一般临床药物代谢动力学回顾

GeneralClinicalPharmacokinetics

Review2UniversityofFloridaWorkingProfessionalDoctorofPharmacyProgramChristineCrain,Pharm.D.,BCPSClinicalQuestion?A60yearoldwomanistakingquinidinefortreatmentofatrialfibrillation.ShehasalsobeenprescribedfluconazolefortreatmentofaCandidaUTIandcisapridetocontrolsxofGERD.Wouldyouexpectanyofthesedrugstointeractbasedonthehepaticenzymesbywhichtheyaremetabolized?

AnswertoClinicalQuestionHepaticEnzyme(s)AffectedQuinidine Inhibits2D6 Metabolizedby3A4Fluconazole Potentialinhibitor of3A4Cisapride Metabolizedby3A4DrugEliminationLiver-PrimarilyMetabolismKidney-PrimarilyExcretionHepaticClearanceTheefficiencyoftheliverinremovingdrugfromthebloodstreamisreferredtoastheextractionratio(E).Withhighextractiondrugs,Eiscloserto1Withlowextractiondrugs,Eiscloserto0HepaticClearanceCLH=QHFpCLiQH+FpCLiQH=HepaticbloodflowFp=FractionoffreedruginplasmaCLi=IntrinsicclearanceHepaticClearanceCLH=QHFpCLiQH+FpCLiDrugswithahighintrinsicclearance=QHFpCLiFpCLiCLH=QHHepaticClearanceCLH=QHFpCLiQH+FpCLi=QHFpCLiQHCLH=FpCLiDrugswithalowintrinsicclearanceIntrinsicClearanceHigh

CLiLow

CLipropranolol warfarinlidocaine phenytoinmorphine theophyllinenitroglycerin diazepamClinicalQuestion?Smokingisknowntoincreasetheenzymesresponsiblefortheophyllinemetabolism.Wouldapatientwithahistoryofsmokinglikelyrequireahigher,lower,orequivalenttheophyllinetotaldailydosecomparedtoanonsmoker?ClinicalQuestion?Apatientinthecoronarycareunitisstartedonlidocainefortreatmentofventriculararrhythmias.Ifthispatienthasseverecongestiveheartfailure(EF<20%),howwouldtherequireddailydosageoflidocainelikelybeaffected?AnswertoClinicalQuestionCLH=QHFpCLiQH+FpCLiCLH=QHCHFDecreasesQHHalf-LifeThetimefortheconcentrationofdrugintheplasmatodecreasebyone-halfconc(logscale)1051**Time(hours)123SteadyStateConcTimeCavg,ssCmax,ssCmin,ssSteadyStateDurationofDrugAdministration(Half-Lives)SteadyStateConcentrationReached1 502 753 87.54 93.755 96.8756 98.47357 99.25ClinicalQuestion?Apatientisadmittedtothehospitalwithcarbamazepinetoxicity.Aninitialserumconcentrationis24mcg/ml.Assumingahalf-lifeofapproximately18hours,howlongwillittakeforthispatienttoreachaserumlevelof6mcg/ml?AnswertoClinicalQuestion24mcg/ml12mcg/ml6mcg/ml18hours18hours36hrsClinicalQuestion?Apatientisstartedondiltiazemforthetreatmentofhypertension.(InitialaverageBP=160/98)Adosageregimenofdiltiazem60mgpotidisinitiated.Approx.howlongwillittakeforthispatienttoreachsteadystate?AnswertoClinicalQuestionHalf-lifeofdiltiazem=6hoursSteadystateshouldbeachievedinapproximately5half-lives.Therefore,thispatientshouldreachsteadystateinapproximately30hours.ClinicalQuestion?A65yearoldmanisstartedondigoxin0.125mgpoqdforthetreatmentofcongestiveheartfailure.Ifaloadingdoseisnotgiven,approxi-matelyhowlongwillittakeforthispatienttoreachasteadystatedigoxinlevel?AnswertoClinicalQuestionThehalf-lifeofdigoxinisapprox.40hoursSteadystateshouldbeachievedbyapproximately5half-lives.Therefore,thispatientshouldachievesteadystatedigoxinlevelsinapproxi-mately8-9days.EliminationRateConstant(ke)Thefractionofdrugremovedperunitoftime(e.g.hour-1)Time(hours)123456512.3DrugConc.(logscale)**(Co,to)(C1,t1)ln

C1=-ket

+ln

CoFirst-OrderEliminationTime(hours)123456512.3DrugConc.(logscale)**(Co,to)(C1,t1)slope=-ke=lnC1-lnCot1-toEliminationRateConstantTime(hours)123456512.3DrugConc.(logscale)**(Co,to)(C1,t1)slope=-ke=ln5-ln12.34

hr-0hrEliminationRateConstantTime(hours)123456512.3DrugConc.(logscale)**(Co,to)(C1,t1)slope=-ke=-0.225hrClinicalQuestion?A45yearoldmanisreceivinggentamicin160mgIVq12hr.Apeakandtroughlevelweredrawnatsteadystate.Peak = 8.4mcg/ml 0900Trough= 1.0mcg/ml 2000Whatisthispatient’seliminationrateconstant(ke)andhalf-life?FirstOrderEliminationconctimekeslope=-ke=lnC1-lnC0T1-T0**AnswertoClinicalQuestion-ke=lnC1-lnC2t1-t2-ke=ln8.4-ln1.011hr=-0.1935/hrRelationshipBetweenDrugEliminationandHalf-lifeke=0.693t1/2AnswertoClinicalQuestionHalf-life=0.693keHalf-life=0.6930.1935=3.58hrClinicalQuestion?A65yearoldpatientisadmittedtothehospitalwithdigoxintoxicity.Thedigoxinserumconc.is5ng/ml.Howlongshouldittakefortheserumdigoxinconc.todecreaseto1ng/ml?First-OrderEquationCt=Coxe

-ketAnswertoClinicalQuestionThehalf-lifeofdigoxinisapproximately40hours.ke=0.693t1/2=0.0173/hrCt=Coxe

-ket1=5xe-.0173t

t=93hoursMaintenanceDoseEquationko=CssxCLsMD=CssxCLsCL=kexVdMD=CssxkexVdRelationshipBetweenClearance,VdandHalf-lifeCL=0.693t1/2VdLDvsMDEquationsLD=CpxVdMD=CssxkexVdClinicalQuestion?A50yearoldmanistobestartedonamaintenancedoseofprocainamideforthetreatmentofseveralepisodesofatrialfibrillation.CalculateaMDthatwillproduceanaveragesteadystateprocainamideconcentrationsofapproximately6mcg/ml.Assumenlrenalfunction.Wt:68kgAnswertoClinicalQuestionMD=CssxkexVd=MD=Cssx0.693xVdt1/2=6mg/Lx0.6934hrx2L/kgx68kg=141mg/hrx24hours=3400mgAnswertoClinicalQuestionAssumingapprox.85%absorption,totaldailydose=4000mgGiveasProcanSR1000mgq6h ORProcanbid2000mgpoq12hRenalEliminationGlomerularFiltrationTubularsecretionTubularreabsorptionCreatinineClearanceCockcroft&GaultEquationCLcr(ml/min,males)=(140-age)(weight)Crsx72CLcr(ml/min,fem)=(0.85)(CLcrinmales)EstimatedEliminationRateConstantforAminoglycosidesGentamicin,Tobramycinke=0.0024(CrCl)+.01Amikacinke=0.00279(CrCl)+.01Vancomycinke=0.00083(CrCl)+.0044ClinicalQuestion?A60yearoldwomanintheintensivecareunithasPseudomonaspneumoniaandisstartedonceftazidimeandtobramycin.Thephysicianasksyoutorecommendaninitialdosageregimenfortobramycin.Wt=52kgHt=5’4”SCr=1.4Whatinitialregimenwouldyourecommend?AnswertoClinicalQuestionIBW=45.5kg+(2.3x4in.)=54.7kgActualweight=52kgDosingweight=52kgLD=CpxVdLD=10mcg/mlx0.3L/kgx52kg=156mg(roundto160mg)AnswertoClinicalQuestionDesiredTrough=0.5mcg/mlEst.ke=0.0024(CrCl)+0.01EstCrCl=48ml/min(x0.85forfemale)=40.8ml/mn

(Pleaseuse48ml/minastheCrClforthisquestionsincetheauthorneglectedtomultiplyby0.85forfemaleandthecalculationscontinueonusing48ml/min.)Estke=0.0024(48)+0.01=0.125140-60yrs x 52kg =1.2 72kgAnswertoClinicalQuestionHalf-life=0.693ke=0.6930.125=5.54hours10mcg/ml5mcg/ml2.5mcg/ml1.25mcg/ml0.625mcg/ml5.54hoursx4=22hoursAnswertoC