DevelopmentandCharacterizationofaPolyester-BasedImplantforControlledDrugRelease

by

HilaryBoucher

AthesissubmittedinconformitywiththerequirementsforthedegreeofMasterofScience

GraduateDepartmentofPharmaceuticalSciencesUniversityofToronto

©CopyrightbyHilaryBoucher[2017]

ii

Abstract

DevelopmentandCharacterizationofaNovelPolyester-BasedImplantfor

ControlledDrugRelease

HilaryBoucher

MasterofScience

PharmaceuticalSciencesUniversityofToronto

2017

There is tremendous interest in developing IDDSs that improve the efficacy of

drugsatthetargetsitewhilesparingoff-targettissuesfromtoxicity.PVLisanattractive

yetunexploredpolyester forpreparing IDDSssincethevalerolactonemonomercanbe

alkylated (PAVL) ingoodyield toaffordapendantarmon thepolymerbackbone that

canbefunctionalizedtotailorthephysicochemicalpropertiestosuittheapplicationof

interest. This thesis describes the first preparation and characterization of polyester-

basedimplantsbasedonPVL-co-PAVLthatarecapableofsustainedreleaseofalibrary

ofdrugsofvaryingphysicochemicalproperties.Drugreleasewasfoundtobecontrolled

bydiffusionbut influencedbythepropertiesofmatrix,thestateofthedrug,anddrug

aqueoussolubility.Finally,thesystemprovedtobehighlybiocompatible,illustratingthe

potentialofPVL-co-PAVLasalong-termIDDS.

iii

ContributionsofAuthors

Thefollowingworkinthisthesisisacollaborativeeffortbetweentheauthorand

Dr.FrantzLeDévédec.InChapter2,Dr.LeDévédecsynthesizedthepoly(valerolactone)-

co-poly(allylvalerolactone)copolymersamplesandcharacterizedthembyDSC,GPC,and

1H NMR. DSC thermograms and XRD spectra of the cross-linked copolymer matrices

werecollectedbyDr.LeDévédec.SolubilityparametersvalueswerecalculatedbyDr.Le

Dévédec. A portion of the buffer penetration experiment was performed by Dr. Le

Dévédec, namely the diffusion of the probe through the hollow cylinder. The swelling

capacity of the cross-linked copolymermatriceswas evaluatedbyDr. LeDévédec and

ShawnWu,anundergraduatestudentworkingundersupervisionoftheauthor.Muchof

theinterpretationsoftheworkperformedbyDr.LeDévédecaretheauthor'sown.

iv

Acknowledgments

Iwouldfirstliketoconveymythankstomysupervisor,Dr.ChristineAllen.Sheis

anintuitivescientist,afiercelycommittedsupervisor,andastrongrolemodel.I'llalways

appreciatethetreatsandhumorshebroughttotheoffice,aswellasherencouragement

andsupportduringtheupsanddownsofgraduatelife.Ifeeltrulyfortunateforhaving

theopportunitytobeapartofherlab.

AnenormousthanksgoesouttoPendantBiosciencesfortheuniqueexperience

ofworking on an industrialMSc project. I'm grateful for the exposure to the start-up

biotechworldandfortheopportunitytomeaningfullycontributetothedevelopmentof

theirtechnology.ThanksaswelltoPendantBiosciencesandDr.Allenforobtainingthe

fundingthatsupportedthisresearch;itwasgenerouslyprovidedbyPendantBiosciences

andtheMitacsAccelerateProgram.

Thankyoutomyadvisorycommitteemembers,Dr.PingLeeandDr.DaveDubins,

for taking their time to offer constructive suggestions on my work, especially where

mathematical modeling was concerned. Dr. Dubins very swiftly wrote a user-friendly

programforfittingmyreleasedataandforthatIamgrateful.

I appreciate the training and technical assistance I received from a number of

people. Thanks to Dr. Frantz Le Dévédec for polymer synthesis and characterization

assistance, Dr. James Evans for cell culture training, Ilya Gourevich for technical

assistancewithESEM,andPeterBrodersenforhoursworthofcryomicrotoming.

I am more than grateful to acknowledge members of the Allen lab for their

support and friendship over the course of my graduate degree. Michael Dunne and

SohyoungHerdeservea truly special thanks for relentlesslymaintainingapositive lab

cultureforeveryone.Thankstothosemore-than-just-workfriendsthataccompaniedme

toRyu'sNoodleBar,Somethin'2TalkAbout,BicerinEspressoBar,TrinityBellwoods,Get

Well, karaoke, Jamie & Daniel's, etc. You all made this time more enjoyable than it

neededtobe.

v

Most importantly, thank you tomyparents,Michael andMichele Boucher, for

their unconditional love and support and for raisingme to be this person that I am. I

aspiretotheirworkethic,generosity,andsincerity.Iwouldalsoliketothanksignificant

membersofmyextendedfamily,namelymyunclesRick&John,foralwayslisteningto

and supportingmewith love. Thankyou to thegroup chat: Steacy, Sam,Chantal, and

Rebecca; the BPMT crew: Kyle, Chelsea, Hilary, and Moh; my life-long best friend

Philippa;Fiona&Brandon;andthatspecialperson,whomayormaynotbementioned

here,forendlesslyinspiringmeandforbeingapartofmylife.

vi

TableofContents

Abstract.......................................................................................................................ii

ContributionsofAuthors.............................................................................................iii

Acknowledgments.......................................................................................................iv

TableofContents.........................................................................................................vi

ListofTables..............................................................................................................viii

ListofFigures...............................................................................................................ix

ListofAbbreviations....................................................................................................xi

1GeneralIntroduction.................................................................................................1

Rationale,GoalandObjectives...............................................................................111.1 RelevantBackground...................................................................................................4

1.1.1 Whatiscontrolleddrugdelivery?...............................................................................41.1.2 Localvs.systemicaction.............................................................................................51.1.3 Historyofimplantabledrugdeliverysystems.............................................................71.1.4 Biodegradablematerialsusedtoprepareimplantabledrugdeliverysystems...........81.1.5 Preparationofpolyester-basedimplantabledrugdeliverysystems........................141.1.6 Degradationofpolyester-basedimplantabledrugdeliverysystems........................171.1.7 Drugreleasemechanismsofpolyester-basedimplantabledrugdeliverysystems..201.1.8 Factorsinvolvedindrugreleasefrompolyester-basedmatrices.............................231.1.9 Mathematicalmodelingofdrugrelease...................................................................241.1.10 Solubilityparameters..............................................................................................261.1.11 Functionalizationofpolyestersasameanstotailorphysicochemicalproperties27

2FactorsInvolvedinDrugReleasefromCross-LinkedPolyester-BasedMatrices........31

Introduction.........................................................................................................3122.1 ExperimentalSection.................................................................................................32

2.1.1 Materials...................................................................................................................322.1.2 SynthesisofPVL-co-PAVLcopolymers......................................................................322.1.3 Preparationofthecross-linkedpolyester-basedmatrices.......................................332.1.4 Characterizationofcopolymersandcross-linkedmatrices......................................332.1.5 Determinationofsolubilityparameters....................................................................342.1.6 Drugloading..............................................................................................................342.1.7 Determinationofdrugsolubility...............................................................................342.1.8 Drugreleaseassay....................................................................................................352.1.9 HPLCanalysisofdrugrelease....................................................................................352.1.10 Degradationofcross-linkedpolyester-basedmatrices..........................................362.1.11 Bufferpenetrationintocross-linkedpolyester-basedmatrices.............................362.1.12 Cellcultureexperiments.........................................................................................37

2.2 ResultsandDiscussion...............................................................................................392.2.1 Bulkcopolymersynthesisandcharacterization........................................................39

vii

2.2.2 IDDSpreparationandcharacterization.....................................................................412.2.3 StabilityandDegradation..........................................................................................462.2.4 BufferPenetration....................................................................................................482.2.5 DeterminationofSolubilityParametersandSwellingCapacity................................502.2.6 EvaluationofPhysicochemicalPropertiesofDrugs..................................................522.2.7 CharacterizationofDrug-LoadedIDDS.....................................................................542.2.8 MechanisticMathematicalTheory...........................................................................592.2.9 DrugRelease.............................................................................................................612.2.10 Biocompatibility......................................................................................................66

3Conclusion...............................................................................................................68

4FutureDirections.....................................................................................................70

5References..............................................................................................................72

Appendices................................................................................................................81

viii

ListofTables

Table1.1SummaryofadvantagesofIDDSsweighedagainsttheirdisadvantages.........................3Table2.1Molecularweightdistribution,thermalcharacteristics,andcrystallinityofthethree

bulkcopolymers(P39K,P15K,P7.5K)andthecross-linkedpolymers(CP7.5K,CP15K,CP39K)..........41Table2.2Partialandtotalsolubilityparameters((MPa)1/2)ofselectdrugloadingsolventsandthe

CPmatricescalculatedusingtheGCM..................................................................................51Table2.3Solubilityparameters,molarvolumes,andlogPvaluesofthedrugsloadedintotheCP

matrices.................................................................................................................................54Table2.4SummaryofdiffusioncoefficientsofdrugsthroughthethreeCPmatricesunder

differentexperimentalconditions(e.g.releasebuffer)........................................................61Table2.5SummaryofFDAapprovedpolyester-basedIDDSs........................................................81

ix

ListofFigures

Figure2.1Thermogramsofthethreebulkcopolymers(P39K,P15K,P7.5K)obtainedbyDSCat10°C/min(2ndcycle).................................................................................................................41

Figure2.2ESEMimagesof(a)cross-sectionand(b)surfacemorphologyoftheCPmatrices.Fromlefttoright,scalebarsrepresent50μm,1mm,and100μm...............................................43

Figure2.3DSCthermogramsofcross-linkedcopolymermatrices(CP39K,CP15K,CP7.5K)andcorrespondingphotographsoftheIDDSs(scalebarsrepresent5mm)................................45

Figure2.4XRDspectraofcross-linkedcopolymermatrices(CP39K,CP15K,CP7.5K)andcorrespondingphotographsoftheIDDSs(scalebarsrepresent5mm)........................................................46

Figure2.5(a)Weightloss(%)ofCP15Kafterincubationin0.1MPBS(pH=7.4)forthreemonths.(b)ChangeinpHofexternalmediaasaresultofCP15K.........................................................47

Figure2.6SurfacemorphologyofCP15Kafterincubationin0.1MPBS(pH=7.4)forthreemonths.Scalebarsrepresent(a)50µmand(b)300µm,respectively.Brightwhitelinesin(b)areKimWipefibres......................................................................................................................48

Figure2.7QualitativedepictionofSRB(0.1mg/mLin0.1MPBS)diffusionintoCP39Kmatricesasafunctionoftimespentunderincubationat37°C.Fluorescencewasmeasuredat540nmbyconfocalmicroscopy..............................................................................................................49

Figure2.8Semi-quantitativeevaluationofthediffusionofSRB(A540nm)acrosstheCP39Kmembraneofahollowcylinderfromtheinternaltoexternalmedia(0.1MPBSpH=7.4,SDS0.5%w/v)...............................................................................................................................50

Figure2.9(a)Plotofthecalculatedaverage[1/3(CP7.5K+CP15K+CP39K)](δCP−δsol)2values(MPa1/2)forthethreecross-linkedmatricesandsolventsconsideredfordrugloadingusingtheGCM.(b)Plotofswellingcapacity(%)asafunctionoftimeforCP39Kcylindersswollenintheindicatedsolvents.Scalebarsrepresent5mm...............................................................51

Figure2.10ChemicalstructuresandMWoffivedrugsselectedforloadingintotheCPmatrices................................................................................................................................................53

Figure2.11Aqueoussolubilityofthefourhydrophobicdrugsin0.1MPBS(pH=7.4)containing0.1%,0.5%,or1%(%w/v)Tween®80orSDS.......................................................................54

Figure2.12Drugloadingcontent(%w/w)oftheCP15Kmatrices..................................................55Figure2.13ESEMimagesofCP15Kmatricesloadedwith(a)TAHor(b)CCM.Scalebarsrepresent1

mm(top)and20µm(bottom)..............................................................................................56Figure2.14ComparisonofDSCthermogramsofCP15Kmatrix(bottom)loadedwitheachofthe

fivedrugs...............................................................................................................................57Figure2.15ComparisonoftheXRDspectraofcrystallineTAHandCCMtothatofthenon-loaded

CP39KmatrixandCP39KmatricesloadedwithTAHorCCM.....................................................58Figure2.16Cumulativedrugrelease(%)asafunctionoftimemeasuringtheimpactofthestate

ofthedrug-loadedmatrices(monolithicsolutionvs.dispersion)ontheresultingdiffusionofdrugfromtheCP15Kmatrices(0.5%SDSw/v).Blacklinesrepresentthepredicteddrugreleasegiventhecalculateddiffusioncoefficient.................................................................62

Figure2.17CumulativeCCMorPTXrelease(%)asafunctionoftimemeasuringtheimpactofdrugaqueoussolubilityontheresultingdiffusionofdrugfromtheCP15Kmatrices.Blacklinesrepresentthepredicteddrugreleasegiventhecalculateddiffusioncoefficient...Error!Bookmarknotdefined.

Figure2.18CumulativeTAArelease(%)asafunctionoftimemeasuringtheimpactofpolymerphysicochemicalpropertiesontheresultingdiffusionofdrugfromthethreeCPmatrices.Blacklinesrepresentthepredicteddrugreleasegiventhecalculateddiffusioncoefficient...................................................................................................Error!Bookmarknotdefined.

x

Figure2.19Invitrocytotoxicityoftheappliedextractdilutionofcylinders(green)orhighdensitypolyethylene(HDPE)(purple)toL929mousefibroblastcells.Cellsincubatedwithmediaalonewereemployedasacontrolandconsideredas100%cellviability.(***,**,and*)indicateslesserviabilityrelativetountreated(p<0.001,0.01,and0.05,respectively);(###,##,and#)indicateslesserviabilityrelativetotreatmentgroup(CP39K)ofsameextractdilutionconcentration(p<0.001,0.01,and0.05,respectively).............Error!Bookmarknotdefined.

Figure2.201HNMRspectrumofP39Kandgelpermeationchromatogramsforthethreeco-polymers................................................................................................................................84

Figure2.21Massofdrugreleasedasafunctionoftimeinvestigatingtheimpactofthestateofthedrug-loadedmatrices(top),aqueoussolubility(middle),andpolymerphysicochemicalproperties(bottom)ondrugrelease.....................................................................................85

xi

ListofAbbreviations

1,6-HDT 1,6-hexanedithiol

5-FU 5-fluorouracil

ACL allyl-ε-caprolactone

ACM acetaminophen

AVL allyl-δ-valerolactone

BCM blockcopolymermicelle

BCNU 1,3-bis(2-chloroethyl)-1-nitrosourea

CCM curcumin

CED cohesiveenergydensity

CLε-caprolactone

CPcross-linkedpolymer

DEX dexamethasone

DLC drugloadingcapacity

DMEM Dulbecco'smodifiedEagle'smedium

DMPA 2,2-dimethoxy-2-phenylacetophenone

DMSO dimethylsulfoxide

DSC differentialscanningcalorimetry

ESEM environmentalscanningelectronmicroscopy

FBS fetalbovineserum

FSH folliclestimulatinghormone

GBM glioblastomamultiforme

GCM groupcontributionmethod

GIT gastrointestinaltract

GnRH gonadotropinhormonereleasinghormone

GPC gelpermeationchromatography

Δ°Hm enthalpyofmelting

HDPE high-densitypolyethylene

HPLC high-pressureliquidchromatography

xii

IDDS implantabledrugdeliverysystem

IV intravenous

LGlactide:glycolide

LHluteinizinghormone

LNG levonorgestrel

logP octanol/waterpartitioncoefficient

MW molecularweight

NLM non-linearminimization

NMP N-methyl-2-pyrrolidone

o/w oil-in-water

PAG poly(allylglycolide)

PBS phosphatebufferedsaline

PCL poly(caprolactone)

PDI polydispersityindex

PDLA poly(D-lacticacid)

PGprogesterone

PGA poly(glycolicacid)

PLA poly(lacticacid)

PLGA poly(lactic-co-glycolicacid)

PLLA poly(L-lacticacid)

PTX paclitaxel

PUpolyurethane

PVA polyvinylalcohol

PVL poly(valerolactone)

ROP ringopeningpolymerization

SDS sodiumdodecylsulfate

SRB sulforhodamineB

TAA triamcinoloneacetonide

TAH triamcinolonehexacetonide

TBD 1,5,7-triazabicyclo[4.4.0]dec-5-ene

xiii

TC crystallizationtransitiontemperature

Tg glasstransitiontemperature

THF tetrahydrofuran

Tm meltingtemperature

UVultraviolet

VLδ-valerolactone

w/o/w water-in-oil-in-water

XRD x-raydiffraction

1

1 GeneralIntroduction

Rationale,GoalandObjectives1Sincetheeraofmoderndrugdiscoverybegan inthe late1800s,pharmacologic

therapyhasplayedacritical role inthetreatmentandmanagementofvariousdisease

states.1Treatmentefficacyisintricatelylinkedwithmanyfactors,includingtherouteof

administrationandthetherapeutic index, i.e. theratioof thedoseofdrugthatcauses

adverse effects at an unacceptable level to the dose that leads to the desired

pharmacologicaleffect.2Mostdrugstendtobeadministeredsystemicallyeitherbythe

oral or intravenous (IV) route. One of the main disadvantages of systemically

administereddrugsistheconstantcyclingbetweenhighandlowplasmaconcentrations

of drug that may cause toxicity or sub-therapeutic drug levels, respectively.1,3Plasma

concentrationshigher than therapeutic valuesmay result in toxicity,while lowplasma

drugconcentrationsmayresultinineffectivenessortheneedforrepeateddosing,which

reduces patient compliance. Also, drugs may suffer from low aqueous solubility, low

permeability, or susceptibility to enzymatic ormetabolic degradation leading to a low

dose fraction reaching the target site and subsequently disappointing in vivo

performance.4,5

Drugsthatareadministeredsystemicallymustalsoovercomeextensivebiological

barriers to reach their sites of action. These barriers include, but are not limited to,

plasma protein binding, transport across the gastrointestinal tract (GIT) membrane,

2

lymphaticsystemremoval,first-passhepaticmetabolism,blood-brainbarriertransport,

anddrugefflux throughmembrane transporters suchasP-glycoprotein.4,6Asaway to

limitadrug'scontactwiththeaforementionedbarriers,onestrategyinvolvesdelivering

thedrugdirectlytothesiteof intendedaction.Thishasbeenaccomplishedbyvarious

routes of administration and dosage forms such as transdermal (e.g. creams and

patches), transmucosal (e.g. suppositories and inhalation aerosols), ocular (e.g. eye

drops), and injection (e.g. solid implants, gels, and particles).6,7 These dosage forms

afford localizeddrugaction toovercomebiological barriersbut further limitations can

arise, such as adverse effects from high plasma drug concentration or the need for

reapplication and increased dosing frequency. These challenges have necessitated the

development of ways to improve the efficacy of the drug at the target site of action

whilesparingoff-targettissuesfromtoxicity.

Dispersingthedruginacarriermaterial,suchasanaturalorsyntheticpolymer,

to yield an implantable drug delivery system (IDDS) can overcome the undesirable

fluctuationsinplasmadrugconcentrationsandoff-targeteffectsbyprovidingtemporal

(e.g. sustained release) and/or spatial (e.g. target to site of action) control of drug

release.8Dependingonthepropertiesofthecarriermaterialandtheextentofpolymer-

drug interactions, drug release can be sustained for hours to several years.9 These

systemsexistinavarietyofformsincludingsolidimplants,gels,andparticlesthatcanbe

inserted into a specific point of interest to bypass the GIT, thereby preventing

degradationoflabiledrugs,improvingtheefficacyofdrugtherapyatthetargetsite,and

improving patient compliance through a reduced dosing frequency.1,10 Table 1.1

3

illustratestheadvantagesofIDDSweighedagainsttheirdisadvantagessincenomethod

ofdeliveringdrugswillbeaonesizefitsallsolution.Implantabledeliverysystemshave

been investigated for the delivery of a great variety of therapeutic agents and for a

number of applications and disease states. The most common areas of use include

cancer therapy11,12 and pain management,13 ophthalmic drug delivery,14 and vascular

applications.15

Table1.1SummaryofadvantagesofIDDSsweighedagainsttheirdisadvantages.

ListofadvantagesofIDDSs ListofdisadvantagesofIDDSs• Improvedpatientcompliance

throughreduceddosingfrequency• Eliminationofpeak-to-trough

plasmadrugconcentrationprofiles• Loweroveralldoserequiredby

deliverytothesiteofaction• Protectionoflabiledrugs(e.g.

thosesusceptibletoenzymatichydrolysisordegradation)

• Fewersystemicsideeffectsbydeliverytothesiteofaction

• Immediateremovalispossibleintheeventofdrugallergy

• Somerequiresurgicalimplant/explantprocedures,whichmayreducepatientacceptance

• Moretrainingrequiredfortheimplant/explantprocedure

• Notascost-effectiveastraditionaldosageforms

• Morecomplexthantraditionaldosageforms;theregulatorypathmaybemorecomplicated

• Potentialpainanddiscomfort,whichwouldlowerpatientacceptance

Given the tremendous versatility in terms of therapeutic applications and the

advantagesthatIDDSpossessovertraditionallyadministeredtherapies,thegoalofthis

thesisistodevelopandcharacterizeanovelIDDSthatiscapableofprovidingcontrolled

releaseofhydrophobicdrugsat therapeutically relevant levels. The specificobjectives

include:

1. Toidentifyarangeofdrugsthatvaryintermsofphysicochemicalpropertiessuch

as octanol/water partition coefficient (logP), molecular weight and aqueous

4

solubility,toincorporateintoacylindricalIDDS

2. ToprepareandcharacterizetheloadedandunloadedIDDS

3. ToevaluatetheinvitrodrugreleasefromthecylindricalIDDS

4. To elucidate the underlying mechanism(s) of drug release from the cylindrical

IDDS

The experiments performed to accomplish these objectives are outlined in

Chapter 2 of this thesis. Briefly, the synthesis of three distinct copolymers based on

poly(δ-valerolactone)-co-poly(allyl-δ-valerolactone) (PVL-co-PAVL) of varying molecular

weightandallylgroupcontentwasperformedbyDr.FrantzLeDévédec.TheIDDSswere

formedfromthesethreecopolymersbyasyringecastingmethod.Then,fivedrugswith

differentphysicochemicalpropertieswereloadedusingasolventswelling/equilibration

method.CharacterizationbySEM,DSC,andXRDwasperformedontheemptyanddrug-

loaded system, and in vitro biocompatibility was determined in the L929 mouse

fibroblastcellline.Finally,invitrodrugreleasewasmeasuredtoevaluatepotentialasa

long-termdrugdeliverydevice.

1.1 RelevantBackground

1.1.1 Whatiscontrolleddrugdelivery?Controlleddrugdeliveryisanymethodofdrugadministrationthatreliesonuse

of advanceddrugdelivery systems (i.e. polymers) tomaximize therapeutic effects and

minimize sideeffects.16Manyconventionaldelivery systemsprovide immediate rather

thancontrolledreleasebyusingslowlydissolvingcoatings(e.g.cellulose)ortheuseof

5

compressed drug tablets. With immediate release formulations, patient-to-patient

variability remains high since drug absorption is easily influenced by environmental

factors such as stomachpH. Furthermore, repeated administration and fluctuations in

plasmadrugconcentrationremainproblematic.17Inrecentyears,thefieldofcontrolled

drugdeliveryhas seen tremendousgrowth inorder toalleviate someof theproblems

associated with conventional dosage forms. There are several reasons for pursuing

controlled release dosage forms, including reduced side effects and toxicity,

administrationofalowerdrugdosebydeliverytothesiteofaction,improvedtreatment

efficacy, and improved patient compliance and convenience. For drugs with short

biological half-lives, slowing the rate at which the drug is released can increase the

apparenthalf-lifeby limitingmetabolism to the released fraction, therebymaintaining

therapeutic levels for a longer duration.16 Drug plasma concentration may fluctuate

aboveorbelowthetherapeuticwindowwhenadministeredasabolusinjectionororal

dosage form, yet controlled release dosage forms can be designed to maintain the

plasmadrugconcentration levelwithinthetherapeuticwindowforthemajorityofthe

releaseperiod.

1.1.2 Localvs.systemicactionInordertofurtherdiscussthefieldofcontrolledrelease,animportantdistinction

must first be made between local and systemic drug delivery. Controlled release

technologiesmayeitherbeadministeredlocally(e.g.certaindepotinjectionsandIDDSs)

withtheintentionofexertingtheireffectlocally,oradministeredlocallywiththeintent

6

ofexertingtheireffectelsewhere i.e. systemically.Thisdistinction iswell illustratedby

the clinical introduction of the first controlled release system for local delivery of the

anti-cancer agent, carmustine (Gliadel®), and a controlled release system for systemic

delivery of goserelin acetate (Zoladex®) for palliative cancer care. Gliadel® is a

biodegradable polyanhydride dime-sized wafer that is used to treat high-grade

malignant glioma or recurrent glioblastoma multiforme (GBM). It is inserted into the

surgicalcavityaftertumorresectionandreleasesthechemotherapeuticagent1,3-bis(2-

chloroethyl)-1-nitrosourea (BCNU, or carmustine) into the surrounding brain tissue to

targetremainingcancercellsoverfivedays.18Inalong-termfollow-upofamulticenter

controlled trial,malignant glioma patients had a 30% reduction in risk of deathwhen

treatedwithGliadel®ascomparedtoplacebo,andGBMpatientshada22%decreased

riskofdeath.19Theseresultsdemonstratethepotentialofa locallyacting IDDSforthe

treatmentofcancer.RanganathandWanghavesincereportedonthedesignandinvivo

evaluation of paclitaxel (PTX)-loaded biodegradable microfiber implants for the local

treatmentofGBM.20AftertreatmentwitheitherTaxol®orthePTX-loadedimplants,C6

glioma cells in the Taxol®-treated group began recovering after 3 days, which is

indicativeofthelimitationsofsystemicdrugadministrationduetoshort-termexposure.

Furthermore, tumor-bearing mice in the implant treatment groups demonstrated a

significantsurvivaladvantagerelativetoplaceboandTaxol®treatedcontrols,confirming

sustainedreleaseofPTX.

Zoladex® is an injectable formulation of goserelin acetate, a gonadotropin

releasing hormone (GnRH) agonist, dispersed in a poly(lactic-co-glycolic acid) (PLGA)

7

matrixusedtotreatadvancedprostatecarcinoma,hormonereceptorpositiveadvanced

breastcancer,andendometriosis.Itcomesintwodoses:a3.6mgimplantdesignedfor

subcutaneous administration by a syringe that releases the drug over a period of 28

days,anda10.8mgimplantdesignedfordeliveryover3months.21,22Itactssystemically

to stimulate the production of follicle-stimulating hormone (FSH) and luteinizing

hormone (LH), which results in GnRH receptor down-regulation and consequently a

reductioninestrogenandtestosterone.Themechanismofactioniseffectiveintreating

hormone-dependentcancersof thebreastandprostate.Arandom,open,multi-center

study was conducted in men with advanced stage D2 prostate cancer that received

eitherZoladex® (3.6mg implant)oranorchiectomy (surgical removalofboth testis).23

The trial confirmed that Zoladex® was equivalent to orchiectomy in terms of survival

advantage and limited adverse effects. The psychological advantage associated with

avoiding orchiectomy and an improved quality of life, asmeasured by the Functional

Living Index-Cancer and the Profile ofMood States instruments, in Zoladex® patients

further confirms the benefit of systemic GnRH therapy administered as a controlled

releasedosageform.Inanothermulticenter,randomizedopenstudy,itwasfoundthat

Zoladex® implants (3.6 mg) were well-tolerated and as effective as danazol, a widely

usedtherapeuticwithundesirableandrogenicandanabolicproperties,inthetreatment

of endometriosis.24 In addition, the dosing frequencywas reduced to a once-a-month

subcutaneousinjectioncomparedtoatwice-a-dayoraldose.

1.1.3 Historyofimplantabledrugdeliverysystems

8

In 1964, Folkman and Long pioneered the first IDDS based on silicone rubber

(Silastic®)capsulesasamethodforprolongedsystemicadministrationofdrugs.25Itwas

foundthatthecapsulescouldprovidesustainedreleaseofavarietyofdrugsandelicited

verylittleinflammatoryresponsewhenimplantedintothecardiacmuscleofdogs.Since

then, the interest indesigninganddeveloping IDDSs for abroad rangeof applications

hasgrownimmensely.Furthermore,anumberofIDDSshaveseenFDAapprovalstarting

with the Norplant® contraceptive implant in 1991, and followed by Implanon®,

Vitrasert®,andGliadel®,tonameafew.10TheNorplant®five-yearcontraceptivesystem

is a direct extension of Folkman and Long's initial work. This system consists of six

flexible, cylindrical Silastic® capsules (34 mm x 2.4 mm), each containing 36 mg

levonorgestrel(LNG),thatareinsertedsubcutaneouslyontheinsideofawoman'supper

arm.10,26-27Theimplantoffershighlyeffectiveprotectionagainstpregnancy(>99%),but

insertionandremovalprovedtobecumbersomeduetoproviderinexperience.28Thisled

to the removal of the Norplant® system from the U.S. market in 2002 in favor of

Implanon®, a single rod implant approved in 2006. A commonality of these two

implantablecontraceptivedevicesistheneedforminorsurgicalremovaloncethedrug

hasbeenreleased,whichmayresultinadecreaseinpatientacceptabilityofthedevice.

SuchSilastic®-based systems representa classofnon-degradable IDDSs,but there is a

keen interest in the substitution of non-degradable systems with those that degrade

uponcompletionofdrugreleasetoavoidtheneedforsurgicalremovaloftheimplant.

1.1.4 Biodegradablematerialsusedtoprepareimplantabledrugdeliverysystems

9

Biodegradablematerialsmaybenaturalorsyntheticinoriginandaredegradedin

vivotoproducebiocompatible,non-toxicby-productsthatcanbeeliminatedbynatural

metabolic pathways.29 Common natural polymers include complex sugars such as

hyaluronicacid, chitosan,andalginate, and inorganicminerals suchashydroxyapatite,

whilepolyestersandpolyanhydrides compose thecommonlyused syntheticpolymers.

PLGA and poly(caprolactone) (PCL) are commonly used polyesters to prepare IDDSs

giventheireaseofsynthesis,malleabilityintoawiderangeofdimensions,degradability,

lack of toxicity, and tailorable release rates.29-34Whenever a therapy is required for a

limited amount of time, the use of degradable materials is preferred over non-

degradablematerialsbecauseiteliminatestheneedforsurgicalremovaloftheimplant

and improves patient acceptability.10 A number of implantable or injectable products

based mainly on PLGA have been approved by the U.S. FDA and have been

commercializedforarangeoftherapeuticapplications(Table2.5).

1.1.4.1 Poly(glycolicacid)andpoly(D,L-lacticacid)Poly(α-esters), such as poly(glycolic acid) (PGA) and poly(D,L-lactic acid) (PLA),

were the first syntheticpolymermaterials tobe investigatedas suturesandnowhave

widespreadpharmaceuticaluse.35Thelactideandglycolidemonomersarecyclicdimers

ofnaturalmetabolitesofthebodyandarethereforehighlybiocompatible.Synthesisof

PGA and PLA can be achieved through polycondensation of the cyclic diesters with

antimonytrioxideyieldingpolymerproductswithlowmolecularweightandundesirable

physicochemical properties for medical applications.35 Therefore, to achieve higher

10

molecularweightpolymers, ring-openingpolymerization (ROP)usingmetal catalysts is

thepreferredmethodofsynthesis.PGAisasemicrystallinepolymer(45-55%,depending

on molecular weight) with good tensile strength (12.5 GPa), a glass transition

temperature (Tg) between 35-40 °C, and amelting temperature (Tm)greater than 220

°C.36 Favorable mechanical properties led to the development of PGA as the first

synthetic degradable suture, Dexon®, by American Cynamid Co. in 1967.35 PGA and

polyesters in general are susceptible to hydrolytic cleavage of their ester linkages

resultinginrapiddegradation,representingabarriertosuccessfuldevelopmentofPGA

homopolymer drug delivery systems. As a result, most efforts to utilize PGA have

focusedonshort-termboneortissueengineeringapplicationswherePGAisfabricated

into degradable scaffolds upon which cells can proliferate.36 The degradation of PGA

results in production of glycolic acid, which enters the tricarboxylic acid cycle and is

eliminated as carbondioxide andwater.However, groupshave shown that significant

production of glycolic acid can lead to an undesired local inflammatory response

followingimplantationofPGA-basedmaterials.37

PLA contains a chiral center and therefore consistsof twoenantiomeric forms:

poly(D-lactic acid) (PDLA) and poly(L-lactic acid) (PLLA). Enantiomerically pure PLA is

semicrystalline (37%), is less mechanically strong than PGA (4.8 GPa), and has a Tg

around55°CandaTmofabout180°C.38Asaresult,materialspreparedfromPLAare

glassyatphysiologicaltemperaturesandmechanicallybrittleinnature.Themethylside

groupinPLAmakeshighmolecularweightPLAhighlyhydrophobic,andthereforewater

diffusion and subsequent degradation by hydrolysis of the ester bonds is very slow in

11

vivo.CamandcolleagueshaveshownthathighmolecularweightPLLAfilmsarenotfully

degradedover200daysinstronglyalkalinemedium(0.01MNaOH),39andthisprocessis

even slower in vivo. Some strategies to reduce degradation time and improve

mechanicalpropertiesincludedecreasingthemolecularweightorcrystallinity.Evenso,

thelowdegradationrateandbrittlenessremainsignificantdrawbackstotheuseofpure

PLAasanIDDS,soblendingorcopolymerizingPLAwithotherpolyesters,suchasPGA,is

awaytoimprovethephysicochemicalpropertiesoftheresultingcopolymer.36,40Indeed,

Vicryl®isacommerciallyavailablesuturethatis10%PLAand90%PGA.

1.1.4.2 Poly(lactic-co-glycolicacid)PLGA, a blend of PLA and PGA at different compositions, is by far the most

extensively explored copolymer for use in drug delivery applications because of its

biodegradability,biocompatibility,andthefactthatPLGA-basedproductsareapproved

byregulatoryagenciesworldwide.41PLGAcombinesthestrengthsofPLAandPGAwhile

improvingontheirindividualweaknessesthroughcopolymerization.SincePLAandPGA

havedifferentphysicochemicalproperties,theabilitytomanipulatethelactide:glycolide

(LG) molar ratio of the resulting PLGA copolymer allows for optimization of

physicochemicalproperties,suchashydrophilicity,crystallinity,andmolecularweight,to

suit the intendeddrugdeliveryapplications.36Polymerhydrophilicitygreatly influences

thedegradationkineticssincePLGAdegradesbyhydrolysisofitsesterlinkagestoyield

lactide and glycolide oligomers and eventuallymonomers. Becauseof themethyl side

group in the lactidemonomer, PLGAwith higher lactide content ismore hydrophobic

12

and stable against hydrolysis than PLGA higher in glycolide, which is susceptible to

hydrolytic degradation. It has been shown that the degradation time of 50:50 PLGA,

75:25PLGA,and85:15PLGAis1-2months,4-5months,and5-6months,respectively,36

highlightingtheabilitytotunethedegradationkineticstosuittheapplicationofinterest.

PLGAcrystallinityisrelatedtotheL:Gmolarratioandthemolecularweightandhasan

impactonthedrugreleasekineticsandmechanicalpropertiesofthesystem.TheTgof

PLGAisbetween40-60°Cdependingonthecomposition,whichimpliesthatPLGA-based

deliverysystemswillbeglassyatphysiologicaltemperatures. Ithasbeenreportedthat

waterimbibitionhasaplasticizingeffectonthePLGAmatrix,decreasingtheTgofPLGA-

based microparticles below physiological temperature, thereby converting a glassy

systemintoarubberystate.42TheTgofPLGAincreaseswithincreasingmolecularweight

due to increased polymer chain interactions, chain entanglement, and dense packing.

Higher lactide content in the copolymer also increases the Tg as a result of the steric

hindrance imposed by the methyl side group and the subsequent reduction in chain

flexibility.43 The physicochemical properties of PLGA change during water penetration

and degradation. This must be taken into consideration when designing PLGA-based

systems.

1.1.4.3 Poly(ε-caprolactone)PCL is a biocompatible, hydrophobic, semi-crystalline homopolymer of ε-

caprolactonewithabroadrangeofapplicationsindrugdeliveryandtissueengineering

13

due to its desirable thermal andmechanical properties.44,45,46 Its compatibility with a

widerangeofdrugs,highpermeability,andgoodsolubilityinmanysolventsalsomakeit

an attractive polymer given its relative ease of use. Furthermore, its FDA approval

enablesfasterclinicaltranslation.47PCLhasaTgofaround-60°CandalowTmbetween

59-64 °C,making it rubbery and flexible at room and physiological temperatures. It is

easily molded into a wide range of dimensions from microparticles and cylinders to

complex, porous scaffolds for tissue engineering.48,49,34 Compared to PLGA, the

crystallinityofPCLismuchhigher,oftenbetween40-70%.Thecrystallinitydependson

the molecular weight, with lower molecular weight PCL displaying higher degrees of

crystallinity than high molecular weight PCL. It has been suggested that this

phenomenon is due to denser polymer chain packing with shorter chain length,50 or

interruption of the crystalline lamella, as longer chains may be able to loop into the

amorphous polymer region and disrupt chain crystallization.51 Unfortunately, drug

releasekineticsfrompurePCL-baseddevicesarenotastailorableasPLGAsincetheonly

variable available to manipulate is the molecular weight. As a result, PCL has been

blendedorco-polymerizedwithotherpolymers,suchaspolylactide,polyurethanes,and

poly(ethylene oxide), to tune the degradation and release kinetics and modify the

physicochemicalpropertiestotheapplicationofinterest.44

1.1.4.4 Poly(δ-valerolactone)

14

Poly(valerolactone) (PVL) is a semi-crystallinepolyester that is similar toPCL in

terms of physicochemical properties and structure,with the only structural difference

beingPCLhavinganadditionalcarbonatominitsbackbone.PVLhasaTgof-67°Canda

Tm between 52-59 °C, making it rubbery and flexible at room and physiological

temperatures, and it has a tendency to undergo crystallization.52 PVL is slightlymore

hydrophilicthanPCL,likelyduetotheincreasedfrequencyofpolarcarbonylbondsanda

shorteraliphaticchaininthebackbone.53Asaresultoftheincreasedhydrophilicity,the

degradation of PVL is slightly faster than PCL, and therefore represents a valuable

polyester intermediate between PLGA and PCL in terms of biodegradation. Given its

compatibility with a wide range of drugs, PVL has also been explored, though not as

extensivelyasPCL,asthehydrophobiccore-formingpolymerinblockcopolymermicelles

(BCMs) for targeted54 or non-targeted55 delivery of chemotherapeutics.56 Drug release

from BCMs is often on the timescale of hours to days, somore long-term PVL-based

deliverysystemshavebeendevelopedintheformofmicroparticlesorimplants.57,58

1.1.5 Preparationofpolyester-basedimplantabledrugdeliverysystemsIDDSs are generally defined as being in themicrometer or larger size range so

theycaneitherbeinjectedbyasyringeorimplantedwithinthebody.IDDSformulations

include microparticles, in situ forming implants, films, and cylinders. The general

preparationmethod involves dissolving the polymer and drug in a compatible solvent

thatisthenevaporatedtoentrapthedrugwithinthepolymermatrix.

Microparticles are most commonly prepared using an emulsion/solvent

15

evaporation technique. Polyesters are hydrophobic in nature, but can encapsulate

hydrophobicorhydrophilicdrugsdependingon themethodofpreparation.Theoil-in-

water (o/w) emulsion/solvent evaporation method is the method of choice for

encapsulatinghydrophobicdrugs.Preparationinvolvesdissolvingthepolymeranddrug

in an organic solvent, such as dichloromethane (the "oil phase"), and emulsifying this

solutioninanaqueousphasecontaininganappropriatestabilizer(e.g.polyvinylalcohol

(PVA)).59Thesolvent is thenevaporated,andtheresultingmicroparticlesarecollected

byfiltration.Theo/wmethodhasbeenusedtopreparePLGAmicroparticlesloadedwith

paclitaxel,60dexamethasone(DXM),61andmethotrexate62tobeusedforthetreatment

ofcancer,inflammatoryreactions,andrheumatoidarthritis,respectively.Forhydrophilic

drugsthatarenotsolubleinorganicsolvent,adoubleemulsionprocessisrequiredfor

encapsulation. The water-in-oil-in-water (w/o/w) emulsion/solvent evaporation

technique involves adding an aqueous solution of the drug to an organic phase

containing thepolymerundervigorousstirring,and thenadding thisprimaryemulsion

(w/o) intoanaqueousphasecontainingPVA.63Thesolvent isthenevaporatedandthe

microparticles are collected in the same fashion aswith the o/w emulsion technique.

The w/o/w method has been used to encapsulate octreotide acetate,64

chlorpheniraminemaleate,65andcalcitonin66inPLGAmicroparticlesforthetreatmentof

acromegaly, allergies, and high blood levels of calcium, respectively. While these

methods are commonly used and PLGA IDDSs prepared using thismethod have been

FDAapproved(Table2.5),drawbacks includethehealthrisksassociatedwithpotential

residual organic solvent, sometimes poor encapsulation efficiencies, and a high burst

16

release due to non-encapsulated drug that is adhered to the surface of the delivery

system.67,68

In situ forming implants are those that form a gel upon contact with the

physiologicalenvironmentafteradministrationbyasyringe.Themethodofpreparation

involvesheat-dissolvingPLGAinabiocompatible,water-misciblesolventanddispersing

thedrug in the cooledmixture, eitherbyhomogenizationor bydissolving thedrug in

another biocompatible, water-miscible solvent that is compatible with the polymer.63

This viscous solution is then injected intramuscularly or subcutaneously, whereby the

polymer precipitates upon contact with the aqueous environment, trapping the drug

withinthegelnetwork.69Theassociateddisadvantageswiththistypeofdeliverysystem

generally outweigh the advantage of convenience to the patient and physician.

Disadvantages include a high burst release due to a lag between injection and in situ

precipitation, the use of controversially biocompatible solvents such as dimethyl

sulfoxide (DMSO) or N-methyl-2-pyrrolidone (NMP), and variable and unpredictable

releaseratesduetotheuncontrolleddistributionofdrugswithinthematrix.70

Thepreparationofpolyester-based filmsandcylinders canbeachievedusinga

solventcasting/compressionmethodwherebythepolymeranddrugaredissolved ina

commonorganic solvent, castat60 °Cuntil solventevaporation,andcompressed ina

moldathighpressuretoformadensenetworkofthedesireddimensions.29Depending

onitssize,theimplantcanthenbeinjectedorimplantedinthedesiredlocation.Macro

scale implantshaveanumberofadvantagesoverdepot formulations.Firstly, theycan

beremovedintheeventofthepatientexperiencinganadversereactiontothedrugor

17

vehicle. Furthermore, the burst release sometimes observed with microparticles as a

result of free drug adhered to the surface can be avoided since the method of

preparationdoesnotrelyondrugpartitioningbetweenanorganicandaqueousphase.

Finally, for long-term drug delivery applications, PCL-based implants are particularly

preferredgiventheirslowinvivodegradationtimeanddiffusion-controlleddrugrelease.

1.1.6 Degradationofpolyester-basedimplantabledrugdeliverysystems

1.1.6.1 PLGA

PLGA-baseddelivery systemsdegradebyhydrolytic chain cleavage at the ester

linkagestoyieldlactideandglycolideoligomersandeventuallymonomers.Twodistinct

types of degradation/erosionoccur: surface erosion (heterogeneous) andbulk erosion

(homogeneous).Inthecaseofbulkerodingsystems,therateofwaterpenetrationinto

thesystemismorerapidthantherateofpolymerdegradation.Consequently,theentire

system is wetted, resulting in hydrolytic cleavage throughout the bulk. In contrast,

polymerchainsinsurfaceerodingsystemsarerapidlydegradedfromtheouterlayerin;

a heterogeneous process. In both instances, an increase in polymer backbone

hydrophilicity,carboxylicacidendgroupsthatarise fromthedegradationprocess,and

less crystallinity further increase the rate of water penetration into the system and

subsequently increase the rate of degradation.71 In contrast to PGA-based systems,

PLGA-based systems are typically regarded as bulk eroding dosage forms since the

lactideportion ismoreresistant tohydrolysisbecausedegradationoccursmoreslowly

than water penetration into the matrix. Fu et al. have shown an acidic microclimate

18

withinPLGAmicrospheresusingconfocalmicroscopy,illustratinghowtheproductionof

lactic and glycolic acids from polymer chain scission can significantly decrease the

environmental pH within the matrix.72 Since ester-bond hydrolysis is catalyzed by

protons, theacidicclimatewithin thesystemcan furtherenhance the rateofpolymer

chaincleavageandsubsequentdegradationofthedevice.42

1.1.6.2 PCL

PCL is known todegradevery slowlydue to itshydrophobicity,making itmore

resistanttowaterpenetrationthanPLGA.ForPCL,theslowdiffusionofwater intothe

matrixistherate-limitingstepfordegradation.PCLdegradesbyhydrolyticchainscission

(i.e. cleavage) of its ester bonds to yield oligomers and eventually monomers.

Autocatalysisofcarboxylicacidendgroupsgeneratedbychainscissionalsocontributes

todegradation.ThedegradationmechanismofPCLisbulkerosionsincetheslowrateof

degradation is independent of device surface area.73 With a total degradation time

between 2-4 years, PCL is suitable for long-term drug delivery applications such as

contraception,where drug release is on the order of years rather thanmonths.74 The

rateofdegradationdependsonthestartingmolecularweight(MW)oftheimplant,with

higher MW PCL having longer chains and more ester groups to cleave in order to

generatedegradationproducts,leadingtoaslowerrateofdegradation.44Incontrastto

PLGA degradation products that are eliminated by natural metabolic pathways,

Woodwardetal.foundthatPCLdegradationproducts(e.g.MW<3000gmol-1)become

sequestered in phagosomes and undergo intracellular degradation,making PCL a fully

19

resorbablepolymeroncehydrolyticdegradationreducestheMWtobelow3000gmol-

1.75 High drug loading content also contributes to a slightly accelerated degradation

relative to control, non-drug loaded systems. Cheng et al. demonstrated 36.5% and

33.2%masslossofhighmolecularweightPCLimplantsat50%and25%drugloading(%

w/w)comparedto12.9%and11.3%masslossat12.5%and6.25%drugloading.76These

differencescanbeexplainedbythevoidcreatedinthespaceonceoccupiedbythedrug

that now forms a passage for enhancedwater uptake and hydrolytic degradation. To

evaluatethepotentialofPCLasalong-termcontraceptivedevice,Sunetal.performeda

3year invivodegradationstudyonaPCL/PluronicF68-basedimplantusingradioactive

labeling.34TheyshowedthatimplantshavinganinitialMWof66000gmol-1maintained

structural integrityover twoyears invivo, andonlybroke into lowMW(8000gmol-1)

piecesattheendof30months.Theauthorstheninvestigatedsafetyandcontraceptive

efficacy of the implants loadedwith LNG. The implants showed no toxicity, sustained

LNGreleaseovertwoyears,andadecreaseintheMWfrom66000gmol-1to15000g

mol-1attheendoftwoyearsinvivo.77Theirworkresultedinapprovalforthedeviceto

undergotestinginphaseIIhumanclinicaltrialsinChina.

1.1.6.3 PVLSince the degradation rate of PVL is slightly faster than that of PCL, PCL-b-PVL

copolymers have been synthesized to enhance the degradation of PCL for biomedical

applications.78 The authors found that with increasing MW (e.g. 12000, 25000, and

40000 gmol-1) and crystallinity, the rate of degradation decreasedbutwas still faster

20

thanPCLhomopolymerofsimilarMW.Linetal.prepareddiclofenacsodiumloadedPVL

microparticles using an oil-in-oil emulsionmethod and compared in vitro release and

degradation to drug-loaded PLGA and PLA microparticles.57 The authors found the

followingorderofreleaseanddegradation:PLGA>PLA>PVL,whichfollowedtheextent

ofwaterhydrationandcrystallinityofthepolymer.Importantly,PVLdemonstratedthe

lowestburstandlongestdurationofdrugrelease(lessthan80%in8days).Fukuzakiet

al.preparedlowmolecularweightPLA-co-PVLimplantsusingamelt-pressingtechnique

andmeasured in vitro and in vivodegradation kinetics.58 After 6weeks in phosphate

bufferedsaline (PBS), implantweight losswas foundtobe~30%fora30/70PLA/PVL

blend,whichwasgreaterthanthehomopolymersalone.Incontrast,theimplantswere

completelydegradedafter2weeks invivo. Imasakaetal. investigatedthepotentialof

PCL-co-PVL implantsascontrolleddeliverysystemsforestramustine.79Theyfoundthat

theimplantswerecapableofestramustinereleaseoveraperiodof20weeks,withless

than30%ofthedrugbeingreleasedatthistime.Theimplantsalsodisplayedfirstorder

degradationkinetics,with15%weightlossobservedafter20weeks,whichwasgreater

than PCL homopolymer. Interestingly, the macroscopic properties of the implants

changedwith varying the PCL or PVL composition,with higher PVL content producing

paste-likebulkmaterialsrelativetothewaxybulkmaterialsobtainedat92%PCL.

1.1.7 Drugreleasemechanismsofpolyester-basedimplantabledrugdeliverysystemsAn understanding of the complex and connected processes involved in drug

releasefromthematricesisrequiredinordertodesigneffectiveIDDSs.Additionally,this

21

knowledgecanbeusedtoselectappropriatemathematicalmodelstopredicttheeffect

ofchangingdesignvariables (e.g.size, shape)ondrug release, therebyeliminating the

needformultipletime-consuminginvitroreleaseexperiments.80Inthisthesis,theterm

releasemechanismwillrefertotherate-controllingmasstransportstepthatgovernsthe

rate at which drug is released i.e. the slowest process. There are various factors

impactingtheoverallreleasemechanism,e.g.polymer-druginteractions,andthesewill

betakenintoconsiderationaswell.

1.1.7.1 PLGA

The two main release mechanisms involved in drug release from PLGA-based

IDDSsarediffusionandbulkandsurfaceerosion.SincePLGAisgenerallyregardedasa

bulk eroding dosage form, drug release is initially diffusion-controlled as the rate of

waterpenetrationismorerapidthanthedegradation/erosion.Drugdiffusioncanoccur

through water-filled pores or, more slowly, through the amorphous polymer regions.

Mechanistically, upon submersion in aqueous media, water penetrates the polymer

matrixandbeginstohydrolyzetheesterbonds, initiatingtheprocessofdegradation.41

Inparallel,drugbeginstodiffusethroughamorphouspolymerregionsnearthesurface

ofthedevice,leadingtoaninitial"burst"release i.e.arapidreleaseofdrugwithinthe

first24hours.Thiseffectcanbeattributedtothedissolutionofnon-encapsulateddrug

at the surface of the device,61 diffusion of drug encapsulated near the surface of the

22

polymer,81thediffusionofdrugthroughsurfaceporesorcavities,12ortheinitialswelling

ofthepolymersystem.82Thenext,moreprolonged,stageofreleaseisacombinationof

diffusionthroughthepolymeranddiffusionthroughwaterfilledporesandchannelsthat

begin to form upon bulk hydrolysis, which induces both physicochemical and

morphological changes in the device.62 Finally, as the polymer degradation products

catalyze further hydrolysis, a third phase of faster release is attributedmainly to the

erosion/degradation of the matrix as oligomers are released and larger pores are

created.62Thisisjustonemechanisticinterpretationofatriphasicreleaseprofile,though

amultitudeoffactorsareatplayateverystageofrelease.

1.1.7.2 PCL

On the basis of its high hydrophobicity, lack of swelling in aqueous media,

crystallinity,and lowrateofdegradation,drugrelease fromPCL-based IDDSs ismostly

controlled by slow diffusion through the PCL matrix, resulting in first order release

profiles. Chenget al. reported the release of praziquantel from injectionmolded high

molecularweight (MW=90000 gmol-1) PCL implantswas a slow, diffusion-controlled

releaseoveranobservationperiodofoneyear.76Additionally,Aydinetal.observeda

lower release rate of doxycycline from 65000 g mol-1 compared to 14000 g mol-1

microparticlesasa resultof the increasedhydrophobicproperties.50 In contrast,when

examining the release rate of hydrophilic drugs from PCL microparticle matrices,

Rosenbergetal.foundtheentiredrugloadwasreleasedwithinthreeweeksduetothe

23

preferential partitioning of the drug into the aqueous micropores of the matrix.83

Therefore,thediffusion-controlledreleaseofdrugsfromPCLmatriceshighlydependson

thedrug'sphysicochemicalpropertiesaswellasthoseofthepolymer.

1.1.8 Factorsinvolvedindrugreleasefrompolyester-basedmatricesPolyesterphysicochemicalpropertiesthatimpactthedegradationofthedevice,

suchashydrophobicity,molecularweight,andcrystallinity,also influencedrug release

kinetics.Waterpenetrationintoadeviceisinverselyrelatedtopolymerhydrophobicity,

whichcanbeevaluatedbywater contactanglemeasurements. Smaller contactangles

lead to an increased water penetration rate, resulting in faster erosion and a higher

releaserate.84Theimpactofpolymermolecularweightondrugreleasedependsonthe

polymer.Forexample,releasefromhighmolecularweightPLGAisslowerthanrelease

fromlowmolecularweightPLGA,butthisobservationisinvertedinthecaseofPCL.Itis

clearthatotherfactorscomeintoplaywithchangesinmolecularweight,suchaserosion

(PLGA) or crystallinity (PCL). To investigate the impact of PCLMWand crystallinity on

drugrelease,Jeongetal.prepareddifferentPCLmicroparticlesloadedwithpapaverine,

amoderatelywatersolubledrug.85Theauthorsfounddrugdiffusionandreleasetobe

mostrapidfromthehighestMWPCLduetothelargeramorphousregionsinhigherMW

PCL.85Aydinetal.alsofoundthatPCLcrystallinitydecreaseswithincreasingMW,though

theyobservedthatdoxycyclinereleasewasfaster fromhigherMWPCLmicroparticles,

which could be explained by the increased drug loading in the amorphous regions.50

Lowercrystallinityalso facilitates theaccessofwater to thematrix, therebyyieldinga

24

fasterdrugrelease.86

Drug solubility and the resulting dissolution in aqueous media is also an

important factor thatgovernsdrug release. Ingeneral, increased solubility leads toan

increase in thedrugdissolution rateandahigher concentrationgradientbetween the

matrix and external media, thereby increasing the driving force for drug release.87

Faisant et al. have shown that the release of 5-fluorouracil (5-FU) from PLGA

microparticlesisinfluencedbythesolubilityof5-FUinthereleasebufferaswellasthe

pH of the release buffer.88 The physical state of the drug within the matrix, which

dependsonpolymer-drug interactions(e.g.hydrogenbonding,dipole interactions,Van

derWaalsforces),processingconditions,anddrug loading levels, impactsreleasefrom

thematrix. It isknownthatdrugsmaybeeithermolecularlydispersed (e.g. crystalline

phase separation) or molecularly dissolved (e.g. amorphously distributed) within the

matrix,yetonlydissolveddrug isavailable fordiffusion.89Therefore, thedissolutionof

crystalline drugsmay act as a rate-limiting step to diffusion of drug from thematrix.

Favourable polymer-drug interactions serve to dissolve the drug in the matrix, and a

greater degree of interaction betweenpolymer and drug can lead to a slower rate of

drugrelease.90

1.1.9 MathematicalmodelingofdrugreleaseMathematical modeling of drug release is a highly useful tool in pre-clinical

formulation optimization. Firstly, it enables quantitative prediction of the effects of

changesinformulationandprocessingparametersontheresultingdrugreleasekinetics.

25

Secondly,itcanprovideinsightintotheunderlyingmechanismscontrollingdrugrelease

from the particular dosage form.80 Consequently, product development can be

acceleratedsincetime-intensiveinvitrotrial-and-errorexperimentscanbereduced.89In

order to select an appropriate mathematical model to quantitatively describe the

experimentally measured drug release, the control and drug-loaded IDDS must be

extensivelycharacterizedbeforeandafterexposuretothereleasemediatodetermine

the type of system and expected mechanisms of release being dealt with. Diffusion,

swelling,anderosionarethemostcommonrate-controllingdrugreleasemechanismsof

commerciallyavailablecontrolledreleasedosageforms.91Forthepurposeofthisthesis,

diffusionwill be considered exclusively. Though a number of processes also inevitably

contributetodrugrelease,themathematicalmodelneedonlyconsiderdiffusionif it is

therate-limitingmasstransportstepinvolvedindrugrelease.

Therearethreemainconsiderationswhenselectinganappropriatemathematical

equationforaparticularsystem:89

1) Thetypeofsystemi.e.isitamatrixorareservoirsystem?

2) Thedrugconcentrationrelativetothedrugsolubilityinthesystemi.e.is

itaboveorbelowthemaximumsolubilityinthewettedsystem?

3) Thegeometryofthesystem,whichisoftenlimitedtoslabs,spheresand

cylinders

Once the appropriate mathematical model is chosen based on these

considerations, unknown parameters such as the diffusion coefficient can be fitted to

theequationusingenoughdatatocharacterizetheshapeofthecurve.Then,themodel

26

can be used to quantitatively predict the impact of a certain parameter (e.g. cylinder

height)ontheresultingdrugreleasekinetics.Thistypeofmodeling isespeciallyuseful

whenlongreleasedurationsaretargetedtoeliminatetheneedformultiple,potentially

years-long,invitroreleaseexperiments.

1.1.10 Solubilityparameters TheHildebrandsolubilityparameter (δ)providesan indicationof thedegreeof

interaction between materials and can be a good indication of material-material

miscibility.92 It is defined as the square root of a molecule's cohesive energy density

(CED),whichisadirectmeasureoftheattractionthatatomsormoleculeshaveforone

another.93

𝛿 = (𝐶𝐸𝐷)!.! = (∆𝐸!/𝑉!)!.! [1]

whereΔEvistheenergyofvaporizationandVmisthemolarvolume.Thetotalsolubility

parameter (i.e. Hansen solubility parameter) can then be divided up into three

components (i.e. partial solubility parameters) accounting for different interatomic or

intermolecular forces.94 The total solubility parameter is then the sum of the

contributionsfromeachoftheseforces:

𝛿! = (𝛿!! + 𝛿!! + 𝛿!!)!/! [2]

where δd, δp, and δh, are the contributions resulting from Van der Waals dispersion

forces, dipole-dipole interactions, and hydrogen bonding, respectively. The partial

solubility parameters can be estimated using the group contribution method (GCM),

which is based on the theory that a molecule's total CED is an additive property.

27

Therefore,thetotalCEDofamoleculecanbecalculatedbysummingthecontributions

fromtheindividualfunctionalgroupsinthemolecule:

𝛿! =∑𝐹!"𝑉!

[3]

𝛿! =∑𝐹!"!

𝑉! [4]

𝛿! =∑𝐸!!𝑉!

[5]

whereFdi,Fpi,andFhiarethecontributionsfromthespecificfunctionalgroupsi.e.vander

Waalsdispersionforces,dipole-dipoleinteractions,andhydrogenbonding,respectively.

Several groups have used solubility parameters to predict polymer-drug compatibility

andinteractions.56,95-97

1.1.11 Functionalizationofpolyestersasameanstotailor physicochemicalpropertiesPolyester co-polymers are most commonly prepared by ROP according to a

varietyofmechanisms(e.g.cationic,anionic,coordinative)andarepropagatedbyactive

hydrogenorzwitterionicspecies.98Traditionally,aliphaticpolyestershavebeenprepared

by ROP using metal catalysts, such as tin and aluminum salts. Unlike the use of the

common catalysts Sn(Oct)2 and Sc(OTtf)2), compounds such as 1,5,7-

triazabicyclo[4.4.0]dec-5-ene(TBD)affordrapidpolymerizationkineticsoflactones(e.g.

valerolactone and caprolactone) in ametal-free environment at ambient temperature

28

and result in polymer products with a low polydispersity index (PDI).99 In designing

effective IDDSs, the careful selection of polymer physicochemical properties is

considered crucial in obtaining good polymer-drug compatibility and desired release

profiles. Moreover, since no polymer-based IDDS can be considered universal for all

therapeutics,thereisanunmetneedfordeliverysystemsthatcanbeeasilytailoredto

suit the current application of interest. Functionalization of traditional aliphatic

polyestersisastrategyusedtoobtaindesiredphysicochemicalpropertiesforcontrolled

release applications. The incorporation of alkene-substituted lactones in polyesters

enablesintegrationofnewchemicalfunctionalitiesandresultsintunablematerialswith

abroad rangeofproperties.100Whenpendantallyl functionality is introduced into the

polyester backbone, a convenient synthetic handle becomes available for post-

polymerization functionalization via radical addition of thiols,101 or attachment of

hydrophilicgroups,drugsorprodrugs,targetinggroups,orfluorophores.99Molanderand

Harris first reported the synthesis of allyl-ε-caprolactone (ACL) by alkylation of ε-

caprolactone(CL)withallylbromideinfairyield(65%).102Darcosetal.thensynthesized

randompoly(CL-co-ACL)copolymersofvaryingmolecularweightandACLmolarratioto

prepare amino-functionalized polyesters by thiol-ene click chemistry.103 Excellent yield

andPDIwasachievedfor12000gmol-1poly(CL-co-ACL)with10%allylfunctionalitybut

unfortunately,effortstoincreasetheACLmolarratioandtargethighermolecularweight

copolymersledtopooryieldsandpolymerproductswithunacceptablyhighPDI.Other

groupshavepreparedpoly(CL-co-ACL)withsimilarsyntheticresults(i.e.yieldandPDI)to

incorporate bromide, epoxide, or silane functionality.100 Leemhuis et al. have

29

synthesized allyl-glycolide with the goal of incorporating allyl-functionality in PLGA

copolymers.104Monomer yields were low (50%), but the resulting poly(allyl-glycolide)

(PAG) could be copolymerized with lactide to yield poly(lactide-co-allyl-glycolide) of

varyingL:AGratios.Notsurprisingly,noTmwasdetectedandtheTgof thecopolymers

decreased with increasing AG content, indicating the copolymers were completely

amorphous since the pendant allyl group disrupts polymer chain crystallization.105 An

alternative approach to designing functionalized polyester copolymers has been

accomplishedusingtheallyl-δ-valerolactone(AVL)monomer,whichcanbesynthesized

byalkylationofVL ingoodyield(>70%).102UnlikePCLandPVL,homopolymersofAVL

are completely amorphous and liquid in bulk at room temperature. Because of this

property,therearenoreportsintheliteratureofAVLhomopolymersbeingexploredfor

drugdeliveryapplications.Parrishetal.haveperformedSn(OTf)2-catalyzedROPofAVL

andVLtopreparethecopolymerPVL-co-PAVLatroomtemperature,inverygoodyields,

andwithlowPDI.105However,theauthorswereonlyabletoachieve8300gmol-1PVL-

co-PAVL,whichwascompletelyamorphousat15%AVLincorporation.

The thiol-ene click reaction is any mechanism by which a thiol is added to a

double bond, most typically through free radical addition in the presence of a

photoinitiator.106 The thiol-ene click reaction can be used to covalently attach thiol

groups to alkene-functionalized polyesters to permit further functionalization or

fabricationofcross-linkedmatricesthatspanawiderangeofapplicationsfromdentistry

to drug delivery. To enhance the hydrophilicity and degradation of poly(CL-co-ACL),

Darcos et al.used the thiol-ene click reaction to add 2-(Boc-amino)ethanethiol to the

30

pendantallylgroup,thendeprotectedtheaminogrouptoobtainamino-functionalized

poly(CL-co-ACL).103 Importantly, themolar ratio of allyl groups to thiol groups can be

tailoredbasedondesiredpolymerproperties,i.e.agreaternumberofaminogroupswill

enhance the hydrophilicity, degradation, and subsequent drug release from the

matrices. Campos et al. highlight the tremendous versatility of the thiol-ene click

reactioninpreparingfunctionalizedpolyesterswithpropertiessuitableforvirtuallyany

applicationinastudyonpoly(CL-co-ACL).107Forexample,thioglycolicacidwascoupled

to the double bond to improve the solubility of the resulting polyester, a

trimethoxysilane group was chosen to allow for coupling to ceramic surfaces or

acid/basecatalyzedcross-linking,andaprotectedaminogroup(Fmoc-C)wasaddedasa

buildingblock for theattachmentofpeptide fragments.107 The thiol-ene click reaction

can alsobeused to cross-link twopendant allyl groups toone another to yield cross-

linked polymeric networks. Boire et al. cross-linked poly(ε-CL-co-α-allyl carboxylate-ε-

caprolactone)(PCL-co-PACCL)with1,6-hexanedioltoyieldshapememorypolymericgel

networksforvascularapplications.108Theyshowedthatthephysicochemicalproperties

of the gel, such as crystallinity, swelling, and mechanical properties, were tunable

dependingonthe%crosslinkingofthematerials.Silversetal.demonstratedtheutility

of thiol-ene coupling to yield nanoparticles of poly(AVL-b-propargyl valerolactone)

(PAVL-b-PgVL) cross-linked with 1,8-diazide-3,6-dioxaoctane.99 When not fully cross-

linked, thependantallylgroupsareamenable to further functionalizationto introduce

other moieties to expand the library of biodegradable materials for tailored and

controlleddrugdeliveryapplications.

31

2 FactorsInvolvedinDrugReleasefromCross-Linked

Polyester-BasedMatrices

Introduction2

The current study reports on the preparation and characterization of novel,

polyester-basedimplantsforsustainedreleasedrugdeliveryapplications.ThreePVL-co-

PAVL copolymers of varying molecular weights and % allyl group content were

synthesized byDr. Frantz LeDevedec. The bulk copolymerswere characterized by gel

permeationchromatography (GPC), 1H-NMRspectroscopy,DSC,andXRD todetermine

relevant physicochemical properties. The IDDS was prepared using a syringe casting

methodinwhichthependantallylgroupswerecross-linkedwith1,6-hexanedithiolusing

athiol-eneclickreaction.Then,the IDDSwasfurthercharacterizedbyDSCandXRDto

determine the impactof the cross-linking reactionon the finaldrugdeliveryplatform.

Fivedrugsofvaryingphysicochemicalproperties,suchaslogP,MW,molecularvolume,

aqueous solubility, and Hansen solubility parameters were loaded into their own

cylinder,andthedrug-loadedsystemswerefurthercharacterizedtoevaluatetheextent

ofpolymer-druginteractionsinordertobetterelucidatetheunderlyingmechanismsof

drugrelease.Invitrodrugreleasewasmeasuredinthepresenceofvarioussurfactants,

and a mathematical model based on Fick's second law of diffusion was fitted to the

experimentally measured release data to determine the diffusion coefficients of the

drugswithinthesystems.Thestability/degradationofthesystemwasassessedinPBS.

Finally, invitrobiocompatibilitywasevaluatedinL929mousefibroblastcellsusingthe

32

extraction dilutionmethod of cytotoxicity testing. Overall, the IDDS presented in this

thesis is capableof sustained,diffusion-controlled releaseofhydrophobicdrugsbased

ontheirsolubilitiesandstateswithinthesystem.

2.1 ExperimentalSection

2.1.1 Materials

Theallyl-valerolactonemonomerwasprovidedbyPolysciencesInc.(Warrington,

PA). Acetaminophen (98-102% powder) was purchased from Sigma-Aldrich Canada

(Oakville,ON). Curcuminwaspurchased fromCaymanChemical Company (AnnArbor,

MI). Paclitaxel was purchased from Ark Pharm Inc. (Libertyville, IL). Triamcinolone

acetonide and triamcinolone hexacetonide (USP powder) were purchased from

Spectrum Chemical MFG Corporation (New Brunswick, NJ). Sulforhodamine B was

purchasedfromCedarlaneLabs(Burlington,ON).AllsolventswereHPLCorACSreagent

grade andwere suppliedbyCaledon Laboratories Ltd. andused as received.All other

materials were supplied by Sigma-Aldrich Canada (Oakville, ON) and were used as

received.

2.1.2 SynthesisofPVL-co-PAVLcopolymers

Polyestercopolymers(PVL-co-PAVL)werepreparedasdescribedbySilversetal.

with slightmodifications.99 Briefly, the catalyst TBD (2mol%, 0.273 g)was added to a

flame-driedroundtwo-neckflaskanddriedundervacuum.Anhydroustoluene(50mL)

andbenzyl alcohol (m=0.027 g)were then combined in the two-neck flaskwith TBD

underargonandstirredfor30min.Distilledmonomers(VL=9mLandAVL=1.01mL)

33

were combined prior to their transfer by cannulation to the reaction vessel under

positive pressureof argon. Polymerizationwas carriedout at room temperature for 6

hours.Theslurrysolutionwasfirstprecipitatedin2Lcoldmethanol,re-dissolvedinTHF

(50mL)andthenprecipitatedin2Lofamixtureofhexane/ethylether(30/70%v/v).

2.1.3 Preparationofthecross-linkedpolyester-basedmatrices

100 mg of copolymer, 0.25 molar equivalents of 2,2-dimethoxy-2-

phenylacetophenone (DMPA), and 0.5 functional group molar equivalents of 1,6-

hexanedithiol (1,6-HDT) were combined in 1 mL dry DMSO and warmed until full

dissolution.Thesolutionwasdrawnintoa1mLsyringe(i.d.4.7mm)andthesyringewas

placed upright to allow for UV crosslinking at 365 nm for 20minutes. The tip of the

syringewasremoved.Then,thecylindricalcross-linkedpolymer(CP)wasremovedfrom

the syringe. In order to remove any unreacted startingmaterials, the CPwaswashed

extensivelyinTHF,thendriedatroomtemperatureandpressure.

2.1.4 Characterizationofcopolymersandcross-linkedmatrices

The 1H NMR spectra were recorded on a Bruker AMX400 or Bruker AC300

spectrometer for the PVL-co-PAVL copolymers. The MW of each copolymer was

determinedbyGPCanalysis in tetrahydrofuran (THF)usingaWaters2695systemthat

includes twoPLgel5μmAgilentcolumnsandaWaters2414RIdetector.Acalibration

curve was constructed using polystyrene standards. DSC measurements of the bulk

copolymers and cross-linked matrices were carried out on a Q100 TA series thermal

analysis system over different temperature rangeswith a common heating rate of 10

34

oC/minundernitrogen(3cycles).%crystallinity(χc)wascalculatedas:

𝜒! = Δ𝐻!/Δ𝐻!° x 100%

whereΔ°Hm=144J/g,theenthalpyoffusionfor100%crystallinePCL.XRDspectraofthe

bulkcopolymersandcross-linkedmatriceswerecollectedonaPhilipsX-rayDiffraction

System.

2.1.5 Determinationofsolubilityparameters

UsingHansen'sapproach,109partialsolubilityparameterswerecalculatedbythe

GCM using equations 3-5 to calculate the total solubility parameter as outlined in

equation2.TotalmolarvolumesofthedrugswerecalculatedusingACDLabssoftware,

while The Hoftyzer-van Krevelen's method110 was used to predict Fdi, Fpi, and Fhi of

polymerrepeatunitsandvariousfunctionalgroupsonthedrugsandpolymers.

2.1.6 Drugloading

All drugs were loaded using a solvent equilibration evaporation technique, as

describedbyLeeetal.,111butwithslightmodifications.Briefly,THFwaschosenasthe

loadingsolventbecauseofitshighdrugsolubilityandCPswelling.Drugsweredissolved

inTHFataconcentrationof30mg/mL.TheCPs(~15mg)wereequilibrated in0.5mL

loading solution for four hours followed by a brief rinse in fresh THF to remove drug

adhered to the surface. The CPswere dried overnight at room temperature and then

weighedonananalyticalbalancetodeterminethedrugloadingcontent(%w/w).

2.1.7 Determinationofdrugsolubility

35

Anexcessofdrug(~4mg)wasaddedto4mLofrespectivemediaandincubated

at37°Cfor48hoursundermagneticstirring.Thesolutionwascentrifugedat8000rpm

for 10minutes, and a 1mL aliquotwaswithdrawn, filtered through a 0.45 µm nylon

membrane,andanalyzedbyhigh-pressure liquidchromatography (HPLC),asdescribed

below.

2.1.8 Drugreleaseassay CPs were placed in a cell strainer to ensure buoyancy and added to beakers

containing100mLofPBSpH7.4witheither0.1%Tween®80or0.5%SDStoenhancethe

solubility of the poorly water soluble drugs. The CPs were incubated at 37°C under

constant stirring, and sink conditions were maintained throughout the entire release

experiment by replacement of the releasemediawith fresh PBSwith the appropriate

surfactantatleasteverythreedays.Atpre-determinedtimepoints,a1mLaliquotwas

removedandimmediatelyfrozenat-20°CforstoragepriortoHPLCanalysis.Inthecase

of incompletedrug releaseduring theobservationperiod, the remainingdrug content

wasdeterminedbyextraction,asfollows:THFwasaddedtothecylindersandtheywere

shakenonanorbitalshakerfor4hours.TheTHFwasremovedandevaporatedbyrotary

evaporationat45°C,and freshTHFwasadded toextract the remainingdrug fromthe

cylinders.Thedrugwasreconstitutedin1mLoftheHPLCmobilephaseandanalyzed,as

described below. The amount remaining was also used to correct the drug loading

content(%w/w)measuredbyanalyticalbalance.

2.1.9 HPLCanalysisofdrugrelease

36

The drug release was analyzed by reverse phase HPLC methods. The system

combinedanAgilent1200HPLCsystemwithAgilentChemStationsoftwarecontrol,an

XDB C18 column (150x4.6mm i.d), and UV detection. For paclitaxel (PTX), themobile

phase consisted of 55:45 acetonitrile:water flowing at 1mL/minwithUVdetection at

227 nm. For triamcinolone acetonide (TAA), the mobile phase consisted of 65:35

methanol:water(0.1%aceticacid)flowingat1.2mL/minwithUVdetectionat240nm.

For triamcinolone hexacetonide (TAH), the mobile phase consisted of 85:15

methanol:water(0.1%aceticacid)flowingat1.2mL/minwithUVdetectionat240nm.

Forcurcumin(CCM),themobilephaseconsistedof55:45acetonitrile:water(0.1%acetic

acid)flowingat1mL/minwithUVdetectionat420nm.Foracetaminophen(ACM),the

mobilephaseconsistedof80:20methanol:waterflowingat1mL/minwithUVdetection

at 250nm.Drug concentrationswerequantifiedusing a prepared calibration curveof

knownconcentrationsbetweentherangeof1to100µg/mL(R2between0.9986-1).

2.1.10 Degradationofcross-linkedpolyester-basedmatrices DegradationoftheCPswasassessedbyincubating~15mgCPslicesinPBSpH7.4

understirringconditions.Atpre-determinedtimepoints,theCPswereremoved,blotted

dry, weighed, the pH of the surrounding media was measured, and the media was

exchangedforfreshbuffer.

2.1.11 Bufferpenetrationintocross-linkedpolyester-basedmatrices Tomonitor buffer penetration into the CPmatrix, cylinders (approximately 2.5

mmx2.5mm)wereincubatedinPBSpH7.4with0.1mg/mLsulforhodamineB(SRB),as

37

describedbyKoenningsetal.112At5,24,and120hours, thecylinderswere removed,

blotteddry,andfrozenat-80°Cuntilcross-sectioning.Cross-sectionswereobtainedby

cuttingintheaxialdirectionwithaLeicaEMUC6/FC6Cryo-Ultramicrotome,andsamples

were placed immediately into a dessicator until microscopic analysis. Confocal

microscopy was performed on a ZEISS confocal laser scanning microscope (ZEISS,

Germany).Inparallel,ahollowend-cappedcylinderwaspreparedandtheinternalcavity

was filledwith 1mg/mL SRB. At pre-determined time points, a samplewas removed

fromtheexternalmediaandtheabsorbanceofSRBwasmeasuredat540nm.

2.1.12 Cellcultureexperiments ThecytotoxicitiesoftheCP15KandCP39KmatriceswereevaluatedinL929mouse

fibroblast cells using the extraction dilutionmethod.113 CPswere incubated in culture

mediumatasurfaceareatovolumeratioof1.25cm2/mLfor48h.Then,themediawas

serialdiluted two-fold to the followingconcentrations:50,25,12.5,6.25,and3.125%.

TheL929fibroblastcellswereculturedandroutinelymaintainedinDulbecco'smodified

Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 1%

penicillin-streptomycin. The cells were grown in a monolayer in tissue culture flasks

incubatedat37°Cand5%CO2at90%relativehumidity.Cellswerecountedandseeded

in 96-well plates at a density of 2000 cells/well, which was determined to be the

optimumcelldensity.After24h incubation,growthmediawasaspiratedandreplaced

with either 150 µL CP15K or CP39K extracts at a surface area to volume ratio of 1.25

cm2/mLor the same volumeof extractionmedia that hadbeen serially diluted to the

38

above concentrations. Following24,48, and72h incubationperiods, cell viabilitywas

evaluatedusingtheMTSassaymethod.Specifically,theextractionmediawasaspirated

andreplacedwith200µLoffreshmediafollowedby20µLofMTSreagent,andthecells

wereincubatedat37°Cfor1.5h.Cellviabilitywasmeasuredbyopticalabsorbanceatλ

=490nmusingaCytation™5CellImagingMulti-ModeReader(BioTek,Vermont).Cells

incubated with media were employed as control and this was considered 100% cell

viability. 100 µM cadmium chloride (CdCl2) was used as a positive control. All

experimentswereconductedintriplicate.

39

2.2 ResultsandDiscussion

2.2.1 BulkcopolymersynthesisandcharacterizationThree random copolymers based on poly(valerolactone)-co-

poly(allyl)valerolactone(PVL-co-PAVL)(P7.5K,P15K,P39K)werepreparedbymetal-freeROP

catalyzedbyTBD.Gelpermeationchromatographyrevealedamonomodaldistribution

for the copolymers and PDI of ≤ 1.5 (Table 2.1.). 1H-NMR spectroscopy was used to

confirm thedegreeof polymerization and the%AVL in the resulting copolymers. The

terminalphenylgroupwasusedasaninternalreference(m,phenylδ=7.33ppm),5.70

ppm(m,CH2=CH),5.03ppm(m,CH2=CH),4.08ppm(m,CH2-OC-O),2.38ppm(m,-CH2-

CH-, and –O=C-CH2-), 1.68 ppm (m, -CH2-CH2- VL, AVL) (Figure 2.21-Appendix). After

precipitation in ether andmethanol, the polymerswere dried under vacuum at room

temperaturewithayieldof80%(~10g).Thethreecopolymershadmolecularweights

(Mn)of7500,15000,39000g.mol-1andcontained23,16and7%AVL, respectively.To

ourknowledge, this is the first time thathighmolecularweight (MW>10000g.mol-1)

PVL-co-PAVLhasbeensynthesized.

Thethermalpropertiesof thecopolymers (Figure2.1)aresummarized inTable

2.1. Inbulk,P7.5K iswaxywhileP15KandP39Karepowders.TheTg forP7.5KandP15Kwas

observed at ≈ -62 °C whereas no Tg was detected for P39K. It is known that with a

decreaseintheMW,thedegreeofchainentanglementdecreasesandsubsequently,the

polymerchainmobilityincreasesandtheTgdecreases.114ThelowMWcopolymer,P7.5K,

containing23%AVLrevealedalargecrystallizationtransition(Tc)at-24°C,indicatingthe

polymercontainsa largeamorphousphase.This isunsurprisinggiventhependantallyl

40

grouphastheabilitytodisruptpolymerchaincrystallization.105,115Meltingtemperatures

ofthethreecopolymersincreasedfrom12.6°Cto47.6°CwithanincreaseinMWanda

decrease in AVL content. It is known that Tm increases with increasingMW.78,116 The

enthalpyofmelting(Δ°Hm)andχcisgreatestforP39K,suggestingitisthemostcrystalline

of the three copolymers. This is due to the low AVL content and subsequently large

crystallineVLdomainsthatarefreetoundergoameltingtransition.117Notsurprisingly,

the three copolymers are less crystalline in nature than PCL homopolymers of similar

molecularweight.This lowercrystallinitymustbeattributedtothependantallylgroup

thatdisruptschaincrystallinitysincePVL(MW=7000gmol-1)homopolymerdisplaysthe

same crystallinity as 100% crystalline PCL. We hypothesize that it is likely the AVL

contentthatcontributesmoresignificantlythanMWtooverall thermalcharacteristics.

ThelargeTcandsmallΔ°HmobservedforP7.5Kwith23%AVLindicatesalargeamorphous

phase.AlthoughtheMWisdoubledforP15K,the7%decreaseinAVLcontent(from23%

to16%)results inasignificant increaseinΔ°Hm, indicatingtheincreasedcrystallinityof

thecopolymerduetothefewernumberofpendantallylgroupsavailabletodisruptthe

crystallinity.Amoresystematicstudyinwhichthemolecularweightofthecopolymersis

held constant while varying the AVL content would be necessary to validate this

hypothesis.

41

Figure2.1Thermogramsofthethreebulkcopolymers(P39K,P15K,P7.5K)obtainedbyDSCat10°C/min(2ndcycle).

Table2.1Molecularweightdistribution,thermalcharacteristics,andcrystallinityofthethree bulk copolymers (P39K, P15K, P7.5K) and the cross-linked polymers (CP7.5K, CP15K,CP39K).

a P7.5K, P15K and P39K refer to the different PVL-co-PAVL copolymers.b Number-averagemolecularweight(g/mol) obtained from GPC analysis. (Polydispersity index = PDI). c Number-average molecular weight(g/mol) obtained by 1H NMR spectroscopy. d Number and percentage (%) of allyl-valerolactone in thecopolymer (% AVL) based on the total molecular weight determined by 1H NMR spectroscopy.e Glasstransition (Tg)

f, melting temperature (Tm)g and enthalpy of melting (Δ°Hm)

h were determined by DSCanalysis(2ndcycle).%crystallinity(χC)

iwasdeterminedusingtheenthalpyof100%crystallinePCL(Δ°Hm=144J/g).

2.2.2 IDDSpreparationandcharacterization Theformationofthecross-linkedIDDS(CPMW)wassuccessfullyachievedbythiol-

eneclickchemistrybetweenthependantallylgroupsonthecopolymerbackboneand

42

1,6-HDT(Scheme1).AfterUV365nmcuringinasyringe,thedissolvedcopolymerformeda

solid transparent cylinder that maintained structural integrity after removal from the

syringe. The CPs were washed three times in excess THF to remove any unreacted

starting materials (e.g. 1,6-HDT, DMPA), and then dried for 48 hours at room

temperature and pressure to yield the final IDDS.Macroscopically, implants prepared

from the 7500 g.mol-1 copolymer (CP7.5K) were transparent and flexible, whereas

implants prepared from the 15000 g.mol-1 and 39000 g.mol-1 copolymers (CP15K and

CP39K) were translucent and rigid. ESEM was used to evaluate the cross-section and

surfacemorphologyof the threematrices (Figure 2.2a-b).Microscopicanalysis reveals

that CPmorphology varieswith theMWandAVL content of the copolymers used for

preparation.Acoral-like,heterogeneoussurfacemorphologywithdenselypackedfolds

wasobserved forCP39K,whereas a smooth surfacewith slight ridgeswasobserved for

CP7.5K.Qualitatively,CPcross-sectionandsurfacemorphologydemonstratedanincrease

in roughness with increasing MW and crystallinity of the copolymers used for

preparation.RegardlessofMW,porositywasunapparentatallmagnifications.

43

Scheme 1 Schematic of the synthetic pathway to produce PVL-co-PAVL via ROP andsubsequentformationofthecross-linkedpolymernetwork.

Figure 2.2 ESEM images of (a) cross-section and (b) surface morphology of the CPmatrices.Fromlefttoright,scalebarsrepresent50μm,1mm,and100μm.

The CPs were characterized by DSC and XRD to determine the impact of the

cross-linkingreactiononthefinal IDDS.DSCthermogramsof thecross-linkedmatrices

44

areshown inFigure2.3. In termsofCP7.5K, thedisappearanceofameltingendotherm

indicated a crystalline-to-amorphous transition of the CP compared to the bulk

copolymer.111This isexpectedas thependantallylgroup interruptedtheorderingand

subsequentcrystallizationof thepolymerchains.117Comparedto thebulkcopolymers,

the cross-linking reaction increased the Tg of all CPs due to the formation of covalent

bonds between the pendant allyl groups and 1,6-HDT cross-linker and a resulting

motionalrestrictionofthepolymerchains.118Cross-linkingthematerialsalsoresultedin

a Tm reduction of 34/39.3-29.8 °C and 41.6/47.6-37.5/47.6 °C for the CP15Kand CP39K

matrices,respectively(Table2.1),indicatingthecrystallinityofthematerialsislowered

post-crosslinking.ThelesserreductionintheTmforCP39KrelativetoCP15KisduetoCP39K

having less AVL than CP15K (7% vs. 16%). Boire et al. also observed a greater post-

crosslinkingreductioninmeltingtemperaturesformaterialshavingahigherpercentage

of pendant groups, confirming that amorphous pendant groups significantly disrupt

crystallinity and simultaneously lower the Tm.108 Though the Δ°Hm and χc are lowered

post-crosslinking indicating a reduction in crystallinity for CP15K, the opposite was

observedforCP39K.Anumberoffactorscaninfluencecrystallinityofpolymermaterials

including the nucleation rate and subsequent crystal growth, the rate of solvent

evaporation,andthepolymerconcentration insolution.Thesolventevaporationstage

ofCP-preparationmayresultinthegreaterdegreeofcrystallinityobservedforCP39K.The

lowAVLcontentalsoenablesCP39KtocrystallizemorereadilyfromsolutionthanCP15K,

sincethependantallylgroupsonCP15Kinhibitthealignmentofpolymerchainsalongthe

crystalgrowthdirection.Incontrast,radiationcross-linkingofPCLfilmsdoesnotresultin

45

a significant decrease in crystallinity or modification of thermal properties relative to

non-crosslinked PCL.119 Therefore, there is a clear dependence of all the thermal

properties of the systemon the AVL content and covalent cross-linkingwith 1,6-HDT.

CP7.5K exhibited a typical broad amorphous XRD pattern, whereas CP15K and CP39K

demonstrated twocharacteristic crystallinepeaksat2θ =21.4°and23.8° (Figure 2.4).

Unsurprisingly,thisdiffractionpatternisalmostidenticaltothatofPCL(χc=53%)(2θ=

21.6°and23.8°),whichdisplaysorthorhombiclatticestructure.120

Figure2.3DSCthermogramsofcross-linkedcopolymermatrices(CP39K,CP15K,CP7.5K)andcorrespondingphotographsoftheIDDSs(scalebarsrepresent5mm).

46

Figure 2.4 XRD spectra of cross-linked copolymer matrices (CP39K, CP15K, CP7.5K) andcorrespondingphotographsoftheIDDSs(scalebarsrepresent5mm).

2.2.3 StabilityandDegradation CP15Kremainedstableoverthreemonthsin0.1MPBS(pH7.4)at37oCwithout

significantweightlossoracidificationofthemediaovertime(Figure2.5).Furthermore,

nosignificantchangesinsurfacemorphologywereobservedafterthreemonths(Figure

2.6). Thedegradation rateof PCL andPVLhomopolymers is known tobe slower than

that of PLGA of similarmolecular weight owing to their high hydrophobicity.121 For a

cylindricalIDDSbasedonPCL66K-F68insertedsubcutaneouslyinrats,Sunetal.observed

lowratesofdegradation,specifically,lessthan10%weightlossafterthreemonthsand

77% weight loss after 30 months.34 Toncheva et al. investigated the hydrolytic

degradationofdiscscomposedofPCL-co-PVLandrevealedminorwateruptake(2-3%),

lowweightloss(1-4%),andnochangeinmolecularweightoveraperiodof20weeks.122

They also showed that water uptake and degradation is more rapid for polymers of

47

lowermolecularweight. Furthermore, even if selectivedegradationof the amorphous

regions can occur prior to degradation of the crystalline domains of the matrix, the

covalentcross-linkingwhichleadstoformationoftheCPmatricesleadstogoodstability

overtimeunderneutralconditions.123

Figure2.5(a)Weight loss(%)ofCP15Kafter incubationin0.1MPBS(pH=7.4)forthreemonths.(b)ChangeinpHofexternalmediaasaresultofCP15K.

48

Figure2.6SurfacemorphologyofCP15Kafterincubationin0.1MPBS(pH=7.4)forthreemonths.Scalebarsrepresent(a)50µmand(b)300µm,respectively.Brightwhitelinesin(b)areKimWipe™fibres.

2.2.4 BufferPenetration In order to gain an understanding of the underlyingmass transport processes

involvedindrugrelease,thediffusionofwatercontaining0.1mg/mLSRBintotheCP39K

matriceswasevaluatedusingmethodsdescribedbyKoenningsetal.112SRB isahighly

watersoluble(100mg/mL)andmoderatelylowMW(559gmol=1)fluorescentprobethat

waschosentoqualitativelymonitorthediffusionofbufferintothesystem.InFigure2.7

theprogressofSRBpenetrationasdeterminedbyconfocalmicroscopycanbefollowed

qualitatively.Anouterringoffluorescencewasvisibleatfivehours,indicatingthatSRB

penetration into thematricesbeginsuponsubmersion inmedia.By120h, thesystem

49

wascompletelypenetratedbySRB,whichconfirmedthatwaterwasabletodiffuseinto

the hydrophobic CPs and facilitate the process of drug release. As described for lipid-

basedimplants,waterdiffusioninordrugdiffusionoutofthesesystemsisreleaserate

controlling,eveninthecaseoflowmaterialhydrophilicityandlimiteddrugsolubility.124

In parallel, a hollowend-cappedCP39K cylinderwas prepared and the cavitywas filled

withanaqueoussolutionofSRB.DiffusionthroughtheCPmatrix(wallthickness=640

µm)wasmonitoredbymeasurementofSRBcontentintheexternalmediaovera72h

period. As shown in Figure 2.8, even at early time points there was transport of the

probeacrossthepolymermatrix.Itislikelythattheprocessofwaterdiffusionintothe

systemoccursatafasterratethanthediffusionofSRBgiventhatwaterismuchlowerin

MW. These experiments provide evidence that the CPs are rapidly wetted upon

submersion in aqueous media, though the true rate of buffer penetration cannot be

discernedfromtheseexperiments.

Figure 2.7 Qualitative depiction of SRB (0.1 mg/mL in 0.1M PBS) diffusion into CP39Kmatrices as a function of time spent under incubation at 37 °C. Fluorescence wasmeasuredat540nmbyconfocalmicroscopy.

50

Figure2.8Semi-quantitativeevaluationofthediffusionofSRB(A540nm)acrosstheCP39Kmembraneofahollowcylinderfromtheinternaltoexternalmedia(0.1MPBSpH=7.4,SDS0.5%w/v).

2.2.5 DeterminationofSolubilityParametersandSwellingCapacity Todeterminethecompatibilitybetweensolvent,cross-linkedmaterials,andthe

selected drugs in order to select the most appropriate drug loading solvent, partial

(δd,p,h)and total (δt) solubilityparametersofall componentswerecalculatedusing the

GCM. TheGCM takes into account the chemical structure and functional groupmolar

attraction constants of themolecule.90, 93 Partial and total solubility parameters were

calculatedforthethreecross-linkedmatrices(includingtheδdcontributionof1,6-HDT)

and compared to values for the partial solubility parameters of select drug loading

solvents (Table 2.2). Materials having similar solubility parameter values will exhibit

similar intermolecular interactions, which will favormiscibility (i.e. the free energy of

mixingthematerials(ΔH) isnegative).95Thesmallerthedifferencebetweenthevalues

51

forδtofthesoluteandsolvent,thegreaterthesolubilityofthesoluteinthesolvent(i.e.

Δδt≤7.5MPa1/2forasolute-solventpair).110GoodswellingoccurswhenΔH<0and(δCP

− δsol)2 is 0, where δCP and δsol are the solubility parameters of the CP and solvent,

respectively.

Table2.2Partialandtotalsolubilityparameters((MPa)1/2)ofselectdrugloadingsolventsandtheCPmatricescalculatedusingtheGCM.

Figure 2.9(a) Plot of the calculated average [1/3( CP7.5K + CP15K + CP39K)] (δCP − δsol)2values (MPa1/2) for the three cross-linked matrices and solvents considered for drugloadingusing theGCM. (b)Plotofswellingcapacity (%)asa functionof time forCP39Kcylindersswollenintheindicatedsolvents.Scalebarsrepresent5mm.

The degree of swelling observed for the three CP matrices was in good

agreementwiththesolvent-networkcompatibilityaspredictedbyvaluesobtainedusing

theGCMmethod(i.e.CH2Cl2>THF>toluene>DMSO>H2O)(Figure2.9a-b).Thesame

trendwasobservedforallcopolymersystems(i.e.CP7.5K–CP39K).WhileCP39K implants

52

swelledwell in CH2Cl2 (swelling ≥ 1100%), CP7.5KandCP15K broke intopieces after two

hours. This is likelydue to the relatively lowMWof thePVL-co-PAVL copolymers that

composedtheCP7.5KandCP15Kmatrices.Duetothehighdegreeofimplantswellingand

drugsolubilityobtainedinTHF,thissolventwaschosenasthesolventfordrugloading.

Given that the differences in solubility parameters for all the drugs and polymer

materials investigated was less than 7.5MPa1/2, polymer-drugmiscibility is expected.

Therefore, drug loading and release results should not depend much on minor

differencesinsolubilityparameters.95

2.2.6 EvaluationofPhysicochemicalPropertiesofDrugs FivedrugsofvaryingphysicochemicalpropertiessuchasMW,molecularvolume,

logP (octanol/waterpartitioncoefficient),aqueoussolubility,andsolubilityparameters

were selected as model drugs for incorporation into the three matrices. The drugs

chosenincludeACM,asimpleanalgesic;CCM,amodelhydrophobicfluorescentprobe;

PTX,achemotherapeuticagent;andTAAandTAH,twocommonlyusedcorticosteroids

for the treatment of osteoarthritis (Figure 2.10). In order to estimate drug-matrix

compatibility and predict the relative release rates of drugs from the matrices, the

physicochemicalpropertiesofthedrugs,includingaqueoussolubility(Figure2.11),logP,

and solubility parameters (Table 2.3) were experimentally determined, as these

propertiesplayakeyroleindrugloadingandrelease.Theshaketubemethodwasused

todeterminethelogPfordrugswithalogPbetween-2to4.ThelogPofTAH(>4)was

determinedusingChemAxon'ssoftwaresincethepartitioningofTAH intotheaqueous

53

phasewasbelow theHPLC limit of detection. Inorder to ensure sink conditionswere

maintainedoverthecourseoftheinvitroreleaseexperiments,theaqueoussolubilityof

the five drugs was determined in the presence of increasing concentrations of two

commonsurfactants(e.g.SDSandTween®80).Then,tocomparedrugreleasefromall

the matrices in similar conditions, 0.5% SDS (% w/v) was chosen to maintain sink

conditionsas it resulted in thehighest increase inaqueoussolubilityof thedrugs.SDS

can also minimize non-specific adsorption at the surface of polymeric matrices and

promotethedissociationofnon-covalentdrug-drugaggregates.125

Figure2.10ChemicalstructuresandMWof fivedrugsselected for loading intotheCPmatrices.

54

Figure 2.11 Aqueous solubility of the four hydrophobic drugs in 0.1M PBS (pH = 7.4)containing0.1%,0.5%,or1%(%w/v)Tween®80orSDS.

Table2.3Solubilityparameters,molarvolumes,andlogPvaluesofthedrugsloadedintotheCPmatrices.

*ValueshavebeendeterminedusingACDLabsandChemAxonsoftware.

2.2.7 CharacterizationofDrug-LoadedIDDS Apost-preparation solvent swelling/equilibrationmethodwasused to load the

fivemodeldrugsintotheCPmatrices.THFwaschosenastheloadingsolventduetoits

high degree of CP swelling and drug solubility. Drugs were dissolved in THF to a

concentration of 30 mg/mL, and the CPs were equilibrated for four hours in 0.5 mL

loading solution. The sorption of a significant amount of loading solution and the

subsequent solvent evaporation resulted in the entrapment of drug within the

matrices.111Thedrug-loadedmatrices(CP-drug)werecharacterizedbyseveralmethods

55

includingESEM,DSC,andXRD.Thedrugloadingcapacity(DLC)ofthematricesforallfive

drugsrangedbetween10-20%(%w/w),asshowninFigure2.12.TheDLCofCP15Kand

CP39Kwasfoundtobesimilar,whereasitwasfoundtobelowerforCP7.5K,likelydueto

theincreasedcross-linkingdensityandlowermolecularweight.111Macroscopically,CPs-

ACM,CPs-PTX,andCPs-CCMweretranslucent liketheunloadedCPs,whereasCPs-TAA

andCPs-TAHwereopaque. ESEM revealed thepresenceof crystallinedrugwithinCP-

TAAandCP-TAH,butnotwithintheCP-ACM,CP-CCM,orCP-PTXpolymer-drugmatrices

(Figure2.13).

Figure2.12Drugloadingcontent(%w/w)oftheCP15Kmatrices.

56

Figure 2.13 ESEM imagesofCP15Kmatrices loadedwith (a)TAHor (b)CCM.Scalebarsrepresent1mm(top)and20µm(bottom).

Thedrug-loadedCP15KmatriceswerethencharacterizedbyDSCtodeterminethe

extent of polymer-drug interactions and level of phase separation between drug and

matrix (Figure 2.14). The incorporationofACM,CCM, andPTX into theCP15Kmatrices

increased the Tg of the system as a result of favourable intermolecular interactions,

generally of an ionic, hydrogenbonding, or dipolar nature, between thedrug and the

polymerand the loweringofpolymerchainmobility.118Thepresenceofa singleTgfor

thesystemsalsoconfirmsthemiscibilitybetweendrugandpolymer.95Additionally,the

disappearanceoftheTcandTmthatwereobservedforthenon-loadedsystemssuggests

these systems are amorphous in nature, indicating a high degree of polymer-drug

57

compatibility and that the drugs are molecularly dissolved within the matrices.126

AccordingtoGreenhalghetal.,basedoncalculationofthesolubilityparametersofthe

CPs(i.e.≈22.2MPa1/2)andadifferenceof1.4-4.5MPa1/2(Δδ<7MPa1/2)betweenthe

solubility parameters for the CPs and drugs, polymer-drug miscibility is expected.97

However,theincorporationof10-15%TAAandTAHintotheCP15Kmatricesdidnotalter

thethermalcharacteristicssignificantlyfromthenon-loadedmaterials.TheTgwasonly

slightly increased upon TAA or TAH incorporation, and the system still displays the

characteristicTcandTmoftheCP15Kmatrix,implyingminimalpolymer-druginteractions.

Figure2.14ComparisonofDSCthermogramsofCP15Kmatrix(bottom)loadedwitheachofthefivedrugs.

InordertoverifythefinalstateofthedrugandvalidateDSCobservations,XRD

spectraofCP39K,crystallineCCMorTAH,andtheloadedsystemsarecomparedinFigure

2.15.ThediffractionpatternsofCCMandTAHdisplayintensepeaksoverthe2θrange,

58

indicating the crystalline nature of the drugs, while CP39K demonstrates two distinct

crystalline peaks at 2θ = 21.4° and 23.8°. CP39K containing CCM depicted a more

amorphous halo band compared to crystalline CCM or semi-crystalline CP39K alone,

confirming the amorphous nature of CP39K-CCM. On the other hand, characteristic

crystallinepeaksofTAHarevisiblepost-incorporationintotheCP39Kmatrix,suggestingat

least someof thedrug loadinghas crystallizedwithin thematrix.127 Thepeaksof TAH

had a slightly reduced intensity in TAH-loaded CP39K, whichmay be attributed to the

partialsolubilizationofTAHwhenincorporatedintothematrix.128Theseresultsfurther

verify the amorphous nature of CPs loadedwith ACM, CCM, or PTXwhile CPs loaded

withTAAandTAHretaincrystallinecharacter.

Figure 2.15 Comparisonof theXRD spectraof crystalline TAHandCCM to thatof thenon-loadedCP39KmatrixandCP39KmatricesloadedwithTAHorCCM.

59

Results obtained by other groups on similar steroidal compounds such as

progesterone (PG) and DXM suggest an inherent preference towards crystallization.

Gomez-Gaeteetal.haveshownthatabove10%loading inPLGA(75:25)nanoparticles,

DXMpreferstoassemblewithotherdrugmoleculesratherthanbeingdispersedwithin

thenanoparticles.129 Similarly, above16.5%PG loading inPLGA (85:15)microparticles,

DSC scans revealed the presence of thermal events characteristic of crystalline PG,

suggesting either a maximum loading threshold or an inherent preference towards

crystallization.130

2.2.8 MechanisticMathematicalTheoryIn order to develop an appropriate mathematical model to quantitatively

describe the experimentally measured drug release, the drug-loaded implants were

extensivelycharacterizedbeforeandafterexposuretothereleasemedia.Sincediffusion

is known to play a major role in the control of drug release from polyester-based

matrices, the releasewasmodeledbasedon the following analytical solution to Fick's

secondlawofdiffusion:131

𝑀!

𝑀!= 1−

32𝜋! ⋅

𝑒𝑥𝑝(−𝑞!!𝐷𝑡/𝑅!)𝑞!!

⋅𝑒𝑥𝑝(−(2𝑝 + 1)!𝜋!𝐷𝑡/𝐻!)

(2𝑝 + 1)!

!

!!!

!

!!!

[6]

whereMtandM∞denote theabsolutecumulativeamountsofdrug releasedat time t

andinfinitetime,respectively;qnarethepositivezerosoftheBesselJfunctionoforder

0,meaningthatJ0(qn)=0foralln;RandHdenotetheradiusandheightofthecylinder;

andD represents thediffusion coefficientof thedrugwithin the system.RProject for

Mac (Version 3.3.3)was used for fitting themathematicalmodel using the non-linear

60

minimization(NLM)fittingroutine.Dwasfittedtoequation6usingenoughdatapoints

todescribetheshapeofthecurve.Themodelconsidersthefollowingassumptions:

1. The implants are cylindrical in shape and the shape does not change over

time

2. Theimplantsdonotsignificantlyswellorerodewithtime

3. Thedrugisinitiallyhomogeneouslydistributedwithinthesystem

4. Perfectsinkconditionsaremaintainedthroughouttheexperiment

5. Diffusion or dissolution through the unstirred boundary layer is rapid

comparedtodrugdiffusionthroughthesystem

RStudioforMac(Version3.3.3)wasusedforfittingthemathematicalmodelto

the data. D was fitted to equation 6 using enough experimentally determined data

pointstocharacterizetheshapeofthecurve,andtheresultingdiffusioncoefficientsfor

eachof the fivedrugs through thesystemaresummarized inTable 2.4.Though these

assumptionsmayholdtrueforACM,CCM,andPTX,assumptionthreeischallengedby

TAAandTAH.ThecrystallinedrugdiffractionpeaksintheXRDspectrumandthemelting

and recrystallization thermal events in the DSC thermograms (Figure 2.14) indicate

crystallinedispersionsofTAAandTAHarepresentwithin thematrices, suggesting the

possibilityoftwophases:anamorphousdrug-polymerphaseandacrystallinedrugonly

phase.95 However, buffer penetration into thematrix has been visualized over 5 days

(Figure 2.7-8), and the dissolution of crystalline drug aggregates serves to create a

homogenous distribution.114 Therefore, the good agreement observed between

experimentandtheorysuggeststhechosenmodelisappropriateforTAAandTAHunder

61

these conditions. The presentedmathematical theory allows to quantitatively predict

theeffectofchangesinimplantgeometryontheresultingdrugreleasekinetics.

Table 2.4 Summary of diffusion coefficients of drugs through the three CP matricesunderdifferentexperimentalconditions(e.g.releasebuffer).

2.2.9 DrugRelease

2.2.9.1 NatureoftheDrug-LoadedSystems ThereleaseprofilesoffivemodeldrugsfromtheCP15Kmatricesinthepresence

of 0.5% SDS (% w/v) are shown in Figure 2.16. The experimentally determined drug

release is presented as closed symbols and the corresponding fitted theory based on

experimental results is presented as solid lines. It can be seen that good agreement

betweenexperimentandtheorywasobtainedforalldrugs,which isan indication,but

notconfirmation,thatdiffusionofwaterordrugistherate-limitingmasstransportstep.

Overall,thereleasebehaviourofalldrugsfromthepolymermatrixexhibitedfirstorder

release kinetics characterized by a fast initial release followed by a slower andmore

sustained release. ACMdemonstrated a rapid release after aweekwhereas themost

62

hydrophobic drug, TAH, was not fully released by the end of the 35-day observation

period(e.g.≤75%release).Variousgroupshaveshownthattheinfluxofwaterintothe

matrix is accelerated by the presence of hydrophilic drugs, which explains the rapid

releaseobservedforACM.132Additionally, thehighdrug loading forACM(≈19%w/w)

results in greater matrix porosity upon drug release and a greater influx of water to

facilitatediffusionthroughwaterfilledchannels.132

Figure2.16Cumulativedrugrelease(%)asafunctionoftimemeasuringthe impactofthe state of the drug-loaded matrices (monolithic solution vs. dispersion) on theresultingdiffusionofdrugfromtheCP15Kmatrices(0.5%SDSw/v).Blacklinesrepresentthepredicteddrugreleasegiventhecalculateddiffusioncoefficient.

Water or drug diffusion is ofmajor importance for the control of drug release

from the matrices. Beyond this, release is influenced both by the distribution and

interactions of drugs within the polymer system and by the solubility of drug in the

polymer.133Polymericmatrixsystemscanbeclassifiedaseitheramonolithicsolutionor

monolithicdispersion.126ResultsobtainedbyDSCandXRDanalysissuggeststhatACM,

CCM,andPTXaredissolvedinthematrixasmonolithicsolutionswhereasTAAandTAH

63

are dissolved and dispersed asmonolithic dispersions.When comparing polymer-drug

systemswithinthesamecategory(i.e.monolithicsolutionsordispersions),drugrelease

willbegovernedprimarilyby thesolubilityanddiffusioncoefficientof thedrugwithin

the system. For the steroid-loaded matrices that contain both dissolved and non-

dissolveddrug,TAAreleasesfasterthanTAHbecauseofitshigheraqueoussolubilityof

287.04±56.86µg/mLcomparedto37.89±0.40µg/mLforTAH.Ontheotherhand,CCM

andPTXbehaveasdissolveddrugs given their favourable interactionswith thematrix

andasaresult,theirreleaserateswereslowerthanACMbutfasterthanTAAandTAH.

Therefore, release of CCM and PTX can be compared on the basis of their aqueous

solubility:105.98±0.57µg/mLforPTXand45.63±1.05µg/mLforCCM,whichleadsto

morerapidsolubilizationofPTXcomparedtoCCMandafasterrateofdiffusionfromthe

matrix.

2.2.9.2 ImpactofAqueousSolubility

Theimpactofthereleasemediacontainingdifferenttypesandconcentrationsof

surfactantsonthereleaseprofileofPTXwasalso investigated. Itcanbeseenthat the

releaseratedependsgreatlyondrugsolubilityintheaqueousmedia(Figure2.17).The

solubility of PTX is 50 times greater in 0.5% SDS than in 0.1% Tween® 80. Increased

solubility leads to an increase in the drug dissolution rate and a higher concentration

gradientbetweenthematrixandexternalmedia,therebyincreasingthedrivingforcefor

drugrelease.87InareportbyPintoetal.,releaseofTAAfromapolyurethane-PCL(PU-

PCL)implantundersinkconditionsinPBS(TAAsolubility21µg/mL)revealedalinearTAA

64

releaseofupto64%over8months,whereasinvivo,almost81%ofTAAwasreleasedin

only 45 days.128 Considering the slow degradation of the PU-PCL implant, the lack of

translationbetween in vitro and in vivo data suggests thatdrug solubilityneeds tobe

welldefinedsinceitisakeydeterminantinreleasekinetics.

Figure 2.17CumulativePTX release (%)asa functionof timemeasuring the impactofdrugaqueoussolubilityontheresultingdiffusionofdrugfromtheCP15Kmatrices.Blacklinesrepresentthepredicteddrugreleasegiventhecalculateddiffusioncoefficient.

2.2.9.3 ImpactofMatrixPhysicochemicalProperties

Release of TAA from the CP7.5K and CP39K matrices was also conducted to

determinetheimpactofmatrixcomposition(e.g.MWandAVLcontent)ondrugrelease

(Figure 2.18). As polymermolecularweight increases and AVL content decreases, the

drug release rate decreases. This result is in agreement with many reports of slow

release from high molecular weight polymers.134-136 Drug diffusion through the

completely amorphous implant (e.g. CP7.5K) is the most rapid since there are no

0 5 10 15 20 250

20

40

60

80

100CP15K-PTX-Tween® 80CP15K-PTX-SDS

Time (days)

Cum

ulat

ive

rele

ase

(%)

65

crystallinedomains actingasbarriers todiffusion. The relativelyhighAVL content (i.e.

23%)ofCP7.5Kcontributessignificantlytotheamorphousnatureoftheimplant.Boireet

al.foundthatamorphousallylcarboxylateε-caprolactonedisruptedPCLcrystallinityby

lowering Tm, Δ°Hm, and the degree of crystallinity of the material.108 Covalent cross-

linkingofthependantallylgroupsonCP15K(16mol%)with1,6-HDTledtoadecreasein

the Tm to below physiological temperature as well as a decrease in crystallinity.

Therefore,nosignificantdifferenceswereobservedbetweenCP7.5KandCP15Kasaresult

ofCP15Kbecomingamorphousat37 °C (Tm=29.8 °C).Theopposite canbe saidabout

drug diffusion through the CP39Kmatrix. The higherMW and highly crystalline nature

resultingfromlowAVLcontentofCP39K(χc=68%)servestodecreasetherateofwater

penetrationintothesystemandtheresultingdrugdiffusivity.Releasewassustainedfor

at least an additional two weeks compared to CP15K-TAA. In summary, as polymer

molecular weight increases and the AVL content decreases, crystallinity subsequently

increases,leadingtoadecreaseinthedrugreleaserate.

66

Figure 2.18CumulativeTAArelease (%)asa functionof timemeasuring the impactofpolymerphysicochemicalpropertiesontheresultingdiffusionofdrugfromthethreeCPmatrices.Blacklinesrepresentthepredicteddrugreleasegiventhecalculateddiffusioncoefficient.

2.2.10 Biocompatibility To gain an initial assessment of the biocompatibility of the CP15K and CP39K

matrices, the cytotoxicity to L929mouse fibroblast cellswas evaluatedusing theMTS

assay. The extract dilutionmethod for cytotoxicity testingwas chosen,which involves

incubatingthematerialsinmediacontainingserumfor48handthenexposingthecells

toserialdilutionsof theextractionmedia.Thismethodeliminatesconfounding factors

resulting from physical trauma to the cells due to sample weight and it has been

reported that toxicity resulting from polymeric biomaterials is likely a result of their

leachables.113,137Figure2.19showsL929cellviabilityasafunctionoftheappliedextract

dilution. The CPs demonstrated excellent in vitrobiocompatibility at nearly all extract

concentrationsandtimepointsstudied.Theonlyexceptionswere100%and25%CP15K

at48h,whichresultedinasignificantdecreaseincellviabilityrelativetotheuntreated

control (p<0.001 and p<0.05, respectively), though cell viability recovered at 72 h.

Interestingly, high-density polyethylene (HDPE), the negative control, resulted in a

significant(p<0.001)decreaseincellviabilityrelativetotheuntreatedcontrolbeyond24

h at the highest extract concentration studied. Cell viability was significantly lower

67

(p<0.001) for HDPE relative to the CP15K and CP39K materials at 48 h post-treatment.

TheseresultsaresimilartostudiesinvestigatingtheinvitrobiocompatibilityofPCLfilms

to L929 mouse fibroblast cells, suggesting the PVL-co-PAVL delivery system is highly

biocompatible.138

Figure2.19Invitrocytotoxicityoftheappliedextractdilutionofcylinders(green)orhighdensitypolyethylene(HDPE)(purple)toL929mousefibroblastcells.Cellsincubatedwithmediaalonewereemployedasacontrolandconsideredas100%cellviability.(***,**,and *) indicates lesser viability relative to untreated (p < 0.001, 0.01, and 0.05,respectively);(###,##,and#)indicateslesserviabilityrelativetotreatmentgroup(CP39K)ofsameextractdilutionconcentration(p<0.001,0.01,and0.05,respectively).

68

3 Conclusion Implantable biomaterials are in growing demand for various therapeutic

applicationsgiventheproblemsthatpersistwith traditional immediatereleasedosage

forms such as drug bioavailability, plasma concentrations fluctuating either above or

below the therapeutic window, reduced patient compliance, and poor cost-

effectiveness.Thereisaneedtodeveloppolymermaterialsthatcanbefunctionalizedin

goodyieldtoproducemorecustomizablebiodegradabledrugdeliverysystemstobetter

suit theapplicationof interest.WehavedevelopedonesuchsystembasedonPVL-co-

PAVL,cross-linkedwith1,6-hexanedithiol,whichpossessesseveraladvantagesincluding

high drug loading with a post-loading method, sustained and reproducible diffusion-

controlled release kinetics, and good in vitro biocompatibility. PVL is not yet FDA

approved,butitisalsopossibletopreparePCL-co-PAVLaswellasPLA/PGA-co-PAVLco-

polymerssincethepercentageofallylgroupsnecessaryforthepreparationofthecross-

linkedmaterialislow(e.gCP39K≤10%).Wehaveshowntheimportanceofathoroughly

conducted investigation into theunderlyingmechanisms controllingdrug release from

these newmaterials and identified factors secondary to diffusion, including polymer-

drug interactions and solubility parameters, which play a role. Furthermore, we have

demonstrated thepotentialof thisnew IDDSbasedonPVL-co-PAVLasadrugdelivery

systemfornumeroustherapiessincethependantallylfunctionalityprovidesaversatile

backbone for improving polymer-drug compatibility and tailoring release profiles to

meetclinicaldemands.

69

70

4 FutureDirections Theresearchpresentedinthisthesisprimarilyfocusedonthedevelopmentand

thorough characterization of a delivery system based on newly synthesized high

molecular weight PVL-co-PAVL. To build on the present research, the following

recommendationsaresuggestedforfutureevaluation:

(1)Amoresystematicrepertoireofexperiments,holdingkeyvariablesconstant,

to determine the true impact of changes in formulation parameters on performance

characteristics such as drug loading and release. A limitation of the present work

includes changing multiple polymer properties at the same time. For example, to

determinethe impactofthependantallylgroupondrug loadingandrelease,thePVL-

co-PAVL molecular weight could have been held constant while varying the % AVL.

Similarly,todeterminethetrueimpactofmolecularweight,the%AVLcouldhavebeen

heldconstantwhileincreasingordecreasingthemolecularweight.Anotherformulation

variablethatcouldbeexaminedissystematicallyloadingdifferentlevelsofTAAandTAH

todeterminewhendrugcrystallizationandphase separation from thepolymermatrix

occurs. Clinically, only the amorphous drug-polymer systemswould be desirable since

releaseinconsistenciescanarisefromdrugcrystaldissolution.

(2)Enhancingtherateofdegradationofthedeliverysystemtobemorewidely

applicabletoshort-termdrugdelivery.Therateofdegradationofthesystemspresented

was very slow under neutral conditions. This would be desirable for long-term drug

delivery applications such as contraception. For short-term applications, an ideal IDDS

71

would degrade upon the completion of drug release to avoid the need for surgical

removal.Strategiestoenhancetherateofdegradation includeconjugatinghydrophilic

moieties tothependantallylgroup,usingahydrophiliccross-linker,or incorporatinga

PEGylatedPVL-co-PAVLindifferentratiostoenhancetherateofwaterabsorption.99-101,

103-104

(3) Investigating the loadingof drugswithdramatically different total solubility

parameters((δCP−δdrug)2>10MPa1/2)totestwhetherthegroupcontributionmethodis

predictiveofpolymer-drugcompatibilityforthissystem.Suggestedmodeldrugstoload

into the systems to determine whether the solubility parameters are predictive of

polymer-drug compatibility include propranolol HCl (35.5 MPa1/2), cefalexin (31-38

MPa1/2),andcarbamazepine(31.2MPa1/2).93

(4)InvivobiocompatibilityoftheCPstofurtherevaluatetheirclinicalpotential.

GiventheexcellentinvitrobiocompatibilityoftheCPstoL929mousefibroblastcells,we

anticipatethisdeliverysystemtobewelltoleratedinrodents,andplantomoveforward

withconductingapilotanimalstudy.

Thorough characterization and understanding of IDDSs is of fundamental

importance to designing effective delivery systems with desired performance

characteristics. The experimental work conducted in this thesis has identified the

mechanismsandfactorscontrollingdrugreleasefromthesesystemsandvalidatedtheir

biocompatibilityforfuture invivoevaluation.Insummary,webelievethistobeasolid

foundationofbasicresearchuponwhichfutureresearchprojects,suchasthoseoutlined

above,canexpand.

72

5 ReferencesPrimarySources

SecondarySources

UncategorizedReferences

1. Dash,A.K.;CuddworthII,G.C.,Therapeuticapplicationsofimplantabledrugdeliverysystems.JPharmacolToxicol1998,40,1-12.2. Muller,P.Y.;Milton,M.N.,Thedeterminationandinterpretationofthetherapeuticindexindrugdevelopment.NatureReviewsDrugDiscovery2012,11,751-761.3. Meng,E.;Sheybani,R.,Micro-andnano-fabricatedimplantabledrug-deliverysystems:currentstateandfutureperspectives.TherDeliv2014,5(11),1167-1170.4. Desai,P.P.;Date,A.A.;Patravale,V.B.,Overcomingpoororalbioavailabilityusingnanoparticleformulations-opportunitiesandlimitations.DrugDiscovTodayTechnol2012,9(2),e71-e174.5. Chakraborty,S.;Shukla,D.;Mishra,B.;Singh,S.,Lipid--anemergingplatformfororaldeliveryofdrugswithpoorbioavailability.EurJPharmBiopharm2009,73(1),1-15.6. Danckwerts,M.;Fassihi,H.,Implantablecontrolledreleasedrugdeliverysystems:areview.DrugDevIndPharm1991,17(11),1465-1502.7. Pediatrics,A.A.o.,Alternativeroutesofdrugadministration-advantagesanddisadvantages(subjectreview).Pediatrics1997,100,143-142.8. Ulrich,K.E.;Cannizzaro,S.M.;Langer,R.S.;Shakesheff,K.M.,Polymericsystemsforcontrolleddrugrelease.ChemRev1999,99,3181-3198.9. Bhattarai,N.;Gunn,J.;Zhang,M.,Chitosan-basedhydrogelsforcontrolled,localizeddrugdelivery.AdvDrugDelivRev2010,62(1),83-99.10. Kleiner,L.W.;Wright,J.C.;Wang,Y.,Evolutionofimplantableandinsertabledrugdeliverysystems.JControlRelease2014,181,1-10.11. Zahedi,P.;DeSouza,R.;Piquette-Miller,M.;Allen,C.,Chitosan-phospholipidblendforsustainedandlocalizeddeliveryofdocetaxeltotheperitonealcavity.IntJPharm2009,377(1-2),76-84.12. Elkharraz,K.;Faisant,N.;Guse,C.;Siepmann,F.;Arica-Yegin,B.;Oger,J.M.;Gust,R.;Goepferich,A.;Benoit,J.P.;Siepmann,J.,Paclitaxel-loadedmicroparticlesandimplantsforthetreatmentofbraincancer:preparationandphysicochemicalcharacterization.IntJPharm2006,314(2),127-36.13. Lesser,G.J.;Grossman,S.A.;Leong,K.W.;Lo,H.;Eller,S.,Invitroandinvivostudiesofsubcutaneoushydromorphoneimplantsdesignedforthetreatmentofcancerpain.Pain1996,265-272.14. Bourges,J.L.;Bloquel,C.;Froussart,F.;Bochot,A.;Azan,F.;Gurny,R.;BenEzra,D.;Behar-Cohen,F.,Intraocularimplantsforextendeddrugdelivery:therapeuticapplications.AdvDrugDelivRev2006,58(11),1182-1202.15. Nikolsky,E.;Lansky,A.J.;Sudhir,K.;Doostzadeh,J.;Cutlip,D.E.;Piana,R.;Su,D.;

73

White,R.;Simonton,C.;Stone,G.,SPIRITIVtrialdesign:alarge-scalerandomizedcomparisonofeverolimus-elutingstentsandpaclitaxel-elutingstentsinpatientswithcoronaryarterydisease.AmHeartJ2009,158,520-526.16. Thombre,A.G.;amEnde,M.T.;Wu,S.X.Y.,Controlledreleasetechnologyanddesignoforalcontrolledreleasedosageforms.InChemicalengineeringinthepharmaceuticalindustry:R&Dtomanufacturing,amEnde,D.J.,Ed.JohnWiley&Sons:Hoboken,NJ,USA,2010.17. Langer,R.S.;Peppas,N.A.,Presentandfutureapplicationsofbiomaterialsincontrolleddrugdeliverysystems.Biomaterials1981,2,201-214.18. EisaiInc.Gliadel®wafer(carmustineimplantforintracranialuse)prescribinginformation.http://gliadel.com/patient/media/_pdfs/prescribing-information-gliadel.pdf.19. Westphal,M.;Ram,Z.;Riddle,V.;Hilt,D.;Bortey,E.;ExecutiveCommitteeoftheGliadelStudy,G.,Gliadelwaferininitialsurgeryformalignantglioma:long-termfollow-upofamulticentercontrolledtrial.ActaNeurochir(Wien)2006,148(3),269-75;discussion275.20. Ranganath,S.H.;Wang,C.H.,Biodegradablemicrofiberimplantsdeliveringpaclitaxelforpost-surgicalchemotherapyagainstmalignantglioma.Biomaterials2008,29(20),2996-3003.21. AstraZeneca.Zoladex®(goserelinacetateimplant)10.8mgprescribinginformation.http://www.azpicentral.com/zoladex/zoladex10_8.pdf.22. AstraZeneca.Zoladex(goserelinacetateimplant)3.6mgprescribinginformation.http://www.azpicentral.com/zoladex-36/zoladex3_6.pdf.23. Vogelzang,N.J.;Chodak,G.W.;Soloway,M.S.;Block,N.L.;Schellhammer,P.F.;Smith,J.A.;Caplan,R.J.;Kennealey,G.T.,Goserelinversusorchiectomyinthetreatmentofadvancedprostatecancer:finalresultsofarandomizedtrial.Urology1995,46(2),220-226.24. Rock,J.A.;Truglia,J.A.;Caplan,R.J.,Zoladex(goserelinacetateimplant)inthetreatmentofendometriosos:arandomizedcomparisonwithdanazol.ObstetGynecol1993,82,198-205.25. Folkman,J.;Long,D.M.,Theuseofsiliconerubberasacarrierforprolongeddrugtherapy.JSurgRes1964,4(3),139-142.26. Benagiano,G.;Gabelnick,H.;Farris,M.,Contraceptivedevices:subcutaneousdeliverysystems.ExpertRevMedDevices2008,5(5),623-637.27. Peralta,O.;Diaz,S.;Croxatto,H.,Subdermalcontraceptiveimplants.JSteroidBiochemMolecBiol1995,53(1-6),223-226.28. Gu,S.;Sivin,I.;Du,M.;Zhang,L.;Ying,L.;Meng,F.;Wu,S.;Wang,P.;Gao,Y.;He,X.;Qi,L.;Chen,C.;Liu,Y.;Wang,D.,<EffectivenessofNorplantimplantsthroughsevenyears:alarge-scalestudyinChina.Contraception1995,52,99-103.29. Makadia,H.K.;Siegel,S.J.,PolyLactic-co-GlycolicAcid(PLGA)asBiodegradableControlledDrugDeliveryCarrier.Polymers(Basel)2011,3(3),1377-1397.30. Shah,N.H.;Railkar,A.S.;Chen,F.C.;Tarantino,R.;Kumar,S.;Murjani,M.;Palmer,D.;Infeld,M.H.;Malick,A.W.,Abiodegradableinjectableimplantfordeliveringmicroandmacromoleculesusingpoly(lactic-co-glycolic)acid(PLGA)copolymers.J

74

ControlRelease1993,27,139-147.31. Kulkarni,R.K.;Pani,K.C.;Neuman,B.S.;Leonard,F.,Polylacticacidforsurgicalimplants.ArchSurg1966,93,839-843.32. Tyler,B.;Gullotti,D.;Mangraviti,A.;Utsuki,T.;Brem,H.,Polylacticacid(PLA)controlleddeliverycarriersforbiomedicalapplications.AdvDrugDelivRev2016,107,163-175.33. Pitt,C.G.;Chasalow,F.I.;Y.M.,H.;Klimas,D.M.;Schindler,A.,Aliphaticpolyesters.I.Thedegradationofpoly(ε-caprolactone)invivo.JApplPolymSci1981,26,3779-3787.34. Sun,H.;Mei,L.;Song,C.;Cui,X.;Wang,P.,Theinvivodegradation,absorptionandexcretionofPCL-basedimplant.Biomaterials2006,27(9),1735-40.35. Gilding,D.K.;Reed,A.M.,Biodegradablepolymersforuseinsurgery--polyglycolic/poly(lacticacid)homo-andcopolymers:1.Polymer1979,20(12),1459-1464.36. Ulery,B.D.;Nair,L.S.;Laurencin,C.T.,BiomedicalApplicationsofBiodegradablePolymers.JPolymSciBPolymPhys2011,49(12),832-864.37. Ceonzo,K.;Gaynor,A.;Shaffer,L.;Kojima,K.;Vacanti,C.A.;Stahl,G.L.,PolyglycolicAcid-InducedInflammation:RoleofHydrolysisandResultingComplementActivation.TissueEngineering2006,12(2),301-308.38. Sodergard,A.;Stolt,M.,Propertiesoflacticacidbasedpolymersandtheircorrelationwithcomposition.ProgPolymSci2002,27,1123-1163.39. Cam,D.;Hyon,S.-H.;Ikada,Y.,Degradationofhighmolecularweightpoly(L-lactide)inalkalinemedium.Biomaterials1995,16(11),833-843.40. AbdAlsaheb,R.A.;Aladdin,A.;Othman,N.Z.;AbdMalek,R.;Leng,O.M.;Aziz,R.;ElEnshasy,H.A.,Recentapplicationsofpolylacticacidinpharmaceuticalandmedicalindustries.JChemPharmRes2015,7(12),51-63.41. Fredenberg,S.;Wahlgren,M.;Reslow,M.;Axelsson,A.,Themechanismsofdrugreleaseinpoly(lactic-co-glycolicacid)-baseddrugdeliverysystems--areview.IntJPharm2011,415(1-2),34-52.42. Faisant,N.;Siepmann,J.;Benoit,J.P.,PLGA-basedmicroparticles:elucidationofmechanismsandanew,simplemathematicalmodelquantifyingdrugrelease.EurJPharmSci2002,15,355-366.43. Wang,N.;Wu,X.S.;Li,C.;Feng,M.F.,Synthesis,characterization,biodegradationand,drugdeliveryapplicationofbiodegradablelacticglycolicacidpolymers:I.Synthesisandcharacterization.JBiomatSci-PolymE2000,11(3),301-318.44. Woodruff,M.A.;Hutmacher,D.W.,Thereturnofaforgottenpolymer—Polycaprolactoneinthe21stcentury.ProgressinPolymerScience2010,35(10),1217-1256.45. Dash,T.K.;Konkimalla,V.B.,Poly-caprolactonebasedformulationsfordrugdeliveryandtissueengineering:Areview.JControlRelease2012,158(1),15-33.46. MasEstelles,J.;Vidaurre,A.;MeseguerDuenas,J.M.;CastillaCortazar,I.,Physicalcharacterizationofpolycaprolactonescaffolds.JMaterSciMaterMed2008,19(1),189-95.47. Koleske,J.,Blendscontainingpoly(caprolactone)andrelatedpolymers.Academic

75

PressInc.:NewYork,1978.48. Coombes,A.G.A.;Rizzi,S.C.;Williamson,M.;Barralet,J.E.;Downes,S.;Wallace,W.A.,Precipitationcastingofpolycaprolactoneforapplicationsintissueengineeringanddrugdelivery.Biomaterials2004,25(2),315-325.49. Perez,M.H.;Zinutti,C.;Lamprecht,A.;Ubrich,N.;Astier,A.;Hoffman,M.;Bodmeier,R.;Maincent,P.,Thepreparationandevaluationofpoly(caprolactone)microparticlescontainingbothalipophilicandhydrophilicdrug.JControlRelease2000,65,429-438.50. Aydin,O.;Aydin,B.;Tezcaner,A.;Keskin,D.,Studyonphysiochemicalstructureandinvitroreleasebehaviorsofdoxycycline-loadedPCLmicrospheres.JournalofAppliedPolymerScience2015,132(14).51. Jenkins,M.J.;Harrison,K.L.,Theeffectofmolecularweightonthecrystallizationkineticsofpolycaprolactone.PolymersforAdvancedTechnologies2006,17(6),474-478.52. Aubin,M.;Prud'homme,R.E.,Preparationandpropertiesofpoly(valerolactone).Polymer1981,22,1223-1226.53. Gagliardi,M.;DiMichele,F.;Mazzolai,B.;Bifone,A.,ChemicalsynthesisofabiodegradablePEGylatedcopolymerfromε-caprolactoneandγ-valerolactone:evaluationofreactionandfunctionalproperties.JournalofPolymerResearch2015,22(2).54. Zeng,F.;Lee,H.;Allen,C.,Epidermalgrowthfactor-conjugatedpoly(ethyleneglycol)-block-poly(valerolactone)copolymermicellesfortargeteddeliveryofchemotherapeutics.BioconjugateChem2006,17,399-409.55. Lee,H.;Zeng,F.;Dunne,M.;Allen,C.,Methoxypoly(ethyleneglycol)-block-poly(valerolactone)copolymermicellesforformulationofhydrophobicdrugs.Biomacromolecules2005,6,3119-3128.56. Liu,J.;Xiao,Y.;Allen,C.,Polymer-drugcompatibility:aguidetothedevelopmentofdeliverysystemsfortheanticanceragent,ellipticine.JPharmSci2003,93(1),132-143.57. Lin,S.-Y.;Chen,K.-S.;Teng,H.-H.;Li,M.-J.,Invitrodegradationanddissolutionbehavioursofmicrospherespreparedbythreelowmolecularweightpolyesters.JournalofMicroencapsulation2000,17(5),577-586.58. Fukuzaki,H.;Yoshida,M.;Asano,M.;Kumakura,M.;Mashimo,T.;Yuasa,H.;Imai,K.;Yamanaka,H.;Kawaharada,U.;Suzuki,K.,Anewbiodegradablepasty-typecopolymerofl-lacticacidandδ-valerolactonewithrelativelylowmolecularweightforapplicationindrugdeliverysystems.JControlRelease1989,10,293-303.59. Wischke,C.;Schwendeman,S.P.,PrinciplesofencapsulatinghydrophobicdrugsinPLA/PLGAmicroparticles.IntJPharm2008,364(2),298-327.60. Mu,L.;Feng,S.S.,Fabrication,characterizationandinvitroreleaseofpaclitaxel(Taxol)loadedpoly(lactic-co-glycolicacid)microspherespreparedbyspraydryingwithlipid/cholesterolemulsifiers.JControlRelease2001,76,239-254.61. Hickey,T.;Kreutzer,D.;Burgess,D.J.;Moussy,F.,Dexamethasone/PLGAmicrospheresforcontinuousdeliveryofananti-inflammatorydrugforimplantablemedicaldevices.Biomaterials2002,23,1649-1656.62. Li,J.;Jiang,G.;Ding,F.,Effectsofpolymerdegradationondrugreleasefrom

76

PLGA-mPEGmicroparticles:Adynamicstudyofmicroparticlemorphologicalandphysicochemicalproperties.JournalofAppliedPolymerScience2008,108(4),2458-2466.63. Jain,R.A.,Themanufacturingtechniquesofvariousdrugloadedbiodegradablepoly(lactide-co-glycolide)(PLGA)devices.Biomaterials2000,21,2475-2490.64. Wang,J.;Wang,B.M.;Schwendeman,S.P.,Characterizationoftheinitialburstreleaseofamodelpeptidefrompoly(D,L-lactide-co-glycolide)microspheres.JControlRelease2002,82,289-307.65. Iwata,M.;McGinity,J.W.,Dissolution,stabilityandmorphologicalpropertiesofconventionalandmultiphasepoly(dl-lactic-co-glycolicacid)microspherescontainingwater-solublecompounds.PharmRes1992,10(8),1219-1227.66. Jeyanthi,R.;Thanoo,B.C.;Metha,R.C.;DeLuca,P.P.,Effectofsolventremovaltechniqueonthematrixcharacteristicsofpolylactide/glycolidemicrospheresforpeptidedelivery.JControlRelease1996,38,235-244.67. Cleland,J.L.,Solventevaporationprocessesfortheproductionofcontrolledreleasebiodegradablemicrosphereformulationsfortherapeuticsandvaccines.BiotechnolProg1998,14,102-107.68. S.,C.;Yoshioka,T.;Lucarelli,M.;Hwang,L.H.;Langer,R.S.,Controlleddeliverysystemsforproteinsbasedonpoly(lactic/glycolicacid)microspheres.PharmRes1991,8,713-720.69. Eliaz,R.E.;Kost,J.,CharacterizationofapolymericPLGA-injectableimplantdeliverysystemforthecontrolledreleaseofproteins.JBiomedMatRes2000,50(3),388-396.70. Hatefi,A.;Amsden,B.,Biodegradableinjectableinsituformingdrugdeliverysystems.JControlRelease2002,80(1),9-28.71. Alexis,F.;Venkatraman,S.;KumarRath,S.;Gan,L.-H.,Someinsightintohydrolyticscissionmechanismsinbioerodiblepolyesters.JournalofAppliedPolymerScience2006,102(4),3111-3117.72. Fu,K.;Pack,D.W.;Klibanov,A.M.;Langer,R.S.,Visualevidenceofacidicenvironmentwithindegradingpoly(lactic-co-glycolicacid)(PLGA)microspheres.PharmRes2000,17(1),100-106.73. Pitt,C.G.;Chasalow,F.I.;Hibionada,Y.M.;Klimas,D.M.;Schindler,A.,Aliphaticpolyesters.I.thedegradationofpoly(caprolactone)invivo.JournalofAppliedPolymerScience1981,26,3779-3787.74. Dordunoo,S.K.;Jackson,J.K.;Arsenault,L.A.;Oktaba,A.M.C.;Hunter,W.L.;Burt,H.M.,Taxolencapsulationinpoly(caprolactone)microspheres.CancerChemotherPharmacol1995,36,279-282.75. Woodward,S.C.;Brewer,P.S.;Moatamed,F.;Schindler,A.;Pitt,C.G.,Theintracellulardegradationofpoly-epsilon-caprolactone.JournalofBiomedicalMaterialsResearch1985,19,437-444.76. Cheng,L.;Guo,S.;W.,W.,Characterizationandinvitroreleaseofpraziquantelfrompoly(caprolactone)implants.IntJPharm2009,377,112-119.77. Ma,G.;Song,C.;Sun,H.;Yang,J.;Leng,X.,Abiodegradablelevonorgestrel-releasingimplantmadeofPCL/F68compoundastestedinratsanddogs.Contraception

77

2006,74(2),141-7.78. Loriot,M.;Linossier,I.;Vallée-Réhel,K.;Faÿ,F.,Syntheses,characterization,andhydrolyticdegradationofP(ε-caprolactone-co-δ-valerolactone)copolymers:Influenceofmolecularweight.JournalofAppliedPolymerScience2016,133(7),n/a-n/a.79. Imasaka,K.;Yoshida,M.;Fukuzaki,H.;Asano,M.;Kumakura,M.;Mashimo,T.;Yamanaka,H.;Nagai,T.,Anewbiodegradableimplantconsistingofwaxy-typepoly(ε-caprolactone-co-δ-valerolactone)andestramustine.IntJPharm1991,68,87-95.80. Siepmann,J.;Siepmann,F.,Mathematicalmodelingofdrugdelivery.IntJPharm2008,364(2),328-43.81. Huang,X.;Brazel,C.S.,Ontheimportanceandmechanismsofburstreleaseinmatrix-controlleddrugdeliverysystems.JControlRelease2001,73,121-136.82. Messaritaki,A.;Black,S.J.;vanderWalle,C.F.;Rigby,S.P.,NMRandconfocalmicroscopystudiesofthemechanismsofburstdrugreleasefromPLGAmicrospheres.JControlRelease2005,108(2-3),271-81.83. Rosenberg,R.;Devenny,W.;Siegel,S.;Dan,N.,Anomalousreleaseofhydrophilicdrugsfrompoly(caprolactone)matrices.MolPharm2007,4(6),943-948.84. Dipasree,S.R.;Rohera,B.D.,ComparativeevaluationofrateofhydrationandmatrixerosionofHECandHPCandstudyofdrugreleasefromtheirmatrices.EurJPharmSci2002,16,193-199.85. Jeong,J.-C.;Lee,J.;Cho,K.,Effectsofcrystallinemicrostructureondrugreleasebehaviorofpoly(ε-caprolactone)microspheres.JournalofControlledRelease2003,92(3),249-258.86. Barbato,F.;ImmacolataLaRotonda,M.;Maglio,G.;Palumbo,R.;Quaglia,F.,Biodegradablemicrospheresofnovelsegmentedpoly(ether-ester-amide)sbasedonpoly(caprolactone)forthedeliveryofbioactivecompounds.Biomaterials2001,22,1371-1378.87. Grund,J.;Korber,M.;Bodmeier,R.,Predictabilityofdrugreleasefromwater-insolublepolymericmatrixtablets.EurJPharmBiopharm2013,85(3PtA),650-5.88. Faisant,N.;Akiki,J.;Siepmann,F.;Benoit,J.P.;Siepmann,J.,EffectsofthetypeofreleasemediumondrugreleasefromPLGA-basedmicroparticles:experimentandtheory.IntJPharm2006,314(2),189-97.89. Siepmann,J.;Siepmann,F.,Modelingofdiffusioncontrolleddrugdelivery.JControlRelease2012,161(2),351-62.90. Liu,J.;Xiao,Y.;Allen,C.,Polymer-drugcompatibility:aguidetothedevelopmentofdeliverysystemsfortheanticanceragent,ellipticine.JPharmSci2004,93(1),132-143.91. Siepmann,J.;Peppas,N.A.,Modelingofdrugreleasefromdeliverysystemsbasedonhydroxypropylmethylcellulose(HPMC).AdvDrugDelivRev2001,48,139-157.92. Hildebrand,J.H.;Scott,R.L.,Thesolubilityofnonelectrolytes.DoverPublications:NewYork,1950.93. Hancock,B.C.;York,P.;Rowe,R.C.,Theuseofsolubilityparametersinpharmaceuticaldosageformdesign.IntJPharm1997,148,1-21.94. Karger,B.L.;Snyder,L.R.;Eon,C.,Expandedsolubilityparametertreatmentforclassificationanduseofchromatographicsolventsandadsorbents.AnalChem1978,50,

78

2126-2136.95. Nair,R.;Nyamweya,N.;Gonen,S.;Martinez-Miranda,L.J.;Hoag,S.W.,Influenceofvariousdrugsontheglasstransitiontemperatureofpoly(vinylpyrrolidone):athermodynamicandspectroscopicinvestigation.IntJPharm2001,225,83-96.96. Marsac,P.J.;Shamblin,S.L.;Taylor,L.S.,Theoreticalandpracticalapproachesforpredictionofdrug-polymermiscibilityandsolubility.PharmRes2006,23(10),2417-26.97. Greenhalgh,D.J.;Wiliams,A.C.;Timmins,P.;York,P.,Solubilityparametersaspredictorsofmiscibilityinsoliddispersions.JPharmSci1999,88,1182-1190.98. Löfgren,A.;Albertsson,A.-C.;Dubois,P.;Jérôme,R.,RecentAdvancesinRing-OpeningPolymerizationofLactonesandRelatedCompounds.JournalofMacromolecularScience,PartC1995,35(3),379-418.99. Silvers,A.L.;Chang,C.-C.;Emrick,T.,Functionalaliphaticpolyestersandnanoparticlespreparedbyorganocatalysisandorthogonalgraftingchemistry.JournalofPolymerSciencePartA:PolymerChemistry2012,50(17),3517-3529.100. Mecerreyes,D.;Miller,R.D.;Hedrick,J.L.;Detrembleur,C.;Jerome,R.,Ring-openingpolymerizationof6-hydroxynon-8-enoicacidlactone:novelbiodegradablecopolymerscontainingallylpendentgroups.JournalofPolymerSciencePartA:PolymerChemistry2000,38,870-875.101. Tempelaar,S.;Mespouille,L.;Dubois,P.;Dove,A.P.,OrganocatalyticSynthesisandPostpolymerizationFunctionalizationofAllyl-FunctionalPoly(carbonate)s.Macromolecules2011,44(7),2084-2091.102. Molander,G.A.;Harris,C.R.,SequencedReactionswithSamarium(II)Iodide.TandemIntramolecularNucleophilicAcylSubstitution/IntramolecularBarbierCyclizations.JAmChemSoc1995,117,3705-3716.103. Darcos,V.;Antoniacomi,S.;Paniagua,C.;Coudane,J.,Cationicpolyestersbearingpendentaminogroupspreparedbythiol–enechemistry.Polym.Chem.2012,3(2),362-368.104. Leemhuis,M.;Akeroyd,N.;Kruijtzer,J.A.W.;vanNostrum,C.F.;Hennink,W.E.,Synthesisandcharacterizationofallylfunctionalizedpoly(α-hydroxy)acidsandtheirfurtherdihydroxylationandepoxidation.EuropeanPolymerJournal2008,44(2),308-317.105. Parrish,B.;Quansah,J.K.;Emrick,T.,Functionalpolyesterspreparedbypolymerizationof?-allyl(valerolactone)anditscopolymerizationwith?-caprolactoneand?-valerolactone.JournalofPolymerSciencePartA:PolymerChemistry2002,40(12),1983-1990.106. Hoyle,C.E.;Bowman,C.N.,Thiol-eneclickchemistry.AngewChemIntEdEngl2010,49(9),1540-73.107. Campos,L.M.;Killops,K.L.;Sakai,R.;Paulusse,J.M.J.;Damiron,D.;Drokenmuller,E.;Messmore,B.W.;Hawker,C.J.,DevelopmentofThermalandPhotochemicalStrategiesforThiol-EneClickPolymerFunctionalization.Macromolecules2008,41,7063-7070.108. Boire,T.C.;Gupta,M.K.;Zachman,A.L.;Lee,S.H.;Balikov,D.A.;Kim,K.;Bellan,L.M.;Sung,H.J.,Pendantallylcrosslinkingasatunableshapememoryactuatorfor

79

vascularapplications.ActaBiomater2015,24,53-63.109. Hansen,C.M.,Thethreedimensionalsolubilityparameter--Keytopaintcomponentaffinities.1:Solvents,plasticisers,polymersandresins.JPaintTechnol1967,39,104-117.110. VanKrevelen,D.W.;Hoftyzer,P.J.,Propertiesofpolymers:theirestimationandcorrelationwithchemicalstructure.Elsevier:Amsterdam,1976.111. Sun,D.D.;Ju,T.C.;Lee,P.I.,Enhancedkineticsolubilityprofilesofindomethacinamorphoussoliddispersionsinpoly(2-hydroxyethylmethacrylate)hydrogels.EurJPharmBiopharm2012,81(1),149-58.112. Koennings,S.;Tessmar,J.;Blunk,T.;Gopferich,A.,Confocalmicroscopyfortheelucidationofmasstransportmechanismsinvolvedinproteinreleasefromlipid-basedmatrices.PharmRes2007,24(7),1325-35.113. Baek,H.S.;Yoo,J.Y.;Rah,D.K.;Han,D.-W.;Lee,D.H.;Kwon,O.-H.;Park,J.C.,Evaluationoftheextractionmethodforthecytotoxicitytestingoflatexgloves.YonseiMedicalJournal2005,46(4),579-583.114. Klose,D.;Siepmann,F.;Elkharraz,K.;Krenzlin,S.;Siepmann,J.,HowporosityandsizeaffectthedrugreleasemechanismsfromPLGA-basedmicroparticles.IntJPharm2006,314(2),198-206.115. Cohn,D.;Younes,H.;Marom,G.,Amorphousandcrystallinemorphologiesinglycolicacidandlacticacidpolymers.Polymer1987,28,2018-2022.116. Jamshidi,K.;Hyon,S.-H.;Ikada,Y.,Thermalcharacterizationofpolylactides.Polymer1988,29,2229-2234.117. Garle,A.;Kong,S.;Ojha,U.;Budhlall,B.M.,Thermoresponsivesemicrystallinepoly(epsilon-caprolactone)networks:exploitingcross-linkingwithcinnamoylmoietiestodesignpolymerswithtunableshapememory.ACSApplMaterInterfaces2012,4(2),645-57.118. Aharoni,S.M.,Increasedglasstransitiontemperatureinmotionallyconstrainedsemicrystallinepolymers.PolymersforAdvancedTechnologies1998,9,169-201.119. Zhu,G.;Liang,G.;Xu,Q.;Yu,Q.,Shape-memoryeffectsofradiationcrosslinkedpoly(ϵ-caprolactone).JournalofAppliedPolymerScience2003,90,1589-1595.120. Monteiro,M.S.S.B.;Chávez,F.V.;Sebastião,P.J.;Tavares,M.I.B.,1HNMRrelaxometryandX-raystudyofPCL/nevirapinehybrids.PolymerTesting2013,32(3),553-566.121. Lyu,S.;Untereker,D.,Degradabilityofpolymersforimplantablebiomedicaldevices.InternationalJournalofMolecularSciences2009,10,4033-4065.122. Toncheva,V.;VanDenBulcke,A.;Schacht,E.;Mergaert,J.;Swings,J.,Synthesisandenvironmentaldegradationofpolyestersbasedonpoly(caprolactone).JournalofEnvironmentalPolymerDegradation1996,4,71-83.123. Eldsater,C.;Erlandsson,B.;Renstad,R.;Albertsson,A.-C.;Karlsson,S.,Thebiodegradationofamorphousandcrystallineregionsinfilm-blownpoly(e-caprolactone).Polymer2000,41,1297-1304.124. Kreye,F.;Siepmann,F.;Willart,J.F.;Descamps,M.;Siepmann,J.,Drugreleasemechanismsofcastlipidimplants.EurJPharmBiopharm2011,78(3),394-400.125. Crotts,G.;Park,T.G.,Proteindeliveryfrompoly(lactic-co-glycolicacid)

80

biodegradablemicrospheres:Releasekineticsandstabilityissues.JournalofMicroencapsulation1998,15(6),699-713.126. Baker,R.W.,Controlledreleaseofbiologicallyactiveagents.NewYork,1987.127. Sun,D.D.;Lee,P.I.,Probingthemechanismsofdrugreleasefromamorphoussoliddispersionsinmedium-solubleandmedium-insolublecarriers.JControlRelease2015,211,85-93.128. Pinto,F.C.;DaSilva-CunhaJunior,A.;Orefice,R.L.;Ayres,E.;Andrade,S.P.;Lima,L.D.;Moura,S.A.;DaSilva,G.R.,Controlledreleaseoftriamcinoloneacetonidefrompolyurethaneimplantabledevices:applicationforinhibitionofinflammatory-angiogenesis.JMaterSciMaterMed2012,23(6),1431-45.129. Gomez-Gaete,C.;Tsapis,N.;Besnard,M.;Bochot,A.;Fattal,E.,Encapsulationofdexamethasoneintobiodegradablepolymericnanoparticles.IntJPharm2007,331(2),153-9.130. Rosilio,V.;Benoit,J.P.;Deyme,M.;Thies,C.;Madelmont,G.,Physicochemicalstudyofthemorphologyofprogesterone-loadedmicrospheresfabricatedfrompoly(D,L-lactide-co-glycolide).JournalofBiomedicalMaterialsResearch1991,25,667-682.131. Vergnaud,J.M.,Controlleddrugreleaseoforaldosageforms.EllisHorwood:NewYork,1993.132. Guse,C.;Koennings,S.;Kreye,F.;Siepmann,F.;Goepferich,A.;Siepmann,J.,Drugreleasefromlipid-basedimplants:elucidationoftheunderlyingmasstransportmechanisms.IntJPharm2006,314(2),137-44.133. Bodmeier,R.;Paeratakul,O.,Drugreleasefromlaminatedpolymericfilmspreparedfromaqueouslatexes.JPharmSci1990,79(1),32-36.134. Omelczuk,M.O.;McGinity,J.W.,Theinfluenceofpolymerglasstransitiontemperatureandmolecularweightondrugreleasefromtabletscontainingpoly(DL-lacticacid).PharmRes1992,9(1),26-32.135. Miyajima,M.;Koshika,A.;Okada,J.;Ikeda,M.;Nishimura,K.,Effectofpolymercrystallinityonpapaverinereleasefrompoly(L-lacticacid)matrix.JControlRelease1997,49,207-215.136. Heya,T.;Okada,H.;Ogawa,Y.;Toguchi,H.,FactorsinfluencingtheprofilesofTRHreleasefromcopoly(DL-lactic/glycolicacid)microspheres.IntJPharm1991,72,199-205.137. Nishi,C.;Nakajima,N.;Ikada,Y.,Invitroevaluationofdiepoxycompoundsusedforbiomaterialmodification.JournalofBiomedicalMaterialsResearch1995,29,829-834.138. Serrano,M.C.;Pagani,R.;Vallet-Regi,M.;Pena,J.;Ramila,A.;Izquierdo,I.;Portoles,M.T.,Invitrobiocompatibilityassessmentofpoly(epsilon-caprolactone)filmsusingL929mousefibroblasts.Biomaterials2004,25(25),5603-11.

81

Appendices

Table2.5SummaryofFDAapprovedpolyester-basedIDDSs.

ProductName

Polymer ActiveIngredient

DosageForm

Indication Dose Duration ApprovalDate

Arestin PLGA

MinocyclineHCl

Microparticle

Adultperidontitis

0.5mg/week

2weeks

2001

Atridox PLA

Doxycyclinehyclate

Insituforminggel

Adultperidontitis

50mg/week

1week

1998

Bydureon PLGA

Exenatide

Microparticle;SC

TypeIIdiabetes

2mg/week

2012

Capronor PCL Levonorgestrel

Implant Contraception

Eligard PLGA

Leuprolideacetate

Insituforminggel;SC

Palliativetreatmentofadvancedprostatecancer

7.5mg/month

1-6months

2002(7.5&22.5mg),2003(30mg),2004(45mg)

LupanataPack

PLGA

Leuprolideacetate;norethindroneacetate

Microparticle;IM

Endometriosis

3.75mg/month

3months

2012

Lupron PLGA

Leuprolideacetate

Microparticle;IM

Endometriosis

3.75mg/month

1995

LupronDepot

PLGA/PLA

Leuprolideacetate

Microparticle;IM

Palliativetreatmentofadvancedprostatecarcinoma

7.5mg/month

1-6months

1989

82

LupronDepot-PED

PLGA/PLA

Leuprolideacetate

Microparticle;IM

Centralprecociouspuberty

7.5,11.25or15mg/month;11.25or30mg/3months

1-3months

1993

NutropinDepot

PLGA

Recombinanthumangrowthhormone

Microparticle;SC

Growthhormonedeficiency,pituitarydwarfism

13.5,18,or22.5mg

1month

1999

Ozurdex PLGA

Dexamethasone

Solidimplant

(Diabetic)macularedema,non-infectiousuvetis,retinalveinocclusion

0.23mg/month

3months

2009

Propel PLGA/PEG

Mometasonefuroate

Solidimplant

Chronicsinusitispatientsundergoingsurgery

370ug/month

1month

2011

RisperdalConsta

PLGA

Risperidone

Microparticle;IM

SchizophreniaandbipolarIdisorder

12.5,25,37.5,or50mg/2weeks

2weeks

2003

SandostatinLAR

PLGA

Octreotide

Microparticle;IM

Acromegaly,diarrheaassociatedwithmetastaticcarcinoidorVIP-secretingtumors

10,20,or30mg/month

1month

1998

SigniforLAR

PLGA

Pasireotideparnoate

Microparticle;IM

Acromegaly

20,40,or60mg/28days

28days

2014

83

SomatulineDepot

PLGA

Lanreotide

Microparticle;SC

Acromegaly,gastrointestinal&pancreaticneuroendocrinetumors

60,90or120mg/month

1month

2007

SuprefactDepot

PLGA Buserelinacetate

Microparticle;SC

Advancedprostatecarcinoma

6.3mg/2monthsor9.45mg/3months

2-3months

2000

Trelstar PLGA Triptorelinpamoate

Microparticle;IM

Palliativetreatmentofadvancedprostatecarcinoma

3.75mg/month

1-6months

2000

Vivitrol PLGA Naltrexone

Microparticle;IM

Alcoholdependence

380mg/month

1984

Zoladex PLGA Goserelinacetate

Solidimplant;SC

Advancedprostatecarcinoma

3.6or10.8mg/month

1989(3.6mg)and1996(10.8mg)

84

Figure2.201HNMRspectrumofP39Kandgelpermeationchromatogramsforthethreeco-polymers.

85

Figure2.21Massofdrugreleasedasafunctionoftimeinvestigatingtheimpactofthestate of the drug-loaded matrices (top), aqueous solubility (middle), and polymerphysicochemicalproperties(bottom)ondrugrelease.