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Pharmacology Notes Pharmacology–drugaction,howmedicineswork,howbodyprocessesthem
• Pharmacodynamicsàeffectsofdrugonbody• Pharmacokineticsanddrugmetabolismàeffectsofbodyondrug
GlossaryofTermsTerms Definition/DescriptionAgonist Ligand/smallmoleculethatbindstoreceptor(usuallyorthosteric
bindingsite)andactivatesreceptore.g.acetylcholineAntagonist Bindstoorthostericsiteofreceptor,buthasintrinsicactivityof0so
doesnotactivateite.g.atropineLigand Amoleculethatbindstoanother(usuallylarger)molecule
Dose-responseorconcentrationeffectrelationship
Dose-response–referstoaddingdrugtoalivingsystemlikeabodyConcentration-effecte.g.guinea-pigileum
Orthostericbinding BindingtotheactivesiteAllostericbinding Sitethatdiffersfromtheorthostericbindingsite
Competitivebinding WillcompeteforthebindingsiteNon-competitivebinding Bindingtoadifferentsitefromtheorthostericsite–doesn’t
necessarilymeanithastobindareceptor–couldbinddownstreame.g.interactwithGproteinsorionchannel
Efficacy AbilitytoinitiateamaximumresponseafterreceptorbindingPotency Measuredbymeasuringfunctionalphysiologicalresponseandfinding
EC50,thelowerthevalue,themorepotentthedrugEC50 EffectiveconcentrationtocauseahalfmaximalresponsepD2 -logEC50KD Dissociationconstant-concentrationofligandthatresultsinhalf
occupancyofreceptorsformaligand-receptorcomplex(appliesa1:1ligand:receptorratio)Indicatorofaffinity,measuredbyaradioactiveligand(KBforantagonist)
Affinity Strengthofbindingofdrugtoitstarget,preferabilitywithhighspecificity
KB DissociationconstantofantagonistFoundfromschildplotordirectbindingexperiment
Desensitizationv.tachyphylaxis
Desensitisation–biologicalresponsetodrugdiminisheswhenitisgivencontinuouslyorrepeatedly,downregulationofreceptor–donethroughkinasesàarrestinbindingetc.Tachyphylaxis–acuterapiddecreaseinresponsetodrugafteritsadministration,liketoleranceTermsmeanthesamething
Selectivity Abilityofdrugtobindpreferentiallytoonesiteoveranother.Asnodrugsareentirelyspecific,yougetthatonedrugismoreselectivetoareceptoroveranotheràhaspreferencetoonesiteoveranother,e.g.muscarineismoreselectiveformuscarinicovernicotinic,betablockersareantagoniststhatbindtoBreceptors,non-selectivewouldbesomethingthatbindstoeitherB1orB2,orsomethingelsemaybeselectiveoverB1thanB2
Specificity Abilityofagivendrugconcentrationtoproduceoneeffectoveranother.Chemicalbindinginteractionbetweentheligandandthe
receptor,desiredoutcomeistohavehighspecificityorbindingbutoftenneverreallyachieved,canalsoworkinreverse,thereceptor
Principles of drug action
• Cellsurfacetargets–transporters,receptors,ionchannels• Intracellulartargets–enzymes,nuclearreceptors,proteinsynthesis• Drugscanincreaseordecreaseactivityoftheirtargets:
o Receptors–agonists,antagonists,modulatorso Ionchannels–blockers,modulatorso Enzymes–inhibitors,falsesubstrateso Transporters–blockers,falsesubstrates
• Drugs:o Smallhydrophilic–actatcellsurfaceo Lipidpermeable–crossmembranetoinhibitanenzymeorregulategeneexpression
Provideexamplesofdrugsandhowtheyworkateachofthefollowingtargets:
• Aneurotransmittertransportero Antidepressantse.g.fluoxetine,nortriptyline
§ Blockserotonin,dopamineandnoradrenalinetransporters§ Moreofthosehormonesstuckinextracellularspace§ Nortriptylinebindsatthebottomofanexternalcavity,preventingdopaminefrom
bindingandpreventingclosureoftheexternalvestibuleo Cocaine
§ Blocksdopamineandserotonintransporters§ Stimulant§ Appetitesuppressant§ Topicalanaesthetic,usedineye,noseandthroatsurgery§ Dopaminetransportinhibitor§ Na+channelblocker
o Amphetamine§ Substrateofdopamine,noradrenaline
transporters§ Moreincell–transportedmoreefficiently
thantheotherhormonessoistransportedinstead(kindofeffectivelyblockingtheuptakeoftheothers)
• Anintracellularenzyme
o Increaserateofchemicalreactionsinthebodyo Aspirin
§ Cyclooxygenase-enzymeassociatedwithendoplasmicreticulum§ Catalysesfirststepinconversionofarachnoidicacidtoprostaglandinsandarange
ofinflammatorycompounds§ Extractedfrombarkofwillowtreeàsalicylicacidàaspirin
§ LowdoseofaspirinusedtoirreversiblyinhibitCOXinplatelets–stopsplatelets
fromclottingblood§ Aspirinbindstoactivesite
o Indomethacin§ VerypotentCOXinhibitor
o Paracetamol• AG-proteincoupledreceptor
o ~50%ofdrugsworkonGPCRso Salbutamol-β2adrenergicreceptoragonist
§ NAdnormallyactivatesβandαadrenergicreceptors§ Insalbutamol,theN-trimethylgrouphasselectivityforβ2adrenergicreceptors
§ Stimulatesβ2–causesbronchodilationandisatreatmentforasthma§ Stimulationofβ2receptorsinduceschangeinreceptorassociationwiththeα
subunitoftheGprotein§ Activatedαsubunitandβsubunitsdissociate§ LeadstoactivationofenzymeadenylcyclasethatformscyclicAMPfromATP§ cAMPrelaxesbronchialsmoothmuscleanddecreasesairwayresistance
• Thereceptor-agonistcomplexinducesconformationalchangeinGprotein,causingαsubunittobindGTP.α-GTPcomplexdissociatesfromGproteinandactivates(orinhibits)membraneenzymeorchannel.Signalendsbecauseα-GTPhasinstrinsicDTPaseactivityandturnsitselfoffbyhydrolysingGTPtoGDP.α-GDPthenreassociateswithβγsubunits
• Anionchannelo Allowreadytransferofionsdowntheirelectrochemicalgradiento Controlionicgradientsacrosscellmembrane–Na+blockerwillblockinfluxofNa+ionsà
canreduceactivityofneurons§ Open-closedstateiscontrolledbymembranepotential(voltage-gatedchannels)or
bytransmittersubstances(ligand-gatedchannels)§ Mostlocalanaestheticsareweakbasesthatexistmainlyinprotonatedformatbody
pH–drugspenetratenerveinnon-ionised(lipophilic)formbutonceinsideaxon,becomeprotonatedandblockNa+channelspreventinggenerationofAPs
o E.g.lidocaineandcocaine§ Ionchannelscontroltheionicgradientsacrossthecellmembrane§ LettingNa+in,allowsthemtomovedownconcentrationgradientandchange
membranepotentialbymakingcellpositive§ Localanaestheticsareweakbases§ Theneutralformcancrossthemembraneandaccessthedrugbindingsite§ Byblockingionchannels,theyreducetheactivityofneuronsandsensory
informationisstopped
• Bacterialproteinsynthesiso Targetbacterialribosomes(bacterialribosomesaredifferenttoeukaryoticones)o Don’tactuallykillthecell–juststopbacteriafromgrowingo Tetracycline
§ InterferewithattachmentoftRNAtomRNA-ribosomecomplexo Chloramphenicol
§ Bindsto50Sportionofribosomeandinhibitsformationofpeptidebond
o Erythromycin§ Bindsto50Sportion,preventstranslocation-movementofribosomealongmRNA
o Streptomycin§ Changesshapeof30Sportion,causescodeonmRNAtobereadincorrectly
• Anuclearreceptor
o Tamoxifen§ Tamoxifenwasfirstmadein1962andwasdesignedtobeananti-fertilitydrug-but
itstimulatedovulationinsteadofsuppressingit§ 1973–foundusefultotreatEstrogenreceptorpositivebreastcancer§ Preventestrogendependentstimulationofbreastcancercellgrowth§ Estrogenreceptorantagonist§ Stimulatesovulationandusedfortreatingestrogenreceptorpositivebreastcancer
aspreventsestrogendependentstimulationofbreastcancercellgrowth§ Bindstoestrogenreceptorandhence,whenestrogenenterscell,itcan’tbindtothe
receptor§ Hydrophobicandnotmanychargedgroups§ Estrogenreceptormediatedgeneregulation
• Estrogenboundtoestrogenreceptordimer• TheligandboundreceptorbindstoreceptorresponseelementsinDNA
sequencetoregulategeneexpression• Tamoxifenboundtoreceptordoesnotrecognisethereceptorresponse
element–geneexpressioncan’toccur
Comparetheactionsofamphetamineandcocaineonthedopaminetransporter• Cocaineblocksdopamineandserotonintransporters,leavingthesehormonesstuckinthe
extracellularspace• Amphetaminesareasubstrateofthetransporter,andhence,competeforreuptakewithdopamine
HowdoeslidocaineaccessitsbindingsiteonNa+Channels?
• MostlocalanaestheticsareweakbasesthatexistintheprotonatedformatbodypH• Thedrugspenetratethenerveinthenon-ionisedform(asalipidsolublefree-base)butonceinside
theaxon,sometimere-joinwiththeproton• ThesethenenterandplugtheNa+channelsafterbindingtoareceptor
Howdoestamoxifeninhibitcancercellgrowth?
• Tamoxifenentersacancercellandbindstotheestrogenreceptors• Whenestrogenentersthecell,itcan’tbindtothereceptors• Cancercellproliferationispreventedasitneedsthisestrogenbindingtoregulategeneexpression
Whatmakessalbutamolselectiveforbadrenergicreceptorsoveraadrenergicreceptors?
• It’sN-trimethylgroupisselectiveforB2adrenergicreceptorsbutnotaComparethemechanismsofactionofaspirinandindomethacininhibitionofthecyclooxygenaseenzyme?
• IndomethacinisaverypotentCOXinhibitorbuthassignificantrisks–cardiovascularandGItoxicityo Itisareversibleinhibitoro Goodfortheshortterm
• Aspirinisanirreversibleinhibitor–can’tretrieveplateletactivityo ProlongedinhibitionofCOX(sitsinmembraneofendoplasmicreticulum)inplatelets–can
beusedtopreventclotting,cancauseexcessiveGIbleeding
Drug-receptor interactions 1: agonists Antagonist–combinewithreceptors,butdonotactivatethem–reduceprobabilityofagonist/transmittersubstancecombiningwithreceptor
Agonist–smallmoleculethatactivatesareceptorthathasaphysiologicalregulatoryfunction• Activationmeansreceptorisaffectedbyboundmoleculeinwayastoalterreceptor’sbehaviour
towardscellandelicitatissueresponse• E.g.acetylcholine(ACh)
o Endogenousandreleasedmainlyfromnerveso Activatesanorthostericbindingsiteonseveralreceptors
§ Muscarinic§ Nicotinic
o ChangesphysiologicalstateofcellthathasreceptorsthatbindtoAch
• Bindingforcesbetweenagonistandreceptor
o Electrostatic(charge)–longrangebinding,initialattraction• Thenifmoleculesuitablyshapedtofitbindingsiteofreceptor
§ Hbonding§ Vanderwaals
o HydrophobicLawofmassaction–at equilibrium, rate of chemical reaction is proportional to product of concentration of reactants (drug and free receptor)
• Assumesbindingisreversible• Ifyouwanttoincreasereactiongoingright,increaseamountofdrugORincreasenumberof
receptors
Affinity–indicatedbythedissociationconstant
• Indicatespotencyof‘drug’andgenerallyasaffinityincreasessotoodoesbindingspecificity• Wewantdrugswithhighaffinity–ifhighaffinity,willbindatlowconcentration• Dissociationconstantisinverseofanassociationconstant• Indicatesaffinityofamoleculethatbinds• KDdenotesthedissociationconstant,butsometimeswrittenasKA(M=mol/L)• KD–concentrationofligandthatresultsinhalfoccupancyofreceptorsformaligand-receptor
complex(appliesa1:1ligand:receptorratio)• Reflectpropertiesofligandandreceptor
Relationbetweenligandconcentrationandamountbound
• Bindingassay–proportionaloccupancyvsdrugconcentration• Functionalassay–shortening(%max)vsdrugconcentration
Scatchardplot–determiningKDandreceptornumber
• Relationshipbetweenamountbound(B)andligandconcentration(Xa)
• Bmax–largernumbermeansmorereceptors• KD–smallernumbermeansgreateraffinity
Concentration-responsecurves
• Ifoccupancyisproportionaltoresponse,thenhill-langmuirequationcanalsobeusedtodescriberelationshipbetweenconcentrationandresponse
• SimilarrelationshipbutKD=EC50
o Maxresponsecanoccurwithoutfullreceptoroccupancyo Transductionpathwayscanamplifytheeffecto KDisforagonistsandantagonistsbecauseitreflectsaffinityoftheligandandnotintrinsic
efficacy(transductionefficiency)o EC50istheeffectiveconcentrationtocauseahalfmaximalresponse
negative log!!
Shapeofresponsecurves
PA – proportion of ligand bound XA – [Ligand] KD – dissociation constant