w w w . h t c t . c o m . b r
Hematology, Transfusion and Cell Therapy
Review article
Targeted therapy with a selective BCL-2 inhibitor in older patients with acute myeloid leukemia
Elisabete do Vale Campos
a,∗, Ricardo Pinto
baUniversidadedoPorto,FaculdadedeMedicina,Porto,Portugal
bCentroHospitalarSãoJoão,Porto,Portugal
a r t i c l e i n f o
Articlehistory:
Received19March2018 Accepted11September2018 Availableonline29December2018
Keywords:
AML
BH3-mimetics BCL-2 Targettherapy ABT-199 Venetoclax
a bs t r a c t
Background:Olderpatientswithacutemyeloidleukemiaareparticularlydifficulttotreat, astheyhaveahighriskofcomorbidities,poorperformancestatusandlesstolerabilityto chemotherapy,aswellasamoreaggressivediseasebiology,responsiblefortheresistance totreatment.Thereisaneedtoexplorenoveltherapeuticagentsthataremoreeffective andtolerable.Venetoclax,aBCL-2inhibitorisapromisingagent,asBCL-2overexpression ispresentin84%ofacutemyeloidleukemiapatientsatdiagnosisand95%ofpatientsat relapseandhasbeenassociatedwithleukemiacellsurvival,chemotherapyresistanceand poorprognosis.
Objective:Toreviewtheavailabledataaboutvenetoclaxinacutemyeloidleukemiaandhow itcaninfluencethetreatmentinolderpatients.
Methods:UsingthePubmeddatabase,weselected29articlespublishedwithinthelast15 years,consideringpreclinicalandclinicaltrialsandreviewstudiesthatcombinedveneto- claxwithacutemyeloidleukemia.
Results:Venetoclaxhasdemonstratedpromisingresultsinpreclinicalandclinicaltrials, especiallyinpatientswithpoorprognosisandtheIDHmutation,withanexcellentside- effectprofile.However,resistanceseemstodeveloprapidlywithvenetoclaxmonotherapy, becauseofantiapoptoticescapemechanisms.
Conclusions: Whiletheresultswiththeuseofvenetoclaxseemencouraging,itisnotlikely thattargetingasinglepathwaywillresultinlong-termdiseasecontrol.Thesolutionincludes theuseofcombinedtherapytoblockresistancemechanismsandenhanceapoptosis,by reducingMCL-1,increasingBIMorinhibitingthecomplexIVinthemitochondria.
©2018Associac¸ ˜aoBrasileiradeHematologia,HemoterapiaeTerapiaCelular.Published byElsevierEditoraLtda.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).
∗ Correspondingauthorat:FaculdadedeMedicinadaUniversidadedoPorto,RuadoMonte,n◦169,4425-133ÁguasSantas,Maia,Portugal.
Tel.:+351961899951.
E-mailaddress:[email protected](E.V.Campos).
https://doi.org/10.1016/j.htct.2018.09.001
2531-1379/©2018Associac¸ ˜aoBrasileiradeHematologia,HemoterapiaeTerapiaCelular.PublishedbyElsevierEditoraLtda.Thisisan openaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Introduction
Acutemyeloidleukemia(AML)isahematopoieticneoplasm involvingprecursorcellscommittedtothemyeloidlineageof cellulardevelopmentthatfacesaseriesofgeneticchanges, which resultsin clonalproliferation ofmyeloid precursors withimpairedcapacitytodifferentiateinto normalmature cells.1,2Thisresultsinanaccumulationofleukemicblastsor immatureformsinthe bonemarrow,peripheralblood and other tissues and reduction of normal cells, expressed as systemicsigns,suchanemia,bleedingandincreasedriskof infection.1
AMLrepresents 80 percent of acute leukemia in adults .1 and annually,morethan onequarterofa millionadults throughoutthe worldare diagnosed with AML,3 with 60%
beingover60yearsofage.4.Althoughitrepresentsthemost commontypeofleukemiainadults,itcontinuestohavethe lowestsurvivalrateofall.5Infact,two-thirdsofyoungadults and90%ofolderadultsstilldieofthisdisease,3withonly20%
ofolderpatientssurviving1year.6
ThatiswhyAMLremainsachallengetobothpatientsand clinicians.Infact,despiteadvancesinourunderstandingof theAMLpathogenesis,classification,genomic,andprognostic factors,treatmentminimallychangedinthelast40yearsand outcomesremainpoorforthemajorityofolderpatients.7
The disappointing outcomes in older patients are explained not only by a poor performance status and a higher prevalence of comorbidities, which make intensive chemotherapy more toxic and less tolerated, but also by adverse disease characteristics, such as unfavorable cyto- genetics (−7/del(7), −5/del(5q) or 5p, inv(3)/t(3;3), t(6;9), complex karyotype, monosomal karyotype, 12p, 17p, and 11q23/mixed-lineageleukemiaaberrations,excludingt(9;11)) and unfavorable gene mutations(DNMT3A, ASXL1, RUNX1 and TP53 mutations), higher incidence of secondary AML, including AML with prior myelodysplasia and therapy- related AML, and multidrug resistance (MDR) phenotypes, determined by the expression ofMDR1 (or p-glycoprotein) in 71% of older AML patients, higher prevalence of the CD34+ phenotype (65%), and higher rates of drug efflux (58%), which make the leukemic cells more resistant to chemotherapy.4,6,8–14
Moreover,cellsfromolderpatientswithAMLhavelower apoptotic rates in culture than those of younger patients with AML, and lower rates of apoptosis after exposure to chemotherapy.6Infact,overexpressionoftheanti-apoptotic Bcell leukemia/lymphoma-2(BCL-2)geneisupregulated in 84%ofAMLpatientsatdiagnosisand95%atrelapse,15and isassociatedwithpoorprognosis,lowercompleteremission (CR)rate,significantlyshortersurvival,2,16AMLcellsurvival andchemotherapyresistance.17
Therefore, olderadultshavelower rates ofCRand only 38–62%of patientsover 60 years ofage with de novo AML achieveCR,comparedto65–73%oftotalpatients,withstan- dardinductiontherapy.16
Thereisaneedfornewertherapiesandamoreindividu- alizedapproachforthetreatmentofAML.18Recently,thanks toabetterknowledgeofthemolecularpathogenesisofAML, therehave been anincreasingnumber of potentialtargets
andpathwaysthatcanbeusedforspecifictargetedtherapy inAML.7,19
TheBCL-2familyregulatesthemitochondrialpathwayof apoptosis.20,21 Thebalancebetweenpro-apoptoticBH3-only proteins and anti-apoptotic BCL-2proteins determines the lifeordeathofacell.21Agentsthatinhibittheanti-apoptotic BCL-2familyproteinsbindtotheBH3bindinggroove,mimick- ingtheBH3domainofBH3-onlyproteins,therebyliberating theproapoptoticproteinsBCL-2antagonist/killer1(BAK)and BCL-2-associatedXprotein(BAX)totriggerapoptosis,andare designatedasBH3mimetics.18,22,23EarlierinvestigationalBH3 mimeticswerefoundtobindefficientlytoseveralantiapop- toticproteins,suchastheBCL-2,B-celllymphoma-extralarge (BCL-XL)andthemyeloidcellleukemiasequence1(MCL-1), but wereassociatedwithon-targettoxicityand thrombocy- topenia because platelets depend on the BCL-XL for their survival.18 Thatisthecaseofnavitoclax(ABT-263),aBCL-2 andBCL-XLinhibitor.However,theclarificationofthemecha- nismbywhichnavitoclaxcausesthrombocytopeniasuggested thatamoreselectiveBCL-2inhibitorcouldpreventthistox- icityandenableahigherdosingtoincreaseclinicalefficacy.
Thisledtotherationalreverseengineeringofnavitoclaxto createvenetoclax(ABT-199).24,25
Treatmentwithvenetoclaxseemssuitableincancerswith the marked overexpression of BCL-2,18,21–23 such as AML, andmaybeusefulinincreasingtheapoptoticresponseand improveclinicaloutcomes.
Inthisstudy,wewillreviewtheavailabledataonveneto- claxinAMLandhowitcaninfluencethetreatmentofAMLin olderpatients.
Methods
In this review we usedthe database and searchedfor the MedicalSubject Heading(MeSH)terms (“BCL-2inhibitor”OR
“Venetoclax”OR“ABT-199”)AND (“acutemyeloid leukemia”OR
“AML”).ThelimitsconsideredweretheEnglishlanguageand articlespublishedwithinthelast15years.Thefinalupdateof thesearchwasconductedinFebruary2018.
Intheselectionofthearticles,weanalyzedthetitleandthe abstract,followedbythefulltextofthepreviouslyselected studies,and excluded those whichwere notrelated tothe theme,consideringpreclinicalandclinicaltrialsandreview studiesthatcombinedacutemyeloidleukemiawithveneto- claxorABT-199.
Additional articles were further investigated through a manualsearchinrelevantreferencelists.Thedetailedsearch strategyispresentedinFigure1.
Overall,thisresearchresultedinatotalof29articles,which comprehendstheliterarybasisofthisreview.
Results
Venetoclax(ABT-199)
Mechanismofaction
Venetoclax(ABT-199),amoreadvancedBH3 mimetic,isan orally bioavailable BCL-2-specific inhibitor that selectively inhibits BCL-2, but not BCL-XL, and thus does not cause
Records identified through databases searching
(n = 89)
Identification
Initial screening excluded duplicates and irrelevant studies
(n = 53) Title and abstract review
(n = 36)
Screening
Records excluded (n = 3) Studies not focused on Venetoclax Full-text articles review
(n = 33)
Eligibility
Articles included from reference lists (n = 2)
Full-text articles excluded (n = 6) Studies not focused on Venetoclax
in AML
Included
Included articles (n = 29)
Figure1–Flowdiagramoftheincludedstudiesselection.
thrombocytopenia, and may be useful in malignancies in whichBCL-2playsamorecentralrolethanBCL-XL,suchas AML. Ithas an excellent side-effect profileand, compared tonavitoclax,hasaslightlyhigheravidityfortheBCL-2and threeordersofmagnitudelessavidityfortheBCL-XL.18,22–25
The AMLblasts and AML stem cells are dependent on theBCL-2forsurvival,whilenormalhematopoieticcellsare dependent on the MCL-1.18 Hence, importantly, the BCL-2 inhibitionrelativelysparesnormalhematopoieticstemcells, whicharemoredependentontheMCL-1fortheirsurvival.26
Venetoclax-based therapy could provide significant sur- vival prolongation for older patients with AML who are ineligibleformoreaggressivetherapiesand,therefore,hasthe potentialtorevolutionizetheAMLtherapy.25
Preclinicalstudies
Inapreclinicalstudy,Panetal.26testedtheactivityofvene- toclaxinleukemiacelllines,murineprimaryxenograftsand primarysamplesofpatientswithAML. Thisstudydemon- stratedsignificant activity ofvenetoclax,as asingleagent, inallthreeAMLmodelsystems(Table1).18,26 Inasensitive AML cell line, cell growth was inhibited and cell apopto- siswasobservedwithinjust2hofexposuretovenetoclax.
Invivoexposuretovenetoclax,onmurineprimaryxenografts, showed inhibition of leukemia progression and prolonged overall survival(OS). Furthermore, inprimary patient AML cells, including AML cells with diploid cytogenetics and mutationsin FMS-like tyrosinekinase-3 (FLT3), NRAS, and nucleophosmin(NPM1)genes,20outof25(80%)weresensitive tovenetoclax,while5sampleswereresistant.
Kontroet al.27 evaluated the exvivo sensitivityoffresh leukemic cells from 73 diagnosed and relapsed/refractory
AMLpatients,subsequentlyanalyzingiftheresponsescorre- latedtospecificmutationsorgeneexpression.Thestrongest responseswereobservedin15%oftheAMLpatientsamples, 32%wereresistant,andtheremainingpresentedintermediate responsestovenetoclax(Table1).
Anotherstudy,developedbyNiuetal.,28concludedthat venetoclaxwasabletoinduceapoptosisinadose-dependent mannerinfourofthefivecelllinestested.Similartothecell lineresults,venetoclaxwasabletoinduceadose-dependent increaseinapoptosisintwoAMLpatientsamples(Table1).28 Overall,inpreclinicalstudies,venetoclaxhasshownpotent antileukemicactivity inAMLcelllinesand primarypatient samplesinvitroandinmurinexenograftmodels.25,29 These dataproviderationalefortargetedBCL-2inhibitionwithvene- toclaxinAMLinclinicaltrials.25,29
Clinicalstudies
In a phase II clinical study,29 to evaluate the efficacy of single-agent venetoclax in patients with high-risk relapsed/refractory AML or in those unfit for intensive chemotherapy,32 patientswereevaluatedandtheiroverall responserate(ORR)bytheInternationalWorkingGroup(IWG) was19%,with6%achievingaCRand13%achievingaCRwith incomplete bloodcountrecovery(CRi).Exceptfor1CRi, all objectiveresponseswereachievedbytheweek4assessment andallIWG-definedresponseswereobservedinpatientswho hadbeenpreviouslytreatedforAML(Table1).29
Additionally19%ofpatientsdemonstratedanti-leukemic activitynotmeetingIWGcriteria,withpartialbonemarrow responseandincompletehematologicrecovery.29
Also,venetoclaxappearedtobesafeand well-tolerated, with asimilar range ofadverse events(AEs) seen inother
Table1–Venetoclaxinmonotherapy.
Typeofstudy References Results
Responseto venetoclax treatment
BCL-2inhibitor sensitivitywas directly associatedwith:
BCL-2inhibitor sensitivitywas inversely associatedwith:
Preclinical studies
Panetal.26 AMLcelllines:
•Single-agentcytotoxicactivityin6/12 AMLcelllines
•Inhibitionofcellgrowth Cellapoptosisinfewhours
Murineprimaryxenografts:
•Inhibitionofleukemiaprogression
•Prolongedoverallsurvival
AMLpatientsamples:
•20/25(80%)sensitivetovenetoclax
•BCL-2 •BCL-XL
•MCL-1
•JAK2mutation
•Complex cytogenetics
Kontroetal.27 AMLpatientsamples:
•15%hadstrongresponses
•32%wereresistant
•53%hadintermediateresponses
•WT1mutation
•IDH1/IDH2mutation
•HOXAandHOXBoverexpression
•
2-microglobulin
•BCL-XL
•BAX
Chanetal.30 •IDH1/IDH2mutation
Niuetal.28 AMLcelllines:
•Apoptosisin4/5celllines
AMLpatientsamples:
•Increasedapoptosisin2/16AMLpatient samples
•AMLcelllinesharboringmixedlineage leukemia(MLL)fusiongenes
•APLphenotype
•BCL-XL
•MCL-1
ORR LFS OS AE
Clinical studies pHaseII clinical study
Konopleva etal.29
•19%(6%CR;13%CRi)
•Additionally19%ofpatients
demonstratedanti-leukemicactivitywith partialbonemarrowresponseand incompletehematologicrecovery
•33%ofpatientswithIDH1/2mutations achievedCR/CRi
•6-monthLFS rate-10%
•MedianLFS– 2.3months
•Mediantimeto progression–2.5 months
•6-monthOS– 36%
•MedianOS–4.7 months
•Nausea
•Diarrhea
•Vomiting
•Headache
•Febrile neutropenia
•Hypokalemia
•Pneumonia
•Hypotension
•Urinarytract infection
AML:acutemyeloidleukemia;ORR:overallresponserate;LFS:leukemiafreesurvival;OS:overallsurvival;AE:adverseevents;CR:complete remission;CRi:completeremissionwithincompletebloodcountrecovery.
studies,with84%presentingsevereAEs,being febrileneu- tropeniathemostcommon(31%).TheseAEswereconsistent withexpectationsinthispopulation.Therewerenoeventsof tumorlysissyndrome(TLS)andnoAEsleadingtodeath.29
Suchresults,althoughseeminglylimited, havenotbeen observedwithanyotheroralmonotherapyforAMLandare particularly impressive considering that their cohort con- sisted of older patients and included many patients with high-riskfeatures.25 Infact, mostpatients hadpre-existing hematologicdisordersormyeloproliferativeneoplasms(41%), therapy-related AML with complex cytogenetics (13%) and olderage(median71years).Also,thesepatientswereheavily pre-treated,41%havingprogressedthroughatleastthreeprior treatments, 53% havingreceived standard “7+3 induction therapy (cytarabine+anthracycline) and 75% having failed hypomethylatingagents.29
Moreover,mutationsinisocitratedehydrogenase(IDH)1 or2wereidentifiedin38%ofpatients. Amongthepatients with IDH1/2 mutations, 33% achieved CR/CRi and several otherpatientswithIDHmutationshadsignificantblastcount reductionthatdidnotmeetIWG responsecriteria.29 These clinical observationsconfirminvitroobservationsthat sug- gest anincreasedsensitivityofIDH-mutatedAMLtoBCL-2 inhibition.18,30
Noneofthepatientswiththerapy-relatedAMLexperienced anti-leukemicactivityonvenetoclax.29
The6-monthleukemia-freesurvival(LFS)ratewas10%and themedianLFSwas2.3months.The6-monthOSestimatewas 36%andmedianOSwas4.7months.29
Insummary,relapsed/refractoryAMLinolderpatientshas anextremelypoorprognosis,withOSlessthan6months.So, a responserateof19% plus additionalcasesofmyeloblast
reduction,witharelativelywelltoleratedoralsingleagent,is apromisingclinicalaccomplishment.29However,allpatients treatedwithvenetoclaxrelapsed,despitethepotentialinitial response,withamediantimetoprogressionof2.5months.
Therefore,resistanceappearstodeveloprapidlywithveneto- claxmonotherapy.25
Predictorsofresponse
Predictive biomarkers may be used to identify and select patients more responsive to venetoclax therapy, therefore potentiallyimprovingtheresponserate.29
InthepreclinicalstudyofPanetal.,26thecelllinesmore sensitivetovenetoclaxwerethosewithhighlevelsofBCL-2 proteinandrelativelylowBCL-XLandMCL-1expression.Sim- ilarresultswerefoundinprimaryAMLmyeloblasts.Moreover, AMLmyeloblastsdemonstratedhigherBCL-2mRNAexpres- sionthannormalbonemarrow.Samplesfrompatientswith complexcytogeneticsandjanuskinase2(JAK2)mutationwere largelyinsensitivetovenetoclaxandnocorrelationwasfound betweenvenetoclaxsensitivityandFrench-American-British (FAB)classification(Table1).26
Thein vitroresults suggestedthat there willbe hetero- geneityinclinicalresponse,soapredictivebiomarkerwould beofgreatutility.Inthiscontext,theypresentedfourmeth- odsthatmaybepredictiveofclinicalresponsetovenetoclax.
Thefirstmethodiscytogenetics,cellulardeathresponseto venetoclaxappearstobelargelyindependentofcytogenetic andgeneticmutationstatus,exceptforcomplexkaryotype andJAK2mutationthatareresistant,suggestingthattreat- mentwithvenetoclaxcouldbeusefulforalargeproportion ofpatientswithpoorprognosticfactors.Asecondmethodis anexvivoshort-termcultureoftheprimarypatientsamples withvenetoclax.Thethirdpredictivebiomarkermethodisto measureBCL-2levelsbyWesternblotting,astheyshowedthat increasedexpressionofBCL-2isassociated withincreased sensitivitytovenetoclax.Thefourthmethod,BH3 profiling, that is a functional assay which accounts for the relative amountsandinteractionsofalloftheBCL-2familymembers, canfunctionasapredictivebiomarkersinceitcoulddiscrim- inateinvivosensitivityofhumanAMLcellstovenetoclax.26 Therefore,BH3 profilingofpre-treatmentAMLsamplescan beusedtoidentifyandselectAMLcasesthataremostlyBCL- 2dependentandsomoreresponsivetovenetoclaxtreatment, potentiallyimprovingtheresponserate.26,29
ToalsoidentifyrobustbiomarkersforBCL-2inhibitorsensi- tivity,Kontroetal.27developedapreclinicalstudyconcluding thatBCL-2inhibitorsensitivitywasassociatedwithgenetic aberrationsinchromatinmodifiers,wilmstumor1(WT1)and IDH,homeobox(HOX)AandBgenetranscriptsoverexpression andBAXexpression.Significantinversecorrelationresponses tovenetoclaxwererelatedto2-microglobulinandinalesser degreetoBCL-XLexpression(Table1).27
A study of Chan et al.30 also showed that IDH1- and IDH2-mutantprimaryhumanAMLcellsweremoresensitive thanIDH1/2wild-typecellstovenetoclaxbothexvivoandin xenotransplantmodels(Table1).Thishasbeenattributedto suppressionofcomplexIV,alsoreferredtoascytochromec oxidase,inthemitochondrialelectrontransportchain(ECT) bytheoncometabolite2-hydroxyglutarate(2-HG),generated byIDHmutations,thatlowersthemitochondrialthresholdto
triggerapoptosisuponBCL-2inhibition.The2-HGaccumula- tioninAMLcellswiththeIDH1/2mutationdirectlyinhibits thecomplexIVandthus,increasesthedependencyonBCL-2 topreventapoptosis.Hence,theIDH1/2mutationstatusmay identifypatientsthatarelikelytorespondtopharmacologic BCL-2inhibition,consideringthatapproximately15%ofAML patientshavemutationsineitherIDH1orIDH2.30However, thestudyofChanetal.30showedthatcomplexIVinhibition issufficienttosensitizeAMLcellstoapoptosis,uponveneto- claxtreatment,andoffersanovelstrategytoenhanceclinical efficacy andovercomepotentialresistanceinpatientswith unmutatedIDH.30,31
Anotherstudy,developedbyNiuetal.,28whoseobjective wastoidentifygeneticsubgroupswhichmaybetterrespondto venetoclaxandbiomarkerspredictingvenetoclaxsensitivity inAML,concludedthatAMLcelllinesharboringmixedlin- eageleukemia(MLL)fusiongeneswereespeciallysensitiveto venetoclax(Table1).RearrangementsintheMLLgeneonchro- mosome11q23occurinupto10%ofAMLandaregenerally associatedwitharelativelyunfavorableprognosis.Resistance totherapyforAMLingeneral,andMLLleukemiasinparticular, mayalsostemfromtheaberrantexpressionofBCL-2family proteins.28
Patient samples with the acute promyelocyticleukemia (APL)phenotypeweresignificantlymoresensitivetosingle- agentvenetoclax.Moreover,inthecombinedcohortofAML cell lines and patient samples were the overexpressionof BCL-XLandMCL-1attenuatedvenetoclax-inducedapoptosis.
ThesesuggestthattheratiosofBCL-2/MCL-1transcriptsmay representagoodbiomarkerpredictingvenetoclaxsensitivity inAML.28
Mechanismsofresistancetovenetoclax
Although venetoclax is the first BCL-2-selective inhibitor that has demonstrated promising results in multiple can- cers, including AML, intrinsic drug resistance remains a concern.32,33 In fact, venetoclax has demonstrated limited efficacy inBCL-XL andMCL-1-dependentmalignancies.Niu etal.,34Choudharyetal.andLinetal.,35demonstratedthat intrinsicallyresistantAMLcellshaveincreasedlevelsofMCL- 1 and BCL-XL following venetoclax treatment, resulting in increasedsequestrationofBCL-2-like11(BIM)byMCL-1.34,35
Venetoclaxtreatmentwasfoundtoresultindissociation ofpro-apoptotic BIMfrom BCL-2,demonstratingits role in inducingapoptosisinAMLcells,butalsoresultedinincreased sequestrationofBIMbyMCL-1,preventingBIMfrominduc- ingapoptosis,whilealsoresultinginincreasedMCL-1protein levels invenetoclax-resistantAMLcells.These resultssug- gestthatMCL-1isakeyplayerintheintrinsicresistanceto venetoclax inAMLcells. Sincevenetoclax does notinhibit MCL-1,increasedexpressionofMCL-1 could beapotential sourceofupfrontresistancetoBCL-2inhibitionbyvenetoclax.
Confirmingthis,theCRISPRknockdownofMCL-1significantly enhancedvenetoclax-inducedapoptosisinAMLcells.26,32,33
ThereforeLinetal.,35demonstratedthatbytargetingMCL- 1andBCL-XL,resistantAMLcelllinescouldberesensitized tovenetoclax.Furthermore,targetingMCL-1 and/orBCL-XL alongwithadministrationofvenetoclaxwascapableofdelay- ingorpreventingtheacquisitionofdrugresistance.35
Table2–Combinedtherapy.
Agents Mechanismstoovercomevenetoclax
resistance
IncreasedDNA damage
Synergisticinductionof cellapoptosis
Overallresponse rate
Venetoclaxwith ↓MCL-1↑BIMBAXactivation Conventionalchemotherapy
(cytarabineordaunorubicinor idarubicin)
√ √ √
LDAC √ √ √ 44%
Hypomethylatingagents decitabineor5-azacytidine
√ √ 61%
CDKinhibitor(LS-007oralvocidib) √ √ √
MCL-1-selectiveinhibitor A-1210477
√ √
CHK1inhibitorLY2603618 √ √ √
PI3K/mTorinhibitors (VS-5584)+MEKinhibitors (SCH772984)
√ √ √ √
HDACI √ √
GLS1inhibitor √
MDM2antagonistidasanutlin √ √
NAEinhibitorMLN4924 (pevonedistat)
√ √ √
GalectininhibitorGCS-100 √ √
Niuetal.34provedalsothatthismechanismofresistance canbeovercomebycombiningvenetoclaxwithDNAdamag- ingagentsdaunorubicinorcytarabine,whichreduceMCL-1 levelsandenhancevenetoclaxactivityinvenetoclax-resistant AMLcells.Itisplausiblethatinthevenetoclax-sensitivecells moreBIMis released from BCL-2than can besequestered by MCL-1 (high BCL-2/MCL-1 transcript ratio), resulting in freeBIM,whichwouldthenactivatetheapoptosispathway, while in venetoclax-resistant cells (low BCL-2/MCL-1 tran- scriptratios)itislikelythatthereisenoughMCL-1tosequester BIMandinhibitapoptosis.Forthecombineddrugtreatments inresistantcells,MCL-1downregulationlikelyresultsinfree BIM, allowing for activation of BAK/BAX and triggering of apoptosis.34Additionally,theydemonstratedthatthesecom- binations are synergistic in cell lines and primary patient samplesindependentoftheirsensitivitiestovenetoclax,thus providingevidencethatscreeningforvenetoclaxresistance is not necessary, and therefore supporting clinical testing regardlessofvenetoclaxsensitivity.34
Overall,itisunlikelythatanysingleapproach,including themosttargeted,willbesuccessfulineradicatingAMLcells, dueto the related problemsofintrinsic or acquired forms ofresistance.Oneapproachtocircumventing this problem involvescombiningagentsthatactincomplementarywaysto promotecelldeathortosuppresscellularescapemechanisms thatfacilitatetheacquisitionofdrugresistance.7,35
Combinedtherapy
OnceMCL-1hasbeenrelatedtointrinsicresistancetovene- toclax in AML cells, drugs that act via MCL-1 inhibition are being explored, as a manner in which to overcome thisresistancemechanism.DecreasedMCL-1 proteinlevels were demonstrated in preclinical studies in the combina- tionofvenetoclaxwithchemotherapeuticagentscytarabine
and/or daunorubicinoridarubicin, hypomethylatingagents decitabineor5-azacytidine,CDK9inhibitorsLS-007andalvo- cidib,MCL-1selectiveinhibitorA-1210477,Checkpointkinase 1(CHK1)inhibitorLY2603618,PI3K/mTORinhibitorsVS-5584 plus MEK inhibitors SCH772984, Mouse double minute 2 homolog (MDM2) antagonist idasanutlin, Nedd8 activating enzyme(NAE)inhibitorMLN4924(pevonedistat)andGalectin inhibitorGCS-100(Table2).24,32,36–42
Another approach would be to upregulate BIM levels, leadingtoanincreasedBIM/MCL-1ratiofavoringapoptosis.
IncreasedBIMlevelsweredemonstratedinpreclinicalstud- ies in the combination of venetoclax with CDK9 inhibitor alvocidib,PI3K/mTORinhibitorsVS-5584plusMEKinhibitors SCH772984 and Histone deacetylase inhibitors (HDACI) (panobinostat)(Table2).33,37,39
All the combinations described above, plus the com- bination of venetoclax with GLS1 inhibitor, resulted in synergistic induction of apoptosis in preclinical studies (Table 2),16,32–34,36–45 suggestingpotential novel therapeutic strategiesforAML.However,thebiggestchallengeischoosing themostpromisingcombinationsandbringingthemforward intoclinicaltrials,whicharecostlyandtime-consuming.20
Untilnow,clinicalstudiescombiningvenetoclaxwerecon- ductedwithlow-dosecytarabine(LDAC)andhypomethylating agents.Inaphase1b/2study,44with18olderpatientswith AML,noteligibleforintensivechemotherapy,venetoclaxcom- binedwithLDACachievedanORRof44%(CRof22%andCRi of22%).Tolerabilitywasacceptable,withfebrileneutropenia beingthemostcommonsevereAE(33.3%).NosignificantTLS wasobserved.Twodeathsoccurred.17,25,44
In a phase Ib clinical trial,46 with a cohort of57 older patientswithAMLineligibleforstandardchemotherapy,vene- toclaxcombinedwithhypomethylatingagentsdecitabineor 5-azacytidine achievedan ORRof 61% (CRof24% and CRi of 37%), without differences in response between patients
whoreceiveddecitabineor5-azacitidine.Poor-riskcytogenet- icswaspresentedin38% ofpatients, withanORRof52%.
TheIDH1/2mutationswerereportedin31%ofpatients,with an ORRof 59%.These drug combinations were well toler- ated,withfebrileneutropeniabeingthemostfrequentsevere AE(31%).NoeventsofTLSwerereported.Twenty-sixdeaths occurred,10fromadverseevents.Mediandurationofresponse was8.4monthswithamedianOSof12.3months.46
Discussion and conclusion
Venetoclaxhasdemonstratedpromisingresultsinpreclinical and clinicaltrials, especiallyinpatients withpoorprogno- sis, and with anexcellent side-effect profile. However,the responsetovenetoclaxisheterogeneous,withsensitivityto theBCL-2inhibitorbeingdirectlyassociatedwithBCL-2and HOXA/Boverexpression,BAXexpressionandWT1andIDH1/2 mutation,andinverselyassociatedwithexpressionofMCL-1, BCL-XL,JAK2mutation,2-microglobulinandcomplexcyto- genetics.
Indeed, cells with intrinsic or acquired resistance to venetoclax exhibited increased MCL-1 and BCL-XL levels, leading to increased sequestration of BIM. Unfortunately, resistancemechanismsseemtodeveloprapidlywithveneto- claxmonotherapy,so,targetingMCL-1and/orBCL-XL,along withthe administration ofvenetoclax,has the capacity to delay or prevent the acquisition of drug resistance. How- ever,BCL-XLinhibitionisnotagoodstrategybecauseitwas associatedwithon-targettoxicityandthrombocytopenia.On the other hand,MCL-1 is notinhibited byvenetoclax, and increasedMCL-1isanimportantantiapoptoticescapemech- anism.Therefore,MCL-1inhibitionseemstobeanimportant approachtoovercomevenetoclaxresistance,aswellastopro- motetheupregulationofBIM.
ConcerningtheIDHmutation,itsprevalenceintheana- lyzedstudies(38%inthestudyofKonoplevaetal.29and31%
inthestudyofDiNardoetal.46)raisedthepossibilitythatthis mutationwasmoreprevalentinolderpatientswithAML,tak- ingintoaccountthatChanetal.,30Stocketal.2andSchiffer etal.1referaregularprevalenceof15%.However,thesestud- iescorrespondtoasmallnumberofpatients,andTsaietal.,14 analyzinggenemutationsinalargercohortofAMLpatients (462patients),reportedaprevalenceoftheIDH1mutationin 6.8%andIDH2mutationin14.7%ofolderpatients.Moreover, Sayginetal.17reportedaprevalencerangeof5–10%intheIDH1 mutationand10–15%inIDH2mutationandtherefore,inthe studyofTsaietal.,14thepercentagesofthe IDH1/2muta- tioninolderpatientswerewithinthenormalrange.Hence,a higherprevalenceoftheIDHmutationinolderpatientswith AMLwasnotproved.However,itisclearthatpatientswith theIDH1/2mutationdemonstratedsuperiorresponseswith thevenetoclaxtreatment,firmlyindicatingitsuseinpatients withtheIDHmutation.
Taking into account that inthese casesthe use ofIDH inhibitors may also be indicated, we raised the possibility ofacombined therapywithvenetoclaxandIDH inhibitors, toenhanceclinicalresponses.However,intheory,thatmay
result in increased resistance,since the production of the oncometabolite2-HG,proportionatedbythemutationinthe IDH,suppressesthecomplexIV,loweringthemitochondrial thresholdandtriggeringapoptosisupontheBCL-2inhibition, andtheIDHinhibitorsdiminishtheoncometabolite2-HG,so it willalsoreduce complexIVinhibitionand therefore the sensitivitytovenetoclax.Therefore,intheory,combinedther- apywithvenetoclaxandIDHinhibitorsmaybenotindicated, unlessthereisathirdcomponentthatinhibitscomplex IV.
Nevertheless,venetoclaxcouldbeusedinacomplementary way,inpatientswhodemonstratedpriorresistancetotreat- mentwiththeIDHinhibitors.
Giventhis perspective,inpatientswithunmutatedIDH, Chanetal.30defendthatcomplexIVinhibitionissufficient to sensitize AMLcells to apoptosis upon venetoclaxtreat- ment,so the combineduse ofcomplex IVinhibitors could beapossibilitytoenhancetheresponsetovenetoclax.Com- plexIVinhibitionmimicsastateofoxygendeprivation,which canactivateBAX/BAKtotriggerapoptosis,ifnotantagonized byBCL-2. Treatment withvenetoclaxdisrupts the complex formation betweenBCL-2 and BAX/BAK and,consequently, promotestheactivationofproapoptoticproteins,leadingto apoptosis.30 In this context, tigecycline, an anti-microbial agentofthe glycylcyclineclass, currentlyusedasa broad- spectrumantibiotic,isapromisingagentinthetreatmentof AML,functioningasaninhibitorofsubunits1and2ofcom- plexIV.47
Inapreclinicaltrial,tigecyclineselectivelykilledAMLcells atallstagesofdevelopment,aswellasshowinganti-leukemic activity inmouse modelsof humanleukemia, whilespar- ing normal hematopoietic cells. Leukemic cells owe their heightened sensitivity to tigecycline due to an increased dependenceonmitochondrialfunctionwithenhancedbasal oxygen consumption, compared to normal hematopoietic cells, demonstrating that the mechanism of tigecycline- mediatedlethalityismitochondrialtranslationinhibition.47
Inaphase1study,with27patients(medianage70years) withrelapsedandrefractoryAML,tigecyclineadministration showedafavorablesafetyprofile,butnoneofthepatientshad evidenceofdiseaseresponse.48Although,theauthorstryto justifytheseresultsasalackofeffectivesteady-statelevelsof tigecycline,itcouldalsobeduetoanincreaseddependencyon BCL-2,preventingapoptosisandthus,combiningtigecycline withvenetoclaxcouldenableasuperiorclinicalefficacy.
Therefore, the use of venetoclax has the potential to improvecurrentlyavailabletherapies,but,whiletheresults withtheuseofvenetoclaxseemencouraging,itisnotlikely thattargetingasinglepathwaywillresultinlong-termdisease control.Thesolutionincludestheuseofcombinedtherapy toblockresistancemechanismsandenhanceapoptosis,by reducing MCL-1,increasingBIMorinhibiting complexIVin themitochondria.
Inthiscontext,largerandlongerclinicaltrials,combining venetoclaxwithLDACandhypomethylatingagents,aswellas withtigecycline,couldsupplytheresultsneededtointegrate theuseofcombinedtherapywithvenetoclaxasatreatment optioninolderpatientswithAML,ineligibleforconventional chemotherapy.
Conflicts of interest
Theauthorsdeclarenoconflictsofinterest.
references
1. Clinicalmanifestations,pathologicfeatures,anddiagnosisof acutemyeloidleukemia[Internet];2017.Availablefrom:
www.uptodate.com
2. Moleculargeneticsofacutemyeloidleukemia[Internet];
2017.Availablefrom:www.uptodate.com
3. RoweJ,TallmanMS,HowI.treatacutemyeloidleukemia.
Blood.2010;116(17):3147–56.
4. GodwinJ,SmithSE.Acutemyeloidleukemiaintheolder patient.CritRevOncol/Hematol.2003;48S:S17–26.
5. DeschlerB,LübbertM.Acutemyeloidleukemia:epidemiology andetiology.Cancer.2006;107(9):2099–107.
6. LichtmanM,RoweJM.Therelationshipofpatientagetothe pathobiologyoftheclonalmyeloiddiseases.SemOncol.
2004;31(2):185–97.
7. ShaferD,GrantS.Updateonrationaltargetedtherapyin AML.BloodRev.2016;30(4):275–83.
8. TheinM,ErshlerWB,JemalA,YatesW,BaerJMR.Outcomeof olderpatientswithacutemyeloidleukemia:ananalysisof SEERdataoverthreedecades.Cancer.2013;119(15):2720–7.
9. AmadoriS,PrincipeMI,VendittiA.Advancesinthetreatment ofelderlyandfrailpatientswithacutemyeloidleukemia.
CurrOpinOncol.2014;26(6):663–9.
10.ErbaH.Hastherebeenprogressinthetreatmentofolder patientswithacutemyeloidleukemia?BestPractResClin Haematol.2010;23:495–501.
11.CreutzigU,ZimmermannM,ReinhardtD,RascheM,Neuhoff Cv,AlpermannT,etal.Changesincytogeneticsand
moleculargeneticsinacutemyeloidleukemiafrom childhoodtoadultagegroups.Cancer.2016;122(24):3821–30.
12.LazarevicV,HorstedtA-S,JohanssonB,AntunovicP,Billstrom R,DerolfA,etal.Incidenceprognosticsignificanceof karyotypicsubgroupsinolderpatientswithacutemyeloid leukemia:theSwedishpopulation-basedexperience.Blood CancerJ.2014;4:e188.
13.BüchnerT,HiddemannW,BerdelW,WörmannB,SchochC, LöfflerH,etal.Acutemyeloidleukemia:treatmentover60.
RevClinExpHematol.2002;(1):46–59,discussion86–7.
14.TsaiCH,HouH-A,TangJ-L,LiuC-Y,LinC-C,ChouW-C,etal.
Geneticalterationsandtheirclinicalimplicationsinolder patientswithacutemyeloidleukemia.Leukemia.
2016;30(7):1485–92.
15.MoonJH,SohnaSK,LeebM-H,JangcJH,KimcK,JungcC,etal.
BCL2genepolymorphismcouldpredictthetreatment outcomesinacutemyeloidleukemiapatients.LeukRes.
2010;34(2):166–72.
16.MedingerM,LengerkeC,PasswegJ.Novelprognosticand therapeuticmutationsinacutemyeloidleukemia.Cancer GenomProteom.2016;13(5):317–29.
17.SayginC,CarrawayHE.Emergingtherapiesforacutemyeloid leukemia.JHematolOncol.2017;10(1):93.
18.KadiaTM,RavandiF,CortesJ,KantarjianH.Newdrugsin acutemyeloidleukemia.AnnOncol.2016;27(5):770–8.
19.TallmanMS,GillilandDG,RoweJM.Drugtherapyforacute myeloidleukemia.Blood.2005;106(4):1154–63.
20.BoseP,GrantS.Rationalcombinationsoftargetedagentsin AML.JClinMed.2015;4(4):634–64.
21.HotchkissRS,StrasserA,McDunnJE,SwansonPE.Celldeath.
NEnglJMed.2009;361(16):1570–83.
22.HataAN,EngelmanJA,FaberAC.TheBCL2family:key mediatorsoftheapoptoticresponsetotargetedanticancer therapeutics.CancerDiscov.2015;5(5):475–87.
23.BesbesS,MirshahiM,PocardM,BillardC.Newdimensionin therapeutictargetingofBCL-2familyproteins.Oncotarget.
2015;6(15):12862–71.
24.GibsonCJ,DavidsMS.BCL-2antagonismtotargetthe intrinsicmitochondrialpathwayofapoptosis.ClinCancer Res.2015;21(22):5021–9.
25.PullarkatV,NewmanEM.BCL2inhibitionbyvenetoclax:
targetingtheAchilles’heeloftheacutemyeloidleukemia stemcell?CancerDiscov.2016;6(10):1082–3.
26.PanR,HogdalLJ,BenitoJM,BucciD,HanL,BorthakurG,etal.
SelectiveBCL-2inhibitionbyABT-199causeson-targetcell deathinacutemyeloidleukemia.CancerDiscov.
2013;4(3):362–75.
27.KontroM,KumarA,MajumderMM,EldforsS,ParsonsA, PemovskaT,etal.HOXgeneexpressionpredictsresponseto BCL-2inhibitioninacutemyeloidleukemia.Leukemia.
2016;31(2):301–9.
28.NiuX,WangG,WangY,CaldwellJT,EdwardsH,XieC,etal.
AcutemyeloidleukemiacellsharboringMLLfusiongenesor withtheacutepromyelocyticleukemiaphenotypeare sensitivetotheBcl-2-selectiveinhibitorABT-199.Leukemia.
2014;28(7):1557–60.
29.KonoplevaM,PollyeaDA,PotluriJ,ChylaB,HogdalL,Busman T,etal.Efficacyandbiologicalcorrelatesofresponseina phaseIIstudyofvenetoclaxmonotherapyinpatientswith acutemyelogenousleukemia.CancerDiscov.
2016;6(10):1106–17.
30.ChanSM,ThomasD,Corces-ZimmermanMR,XavyS,Rastogi S,HongW-J,etal.Isocitratedehydrogenase1and2mutations induceBCL-2dependenceinacutemyeloidleukemia.Nat Med.2015;21(2):178–84.
31.VermaA,SteidlU.Asyntheticlethalapproachtargeting mutantisocitratedehydrogenaseinacutemyeloidleukemia.
NatMed.2015;21(2):113–4.
32.ZhaoJ,NiuX,LiX,EdwardsH,WangG,WangY,etal.
InhibitionofCHK1enhancescelldeathinducedbythe Bcl-2-selectiveinhibitorABT-199inacutemyeloidleukemia cells.Oncotarget.2016;7(23):34785–99.
33.SchwartzJ,NiuX,WaltonE,HurleyL,LinH,EdwardsH,etal.
Synergisticanti-leukemicinteractionsbetweenABT-199and panobinostatinacutemyeloidleukemiaexvivo.AmJTransl Res.2016;8(9):3893–902.
34.NiuX,ZhaoJ,MaJ,XieC,EdwardsH,WangG,etal.Bindingof releasedBimtoMcl-1isamechanismofintrinsicresistance toABT-199whichcanbeovercomebycombinationwith daunorubicinorcytarabineinAMLCells.ClinCancerRes.
2016;22(17):4440–51.
35.LinKH,WinterPS,XieA,RothC,MartzCA,SteinEM,etal.
TargetingMCL-1/BCL-XLforestallstheacquisitionof resistancetoABT-199inacutemyeloidleukemia.SciRep.
2016;6:27696.
36.LuedtkeDA,NiuX,PanY,ZhaoJ,LiuS,EdwardsH,etal.
InhibitionofMcl-1enhancescelldeathinducedbythe Bcl-2-selectiveinhibitorABT-199inacutemyeloidleukemia cells.SignalTransductTargetTher.2017;2:17012.
37.BogenbergerJ,WhatcottC,HansenN,DelmanD,ShiCX,Kim W,etal.Combinedvenetoclaxandalvocidibinacutemyeloid leukemia.Oncotarget.2017;8(63):107206–22.
38.XieS,JiangH,ZhaiX-w,WeiF,WangS-d,DingJ,etal.
AntitumoractionofCDKinhibitorLS-007asasingleagent andincombinationwithABT-199againsthumanacute leukemiacells.ActaPharmacolSin.2016;37:1481–9.
39.SuY,LiX,MaJ,ZhaoJ,LiuS,WangG<ET-AL.TargetingPI3K, mTOR,ERK,andBcl-2signalingnetworkshowssuperior
antileukemicactivityagainstAMLexvivo.Biochem Pharmacol.2018;148:13–26.
40.LehmannC,FriessT,BirzeleF,KiialainenA,DanglM.Superior anti-tumoractivityoftheMDM2antagonistidasanutlinand theBcl-2inhibitorvenetoclaxinp53wild-typeacutemyeloid leukemiamodels.JHematolOncol.2016;9(1):50.
41.KnorrK,SchneiderP,MengX,DaiH,SmithB,HessA,etal.
MLN4924inducesNoxaupregulationinacutemyelogenous leukemiaandsynergizeswithBcl-2inhibitors.CellDeath Diff.2015;22:2133–42.
42.RuvoloP,RuvoloVR,BentonCB,AlRawiA,BurksJK,Schober W,etal.CombinationofgalectininhibitorGCS-100andBH3 mimeticseliminatesbothp53wildtypeandp53nullAML cells.BiochimBiophysActa.2016;1863(4):562–71.
43.TehTC,NguyenNY,MoujalledDM,SegalD,PomilioG,RijalS, etal.Enhancingvenetoclaxactivityinacutemyeloid leukemiabyco-targetingMCL1.Leukemia.2018;32(2):303–12.
44.LinTL,StricklandSA,FiedlerW,WalterRB,HouJ-Z,RobozGJ, etal.PhaseIb/2studyofvenetoclaxwithlow-dosecytarabine
intreatment-naivepatientsage≥65withacutemyelogenous leukemia.JClinOncol.2016;3415Suppl,7007.
45.BogenbergerJM,DelmanD,HansenN,ValdezR,FaubleV, MesaRA,etal.ExvivoactivityofBCL-2familyinhibitors ABT-199andABT-737combinedwith5-azacytidinein myeloidmalignancies.LeukLymphoma.2014;56(1):226–9.
46.DiNardoCD,PratzKW,LetaiA,JonasBA,WeiAH,ThirmanM, etal.Safetyandpreliminaryefficacyofvenetoclaxwith decitabineorazacitidineinelderlypatientswithpreviously untreatedacutemyeloidleukaemia:anon-randomised, open-label,phase1bstudy.LancetOncol.2018;19(2):216–28.
47.SkrticM,SriskanthadevanS,JhasB,GebbiaM,WangX,Wang Z,etal.Inhibitionofmitochondrialtranslationasa
therapeuticstrategyforhumanacutemyeloidleukemia.
CancerCell.2011;20(5):674–88.
48.ReedGA,SchillerGJ,KambhampatiS,TallmanMS,DouerD, MindenMD,etal.APhase1studyofintravenousinfusionsof tigecyclineinpatientswithacutemyeloidleukemia.Cancer Med.2016;5(11):3031–40.
