi asrervheerae, pdrioasrtrahteitais, ,
OCfilporxoaflcoinxa cin PnePunmeuomccoocccio ci intcitosrnsaucceeen,llaturnaldatiroi ndntrsru,acgcoe cvlloeunrlaacgreendtorrufagtions, nosopcroomstiaatliitnisf,e cntoiosnosc,osmexiaula lly
Ciprofloxacin transmitteddiseases
coavtyepriacgael poaf tahtoygpeincasl pathogens infections, sexually
transmitted diseases
Antibiotics2017,6,26 5of24
Table1.Cont.
Administrationand
Generations MicrobiologicActivity Indications
Characteristics
Thirdgeneration Oralandintravenous Similarindicationsasfor
Enterobacteriaceae,
Levofloxacin, administration,similartoclassII second-generation.
atypicalpathogens,
Sparfloxacin second-generationbutwith Community-acquiredpneumonia
streptococci.
(Discontinued) modeststreptococcalcoverage. inhospitalizedpatientsorif
PneumoccociMIC:
Gatifloxacin Increasedhepaticmetabolism atypicalpathogensare
0.25–0.5µg/mL
(Discontinued) (sparfloxacin) stronglysuspected
Fourthgeneration Enterobacteriaceae, Oralandintravenous
Trovafloxacin P.aeruginosa,atypical administration.Similarto
Considerfortreatmentof
(Discontinued) pathogens,MSSA, third-generation,butwith
intra-abdominalinfections
Moxifloxacin streptococci,anaerobes, improvedgram-positiveand
Gemifloxacin Pneumoccoci anaerobiccoverages
3. ClassicalAntimicrobialActivityofFluoroquinolones
TheantimicrobialactivityofFQsagainstawiderangeoforganismsandtheirclinicaluseinthe
treatmentofurinarytractinfections(UTIs),prostatitis,bacterialentericinfections,biliarytractinfections,
sexuallytransmitteddiseases,prophylaxisinimmune-compromisedneutropenicpatients,andahostof
otherclinicalconditionsconfirmtheirbroad-spectrumofactivity[38,39].Whileefficacyandspectrum
ofactivityhaveimprovedacrossthegenerations,tolerabilityandpharmacokineticparameters,suchas
tissuepenetration,bioavailability,andserumhalf-lifehavealsobeenoptimized[40].Theantimicrobial
spectrum of the first-generation was largely limited to aerobic Gram-negative bacillary infections,
particularlyintheurinarytract, whilethesecondgenerationFQshaveenhancedactivity(1000-fold)
againstaerobicGram-negativeandGram-positivebacteria[41,42]. Ciprofloxacin,asecondgeneration
FQ, is the most successful of all FQs to date, both economically and clinically [3]. Newer FQs, such
asgatifloxacin,levofloxacin,gemifloxacin,moxifloxacin,etc.(Figure4)offerenhancedactivityagainst
aerobic Gram-negative bacilli and improved Gram-positive activity over ciprofloxacin (e.g., against
StreptococcuspneumoniaeandStaphylococcusaureus)[43], butciprofloxacinandmoxifloxacinmaintain
thebestinvitroactivityagainstPseudomonasaeruginosa[43,44].ItisinterestingtonotethatP.aeruginosa
produces2-heptyl-3-hydroxy-4(1H)-quinolone(PQS),aquorum-sensingsignalmoleculethatsharessome
corestructuralfeatureswithfluoroquinolones,toregulatenumerousvirulencegenes,includingthose
involvedinironscavenging[45].Intermsofpotency,moxifloxacinismoreeffectiveagainstGram-positive
andanaerobesthanciprofloxacinandlevofloxacin. Moxifloxacinisoftenconsideredasa“respiratory
quinolone”becauseofitssignificantpotencyagainsttherespiratorypathogenS.pneumoniae[46]. Itis
currentlybeinginvestigatedasaBPaMZ(bedaquiline+pretomanid+moxifloxacin+pyrazinamide)
regimenforthetreatmentofmultidrugresistanttuberculosis,aneffortthatcouldshortentheduration
of tuberculosis treatment from six to four months if successful [47,48]. When compared to other
FQs, moxifloxacin appears to be less affected by the bacterial efflux system because of its bulky C-7
substituents[49–52]andtheiroptimized8-methoxysubstituent(Figure4)[53,54].
ThenewergenerationsFQsdisplaypotentactivityagainstpenicillin-resistantandmultidrug-resistant
(MDR) pneumococcus and anaerobes, while still retaining their activity against aerobes [55,56].
Several quinolones, most of which are FQs, are also currently at different stages of clinical
development. For example, nemonoxacin (Figure 5) is a non-fluorinated broad spectrum quinolone
(isothiazoloquinolone) that displays comparable invitro Gram-negative activity as ciprofloxacin,
levofloxacin, and moxifloxacin, but an enhanced potency against Gram-positive bacteria (including
MRSA and MDR S. pneumoniae) [57]. It is currently under development for oral and intravenous
treatmentofcommunityacquiredpneumonia(approvedinTaiwan), aswellastheoraltreatmentof
diabeticfootulcerinfectionsandskinandsofttissueinfections[57].Also,in2014,finafloxacin(Figure5)
wasapprovedbyFDAasatopicaloticsuspensionforthetreatmentofacuteotitiseterna(swimmer’s
ear),whichiscausedbysusceptiblestrainsofP.aeruginosaandS.aureus[58].Delafloxacin(Figure5)was
approvedin2017forthesystemictreatmentofacutebacterialskinandskinstructureinfectionscaused
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byarangeofsusceptibleGram-positiveandGram-negativebacteria(includingESKAPEpathogens)in
Antibiotics 2017, 6, 26 6 of 23
adults[59].The“ESKAPE”pathogens—encompassingEnterococcusfaecium,S.aureus,Klebsiellapneumonia,
many serious infections in hospitals [60]. Interestingly, unlike ciprofloxacin, moxifloxacin, and
Acinetobacterbaumanii,P.aeruginosa,andEnterobacterspp—areresponsibleformanyseriousinfectionsin
levmoaflnoyx asceirni,o uws hiincfhec teioxnhsi biint hreodspuicteadls a[6c0ti]v. iItnyt earet stsinligglhyt,l yu nalickide icc ipprHof lo(x5a.0ci–n6,. 5m), oxfiinflaofxlaocxianc, ina ndan d
dhoellseapvfilotoafxllosaxc[6ain0ci ]ne.x,I nhwtiebhrieitc sehtin nhegxalyhn,icbueindt l iprkeoedtceuincpecrdyo falaotc xttahivciiisnt yp, mHa otl xesivflilegol,hx amtlcyai nka,icaningdd itchl eepvmHof slou(5xi.ta0ac–bi6nl.e,5 w)f,o hrfi ictnhhaefe lxeohrxaiabdciiitncra etadinoudnc eodf
Sac. tdaiuveirltaeyfulsao txfoasluciignnhd et lxiynh aiabcciiitdd eiicncph eaHnnvc(i5er.do0 –np6mo.5tee)n,nftci.yn aaftl othxaisc ipnHa nledvdele, lmafalokxinacgi ntheexmh isbuititeanbhlea nfocre dthpeo etreandciycaattiothni sofp H
levSe.l ,aumreauksi nfogutnhdem ins auciitdaibcl eenfovrirtohnemeerandt.i cationofS.aureusfoundinacidicenvironment.

Figure 4. Structures of select fluoroquinolones.
FFiigguurree 44.. SSttrruuccttuurreess ooff sseelleecctt fflluuoorrooqquuiinnoolloonneess..

Figure 5. Structures of two recently approved quinolones and one in clinical development.
Figure 5. Structures of two recently approved quinolones and one in clinical development.
Figure5.Structuresoftworecentlyapprovedquinolonesandoneinclinicaldevelopment.
The comprehensive knowledge of the SAR of FQs (Figure 6) is central to the optimization of this
claTssh oe fc dormupgsr e[h43e,n6s1i–v6e3 k].n Tohwe lpedhagrem oaf ctohpeh SoAriRc gorfo FuQps o (fF qiguuinroe l6o)n iess cheanst rbaele tno itdheen otipfiteimd iazsa ati ocenn otrfa tlh is
ThecomprehensiveknowledgeoftheSARofFQs(Figure6)iscentraltotheoptimizationofthisclass
clabsisc yocfl idcr ruinggs w[4i3th,6 h1y–6d3ro].g Tenh ea tp phoasrimtioanc-o2p, ah ocarircb ogxryolu gpr oouf pq auti npoosloitnioens- h3 aasn db eae kne tiod egnrotiufipe dat apso asi tcieonnt-ral
ofdrugs[43,61–63].Thepharmacophoricgroupofquinoloneshasbeenidentifiedasacentralbicyclicring
bic4y. cTlihci sr iins gk nwoiwthn h aysd trhoeg qeuni nato lpoonsei tcioorne- 2(F, aig cuarreb 1o)x aynl dg rito cuapn anto pt obsei atilotenr-e3d a innd a an yk ewtoa yg rwoiuthpo autt ploossiintigo n-
withhydrogenatposition-2,acarboxylgroupatposition-3andaketogroupatposition-4.Thisisknownas
potency [43,64]. However, quinazolinediones (Figure 1) that share similar structural homology but
4. This is known as the quinolone core (Figure 1) and it cannot be altered in any way without losing
thequinolonecore(Figure1)anditcannotbealteredinanywaywithoutlosingpotency[43,64].However,
lack the C3/C4 keto acid have now been demonstrated to be capable of overcoming quinolone
potency [43,64]. However, quinazolinediones (Figure 1) that share similar structural homology but
qurinesaizsotalninceed [i6o5n–e6s7(]F. iMguerceh1a)ntihstaict sshtuardeiessim reilvaeraslterdu ctthuarta lqhuoinmaozloolignyedbiuotnleasc kotvheercCo3m/eC r4eksiesttoanacceid vhiaa ve
lack the C3/C4 keto acid have now been demonstrated to be capable of overcoming quinolone
noawddbieteionndael mdroungs-ternatzeydmteo cboenctaapctasb mleeodfioatveedr cboym tihnegirq u“uinnoulsounael”re Csi7s tasunbcest[i6tu5e–n6t7 ][.5M,65e–c6h7a]n, iastnidc stthuadt ies
resistance [65–67]. Mechanistic studies revealed that quinazolinediones overcome resistance via
revseuablsetditutheanttsq uaitn apzoosliitnioend-i7o ngerseoavtleyr cionmflueernecseis tpanocteenvciya, asdpdeicttirounmal, darnudg -esnafzeytmy eocf oFnQtasc ts[4m3]e. dTiahteesde by
additional drug-enzyme contacts mediated by their “unusual” C7 substituent [5,65–67], and that
thesiurb“sutintuuesnutasl ”arCe 7cosnusbisdteitrueden ats[ 5th,6e5 p–6o7r]ti,oann dthatht abtinsudb tsot itthuee nsutsbautnpito Bs iotifo tnh-e7 DgrNeaAt-lyeniznyflmuee nccoemppoletexn, cy,
substituents at position-7 greatly influence potency, spectrum, and safety of FQs [43]. These
speacntdru cmha,nangdess aatf eCty-7o afnFdQ Cs-[84 3o].f Tqhueinseolsounbesst iatupepnetasr atroe pcloanys iad seirgendifaiscatnhte rpoolert iionn ththe attabrginedt ptoretfheeresnucbeus nit
substituents are considered as the portion that bind to the subunit B of the DNA-enzyme complex,
Booff tthheisD cNlaAss- eonf zdyrmugesc [o6m8]p. lex,andchangesatC-7andC-8ofquinolonesappeartoplayasignificantrole
and changes at C-7 and C-8 of quinolones appear to play a significant role in the target preferences
inthetargetpreferencesofthisclassofdrugs[68].
of this class of drugs [68].
Antibiotics2017,6,26 7of24
Antibiotics 2017, 6, 26 7 of 23

Figure 6. Structure-activity relationships of fluoroquinolones (adapted from reference [43]).
Figure6.Structure-activityrelationshipsoffluoroquinolones(adaptedfromreference[43]).
4. Non-Classical Antiproliferative Activity of Fluoroquinolones
4. Non-ClassicalAntiproliferativeActivityofFluoroquinolones
FQs were optimized and developed as antimicrobial agents, but several reports have shown that
FQs were optimized and developed as antimicrobial agents, but several reports have shown
their potentials might be more than just antimicrobial actions [69]. Some FQs have been reported to
that their potentials might be more than just antimicrobial actions [69]. Some FQs have been
display in vitro antiproliferative properties by inducing apoptosis, disrupting biochemical
reptroarntsefdortmoadtiisopnl aoyf inpovtietrnotiaalnlyti pcraonlicfeerroautisv ecepllrso, peenrthiaenscbiyngin tdhuec iunpgtaakpeo potfo soitsh,edri scrhuepmtinotghberioapcheuemtici cal
traangsefnotrsm, aantido/noro mfpedotiaetnintiga lilmymcaunncoermooudsuclealtlos,rye nrhesapnocninsegst [h7e0–u7p2t]a. k eofotherchemotherapeuticagents,
and/ormediatingimmunomodulatoryresponses[70–72].
4.1. Ciprofloxacin
4.1. Ciprofloxacin
The antitumor efficacy of ciprofloxacin has been attributed to both intrinsic apoptosis and cell
cycTleh eararnestti tuthmaot rcoeuffildca cbye orefvceirpsreodfl ouxpaocnin rehmasovbaele nof atthtrei bquuteindoltooneb o[t1h2]i.n Ittr inwsaics asphoowptno stihsaat nd
cecllipcryocflleoxaarcriens tintdhuactecdo ua ldtimbee- raenvde rsdeodseu-dpeopnenrdeemnot vgarlowoftht hienhqiubiitnioonlo, naend[1 2ap].opIttowsisa sofs hvoawrionutsh at
cipcarorcflinooxmacain, oisntedouscaercdomaat,i maned- laenudkedmoiase c-edlle pliennesd [e7n2t]. gTrhoew itnhhiibnihtiiobni toiof nm,aamnmdaalipaonp cteolsl igsroowf tvha briyo us
cacrcipinroofmloax,acoisnt evoisaa trhceo minad,uacntidonl eoufk teummioarc gelrlowlinthe sfa[7ct2o]r. (TThGeFi)n βh-i1b ibtiyo ncoolofnmica empmithaelliiaanl cceelllls gwroaws th
byrecpiporrotefldo xata ccionncveinattrhateioinn daus cltoiwon aos f1t0u μmgo·mrLg−r1o [w12th]. fTahcitso irs (aT GclFin)icβa-l1lyb aychcoielvoanbiclee cpointhceenlitarlatcieolnls inw as
rephuormteadn atitsscuoensc uensitnrga tsiotanndaasrldo wdoasisn1g0 reµggim·meLn −[112[]1. 2M].oTrehoivseirs, oaf calliln oicfa tlhlye dacifhfeierevnatb FleQcso tnhcaet nwtreartei on
tested in a cell free system, ciprofloxacin was the most potent inhibitor of mammalian DNA
in human tissues using standard dosing regimen [12]. Moreover, of all of the different FQs that
topoisomerase and polymerase [73]. Similarly, ciprofloxacin has been found to induce cell cycle arrest
were tested in a cell free system, ciprofloxacin was the most potent inhibitor of mammalian DNA
at the S and G2/M phases of androgen-independent carcinoma PC3, while sparing non-tumorigenic
topoisomeraseandpolymerase[73]. Similarly,ciprofloxacinhasbeenfoundtoinducecellcyclearrest
prostate epithelial cells [74]. This is particularly interesting because most chemotherapeutic agents
attheSandG2/Mphasesofandrogen-independentcarcinomaPC3,whilesparingnon-tumorigenic
that were used for treating advanced hormone resistant prostate cancer often result in 100%
prostateepithelialcells[74]. Thisisparticularlyinterestingbecausemostchemotherapeuticagents
mortality, with a mean survival time of 7–8 months, as well as several associated toxicities [74].
thatwereusedfortreatingadvancedhormoneresistantprostatecanceroftenresultin100%mortality,
withameansurvivaltimeof7–8months,aswellasseveralassociatedtoxicities[74].
4.2. Enoxacin
4.2. EnEoxnaocxinacin, a second-generation FQ, is one of the few FQs that retain the original 1,8-
naphthyridone core of nalidixic acid. It has been used to treat bacterial infections ranging from
Enoxacin,asecond-generationFQ,isoneofthefewFQsthatretaintheoriginal1,8-naphthyridone
gonorrhea to urinary tract infections [75] but has now been discontinued in the United States due to
coreofnalidixicacid. Ithasbeenusedtotreatbacterialinfectionsrangingfromgonorrheatourinary
its severe side effects of insomnia and ability to trigger seizures or lower seizure threshold [76,77].
Antibiotics2017,6,26 8of24
tractinfections[75]buthasnowbeendiscontinuedintheUnitedStatesduetoitsseveresideeffectsof
insomniaandabilitytotriggerseizuresorlowerseizurethreshold[76,77].Thehumanbreastcancercell
line,MCF-7,wasshowntobehighlyresponsivetotreatmentwithenoxacin,andgrowthinhibitionwas
dose-andtime-dependent,andirreversibleinnature[78].Thisisincontrasttociprofloxacin,whereits
effectcouldbereversedupontheremovalofthedrug[12].Aseparatestudyalsoshowedthatenoxacin
wasabletoenhancetheproductionofmicroribonucleicacids(miRNAs)thatsuppressesthetumorby
bindingtothemiRNAbiosynthesisprotein,trans-activationresponseRNA-bindingprotein-2(TRBP)[79].
MicroRNAsaresmallRNAmoleculesthatregulategeneexpressionatthepost-transcriptionalleveland
arecriticalformanycellularpathways,withtheirdisruptionoftenbeingassociatedwiththedevelopment
ofhumantumors[80–82].Unexpectedly,ofthetenFQsthatwereanalyzed,enoxacinwastheonlyone
thatwascapableofselectivelystimulatingmiRNAexpressionwithamedianeffectiveconcentrationof
~30µM[83]. Importantly,enoxacinwasfoundtoberelativelynon-toxicevenatahighconcentration
of 150 µM [83], which is lower than its clinical dose [84]. The effect of enoxacin was also observed
invivo using a transgenic mouse line, and its miRNA-enhancing activity was TRBP-dependent [83].
ModificationsandsubstitutionsattheN-1,C-6,andC-7positionsofenoxacinsignificantlyinterferedwith
itsmiRNA-enhancingactivity,suggestingthatthecompoundformsaspecificcomplexthatisdistinct
fromknowntargetsofquinolones[83]. ItalsohintsthatthemiRNAbiogenesis-enhancingactivityof
enoxacinmightprobablynotdependonthegeneralFQactivities,butratherontheuniquechemical
structureofenoxacin.Thecancer-selectivepropertiesofthismoleculeanditsproposedcellcyclearrestof
enhancingmiRNAmachinerycouldthereforerepresentauniquesteptowardsthepotentialapplication
ofmiRNA-basedtherapyinthetreatmentofhumancancer[79].
4.3. Moxifloxacin
Moxifloxacinisafourth-generationFQantibioticthatdiffersmainlyfromciprofloxacin,inthat
the piperazine moiety in ciprofloxacin has been replaced by a (1S,2S)-2,8-diazabicyclo[4.3.0]nonyl
moiety. Moxifloxacinisalsoavailableinophthalmicformulationsthatiswidelyusedasprophylaxis
for managing endophthalmitis after cataract surgery [85,86]. However, intraocular penetration of
moxifloxacinwasfoundtobeinefficientforpostoperativebacterialendopthalmitis,henceitsusevia
anintracameralrouteofadministration[87,88]. Basedonthisdifferentrouteofdrugdelivery,theeffect
andsafetyofmoxifloxacinonretinalganglioncells(RGC5)ofratswereexamined. Moxifloxacinwas
foundtoexhibitbothcytotoxicandanti-proliferativeactivityinvitroataconcentration>50µg/mL[89].
Althoughalittlehigherthantheexpectedprophylacticconcentrationof≤50µg/mL,anapparentlack
oftoxicitytoothernormalcellularactivities[87,90]revealedtheprospectsofmoxifloxacintotreat
glaucomapatientswithanincreasedriskofganglioncelldamage,wheretheymaybeusedatsuch
concentrationadvisedly.
4.4. OtherQuinolones
SeveralnoveltetracyclicFQshavebeenfoundtodisplayanticancerpropertiesagainstvarious
human cells, such as breast cancer (MCF-7) and non-small cell lung cancer (A549), and were
non-toxic to normal human-derm fibroblasts (HuDe) [68]. For example, 6,8-difluoroquinolones
have been shown to be potent effectors of eukaryotic topoisomerase, as evident in the increased
levelofcleavageintermediateswithoutimpairingtheDNAreligationreactionoftheenzyme[91].
Oneofthedifluorocompoundsexamined,CP-115,953,(Figure7)wastwiceaspotentasetoposide
at enhancing Top 2-mediated DNA cleavage in eukaryotic cells, while retaining potency against
DNAgyrase[92]. Tasquinimod(Figure7),anovelsmall-moleculeinhibitorandsecond-generation
oralquinolone-3-carboxamide,hasalsobeenreportedtohaveanti-angiogenicpropertiesandtumor
growth-inhibitingactivitiesagainsthumanprostatecancer. Itisbelievedtomediateitsactivitiesvia
angiogenesisandimmunomodulation,withapotencyofabout30-to60-timesofitsstructuralanalog,
linomide[93,94].
Antibiotics2017,6,26 9of24
Antibiotics 2017, 6, 26 9 of 23

Figure 7. Structures of some fluoroquinolone-derivatives with enhanced antiproliferative activity.
Figure7.Structuresofsomefluoroquinolone-derivativeswithenhancedantiproliferativeactivity.
In summary, most FQs tend to induce nearly similar types of morphological alterations where
Insummary,mostFQstendtoinducenearlysimilartypesofmorphologicalalterationswhere
cells become rounded, detached, and show cell membrane blebbing, a typical morphological change
cellsbecomerounded,detached,andshowcellmembraneblebbing,atypicalmorphologicalchange
that signals the initiation of apoptotic processes [95].
thatsignalstheinitiationofapoptoticprocesses[95].
5. Topoisomerases as Targets for Fluoroquinolone Actions: Prokaryotes versus Eukaryotes
5. TopoisomerasesasTargetsforFluoroquinoloneActions: ProkaryotesversusEukaryotes
Topoisomerases are large proteins found in humans and bacteria, with functional sizes of their
Topoisomerasesarelargeproteinsfoundinhumansandbacteria,withfunctionalsizesoftheir
enzyme-complex assembly ranging from 70 to 400 kDa [96]. Since the circumference of an average
enzyme-complexassemblyrangingfrom70to400kDa[96]. Sincethecircumferenceofanaverage
mammalian cell nucleus is almost one million times smaller than the length of the genome that needs
mammaliancellnucleusisalmostonemilliontimessmallerthanthelengthofthegenomethatneedsto
to be packed into it, DNA compaction is necessary to squeeze these base pairs into the nucleus.
bepackedintoit,DNAcompactionisnecessarytosqueezethesebasepairsintothenucleus.Specifically,
Specifically, the entire genome of a single human cell (3 × 109 base pairs, corresponding to
theentiregenomeofasinglehumancell(3×109basepairs,correspondingtoapproximately1.8m)
approximately 1.8 m) needs to be squeezed into a nucleus with an average diameter of 6 μm [96].
needstobesqueezedintoanucleuswithanaveragediameterof6µm[96]. Eventhesmallercircular
Even the smaller circular E. coli genome (4.7 × 106 base pairs) needs to be compacted within the
E.coligenome(4.7×106basepairs)needstobecompactedwithinthenucleoidspaceofbacteriawhose
nucleoid space of bacteria whose average circumference is 3000 times smaller [96]. The primary
averagecircumferenceis3000timessmaller[96]. Theprimaryfunctionoftopoisomerasesistherefore
function of topoisomerases is therefore to introduce positive and/or negative supercoil during DNA
tointroducepositiveand/ornegativesupercoilduringDNAtranscriptionandreplicationtoavoid
transcription and replication to avoid super-helical tension and knots. Both human and bacteria
super-helicaltensionandknots. BothhumanandbacteriatopoisomerasesaredividedintotypeIandII,
topoisomerases are divided into type I and II, each with further subdivisions (Figure 8). Surprisingly,
eachwithfurthersubdivisions(Figure8). Surprisingly,despitelittleornosequencehomology,both
despite little or no sequence homology, both type IA and type IIA from prokaryotes and the type 2A
typeIAandtypeIIAfromprokaryotesandthetype2Aenzymesfromeukaryotessharestructural
enzymes from eukaryotes share structural folds that appear to reflect functional motifs within the
foldsthatappeartoreflectfunctionalmotifswithinthecriticalregionsoftheenzymes[97]. Several
critical regions of the enzymes [97]. Several excellent reviews have been published on DNA
excellent reviews have been published on DNA topoisomerases and their inhibitors [98–102]. FQs
topoisomerases and their inhibitors [98–102]. FQs are safe in humans because they can discriminate
aresafeinhumansbecausetheycandiscriminatebetweenprokaryoticandeukaryotictopoisomerase
between prokaryotic and eukaryotic topoisomerase targets. Whereas, gyrase and Top IV appear to
targets. Whereas,gyraseandTopIVappeartobethepreferentialtargetsofFQsinGram-negativeand
be the preferential targets of FQs in Gram-negative and Gram-positive bacteria, respectively [69,103],
Gram-positivebacteria, respectively[69,103], otherstudiesindicatethattheprimarytargetsofFQs
other studies indicate that the primary targets of FQs are more drug-dependent than Gram-
aremoredrug-dependentthanGram-classification-dependent[104–106]. However,bothprokaryotic
classification-dependent [104–106]. However, both prokaryotic enzymes can be targeted
enzymescanbetargetedsimultaneouslyduetothehighdegreeofhomologybetweenthem. Inhibition
simultaneously due to the high degree of homology between them. Inhibition of eukaryotic Top 2
ofeukaryoticTop2correlateswithcytotoxicityinthosecells,andclinically-relevantFQsareknown
correlates with cytotoxicity in those cells, and clinically-relevant FQs are known to be safe and
to be safe and tolerated at concentrations that far exceed their cytotoxic threshold without bearing
tolerated at concentrations that far exceed their cytotoxic threshold without bearing cytotoxic,
cytotoxic,genotoxicorcarcinogenicpotential. SinceFQsalsomediateantitumoractivitiesinhumansat
genotoxic or carcinogenic potential. Since FQs also mediate antitumor activities in humans at
clinicallyachievableconcentrations,howtheyperformtheseextendedrolesisunclear.
clinically achievable concentrations, how they perform these extended roles is unclear.
5.1. ProkaryoticTopoisomeraseTypeIIA
5.1. Prokaryotic Topoisomerase Type IIA
Incontrasttomostanti-infectivedrugs,quinolonesdonotkillbacteriabyinterferingwithacritical
In contrast to most anti-infective drugs, quinolones do not kill bacteria by interfering with a
cellular process, rather, they corrupt the activities of two essential enzymes: DNA gyrase and Top
critical cellular process, rather, they corrupt the activities of two essential enzymes: DNA gyrase and
IV, and subsequently induce them to kill cells by generating high levels of double-stranded DNA
Top IV, and subsequently induce them to kill cells by generating high levels of double-stranded DNA
breaks[5,96,107–111]. Ordinarily, therelaxedbacterialDNAistoolongtofitinsideabacterialcell,
breaks [5,96,107–111]. Ordinarily, the relaxed bacterial DNA is too long to fit inside a bacterial cell,
andmustthereforebefoldedandcompacted.Supercoiling(overwindorunwind)isanessentialprocess
and must therefore be folded and compacted. Supercoiling (overwind or unwind) is an essential
duringchromosomecompaction,butalsoduringtranscription,replication,andDNArepair[111]. Due
process during chromosome compaction, but also during transcription, replication, and DNA repair
tothestrainthatisgeneratedduringunwinding,DNAgyrasetransientlynickseachchromosomal
[111]. Due to the strain that is generated during unwinding, DNA gyrase transiently nicks each
chromosomal domain, introduces a negative supercoil, and rapidly seals the nicked DNA before Top
Antibiotics2017,6,26 10of24
Antibiotics 2017, 6, 26 10 of 23
domain,introducesanegativesupercoil,andrapidlysealsthenickedDNAbeforeTopIVseparates
tIhVe sleipnakreadteds atuhge hlitnekreDdN dAaumghotleerc uDlNesAa fmteorlerecuplleicsa atifotenr. rBepoltihcaDtiNonA. Bgoytrha sDeNaAnd gyTorapseI Vanadre Tcolpa sIsVif iaerde
aclsaspsriofikeadr ayso tpircoTkoapryIoIti(cF Tigoupr IeI (8F),igaunrde 8t)h, eayndp ltahyeyt wploaye tswseon etisaslenrotilaels :ro(lie)s:s (ui)p seurcpoeirlcinogilin(ign v(ionlvvoeldveind
cinh rcohmroosmoomseomrepe lirceaptiloicna)taionnd) (iain)dde c(iait)e ndaetciaotnen(iantvioonlv e(idnivnoclhvreodm ions ocmhreopmaortsiotimonei npgasrutictihoansintogp soulocghi caasl
rtoespoolluotgioicnala rnedsotoluptoiognra apnhdic taolpsoeggrraepghatiicoanl )se[9g6r]e.gCatriyosnt)a l[9s6tr]u. Cctruyrsetsals hstorwuctthuaretsq suhinoowlo tnheast bqiunidnotloonthees
tbeirnnda rtoy tchoem tperlenxaersy fcoormmpedlebxeestw foeremneTdo pbeIItwaneednt hTeopsu IpI earncdoi tlehde DsuNpAer,choeilnecde ,DsNtaAbi,l ihzeinngcea, sptraobcieliszsinthga at
sphroouceldss htahvaet sohthoeurlwd ihsaevbee eonthveerwryistera bneseienn vte[r1y0 8t–r1a1n0s,i1e1n2t ][.1T0h8–is11s0ta,1b1il2i]z.a Ttihoins esftfaebcitliivzaeltyioinm epffeedcetisvtehlye
nimorpmedalesb rethaek inngo-rpmasasl inbgr-eraeksienagli-npgaspsrioncge-srseesseaolfinthge pbraoccteersisaelsD oNf Athbey cboancvteerritailn gDtNopAo isboym ceoransveesritnintog
ptohpyosiisoolomgeircaasletso xiinntso. pThhyesieoxltoegnisciavle tDoxNinAs. frTahgem eexntteantsioivne thDeNreAfo rferabgemcoemnteastioonv erthwehreeflmorien gbefcoormthees
boavcetrewrihaellrmepinaigr fmore cthhaen biasmctesr,itahlu rsepleaaidr imngecthoabnaicstmersi,a thceulsl dleeaadthin[g1 1to0, 1b1a1c]t.eria cell death [110,111].

Figure 8. Classification of topoisomerases and their poisons.
Figure8.Classificationoftopoisomerasesandtheirpoisons.
Several theories have been propounded as the molecular basis by which quinolones interact with
Severaltheorieshavebeenpropoundedasthemolecularbasisbywhichquinolonesinteractwith
DNA-enzyme complex. One model proposes that quinolones interfere with DNA gyrase activity by
DNA-enzymecomplex. OnemodelproposesthatquinolonesinterferewithDNAgyraseactivityby
selectively and directly interacting with the substrate DNA, rather than to gyrase [113–115]. This
selectively and directly interacting with the substrate DNA, rather than to gyrase [113–115]. This
implies that the enzyme only plays a passive role in the inhibition mechanism [113]. However, the
implies that the enzyme only plays a passive role in the inhibition mechanism [113]. However,
revised and widely accepted model is that quinolones do not bind directly to DNA, but to an enzyme-
the revised and widely accepted model is that quinolones do not bind directly to DNA, but to an
DNA complex [5,96,108–111]. The gyrase itself is believed to bind directly to the DNA, thereby
enzyme-DNAcomplex[5,96,108–111].ThegyraseitselfisbelievedtobinddirectlytotheDNA,thereby
creating a suitable pocket for quinolones to bind and stabilize the complex [116,117]. Stabilization of
creatingasuitablepocketforquinolonestobindandstabilizethecomplex[116,117]. Stabilization
this ternary complex prolongs the normal transient nicking of the chromosome, thus leading to an
ofthisternarycomplexprolongsthenormaltransientnickingofthechromosome,thusleadingto
extensive fragmentation of the DNA. The C3/C4 keto acid of quinolones (Figure 6) have long been
an extensive fragmentation of the DNA. The C3/C4 keto acid of quinolones (Figure 6) have long
known to chelate a divalent metal ion, but it was not until recently that this was captured in a crystal
been known to chelate a divalent metal ion, but it was not until recently that this was captured
structure of a ternary A. baumannii Top IV-cleaved DNA-moxifloxacin complex [112]. The chelated
in a crystal structure of a ternary A. baumannii Top IV-cleaved DNA-moxifloxacin complex [112].
Mg2+ ion appeared to be coordinated to four water molecules; two of which were situated close
ThechelatedMg2+ionappearedtobecoordinatedtofourwatermolecules;twoofwhichweresituated
enough to Ser84 and Glu88 (equivalent of GyrA Ser83 and Glu87 in E. coli) to form hydrogen bonds
closeenoughtoSer84andGlu88(equivalentofGyrASer83andGlu87inE.coli)toformhydrogen
[112]. The serine (Ser83 in E. coli) and acidic amino acid (Glu87 in E. coli) residues act as the anchor
bonds[112]. Theserine(Ser83inE.coli)andacidicaminoacid(Glu87inE.coli)residuesactasthe
points that coordinate the bridge to the enzyme. Mutations at these highly conserved residues in the
anchorpointsthatcoordinatethebridgetotheenzyme. Mutationsatthesehighlyconservedresidues
A subunit of Gyrase or Top IV are therefore the dominant mechanism of the resistance to FQs [5,64].
in the A subunit of Gyrase or Top IV are therefore the dominant mechanism of the resistance to
This supports the observation that water-metal ion interaction that bridges FQ to the enzyme plays a
FQs [5,64]. This supports the observation that water-metal ion interaction that bridges FQ to the
significant role in mediating quinolone activity [64]. The moxifloxacin binding model gives a
enzymeplaysasignificantroleinmediatingquinoloneactivity[64]. Themoxifloxacinbindingmodel
structural explanation for key quinolone structure-activity relationships (Figure 6) [3] and the bulky
gives a structural explanation for key quinolone structure-activity relationships (Figure 6) [3] and
substituent at position 7 of moxifloxacin was found to occupy a large and open pocket between the
the bulky substituent at position 7 of moxifloxacin was found to occupy a large and open pocket
DNA and the ParE subunit [112]. This is consistent with the C7 position being the most versatile
betweentheDNAandtheParEsubunit[112]. ThisisconsistentwiththeC7positionbeingthemost
position for substitution in quinolones [44], suggesting that the basic substituent at this position
versatile position for substitution in quinolones [44], suggesting that the basic substituent at this
possibly interacts with ParE/GyrB, thus explaining the effect of a ParE/GyrB mutation on quinolone
sensitivity [118]. On the contrary, a recent crystal structure of gyrase-FQ cleaved complexes from
Mycobacterium tuberculosis showed that moieties appended to the C7 position of FQs have no specific