pH-dependent ileocolonic drug delivery, part I: in vitro and clinical evaluation of novel systems

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University of Groningen

pH-dependent ileocolonic drug delivery, part I

Broesder, Annemarie; Woerdenbag, Herman J; Prins, Grietje H; Nguyen, Duong N; Frijlink,

Henderik W; Hinrichs, Wouter L J

Published in:

Drug Discovery Today

DOI:

10.1016/j.drudis.2020.06.011

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Citation for published version (APA):

Broesder, A., Woerdenbag, H. J., Prins, G. H., Nguyen, D. N., Frijlink, H. W., & Hinrichs, W. L. J. (2020).

pH-dependent ileocolonic drug delivery, part I: in vitro and clinical evaluation of novel systems. Drug

Discovery Today, 25(8), 1362-1373. https://doi.org/10.1016/j.drudis.2020.06.011

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REVIEWS DrugDiscoveryTodayVolume25,Number8August2020

Teaser

After

the

pH

dependency

of

novel

pH-dependent

ileo-colonic

drug

delivery

systems

is

confirmed

in

vitro,

their

performance

should

be

evaluated

in

human

volunteers.

pH-dependent

ileocolonic

drug

delivery,

part

I:

_in

_vitro

and

clinical

evaluation

of

novel

systems

Annemarie

Broesder,

Herman

J. Woerdenbag,

Grietje

H. Prins,

Duong

N. Nguyen,

Henderik

W. Frijlink

and

Wouter

L.J.

Hinrichs

UniversityofGroningen,GroningenResearchInstituteofPharmacy,DepartmentofPharmaceuticalTechnology andBiopharmacy,AntoniusDeusinglaan1,9713AVGroningen,TheNetherlands

pH-dependent

ileocolonic

drug

delivery

systems

rely

on

the

sharp

pH

peak

reaching

7.2–7.7

usually

found

in

the

ileum

of

healthy

individuals

and

patients

with

colonic

diseases.

The

pH

dependency

of

novel

drug

delivery

systems

should

first

be

evaluated

in

vitro

dissolution

tests

mimicking

the

human

gastrointestinal

pH

profile

and

buffer

composition.

When

proven

successful

in

vitro,

the

clinical

applicability

of

a

novel

system

should

be

confirmed

in

humans.

Various

methods

have

been

published

to

verify

ileocolonic

drug

delivery

in

humans.

Of

those,

we

recommend

the

caffeine-sulfasalazine

method

and

the

dual

stable

isotope

approach.

Introduction

Drugdeliverytotheileocolonicregionisreceivingsubstantialattentionbecauseitcouldimprove

the local treatment of disorders, such as ulcerative colitis (UC), Crohn’s disease (CD), and

colorectal cancer (CRC) [1]. In addition, the potential of ileocolonic delivery for systemic

treatmentisasubjectofinvestigation.Examplesarethesystemicdeliveryofproteinandpeptide

drugs,whicharesparedbecauseofthelowproteolyticactivityinthecolon[2],andthedelayed

deliveryofdrugstotreatdiseasesthatfollowacircadianrhythm,suchasasthma,anginapectoris,

andrheumatoidarthritis[3].

Severalapproacheshaveemergedtoachieveileocolonictargeteddrugdelivery.Theyinclude

systemsthataredependentontime,pressure,enzymes,pH,andcombinationsthereof.

Com-prehensive overviews on this topic have been published elsewhere [1,4,5]. Although time-,

pressure-, and enzyme-basedsystemshave also showntheir potential,herewe focuson

pH-dependent systemsforileocolonicdrug delivery.Ingeneral, thesepH-dependentsystemsare

basedonpH-sensitivepolymericcoatingssurroundingthedrugorpH-sensitivematricesinwhich

thedrugisembedded.WedescribedifferentmethodstomeasurethepHinthevariouspartsofthe

humangastrointestinal(GI)tractandcomparethereportedvaluesforhealthyindividualsand

Reviews KEYNO TE REVIEW AnnemarieBroesder receivedherMScin

pharmacyfromthe

UniversityofGroningen

(TheNetherlands).

Currently,sheispursuing

herPhDintheDepartment

ofPharmaceutical

Technologyand

BiopharmacyattheUniversityofGroningenunder

thesupervisionofProf.DrH.W.FrijlinkandDr.W.L.

J.Hinrichs.Herresearchfocusesonoral

pH-dependentileocolonicdrugdeliveryandthesuitability

ofthesesystemsfordifferentdosageforms.

WouterL.J.Hinrichs

studiedchemistryatthe

UniversityofGroningenand

receivedhisPhDfromthe

UniversityofTwente(The

Netherlands)in1993.

Thereafter,heworkedasa

postdocintheUniversityof

TwenteandUtrecht

University(TheNetherlands)until1998.Sincethen,

hehasbeenanassistantprofessorattheDepartment

ofPharmaceuticalTechnologyandBiopharmacyat

GroningenResearchInstituteofPharmacy(The

Netherlands).Oneofhisresearchinterestsisoral

dosage forms. He has (co-)authored more than 140

originalarticlesinpeer-reviewedjournalsandbook

chapters.

HenderikW.Frijlinkhas

beenaprofessorand

chairmanoftheDepartment

ofPharmaceutical

TechnologyandBiopharmacy

attheGroningenResearch

InstituteofPharmacysince

1998.Heisinterestedinsolid

oraldosageforms,their

technologicalandbiopharmaceuticalaspectsand

gastrointestinaltargetingstrategy.Furtherinterestsof

hiscurrentresearcharetechnicaland

biopharmaceuticalaspectsofpulmonarydrug

delivery.Theformulationofbiopharmaceuticals,such

aspeptides,proteins,vaccines,andgene-based

products,usingstabilizingsugarglassesisafurther

researchtopicinhisdepartment.Hehaspublished

over200peer-reviewedpapersinthefield.

Correspondingauthor:Hinrichs,WouterL.J. (w.l.j.hinrichs@rug.nl)

1362 www.drugdiscoverytoday.com

1359-6446/ã2020TheAuthor(s).PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).

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patientswithvariouscolonicdiseases.Thereafter,current

meth-odstoevaluatenovelsystemsinvitroaswellasinclinicaltrialsare

describedandcriticallydiscussed.Finally,wesuggestmethodsthat

aremostsuitableforevaluatingtheperformanceofnovelsystems.

pH

in

the

lumen

of

the

human

gastrointestinal

tract

Methods

to

determine

the

pH

in

the

human

gastrointestinal

tract

Forthedevelopment andperformanceofpH-dependent

ileoco-lonicdrugdeliverysystems,itisaprerequisitetoknowthepHof

the content in the different segments of the GI tract of both

healthyanddiseasedhumans.Threedifferentmethodsareused

tomeasurethesepHvalues.Thefirstisaspiration,duringwhichGI

fluidiscollectedandthepHofthecollectedfluidismeasuredex

vivowithapHelectrode[6–16].Thiscanbeperformedviatheoral

routeforthestomach,duodenal,and jejunalfluid [6–14]orvia

colonoscopyfortheilealandcolonicfluid[15,16].The

disadvan-tageofcolonoscopyisthatitrequirestheadministrationofeither

bisacodylorKlean-Prep1beforetheprocedure,whichaltersthe

contentsoftheGItractand,therefore,couldalterthepH.Inthe

secondmethod,atetheredpHelectrodeisusedtomeasurethepH

insituofthestomach,duodenum,orjejunum[17–24].Inthethird

method, a pH-sensitive radio telemetry capsule is used, which

measures the pH during itstransit throughthe entireGI tract

[25–45].ThemajoradvantageofatetheredpHelectrodeora

pH-sensitiveradiotelemetrycapsuleisthatthepHchangesovertime

ofthedifferentsegmentsintheGItractcanbemeasured,whereas

the aspiration and colonoscopy methods only generate mean

values.Furthermore, pH-sensitiveradiotelemetry capsules

mea-surethepHduringitstransitthroughtheentireGItract.Thus,this

methodrepresentsadynamicmeasurement.Examplesof

teleme-try capsules arethe Heidelberg capsule, SmartPill, BravoTM _pH

monitoringcapsule,and theIntelliCap1(Table1) [46–49].The

SmartPillandIntelliCap1measurenotonlythepHintheGItract,

butalsothetemperature,whichishelpfultodeterminewhenthe

capsule exits the body. Furthermore, when pyloric passage is

assumed, ice-cold water can be orally administeredand, when

noimmediatetemperaturedecrease ismeasured,itcanbe

con-cluded that the telemetry capsule has passed the stomach

[47,49,50]. The IntelliCap1 also containsa fluid reservoirthat

can be used to locally deliver compounds to the GI tract, for

instancetoenablepharmacokineticstudies[49,50].Thedownside

ofthissystemisthatthecapsuleisconsiderablylargerthanthe

othersystemsandhasnotbeencommerciallyavailablesince2017.

TheBravoTM_system_was_originally_developed_to_be_attached_to_the

esophagealmucosaviaendoscopicinterventiontomeasure

esoph-agealacidexposureovertime[48].Withoutendoscopic

interven-tion,thecapsulebecomesafreefallsystemandthe pHoverthe

entireGItractcanbemeasured[30].Overall,thereisnosystem

superiortoanother,andthechoiceofthesystemdependsonthe

specificresearchquestionorpreference.

Besides measuring pH, temperature, and intestinal pressure,

radiotelemetrycapsulescanbeusedtomeasurethetransittime

throughthe differentregionsoftheGItract,withouttheaidof

imagingtechniques.Thetransittimes,basedonpHreadings,have

been found to be comparable to those obtained with imaging

techniques[37,42,51].

pH

values

in

the

gastrointestinal

tract

of

healthy

human

individuals

Intheliterature,36studieswerefoundin whichthe pHofthe

differentsegmentsoftheGItractofhealthyhumanswasmeasured

[6–14,17–23,25,28–45,52].When compilingan overview of the

meanpHvaluesofthevarioussegmentsoftheGItract,both

intra-andinterindividualvariationhastobetakenintoaccount.For

pH-dependentileocolonicdrugdelivery,thepHvaluesinthe small

intestineandascendingcolonaremostimportant.

Regardingintraindividualvariation,Mikolajczyketal.showed

that, over 24h, pH fluctuations in a single subject were only

minor, with pH variations in the colon being slightly higher

(DpH: 0.45) than in the proximal small intestine (DpH: 0.14)

anddistalsmallintestine(DpH:0.22)[43].AccordingtoIbekwe

etal.theintraindividualvariabilityinpHispartiallytheresultof

differencesin transittimes ofthetelemetry capsules[30].With

telemetry capsules,thetransittimedeterminesthenumberand

locationofpHmeasurementstakenineachregionoftheGItract.

Therefore,differencesintransittime,includingstasisand

retro-pulsion,couldinfluencethemeanpHvalue.Withaspiration,the

intestinalfluidishomogenizedandthemeanpHisbasedonthe

aspiratedsampletaken;therefore,thelocationofaspirationcould

influencethemeanpH.Kozioleketal.investigatedthe

interindi-vidualvariabilityandshowedthatthepHwashighlyvariablein

thestomachandcolon,butonlysmalldifferenceswereseeninthe

proximalsmallintestineandevensmallerdifferencesoccurredin

thedistalsmallintestine[31].Thisisinlinewithotherstudies,

whichshowedthatgastricandcolonicpHshowedlarger

interin-dividualvariations[25,41].

The meanor medianpHfoundinthe individualstudies,for

boththefastedandfed stateinhealthyadults,aredisplayedin

Fig.1asdots.WhenmultiplepHvaluesweregivenforaGItract

segment,forinstancethefundusandantrumofthestomach,the

meanofthesevaluesisshown.TherangeoffoundpHvaluesis

shownasbars,inwhichaverticallineshowsthemeanpHofall

studies with healthy adults. To calculate this mean value, the

DrugDiscoveryTodayVolume25,Number8August2020 REVIEWS

TABLE1

OverviewofpH-measuringradiotelemetrycapsules

Brandradiotelemetrycapsule Dimensions(mm) Telemetry Fluidreservoir Commerciallyavailable Refs

pH Temperature Intestinalpressure

Heidelberg 818 X X [46] IntelliCap1 1127 X X X [49,50] BravoTM 65.525 X X [48] SmartPill 11.722 X X X X [47] www.drugdiscoverytoday.com 1363 Reviews KEYNO TE REVIEW

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mean pHvalueswereused ifthey werementioned inthe

indi-vidualstudies.Meanvalueswerepreferredtopreventdatalossof

individualswithoutlyingpHvalues.Ifnomeanvaluewasgiven,

themedianvaluewasused.Thenumberofsubjectsintheseparate

studieswasnotusedinthecalculations.StudiesonlyreportingpH

ranges wereexcluded fromthe overview and fromour

calcula-tions.Wealsoexcludedstudiesthatusedcolonoscopytoaccess

thepHvalueofthelowerGItract,becauseoftheuseofBisacodyl

ofKlean-Prep_1. Wedid notfindmarked differencesin thepH

valuesobtainedwithaspiration,tetheredpH-electrodes,or

telem-etrycapsules.

ThepHrisesduringtransitfromthestomachtotheileum,after

whichitdropsinthececumandrisesagainslightlyinthecolon

(Fig.1).ThepreciselocationofthepostileumpHdropof1.5and

1.2unitsinthefastedandfedstate,respectively,wasfoundtobein

theproximalcolon.Thedropcanoccureitherinthececum,the

ascending colon, or during the transit from the cecum to the

ascending colon [37]. This pH drop can be explained by the

bacterial fermentation of polysaccharides to short-chain fatty

acids[53].Thefoodstatusofthesubjectsonlyappearedto

influ-encethepHofthestomach;thatis,thepHissubstantiallyhigher

afterfoodintake.However,fewerdatawereavailableonthefed

statethanonthe fastedstate.Furthermore,inthe postprandial

statethe stomachhas regionsof differentpH values, namelya

proximalacidlayer(pH2.9),abufferedlayer(pH5.0),andadistal

acidlayer(pH2.3)[23].

Fallingborgetal.studiedthepHintheGItractoffastedhealthy

children,aged8–14years[26].ThemeanpHvalues,indicatedby

whitesquaresinFig.1,onlyslightlydifferedfromthevaluesof

fastedhealthyadults.Forhealthyoldersubjects,aged62–83years,

threestudieswerefoundinwhichthegastricand/orduodenalpH

wasstudied.TheseareindicatedbygraysquaresinFig.1[7,24,40].

Contradictoryresultswerefoundontheinfluenceofageonthe

gastric and duodenal pH. Comparative studies from the same

group indicated that the gastric fasted and peak-fed pH were

significantly lower forolder (65–83 years) [24] than for young

individuals(21-35years)[19].Bycontrast,Mojaverianetal.found

that the postprandial pH values in the stomach of the older

subjects (65–79 years) were significantly higher than those of

youngindividuals(2–34years)[40].Inthefastedstate,no

signifi-cantdifferencesingastricpHbetweenthreedifferentagegroups

(20–39,40–59,and60–70years)wasfound[10].TheduodenalpH

Stomach Duodenum Jejunum Ileum Cecum Colon Rectum 0 1 2 3 4 5 6 7 8 Luminal pH Adults Children (8-14 years) Elderly (62-83 years) Healthy - fed Healthy - fasted Diseased - fasted or fed

Drug Discovery Today

FIGURE1

pHvaluesofvariouspartsofthegastrointestinal(GI)tractofhealthyanddiseasedhumans.Themaximum,minimum,andmeanpHofthestomach[9–12,17– 19,21–23,28–33,36,40,42,52],duodenum[6–9,12–14,18–20,25,30–35,39,41,45],jejunum[8,18,30,32,34–36,38,43,45],ileum[25,30–39,41,43–45],cecum [28,33,34,36,37,44],colon[25,28,30–36,38,41–43,45],andrectum[34,36,43,45]aregivenforhealthyindividualsinthefed(blue)andfasted(orange)state.ThepH valuesofdiseasedhumanindividuals(gray)aregroupedforthefastedandfedstate;themeanpHvalueisnotgiven[33,35,36,44].Themaximumandminimum pHvaluesareindicatedbythebars,inwhichaverticallineindicatesthemeanpHofhealthyindividuals,andthedotsthemeanormedianpHvaluesofthe individualstudies.Thewhitesquares,inthefastedstate,indicatethepHvaluesoftheGItractofhealthychildrenaged8–14years[26].Thegraysquares,inthe fastedandfedstate,indicatethepHvaluesoftheGItractofoldersubjects,aged62–83years[7,24,40].Giventhelimitedamountofstudiesinthefastedstatefor thepHintherectum(onestudy),thebarandverticallineoverlap.

1364 www.drugdiscoverytoday.com Reviews KEYNO TE REVIEW

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inthefastedandfedstatewassignificantlyhigherinolder(65–83

years)[24]thanyoung(21–35years)individuals[19].Bycontrast,

Annaertetal.foundnosignificantdifferencesbetweenduodenal

pHvaluesfortheolder(62–72years)andyoungerindividuals(18–

25years)inthefastedstate[7].

Next to age, gender differences in GI pH values have been

investigated.Nosignificantdifferenceswerefoundforthefasted

and fed statesfor gastric [18,19,24,40], duodenal[18,19,24,35],

andjejunal[18,35]pH.Fortheileumandcolon,onlyfastedstate

datawereavailable,thatshowednosignificantgendereffect[35].

Tothebestofourknowledge,noinfluenceofethnicityontheGI

pHhasbeenreported inliterature.The influenceofage-related

diseasesonGIpHhasbeensparselyaddressedand,therefore,was

notincludedinthisoverview[54,55].

Thesharp,localpHpeakintheterminalileumtovalues_>7.2

(Fig.1)hasbeenusedforthedevelopmentofcolontargeteddrug

deliverysystems.WhenasystemusesthispHvalueasatriggerto

initiatedrugrelease,releasewillstartintheterminalileumwhich

isincloseproximityofthececum(i.e.,thebeginningofthelarge

intestine)[1].Thus,asstatedbyIbekweetal.,theterm‘ileocolonic

drugtargeting’ismoreappropriateforthesesystemsthantheterm

‘colonic drugtargeting’ [56].Whencontinuation ofthe release

dependson the presence ofthe highpHvaluein the terminal

ileum,thefullreleaseshouldoccurquicklybecausethepHdrops

againinthececum(pH6.1).Thiswillslowdownorhaltfurther

disintegration and/or dissolution ofthe pH-dependent

compo-nentinthesystem.AlthoughthemeanandminimumpHinthe

jejunumwerefoundtobe6.5and4.9,respectively,amaximum

pHof7.4wasfoundinthisGIsegment.Therefore,thereisariskof

prematuredrugrelease.

pH

values

in

the

gastrointestinal

tract

of

humans

with

colonic

diseases

Forasuccessfultherapeuticapplicationofanileocolonictargeted

drugdeliverysystems,itisimportanttohaveinsightintothepHof

theGItractof patientswith GItract diseases,suchasCD, UC,

irritable bowel syndrome (IBS), and CRC. Therefore, here, we

comparethe pHofvarious segmentsofthe GItractofhealthy

subjects(Fig.1)withthoseoftheaforementionedpatientgroups.

To visualize the overall differences more clearly, pH values of

diseasedindividualsarealsogiveninFig.1,inwhichthedifferent

diseasesaregroupedtogether.

ThepHvaluesoftheGItractofindividualswithCDdidnot

differfromthoseofhealthyindividuals,includingthemedianpH

peak of 7.4 in the terminal ileum [36]. For patients with UC,

contradictorypHvalueshavebeenreported.Pressetal.founda

medianpHof7.95intheileumand6.95inthececumofpatients

withUC,whichwashigherthanthemaximumvaluesof7.70and

6.25,respectively,foundinhealthyindividuals[33].Bycontrast,

inastudybyEweetal.,amedianpHof6.8intheileumand5.5in

thececumofpatientswithUCwasfound[36],whichwaslower

thantheminimumvaluesof7.2and5.7,respectivelyinhealthy

individuals. However, the median pH increased to 7.3 in the

ascending colon, which is higher thanthe maximumvalue of

7.1foundinthecolonofhealthyindividuals,making

pH-depen-dentcolonictargeteddrugdeliverypossible.InpatientswithIBS,

themeanpHintheileumwas7.7[44],whichfallswithinthepH

range(7.2–7.7)oftheileumofhealthyindividuals.ThemeanpH

inthececuminpatientswithIBSwas5.1,whichislowerthanthe

minimumvalueof5.7foundintheileumofhealthyindividuals.

ThislowerpHcouldbeattributedtoexcessivebacterial

fermenta-tionofpolysaccharidesleadingtotheproductionofshort-chain

fattyacids.Lastly,patientswithcolorectaladenomaorcarcinoma

were foundto have pH valuescomparable to those in healthy

volunteersinthevarioussegmentsoftheGItract[35].

Based on the data from the studies described above, it is

expected that pH-dependent ileocolonic drug delivery systems

cansuccessfullybeappliedinpatientswiththeabove-mentioned

chronic GI diseases. Therefore,itis not surprisingthat various

productsareavailableon themarketthat utilizepH-dependent

excipientsforileocolonicdrugtargeting[e.g.,Budenofalk₁

(Bude-sonide; Dr. Falk Pharma), Lialda₁ (Mesalazine; Shire), and

Asacol₁(Mesalazine;Allergan)].

In

vitro

methods

to

simulate

the

human

gastrointestinal

tract

Beforeclinicalevaluation,novelpH-dependentsystemsare

gen-erally first tested in in vitro dissolution studies. Obviously, an

optimal invitrodissolutiontestmimics thephysiologicalpHof

thevarioussegmentsoftheGItractascloseaspossible(Fig.1).In

addition,thetypeofbufferandexposuretimestothedifferentpH

valuesshouldbecarefullychosen.

Table2presentsanoverviewofclinicalstudiestogetherwith

informationaboutthedrugdeliverysystemanddetailsaboutthe

invitrodissolutiontest,ifapplicable.Asexpected,thepHofthe

dissolutionmediumisoftenadjustedduringthedissolutiontestto

mimic the in vivo conditions. This can be achieved by either

transferringthedrugdeliverysystemfromonevesseltoanother

vesselcontainingmediumwithadifferentpHorbychangingthe

pHofthedissolutionmediumbyaddinganothersolutiontothe

original vessel. The latter option is preferred becausethe drug

deliverysystemwillthenonlyexperiencethepHchange.

TheGItractisparticularlybufferedbybicarbonateinthefasted

state,whereasinthefedstatedifferentbufferspeciesarepresent

[57].Systemsthatusebicarbonatebuffers withbuffercapacities

reflectingtheinvivosituationhavebeenfoundtobetterrepresent

the in vivo situationcompared with the more commonlyused

phosphatebuffers(Table2)[56,58–61].AccordingtoAmaralSilva

etal.,amajordifferenceisthatbicarbonatehasalowerbuffering

capacityinthediffusionlayernearthesurfaceofthedrugdelivery

system compared with phosphate, resulting in a better in vivo

predictabilityofbicarbonatesystems[62].Acomplicatingfactor

of bicarbonate buffers in in vitro studies is their pH instability

becauseofthelossofCO2asagasfromthesystem,whichaltersthe

pH.Garbaczet al.utilized thisintheir pHysio-grad1devicein

which the pH during the dissolution test can be dynamically

alteredby purgingN2orCO2gasintothesystemtoincreaseor

decrease the pH, respectively, thereby eliminating the major

drawbackofbicarbonatesystems[63].Purgingofgassesandloss

ofCO2gasinthesystemintroducesbubblesintothedissolution

medium,whichcould influencethe drugrelease[62].Toavoid

effects of CO2 bubbles on the drug release when bicarbonate

buffers are used,alterationof the buffer capacityof phosphate

buffershasbeenproposed.However,notonlythebuffercapacity

influencesthereleaseofpH-dependentsystems,butalsothebuffer

type and the ionic strength [61,64]. An investigation into the

www.drugdiscoverytoday.com 1365

Reviews

KEYNO

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REVIEWS Drug Discover y Today V olume 25, Number 8 August 2020 TABLE 2

Overview of pH-dependent ileocolonic drug delivery systems tested in clinical trials

Drug delivery system In vitro dissolution test In vivo test Refs

pH (t in h) Buffer type

pH-dependent matrix (M) and/or coating (C)

Drug/marker Including shifts Fixed Readout Colon arrival determined

with Cellulose acetophtalate

and diethylphtalate (C)

Riboflavin

monophosphate,13 C-urea, and15_N-urea

1.2; 5.0, 6.0, and 7.0 Phosphate Breath and urine samples Breath and urine samples and confirmed with inulin, inulin-14_{C-carboxylic acid,} and lactose-13_C-ureide breath tests

[106]

Enteric acrylic resin (C) Mesalazine and153_Sm _{1.2 (1); 2.5 (2); 4.5 (1.5); 7} (1.5); 7.2 (2)

N/A

g

-Scintigraphy; plasma and

urine samples

g

-Scintigraphy [78,111]

Eudragit E and L (C)e 111In 1.2 (4); 6.8 (4); 5 (1) Citrate-phosphate

and phosphate

g

-Scintigraphy

g

-Scintigraphy [82]

Eudragit E and S (C)e 153Sm 1.2 (2); 7.4 (2); 6.4 among

othersc

Citrate-phosphate and phosphate

g

-Scintigraphy

g

-Scintigraphy [94]

Eudragit FS 30 D (C) Caffeine 1.2 (2); 6.5, 7.0, 7.5 (12) Phosphate Plasma and breath samples Lactose-13C-ureide breath

test

[98,112]

Paracetamol;153Sm 1.2 (2); 6.8 (1); 7.4 Phosphate

g

-Scintigraphy

g

-Scintigraphy [80]

Eudragit FS 30 D (C) Diclofenac sodium; barium sulfate

1.2 (2); 6.5 (1); 6.8 (2); 7.2 Phosphate Radiography Radiography [113]

Eudragit L (C) Mesalazine and153_Sm _{1.2 (2); 6.8 (1 and 5}d₎ _N/A

g

_{-Scintigraphy; plasma and}

urine samples

g

-Scintigraphy [85]

Mesalazine and111_In _N/Ab _N/A _N/A

g

_{-Scintigraphy; plasma}

samples

g

-Scintigraphy [92]

Eudragit L 100-55 (C) 2_H

8-budesonide; budesonide and111_In

N/A N/A N/A

g

-Scintigraphy; plasma,

urine, and fecal samples

g

2_H

8-budesonide; budesonide and111_In

N/A N/A N/A

g

-Scintigraphy; plasma,

urine, and fecal samples

g

Eudragit L 30 D-55 (C) Paracetamol;153_Sm _{1.2 (2); 6.8} _Phosphate

g

_{-Scintigraphy}

g

_{-Scintigraphy} _[80]

Eudragit L or Eudragit L/S (C)

Barium sulfate 1.2 (2); 6.8, 7.5c _Phosphate _Fluoroscopy _Fluoroscopy _[76]

Eudragit L/S (C) Mesalazine and153Sm N/A N/A N/A

g

urine samples

g

-Scintigraphy [79]

Eudragit L (C) Mesalazine and111In N/A N/A N/A

g

-Scintigraphy; plasma

samples

g

-Scintigraphy [88]

Eudragit L 100-55 (C) Beclomethasone dipropionate and153Sm

N/A N/A N/A

g

urine samples

g

-Scintigraphy [83]

Eudragit S (C) Salicylic acid; DTPAa labeled with99mTc

1.1 (2); 6.1 (1); 7.0 (2); 6.5 (2), 1.1 (2); 7.2 (1); 7.8 (2)

7.5 Phosphate

g

-Scintigraphy

g

Sulfapyridine and barium sulfate

N/A N/A N/A Radiography; plasma

samples Radiography [75] 99m_Tc-DTPA _{1.2 (0.5 or 2); 6.8, 7.0, 7.2, 7.4} (6) Phosphate and bicarbonate

g

-Scintigraphy

g

Mesalazine N/A N/A N/A Fecal dialysate; urine

samples and samples

N/A [117]

Theophylline 1.2 (2); 6.8 (2); 7.4 (2) Phosphate

g

-Scintigraphy; plasma

samples

g

-Scintigraphy [95]

3_{H-Prednisolone} _N/A _N/A _N/A _{Radiography; urine samples} _Radiography _[118]

Prednisolone and99m Tc-DTPA or111In-DTPA 1.2 (2); 7.4; 1.2 (2); 6.0 (1); 7.2 Phosphate and bicarbonate

g

-Scintigraphy

g

-Scintigraphy [84] 1366 www .drugdisco verytoday .com Reviews KEYNOTE REVIEW

(7)

i- Drug Discove ry Toda y V olume 25, Number 8 A ugust 2020 REVIEWS TABLE 2 (Continued )

Drug delivery system In vitro dissolution test In vivo test Refs

pH (t in h) Buffer type

pH-dependent matrix (M) and/or coating (C)

Drug/marker Including shifts Fixed Readout Colon arrival determined

with Eudragit S (M) with

Aqoat AS-HF (C)

Ibuprofen 5.8; 6.8, 7.4 Phosphate Plasma samples N/A [119]

Eudragit S or Eudragit FS (C) Prednisolone;99m Tc-DTPA;111In-DTPA 1.2 (0.5 or 2); 6.8, 7.0, 7.2 and 7.4 (6) Phosphate and bicarbonate

g

-Scintigraphy

g

-Scintigraphy [56,60] Eudragit S or Eudragit L (C) Prednisolone metasulfabenzoate; prednisolone acetate

N/A N/A N/A Plasma samples and

samples; clinical symptoms

N/A [120]

Mesalazine and barium sulfate

N/A N/A N/A Radiography; plasma and

urine samples

Radiography [121]

Eudragit S, Eudragit L, or Aqoat AS-HF (M) with Aquateric or Aqoat AS-HF (C)

Ibuprofen; furosemide 5.0; 6.8; and 7.4 Phosphate Plasma samples Literature [122]

Eudragit S/NE 30 D for granules (C) and Eudragit L for capsules (C)

Mesalazine 1.2 (2); 6.4 (1); 7.2 (1) N/A Plasma and urine samples Sulfasalazine method [123]

Eudragit S (C) 13C-urea 1.2 (2); 6.8 (2); 7.5 (0.5); 6.0

(1.5)

Phosphate Breath and urine samples Breath and urine samples,

confirmed with IntelliCapJ [28,124] 13 C-urea 1.2 (2); 6.8 (2); 7.5 (0.5); 6.0 (1.5)

Phosphate Breath and urine samples Breath and urine samples [108,124]

Mesalazine;13C6-glucose 1.2 (2); 6.8 (2); 7.5 (0.5); 6.0 (1.5)

Phosphate Breath samples Literature (based on intake

of a‘subsequent meal’)

[105,124]

Bee venom peptide; labeled bee venom with 99m

Tc-MIBI

1.2 (2); 6.8 (3); 7.4 (3) Phosphate

g

-Scintigraphy

g

-Scintigraphy [90]

Methacrylic acid copolymer soluble_{> pH 7.0 (C)}

Insulin; barium sulfate N/A N/A N/A Plasma samples;

radiography

Radiography [125]

Polymethacrylate (C) Mesalazine and153_Sm _N/A _N/A _N/A

g

urine samples

g

-Scintigraphy [77]

Budesonide and153_Sm _N/A _N/A _N/A

g

urine samples

g

-Scintigraphy [81]

a

Diethylenetriamine penta-acetic acid.

b

Not available.

c

In vitro disintegration test.

d

1 h for tablets and 5 h for pellets.

e

Inner coating Eudragit E.

www

.drugdisco

verytoday

.com

1367

(8)

nfluence of pH, electrolyte composition, and ionic strength

showedthatthedrugreleasefromanenteric-coatedaspirin

for-mulationincreasedwithanincreasingionicstrength[64].Nextto

this,itwasshownthataphosphateandabicarbonatebufferwith

anequalpHandasimilarbuffercapacitydidnotresultinasimilar

dissolutionprofile.Similarly,Faddaetal.foundthataphosphate

buffer with a buffer capacityand ionic strength comparable to

Hanksbuffer,abicarbonatebuffer,didnotgiveasimilar

dissolu-tionprofileofAsacoltablets[61].Therefore,oneshouldbe

reluc-tanttosubstitutethebuffertype,evenifthebuffercapacityand

ionicstrengthiskeptthesame,becausethiscouldresultinapoor

invitro–invivocorrelation.

ToaccuratelytestthepHdependencyofasystem,thepHprofile

ofaninvitrodissolutiontestshouldmimictheinvivosituationand

simulate challenging conditions. The exposure time and pH

(whichwerefertoaspHprofileintheremainderofthisreview),

but preferablyalso arepresentative buffertype, buffer capacity,

andionicstrength,havetobetakenintoaccount[57,61].

Inthestomach,acidcanpenetratethepH-dependentsystem

andtheamountofacidisdependentontheresidencetime[30,56].

Thisacidpenetrationcaninfluencetheperformanceofthesystem

becauseitisbasedonneutralizingitsacidiccomponents.

Substan-tiallydifferentvaluesforthemeangastricresidencetimeinthe

fastedstatearegivenintwodifferentsystematicliteraturereviews

[65,66].Pilaretal.showedthat,whenamealwasadministered

beforethedosageformhadexitedthestomach,gastricresidence

time was longer thanwhen a meal was administeredafter the

dosage formexcitedthe stomach[65].This indicatesthat food

administrationwhilethedosageformisinthestomachdoesnot

truly reflecta fastedstatetransittime.Therefore,to obtain the

gastric residence time in the fasted state, they only included

studiesinwhichamealwasadministeredafter4h,toensurethat

thedosageformhadpassedintothesmallintestine.Basedonthese

studies,a fastedgastricresidence timeof48min wasfound.In

theirreview,Abuhelwaetal.didnotconsiderwhetherthedosage

formexitedthestomachatthetimeamealwasadministered[66].

Therefore,the highergastric residencetime of1.37h that they

found for the fasted statecould be explained by the fact that

studieswereincludedwhereamealwasadministeredwhilethe

dosageformwasstillinthestomach.Sizeofthedosageformdid

notaffectgastricresidencetimeinthefastedstate[66,67].Inthe

fedstate,thegastricresidencetimewasfoundtobe2.5and3.5h

foralight(300kcal)andheavy(700kcal)breakfast,respectively

[66,67].Largerdosageforms(generallysingleunit)remaininthe

stomach longer thando smaller dosage forms (generally multi

unit)inthefedstate[66,67].BasedonthesedataandthepHdata

showninFig.1,werecommendaresidencetimeof50minatapH

of1.85oraresidencetimeof2.5hatapHof3.30forthefastedor

fedstate,respectively,forsimulationofthestomachinaninvitro

dissolutiontest.

Apropersimulationofthesmallintestinerequiresthatboththe

proximal and distal intestineshould betaken intoaccount, in

other words pH values just below and at the pH peak in the

terminalileumshouldbesimulated.Exposureofadrugdelivery

systemtoapHjustbelowthepHpeakmightresultindrugrelease

becauseofslowdissolution,disintegration,and/orswellingofthe

pH-dependentmatrixorcoating.Anoptimaldrugdeliverysystem

shouldhavethecapacitytowithstandexposuretothispHfora

biologicallyrelevantamount oftime.The totalsmallintestinal

transittimeisunaffectedbyfoodstatusandisaround3–4h[65–

68].Thereisnodifferenceinsmallintestinaltransittimeforlarger

dosage forms (generally single unit) and smaller dosage forms

(generallymultiunit)[66–68].Thetransittimecanbedecreased

by50%whenamealisgivenatthetimethedrugdeliverysystem

reachestheproximalsmallintestine[69].Therefore,exposureof

the drug delivery system for 3h at a pH of 6.8 would be a

challengingcondition tosimulatetheproximalsmallintestine.

Forthe ileocecal junction,a range ofresidence times hasbeen

found, from almost instantaneous to >10h [30,65]. The drug

deliverysystemissupposedtostartreleasingitsdrugcontentin

the terminal ileum.When the peakpH value determinesdrug

release,ashortexposuretimeandarelativelylowpHvaluewithin

therangefoundintheterminalileum(7.2–7.7)arechallenging

conditions,whichmeans,forexample,30minatapHof7.3.

Simulationofthelargeintestinerequirestheintroductionofthe

pHdropthatoccursinvivo,withexposuretimebeinglessrelevant.

Asaresult,apHof6.50untiltheendofthetestisrecommended.

Table3detailsthepHprofilesrecommendedforsimulatingthe

dissolutiontestinafastedandfedstate.

Different pH profiles have been used in the various studies

(Table2).Inouropinion,theseconditionsareoftennot

challeng-ingenough.Simpleand,thus,easilyimplementabledissolution

modelsonlymimictheGItransittimeandpH,butlacksimulation

of,forexample,motility,intestinalpressure,andviscosity,and,

thus, are limiting. However, when a challenging pH profile is

chosen, valuable information is obtained. The recommended

pHprofile presentedin Table3 doesnottakeintoaccountthe

intra-andinterindividualvariations[30,65–71].Ifmore

informa-tionabouttherobustnessofthedrugdeliverysystemisdesired,

individualpHprofilesobtainedinvivocouldbesimulatedwith,for

example,thepHysio-grad1system[72].Otherphysiological

pa-rameters, such asmotility, intestinal pressure, the presence of

enzymesand bile, fluid volumes,and viscosity, areusually not

simulated.ThecomplicatedandnoteasilyimplementableTIM-I

andTIM-IIsystem[73]areexceptions.Eventhesesystemsarea

simplificationoftheinvivosituationandtheirfullinvivo

predict-ability has yet to be established. Therefore, novel ileocolonic

deliverysystemsalwayshavetobetestedinhumansaswell.

In

vivo

methods

to

investigate

or

verify

ileocolonic

targeting

in

humans

Variousmethods have been usedto investigate the ileocolonic

drugdeliveryinhumans,aslistedinTable2.Theyincludeimaging

techniques,suchas

g-scintigraphy,

radiography,andfluoroscopy

(continuousX-rayimaging),andindirectmethods,suchas

deter-mination ofplasma drug concentrations or stable isotope

con-centrationsinbreathandurinesamples.

TABLE3

pHprofiledissolutiontesta

SegmentGItract Fasted(pH;time) Fed(pH;time)

Stomach 1.85;50min 3.30;150min

Proximalsmallintestine 6.80;180min 6.80;180min

Distalsmallintestine 7.30;30min 7.30;30min

Colon 6.50;endoftest 6.50;endoftest

a

pHvaluesmightvary0.10pHunit.

(9)

Imaging

techniques

Anadvantageofimagingtechniquesforthevisualizationofdrug

deliverysystemsintheGItractisthattheyarenon-invasiveand

direct.Themostfrequentlyusedimagingtechniqueis

g-scintig-raphyfollowedbyX-rayimaging(radiographyandfluoroscopy)

(Table2).Forinvivovisualizationby

g-scintigraphy

aradionuclide

(e.g.,99mTcor111In)isincorporatedintotheformulation,whereas,

for radiography or fluoroscopy, a contrast agent (e.g., barium

sulfate)isneeded[74–76].Suchnonabsorbablemarkerscaneither

beformulatedaloneinthedrugdeliverysystemortogetherwith

the drug, depending on the researchquestion. Various studies

have confirmed that the markers do not influence the in vitro

releaseprofileofthedrug[30,60,77–88].Althoughitisgenerally

assumed that marker and drug would show the same release

behavior,andthesiteofmarkerreleaseindicatesthesiteofdrug

release,this isnotnecessarily the case[77].Therefore,imaging

techniques by themselves cannot truly verify ileocolonic drug

delivery,buttheyprovideagenerallyusefulindication.

Aswithalldrugadministrationsandmedicalprocedures,

pos-sible drug interactions, adverse effects(including allergic

reac-tions), contra-indications, and complications because of the

procedure have to be considered. Barium sulfate is used as a

diagnostic contrast agent for radiographic visualization of the

GItract,in whicha suspension of 150–180gof bariumsulfate

isadministered.Atthesehighamounts,bariumsulfatemightbe

responsiblefordisturbedabsorptionofdrugs,allergicreactions,

and gastrointestinal adverse effects, such as diarrhea, nausea,

abdominalpainordistention,andconstipation[87].Theamounts

used in the pH-dependentdrug delivery systemsare small; for

example,Aimoneetal.onlyused2.1g[76],thusnoadverseeffects

areexpected.BothX-rayimagingand

g-scintigraphy

makeuseof

radiationtovisualizethedosageformand,thus,radiation

expo-sureshouldbetakenintoconsideration.

Anadvantageof

g-scintigraphy

isthattheradiationexposure

doesnotincreasewithanincreasingnumberofimages,because

theradionuclideistheradiationsource.Influoroscopyand

radi-ography,anexternalionizingradiationsource,X-rays,isusedfor

visualization.Therefore,theexposuretoradiationincreaseswith

eachadditionalimagetaken,whichresultsinsubstantiallyhigher

dosesofradiationthanwith

g-scintigraphy

[74].Otheradvantages

of

g-scintigraphy

include a moreaccurate quantitative analysis

andanincreasedsensitivity[74].Thehighersensitivitysimplifies

theidentificationofthesiteofrelease,eventhoughthelocationof

organsisnotvisiblewith

g-scintigraphy

(incontrastto

radiogra-phyandfluoroscopy)[74].However,visualizationoforgansisnot

strictlynecessarywhenevaluatingileocolonicdrugdelivery

sys-tems,becausethehumanlargeintestinehasaninvertedUshape

[77,79,81,82,85,88].ThisinvertedUshapebecomesprominently

visiblewhentheradionuclideisreleasedinthelargeintestinefrom

asingleunitdrugdeliverysystem,becausethemarkerspreadsover

thecolonwhenreleasedfromthesystem[82,85,89].Formultiunit

drug delivery systems, thisis more complicatedbecause ofthe

inevitablespread ofthe individual particles beforedrug release

occurs[90].Ifanatomicalinformationisdesiredwith

g-scintigra-phy, an aqueous solution of 99mTc-labeled diethylenetriamine

penta-aceticacid (DTPA)or 99mTc-labeled colloidcan be taken

togetherwith111Inincorporatedinadrugdeliverysystem,suchas

amultiunitformulation[88,89,91,92].Thedeliverysystem can

thenbevisualizedtogetherwiththedifferentsegmentsoftheGI

tractoutlinedby99m_Tc._A_disadvantage_of

g-scintigraphy

_is_that

theshelf-lifeoftheformulationislimiteddependingonthe

half-lifeoftheradionuclide.Furthermore,forsafe manufacturingof

radiopharmaceutical dosage forms, specific conditions are

re-quired and often the production process must be downscaled

[93]. This can be circumvented, for example, by using stable

152

Sm-oxideduringtheproductionprocess,whichcanbeneutron

activated to the radionuclide 153Sm before administration [77–

81,85,93,94].Amajordrawbackremainsthat,forisotope

techni-ques,specializedequipmentisrequiredtoproduce,measure,or

visualize the isotopes, which complicates broad applicability.

Furthermore,simultaneousreleaseofthemarkeranddrugmust

be verified to draw definite conclusions about the ileocolonic

targetingabilityoftheformulation.

Plasma

samples

Whereas imaging techniques generate information about the

locationofthedrugdeliverysystem,plasmadrugconcentrations

willrender valuableinformation aboutthedrugrelease, onthe

conditionthatthedrugcanbeabsorbedovertheentirelengthof

theGItract[77,79–81,85,88,89,91,95].Combiningimaging

tech-niquesandbloodsamplingallowsforverificationofileocolonic

drugdelivery.Plasmasamplesassucharegenerallyinsufficientto

verifyileocolonicdrugdelivery,becausetheappearanceofdrugin

theplasmadoesnotgiveinformationaboutthelocationofdrug

release.Anexception tothisisanadjustedmethodofKennedy

etal.,inwhichsulfasalazineisusedincombinationwitha

com-pound with goodsolubility and permeability across the entire

intestinalmembrane(e.g.,theophylline)[96–98].Sulfasalazineis

poorlyabsorbedfromtheGItractbutismetabolizedbybacteriain

the colon into sulfapyridine, which is subsequently rapidly

absorbed. Therefore,the occurrence ofsulfapyridine in plasma

pointstocolonicarrival[99,100].Bycontrast,theophyllineiswell

absorbedovertheentireGItractand,therefore,itsappearancein

plasmaindicatesdrugreleasefromthedrugdeliverysystem[97].

Based oncomparisonoftheplasmacurves oftheophyllineand

sulfapyridine,itcanbedeterminedwhethertheformulationhas

releaseditscontentsolelyinthecolon.Whendrugreleaseoccurs

inthecolon,theophyllineandsulfapyridineoccursimultaneously

inplasma.Whendrugreleaseoccursbeforetheileocolonicregion,

theophyllinewillappearintheplasmaprevioustosulfapyridine.

Themethodwassuccessfullyusedinbeagledogs[97],butneverin

humans.Theoretically,itcanbeappliedinhumans,becausethe

sulfasalazine–sulfapyridinemethodwasdevelopedandvalidated

forhumans[96,101].Replacingtheophyllinewith,forexample,

caffeine (whichisalsowellabsorbedovertheentireGItract)is

advisedtocircumventpossibletoxicityissuesrelatedto

theophyl-line [98,102]. Furthermore, to avoid the invasive character of

bloodsampling,salivasamplescouldpossiblybeused[103,104].

Breath

and

urine

samples

Formulationscontainingstableisotopeshavebeenusedtoassess

ileocolonicdrugdeliveryina non-invasivemanner withbreath

andurinesamples.Schellekensetal.used13C6-glucose,whichis

absorbedrapidlyfromtheGItractandthenpartlymetabolized

into13CO2inthesystemiccirculation[105].Giventhatglucose

metabolismdoesnotdependonbacteria,theappearanceof13CO2

Reviews

KEYNO

TE

(10)

in breath is an indicator ofdrug releaseand nota markerfor

colonicdrugrelease.Inthestudy,themotilityoftheGItractwas

activated by giving the test subjectsa subsequent meal,which

causedthedosageformtopasstheileocecaljunction.Inthisway,

theauthorsobtainedanindicationofthelocationofdrugrelease.

Inanotherstudy,13C-ureaand15N-ureawereincorporatedintoan

ileocolonicdrugdeliverysystem[106].Theauthorsinvestigated

the appearanceof13CO2 (from13C-urea)and 15NH3(from 15

N-urea)inthebreathandurine,respectively.Released13C-ureainthe

colonwill bepartiallyfermented by bacteriagenerating13CO2,

whichisexhaled,whereastheremainderofthe13C-ureais

excret-edintotheurine.Whentheformulationreleasesitscontentbefore

itreachestheileocolonicregion,no13_CO

2willbeformedandall

13_C-urea_is_excreted_into_the_urine._In_case_of_failed_release,_the

complete13_C-urea_dose_will_be_excreted_into_the_feces._When_no

13_CO

2isdetectedinthebreath,thepresenceorabsenceof15NH3

inurineindicatesprematurereleaseinthe smallintestineorno

releaseatall,respectively.However,thismethodlacksaninternal

standard for variations in 13_C-urea _metabolism. _Maurer _et _al.

optimizedthedualstableisotopeapproach[28,107,108].Inthis

method, anileocolonictargeteddrugdelivery systemwith 13

C-ureaincorporatedandanimmediatereleaseformulationwith15

N-ureaincorporatedwereadministeredsimultaneously.The

imme-diate releaseformulationwith15N-ureawasusedasaninternal

standard.Itreflectsvariationinureametabolismand,therefore,

givesareferencevaluefor100%absorptionof13C-urea.Thus,by

comparingthe13CO2amountinbreathandthe13C-ureaand15N2

-ureainurine,itispossibleto verifywhetheror notileocolonic

deliverywassuccessful.

Amajoradvantageofutilizingstableisotopesisthatvolunteers

orpatientsarenotexposedtoirradiation(incontrastto

g-scintig-raphy, radiography, and fluoroscopy), and the method is

non-invasive(incontrasttoplasmasampling)[109].

Performance

of

pH-dependent

ileocolonic

drug

delivery

systems

pH-dependent ileocolonic drug delivery systems often fail in

humans,even thoughin vitroreleasestudiesshowed promising

results, as described in a review by Maroni et al. [110]. This

discrepancycouldbeexplainedbythefactthatthesystemswere

testedwithphosphatebuffers andnotbicarbonatebuffers,thus

failingtoidentifysuboptimalsystems.Anotherexplanationcould

bethatthepHprofileusedintheinvitrodissolutiontestdoesnot

adequatelyreflectthepHprofileinhumans(Table2).

However,therearefoursystems, givenin Table2,thatshow

goodperformanceinhumans(Table2).Thefirstsystemcontainsa

double coating layer. The inner coating layercomprises a

pH-sensitive polymer, EudragitS,which wasneutralizedto pH8.0

withphosphatesalts[84].Theouterlayercomprisesa

non-neu-tralized coating of Eudragit S. The authors hypothesized the

followingmechanismforrapiddrugreleaseoncethepHthreshold

ispassed.Whentheoutercoatingstartstodissolveintheterminal

ileum,watercanpass throughthislayerand dissolvetheinner

coatingwiththebuffersalts.ThisresultsinahighlocalpHwitha

highbuffercapacity,whichenhancesthedissolutionrateofthe

inner layer. As a result of diffusion,the pH in the outer layer

increases,whichfacilitatesdissolutionofthislayer.Theauthors

showedthat thedoublecoatinglayeryieldedsuperiortargeting

capabilities in humans compared with a single coating layer

system.They alsoshowed that anin vitrodissolution test with

phosphate buffers was incapable of distinguishing the release

profilesofthe singleand doublecoatedformulation,whereasa

bicarbonate buffered system reflected the situation in humans

moreaccurately.Thesecondsystemshowingtherequired

behav-iorin humansisthe ColoPulsesystemin whichadisintegrant,

such as sodium starch glycolate or croscarmellose sodium, is

incorporated into a pH-sensitive polymeric coating layer in a

nonpercolating manner [105]. Once the pH-sensitive polymer,

Eudragit S, starts to dissolve, the protective layer around the

disintegrantisremovedand thedisintegrantswellsand quickly

rupturesthecoatinglayerresultinginafastreleaseofthedrug.The

thirdsystemisbasedonacombinationofEudragitLandEudragit

S,whichresultsinapHthresholdbetween6and7dependingon

the ratio of the two polymers [76,79]. By combining the two

polymers,the disadvantages ofthe individualpolymersare

cir-cumvented,thatis,thepass-throughofintacttabletswith

Eudra-gitSandprematurereleasewithEudragitL.Thefourthsystemis

basedonanouterEudragitScoatinglayerandaninnerEudragitE

coatinglayer(acid-solublelayer)[94].TheouterlayerofEudragitS

dissolvesintheterminalileum(pH>7.0)andtheinnerlayerin

the more acidic pH of the cecum. If the outer coating opens

prematurely,suchasinthejejunum,theinnercoatingwill

pre-ventprematurerelease.

Thefirstthreesystemsaredesignedtoresultinfastdisruptionof

theusually slowdissolvingEudragitScoating [56,60,105],thus

decreasingtherisksofpass-throughofintactdosageformsornot

fullyopenedsystems,whereasthefourthsystemprevents

prema-turerelease.

Concluding

remarks

BasedonthetemporarypHincreasetovaluesbetween7.2and7.7

ineithertheterminalileumoratthestartoftheascendingcolon

inbothhealthyvolunteersandinpatientswithCD,UC,IBS,or

CRC, site-specific drug release from pH-dependent ileocolonic

targeted drug delivery systems can be attained. To investigate

the performance of novel pH-dependent drug delivery systems

invitro,thepHprofileinthedissolutionmediumshouldmimicthe

pHprofileofthehumanGItract,withashortpHpeakofpH7.3

beingcrucial.Theuseofdissolutionmediabasedonbicarbonate

buffers,suchas the pHysio-grad1 system,is recommended

in-stead of the frequently used phosphate buffers. For studies in

humans,thecaffeine-sulfasalazinemethodisaneasy

implemen-tablemethodtoverifyileocolonicdrugdelivery.However,a

non-invasivemethod,suchasthedualstableisotopeapproachwithan

internalstandard,whichrequiresonlybreathandurinesamplesto

verifyileocolonicdrugdelivery,mightbeanattractivealternative.

Declaration

of

Competing

Interest

The authors declare the following financial interests/personal

relationshipswhich maybe considered aspotential competing

interests:

Potentialconflictofinterest:HWFisoneoftheinventorsofa

patent(WO2007/013794)describingamethodforcolon

target-ing,whichisheldbyhisemployer.Theotherauthorsdeclarethat

they have no known competing financialinterests or personal

relationships that could have appeared to influence the work

(11)

reportedinthispaper.Theliteraturesearchwaspartiallyfundedby

JanssenPharmaceutica.JanssenPharmaceuticahadneitherrolein

studydesign,inthecollection,analysis,andinterpretationofdata,

norinthewritingofthereportandinthedecisiontosubmitthe

paperforpublication.

Acknowledgments

ThisliteraturesearchwaspartiallyfundedbyJanssen

Pharmaceutica.JanssenPharmaceuticahadneitherroleinstudy

design,inthecollection,analysis,andinterpretationofdata,nor

inthewritingofthereportandinthedecisiontosubmitthepaper

forpublication.

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