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PolymericDrugDelivery PolymericDrugDelivery ControlledReleasevs SustainedRelease SustainedreleaseComplexation slowlydissolvingcoatings useofderivativeswithreducedsolubilitySensitivetoenvironmentalconditionstowhichtheyareexposedControlledreleaseReleaserateisdeterminedbythedeviceitselfMoreaccurate predictableadministrationrate PolymericDrugDeliverySystems IncorporatedrugintoapolymericmatrixReleasedrugataknownrateoveraprolongeddurationReleasedrugdirectlytothesiteofactionConstantrelease oftenthegoal difficulttoachieveDeliverdrugsuchthatconcentrationintissueisinappropriaterangeProtectionofthedrugfromenzymaticdegradation particularlyapplicabletopeptideandproteindrugs TypesofDrugDeliverySystems Matrixsystems monolithicdevicesRatecontrollingmembranes reservoirdevicesDegradablepolymersVarietyofconfigurationsReleaseratesgenerallydeterminedbysolutionofFick sLawswithappropriateboundaryconditions Membranes Mostimportantclassisnonporous homogeneouspolymericfilmsTransportoccursbydissolutionofpermeatingspeciesinthepolymeratoneinterfaceanddiffusiondownagradientinthermodynamicactivityMeasurablypermeabletodrugswithMWlessthan400 OfftheShelfPolymersUsedinDrugDelivery EVAPDMSpHEMAPVA TransportgovernedbyFick sLawSteadystateversionofequation AssumingthatthepermeantoneithersideofthemembraneisinequilibriumwiththerespectivesurfacelayerConcentrationjustinsidethemembranecanberelatedtotheconcentrationintheadjacentsolution AssumingthatDandKareconstant goodassumptionsincedrugshavelowsolubilityinpolymers ReleaseratesattainablefromsolutiondiffusionmembranecontrolleddevicesconstrainedbyphysicallimitationsDevicethicknessMolecularweightofdrugisgreaterthan500 mustexpectasubstantialdecreaseintheachievablereleaserateReleaseratesbetween1and200mg cm2hexpected MonolithDevices DrugdispersedordissolvedinasuitablepolymerReleasediffusionofdrugthroughthepolymerdiffusionthroughporesinthepolymerstructureDifferentreleaseprofilesresult DissolvedDrug ConsideramatrixsystemcontainingdrugThissystemisplacedinasolutioncontainingnodrugandthedrugdiffusesfromthesystemtothesolutionReleasewillbeafunctionoftimeandspaceWhatdoesthereleaseprofile amountofdrugreleasedfromthesystemperunittime looklike PossibletosolveFickiandiffusionequationanalyticallyforspecificcasesandspecificdevicegeometriesInterestedinreleaserateasfunctionoftimeBoundaryconditions SolutionofFick sLawwithappropriateboundaryconditionsExpressdesorptionofdissolveddrugfromtheslabbyeitheroftheseries Simplificationscanbemadewhichapplyoverdifferentrangesofthedesorptioncurve accurateto1 Derivedfrom2 fortheearlyportionofthedesorptioncurve Derivedfrom1 forthelateportion ThedrugreleaserateatanytimeisalsoofinterestObtainedfromdifferentiationofapproximationequationstogive Earlytime Latetime Timetoreleasehalfofthedrug halflifeofthedevice Releaserateathalftime TheoryversusExperimental Earlytimeapproximationforcylinder Latetimeapproximationforcylinder 0 4 0 6 Earlytimeapproximationforsphere Latetimeapproximationforsphere 0 4 0 6 DispersedDrug Drugdispersedasasolidinthemembranephaseinsteadofbeingdissolved releasekineticsalteredTotalconcentrationofdrugCo dissolved dispersed largerthanthesolubilityofthedruginthemembrane CsHiguchi JPharmSci50874 1961 Releaserateandmassofdrugreleasedatanytimearegivenby ReservoirDevices RateControllingMembranes ReservoirDevices RateControllingMembranes Assumethattheconcentrationinthereservoirisveryhigh assumedconstant andtheconcentrationinthesinkisverylow approximateaszero Afteraninitialunsteadyperiod wewillreachsteadystateZeroorderrelease constantrateofdrugreleasefromdevice Duringtheunsteadyperiod Canbesolvedtogive Forsufficientlylarget RateControllingMembrane Forsufficientlylarget Whichhassolution RateControllingMembranes Therateofdrugdeliveryisgivenby Assumingaconstantactivityinthedevice constantreleasewillbeachieved Similarequationsderivedforbothacylinderandasphere Cylinder Sphere OcusertSystem BurstandLagEffect InitiallyexhibitreleaserateshigherorlowerthanthesteadystatevalueImmediateuse timerequiredforestablishmentofconcentrationgradientinthemembrane LagTimebeforeuse drugwillsaturatethemembrane insolutionwillresultinaninitiallyhigherrateofrelease BurstSolutionofFick slawunderunsteadyconditions DeliverySystemsforWaterSolubleDrugsandProteins OfconsiderableinterestsinceproteindrugsareOfgrowingimportanceHighlyunstableinbiologicalmediaMechanismofdrugreleasetendstobeindependentofsizeofthemoleculeGenerallyloadedbydispersingsolidparticlesthroughoutthepolymerReleasefollowst1 2kinetics DegradableDeliverySystems ReleaseviathreedifferentmechanismsdegradationofmatrixsurroundingthedrugdegradationofbondsbywhichadrugisjoinedtopolymermatrixdiffusionofdrugfromthesystemDispersedanddissolvedDissolvedonlyDegradationproductsmustbereadilymetabolizableandexcretable DegradableDeliverySystems Goodas surgicalleavebehind Suitedwelltohighmolecularweightdrugsanddrugswhicharenotsolubleinpolymer DegradableDeliverySystems TwomechanismsofpolymerdegradationSurfacedegradation moreconstantreleaseHydrolyticdegr
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