New Horizons in Antifungal Therapy

24 Pages·2016·0.72 MB·English

Journal of
Fungi
Review
New Horizons in Antifungal Therapy
KailaM.Pianalto1andJ.AndrewAlspaugh1,2,*
1 DepartmentofMolecularGeneticsandMicrobiology,DukeUniversitySchoolofMedicine,Durham,
NC27710,USA;[email protected]
2 DepartmentofMedicine/InfectiousDiseases,DukeUniversitySchoolofMedicine,Durham,NC27710,USA
* Correspondence:[email protected];Tel.:[phone]
AcademicEditor:MaurizioDelPoeta
Received:18July2016;Accepted:20September2016;Published:2October2016
Abstract:Recentinvestigationshaveyieldedbothprofoundinsightsintothemechanismsrequiredby
pathogenicfungiforvirulencewithinthehumanhost,aswellasnovelpotentialtargetsforantifungal
therapeutics. Someofthesestudieshaveresultedintheidentificationofnovelcompoundsthatact
against these pathways and also demonstrate potent antifungal activity. However, considerable
effort is required to move from pre-clinical compound testing to true clinical trials, a necessary
step toward ultimately bringing new drugs to market. The rising incidence of invasive fungal
infectionsmandatescontinuedeffortstoidentifynewstrategiesforantifungaltherapy. Moreover,
theselife-threateninginfectionsoftenoccurinourmostvulnerablepatientpopulations. Inaddition
tofindingcompletelynovelantifungalcompounds,thereisalsoarenewedefforttoredirectexisting
drugsforuseasantifungalagents. Severalrecentscreenshaveidentifiedpotentantifungalactivity
in compounds previously indicated for other uses in humans. Together, the combined efforts of
academicinvestigatorsandthepharmaceuticalindustryisresultinginexcitingnewpossibilitiesfor
thetreatmentofinvasivefungalinfections.
Keywords: amphotericin; polyene; azole; echinocandin; flucytosine; aspergillosis; candidiasis;
cryptococcosis;fungalinfection;mycosis
1. Introduction
Fungalinfectionsareaworldwideglobalhealthproblem,affectingmillionsofpatientsperyear[1].
Ofthese,approximately1.5millionaredisseminatedorinvasivefungalinfections(IFIs),requiring
advancedtreatmentandhospitalization[1]. Unfortunately,thishighnumberofinfectionsisassociated
withhighmortalityrates,withsomefungalinfectionshavingmortalityratesnearing90%–95%[2,3].
A summary of some of the most common fungal diseases along with their rates of incidence and
mortality can be seen in Table 2. Worldwide, most IFIs are caused by the Candida, Cryptococcus,
Aspergillus and Pneumocystis species, although diseases caused by rarer fungi are becoming more
common. Additionally,thethermally-dimorphic,orendemic,fungi,whichtendtobemoreprevalent
inspecificgeographiczones,havehighratesofunreportedinfectionduetothefrequencyofsubclinical
infection[4,5].
Table1.Estimatedyearlyincidencesofinvasivefungalinfections.
EstimatedMortalityRates
FungalDisease EstimatedCasesperYear
(%ofInfected)1
Cryptococcosis >1,000,000[6] 20%–70%[6]
Candidiasis >400,000[7] 10%–75%[8]
Aspergillosis >200,000[9,10] 30%–95%[1,10–12]
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Table2.Estimatedyearlyincidencesofinvasivefungalinfections.
EstimatedMortalityRates
FungalDisease EstimatedCasesperYear
(%ofInfected)1
PneumocystisPneumonia >400,000[1,13] 20%–80%[13–15]
Mucormycosis(zygomycosis) >11,000[16] 30%–90%[16,17]
Endemic/DimorphicFungi2
Blastomycosis ~3000[4] <2%–68%[1,18,19]
Coccidioidomycosis ~20,000[20] <1%–70%[21]
Histoplasmosis ~25,000[22] 28%–50%[5]
Paracoccidioidomycosis ~4000[23] 5%–27%[23]
Penicilliosis >8000[1] 2%–75%[24,25]
1Estimatedmortalityrateswillvarywidelydependingontheimmunecompetenceofthehostandgeographical
region(adaptedfrom[1],withadditionalcitationslistedinthetable);2estimatedcasesinendemicregions.
BlastomycosisandhistoplasmosisareendemicintheMidwesternU.S.andtheMississippiandOhioRiver
Valleys,coccidioidomycosisintheSouthwesternU.S.,paracoccidioidomycosisinBrazilandpenicilliosisin
SoutheastAsia.
Generally, IFIs are infections of immunocompromised hosts. The standard definition of the
immunocompromisedhostisexpandingfromthetraditionalsetofpatientswithAIDS,patientswith
cancerwhoareundergoingimmunosuppressivechemotherapyortransplantpatientswhoseimmune
systemsaresuppressedtopreventorganrejection[26]. IFIscanalsobeseenduringtreatmentwith
newbiologically-activeagents,suchasTNF-αinhibitors,usedtotreatautoimmuneorinflammatory
diseases. Theseinhibitorsdampeninflammationandhelptreatdiseasesymptoms,buttheycanalso
leadtoopportunisticinfections[27]. Additionally,IFIsarealsoobservedinpatientswhoarehealthy
andapparentlyimmunocompetent,butwhohaveunderlying,asymptomaticconditionsthatmight
alterimmunefunctionandpredisposetowardinfection,suchasthepresenceofautoantibodiesagainst
cytokines,suchasGM-CSF[28].
Due to the high global health burden associated with fungal disease, the treatment of these
infectionsneedstobepotentandeffective.Indeed,manyofthecurrentlyavailableclassesofantifungal
drugsarehighlyeffectiveintheappropriatecontexts. However,thesedrugs, aswithanytherapy,
havelimitationsandcaveats. Forexample,thetoxicitiesassociatedwiththeuseofsomeantifungal
agents can be prohibitive toward use or must be accepted in order to effectively treat the patient.
Additionally,therearefewapprovedantifungalagentsinonlyfourdrugclassesforthetreatment
ofIFIs. Furtherlimitingisthesmallnumberoftargetsthatthesedrugsactupon,owingtothehigh
levelsofsimilaritybetweentheeukaryoticfungalpathogensandthehumanhosts. Forexample,ofthe
four classes of antifungal drug approved for treatment of invasive fungal infections, two of these,
polyenesandtheazoles,targetthesamecomponentofthefungalcellmembrane[29,30]. Thissmall
numberofcellulartargetsincreasesopportunitiesforfungitodevelopresistancetooneormoreof
theavailableantifungals. Furthermore,fewofthecurrentlyavailabledrugsareactuallyfungicidal.
Finally,severalofthesedrugshavelimitationsingeographicavailability,particularlyinareaswith
highratesoffungalinfections[31]. ThisleadstohighermortalityratesforIFIsduetotreatmentwith
lesseffectiveantifungaldrugs.
Moreover, the problem of antifungal resistance is on the rise: both that which has evolved in
formerlysensitivespecies,aswellastheprevalenceofintrinsically-resistantspeciesoffungi. Todate,
resistanceexiststoallofthecurrentlyavailableclassesofantifungalagent[32–36]. Candidaspecies
have a high prevalence of azole resistance, largely attributed to the cytostatic nature of these
drugs[37,38]. Similarly,AspergillusandCryptococcusstrainshaverecentlyalsodemonstratedazole
resistance [35,39–41]. Only a few years ago, echinocandins were considered effective therapy for
most clinically-relevant Candida isolates. However, with increased use of these antifungal agents,
echinocandin resistance in Candida species has also become more prevalent [36]. Additionally,
theintrinsicallydrug-resistantfungi,suchasScedosporiumspecies,continuetocauseabackground
of infections in highly immunosuppressed patients, especially those who are heavily treated with
J.Fungi2016,2,26 3of24
antifungals. Theseinfectionsareoftenassociatedwithpoorpatientoutcomes[42,43]. Duetothese
limitations,thereisanurgentneedfornewantifungalagents.
Researchgoalsfornovelantifungalagentshaveemphasizedafewmajorpoints. First,potencyis
akeycharacteristicofanoveldrug. Newdrugsmustbeabletoeffectivelycontrolfungalgrowthinthe
contextofthepatient,atcompoundlevelsthatarereadilyachievableatinfectionsites. Additionally,
idealnovelantifungalagentsshouldpossesslittletonohosttoxicity. Selectivityisalsocrucial,asthe
differencesbetweenthefungalpathogenandthehumanhostareevolutionarilymuchsmallerthan
thosebetweenbacterialpathogensandhumans. Ideally,novelagentswouldbebroadspectrumand
abletotreatmultiplespeciesoffungi. However,manyantifungalcompoundsthatareindevelopment
havepotent,butveryspecialized,activity.
2. AntifungalAgentsApprovedforClinicalUsage
Currently,therearefourmajorclassesofantifungaldrugsthatareindicatedforthetreatment
ofinvasivefungalinfections. Whenusedasindicated,thesedrugscanbehighlyeffectiveattreating
IFIs,withsignificantbeneficialeffectsonpatientmortality. Ashortsummaryofthesedrugsandtheir
primaryindicationsandusagescanbefoundinTable3.
Table3.Approvedantifungaldrugsforthetreatmentofinvasivefungalinfections.
Drug Indication
Polyenes
Life-threateningfungalinfections,includingcryptococcalmeningitis,
AmphotericinB
aspergillosis,blastomycosisandmucormycosis
Azoles
Fluconazole InvasiveinfectionsduetosusceptibleCandidaspecies;cryptococcosis
Blastomycosis,histoplasmosis,aspergillosisinpatientsrefractoryto
Itraconazole
AmphotericinB
Invasiveaspergillosis;non-neutropeniccandidiasis;serious
Voriconazole
ScedosporiumorFusariuminfectionsrefractorytootheragents
PreventionofinvasivefungalinfectionsinneutropenicorHSC1
Posaconazole
transplantrecipients
Invasiveyeastandmoldinfections,includingaspergillosis
Isavuconazole
andmucormycosis
Echinocandins
Caspofungin Candidemia;refractoryaspergillosis
Micafungin Candidiasis
Anidulafungin Candidiasis(adjunctivetherapywithvoriconazoleforaspergillosis)
Anti-metabolites
AdjunctivetherapyinCryptococcusneoformansmeningitisandCandida
Flucytosine
septicemiaandendocarditis(incombinationwithamphotericinB)
1HSC=hematopoieticstemcell.
2.1. Polyenes: AmphotericinBandItsderivatives
AmphotericinBanditsnewerlipidformulationsarepolyeneantifungalsthattargetthefungal
plasmamembrane. Recentmodelspositthatthesedrugsactas“sponges”thatbindtoandremove
ergosterolfromtheplasmamembrane,reducingmembraneintegrity[30,44]. Duetoitsmechanism
of action, amphotericin B is broad spectrum and indicated for the treatment of severe infections
caused by Candida species, Cryptococcus species, Zygomycetes and as an alternative therapy for
aspergillosis [45]. Amphotericin B is also used to treat many life-threatening IFIs due to other
filamentous molds, aswellas the thermally-dimorphic fungi, such as Histoplasma, Coccidioides
and Blastomyces. Amphotericin B is cytocidal for most fungi. As amphotericin B is not highly
bioavailablewhenadministeredorally,onlyintravenous(IV)formulationsareusedclinically.However,
amphotericinBcanhaveseveresideeffects,suchasnephrotoxicityduetooff-targetbindingofhost
J.Fungi2016,2,26 4of24
membranes,limitingitsusagetopatientswithlife-threateninginfections[46]. Newerformulationsof
thisdrug,suchasthelipid-associatedandliposomalformulations,demonstratemoreselectivefungal
targetingandlesshosttoxicity[46].
2.2. AzolesandTriazoles
Antifungalagentsintheazoleclasstargetthefungalplasmamembranethroughinhibitionofthe
biosynthesisofergosterol,afungalplasmamembranecomponentthatissimilartocholesterolfound
in mammalian cell membranes. This occurs through the inhibition of the sterol 14α-demethylase
(cytochrome P450 51 or CYP51), which catalyzes the final step in ergosterol biosynthesis [29].
The inhibition of this enzyme leads to defects in fungal plasma membrane integrity and cellular
integrity. Themostcommonly-usedazolesfortreatingIFIscanbefunctionallydividedbetweenagents
withprimaryactivityagainstyeast-likefungi(yeast-activeazoles),andthosewithexpandedactivity
againstfungithatoftengrowasmolds(mold-activeazoles).
Fluconazoleisthemostwidely-usedyeast-activeazole,anditisoftenveryeffectivefortreating
infectionscausedbyCryptococcusandCandidaspecies.Importantly,fluconazoleresistancecanpresenta
significantclinicalissueinsystemiccandidiasis:someCandidaspecies,suchasC.krusei,areintrinsically
resistanttothisdrug,andotherCandidaisolatesareoftensusceptibletothisdrugathighconcentrations.
Therefore,precisespeciesidentificationandtargetedantifungalsusceptibilitytestingforclinically-relevant
isolatesareveryimportantcomponentsofthecareofpatientswithCandidaIFIs.
The mold-active azoles include itraconazole, voriconazole, posaconazole and isavuconazole.
InadditiontoretainingactivityagainstCandidaandCryptococcusyeasts,theseagentsalsoinhibitmany
filamentousfungi. Itraconazolewasthefirstavailableazolewithsignificantactivityagainstmolds,
suchasAspergillusfumigatus. However,issueswithbioavailabilityandtoxicitylimititscurrentusefor
IFIs. Twoneweragents,voriconazoleandposaconazole,aremorewidelyusedfortheseinfections,
especiallyinhighlyimmunocompromisedpatients. Voriconazolehasbecomethefirst-lineantifungal
drugfortreatmentofinvasiveaspergillosisduetoAspergillusfumigatus. Comparativetrialsindicate
thatvoriconazoleissuperiortomanyotherantifungalagentsforthisinfection[47]. Posaconazoleis
indicatedforthepreventionofIFIs,especiallyinthesettingofprolongedneutropeniaafterhighdose
cancerchemotherapy. Theuseofthesedrugshaslikelygreatlyimprovedoutcomesinpatientswith
invasivemoldinfections. However,bothdrugshavethepotentialtointeractwithothermedications
duetotheirinhibitionofhepaticcytochromeP-450-dependentmetabolism.Moreover,manyazolescan
resultincardiacconductionchanges,andtheQTintervalshouldbemonitoredduringtherapy[48].
Isavuconazole(Cresemba®,AstellasPharmaceuticals,Tokyo,Japan)isthemostrecentlyapproved
triazole antifungal drug. It differs from other approved azoles in several clinically-relevant ways.
First, it has expanded invitro activity that includes the Mucorales molds (Zygomycetes), such as
Rhizopus, Mucor and Cunninghamella species, and it may, therefore, be an effective component of
the complex, medical-surgical treatment of mucormycosis [49]. Additionally, the intravenous (IV)
formulationofisavuconazolelackscyclodextrin,asolubilizingagentusedwithothertriazolesthatis
associatedwithnephrotoxicityinpatientswithrenalinsufficiency. Additionally,unlikeotherazole
drugsisavuconazoledoesnotappeartoexacerbateQTprolongation,anditmayactuallyshortenthe
QTintervalinsomepatients[50].
2.3. Echinocandins
Theechinocandinsrepresentthenewestclassofantifungals. Currently,threedrugsfromthisclass
areapprovedforclinicalusage: caspofungin,micafunginandanidulafungin. Echinocandinsaffectcell
wallbiosynthesisthroughthenoncompetitiveinhibitionofβ-1,3-glucansynthase[51,52]. Thisenzyme
isinvolvedinthebiosynthesisofoneofthemostabundantfungalcellwallcomponents. Therefore,
treatmentwithechinocandinsleadstodefectsinfungalcellintegrity. Thesedrugsareprimarilyused
forthetreatmentofinvasivecandidiasisandasanalternativetherapyfortreatmentofaspergillosis[53].
Echinocandins have low host toxicity and few drug interactions. However, they have no activity
againstCryptococcusspecies,andtheyaredecidedlypooragentsfortreatmentoftheendemicmycoses.
J.Fungi2016,2,26 5of24
Additionally,theyarenotorallybioavailable,likelyduetotheirlargemolecularsize,andso,areonly
availableinIVformulations.
2.4. 5-Fluorocytosine
5-Jfl. Fuunogri o20c1y6,t 2o, sx i ne (flucytosine) is a fluoridated pyrimidine analog, which inhibits5 Dof N23 A and
RNAsynthesisbyincorporatingintothegrowingnucleicacidchain,preventingfurtherextension.
2.4. 5-Fluorocytosine
Thisnucleicaciddamageeventuallyleadstocellulardefectsinproteinbiosynthesisandcelldivision.
Thisantifu5n-gflauloarogceynttoshinase (bfeluecnytaotstirnibe)u itse ad flwuoitrhidcaytetdo sptyartiimceidffineec tasnaanlodg,h wighhichra itnehsiboiftsr eDsNisAta anncde RdNevAe loping
duringsymntohnesoist hbeyr ainpcyo.rpTorhaetirnegf oinreto, tflhue cgyrtoowsiinnge niuscrleairce alycidu csheadina, spraevseinntginleg faugrethnetr feoxrtetnhseiontr. eTahtims ent of
nucleic acid damage eventually leads to cellular defects in protein biosynthesis and cell division. This
fungal infections. However, it has been shown in multiple clinical trials to be highly effective
antifungal agent has been attributed with cytostatic effects and high rates of resistance developing
in combination with amphotericin B for the treatment of cryptococcal meningitis [54,55]. Indeed,
during monotherapy. Therefore, flucytosine is rarely used as a single agent for the treatment of fungal
amphotericinBplusflucytosineisthefirst-linetreatmentforCryptococcuscentralnervoussystem(CNS)
infections. However, it has been shown in multiple clinical trials to be highly effective in combination
infectiwonitsh [a5m6p].hFotleurcicyinto Bs fionre thcea ntreaaltsmoebnet ouf scerydpitonccooccmalb minenaitniognitisw [5it4h,55o]t.h Inedreaendt, iafmunphgoatlesritcoint rBe paltuCs andida
infectioflnuscy,ttohsoinueg ihs tthheis fiirssat-llienses trceoamtmmenotn foprr aCcrtyipcteo.coAccduvs ecresnetreaflf necetrsvofousr flsyuscteymto (sCinNeSi)n icnlfuecdtieonbso n[5e6]m. arrow
toxicitFy,luecsyptoesciinael lcyanin altshoe bep uresesden inc ecoomfbriennaatiloinm wpitahi romtheenr at.nHtifouwngeavlse tro, torneaet oCfanthdiedat rinufleyctliiomnsi,t itnhogufgahc torsof
this is a less common practice. Adverse effects for flucytosine include bone marrow toxicity,
thisdrugisitslimitedavailabilityincountrieswiththehighestincidenceofcryptococcosis[31]. This,
especially in the presence of renal impairment. However, one of the truly limiting factors of this drug
unfortunately,limitstheeffectivenessofcryptococcalmeningitistherapyinthoseregionsoftheworld
is its limited availability in countries with the highest incidence of cryptococcosis [31]. This,
inwhichitismostprevalent,likelyincreasingratesofmortalityinthisdisease.
unfortunately, limits the effectiveness of cryptococcal meningitis therapy in those regions of the
world in which it is most prevalent, likely increasing rates of mortality in this disease.
3. AntifungalAgentsinDevelopmentwithNovelModesofAction
3. Antifungal Agents in Development with Novel Modes of Action
Inadditiontothecurrentlyapprovedantifungalmedications,novelcompoundsareinvarious
stages of cIlni naidcdailtiodne vtoe ltohep mcuerrnetntfloy rapthperovtreeda atmntiefnuntgoafl mIFeIdsi.caTtiohness,e nonveewl coamgepnotusndhsa avree ibne veanrioidues ntified
in largseta-sgceas loef, culinnbiciaals deedveslcorpemenenstf foorr athnet itfrueantgmaelnat cotfi vIFitIys., Tahsewsee nllewas aignenttasr hgaevtee dbeiennv iedsetnigtiafiteido nins based
large-scale, unbiased screens for antifungal activity, as well as in targeted investigations based on
on detailed studies of fungal-specific cellular processes. In this review, we discuss selected novel
detailed studies of fungal-specific cellular processes. In this review, we discuss selected novel
antifungalcompoundsthathaveeitherbeguntobeassessedinclinicaltrialsorthatrepresentnovel
antifungal compounds that have either begun to be assessed in clinical trials or that represent novel
biologicaltargetswithinfungi.Thesenewcompoundsarediscussedbasedontheirknownorpredicted
biological targets within fungi. These new compounds are discussed based on their known or
molecuplraerditcatregde mt,oillleucustlarar tteadrgient, Fililguustrreat1e.d Ainl tFhioguurgeh 1m. Aalnthyoiungvhe smtiagnayt oinrvseasrtiegaatlosros satrued aylsion gsthudoywintgo better
harneshsotwh etoh boesttteirm hmarunenses rtehsep hoonsts eimfmoruenfef ercestipvoenstree faotrm efefnecttiovfe ItFreIsa,tmweentw oifl lIFbIes, fwoceu wsiilnl gbeo fnoctuhseirnagp eutics
targeteodn athgearianpsetutthices ftuarnggeateldp aagtahiongste tnh.e Afunbgriael fpsauthmogmeanr. yA obfritehf esuamntmifaurny goaf lthaeg eannttisfutnhgaatl wagilelnbtse tdhaist cussed
will be discussed in this review, along with their activities based on minimum inhibitory
in this review, along with their activities based on minimum inhibitory concentration (MIC) and
concentration (MIC) and primary indications, can be found in Table 3.
primaryindications,canbefoundinTable4.

Figure1.Thisdiagramofafungalcellindicatesthesitesofactionforcurrentlyapprovedantifungal
Figure 1. This diagram of a fungal cell indicates the sites of action for currently approved antifungal
drugs(green)aswellasotheragentsinvariousstagesofdevelopment(blue).
drugs (green) as well as other agents in various stages of development (blue).
J.Fungi2016,2,26 6of24
Table4.Newantifungalagentsindevelopment.
AntifungalCompound Indication(s)1 Activity(MIC) References
Cryptococcusneoformans 4µg/mL
AR-12 [57]
Candidaalbicans 4µg/mL
Cryptococcusneoformans 0.25–8µg/mL
Cryptococcusgattii 0.5–2µg/mL
Candidaglabrata 0.125–>32µg/mL
BHBM [58]
Blastomycesdermatitidis 0.5–1µg/mL
Histoplasmacapsulatum 0.125–1µg/mL
Pneumocystisjirovecii 0.072–0.912µg/mL2
Candidemia3,* ≤0.008–2µg/mL4
CD101 [59–61]
Aspergillusspecies5 ≤0.008–0.03µg/mL4
Aspergillusspecies5 ≤0.008–0.25µg/mL
Candidaspecies6 ≤0.002–0.25µg/mL
E1210/1211 [62–65]
Scedosporiumspecies 0.03–0.25µg/mL
Fusariumspecies 0.015–0.25µg/mL
F901318 Aspergillusspecies5 <0.03µg/mL [66]
Candidaspecies7 0.01–5µg/mL
IlicicolinH Aspergillusfumigatus 0.08µg/mL [67]
Cryptococcusneoformans 0.2–1.56µg/mL
NikkomycinZ Coccidioidomycosis* 0.125µg/mL [68]
Cryptococcusneoformans <0.05µg/mL
Candidaalbicans 3.1µg/mL
Sampangine Candidaglabrata 3.1µg/mL [69]
Candidakrusei 6.2µg/mL
Aspergillusfumigatus 6.2µg/mL
Invasivecandidiasis* 0.03–2µg/mL8
SCY-078 [70–73]
Aspergillusspecies5 0.03–0.25µg/mL4
Sertraline Cryptococcusspecies* 2–6µg/mL4 [74]
Candidaspecies9 0.00025–0.0078µg/mL
Cryptococcusneoformans 0.0039–0.0625µg/mL
T-2307 Aspergillusspecies5,10 0.0156–2µg/mL4 [75–77]
Fusariumsolani 0.125µg/mL
Mucorracemosus 2µg/mL
Cryptococcusneoformans 64µg/mL
Tamoxifen Candidaalbicans 32µg/mL [78]
Candidaglabrata 8µg/mL
Invasiveaspergillosis* 1–4µg/mL4,9
Candidaglabrata ≤2µg/mL
VL-2397 [79]
Candidakefyr ≤2µg/mL
Cryptococcusneoformans ≤2µg/mL
Cryptococcalmeningitis* <0.0001–0.25µg/mL4
VT-1129 [80–82]
Candidaspecies11 <0.0001–1µg/mL
VT-1598 Coccidioidomycosis NA12 NA
1 Indication for agents in clinical trials, denoted by *. For preclinical agents, organisms for which there
is significant antifungal activity are listed. 2 IC50 = 50% inhibition. MIC not determined due to culture
conditions. 3 IncludesCandidaalbicans,C.glabrata,C.tropicalis,C.krusei,C.parapsilosis,C.dubliniensisand
C.orthopsilosisisolates. 4 RepresentsMIC90,orMICfor≥90%ofisolatestested,withoccasionalresistant
isolates observed. 5 Includes Aspergillus fumigatus, A. terreus, A. flavus and A. niger isolates. 6 Includes
Candidaalbicans,C.glabrata,C.tropicalis,C.parapsilosisandC.dubliniensisisolates.7IncludesCandidaalbicans,
C. glabrata, C. guillermondii, C. krusei, C. lusitaniae and C. parapsilosis isolates. 8 Includes Candida albicans,
C.glabrata,C.tropicalis,C.parapsilosisandC.kruseiisolates.9IncludesCandidaalbicans,C.dubliniensis,C.glabrata,
C.guillermondii,C.krusei,C.parapsilosisandC.tropicalisisolates. 10IncludesAspergillusfumigatus,A.flavus,
A.terreusandA.nidulansisolates.11IncludesCandidaalbicans,C.glabrataandC.kruseiisolates.12NA:MIC
informationnotpubliclyavailable.
J.Fungi2016,2,26 7of24
3.1. CellMembraneasAntifungalTarget
3.1.1. VT-1129,VT-1598andVT-1161(ViametPharmaceuticals)
VT-1129,VT-1598andVT-1161arecompoundsinanovelclassofmetalloenzymeinhibitorsthat,
liketheazolesandtriazoles,inhibittheenzymeresponsibleforthefinalstepofergosterolbiosynthesis:
thefungalsterol14α-demethylase(CYP51). Thesecompoundswereidentifiedaspartofaneffortto
decreaseoff-targetbindingofhumanCYPenzymes,includinghumanCYP51. Thishasbeenachieved
byidentifyingmolecules,liketheViametcompounds,thataremorespecificforthefungalenzyme
active site. Additionally, unlike the older generation azoles, whose high affinity for heme groups
ledtooff-targetinhibitionofhumanCYPenzymes,theViametcompoundshaveloweraffinityfor
heme[83]. ThesefeaturesallowVT-1129andVT-1161toselectivelyinhibitfungalCYP51overhuman
CYP51. VT-1129isapproximately3000-foldmoreselectivefortheCryptococcusisoformsofCYP51over
humanCYP51invitro,whileVT-1161isgreaterthan1000-foldmoreselectivefortheCandidaenzyme.
Theoretically, thisincreasedfungalselectivitymaydecreasetheriskfortoxicityathigherdosesof
drug[81,84].
These compounds, like many of the azoles, are available in oral and intravenous forms.
VT-1129 inhibits the growth of many Cryptococcus isolates, including both C. neoformans and
C.gattii[80,81,85].Additionally,inamousemodelofcryptococcalmeningitis,treatmentwithVT-1129
ledtodose-dependentclearanceofCryptococcusfromthebrain,performingbetterthanfluconazoleat
similardrugconcentrations[86]. Therefore,VT-1129hasbeengrantedQualifiedInfectiousDisease
Product (QIDP) designation, allowing expedited review for approval. VT-1129 is currently in
phase1clinicaltrialsforthetreatmentofcryptococcalmeningitis.
VT-1598 is in preclinical development for treatment of coccidioidomycosis. Additionally, as
part of Viamet’s extended platform, VT-1161, which has been shown to be effective against
fluconazole-resistantCandidaisolates,isinphase2bclinicaltrialsforthetreatmentofonychomycoses
andrecurrentvulvovaginalcandidiasis[87].
3.1.2. InhibitionofMembrane-AssociatedLipids
N(cid:48)-(3-bromo-4-hydroxybenzylidene)-2-methylbenzohydrazide (BHBM) and 3-bromo-N(cid:48)-(3-
bromo-4-hydroxybenzylidene)benzohydrazide(D0)representanewclassofantifungalcompounds,
termed“hydrazycins”. Theseagentswereidentifiedinascreenofsyntheticcompoundsinhibiting
sphingolipid biosynthesis in C. neoformans, a process demonstrated to be required for fungal
growth invivo [58]. These compounds inhibit vesicle trafficking of precursor lipids, such as
ceramides, to the cell surface, thereby inhibiting glucosylceramide biosynthesis and cell division.
Additionally,thesecompoundsspecificallyinhibitfungal,butnothuman,glucosylceramidesynthesis,
suggestingfungal-specificcellularinhibitionforthisnovelclassofcompounds.
BHBM and D0 were identified in a screen for antifungal activity against C. neoformans at
alkaline,butnotacidicpH[88]. BHBMshowedpromisinginvitroinhibitoryactivityagainstmultiple
isolates of two Cryptococcus species (C. neoformans and C. gattii), as well as Histoplasma capsulatum,
Blastomycesdermatitidis,PneumocystismurinumandPneumocystisjirovecii. Notably,thestrainstested
includedfluconazole-resistantCryptococcusstrains. InvivoactivityforBHBMandD0wasassessedin
murinemodelsofinfectionduetoCryptococcusneoformans,CandidaalbicansandPneumocystismurinum,
resultinginsignificantincreasesinsurvivaltimescomparedtocontrols. Additionally,thesedrugs
showedsynergywithbothfluconazoleandamphotericinB,suggestingpotentialforcombinatorial
therapy. These hydrazycins were well tolerated in animal models of invasive fungal infection,
thoughsomeinteractionwasobservedwiththeimmunosuppressivecorticosteroiddexamethasone,
whichresultedinadecreasedcompoundhalf-lifeinvivo[58].
J.Fungi2016,2,26 8of24
3.2. CellWallSynthesisInhibitors
3.2.1. CD101(Biafungin)(CidaraTherapeutics)
CD101,orbiafungin,isanovelechinocandinformulatedforbothintravenousandtopicaluse.
It is similarly effective when compared to anidulafungin and caspofungin against Aspergillus and
Candidaspeciesinvitro[59–61]. However,theadvantageofCD101overexistingechinocandindrugs
lies in its pharmacokinetics. The half-life of this drug is 81 h invivo. In contrast, the half-life of
anidulafungin, thelongest-actingechinocandintodate, isapproximately24h[89,90]. Thisallows
biafungintopotentiallybeadministeredwithonce-weeklyintravenousdoses,ratherthandailydoses,
betterfacilitatingpatientcareandcompliancewhilepotentiallydecreasingdrugadministrationcosts.
Biafungin,likeotherechinocandins,demonstratesfewdruginteractionsandanexcellentsafetyprofile.
Thisdrugiscurrentlyinphase2clinicaltrialsforthetreatmentofcandidemia.
3.2.2. SCY-078(Scynexis)
SCY-078isanovelβ-1,3-glucansynthaseinhibitorthatisstructurallydistinctfromthecurrently
availableechinocandinglucansynthaseinhibitors. Itisderivedfromenfumafungin,anovelnatural
product. Thereby, SCY-078 is a first-in-class, orally-available β-1,3-glucan synthase inhibitor that
has received QIDP designation. An intravenous formulation is also in development. SCY-078
showsinvitroactivityagainstisolatesofCandidaandAspergillusspeciesatMICorMEC(Minimum
EffectiveConcentration)levelsbelow0.5µg/mL,includingseveralfluconazole-resistantstrains[70,72].
Additionally,SCY-078remainsactiveagainstcertainechinocandin-resistantstrainsofCandidaand
Aspergillus [71,73]. When SCY-078 was tested invitro against non-Aspergillus molds, SCY-078 was
the only β-1,3-glucan synthase inhibitor with activity against the notoriously pan-resistant mold
Scedosporiumprolificans[91]. Inamurinemodelofinvasivecandidiasis,treatmentwithSCY-078led
toadose-dependentdecreaseinfungalburdeninthekidneysacrossCandidaspecies[72]. Thisdrug
iscurrentlyinphase2clinicaltrialsinitsoralformulationforthetreatmentofinvasivecandidiasis,
whiletheIVformulationisinphase1clinicaldevelopment.
3.2.3. NikkomycinZ(UniversityofArizona)
NikkomycinZisanolderdrugthathasresurfacedrecentlyduetoanincreasedinterestinanti-cell
wallantifungaldrugs. NikkomycinZisacompetitiveinhibitorofchitinsynthases,actingtodecrease
cellwallstability. Recently, itreceivedOrphanDrugStatusforitsdevelopmentasatreatmentfor
coccidioidomycosis. Inthepast,thisdrugshowedpromiseagainstthethermally-dimorphicfungi,
including Coccidioides immitis, Histoplasma capsulatum and Blastomyces dermatitidis [68]. However,
the clinical development of this drug was terminated due to difficulties in production. The drug
wasre-licensedtotheUniversityofArizona,allowingforthereinstitutionofclinicaldevelopment.
NikkomycinZiscurrentlybeingtestedinphase1/2clinicaltrialsfortreatmentofcoccidioidomycosis.
TherenewedinterestinNikkomycinZreflectsmomentumexploringthefungalcellwallasan
antifungaltargetingeneral. Thisfungal-specificcellularfeaturemakesforanidealtargetforlesstoxic
antifungaldrugdevelopment.
3.3. MitochondriaasanAntifungalTarget
3.3.1. T-2307(ToyamaChemicals,Tokyo,Japan)
T-2307 is a novel arylamidine compound that inhibits fungal growth by interfering with
fungalmetabolism. Thiscompoundspecificallycollapsesfungalmitochondrialmembranepotential,
whichpreventsfungifromperformingcellularrespiration,thuscompromisingenergyproduction
for essential cellular processes [92]. Furthermore, this anti-mitochondrial activity is specific to
fungi,andthisdrugdoesnotcollapsemammalianmitochondrialmembranepotentialatveryhigh
concentrations. Themechanismforthisselectivityhasbeenpositedtobeduetoselectiveuptakeof
J.Fungi2016,2,26 9of24
thisdrugbythehigh-affinity,fungal-specificAgp2spermine/spermidinetransporter[93]. T-2307,
likemanyavailableantifungaldrugs,hasafungistaticmechanismofaction. However,ithaspotent
invitroactivityagainstCandidaspecies,Cryptococcusneoformans,AspergillusspeciesandFusariumsolani,
includingechinocandin-resistantCandidaisolates[75–77]. TheinvitroactivityofT-2307iscomparable
to the activity of voriconazole and micafungin for Aspergillus species and more effective at lower
concentrationsofdrugthanfluconazole,voriconazoleandmicafunginagainstCandida,Cryptococcus
andFusarium[75]. Additionally,inamurinemodelofsystemiccandidiasis,T-2307performedaswell
asmicafunginoramphotericinB,butatlowerconcentrationsofcompound[75].
3.3.2. IlicicolinH
IlicicolinHactsonthemitochondriabyspecificallyinhibitingtheactivityofthecytochromebc1
complex. Thisinhibitionofenzymaticactivitydecreasesfungalmitochondrialrespiration,preventing
thebiosynthesisofATP.Basedoninvitrodata,fungalcytochromebc1is50-foldmoresensitiveto
ilicicolin H inhibition than the bovine enzyme and 1000-fold more sensitive than rat cytochrome
bc1[94,95]. Unfortunately,insomecases,resistancetothisdrugdoesoccur[67]. Yet,thismolecule
isapromisingstartingpointforfuturestructuralanalogswiththepotentialformorespecificityand
potencyagainstmitochondrialprocesses.
3.4. OtherMechanisms/UnknownMechanisms
3.4.1. VL-2397(Vical,SanDiego,CA,USA)
VL-2397representsanewclassofantifungalcompoundthathasreceivedQIDPdesignationfor
developmentfortreatmentofaspergillosis. Althoughthemechanismoffungalcellinhibitionhas
yettobedetermined,thiscompoundsexhibitssignificantinvitroactivityagainstAspergillusspecies,
Cryptococcusneoformans,Candidaglabrata,CandidakefyrandTrichosporonasahii,aswellasmodestactivity
againstFusariumsolani[79]. Additionally,VL-2397showsfungicidalactivityagainstAspergillusspecies.
However,thiscompoundhasnoactivityagainstotherCandidaspecies,includingC.albicans,oranyof
theMucoralesfungi[79]. Interestingly,theimportofthedrugintofungalcellsrequiresthesiderophore
transporterSit1. Asmammaliancellslackthistransporter,thismechanismofentrycouldresultin
specificityforthisdrugtowardfungalpathogens.
VL-2397haspotentinvivoeffectsagainstazole-refractoryAspergillusfumigatusinamurinemodel
ofinvasiveaspergillosis[96].Treatmentwiththisdrugresultedinasignificantimprovementinsurvival
ininfectedmicecomparedwithapprovedantifungaldrugs,suchasamphotericinB,posaconazole
and caspofungin alone. Additionally, it is effective in combination with posaconazole invivo,
andposaconazoledoesnotantagonizeVL-2397activity[96]. VL-2397iscurrentlyinphase1clinical
trialsforthetreatmentofinvasiveaspergillosis. Althoughitdemonstratesfairlynarrow-spectrum
activity, VL-2397 shows promise as a novel agent for combinatorial therapy, especially against
Aspergillusspecies.
3.4.2. AR-12(ArnoTherapeutics,Flemington,NJ,USA)
AR-12wasinitiallydevelopedasananticanceragentin2006. Thisdrugisapotentinhibitorofthe
phosphoinositide-dependentkinasePDK1inhumansandinducescell-deathpromotingendoplasmic
reticulumstress,inhibitingproliferativecellgrowth[97]. However,AR-12(OSU-03012)wasidentified
through a repurposing screen of protein kinase inhibitors looking for agents that induced fungal
celllysis. Thisdrugwastherebydemonstratedtohavepotentantifungalactivity[57]. Ithassince
beengrantedtheEuropeanOrphanDrugDesignationforthetreatmentofcryptococcosis. Thougha
strongPDK1inhibitorinhumans,thisdrugdoesnotinhibittheC.neoformansPDK1[98]. However,
based on haploinsufficiency profiling in S. cerevisiae and C. albicans (a microbial genetic technique
usedtotestforpotentialmechanismsofantifungalaction),itissuggestedthatAR-12inhibitsfungal
carbonmetabolism,specificallyacetyl-CoAsynthetase(ACS2)[98]. InhibitionofACS2leadstodefects
J.Fungi2016,2,26 10of24
in several cellular processes, including histone acetylation, ribosome assembly and regulation of
autophagy,inadditiontoitsrolesincarbonmetabolism[98]. AR-12showsactivityagainstC.albicans
biofilms, important structures in mucosal and catheter-associated infections. It is in preclinical
developmentasanorally-availableantifungalforthetreatmentofcryptococcosis.
3.4.3. F901318(F2GLtd.,Manchester,UK)
F901318 is a member of the novel orotomide class of antifungal drug that is formulated for
bothintravenousandoraldelivery. Itinhibitspyrimidinebiosynthesisbyblockingdihydroorotate
dehydrogenaseactivity,preventingnucleotidebiosynthesis. F901318inhibitsthegrowthofAspergillus
species invitro, with MEC levels below 0.5 µg/mL [66]. It is even effective against azole- and
amphotericinB-resistantAspergillusstrains[66]. Inamurinemodelofinvasiveaspergillosis,F901318
treatmentledtoreducedgalactomannanlevelsinmice,whichhasbeenshowntobeassociatedwith
betterclinicaloutcomesinpatients[99,100]. F901318iscurrentlyinphase1clinicaltrialsassessingthe
safetyoftheIVformulation. Thusfar,ithasdemonstratedanexcellentsafetyprofileandwaswell
toleratedattherapeutically-relevantdoses[101].
3.4.4. E1210/1211
E1210 and E1211, which represent the active compound and its pro-drug, respectively,
are inhibitors of fungal, but not human, glycophosphatidylinositol (GPI) anchor biosynthesis.
This cellular process is required for the anchoring of proteins to both the fungal cell wall and cell
membrane [64]. The anti-GPI activity of E1210 is achieved through inhibition of the fungal Gwt1
protein, whichcatalyzesanearlystepinthecreationoftheGPIanchor, whilethehumanenzyme
appearsunaffected[64]. InC.albicans,treatmentwithE1210inhibitsgermtubeformation,aswellas
biofilm formation and adherence to plastics [64]. E1210 has activity against Candida, Aspergillus,
FusariumandevenScedosporiumspeciesinvitro,intheng/mLrangeforallstrainstested[62,65].E1210
alsoshowsactivityagainstazole-andechinocandin-resistantstrainsofCandida[102]. At1–2µg/mL,
theactivityappearstobefungicidal[63].
In a murine model of disseminated candidiasis, orally-administered E1210 had a significant,
beneficialsurvivaleffectinanimalsinfectedwithanazole-resistantstrainofC.albicans.Italsoincreased
survivalinmousemodelsofdisseminatedcandidiasisandpulmonaryaspergillosis,althoughthis
effectwasseenathigherconcentrationsthanthatrequiredforcaspofungin,liposomalamphotericin
Borfluconazole/voriconazole. E1210alsoaidedclearanceversusanuntreatedcontrolinamurine
modelofdisseminatedfusariosis[103].
3.4.5. Sampangine
Sampangine is a copyrine alkaloid natural product which inhibits heme biosynthesis
invivo [69,104,105]. Because of this activity, treatment with this compound leads to defects in all
heme-requiringpathways,includingcellularrespirationandergosterolbiosynthesis[69]. Thisnatural
product has very low MIC values for Cryptococcus neoformans (0.05 µg/mL), and it demonstrates
inhibitoryactivityatconcentrationsof3–6µg/mLforCandidaandAspergillusspecies[69]. Synthesisof
thisandotheralkaloidshasbeenindevelopmentinthesearchfornovelantifungalsandantimicrobials,
asspecifichemebiosynthesisinhibitioncouldproveapotenttargetforantifungaldevelopment[106].
Indeed,someanalogsthathavebeencreatedhavepotentinvitroactivityagainstC.neoformansand
A.fumigatus[107].
3.5. OldDrugs,NewTricks
Asdemonstratedabove,severalavenuesofresearchhaveidentifiednovelcompoundsorclasses
of compounds that havesignificant antifungal effects, with thepotential to develop toward novel
therapeuticstobeappliedinaclinicalsetting. However,giventheconsiderableresourcesrequiredto
developcompletelynovelantimicrobialdrugs,manyinvestigatorshavepursuedaparallelstrategyto