curr.opin.drug.discov.devel.2000.p549

549Signaltransductiondrugdiscovery:Targets,mechanismsandstructure-baseddesignDavidCDalgarno*,ChesterAMetcalfIII,WilliamCShakespeare&TomiKSawyerAddressARIADPharmaceuticalsInc26LandsdowneStreetCambridgeMA02139USAEmail:david.dalgarno@ariad.com*TowhomcorrespondenceshouldbeaddressedCurrentOpinioninDrugDiscovery&Development20003(5):549-564©PharmaPressLtdISSN1367-6733Signaltransductiontargetsincludecatalyticand/ornon-catalyticdomains,whicharecriticaltovariousaspectsofcellgrowth,differentiation,metabolismandfunction,mitogenesis,motilityandgenetranscription.Specificexamplesofmoleculartargetsincludethecatalyticdomainsofproteintyrosinekinases(PTKs)andofproteintyrosinephosphatases(PTPases),aswellasrelatedprotein-proteininteractionmotifs(eg,SH2,PTBandSH3domains).FromtherelationshipoftyrosinephosphorylationtointracellularpathwayregulationbyPTKsandPTPases,thedynamicandreversiblebindinginteractionsofSH2andPTBdomain-containingproteinswiththeircognatephosphotyrosine(pTyr)-containingproteins,provideanadditionaldimensiontothemodulationofsignaltransductionpathwayswhichexistasaresultofpTyrformation,degradationandmolecularrecognitionevents.Thisreviewfocusesonourcurrentunderstandingofkeysignaltransductiontargets,bothcatalyticandnon-catalytic,relativetorecentreportswhichhaveprovidedfurtherinsightintotheirthree-dimensionalstructureandmechanism.Thisreviewalsohighlightsrecentprogressinthedesignandoptimizationofmolecularmechanism-basedsignaltransductioninhibitors.KeywordsPhosphotyrosine,SH2,signaltransduction,structure-baseddrugdesign,tyrosinekinase,tyrosinephosphataseIntroductionCellsrespondtotheirenvironmentbyrelayingthechemicalsignalsreceivedbyexternalreceptorsthroughcomplexsetsofintracellularsignaltransductionpathways,whichultimatelyaffectnumerouscellularprocessesincludinggrowth,motility,apoptosis,genetranscription,metabolismandimmuneresponse.Thesignaltransductionprocessinthesimplestviewactstotransmittheextracellularstimulus,eg,theoccupancyofareceptorbyitsligand,toanintracellularsignalingpathwayviasequentialformationofaseriesofprotein-proteininteractionsresultinginenhancementorinhibitionofaseriesofenzymes[1].Manyintracellularsignaltransductionpathwaysineukaryoticcellsaremediatedthroughthereversibleprocessesoftyrosinephosphorylation(viatyrosinekinases)andtyrosinedephosphorylation(viatyrosinephosphatases).Indeed,thenumberofhumangenesencodingtyrosinekinasesisestimatedtobemorethan1%ofthegenome-morethan1000kinases-implyingthatthesekinases,togetherwiththeircomplementaryphosphatases,playakeyroleintheregulationofmultiplesignaltransductionpathwaysandhenceinmultiplediseasestatesassociatedwithabnormalsignaling[2].Theprocessesoftyrosinephosphorylationanddephosphorylationresultinboththecreationandremovalofsitesofprotein-proteininteraction,withconsequencestoproteinlocalizationinsignalingcomplexes,andinmoredirecteffectsonenzymaticactivity.Inthisreview,wewillillustratethestructuralnatureofphosphotyrosine-associatedsignaltransductiontargets,especiallyintermsofinhibitorymechanisms,andthenwewillfocusoncurrentdrugdiscoveryapproachestospecificsignaltransductiondomains.Mutationsinthesrcgenegaverisetotheprototypeoncogene,v-src,intheRoussarcomavirus.Aconstitutivelyactivatedtyrosinekinaseisencodedbyv-src,producingenhancedcellulartyrosinephosphorylationandpathwayactivation,thuslinkingabnormalsignalingeventstodisease[3-5].SrcandSrc-familykinasesremainmodelsofthestructuralbasisofregulationandafocusofsignaltransductionresearch[6].Otheroncogenesencodeactivatedreceptortyrosinekinases,particularlythoseintheEGFrandPDGFrfamilies,whichledtoanearlyfocusontheinhibitionoftheseandrelatedtyrosinekinasesincancertherapy.Morerecenteffortsfocusonabroaderrangeofmoleculartargetsanddiseases,eg,Srcinosteoporosis,LckandZAP-70inimmunosuppression,andVEGFr(Flt/KDR)andFGFrforanti-angiogenesis[2,7-11].Tyrosinephosphataseinhibitionalsooffersameanstointerveneinabnormalsignalingevents;thePTPasefamilycurrentlycomprisesabout100enzymeswhichareeitherreceptor-likeorcytoplasmic[12],however,estimatesfromthecurrenthumangenomeputthenumberataround500genes[13].Inparticular,thereisintenseinterestinthetyrosinephosphatasePTP1B,whichdephosphorylatesactivatedinsulinreceptorkinase.PTP1BhasbeenidentifiedasamoleculartargettoimproveinsulinsensitivityintypeIIdiabetes[14].Acharacteristicfeatureofintracellularsignaltransductionpathwaysisthepresenceofmulti-proteinclustersthatformoncellularstimulus.Theproteincomplexescancontainbothtyrosinekinasesandphosphatases,theirsubstrateproteins,bridgingadaptermolecules,aswellaslinkstomembraneandcytoskeleton.Proteinsassociatedwithsuchcomplexespossessadomainstructure,withacatalyticdomainandanumberofprotein-proteininteractionmodules.Formationofasignalingcomplexcaninvolvelocalizationofacatalyticdomainandsubstrateviaformationofnewprotein-proteininteractionsites,modulationofcatalyticactivityviaphosphorylation,andfinally,additionalmodulationofcatalyticactivityoftheenzymeseitherthroughdirectorpartneredassociationofinhibitorymolecules.Althoughthemolecularstructuresoftheisolateddomainsarenowwell550CurrentOpinioninDrugDiscovery&Development2000Vol3No5characterized,theintramolecularmechanismsforregulationareonlynowbeingrevealed.Knowledgeofthespecificproteinpartnersandmolecularstructureoftheprotein-proteininteractionsinintactsignalingcomplexesremainsincompleteandthesubjectofintensestudy,leavingthepresentchallengeofidentifyingspecificsignaltransductionproteinsorproteindomainsforpharmacologicalintervention.Tyrosinekinasesandphosphatasescontainanumberofwell-characterizedproteindomainswhoseindividualmolecularstructureshavebeendeterminedbyX-raycrystallographyandNMRspectroscopy.Severalnon-catalyticprotein-bindingdomainsoccurfrequentlyinkinasesandphosphatases,ofteninmultiplecopies;theSH2domain,aproteindomainwhichrecognizespTyr-containingpeptides,thePTBdomainwhichalsorecognizespTyrpeptides,andtheSH3domainwhichrecognizesPro-richsequences[1].Asurveyoftheproteindatabankindicatesthatmorethan50SH2andSH3domainstructureshavebeendeposited,thesestructuralmotifsarewell-characterizedandarereviewedindetailelsewhere[15-18].Thenon-catalyticdomainshavedualfunctionstomediateprotein-proteininteractionsandtoregulateenzymeactivity.Crystallographicstructuresofseveraltyrosinekinasecatalyticdomainsarenowknown,showingsignificantstructuralhomologytotheirSer/Thrkinasecounterparts.Theseisolatedtyrosinekinasecatalyticdomainstructures,togetherwiththecrystallographicstructuresoftheSH3-SH2-catalyticdomainsforSrcandHck,revealdetailsofthecomplexmolecularbasisfortheenzymaticregulationofSrcfamilykinases[19,20].TheSH3-SH2-kinasedomainsweresolvedinthe'inactive'orauto-inhibitedform-hereaC-terminal'tail'regiontyrosineisphosphorylated(Tyr527),andtheSH2andSH3domainsareboundtotheoppositesideofthecatalyticdomaintotheactivesite.ThephosphorylatedtyrosineisboundtotheSH2domain;furthermore,theSH3domainisboundtoboththeSH2domainkinase-linkerandthecatalyticdomain.TheactivesiteofthekinaseisdisruptedbythemovementofahelixintheN-terminalcatalyticlobe;theactivationsegmentofthecatalyticdomain(includingtheunphosphorylatedTyr416)movesinward.Incontrast,inthestructureoftheactivatedformofthecatalyticdomainofhomologouskinaseLck,thetyrosineintheactivationloopisphosphorylatedandtheactivationloopismovedawaysothatitdoesnotinterferewithpeptidesubstratebinding[21].ThekinaseactivityofisolatedSrc(andHck)isthereforecontrolledbymultiplefactors;thephosphorylationstateoftwotyrosineresidues,Tyr527andTyr416,andwhethertheintramolecularassociationsoftheSH2andSH3domainsaredisruptedbyexternal(higheraffinity)ligands.Ofthesefactors,thephosphorylationstateofTyr416appearstobemostcritical.Themolecularbasisfortheactivationmechanismsofothertyrosinekinasesremainforthemostpartunknown,althoughphosphorylationofactivationsegmentsplaysacriticalrole,eg,bytransphosphorylationinresponsetoreceptorclustering[22].Thesituationinvivoisfurthercomplicatedbythepresenceofadditionalproteinsinsignalingcomplexes,whichcanmodulatecatalyticactivityeitherenzymatically,orbyprovidingadditionalSH2andSH3bindingsitesorbindingdomains.Theroleofthesignalingcomplexinbringingenzymesandtheirsubstratesintocloseproximityshouldbestressed.Althoughknowledgeofupstreamanddownstreamsignalingpartnerscontinuestoadvance,atpresentthemoleculartopologyofthesecomplexinteractionsisingeneralunknown.However,themodularnatureofsignaltransductiontargetsandtheseparationofcatalyticandnon-catalytic(proteinrecognition)functionsindicatesthattherearetwobroadapproachestoinhibitingabnormalsignalingevents.Signaltransductioninhibitorscanpreventformationof,ordisruptsignalingcomplexesthroughinhibitionofprotein-proteincomplexformation,orelsetheycanactdirectlyasenzymeinhibitors.Inthenextsectionswesurveyrecentprogressinthedevelopmentofbothgeneralclassesofsignaltransductioninhibitors,specificallydescribingprogressinSH2,tyrosinekinaseandtyrosinephosphataseinhibition.Signaltransductioninhibitors:Protein-proteininteractionsSrcSH2andZAPSH2inhibitorsThedesignofSH2inhibitorsposesseveralchallenges,asnumerousSH2domainsarepresentinacell.Therefore,theinhibitorsmustbeselectiveandfurthermore,theyneedtoshowenhancedbindingoverpeptidicphosphotyrosine-containingligands.Finally,thephosphotyrosinemoietymustbereplacedbyanon-hydrolyzableanalogwhileretainingpotency.Anadditionalcomplicationfordrugdesignisthetwo-lobednatureofthebindingsite[15].Awell-utilizedapproachforthedesignofsmallmoleculeSrcSH2inhibitorsistoreplacetheGlu-GluportionofthepTyr-Glu-Glu-Ilepreferredbindingmotifwithrigid,non-peptidetemplateunits.Usinga2,4-substitutedthiazoletemplate[23],aseriesofcompoundsweresynthesizedtobindtoSrcSH2.Thethiazole-containingcompoundsweredesignedtoaccessboththepTyrandpTyr+3pockets,whilealsogainingadditionalhydrophobiccontactswiththeTyr205residuethroughthethiazolemoiety.CrystallographicandNMRspectroscopicanalysisoftheparentcompound1(Figure1)verifiedthelegitimacyofthedesignconceptandledtoSAReffortscenteredaroundcompoundscontaining2,4-and2,5-substitutedthiazoles,aswellas1,2,4-substitutedoxadiazoles[24].AlthoughallthreeheterocyclicringsprovidedsimilarIC50valuesforanalogouscompounds,thehighestaffinitycompoundcouldbefoundintheoxadiazoleseries(2)containingann-heptylsidechaininthepTyr+3position.SubstitutionatthepTyr+1positionwitheitherL-Gln,L-Abu(2-aminobutyricacid)orL-Trp-derivedfunctionality(incorporatedintothecompoundsviatheirrespectiveaminoacids),allresultedinreducedbindingaffinitiesforSrcSH2relativetoL-Glu.OneparticularlyweakSrcSH2binderinthisseries,3(SrcSH2IC50=135µM),demonstratedasurprisinglyincreasedselectivityforZAP-70SH2(ZAP-70SH2IC50=55µM)[25].SARstudieseventuallyledtothehighlyselective(versusSyk,SrcandGrb2)andpotentZAP-70SH2inhibitor,4(ZAP-70SH2IC50=1µM).Compound4bindswithapproximately400-foldincreasedaffinityoverthenativetetrapeptidesequences(pTyr-Asp-Val-LeuandpTyr-Asn-Glu-Leu),andbinds25-foldlesseffectivelythanthecognate,doublyphosphorylatedζ-ITAMpeptidesequence.Signaltransductiondrugdiscovery:Targets,mechanismandstructure-baseddesignDalgarnoetal551Figure1.Thiazoleandoxadiazole-containingSH2inhibitorsforSrcandZAPdesignedusingstructure-basedmethods.NSNHOPOOHOHNHOCH3OOOH1SrcSH2IC50=26µM2SrcSH2IC50=7µMNNONHOPOOHOHNHOCH3OOOHCH33ZAP-70SH2IC50=55µMNNONHOPOOHOHNHOCH3OOOH4ZAP-70SH2IC50=1µMSyk/Src/Grb2SH2IC50=>500,68,368µMNNONHOPOOHOHNHOOHOThedenovodesign[26••]of5(Figure2)representsoneofthefirstsuccessfulattemptstomimictheinteractionsofthepTyr-Glu-Glu-Ilepreferredbindingsequencewithsmallmoleculenon-peptides.AcomplexedX-raystructureof5revealedauniquemodeofbindinginthepTyrpocket,aswellasverificationofproteininteractionsinvolvingthebenzamidefunctionalitywhichwereconceivedintheoriginaldesign.Thisseriesprogressedtothehighaffinitycompound6(SrcSH2IC50=0.3µM)[27],containingthebiologicallystableF2Pmpgroup[28].Usingthiswell-definedbenzamidetemplate,aseriesofcompoundsweresynthesizedbysolid-phaseparallelsynthesistoexplorehydrophobicbindinginthepTyr+3bindingpocketofSrcSH2[29••].TheFLOdockingprogramwasusedinthecontextoftheparentcompound7toselectappropriatepTyr+3substituents,whichledtoanenhancementinbindingaffinity,realizedbythebis-allyl-containingcompound8(SrcSH2IC50=0.9µM).Figure2.DenovodesignedSrcSH2inhibitorsmimickingthepreferredbindingsequencepTyr-Glu-Glu-Ile.NHOOPOHOOHNH2OOCH3CH35SrcSH2IC50=6.6µM6SrcSH2IC50=0.3µMNHPOHOOHFFOCH3NH2OONHOCH37SrcSH2IC50=1.9µM8SrcSH2IC50=0.9µMNHOCH3NHOPOHOOHOCH3NH2OONHOCH3NHOPOHOOHOCH3NH2OOCH2CH2OptimizationofthepTyr+3positionwasaccomplishedusingaFLOdockingmethodandsolid-phaseparallelsynthesis.552CurrentOpinioninDrugDiscovery&Development2000Vol3No5Figure3.CompoundstargetingCys188inthepTyrpocketofSrcSH2toachievebiochemicalselectivity,andpotentbenzamide-basedcompoundscontainingnon-hydrolyzable,bone-targetingphosphotyrosinemimicstoprovideinvivotissueselectivity.9SrcSH2Kd=0.2µMNHOPOOHOHNHOCH3OOOHOHONCH3CH310SrcSH2IC50=0.24µMPPOHOOHOOHOHNHNHOCH3OCH3NH2OO11SrcSH2IC50=0.35µM12SrcSH2IC50=0.3µMNHPNHPCH3OOOOHOHOOHOHNH2OONHNHCH3OOHONH2OOOOHThehighlyconservednatureoftheSH2domainamongtheSrcfamilytyrosinekinaseshaspromptedseveralgroups[30,31,101]topursueabiochemicalapproachtoachievingSrcSH2bindingselectivity.Thisstrategyinvolvesthespecifictargetingofacysteineresidue(Cys188),residinginthephosphotyrosine-bindingpocket,thatisuniquetoSrckinase.Theprecedentforthisapproachstemsfromthehostofcysteineproteaseinhibitorsthathavedemonstratedbothhighaffinityandreversiblebindingthroughengagementofareactivethiolintheactivesiteoftheenzyme[32-35].Evidencethatanappropriatelyplacedformylgroup(9;Figure3)canactasanelectrophilicsiteforthethiolfunctionalityofCys188inSrcSH2hasbeenreported[31].CrystallographicandNMRspectroscopicanalysisrevealedthatthethiolwasindeedcapturedbythealdehydefunctionalityin9.Nonetheless,9onlyexhibiteda2-foldincreasedaffinityforSrcSH2relativetoitsnon-aldehyde-containingcounterpart.Recently,inapaperdescribingthedesign,synthesisandbiologicalactivityof10(Figure3),researchersatARIADreporteda>100-foldincreasedselectivityof10forSrcSH2versusthehighlyhomologousYesSH2domain,anSrcfamilymember[36••].Compound10alsodemonstratedefficacyinseveralmechanism-basedcellularassaysandinhibitedrabbitosteoclast-mediatedresorptionofdentineslices(IC50=50µM).Inadditiontothe3-formylmolecularrecognitionelement,compound10containsabicyclictemplatedesignedtoenhancebindingaffinitythroughfavorablehydrophobicinteractionswithTyr205oftheSrcSH2domain.Theadvancementofaseriesofcompoundsdesignedaroundtheaforementionedbenzamidetemplatehasledtothedisclosureofseveralhigh-affinity,cell-activeSrcSH2inhibitorsexemplifiedbycompounds11and12.Bothcompoundsarecharacterizedbynon-hydrolyzable,bisphosphono-phosphotyrosinebindingelementsthatimpartinvivotissueselectivitytothecompoundsbyvirtueoftheirbone-targetingproperties[37,38••].Inthecaseofcompound12,bonelocalizationandpotentSrcSH2inhibition(achievedthroughhydrophobicproteinbicyclic-bindinginteractions;Figure4)havetranslatedintosignificantinvivoactivityinaTPTXanimalmodel,measuringinhibitionofPTH-inducedserumcalciumelevationinrats.Thisinvivoresultfor12,afirstintheareaofSH2inhibition,isakeyresultfordrugdiscoveryapproachestargetingSH2-associatedsignaltransductionpathways,andvalidatestheuseofSrcSH2inhibitorsaspossibletherapeuticsforbone-relateddiseasestates,suchasosteoporosis.Grb2SH2inhibitorsAvarietyofpeptidomimeticshaveappearedintheliteraturebasedontheminimalpeptidesequenceAc-pTyr-X+1-Asn-NH2thatbindtotheGrb2SH2domain[39-43].Recently,thefirstexamplesofsmallmoleculeGrb2SH2inhibitorswithsignificantlyreducedpeptidenaturewerereported,asexemplifiedbycompound13(Figure5)[44••].Usingstructure-baseddesign,the(3-aminomethylphenyl)ureamoietywasselectedasasuitablereplacementfortheX+1-Asnsequence,mimickingthelow-energytype-Iβ-turn-bindingconformationthatischaracteristicofGrb2SH2peptideligands.Inaddition,the3-aminomethylphenyltemplateallowstheintroductionofbenzylicsubstituentstomimicthesidechaininteractionsoftheX+1residue.Asimilardesignconceptwasutilizedintheconstructionofthenon-peptideGrb2SH2inhibitor,14(Figure5)[45].The4-Signaltransductiondrugdiscovery:Targets,mechanismandstructure-baseddesignDalgarnoetal553Figure4.Crystallographicco-structureofcompound12withtheSH2domainofLck(S164C)[38].Thebicyclicbenzamidelinkerportionoftheligandishighlighted,spanningthepTyrandpTyr+3pockets,attherightandleftofthefigure,respectively.Thelinkerisoptimizedtofittheproteinsurfaceviahydrophobiccontactsandhydrogen-bonding.aminopyrimidinegroupwasdesignedtomimicthepseudo6-memberedringconformationofthephosphopeptideAsnresidue,whilethethiazoleringservesasaspacerelementtoallowaccesstoboththephosphotyrosineandX+2asparagineinteractions.LckSH2inhibitorsAnumberofhighaffinity(Kd=0.2µM)dipeptideLckSH2inhibitorshavebeenreportedintheliteratureandwillnotbediscussedinthisreview[46-48].ThetechniqueSARbyNMR™wasrecentlyappliedtothesearchforphosphotyrosinemimicsbindingtotheLckSH2domain[49].Thismethodcanidentifyavarietyofmoleculesthatbindinthemillimolartomicromolarrangebyanalyzingthechemicalshiftchangesofspecific,proteinbackboneamideresonances.Severalcompoundswerediscoveredwithcomparable,andinsomecasesincreased,bindingaffinityversusphosphotyrosine(LckSH2Kd=0.3mM,Figure6).Signaltransductioninhibitors:EnzymeinhibitorsTyrosinekinasesMajoreffortswithinthepharmaceuticalandbiotechnologyindustrieshavebeenmadetodiscoveranddevelopnoveltyrosinekinaseinhibitors[2,7-11].Duetoapaucityofinformationonsubstratesequencesandco-structuraldataonenzyme:substratecomplexes,thiseffortisfocusedheavilyonATP-competitiveinhibitors.Thediscoveryoftheanticancer-targetedtyrosinekinaseinhibitorshasbeendirectedtowardthetyrosinekinasesoftheEGFreceptor,oroneoftherelatedErbBfamilymembers,thePDGFreceptor,andmoreFigure5.Grb2SH2inhibitorsdesignedtomimicthelow-energytype-Iβ-turn-bindingconformationadoptedbythecognatepeptideligands.NHOPOHOOHNHOCH3OCH3CH3NHNH2O13Grb2SH2IC50=6.2µM14Grb2SH2IC50=26µMNNSNOPOHOOHCH3NH2554CurrentOpinioninDrugDiscovery&Development2000Vol3No5Figure6.PhosphotyrosinemimicsforLckSH2discoveredusingtheSARbyNMRTMmethod.OPOHOOHNHOOCH3CH3OPOHOOHCH3POOHOHOHOOHOOOHNNHCH3CH3OOHOOHOONHNCH3CH319LckKd=0.06mM18LckKd=0.2mM16LckKd=1.0mM17LckKd=1.0mM15LckKd=0.3mMrecently,onangiogenesisinhibitorstargetingthetyrosinekinasesofFGFreceptorand/orVEGFreceptor.Inadditiontothesereceptortypetyrosinekinases,thenon-receptortyrosinekinaseshavebeenafocusofdrugdiscovery,includinginhibitorsofSrcand/orrelatedSrcfamilymembers(eg,Lck,Hck),Abl(withinthecontextofBCR-Abl,ZAP-70)andSyk.Suchtargetshavefurtherexpandedthetherapeuticapplicationsoftyrosinekinaseinhibitorstoosteoporosis,andimmuneandinflammatorydiseases.Keyleadswhichhaveadvancedinpreclinicalandclinicaldevelopmentagainstthesetyrosinekinasetargetsinclude:theEGFreceptorTKinhibitors,ZD-1839(20,AstraZeneca;Figure7)[102]andOSI-774(21,OSIPharmaceuticals,formerlyknownasCP-358774(Pfizer);Figure7)[103];thePDGFreceptorTKinhibitorSU-101(22,SUGEN;Figure7)Figure7.Leadtyrosinekinaseinhibitorstructures.NHNNOOCH3FClNONNHNOOCOOCHCH3CH3NHOOCH3NFFFNNNHNCH3NHONNCH326CGP-57148(Novartis)NHONHCH3CH328SU-5416(SUGEN)20ZD-1839(AstraZeneca)23PP-1(Pfizer)27PD-166866(Pfizer)25PD-163510(Pfizer)24CGP-77675(Novartis)21CP-358774/OSI-774(Pfizer/OSIPharmaceuticals)22SU-101(SUGEN)NNNNH2OCH3NOHNNNCH3NNHCH3ClClNHONHCH3NNNNH2CH3CH3NHONHCH3CH3CH3CH3CH3NNNNCH3CH3CH3CH3NH21345Signaltransductiondrugdiscovery:Targets,mechanismandstructure-baseddesignDalgarnoetal555[104],whichisstructurallyidenticaltoHoechst'sleflunomidethatwaspreviouslyidentifiedasananti-inflammatoryagent;theSrcfamilyTKinhibitorsPP-1(23,Pfizer;Figure7)[50•,105],CGP-77675(24,Novartis;Figure7)[51••]andPD-163510(25,Parke-Davis;Figure7)[52••,106];theAblandPDGFreceptorTKinhibitorCGP-57148(26,Novartis;Figure7)[53••,107];andtheFGFreceptorTKinhibitors,PD-166866(27,Parke-Davis;Figure7)[52••,106]andSU-5416(28,SUGEN;Figure7)[54••].Theaboveexamplesoftyrosinekinaseinhibitorsillustratethechemicaldiversitythathasbeenachievedfr