Stem Cell Therapies in Clinical Trials Progress and Challenges

Review

StemCellTherapiesinClinicalTrials:ProgressandChallenges

AlanTrounson1,*andCourtneyMcDonald1InstituteforMedicalResearch,27-31WrightStreet,Clayton,VIC3168,Australia

*Correspondence:alan.trounson@hudson.org.auhttp://dx.doi.org/10.1016/j.stem.2015.06.007

1Hudson

Clinicalinvestigationsusingstemcellproductsinregenerativemedicineareaddressingawidespectrumofconditionsusingavarietyofstemcelltypes.Todate,therehavebeenfewreportsofsafetyissuesarisingfromautologousorallogeneictransplants.Manycellsadministeredshowtransientpresenceforafewdayswithtrophicinfluencesonimmuneorinflammatoryresponses.LimbalstemcellshavebeenregisteredasaproductforeyeburnsinEuropeandmesenchymalstemcellshavebeenapprovedforpediatricgraftversushostdiseaseinCanadaandNewZealand.Manyotherapplicationsareprogressingintrials,somewithearlybenefitstopatients.

Introduction

Stemcelltherapieshavebeenexpectedtobringsubstantialbenefittopatientssufferingawiderangeofdiseasesandin-juries.Itwasexpectedthatthebenefitsofbonemarrowtrans-plantsforpatientsneedingreconstructionoftheirhematopoieticandimmunesystemswouldapplytostemcelltransplantsofothercelltypes,andoptimismhasbeenhighfortheutilizationofpluripotentstemcelltypes(embryonicstemcells[ESCs]andinducedpluripotentstemcells[iPSCs])foravarietyofappli-cations.

Fortheentireareaofcelltherapiesin2014,70%oftrialsweresponsoredbyacademicinstitutions(publicfunding)and30%bycompanies(theprivatesector)(Bersenev,2015).Acombinationofpublicandprivatefundingwasstronglyadvocatedinthefund-ingoftranslationandclinicalstemcelltrialsforsustainedcapac-itybytheCalifornianInstituteforRegenerativeMedicine(CIRM)(Trounsonetal.,2010).Thereisaverysignificantinvestmentoverallinthefieldofstemcellclinicaltrialsthatdeservesmoni-toringandevaluation.Successfulnewtherapiescomeataconsiderablecostthatcannoteasilybesustainedwithoutevalu-ationandguidance.Wehaveexploredtheclinicaltrialsinwhichdatahavenowbeenpublishedandwhichthegeneralandsci-encecommunityareanxioustofollowgiventheinvestmentofresearchresourcesandfinance.Thesetrialoutcomeswilldeservecontinuousevaluationtoenableanunderstandingoftheextendedtimeframesinvolvedinrealizingsuccessfulprod-uctsandanunderstandingofthecollaterallossesforpotentialproductsthatareunabletomeetthedemandsoftheregulatorysystemandclinicalefficacyoftherapy.

Sincepreviousreviewsofstemcellsinclinicaltrials(Ratcliffeetal.,2013;Trounsonetal.,2011),therehasbeenacontinuingexpansioninthenumberandtypeofstemcellsunderstudy.WeexaminedthereportsofclinicaltrialsintheNIHandEuro-peandatabasestoclassifythembystemcelltypeanddiseaseapplication.Wesearchedforclinicaltrialdatapublishedinpeer-reviewedjournalsandsoughtoutpublicallyavailableinfor-mationontrialsperformedbycompanies.Dataforsomestudiesareregularlypublished,butformanytrialsdataareunavailableornoteasilyaccessed.Giventherelativeimmaturityofthecelltherapyfield,itisimportanttoknowtheoutcomesofearlyclin-icaltrialstohelpguideothersintheprocesses.Theoverallimpressionisthatconsiderableinvestmenthasbeenmadeinpreclinicalresearchandclinicaltrials,butasyetthereisonlyamodicumofsuccessbeingachieved.However,clinicalreportswillcontinuetoevolveandgeneraltrendswillemerge.Itisclearthatlimbalstemcellshavematured,neuralstemcellsshowconsiderablepromiseforregenerativerepair,pluripotentstemcellshaveanabundantpotentialinregenerativemedicine,andmesenchymalstemcells(MSCs)arenumericallythemostfavoredcelltypepresentlyunderclinicaltrial.Thesestudiesattractalotofattentionforcommentaryinthepressandpatientexpectationsofsubstantialbenefits.Wealsoconsideredplacental-derivedstemcellsthatarefrequentlyMSC-likeandendothelialstemorprogenitorcellsbecauseoftheircloserelationshiptoperivascularrepairnecessaryinregenerativemedicine.

Theprevalenceofcelltherapiesforinjuryanddiseaseoftheeyeisanotabletrend.Thisprogressisaresultofafewfactors,includingtherelativelysmallnumbersofcellsrequired,easyaccessibilityforsurgery,andstraightforwardassessmentandvisualizationofgrafts.Therealsoappearstobesomeimmuneprivilegeforallogeneictransplantstotheeye.Furthermore,oneeyecanbeusedasacontrolwhencelltherapyisappliedtotheothereyebecausediseaseisgenerallybilateral.Itisalsorelativelyeasytodifferentiatepluripotentcellsintocelltypesneededforregenerativepurposesintheeye.Consequentlynumeroustypesofcellshavebeenusedinclinicaltrialsforeyediseaseandinjury.

ThepresentReviewhasbeenconfinedtopublishedreportsofstemcellclinicaltrialsandexcludestheverysubstantiallitera-tureonhematopoietic(blood)stemcellsandassociatedgenetherapiesandcancercelltherapies.Thelatterhasdramaticallyexpandedwiththesuccessofchimericantigenreceptortech-nologybutpresentlydoesn’tinvolvestemcells.Thepublisheddataonregisteredclinicaltrialsinvolvingstemcellsotherthanhematopoieticstemcellsisrelativelyleanandweexamineanddiscussthesafetyandefficacydatathathavebeenprovidedinpublicationsorcompanyreports.Inthisyoungfieldwithconsiderablepromise,thereareexcitingprospectsformanydifferentstemcelltherapiesthatarearisingfromtheearlytrials

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Review

Table1.ESCTrialsTrialSponsor(Location)ChabiotechCo.Ltd.(S.Korea)OcataTherapeutics(MA,USA)DiseaseTargetmaculardegenerationStargardt’smaculardystrophymaculardegenerationmyopicmaculardegenerationPfizer(UK)CellCureNeurosciencesLtd.(Israel)ViaCyte(CA,USA)AssistancePublique-HopitauxdeParis(France)InternationalStemCellCorp.(Australia)AsteriasBiotherapeutics(CA,USA)maculardegenerationmaculardegenerationtypeIdiabetesmellitusheartfailureParkinson’sdiseasespinalcordinjuryCellTherapyhuman-ESC-derivedRPEhuman-ESC-derivedRPEhuman-ESC-derivedRPEhuman-ESC-derivedRPEhuman-ESC-derivedRPEhuman-ESC-derivedRPEhuman-ESC-derivedpancreaticendodermcellhuman-ESC-derivedCD15+Isl-1+progenitorshumanparthenogenetic-derivedneuralstemcellshuman-ESC-derivedoligodendrocyteprecursorcellsNo.Patients121616unknown1015406unknown13PhasephaseI/IIphaseI/IIphaseI/IIphaseI/IIphaseIphaseI/IIphaseI/IIphaseIphaseI/IIphaseI/IICellStemCell

aswellassomedisappointmentsthataretemperingtheopti-mismfornewcuresoccurringrapidlyacrossalargespectrumofdiseaseandinjury.

PluripotentStemCells

BothESCsandiPSCsaremakingtheirwayintoclinicaltrials(Table1)afterconsiderableoptimismfortheirtherapeuticpoten-tial.Theyareofmostinterestwherefunctionaladultstemcelltypesaredifficulttoaccess,expand,orderive.Itappearsthatapplicationsintheeye,pancreas,andvariousneuraldegenera-tivedisordersorinjuriessuchasParkinson’sdisease,amyotro-phiclateralsclerosis(ALS),andspinalcordinjuryareleadingcandidatesforpluripotentstemcell-basedcelltherapy.

TheoriginalGeronInc.clinicaltrialofhumanESC(hESC)-derivedoligodendrocyteprogenitorsforthetreatmentofspinalcordinjury,thefirstofitskindtouseESCs,wasterminatedwhenthecompanydecidedtodiscontinuetheircelltherapypro-gramtoconcentrateoncancertreatments.Theytreatedfivepatientswithoutanyadversefindingswithadosebelowthatex-pectedtoshowanyefficacy.Theestimatedcostsofthepreclin-icalstudies(US$200million)(Keirstead,2012,Spinalcordinjury:whatarethebarrierstocure?BedfordCenterWorkshop)wereveryhighbecauseofthepioneeringdatarequiredtoshowsafetyagainstteratomaformationandanimalmodelefficacyneededforthefirstinhumanstudiesofhESCs.Theactualclinicaltrialswerealsoverycostlybecauseofthenumberoftrialsites,trainingforthetransplantprocedure,andcellmanufacturing.ThestudyisnowcontinuingunderthedirectionofAsteriusBiotherapeuticswhohaveregisteredadoseescalationphaseIsafetytrialwiththeFDA,usingthesamehESC-derivedoligodendrocyteprogen-itorcellsadministered1–2weeksafterspinalcordinjury.

PreliminaryresultshavebeenreportedbythecompanyOcataTherapeutics(formallyACT)onclinicalsafetytrialsfortheuseofretinalpigmentedepithelialcells(RPEs)derivedfromhESCsfordrymaculardegenerationandStargardt’smaculardystrophy(Schwartzetal.,2012,2015).TheywereabletoderiveRPEsthatwere99%purefromhESCs.Therewere9patientswithmaculardegenerationand9patientswithStargardt’sdiseaseindosecohortsof50,000,100,000,and150,000cellsadminis-teredtooneeyeofeachpatient.Theonlyadverseeventswere

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associatedwithsurgeryandimmunosuppression.Visualacuityimprovedin10treatedeyes,wasstablein7eyes,anddecreasedin1eyeover22months.Patchesofregeneratedretinalepithe-liumwereobservedin72%ofthepatientsalthoughthespreadwasvariableandincomplete,suggestingthatimprovementscouldbemadetothemethodologytoachievegreaterRPEmonolayercoverageofthemacula.Otherclinicalstudiesareemergingonmaculardegeneration,whichincludestudiesintheUKandinCalifornia,whereESC-derivedRPEsaregrownasamonolayeronultra-thinscaffoldsandinsertedunderthephotoreceptorcellstocovertheentiremacula.ThereisalsoareportofoneJapanesepatientwhoreceivedatransplantofasheetofiPSC-derivedRPE(Cyranoski,2014).ThisisthefirstinhumanstudythatmaybetheleaderinmanyotherapplicationsofiPSCderivatives.WhileitmightbeexpectedthatiPSCswillbeusedasautologoustherapies,thereisastrongmovementfortheiruseasallogeneictransplantsoraspartiallycompatibleHLAhaplotypedderivatives(Turneretal.,2013).

Reportsofprogressforotherindicationsundergoingtreat-mentwithpluripotentstemcellsarenotavailableasyet.Thisin-cludestheuseofinsulin-producingbIsletcellscontainedinasubcutaneouscapsuletopreventcell-mediatedautoimmunityinpatientswithtypeIdiabetes(Schulzetal.,2012).Thisstudyisanopenlabeldose-escalatingphaseI/IItrialinvolving40pa-tients.ThefirstpatientreceivedatransplantinNovember2014oftwosubcutaneouscapsulesofbIsletprogenitorsdifferenti-atedfromESCs.ThisstudyisbeingconductedbythecompanyViaCyteinCaliforniaandtheresultswillbeinterestingbecausetherewillbeafunctionalreadoutofthetransplantedcellscon-trollingdiabetes.IntheViacytestudiesthefinalstagesofmatu-rationtoglucose-responsiveinsulin-producingcellsneedstobedoneinvivo.MorerecentlytworesearchgroupsreportedtheapparentcompletematurationofcellsintobIsletcellsinvitro,butatthisstageitisnotknownifthismaybenefittheirefficacywhenusedinclinicaltrialsornot(Kushneretal.,2014).

TherewasalsoarecentannouncementbyInternationalStemCellCorporationthattheywillbeginclinicalstudiesinAustraliain2015.ThestudywilluseparthenogeneticESC-derivedneuralcellsforthetreatmentofParkinson’sdisease(ISCO,2015).Themainconcernforthisstudymaybehowwell

CellReview

StemCell

thedifferentiationprotocolisabletoderivepureA9dopami-nergicneurons,giventhepotentialforassociateddyskinesiafromthecontaminationofserotoninergicneurons(CIRM,2013).Attemptstoreplicatethepositiveoutcomesfromhumanfetalventralmesencephalontissuetransplants,usingpluripotentstemcells,areamajorfocusthathaschallengedresearcherstoderivethenigralA9dopaminergicneuronsthatareabletofunc-tionallyrecoverthe6-OHDAanimalmodelofParkinson’sdis-easewhengraftedatadoseequivalenttothatusedforfetalVM,asrecentlyshownbyGrealishetal.(2014).Thesechal-lengeshavebeensummarizedbyBarker(2014)indiscussingtheconsortiainvolvedinfetalbraintransplantsandthenewlyformedParkinson’sGlobalForce(Abbott,2014)thatisguidingresearcherstooptimizetheircellcharacterizationandthepatientsubgroupslikelytorespondtotherapeutictransplants.Thereisconcernthatclinicaltrialsbasedonlessrobustcriteriamayagainsetbacktheclinicalprogresstowardsuccessfultherapyforthisdisease.

LimbalStemCells

Limbalstemcelltransplantshavebeenusedverysuccessfullytorestorefunctionalcornealepitheliumthatistransparentandself-renewinginpatientswhohavecornealdestruction,usuallyduetoburns(Ramaetal.,2010).Thelimbalcellsareautologoustransplantsobtainedfromthelimbus,providingevenatinypartineitherofthepatient’stwoeyeshasbeensparedfrominjury,andtheyareexpandedincultureonfibrin(Ramaetal.,2010)orhumanamnioticmembrane(Kollietal.,2010)thatpre-servesandenablesexpansionoftheholoclonecells.Allogeneiclimbalcellsdonotpersistlong-termaftertransplantation(Pelle-griniandDeLuca,2014).

iPSCderivationofthelimballineagemayeventuallyprovideatherapeuticsourceoflimbalcellsforpatientsforwhomcompletebilateraldestructionorlossorabsenceofthelimbushasoccurred.Recentadvancesintheidentificationofkeyregulatorygenesinlimbaldevelopment,differentiation,andexpansionarelikelytoacceleratethistherapeuticopportunity(Pellegrinietal.,2014).Inthemeantime,transplantationofinvitroculturednon-ocularautologousoralmucosalepitheliumhasbeenusedinpatientswithbilaterallimbalcelldeficiency,particularlyforacutechemicalburns.Thisprocedurecangivesatisfactoryresults(Burillonetal.,2012;Chenetal.,2009)andprovideadequatere-epithelializationandstabilizationofthesurfaceofthecorneaiftheculturedautologousthinsheets(withafewcells’worthofthickness)oforalmucosalepitheliumareliftedwithtemperaturesensitivecellsheet-liftingtechnologythatpreservestheculturedcell-celljunctionsandanintactbasementmembranefortrans-plantation(Burillonetal.,2012).Thismethodappearstopreventthevascularizationandlossofcornealopacitypreviouslyasso-ciatedwithoralmucosaltransplantsthatoftenhadmultipleepitheliallayersofstratificationandnumerousmucosalcellstructures(Chenetal.,2009).

Cadavericlimbaltissuecanbepreservedwithintactstructureat4󰀁Cforupto8daysbyairliftculture(whichisamethodwhereinthestromalcomponentofthecorneaissubmergedinliquidculturemediumandtheepitheliumisexposedtoairforcul-tureorcoldpreservation)andusedforallogeneictransplantsthatprovidecompletecornealre-epithelializationforatleast1month(Lietal.,2013).Thisresultsuggestsallogeneiclimbal

stemcelltherapiesaresteadilyprogressing.However,arecentstudyonallograftsinAniridiaandStevens-JohnsonSyndromethatusedadefinedoutcomessetprocedureforculturedlimbalcellepitheliumtransplantsforbilaterallimbalcelldeficiencyshowedimprovementsinepithelialintegrityandvisualacuityupto12monthsbutthenagradualdeclineover3years(Shorttetal.,2014).Typicallyrestorationofthecornealepitheliumcannowbeachievedinthemajority(󰀃67%)ofauto-andallogeneictransplantsforpartialandtotallimbalstemcelldeficiencywithouteasilydetectablealterationtovisualacuity(Zakariaetal.,2014).

NeuralStemCells

Neuralstemcellderivativesareinanumberofclinicaltrialappli-cations(Table2).Theapplicationsareprimarilyaimedatrepair-ingthedamagedcentralnervoussystem.Thebesttypeofneuralcellforregenerativerepairofthecentralnervoussystemisstillyettobedeterminedandmayvaryaccordingtothediseaseorinjury.Itisgenerallyconsideredthataneuralstemcellisprob-ablyidealforestablishingastemcellpoolforcontinuoussupplyofthedesiredneuron,astrocyte,oroligodendrocyte.However,matureneuralcellsmayalsobenecessarywhenspecificcelltypesareneededtoachievenormalfunction.Forexample,nigralA9dopaminergicneuronsareneededinParkinson’sdisease.ItcanbesurprisingthatabrainneuralstemcellcanrepairlossofRPEinmaculardegeneration.Clinicalstudieswillalsoeventuallydeterminewhetheraneuralstemcelloroligodendroctyepro-genitorisbestforspinalcordrepair.

StudiesbytheStemCellsInc.grouphaveusedhumanfetal-derivedneuralstemprogenitors,whichshowlifelonglysosomalenzymeproduction,totreatveryadvancedinfantileorlate-infan-tileceroidlipofuscinosis(Batten’sdisease)inchildrenwithoutanydetectableadverseevents(Seldenetal.,2013).Dosesof1billioncellsintothebrainwerewelltoleratedinthesepatients.Furthermore,theallogeneiccellsshowedlong-termsurvivalofthetransplantedcellsinautopsy2.5yearslaterand1.5yearsaftertheterminationofimmunosuppression.Threepatients(ofsix)aremorethan5yearspost-transplant.ThecompanyhasalsotreatedyoungpatientswiththefatalgeneticdemyelinationconditionknownasPelizaeus-Merzbacherdisease(PMD)(Guptaetal.,2012).Theneuralprogenitorsweresurgicallyim-plantedintothefrontallobewhitematterinfouryoungpatientswithanearly-onsetsevereformofPMDandsomemodestgainsinneurologicalfunctionwereobservedinthreepatients.CranialmagneticresonanceimagingandMRspectroscopyindictedthatmyelinationoccurredinthesiteoftransplantationwhencomparedtowhitemattersitesdistanttotransplantation.Furtherprogresshasbeenlimitedbecauseitisdifficulttorecruityoungpatientspriortotheexhibitionoftheadvanceddiseasephenotype.Thesestudiesindicatethatitmaybepossibletohalttheadvanceofseveregeneticdisordersbyprovidingneuralstemcelltransplantsthatarevehiclesfordeliveryofthecorrectprotein.

Perhapsmorechallengingisthetreatmentofstrokeandotherdisordersofthebrainandspinalcord.Despiteaverylargenum-berofstudiesinanimalsusingneural,embryonic,mesenchymal,bonemarrow,andcordbloodcelltypes,thereappears‘‘littleevidencethattransplantedstemcellsortheirderivativescanreplacedamagedcells,reconstructneuralcircuits,orimprove

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Review

Table2.NeuralStemCelltrialsTrialSponsor(Location)CityofHope(CA,USA)DiseaseTargetrecurrenthighgradegliomasrecurrenthighgradegliomasNeuralstemInc.(MD,USA)ALSALSchronicspinalcordinjuryReNeuronLtd.(UK)strokestrokelowerlimbischemiaStemCellsInc.(CA,USA)neuronalceroidlipofuscinosiscervicalspinalcordinjurymaculardegenerationthoracicspinalcordinjuryPelizaeus-MerzbacherdiseaseTRANSEURO(UK)WroclawMedicalUniversity(Poland)Parkinson’sdiseasespinalcordinjuryCellTherapyE.ColiCD-expressingneuralstemcellscarboxylesterase-expressingneuralstemcellsfetal-derivedneuralstemcellsfetal-derivedneuralstemcellsfetal-derivedneuralstemcellshumanneuralstemcellshumanneuralstemcellshumanneuralstemcellshumanCNSstemcellshumanCNSstemcellshumanCNSstemcellshumanCNSstemcellshumanCNSstemcellsfetal-deriveddopaminergiccellsolfactoryensheathingcells,autologousNo.Patients24531818412419650151244010Phasephase1phaseIphaseIphaseIIphaseIphaseIphaseIIphaseIphaseIphaseIIphaseI/IIphaseI/IIphaseIphaseIphaseICellStemCell

lossoffunctionfollowingstroke’’(Huoetal.,2014).However,thecompanyReNeuronusesimmortalizedhumanfetalneuralstemcellsfortransplantationtostrokepatientsandhasreportednocell-relatedorimmunologicaladverseeventsinaphaseIstudyof11patientsintheirfollowupafter12months(ReNeuron,2014).Theseimmortalizedhumanneuralstemcellsdonotcolo-nizebraintissueandareatransientpopulationthatappearstohaveatrophicinfluenceonbrainfunction.Whilethestudywasnotdesignedtomeasureefficacy,someimprovementswereobservedinpatientspasticityandneurologicalimpairment.AsecondphaseIItrialisunderwayfortheevaluationofthebenefitoftheseneuralstemcelltransplants2–4monthsafterstroke,involvingafutilitystudyof41patients(nocontrols)intwoseparatetrialcohorts.

ALSisanadultonsetneurodegenerativediseasethatisaresultofmotorneurondegenerationinthecerebralcortex,brainstem,andspinalcord.AphaseIstudybythecompanyNeural-stemforambulatoryandnon-ambulatoryALSpatientsusedin-traspinalinjectionof500,000to1millionhumanfetalspinalcordneuralstemcellsintointhelumbarandcervicalregionsofthespinalcord.Thisstudyshowednocell-relatedadverseeventsbutlittleindicationofimprovedsurvivalbenefit(Feldmanetal.,2014;Glassetal.,2012;Rileyetal.,2012).AnambulatoryphaseIIstudyinvolvedmultiplecervicalinjectionsof2–8millioncellsor160millioncellsintothelumbarandcervicalspinalcordregions(Thomsenetal.,2014).WhilepreclinicalALSanimalmodelstudiesshowsomeefficacyofprolongedsurvivalafterneuralcelltransplants(Leeetal.,2014a;Thomsenetal.,2014),significantbenefitisstillawaitedforALSinhumanphaseIIclin-icaltrials.

Neuralstemcellsarealsoinclinicaltrialsforspinalcordrepair.Theuseofadultolfactorynasalensheathingcellsfordifferentia-tionintoneuralprogenitorsforspinaltransplantation(Tabakowetal.,2013)iscomplicatedbythepossibleretentionoftheunwantedgrowthofamulticysticmassofmixednasalcellphe-notypes(Dlouhyetal.,2014).Thishasnotbeentheexperience

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withtransplantsoffetal-brain-derivedneuralstemcellprogeni-tors.ThecompanyStemCellsInc.hasbeenundertakingaPhaseI/IIclinicaltrialoftheirfetalneuralstemcellprogenitorsin12chronicstage(ASIA-AandB)patientswithT2–T11injuriesinSwitzerland.Theyreceivedatotaldoseof20millioncellsinfourinjectionsaboveandbelowthesiteofinjury.Therehavebeennoadversecell-relatedeffectsreportedinthesepatients,andtherehavebeensegmentalgainsinsensoryandelectro-physiologicalresponsethatoccurovertime.SeveralpatientsprogressedfromASIA-AtoB(patientswithsomerectalcontrol)andsomehadareturnofminormotorcontrolcapacity.PatientsarenowbeingrecruitedforphaseIItrialsinCanadaandtheUSAinvolvingthemorecommoncervicalinjuries(StemCellsInc.,2014).

Basedonevidenceinanimalstudiesforthecapacityofhumanneuralbrainstemcellstoprotectagainstretinaldegenerationandmaintenanceofphotoreceptorhealth(McGilletal.2012;Cuencaetal.,2013),thecompanyStemCellsInc.hasalsouseditsneuralstemcellsfortreatinghumanblindnessduetodrymaculardegeneration.InaphaseI/IIdoseescalationstudy(200,000to1millioncells),theyshowedstableandimprovedvisualacuityinsubjectsat6and12monthsaftersubretinalinjectionofneuralstemcellswithreducedgrowthofretinalgeographicatrophyofthetreatedeyeversesthenon-treatedeye(StemCellsInc.,2015).AphaseIIproof-of-conceptstudyisnowunderwaytoconfirmthemaintenanceofsightinthistypeofpatient.

Transformedneuralstemcelllineshavealsobeenusedtocarryapayloadofcytotoxicdrugstothesiteofglioblastomatumorsinthebrain.Thesestudiesrelyonthehomingcharacter-isticsofneuralstemcellstotumors(Aboodyetal.,2000).ResearchersattheCityofHope,LAhavegeneticallymodifiedneuralstemcellswithenzymesthatareabletoconvertprodrugsadministeredtopatientstohighlypotentcytotoxinsinthelocal-izedsitesofthetumor(Aboodyetal.,2013).Theprimaryclinicalstudieshaveshownconversionoftheprodrugflucytosine(5-FC)

CellStemCell

Review

Table3.PlacentalStemCellTrialsTrialSponsor(Location)CelgeneCorporation(NJ,USA)DiseaseTargetstroke(terminated)pulmonarysarcoidosis(terminated)CDMSperipheralarterydiseaserheumatoidarthritisKarolinskaInstitute(Sweden)PrinceCharlesHospital/MaterMedicalResearchInstitute(Australia)NewYorkMedicalCollege(NY,USA)GVHDhemorrhagiccystitisidiopathicpulmonaryfibrosisimmunedisordersCellTherapyhumanplacenta-derivedcellshumanplacenta-derivedcellshumanplacenta-derivedcellshumanplacenta-derivedcellshumanplacenta-derivedcellshumanplacenta-derivedcellsdecidualstromalcells(MSC-like)decidualstromalcells(MSC-like)placentalmesenchymalstromalcellhumanplacental-derivedstemcells24263012830No.Patients44414PhasephaseIIphaseIphaseIphaseIphaseIphaseIIphaseI/IIphaseI/IIphaseIphaseIto5-fluorouracil(5-FU)andpatienttolerancetoadministrationoftheprodrug.Adoseescalationstudyisunderwaytoassesseffi-cacyofthistreatmentongliomareduction(Hirmand,2014).ThegroupisalsoexploringtheuseofengineeringcarboxylesteraseintoneuralstemcellstoactivatethepotentcytotoxinCPT-11fordestructionoflungandbraincancers(Hongetal.,2013).Therearenodataavailablefromtheseclinicalstudiesasyet.EndothelialStemorProgenitorCells

Therearemany(>60)clinicaltrialslistedontheNIHclinicaltrialswebsite(https://clinicaltrials.gov/)forendothelialstem/progeni-torcells,themajoritywithunknownstatus.Ofthe12studiesofknownstatus,allareautologoustransplants,11arephaseI/IItri-als,and1isphaseII.Inthesestudiesthecellsareeithersourcedfromthebonemarroworperipheralblood.Treatmentsincludetransfusionofendothelialprogenitorcellsinpatientswithidio-pathicpulmonaryarterialhypertension,whereimprovementswereobservedinwalkingdistance,meanpulmonaryarterypres-sure,pulmonaryvascularresistance,andcardiacoutput(Wangetal.,2007).Theyhavealsobeenusedforrefractoryanginainanattempttofosterangiogenesis(Jimenez-Quevedoetal.,2014).Inthistrialof28patients(19treatedand9controls),therewerenoadversecell-relatedeffects(onepatientinthetreatmentgroupandoneinthecontrolgroupdied)buttherewasareduc-tioninthemeannumberofanginaepisodespermonthinthetreatmentgroup.Otherwiseefficacyparameterswerenotdifferent.Somebenefitofautologousendothelialcelltransplantswasseeninpost-mastectomylymphedemavolume,butotherparameterswerenotdifferentfromthoseofcontrols(Maldonadoetal.,2011).Anumberoftrialshavealsobeenundertakenforcriticallimbischemia,usingtheautologousCD34+fraction(endothelialandhematopoieticstemcells)ofgranulocytecol-ony-stimulatingfactor(GC-SF)mobilizedbloodcells(Kawamotoetal.,2009;Kinoshitaetal.,2012).ThesehaveshownstrongfavorabletrendsinefficacyparametersthatencouragefurtherrandomizedandcontrolledphaseIItrialstoprotectagainstseriousdiseasesequealae.Whileagenuinetherapeuticroleforendothelialprogenitorcelltransplantationremainshighlydesir-ableforrepairofvascularinjury,strongevidenceforrepairofthecardiovascularsystembyendothelialprogenitorcellsfromhumanclinicaltrialsremainstobedemonstrated(Mitchelletal.,2015).

ItisofinterestthatbonemarrowMSCsarebeingtrialedforcriticallimbischemiawithoutendothelialcellswithveryminorbenefitsnotedinphaseI/IItrials(Guptaetal.,2013).Seriousadverseeventsoccurredinbothplaceboandtreatmentarms.UmbilicalcordbloodMSCshavealsobeenusedinphaseIstudieswithoutinfluencingseriouseventsandshowingsomeveryminorimprovementsinangiographicscores(Yangetal.,2013).ItwouldbeinterestingtocompareMSCswithandwithoutendothelialcellcombinationtreatments.

PlacentalStemCells

Cellsderivedfromthehumanplacentaareinclinicaltrialsforavarietyoftherapeuticapplications(Table3).AfractionidentifiedasplacentalMSCsbyitsadherencetoplasticandexpressionoftypicalcellsurfacemarkershasbeenusedtotreatpatientswithidiopathicpulmonaryfibrosis.Inasingle-center,non-random-ized,intravenousdoseescalation(1–2millioncells/kg)phaseIstudy,someminorandtransientacuteadverseeffectswereshownandnoobservableimprovementinanyparametersrelatingtothediseaseconditionwasseen6monthsaftertrans-plantation(Chambersetal.,2014).ThecompanyCelgeneisalsousingplacentalderivedcells(MSC-like)totreatCrohn’sdisease(CD)(Mayeretal.,2013)andmultiplesclerosis(MS)(Lublinetal.,2014).Crohn’spatientstreatedwithtwoinjections(1weekapart)of2millioncellsshowedimprovedclinicalresponseandhalfwereinremission6monthsafterplacentalcellinjection.Atahigherdoseof8millioncells,onlyone-thirdofpatientsthatweretreatedresponded,andnonewereinremissionafter6months.Thesedataindicateclinicalvariationsinresponsetocelltherapythatarecriticallydoserelated.Inpatientswithrelapsing-remittingandsecondaryprogressiveMS,thecellswerewelltoleratedbutthetreatmentresultedinavarietyofminoradverseevents.DataforclinicalresponseswerevariableandgenerallyindicatedthatthecellsdidnotimprovetheMScondition.ItisapparentthatmoreinformationisneededonthemechanismofactionofplacentalcellstoenablebetterstrategicdesignforpotentialbenefitstoMSpatientstowarrantexpandedcelltherapytrials.

Thereisinterestinusingcellsoftheamnionforclinicalimprovementoflungfunctioninpretermbabiesandforotheradultrespiratorydisorders(Murphyetal.,2014).However,thesecellscannotbeexpandedinvitroandthereisevidencethat

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Figure1.IndicationsBeingAddressedusingMSCsinClinicalTrials

Datafor352registeredclinicaltrials.

amniocytesobtainedfrompretermplacentashavelimiteddiffer-entiationandreparativecapacityinvitroandinanimalmodelsofpulmonaryfibrosis(Limetal.,2013).Amnioticfluidcellshavealsobeenstudiedinpreclinicalmodelsforawiderangeofther-apeuticapplicationsincludingrepairofbraininjury(Rennieetal.,2013),buttheyhaveneverprogressedtofirstinhumanclinicalstudies.

MSCs

CelltherapiesutilizingMSCsarebeingexploredinalargenum-berofclinicaltrials.ThereisconsiderableheterogeneityinthecellsdescribedasMSCsandavarietyofsourcesusedtoisolateandmanufacturetheMSCpopulationsforclinicaltrials.Impor-tantperspectivesontheverydifferentnatureoftheMSCpopu-lationsinuseclinicallyandtheirvarioussourceshavebeencriticallyaddressedrecentlybyBianco(2014).MSCsareclassi-callythe‘‘postnatal,self-renewing,andmultipotentstemcellsgivingrisetoalltheskeletaltissues’’(Bianco,2014).Theyareclo-nogenicandformstromalprogenyinvitroand,whentrans-planted,formminiatureorganoidsofboneincludingboneandmarrow,withhosthematopoieticcontributions.ThestromalcelltypethatiscalledMSCinuseclinicallyisdefineddifferentlyfromtheclassicalMSCbecausetheyareisolatedfromvarioustissues(includingbulkculturesofbonemarrowstromalcelltypes)bytheiradhesivecharacteristicstoplasticculturedishesandotherplasticvessels.Thesestromalcelltypesallshowcom-monexpressionofgeneralfibroblasticmarkersandwhentrans-planted,theyhavepropertiesofmodulatingthehostimmunesystemandothersystems.Thesearegenerallytransientcellsthatexistbrieflyinthehostandcannotbeidentifiedafterafewdaysorpossiblyaweekortwo.Theirsafetyasallogeneiccelltransplantsmaybecloselyrelatedtotheirshort-termexistence.Theiranti-inflammatoryproperties,homingtositesofdamageandinflammation,andtheirtrophicinfluenceontissuerepairhavemadethemverypopularforclinicalstudy.

IntheNIHclinicaltrialsdatabasethereare374registeredclin-icaltrials(excludingthoseofunknownstatus)usingMSCs(Figure1).Thisisa3-foldexpansionoftrialsoverthenumbernotedin2011(Trounsonetal.,2011),butthedistributionoftrialsbyphaseismuchthesame(Figure2).Thismightsuggestthatproductsarenotmovingoutoftheclinicalpipeline.TheonephaseIVstudyisforumbilicalcordbloodMSCstreatingaplasticanemiathatispresentlyrecruitingpatientsinChina.Themajority

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Figure2.MSCClinicalTrialsClassifiedbyClinicalPhase

Datafor315registeredclinicaltrials.

oftheMSCtrialsarewithallogeneiccellsandthesetrialsarehappeningallovertheworldwiththehighestactivityintheUSA,Europe,andChina.Ofthephase3clinicaltrials,onlythreehavereportedcompletion.

ThepleiotropicpropertiesofMSCsthatincludeanti-apoptosis,angiogenesis,growthfactorproduction,neuropro-tection,anti-fibrosis,andchemo-attractionprovideabroadspectrumfortheirpotentialindiseasetherapies.Thisincludestheirpropertiesofsuppressionofinflammationandtheirabilitiestodownregulatepathogenicimmuneresponsescommonlyobservedwithallogeneictransplantation(GlennandWhartenby,2014).WehavesubdividedthediscussionofMSCclinicaltrialsintoconditionsthatutilizetheirimmunesuppressionproperties,applicationsforregenerationinorgandiseases,osteoarthritisandrelatedlowerbackpain(treatmentsforwhichprimarilyutilizetheiranti-inflammatoryproperties),andrepairofneurodegener-ativediseaseandinjury.

ImmuneSuppressivePropertiesofMSCs

MSCsandtheirderivativeshaveimportantrolesinsuppressingactivatedTcellproliferationandtheircytokineproduction.TheyalsoincreaseRegulatoryTcells(Tregs)thatdampenkillerTcellattackonforeigncellsortissues(seethereviewsofBernardoandFibbe,2013;GlennandWhartenby,2014).ThesefunctionsofMSCshavemadethempopularforstudiesinvolvingcontain-mentofimmunerejectioninallogeneicgrafting.

SystemicimmunosuppressionbybonemarrowMSCs(1million/kg)hasbeenindicatedinkidneyallografttransplantpa-tientsforthepossibleimprovementofrejectionandfibrosisindonororgans(Reindersetal.,2013,2014).Graftversushostdis-ease(GVHD)isaseriousconsequenceofthehostimmunesys-temrecognizingandrejectingallogeneicgrafts.MSCshaveimportantimmunesuppressivepropertiesthatinhibitGVHDandtheyhavebeenstudiedinclinicaltrialsforsteroidresistant,severe,acuteGVHDdiseaseforsometime.EarlyphaseIIstudiesbyLeBlancandcolleaguesshowedthatmorethan50%ofGVHDpatientsrespondedcompletelytoonetofivedosesofameanof1.4millionbonemarrowMSCs(LeBlancetal.,2008).Thesecompleterespondershadlowertransplant-relatedmortalitythanthepatientswithpartialornoresponse(37%verses74%)andhighersurvivalrateafter2years(53%verses16%).ThesedataweresupportedforstageIII–IVGVHDinchildren(0.4–15years)receivingProchymalMSCs(Prasadetal.,2011).Completeresponseswereobservedin58%ofpatientsinfusedwith2–21(mean8)dosesof2or8millionMSCs.Fiveofthetwelve(42%)childrensurvivedforamediumof611days.ThesestudiesledtoregistrationforapprovalfortheuseofProchymalMSCsinsteroidresistantseverepediatricGVHDinCanadaandNewZealand(Ratcliffeetal.,2013).Other

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phaseI/IIstudiesinadultshaveconfirmedthesafetyofMSCinfusionforacuteandchronicGVHDwithvariablebutimprovedresultsforcompleteresponseandsurvival(Herrmannetal.,2012;Intronaetal.,2014;Kuzminaetal.,2012;Liuetal.,2011;

Muroietal.,2013;Pe

´rez-Simonetal.,2011).DatareportedbyJoannaKurtzbergin2015for160childrenwithGVHDnotresponsivetosteroidswhowereenrolledinanexpandedmulti-centeraccessprogramofMesoblastPtyLtd.MSCsagainshowedsurvivalbenefitsforbonemarrowtransplants.Around80%ofgradeB/Cpatientsand50%ofthemoreseveregradeDpatientssurvivedto100days.Only5%–20%oftheseseverepoorprognosispatientswereexpectedtosurvivewithoutMSCtherapy(Mesoblast,2015).

WhileclinicalbenefitswereshowntobeveryminorandnotofclinicalvaluewhenMSCswereco-administeredinhaploidenti-calhematopoieticstemcelltransplantsforleukemia(Maziarzetal.,2015),theAthersysMultistemMSCshaveshownsomeapparentbenefitinreducingGVHDwhenusedasadjunctther-apyformyloablativeallogeneicbonemarrowcelltransplants(Liuetal.,2011).However,purificationofhematopoieticstemcellsand,inparticular,theremovalofalloreactiveTcellscanprobablyaccomplishthisevenmoreeffectively(Loganetal.,2012;Shizuruetal.,1996).

TheimmunosuppressivepropertiesofMSCshavebeenpro-posedastreatmentsforCD,despitethesepropertiesbeingre-tainedbyCDpatients’ownendogenousMSCs(Duijvesteinetal.,2010).ThestudyshowedthatinfusionofdonorMSCsfromhealthynormalpatientshadlittleeffectonrecipientCDpa-rameters,aswouldbeexpected(Duijvesteinetal.,2010).How-ever,anotherstudyshowedadecreaseinendoscopicactiveluminalCD(42%)in7/15patientsgiven2millionMSCs/kg(For-besetal.,2014),suggestingthattheremaybeclinicalbenefitsofMSCtherapyforCD.Likewise,intrathecalorintravenousinjec-tionsofMSCshavebeentriedforadvancedprogressiveandre-lapsingandremittingMS,withlittledetectableclinicalbenefit(Bonabetal.,2012;Connicketal.,2012;Llufriuetal.,2014;Yam-outetal.,2010).Thereweresomeindicationsofminorreduc-tionsinMRIinflammatoryparametersintherelapsingandremittingtypeofMS(Llufriuetal.,2014).MyocardialInjuryBenefitsofMSCs

TherehasbeenaconcertedefforttodemonstrateabenefitofMSCsforcardiovascularrepair,particularlyabenefittopatientswithseveremyocardialinfarct.Generally,autologousbonemarrowcelltransplantshavebeenineffective(Nowbaretal.,2014).AllogeneicMSCtransplantshavehadvariablebenefitstopatientswithischemicheartdisease.LeftventricularejectionfractionwasimprovedinpatientsgivenProchymalMSCswhencomparedtoplaceboinarandomized,doubleblind,doseesca-lationstudy(Hareetal.,2009).RandomizedcomparisonsofautologousandallogeneicMSCs(20–200million)deliveredastransendocardialinjectionsshowedafewdifferences(autolo-gouscellsimproved6minwalking),andonlylowdoseallogeneicMSCsimprovedleftventricularejectionfraction(Hareetal.,2012).Furthermore,autologousMSCsappearedbetterthanautologousbonemarrowcellswhenadministeredastransendo-cardialinjections(Bartuneketal.,2013;Heldmanetal.,2014).IntracoronaryadministrationofMSCshashadaminorbenefitontheleftventricularejectionfraction(Leeetal.,2014b),andameta-analysisofallcelltherapiesbyintracoronaryadministra-tionshowsthatthereisnoclinicalbenefitonleftventricularfunc-tion(Gyo

¨ngyo¨sietal.,2015).Minorbenefitswereobservedinweaningmyocardialinfarctpatientsfromleftventricularassistdeviceswhencomparedwithshamcontrolswhentheformerweretreatedwithintramyocardialinjectionsof25millionofMesoblast’sMSCs(Ascheimetal.,2014).GenuinesustainedbenefitsnowneedtobedemonstratedinphaseIIIstudies.Itisofinteresttonotethatinatleastonecomparisonofheart-derivedcellsandMSCs,thecardiospherecelltypeoutper-formedMSCs(whichwerebonemarrowandadiposederived)inapreclinicalanimalmodel(Lietal.,2012).TherelativebenefitsoftheintramyocardialinjectionoftransientallogeneicMSCsandculturedcadavericcardiospheres(CapricorTherapeutics,2015)inclinicaltrialswillbeinterestingtoevaluate.ItisalsoofsomeinterestthatscarsizereductionandventricularfunctionoccursinsitesofMSCinjectionratherthannon-cellinjectionsites(Sun-cionetal.,2014).Thissuggeststhatthebenefitsofcellsarelocalizedtositesofinjection.

MSCsforOsteoarthritisandLowerBackPain

BonemarrowMSCswouldbeexpectedtocontributetoboneandcartilagerepair.Indelayedbonefractureunion,MSCs,whenmixedwithdemineralizedboneandplatelet-richplasma,halvedthetimetofractureunion(Liebergalletal.,2013).Intra-articularinjectionofMSCsinosteoarthriticpatientsresultedinstrongimprovementincartilagecoverageandqualityinthevastmajorityoftreatedcases(Orozcoetal.,2013).Clinicalpa-rametersofpain,disability,andqualityoflifewereimproved.Likewise,patientswithseverebackpainduetodegenerativediscdiseaseimproveddramatically,with71%ofoptimaleffi-ciencyintheimprovementofclinicalparametersofpainanddisability,butwithoutdischeightrecovery(Orozcoetal.,2011).Mesoblasthasalsoreportedthebenefitsofasingleinjec-tionofMSCs(6or18million)inarandomized,placebo-controlledphaseIIstudyof100patientswithchroniclowbackpainduetodegenerativediscdisease.Asingleinjectionof6millionMSCsgavesubstantialandsustainedpainreliefwith48%havingnoorminimalpainafter24monthscomparedwithonly13%ofpatientswithoutpainwhoreceivedsalinecontrolin-jections(Mesoblast,2015).Theimprovementinpainisaveryimportantclinicalbenefitforthisgroupofpatientsandfurthertrialsshouldconfirmthisbenefitandhopefullyimprovethepro-portionoftreatedpain-freecases.MSCsforPulmonaryDisease

TheuseofMSCsinpediatricbronchopulmonarydisordershasbeenactivelypursued.Inpreterminfantsathighriskofbroncho-pulmonarydysplasia(BPD),umbilicalcordbloodMSCs(1or2millionMSCs/kg)wereadministeredasintratrachealtrans-plants(Changetal.,2014).Levelsofinflammatorycytokinesweresignificantlyreducedinlungaspirates3daysaftertrans-plantationandBPDseveritywaslowerintransplantpatients.Studiesintheadulthavefocusedonacuterespiratorydistresssyndrome(ARDS)treatedwithinfusionsof1,5,or10millionMSCs/kg(Wilsonetal.,2015).Threeofninepatientshadseriousadverseevents,althoughtheywerenotbelievedtoberelatedtocellinfusion.FurtherstudiesareneededtodeterminebenefitsofMSCtherapyforthesetypesofpatients.MSCsforLiverDiseaseandDiabetes

PhaseI/IIclinicaltrialsofMSCtherapyforliverdiseasehaveshownsomeminorimprovementsinliverfunction.Liverinend

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stageliverdisease(cirrhosis)duetohepatitisBandCandalco-holicandcryptogeniccausesshowedsomeresponseinfunctionasassessedbychangesincreatinine,serumalbumin,andbili-rubininresponsetoautologousMSCs(El-Ansaryetal.,2012;Kharazihaetal.,2009).Theanti-fibroticeffectsofautologousMSCs(whichwere50millionMSCsadministeredontwoocca-sions4weeksapart)wereexploredinpatientswithalcoholiclivercirrhosis(Jangetal.,2014).Improvementsinhistologicalliverbiopsysampleswereobservedin55%patientsaswellassomechangeintype-1collagenanda-smoothmuscleactin.FurtherstudiesareneededtoconfirmtheclinicalbenefitsofMSCstopatientswithliverdisease.

SomeclinicalstudiesareunderwayusingMSCsinpatientswithtypeIIdiabetes.Datasuggestsometemporarychangeinmetabolicparametersmayoccurwithintravenousorintra-pancreaticendovascularinjectionofWharton’sjelly(umbilicalcord)MSCs(Liuetal.,2014).

MSCsforIschemicStrokeandALS

MSCsarealsobeingusedinischemicstrokepatients.Fat-derivedallogeneicMSCsarebeingstudiedbyintravenousinjec-tionwithin2weeksofstroke(Dı

´ez-Tejedoretal.,2014).ThecompanyAthersysisalsoundertakingaphaseIIdoubleblindclinicalevaluationofMSCsgivenintravenously24–36hrafterstroke(Hessetal.,2014),butthiswasreportedrecentlytoshownoclinicalbenefit(FierceBiotech,2015).ItisdifficulttounderstandthemechanismofactionoftheseapproachesgiventhetransientnatureandlineagedifferentiationpropertiesofMSCs.ClinicaltrialsarealsounderwayusingMSCsfortherapyinALS(Karussisetal.,2010;Mazzinietal.,2010,2012).NoresponsetoMSCinjectionswasseeninMRIstructuresinthebrainorspinalcordandtherewerenoapparentpostmortemin-dicatorsofbeneficialchange(Mazzinietal.,2010).FewclinicalbenefitswerereportedforeitherintrathecalorintravenousadministrationofautologousMSCs(Karussisetal.,2010;Maz-zinietal.,2012).

FailuresandConcernsforStemCellClinicalTrials

Therehavebeenmanyclaimsformulti-orpluri-potentialityforcellsofvariousorigins,suchastheverysmallcellsofthevascularsystem(multipotentadultprogenitorcells,orMAPCs),umbilicalcordbloodcells,amnioticfluidcells,andothers,thathavenotreallyconvertedtothebroadspectrumofapplicationsinitiallyenvisaged.Thereareautologousbonemarrowstemcelltrialsbeingexploredformanyconditions,includingstrokeandischemicheartdisease.However,inthecaseofthelatter,benefithasbeenrelatedtofactualdiscrepanciesintheclinicaldatacollected,ratherthantheadministeredcelltherapy(Nowbaretal.,2014).

ClinicaltrialfailureshavebeenfrequentforMSCtherapies,and,morerecently(Bersenev,2015),MSCtrialsforulcerativecolitisandischemicstroke(Athersys),cardiacrepair(3studiesbyMiltenyiBiotec;FBCPharmicell,Korea;andStempeuticsResearch),acutekidneyinjury(Allocure),ischemicstroke(Man-ipalAcunova,India),ARDS(China),criticallimbischemia(Ravaletal.,2014),andMS(Spain)havefailedorbeenterminated.Whileclinicaltrialfailuresaretobeexpectedintheearlydaysofclinicalresearch,thesedisappointmentsareareasonablycommonfeatureofthisfirstwaveofcelltherapytrialsinvolvingautologousandallogeneicMSCproducts.

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WhilethereislittlescientificbasisforthedifferentiationofMSCstofunctionalneurons,thereisaregisteredphaseI/IIclin-icaltrialinRussiausingautologousMSC-derivedneuralcells(with‘‘amatrixscaffoldasnecessary’’)in30patientswithtrau-maticspinalcordinjury(Averyanov,2014).Asyet,therearenoresultsavailableforthisstudy,whichbeganrecruitmentinJuly2014;therewerenopreclinicaldataprovidedtosupportthisdubioustypeofapproach.

Incontrasttothecaretakentoensuretheappropriatecelltypeisusedfortransplantationbyothersinthefield,therehasbeenoneclinicaltrialreportedusingundifferentiatedandpartiallydifferentiatedESCstotreatcerebralpalsyinchildreninIndiaunderapprovalofan‘‘independentethicscommittee’’anda‘‘nationalapexbody’’(Shroffetal.,2014).Thecellsweredeliveredbyintravenousorintramuscularinjectionandavarietyofotherroutes.Itispossiblethatmostofthecellsweredeadbecauseofthemethodsused,butthisisapotentiallydangeroustreatmentoftransplantingundifferentiatedpluripo-tentstemcellsthatcouldhaveresultedintheformationofteratomasinthesechildren.Thesetypesofstudiesareverycon-cerningforthefield.

ProgressandCautionaryNotes

TheprogressesofstemcellclinicaltrialsareencouraginggiventhatthemajorityareintheearlyphaseI/IIstageandinmostcases,clinicaldataisstillbeingaccumulated.Itremainstooearlytobeconfidentthatpluripotentstemcellswilldelivertheirconsiderablepromise.AsyetiPSCshavenotreceivedregulatoryapprovaltobeginappearinginfirstinhumanstudies,althoughonepatienthasbeentreatedformaculardegeneration.Neuralstemcellsareprogressingwithsomeinterestingapplications.ItisinterestingthatStemCellsInc.brainstemcellsarecapableofacquiringsomeoftheRPEcellpropertiessuchasphagocy-tosisandsecretionofneuroprotectivefactors.Thereisnoevi-dencethattheymakephotoreceptors,buttheydoseemtopreservesynapticconnectionsbetweenthephotoreceptorsandthebipolarandhorizontalcellsasevidencedbythepres-enceofsynapticribbonsineyesreceivingtransplants(Cuencaetal.,2013).Theneuralstemcellsderivefromprimitivebraintis-sueoftheearlyfetus,whichmayincludeanteriorneuralplatecellsfromwhichRPEsarederived(Graw,2010).Studiesonspi-nalcordinjuryshowsometouchsensitivitybenefitbutlittlemotorresponseforneuralstemcelltherapiesasyet.Thereisalsolittleevidenceofbenefitofneuralstemcelltherapyforstrokepatientsasyet,despitewell-organizedconsensus-basedguidelinesforclinicalstemcelltherapiesforstroke(Savitzetal.,2011,2014).Themajorityofclinicaltrialstodatehaveusednon-neuralcelltypes.

Autologouslimbalstemcellexpandedcultures(Holoclar)havebeenrecentlyformallyapprovedandregisteredforclinicalusebytheEuropeanMedicineAgencyandtheEuropeanCommis-sionasthefirstadvancedtherapymedicinalproductcontainingstemcells(EuropeanMedicinesAgency,2015).Holoclarwillbeusedformoderatetoseverelimbalstemcelldeficiencycausedbyphysicalorchemicalburnstotheeyeinadults.Thisisamajordevelopmentforglobalstemcellmedicine.Researchwillcontinuetoexploretheopportunitytoestablishallogeneiclimbalstemcellsforeffectivecornealtherapiesfrompluripotentstemcellsources.

CellReview

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Autologousendothelialstemorprogenitorcellsshowpreclin-icalvalueforvasculardiseaseanddeficiencysuchasthatseenincriticallimbischemia.ThesecellsincombinationwithMSCsorotherdrugregimesmaybeshowntobeeffectiveastheearlyclinicaltrialscontinuetoevolve.Presentlyitistooearlytomakestrongpredictionsoftheirlikelyclinicalbenefitsbuttherearesomeencouragingdatabeginningtoappearthatwarrantfurtherstudy.

Placental,bonemarrow,andfat-derivedstromalcellsorMSCsfeaturefarandawayinthelargestnumberofclinicaltrialscurrentlyunderway.Theyhavebeenshowntoberemarkablysafealthoughtransitoryintheirpresencefollowingtransplanta-tion.Ithasnotbeendeterminedifthisisduetoarapidprocessofremovingforeigncellsortheirshorthalf-life.Sincetheyareimmunesuppressiveitislikelythattheyarerapidlyturnedoverintissuessuchasthelungwheretheygenerallylodgeinlargenumberswhenadministeredbyintravenousinjection.Thereap-pearstobeverylittledifferencebetweenautologousandalloge-neicMSCsintheiractionsorclinicaleffects,suggestingthattheydeliverapayloadofcytokinesthathaveimmune-modulatoryeffectsandotherinfluencesonendogenoustissueregeneration.Theyappeartomigratetoinflammatorysitesandhaveinflam-matorysuppressingeffectsthatarepro-regenerativefortheaffectedtissue.Theyarerarelyidentifiedascolonizingintissuerepairmechanisms.However,thereisagenerallackofknowl-edgearoundtheiractualmechanismofactionthatisahandicapformodifyingclinicalstrategiestoimprovetheiractions(seediscussionof‘‘Roadblockstotranslationofstemcelltherapies’’inDimmeleretal.,2014).Nevertheless,clinicalbenefitshavebeenobservedinmanyearlyclinicaltrialsthathaveattractedcontinuedfundingforlarger-scaleefficacystudies.Thecontribu-tionsofbonemarrowMSCstoboneandcartilagerepairandreductionofosteoarthriticandlowerbackpainareimpressive.Likewise,theiruseforcontrollingGVHD,particularlyinchildren,isimportantforcancerandtransplantationmedicine.WhetherMSCsfromsourcesotherthanbonemarrowcanalsoshowtheseclinicalbenefitsislesscertain.NotablytheJanssenCom-panyhasMSC-likeumbilicalcordtissuecellsinearlyclinicaltrialformaculardegenerationbuttherearenoclinicaldataavailableasyet.Itisalsotooearlytodecideifplacentalstemcells,whetherstromalorotherwise(suchasamniocytes)haveafuturesignificantroleinclinicalmedicine.Clearlytheresearchwillcontinuetoprobetheseopportunities.

Someofthefailuresinclinicaltrialsmaybepredictedonthebasisthattherewasinsufficientscientificdatatosupportastrongclinicalbenefit(Dimmeleretal.,2014).Inothercases,thereisinsufficientclinicalbenefitapparentinearlyefficacystudiestowarrantfurthercommitmentofrelativelyscarcefinances.ItiscriticallynecessarytoshowclearandsignificantclinicalbenefitinphaseIIstudiesbecausetheheterogeneityofhumandiseaseinmoreextensivephaseIIIorIVstudieswilloftenerodethesignificanceofminorbenefitsapparentinearlytrials.ThenewregulatorypathwayestablishedinJapan(Konomietal.,2015),whereproductsmayenterthemarketplacewithprovisionalapprovalifphaseIIstudiesshowefficacy,willtesttherobustnessoftheentireglobalregulatorysystems.Ifprod-uctsbecomeavailablewithouttestingforsufficientbenefitthenpatientswillnotbeservedwellbytheevolvingcellthera-pies.IftheneedforregulatedphaseIIIstudiescanbedispensed

with,manymoreproductsmaybecomeavailableinashortertimeframeandmorecost-effectivemanner.ThepresentReviewshowsthatcelltherapiesarerapidlyevolvingbutfewatpresentwouldhavedemonstratedsufficientclinicalbenefittowarranttheiradoptionasusefultherapiesinanabbreviatedregulatorysystem.Asstudieswithstrongerscientificevidenceoflikelyclin-icalbenefitanddemonstratedmechanismsofactionevolvefrompreclinicaltrials,itmightbeexpectedthattheconversiontoregisteredstemcelltherapieswillincreasestronglywithtime.WeareoptimisticfromthepresentReviewthattherewillbemanystemcellproductsthatwillmeetthecriteriaforregisteredproductsintheestablishedregulatorysystemsoverthenext5years.

ACKNOWLEDGMENTS

AlanTrounsonisaBoardDirectorofStemCellsInc.andreceivesfinancialcompensationfromthecompanyasaboarddirector.REFERENCES

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