Chemotherapeutic Targets in Parasites

20 Pages·2002·0.15 MB·English

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Chemotherapeutic Targets in
Parasites
Contemporary Strategies
Tag E. Mansour
withtheassistanceof
JoanMacKinnonMansour
iii
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PUBLISHEDBYTHEPRESSSYNDICATEOFTHEUNIVERSITYOFCAMBRIDGE
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(cid:2)C TagE.Mansour2002
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noreproductionofanypartmaytakeplacewithout
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Firstpublished2002
PrintedintheUnitedStatesofAmerica
TypefacesBertholdGaramond10/13pt.andFrutiger SystemLATEX2ε [TB]
AcatalogrecordforthisbookisavailablefromtheBritishLibrary.
LibraryofCongressCataloginginPublicationData
Mansour,TagE.
Chemotherapeutictargetsinparasites:contemporarystrategies/TagE.Mansour.
p. cm.
Includesbibliographicalreferencesandindex.
ISBN0-521-62065-1(hc)
1.Antiparasiticagents. 2.Parasiticdiseases–Chemotherapy. I.Title.
RM412.M365 2002
616.9(cid:3)6061–dc21 2002067057
ISBN0521620651hardback
iv
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Contents
Preface pagexi
Acknowledgments xiv
1 TheSearchforAntiparasiticAgents 1
EarlyBeginningsofChemotherapy 1
PaulEhrlichandthePrincipleofSelectiveToxicity 2
RationalDiscoveryofAntiparasiticAgents 3
BiologyofParasites 4
BiochemicalAdaptationofParasitestotheHost 5
DrugReceptorSelectivity 6
MetabolicPathwaysasTargets 7
EmpiricalScreeningofAntiparasiticAgents 8
UseofAnimalModelsinScreening 9
UseofinvitroParasiteCulturesforScreening 9
DesigningChemicalsforSelectiveTargets 12
RelationshipbetweenChemicalStructureandBiological
ActivityofInhibitors 13
QuantitativeStructureActivityRelationship 13
CombinatorialTechnologyinDrugDiscovery 14
References 16
2 Biophysical,Genomic,andProteomicAnalysisofDrugTargets 19
BiophysicalTechniquesforStudyingDrugTargets 19
X-RayCrystallography 19
NuclearMagneticResonance 20
UseofRecombinantDNATechnologyforStudiesonDrugTargets 22
IdentificationofPotentialDrugTargetsandCandidateAntigens
forVaccinesUsingExpressedSequenceTags 23
v
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vi Contents
GenomicAnalysisofParasiticHelminths 24
CaenorhabditiselegansGenome 25
GenomeAnalysisofTrypanosomatids 25
GenomeAnalysisofPlasmodiumfalciparum 26
StructuralGenomics 27
Proteomics 28
References 30
3 EnergyMetabolisminParasiticHelminths:Targets
forAntiparasiticAgents 33
LifeCyclesofRepresentativeHelminths 33
Schistosomes(BloodFlukes) 34
TheLiverFlukeFasciolahepatica 34
TheIntestinalNematodeAscarissuum 34
EnergyMetabolismofS.mansoni 35
GlucoseTransporters 36
TheGlycolyticPathway 37
InhibitionofGlucoseUtilization 39
Phosphofructokinase(PFK) 40
RegulationofPFK 40
ActivationofPFKandGlycolysisbySerotonin 41
PFKasaTargetforAntischistosomalAntimonials 44
EnzymesInvolvedinPhosphoenolpyruvateMetabolism 46
MitochondrialAnaerobicEnergy-GeneratingPathways
inHelminths 46
PhosphoenolpyruvateCarboxykinase(PEPCK) 49
FumarateReductase 49
PyruvateDehydrogenaseComplex(PDC) 49
AnaerobicMitochondrialMetabolismasPossibleTargets
forAntiparasiticAgents 50
References 54
4 AntimalarialAgentsandTheirTargets 58
QuinineandQuinidine 60
Chloroquine 61
ConcentrationofChloroquineinPlasmodium
FoodVacuoles 62
HemePolymerization:ATargetforChloroquineAction 63
TheProteasesofHemoglobinDigestion 64
ResistancetoChloroquine 70
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Contents vii
AntimalarialsUsedagainstChloroquine-ResistantMalaria 74
HalofantrineandMefloquine 74
Artemisinin(Qinghaosu) 74
Quinine 76
Primaquine 76
AntifolatesandTheirTargets 77
DihydropteroateSynthase 77
DihydrofolateReductase-ThymidylateSynthase(DHFR-TS) 80
MalariaResistancetoPyrimethamine 82
ProteasesandMerozoiteEntranceandExitfromErythrocytes 83
AntisenseOligodeoxynucleotides 84
References 86
5 AntitrypanosomalandAntileishmanialTargets 90
TripanothioneandTripanothioneReductaseinProtozoa 91
TrypanothioneReductaseasaTargetforDrugDesign 93
BiosynthesisofTrypanothione 93
EffectofAntitrypanosomalAgentsontheTrypanothioneSystem 94
InhibitionofTrypanothioneReductasefromLeishmania
byTrivalentAntimonials 96
Tryparedoxins 97
AntiprotozoalEffectsofNifurtimox 97
PolyaminesandOrnithineDecarboxylase 98
OrnithineDecarboxylaseInhibitorsasAntiparasiticAgents 99
Glycolysis:ATargetforAntitrypanosomalAgents 101
TheGlycosome 101
GlucoseTransporters 104
PyruvateTransporters 104
α-GlycerophosphateOxidase 105
OtherGlycolyticTargets 105
SuraminandGlycolysis 106
Melarsoprol 107
MitochondrialDNAasaTargetforAntitrypanosomalAgents 107
PurineandPyrimidineMetabolismasTargets 108
PurineSalvagePathways 109
NucleosideTransporters 111
NucleosideRibohydrolases 112
PurineAnalogsasAntiparasiticAgents 112
PyrimidineMetabolismPathwaysasPossibleTargets 115
PteridinReductaseinLeishmaniamajorasaTarget 116
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viii Contents
AntigenicVariationinAfricanTrypanosomesandtheDesign
ofAntiparasiticAgents 117
ProteinFarnesylationasaDrugTarget 119
CysteineProteasesasTargetsforAntitrypanosomal
andAntileishmanialAgents 120
References 122
6 TargetsinAmitochondrialProtists 129
BiologyofAmitochondrialProtists 129
Entamoebahistolytica 130
Giardiaintestinalis(G.duodenalis) 130
Trichomonasvaginalis 131
CarbohydrateMetabolismofEntamoebahistolytica 131
EnergyMetabolismofGiardiaintestinalis 134
HydrogenosomesandEnergyMetabolisminTrichomonads 135
Pyruvate:FerredoxinOxidoreductase 137
MechanismofAntiprotozoalEffectsofMetronidazole
andRelated5-Nitroimidazoles 138
ResistancetoMetronidazole 141
PotentialsforNewDrugTargets 143
PyrophosphateKinases 143
AlcoholDehydrogenasesandIdentificationofAntiparasitic
DrugsbyComplementationofE.coliMutants 146
PFORasaTarget 147
ProteinasesasTargets 148
References 150
7 NeuromuscularStructuresandMicrotubulesasTargets 156
NeuromuscularPhysiologyofNematodes 156
NicotinicAcetylcholineReceptors(nAChRs) 157
NicotinicAcetylcholineReceptorsinNematodes 158
γ-AminoButyricAcid(GABA) 163
TheActionofPiperazine(Diethylenediamine)onGABAReceptors 164
GlutamateasaNeurotransmitterinNematodes 164
ActionofAvermectinsonGlutamate-GatedChlorideChannels 165
GlutamateReceptorsinthePharynxofNematodes 167
BioamineReceptorsinNematodes 169
MicrotubulesasaTargetforBenzimidazolesinNematodes 170
AntimicrotubuleDrugs 171
BenzimidazolesasAnthelmintics 173
ResistancetoBenzimidazoles 173
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Contents ix
NeuroactivePeptidesinNematodes 174
TheNeuromuscularSystemofFlatworms 175
EvaluationofMotilityofFlatworms 176
5-Hydroxytryptamine(Serotonin)inParasiticFlatworms 176
CholinergicSystemsinFlatworms 178
InfluenceofParasiteMotilityonParasiteLocationintheHost 178
AnthelminticsThatActonMotilityinPlatyhelminths 180
NeuropeptidesinFlatworms 180
PotentialResearchonAnthelmintics 181
References 183
8 TargetsintheTegumentofFlatworms 189
StructureandFunctionofSchistosomeTegument 189
SomeProteinsIdentifiedinSchistosomeTegument 192
ProteinAttachmenttotheTegument 193
GlucoseTransportacrosstheTegument 194
TegumentalAcetylcholineReceptors,Acetylcholinesterase,
andGlucoseUptakebySchistosomes 196
Metrifonate(Bilarcil)asAChEInhibitor
andAntischistosomalAgent 197
PotentialStudiesonTegumentalNicotinicTargets 198
Praziquantel(PZQ) 199
MechanismofActionofPraziquantel 199
AntigensThatAttracttheHost’sImmuneSystem 201
ResistanceofSchistosomestoPZQ 202
PotentialStudiesonCa2+ InfluxacrosstheTegument 204
Oxamniquine 205
DyneinsinSchistosomamansoni 206
TheEffectofTriclabendazoleonFasciolahepaticaTegument 207
DrugsActingontheTegumentofCestodes(Tapeworms) 208
References 209
Epilogue 215
References 219
Index 221
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1
The Search for Antiparasitic
Agents
Early Beginnings of Chemotherapy
Historicalrecordsfromalmosteveryculturehaveexamplesofsuccessfultreat-
ment of different ailments by potions, mainly from vegetable sources and oc-
casionallyfromanimals.Agoodexampleofanantiparasiticagentiscinchona
bark(quinine),whichwasusedagainstmalariabytheearlyPeruviansandintro-
ducedtotheWesternWorldbytheJesuits.Extractsofmalefernandofsantonin
obtainedfromArtemisiamaritimavar.anthelminticawereheldinhighregardas
anthelmintics by Theophratus (370–285 B.C.) and Galen (A.D. 130–201) and
were also recommended as effective anthelmintics until the 1940s and 1950s
(Goodman&Gilman,1955).
Thebeginningofchemotherapyasasciencehadtoawaittheestablishmentof
thegermtheoryofinfectionbyLouisPasteurmorethanacenturyago.Rather
thanacceptingtheconceptofspontaneousgenerationofinfection,Pasteursug-
gestedthatdiseasesarecausedbymicrobesthataretransmittedamonghumans.
Pasteur’sinspirationforthistheorycameduringhisearlystudiesonfermentation
forthewineindustryinFrance.Pasteur’scontributionsinfluencedbacteriology,
immunology, the study of antibiotics, sterilization, epidemiology, and public
health.Thisremarkableerawasfollowedbythediscoveryandidentificationof
many of the infectious microorganisms and methods for their culture in the
laboratory and for infecting experimental animals. The study of parasites was
initiated at the time of Sir Patrick Manson (1844–1922), who established the
first institute for studying and teaching about tropical diseases: The London
SchoolofHygieneandTropicalMedicine.Mansonwasaprominentresearcher
who made the seminal identification of the life cycle of the filarial nematode
Wuchereria bancrofti in humans and the mosquito vector. He emphasized the
importance of studies on tropical diseases and parasitology. The early part of
thetwentiethcenturysawtheidentificationofmanyofthemainparasitesthat
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2 TheSearchforAntiparasiticAgents
afflicthumansinthetropics.Theseincludedtrypanosomiasis(sleepingsickness),
leishmaniasis(kalaazar),lymphaticfilariasis,andmalaria.
Paul Ehrlich and the Principle of Selective Toxicity
ThefatherofchemotherapywasPaulEhrlich(1854–1915),aphysicianinitially
interested in immunology. He won the Nobel Prize in 1908 for his work in
the field of immunology following Emil Behring (1901), Ronald Ross (1902),
andRobertKoch(1905).Ehrlichwasfascinatedbytheselectivityofinteraction
betweenantigensandtheantibodiesthatcouldbeproducedtothembyanimals.
Inthe1870sand1880s,whileusingdifferentdyestostaintissuesandparasites,
hesawasimilarkindofselectivity.Someofthesedyescouldbindtocertaincells
and even to certain parts of the cell whereas other cells remained free of dye.
Hewasattractedtotheideathatselectivityofbindingofchemicalagentsthat
are toxic to parasites but not to human cells could be the basis of identifying
effectivechemotherapeuticagents.Theseobservationsinhislaboratoryinspired
histhoughtsaboutthebasicprinciplesofchemotherapy.ItalsoprovedPasteur’s
famousdictum“Chanceonlyfavorsthepreparedmind.”
During the first decade of the twentieth century there was a great interest
amongEuropeancountriesinexploitingthenaturalresourcesfromtheirAfrican
colonies.AwayhadtobefoundtoprotecttheEuropeancolonizersandthena-
tiveworkerstheyemployedfromendemicdiseasesinthetropics.Attemptstouse
theirknowledgeofimmunologywerenotsuccessfulinprotectingpeopleagainst
parasiticinfections.Inaddition,verylittlewasknownaboutthebiologyofthese
parasites.Therewasagreatneedforchemicalagentstotreattheseinfections.
Germany,atthattime,hadoneofthelargestandmostmodernchemicalin-
dustriesinEurope,particularlyforthemanufactureofdyes.Guidedbyhisearly
observationthatsomedyeshaveselectiveaffinitytocertaincellsandtheavail-
abilityofthousandsofdyesfromtheneighboringHo¨chstlaboratoriesEhrlich
embarked on a program to identify dyes that would have selective affinity to
microorganismsresponsiblefordifferentinfections.Hestartedwithmethylene
blue,whichshowedsomeeffectsagainstmalariabutalsohadaspecificaffinity
to nerve cells and was therefore too toxic to humans. Hundreds of dyes were
examinedagainstbothmalariaandtrypanosomeinfectionsinexperimentalan-
imalswithoutsuccess.
Paul Ehrlich’s new strategy was based on rational screening for identifying
newchemotherapeuticagents.Oneshouldtrytofindchemicalsthathavebeen
proven to be selectively toxic to the microorganisms but not to the human
hosts. Ehrlich’s thinking was greatly influenced by the specificity of antibody
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RationalDiscoveryofAntiparasiticAgents 3
formationinresponsetoantigensfrominfectiousmicroorganisms.Heperceived
ananalogybetweentheprocessofantigen/antibodyinteractionandtheselective
toxicityofchemicalagentsagainstparasites.Headvocatedthesearchforatoxic
chemicalagentthatcanselectivelybindtoareceptorintheparasitebutnotto
oneinthehost.Inanaddresstothe17thInternationalCongressofMedicine
in England (Ehrlich, 1913) he presented his ideas about the rational approach
to chemotherapy. His first principle was “If the law is true in chemistry that
Corporanonaguntnisiliquida(substancesareeffectiveonlyinthefluidstate),then
for chemotherapy the principle is true that Corpora non agunt nisi fixata (only
attached, anchored substances are effective).” A metaphoric term “the magic
bullet”wasgiventotheidealchemotherapeuticagent,adrugthatdisablesthe
“germ”butnotthehostcells.Ehrlichenvisioneda“therapiasterilisansmagna...
that by means of one or at most two injections the body is freed from the
parasites.”ChemotherapyisatermthatwasinventedbyPaulEhrlichandwas
definedastheuseofchemicalagentsto“injure”aninvadingorganismwithout
harmingthehost.
In 1906, after his disappointing work with dyes, Ehrlich shifted his atten-
tion to the spirochetes that cause syphilis (which was widespread in Europe
at that time). Mercury was the only known treatment, but this metal did not
meet Ehrlich’s conception of therapia sterilisans magna. Ehrlich studied arseni-
calsforhisfirstresearchonidentifyingnew,effectivechemotherapeuticagents
against syphilis. At that time Atoxyl (sodium arsanilate) was being used as an
antiprotozoal agent, but because of toxicity, its therapeutic usage was discon-
tinued. Several hundred arsanilate derivatives were examined against an infec-
tionofaspirochete-likeorganism,Spirillummorsusmuris,inlaboratoryanimals.
Compound606(diaminodioxyarsenobenzol)wassuccessfulintreatingSpirilla
infections in chickens, rabbits, rats, and mice because Spirilla readily absorbed
the drug, killing the bacterial cells (Baumler, 1965). Subsequently, Compound
606 was used with a dramatic effect in patients suffering from syphilis. The
human cells did not appear to take up the drug. An improved preparation of
Compound606(neosalvarsan,Nadiaminodihydroxyarsenobenzenemethanol
sulfoxylate),whichwasmoresolubleandthusmoresuitableforinjection,was
subsequentlydiscovered.Withinfiveyearstheincidenceofsyphiliswasgreatly
reducedinmanyEuropeancountries.Thiswasatriumphantsuccess.
Rational Discovery of Antiparasitic Agents
SubsequenttotheworkofPaulEhrlich,astrategywasgraduallyestablishedto
searchforantiparasiticagentsonthebasisofidentifyingdrugsthatareselectively