azathioprine/6-mercaptopurine toxicity: the role of the ... · this review summarizes the outcomes...

251Azathioprine/6-mercaptopurinetoxicity:TheroleoftheTPMTgene

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Review

ANNALS OF GASTROENTEROLOGY 2007, 20(4):251-264

Azathioprine/6-mercaptopurine toxicity: The role of the TPMT geneK.H.Katsanos,E.V.Tsianos

SUMMARY

The thiopurine drugs azathioprine (AZA) and 6-mercap-topurine (6-MP) are widely used in medicine, including in-flammatory bowel diseases (IBD). Thiopurine drugs under-go S-methylation catalysed by thiopurine methyltransferase (TPMT) to methylmercaptopurine or oxidation to thiouric acid via xanthine oxidase (XO). Altered TPMT activity pre-dominantly results from single nucleotide polymorphisms (SNPs) in the TPMT gene, which is located on chromosome 6p22.3.In the general population TPMT enzyme activity is normal in 89%, intermediate in approximately 11% and ab-sent in approximately 0.3% of cases. The prevalence of the most frequent single nucleotide polymorphisms (SNPs) in the TPMT gene has been reported to vary worldwide. The mech-anisms of AZA/6-MP action are currently unknown. Phar-macogenetics of AZA/6-MP represents an interesting field of research with direct implications in clinical practice. AZA/6-MP metabolization steps, the impact of TPMT gene SNPs on toxicity prediction as well as the 6p loci analysis represents a challenging field of research in IBD and other diseases. When possible, TPMT genotyping prior to the initiation of AZA/6-MP should be considered to decrease the risk of severe ad-verse event as well as to identify patients who might benefit from higher doses. Clinicians should be aware that TPMT SNPs do not explain all leucopenic events and that TPMT measurements cannot replace the need for continued moni-toring of leucocyte counts in AZA/6-MP treated patients.

Key words: TPMTgene,singlenucleotidepolymorphism,prev-alence,AZA/6-MP,inflammatoryboweldisease,toxicity.

INTRODUCTION

Thethiopurinedrugsazathioprine(AZA)and6-mer-captopurine(6-MP)arewidelyusedinmedicine,includ-inginflammatoryboweldiseases(IBD).

ThiopurinedrugsundergoS-methylationcatalysedbythiopurinemethyltransferase(TPMT)tomethylmercapto-purineoroxidationtothiouricacidviaxanthineoxidase(XO).1Duringthelasttenyearsalotofclinicalandfun-damentalresearchhasbeenfocusedontheroleofTPMTgeneandTPMTenzymeinpredictingAZA/6-MPefficacyandsideeffects.Indeed,pharmacogeneticsinIBDseemscurrentlytobeofimmediatescientificaswellasofcapitalclinicalimportancealsoinviewofnewtherapies.2-3

Thisreviewsummarizestheoutcomesofthesestud-ies,highlightsthepointsofcurrentinterestinresearchandstressesfundamentalclinicalprinciplesrelatedtoAZA/6-MPtherapy.

IthasbeengenerallyacceptedthatTPMTpolymor-phismsrepresentoneofthebestmodelsforthetranslationofgenomicinformationtoguideIBDpatienttherapeutics

1st Department of Intenal Medicine & Hepato-Gastroenterology Unit, University Hospital of Ioannina, Greece

Author for correspondence:Prof.EpameinondasV.Tsianos,ProfessorofInternalMedicine,1stDepartmentofInternalMedicine,MedicalSchool,UniversityofIoannina,leoforosPanepistimiou,45110Ioannina,Greece,Tel:0030-26510-097501,Fax:00-30-26510-097016e-mail:[email protected]

Abbreviations: AZA/6-MP=azathioprine / 6-mercaptopurineCD=Crohn’s diseaseIBD=Inflammatory Bowel Disease6-MP=6-mercaptopurineSNP(s)=Single nucleotide polymorphism(s)TPMT=thiopurine methyl transferase (gene)TPMT alleles: TPMT*1=G238C,TPMT*3A=A719G & G460A,TPMT *3B= G460A, TPMT *3C=A719GUC=Ulcerative Colitis

252 K.H.KATSAnOS,E.V.TSIAnOS

withandithasbeenalsosuggestedthatTPMTstatusisaneffectivemethodfortailoredthiopurinedrugtherapy.4-5

ClinicallysoundpharmacogeneticstudiesoverthelasttwodecadeshaveshownthatpolymorphismsattheTPMTgenelocusplayasignificantroleintheoccurrenceofvar-ioussideeffectsofthiopurinedrugsincludinglife-threat-eningmyelosuppression,aseriousdose-relatedtoxicity.Inadditiontotoxicity,TPMTpolymorphismshavebeenalsorelatedtoAZA/6-MPtherapeuticefficacy.6-20

AZA/6-MP METABOlIZATION, KINETICS, ACTIONS AND INTERACTIONS

Factors that affect AZA/6-MP pharmacokinetics maybe involved in the following steps: 1) absorption, 2)distribution,3)metabolism(TPMTgene)and4)excretion.FactorsaffectingAZA/6-MPpharmacodynamicsincludereceptorsandtransporters.Todatenocleargeneticfactorsaffecting receptorsor transportershavebeendescribed.Interactions of AZA/6-MP have been described withallopurinol,warfarin21andInfliximab.22

Azathioprinewasintroducedapproximatelyfortyyearsago[Prepn:Hitchings,Elion,USpatent3,056,785(1962)].Azathioprine[6-(1-Methyl-4-nitroimidazol-5-methylthio)purinehasarelativemolecularmassof277.3,itisin-solubleinwaterandveryslightlysolubleinethanolandchloroform.AZA/6-MPmaybedissolvedinwaterwithadditionofonemolarequivalentofalkali.AZA/6-MPisstableinsolutionatneutraloracidicPHbuthydroly-sistomercaptopurineoccursinexcesssodiumhydrox-ide(0.1n).23-26

AZA/6-MPiswellabsorbedfollowingoraladminis-tration.Maximumserumradioactivityoccursatonetotwohoursafteroralingestionanddecayswithahalf-lifeoffivehours.noAZA/6-MPisdetectableinurineafter8hours.Azthioprine/6-MPismoderatelyboundtoserumproteinsandispartiallydialyzable.27-29

Each tablet contains 92.5 to 107.5% of the statedamount.AfteringestionAZA/6-MPisrapidlybrokendowninvivointo6-MPandamethylnitroimidazolemoiety.The6-MPreadilycrossescellmembranesandisconvertedin-tracellularlyintoanumberofpurinethio-analogues,whichincludethemainactivenucleotide,thioinosinicacid.Itisofinterestthattherateofconversionvariesfromoneper-sontoanother.30-34

ThemechanismsofAZA/6-MPactionarepresentlyunknown;manymechanismshavebeensuggestedinor-dertoexplainalsothecommonclinicalplacethatAZA/6-MPtherapeuticeffectsmaybeevidentonlyafterseveral

weeksormonthsoftreatment.Thesemechanismsincludethereleaseof6-MPwhichactsasapurineantimetabo-lite,thepossibleblockadeof–SHgroupsbyalkylation,theinhibitionofmanypathwaysinnucleicacidbiosyn-thesis,hencepreventingproliferationofcellsinvolvedindeterminationandamplificationoftheimmuneresponse,thedamagetodeoxyribonucleicacid(DnA)throughin-corporationofpurinethio-analogues[Figure].

INTRODUCING ThE TPMT GENE AND ITS POlYMORPhISMS

ThehumanTPMTgeneislocatedonchromosome6p22.3andisapproximately34kbinlength,consistingof10exons.TPMTactivityisinheritedasanautosomaldominanttrait.ThehereditarynatureoftheTPMTde-ficiencyinhumanswasinitiallyidentifiedinastudyofTPMTactivityinredbloodcells(RBC).Thisandsubse-quentstudiesdeterminedthedistributionofTPMTactiv-ityinRBCtobetrimodal;90%ofpersonshavehighac-tivity,10%haveintermediateactivityand0.3%havelowornodetectableenzymeactivity.37

AlteredTPMTactivitypredominantlyresultsfromsin-glenucleotidepolymorphisms(SnPs).Todate,numer-ousTPMTalleleshavebeenidentified.38-49Fourprev-alentmutantalleles(TPMT*2,TPMT*3A,TPMT*3B,TPMT*3C)accountfor75%to80%ofTPMTmutationsandareassociatedwithvariousdegreesofreductioninTPMTactivity.

ThemutantalleleTPMT*2isdefinedbyasinglenu-cleotidetransversion(G238C)intheopenreadingframe(ORF),leadingtoanaminoacidsubstitutionatcodon80(Ala>Pro)andresultingina100-foldreductionincatalyticactivitycomparedtothewildtype.Thesecondandmoreprevalentmutantallele,TPMT*3A,containstwonucleo-tidetransitionmutations(G460andA719G)intheORF,leadingtoaminoacidsubstitutionsatcodon154(Ala>Thr)andcodon240(Tyr>Cys).ThealleleTPMT*3BhasonlytheG460Amutationandleadstoanine-foldreductionincatalyticactivity.TPMT*3ChasonlytheA719Gmu-tationandleadstoa1.4reductionincatalyticactivity.Thus,G460AmutationcarriesahigherthanA719Griskoftoxicity.50-54

lABORATORY METhODS OF SCREENING FOR TPMT POlYMORPhISMS

ManymethodsofscreeningforTPMTpolymorphismshavebeendevelopedorareunderevaluation.55-60Thesemethodswillnotbeanalysedinthisreview.


Briefly,thesemethodsincludeanalysisforoneormoreSnPandvisualizationingelsusingpolymerasechainre-action/restrictionfragmentlengthpolymorphismmeth-ods(PCR-RFlP)orothermethodsinvestigatingsimulta-neouslymorethanoneSnP(i.emultiplexPCR).

FinallysequencingoftheTPMTgenestillremainsanaccurate,althoughexpensivemethod.laboratoryexper-tiseandstandardizedtechniqueismandatoryforclear–cutresults.

PREvAlENCE OF TPMT GENE POlYMORPhISMS IN EThNIC POPUlATIONS

Theprevalenceofthemostfrequentsinglenucleotidepolymorphisms(SnPs)intheTPMTgenehasbeenre-portedtovaryworldwide61-86[Table1],howeverapprox-imately90-95%ofthehealthypopulationhasnoTPMTvariantallelewhiletheresthaveoneormoreofthemostfrequent variants. Knowing the prevalence of TPMTSnPsinethnicpopulationsseemstobeofhelpinmoreaccuratelyassessingandpredictingtheriskofAZA/6-MPtoxicity.87-92

PREvAlENCE OF TPMT GENE POlYMORPhISMS IN PATIENT GROUPS

TMPTturnsoutnowadaystobean‘oldstory’butasAZA/6-MParemoreandmoreextensivelyusedinmanydiseasesincludingIBDtheneedforpredictingtoxicityisconstantlyrising.

Itseemsthatthelackoflargescalecomparativestud-iesonTPMTpolymorphismsbetweenpatientgroupsandbackgroundpopulationismainlyduetothefactthatthereisstillconflictingdataonwhetherTPMTgenotypeorTPMTactivityaresafeinpredictingcommonadverseeventstoAZA/6-MP.Infact,therearestudiesinfavouroragainsttheclinicalimportanceofTPMTgenotypinginAZA/6-MPtreatedgroupsofpatients.

Inthestudyfromleuven66ithasbeendemonstratedthatTPMTallelefrequencymaydifferbetweenIBDpa-tientsandhealthycontrolpopulations.ThisstudyisnottheonlyonetosupportthatTPMTpolymorphismsmaydifferbetweenapatientandacontrolgroupfromthesameethnicorigin.AsignificantdifferencefortheTPMT*3Callelehasalreadybeendemonstratedinlupuserythema-tosuspatients,93inchildrenwithacuteleukemia40,94and

Figure. Azathioprine, 6-mercaptopurine and 6-thioguanine metabolization.


patientswithneurologicaldiseases,95 allofthemunderAZA/6-MPmaintenancetherapy[Table2].

TPMThasbeenthoroughlyinvestigatedasitssub-stratesAZA/6-MPcancause,amongothersideeffects,bonemarrowsuppressionandaccordingtoareviewofnineretrospectivestudiestherewasa3.2%overallfre-quencyofleucopeniainIBDpatientstreatedwithAZA/6-MP.96However,noneofthesestudiesprovidedlargecom-parativedataontheprevalenceofTPMTpolymorphismsofIBDpatientscomparedtobackgroundpopulation.

Furthermore,arestrictednumberofstudieshasimpli-catedthatIBDpatientshavethesameprevalenceandthesamepatternofTPMTmutatedallelescomparedtoback-groundpopulations[Table 3].Threestudies97-99suggestedthatthefrequencyofvariantallelesdoesnotdifferbetweenIBDpatientsandhealthycontrolsfromthesameback-groundCaucasianpopulation.Itisofimportancetomen-tionthatTPMTvariantallelesmayvaryamongstudiesinCaucasiansandthismayreflectethnicdifferences.66

Webelievethatthisissuestillremainsdebatableand

Table 1. PrevalenceoffrequentTPMTgeneallelesinethnicgroups.Ethnic group No of al-

lelesTPMT*1 TPMT*2 TPMT*3A TPMT*3C Reference

AmericanCaucasians 564 0.963 0.0017 0.032 0.0017 Honetal.61AmericansAfrican 496 0.954 0.004 0.008 0.024 Honetal.61AsiansSouthwest(British) 198 0.990 0 0.010 0 Collie-Duguidetal.62AsiansSoutheast 600 0.993 0.0017 0 0.050 Changetal.63Asians(British) 170 n/A* n/A* 0.011 0.047 Marinakietal.64Argentinian 294 0.960 0.0068 0.031 0 larovereetal.65Belgians 742 0.894 0.008 0.08 0.0013 Katsanosetal.66BritishCaucasians 398 0.947 n/A 0.045 0.003 Ameyawetal.67BritishCaucasians 398 n/A 0.004 0.045 0.003 Mcleodetal.68BritishCaucasians 398 0.948 0.005 0.045 0.002 Collie-Duguidetal.62Brasilian(mixed) 306 n/A 0.0082 0.0163 0.0212 Reisetal.69Chinese 384 0.977 n/A 0 0.023 Collie-Duguidetal.62Chinese(Han) 550 n/A n/A n/A 0.026 Zhangetal.70Chinese(Han) 624 n/A n/A n/A 0.022 Zhangetal.70Chinese(Jing) 206 n/A n/A n/A 0.019 Zhangetal.70Chinese(Yao) 252 n/A n/A n/A 0.037 Zhangetal.70Chinese(Uygur) 320 n/A n/A 0.00625 0.03125 Zhangetal.71Colombian 280 0.960 0.0038 0.036 0 Isazaetal.72Egyptian 400 0.984 0 0.003 0.013 Hamdyetal.73FrenchCaucasians 382 n/A 0.005 0.057 0.08 DelaMoureyreetal.74FrenchCaucasians 560 n/A 0.01 0.059 0.07 Ganiere-Monteiletal.75Germans 2428 0.898 0.005 0.08 0.006 Schaefeleretal.57Ghanaians 434 0.924 0 0 0.076 Ameyawetal.67Italians 206 n/A 0.005 0.039 0.01 Rossietal.76Indians 400 0.987 0 0.005 0.008 Khametal.77Japanese 1044 0.984 n/A 0 0.015 Kumagaietal.78Japanese 142 0.979 n/A 0 0.014 Andoetal.79Japanese 302 n/A 0 0 0.003 Kubotaetal.80Japanese 192 n/A 0 0 0.008 Hiratsukaetal.81Japanese 88 n/A 0 0 0.022 nishidaetal.82Kenyans 202 0.946 0 0 0.054 Mcleodetal.68Malayans 400 0.975 0 0 0.023 Khametal.77newZealand(Caucasians) 200 0.950 0 0.05 0 Gearryetal.83Polish 716 n/A 0.004 0.027 0.001 Kurzawskietal.84Saami(norway) 388 0.969 0 0 0.033 loennechenetal.85Swedish 1600 0.956 0.00063 0.0375 0.0044 Haglundetal.56Taiwanese 498 0.994 0 0 0.014 Changetal.63Thai 400 0.950 n/A 0 0.050 Shrimartpirometal.86

*N/A=not available or not investigated


Table 2. TheTPMT*3Calleleprevalenceinpatientgroupstreatedwithazathioprine/6-MP.Author Patient group Controls p-value

Okadaetal.93 Systemiclupuseryhtematosus(n=68) 174 0.23

Alvesetal.40 Acutelymphoblasticleukemiachildren (n=43)

43


betweenTPMTgenotypeandthedevelopmentoftheseadverseevents.Additionally,areportsuggestedAZA/6-MPintolerancemaybeimidazole-relatedandisindepend-entofTPMT.110

Anyway,thegreatmajorityofthesestudieswereofretrospectivedesign.WebelievethataprospectivestudymaybemoreinformativeontherealclinicalimpactofscreeningforTPMTpolymorphismsinpatientsonAZA/6-MPtherapy.

ClINICAl vAlUE OF TPMT GENE POlYMORPhISMS IN IBD

AZA/6-MPcancausebonemarrowsuppressionandhasbeenassociatedwithleucopenia,thrombocytopenia,macrocytosiswithoutmegaloblasticanemiaandrarelypureredcellaplasia.17

TPMTseemsofimportanceinIBDasthelargeststudy108onAZAtoxicityreporteda3.8%prevalenceofleucopeniain739patientstreatedwithAZA/6-MP;inthiscohorttwopatientsdiedasaresultofpancytopeniaandsepsis.

Inacosteffectivenessanalysisstudyithasbeensug-gestedthatin1000IBDpatientstreatedwithAZA/6-MP,32willdevelopmyelosuppressionandonewilldiebe-causeofthis.OfthosewhodevelopmyelosuppressionduringAZA/6-MPtherapyonly32%isattributedtolowTPMTenzymeactivity.96

TherearemanystudiessupportingtheclinicalvalueofscreeningforTPMTgeneSnPinIBDpatientsbeforeorduringAZA/6-MPtherapy.Accordingtoastudyhalfofthepatientswithoneortwonon-functionalTPMTmutantal-leleswilldevelopAZA/6-MPintoleranceleadingtowith-drawaloftherapy.99AnotherstudyshowedthatpatientswithCrohn’sdisease(CD)andnormalTPMTactivitywhowerestartedonhighdoseAZA/6-MPandpatientswithin-termediateenzymeactivitywhowerestartedonreduceddosesofAZA/6-MPdidnotdevelopacuteleucopenia.111

EventhoughanotherstudyconfirmedtheefficiencyofTPMTgenotypinginidentifyingpatientsatriskformy-elosuppressionithaditslimitationasonly27%ofpatientscarryingthemutatedTPMTallelehadaparallelenzymedeficiency.9AnotherstudynotfavouringTPMTgenotyp-ingshowedthatonlyoneofthirteenpatientswhoexperi-encedleucopeniawasheterozygousforTPMT.

Thevastmajorityofpatientswithdrugrelatedtoxic-ityhadawildtypeTPMTgenotype.112In56patientswithIBDTPMTgenotypedidnotpredictadversereactionstoAZA/6-MP.83Inaddition,AZA/6-MP-relatedgastrointes-

tinalsideeffectshavebeensuggestedtobeindependentoftheTPMTpolymorphisminaretrospectiveanalysisofIBDpatientstakingAZA/6-MP.49

WehavetostressherethatthesestudiesreferredtothethreemorefrequentTPMTSnPs.However,hematotox-icitymayoccurintheabsenceofTPMT*2,TPMT*3A,TPMT*3BorTPMT*CvariantsduetopresenceofotherrareTPMTvariantsorotherfactorsincludingviralinfec-tions,drugsorenvironment.113-120

TosummarizeitseemsthatTPMTgenotypingisabletopredictsomebutnotthemajorityofAZA/6-MPcaseswhichwilldeveloptoxicityincludingbonemarrowtoxicity.

ClINICAl vAlUE OF TPMT POlYMORPhISMS IN NON-IBD PATIENT GROUPS

Inrheumatoidarthritispatientsgastrointestinalintol-erancehasbeenrelatedtothiopurinemetabolicimbalanceresultinginasignificantrelationshipbetweentoxicityandabnormalTPMTactivity.121

InastudywithneurologicalpatientstakingAZA/6-MPithasbeensuggestedthatTPMTgenotypingispref-erabletoTPMTactivitydetermination;95however,onlyTPMThomozygousdeficiencywasassociatedwithse-veremarrowsuppressioninastudywithlupuserythema-tosuspatients.122

Improvedmethodologyformonitoringthiopurineme-tabolitesinpatientsonthiopurinetherapiesismandatoryinordertofacilitateaccurateclinicaldecisions.123

AZA/6-MP METABOlITES MONITORING

TherehavebeenmanystudiesinfavouroforagainstthemonitoringofAZA/6-MPmetabolitelevels.124-136Infa-vouringstudies,hepatotoxicitycorrelatedwithveryhigh6-MMPlevels.Itisnottheaimofthisreviewtofurtherdiscussthemethodsofmonitoringthesemetabolites.WewouldliketostressherethatAZA/6-MPmetaboliteas-sessmentrequiresexpertise,standardizedmethodsandisindicatedonlyincaseswithunexplaineddose-relatedsideeffectsinpatientswereAZA/6-MPuseismandatoryfordiseaseremission.

TPMT ENZYME lEvElS IN ClINICAl PRACTICE

TPMTenzymeisalreadymatureinbirthandpractical-lynodifferencesexistbetweenchildrenandadults.75


SomebutnotallstudieshaveindicatedthatAZA/6-MPtherapymonitoringwitherythrocyte6-thioguaninenucle-otidesmaybeclinicallyuseful.

Ithasbeensuggestedthatpatientswithloworinter-mediateTPMTactivityaccountforapproximatelyhalfofearlyleucopenia(withinthefirst6monthsofAZA/6-MPinitiation)whereaspatientswithnormalTPMTactivityaccountedforapproximately50%oftheearlyleucopeniaandnearlyallthelate(after6monthsoftherapy)leuco-peniacases.1Itmustbestressedhere,thatsomeauthorsdefineas‘early’allleucopeniacasesdiagnosedthefirst2-3monthsofAZAtherapy,while‘intermediate’casesarethosediagnosedduringthe4-7monthperiodofAZAtherapy.

AstudysuggestedthatTPMTdeficiencysignificantlycorrelateswithhematopoietic15toxicityandanotherstudyshowedthatTPMTactivitywassignificantlylowerinpa-tientswhodiscontinuedAZA/6-MPduetoleucopeniathaninthosewhodiscontinuedduetoothersideeffects.137

Incontrast,anotherstudysupportedthat measurementofTPMTactivityhasnospecificroleinidentifyingriskofhaematologicaltoxicity.88Alongtheselines,inadditiontoahighdegreeofvariabilityinTPMTactivitywithinboththehomozygouswildtypeandheterozygousgroups,someindividualswithaheterozygousgenotypeexhibithighac-tivitywhereassomehomozygouswildtypesubjectsex-hibitanintermediatephenotype;attentionhastobepaidalsototransfusedindividuals.

SuchdiscrepanciesareduetothefactthattheSnPsdiscussedsofararenottheonlyfactorsregulatingcata-lyticactivity.Populationgeneticstudieshaveshownthatthegenotype,whichregulatesTPMTactivity,accountedforapproximatelytwo-thirdsofthetotalvarianceinthelevelofRBCenzymeactivity.Otherfactorssuchaspro-moterpolymorphisms,druginteractions,andenvironmen-talfactorscouldplayanimportantroleinthefinalTPMTactivityphenotype.138-158

Bloodlevelsofthedrugareoflittlepredictiveval-uefortherapysincethemagnitudeanddurationofclini-caleffectsandsideeffectsseemtocorrelatewiththiopu-rinenucleotidelevelsintissuesratherthanwithplasmadruglevels.

AdefiniteconsensusontheclinicalvalueofTPMTen-zymelevelsdeterminationiswishful.Wedobelievelikeothers159-168thatcombininginformationonTPMTgeneSnPsandTPMTenzymelevelscouldbeofimportancenotonlytopredicttoxicitybutalsounrevealingAZ/6-MPkineticsandmechanismofactioninseveraltissues.

FURThER PERSPECTIvES IN IBD RESEARCh: TPMT lOCUS AND OThER lOCI

ScreeningfornewTPMTpolymorphismsisofimme-diateinterestinIBDnotonlyinviewoffirmlyassessingtheriskoftoxicityduringAZA/6-MPtherapybutalsoinviewofunrevealingdiseaseetiopathogenesis.

TPMTgeneislocatedonchromosome6p.Previousas-sociationandlinkageanalysishasprovidedsomeevidenceastotheexistenceofanIBD/CD-susceptibilitylocus,re-ferredtoasIBD3,inthisregion.169-173Itisalsoofinter-estthatonchromosome6parelocatedthehuman-leuko-cyte-antigen(HlA)region,themajorhistocompatibilitycomplex(MHC)174-175regionandthetumournecrosisfac-tor(TnF)-agene,176allofthemassociatedwithIBDsus-ceptibility.Interestingly,thegroupofIBDpedigreesthatcontainedoneofthethreeCARD15variantshadtwosug-gestivelinkageresultsoccurringin6pand10p.177

Bycontrast, otherpolymorphismslocatedonchro-mosome6p,includingalsosomeHlAandMHCpoly-morphisms,werenotassociatedwithanincreasedriskforIBD.178-180

CONClUSIONS

PharmacogeneticsofAZA/6-MPrepresentsaninter-estingfieldofresearchwithdirectimplicationsinclinicalpracticeandfundamentalresearch(Table4).181-187AZA/6-MPmetabolizationsteps,theimpactofTPMTgeneSnPsontoxicitypredictionaswellasthe6plocianalysisrep-resentsachallengingfieldofresearchinIBDaswellasinotherdiseases.

Whenpossible,routineTPMTgenotypingpriortotheinitiationofAZA/6-MPshouldbeconsideredtodecreasetheriskofasevereadverseeventaswellastoidentifypatientswhomightbenefitfromhigherdoses.AlthoughTPMTtestingdeernoteliminatetheriskofbonemarrowtoxicityithasthepotentialtowarnearlyofalife-threat-

Table 4.TheclinicalusefulnessofTPMTtesting.• PossibilitytostartfromveryearlytheoptimaldoseofAZA/6-

mercaptopurine• PossibilitytofurtherincreasethedoseofAZA/6-mercaptopurine

(upto3mg/kg)• Possibilitytopreventearlyorintermediateleucopeniainhighrisk

patients(withgenehomozygousorheterozygouspolymorphisms)• TPMTtestingcannotreplacetheneedforregularperipheralblood

testinginallpatientsonAZA/6-mercaptopurinetherapy.


eningadverseeventduetoverylowTPMTenzymeactiv-ityinhomozygousrecessivepatients.

Thus,cliniciansshouldbeawarethatTPMTSnPsdonotexplainallleucopeniceventsandthatTPMTmeasure-mentcannotreplacetheneedforcontinuedmonitoringofleucocytecountsinAZA/6-MPtreatedpatients.

ACKNOWlEDGMENTS

-DrKonstantinosH.KatsanoswasaninternationalgrantrecipientoftheHellenicSocietyofGastroenterol-ogy(2005-2006)andalsoreceivedagrantfromtheHel-lenicIBDstudyGroup(2006).

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