TherapeuticDrug Management

Mike HallworthACB Training CourseCoventry, 17 June 2013

Pharmacokinetics

Processes involved in drug handling

Dose Prescribed

Dose taken

Drug in blood

Activesite EFFECT

Other tissues

Excreted/inactivated drug

concordance

absorption

Metabolism/elimination

distribution

pharmacokinetics pharmacodynamics

Concordance (compliance)Often poor. Improved by simplifying regimesAssessment:

Was Rx dispensed?Tablet countsDirect observation/ interviewMarkersDrug/metabolite levelsEffect

Bioavailability

Bioavailability, F = Dose absorbedDose administered

(IV dosing, bioavailability = 1)

First-pass metabolismMetabolism en route from gut to systemic circulation –in the LIVER

Drugs with extensive first-pass metabolism:Analgesics (aspirin, morphine, paracetamol, pethidine)

CNS-active drugs (chlormethiazole, chlorpromazine, imipramine. L-dopa, nortriptyline)

Cardiovascular (glyceryl trinitrate, isosorbide dinitrate, lignocaine, nifedipine, propanolol,

verapamil)

Respiratory (salbutamol, terbutaline)

Oral contraceptives

First-pass metabolism

One reason for apparent variations in drug “absorption” between individuals

Reduced in severe liver disease Greatly increased delivery of drug to active site

Salt-conversion factor (S)

Factor relating to the actual concentration of active drug in the preparation being used

e.g. 108 mg phenytoin sodium = 100 mg phenytoine.g. 200 mg aminophylline = 160 mg theophylline(aminophylline is the EDTA salt of theophylline, S = 0.8)

So, DRUG ABSORBED = S x F x DOSE ADMINISTERED

Distribution

We define:

Volume of Distribution, Vd = Amount of drug in body

Plasma concentration

Volume of distribution is the THEORETICAL VOLUME that would contain all the drug in the body if it were present everywhere at the same concentration that is found in plasma

Volume of distributionFor example:

Vd (L/kg)Amiodarone 1.3Digoxin 7.3Phenytoin 0.65Aspirin 0.14

Vd is LOW if – low lipid solubility- high plasma protein binding- low tissue binding

(and vice versa)

Calculation of size of loading dose

DOSE = Vd x desired concentration

E.g. I want to give a 70 kg man a loading dose of digoxin to produce a plasma concentration of 1.5 µg/L. The bioavailability of digoxin is 0.62

DOSE x 0.62 = 7.3 x 70 x 1.5= 1236 µg

(The usual approach would be to give this in divided doses)

Metabolism and Excretion

Metabolism – usually hepatic

Excretion - via kidneys into the urine- via liver into the bile

Both these processes are encompassed in the pharmacokinetic parameter CLEARANCE

Clearance

DefinitionThe theoretical volume of blood which can be completely cleared of drug in unit time(cf. creatinine clearance)

So,

RATE OF ELIMINATION = Clearance x Plasma conc.= Cl x C

At Steady StateRate of administration = Rate of elimination

(definition of steady state)

So,DOSE x S x F = Cl x C

Τwhere Τ is the dosage interval

Clearance = Dose x S x FΤ x C

Or,

Maintenance dose = Cl x C x ΤS x F

Clearance

If renal clearance of a drug is approximately 120 ml/min, this suggests that the drug is completely cleared by the kidneys and GFR is the only limiting factor to excretion.If hepatic clearance is of the order of 500 –1500 ml/min, this reflects hepatic blood flow and suggests that blood flow is the limiting factor on excretion (perfusion-limited clearance) e.g. lignocaine, morphine.

Clearance

Frequently expressed asVolume/unit time/kg body weight

A more accurate basis for comparison between patients is

Volume/unit time/unit body surface area

Surface area can be determined from standard nomograms

First-order kinetics

Rate of elimination ∝ C(assumes Cl is independent of conc)

Amount of drug in = A0e-kt

body at time t

where A0 is the amount at time 0 and k is the elimination rate constant - the percentage elimination per unit time

Elimination rate constant

k = Amount cleared in unit timeTotal amount in body

= Cl x CVd x C

= ClVd

Elimination rate constantOften expressed in terms of half-life, t½

Ao = Ao . e-kt½

2

ln (½) = - k.t½

t½ = 0.693k

Peak and trough concentrations

Conc

Time

Τ

Peak and trough concentrationsAt steady state:

Cssmax = S x F x Dose + Cssmaxe-kΤ

Vd

Cssmax ( 1 – e-kΤ ) = S x F x DoseVd

Cssmax = S x F x DoseVd ( 1 – e-kΤ )

Peak and trough concentrations

Cssmax = S x F x DoseVd ( 1 – e-kΤ )

Cssmin = S x F x Dose . e-kΤ

Vd ( 1 – e-kΤ )

Plasma protein binding

Primarily to ALBUMIN and α1 acid glycoprotein% plasma protein binding

Lithium 0%Gentamicin <10%Digoxin 20%Theophylline 60%Carbamazepine 70-80%Phenytoin 90-94%

Reduced plasma protein binding

Reduced protein binding (more free drug)

Low protein (liver disease, pregnancy, nephrotic syndrome)Displacement (renal failure, other drugs)Abnormal binding proteinsConcentration-dependent binding

0

50

100

150

0

Plasmaconc.

umol/L

100 200 300 400 500 600 700

GW

PHe

Dose mg/day

PHo

Non-linear (saturation) kinetics

Non-linear (saturation) kineticsMichaelis-Menten equation

Rate of elimination = Vmax x CKm + C

If C << Km Rate = Vmax x C FIRSTKm ORDER

If C>> Km Rate = Vmax x C = VmaxC

ZERO ORDER

Non-linear kinetics

Phenytoin:Km ranges from 0.5 to 15 mg/LVmax ranges from 3 to 12 mg/kg/day

Q1:

How much Chateau Plonque is required to put a 60 kg woman above the legal driving limit 1 hour after ingestion?

(assume:

Rapid ingestion and absorption

Elimination = 100 mg/kg/h

Total body water in women = 55% of total body weight

Chateau Plonque = 13% alcohol by volume

Density of ethanol = 0.8 g/mL

Atmospheric pressure at sea level = 101.325 kPa)

Therapeutic drug management

• Conventional TDM• Biomarkers of drug effect

• Pharmacodynamic monitoring

• Pharmacogenomics

Therapeutic Drug Monitoring

Definition:“Measuring drug or metabolite concentrations in

body fluids as an aid to optimising therapy”TIAFT definition (1997):“Measurement made in the laboratory of a

parameter that, with appropriate interpretation, will directly influence prescribing”

Patient

Drug

Initial dose

Revise dose

EffectPlasmaconcentration

Measure Measure

Interpret

Criteria for valid classical TDM• Drug has reversible action at receptor site• Dose has poor correlation with effect• Plasma concentration correlates well with

effect• Narrow therapeutic ratio• Well-established therapeutic range

Drugs for routine measurement

Established value• Aminoglycosides• Carbamazepine• Ciclosporin• Digoxin• Lithium• Methotrexate

• Phenytoin• Sirolimus• Tacrolimus• Theophylline• Vancomycin

Drugs for routine measurementLess well -established

• Amiodarone• Anti-retrovirals• Caffeine• Chloramphenicol• Clozapine• Disopyramide• Flecainide• Flucytosine• Haloperidol

• Lamotrigine• Mycophenolate• Olanzapine• Phenobarbital• Procainamide• Quinidine• Tricyclics• Valproate

Epilepsy

• Commonest neurological disorder• Highest therapeutic potential• 0.5-1% of population (0.75% UK)• 50 million worldwide• 5% will have fits at sometime during

life (excluding febrile convulsion)• 30-50% of patients still have fits

despite Rx

Antiepileptic drugs (UK)Bromide 1857Phenobarbitone 1912Phenytoin 1938Ethosuximide 1960Carbamazepine 1972Valproate 1972Clonazepam 1974Clobazam 1978Acetazolamide 1988Vigabatrin 1989

Lamotrigine 1991Gabapentin 1993Topiramate 1996Tiagabine 1998Oxcarbazepine 2000Levetiracetam 2000Pregabalin 2004Zonisamide 2005Rufinamide 2007Lacosamide 2008Eslicarbazine 2009Retigabine 2011

Epilepsy Rx (NICE, 2012)

ABSENCE GENERALISED FOCAL MYOCLONIC TONIC/TONIC-CLONIC (PARTIAL) ATONIC

Ethosuximide Valproate* CBZ Valproate* Valproate*Valproate* Lamotrigine Lamotrigine LevetiracetamLamotrigine Carbamazepine Levetiracetam Topiramate

Oxcarbazepine (cost)

OxcarbazepineValproate* _

Clobazam Clobazam Clobazam Clobazam LamotrigineClonazepam Levetiracetam Gabapentin Clonazepam RufinamideTopiramate Topiramate Topiramate Piracetam TopiramateZonisamide (Vigabatrin) ZonisamideLevetiracetam Retigabine

* NB teratogenicity

Epilepsy - Rx

Stabilize neuronal resting potential(mechanisms not well understood)

Inhibit excitation (Na/Ca channel blockers)(e.g. lamotrigine, phenytoin)

Enhance inhibition (GABA enhancers)(e.g. benzodiazepines, tiagabine, vigabatrin)

Modify cell excitability

Krebs

Glutamate Succinate

GABA GABA-TGAD

GABA(inhibitory)

presynapticneurone

Vigabatrin(inhibition)

Tiagabine(blocking)

BenzodiazepinesBarbiturates(potentiation)

postsynapticneurone

PhenytoinLong half-life (20 – 40 h, up to 100h at high concs)

timing unimportantSaturation kineticsMetabolism – hepatic oxidation

Faster in infants/children, slower in pretermInc by ethanol and carbamazepine, dec by enzyme inducers (e.g. cimetidine)

90-93% protein-boundSide effects – neurotoxicity (nausea, ataxia, drowsiness)Target range 10-20 mg/L (40-80 umol/L)

Only a guide – wide variationVery strong case for routine monitoring

0

50

100

150

0

Plasmaconc.

umol/L

100 200 300 400 500 600 700

GW

PHe

Dose mg/day

Phenytoin

PHo

(Richens and Dunlop. 1975)

CarbamazepineShort half-life (8-24h)

Trough samples preferableTarget range 4-10 mg/L (17-42 umol/L)Lower limit of range difficult to defineProtein binding 22-30%Side effects – rash (5%), haematological (rare), neurotoxicity (mild), ADH stimulation (hyponatraemia)Induces own metabolism (hepatic oxidation)Clearance inc by phenytoin/phenobarbitone, dec by enzyme inhibitorsMonitor if control difficultActive metabolite (CBZ 10,11 epoxide)

Metabolism of carbamazepine and oxcarbazepine

N

O

NH2O

N

NH2OOxcarbazepine Carbamazepine

ACTIVE

N

HO

NH2O

N

NH2O

O

10-hydroxy carbazepineACTIVE

Carbamazepine10,11 epoxideACTIVE

N

NH2O

HO OH

Carbamazepinetrans-diol

VigabatrinGamma-vinyl GABASuicide inhibitor of GABA transaminaseIrreversible - long pharmacodynamic half-lifeMonitoring not helpfulVisual field defects in 30%

GABA + α-ketoglutarate

Succinic semialdehyde + glutamate

VigabatrinRapidly absorbedElimination half-life 5-7 hoursBioavailability 80%Not protein-boundRenal eliminationChiral - only S-enantiomer activeVisual field defects limit use

HC

CH CH2

H2N

H2C C

OH

OCH2

LamotrigineRapidly absorbedBioavailability >95%Protein binding 55%Half life 24 - 35 h (varies on combination Rx)Hepatic metabolismTarget range

<15 mg/L ??

ClCl

N

NN

NH2H2N

TopiramateBioavailability 85%Protein binding 15%Half life 20 - 30 hTarget range ?? O

CH2OSO2NH2

OO CH3

CH3

O

OH3C

H3C

GabapentinBioavailability 60%Not protein boundRenal eliminationNo metabolitesHalf life 5 - 7hNo definite conc-effect relationship

NH2 COOH

ValproateHalf-life 8-15h Wide diurnal variation (rapid absorption and elimination)Hepatic metabolism (CBZ, phenytoin increase clearance)Very poor evidence for target range

Clinical effect may take weeks to developAntiepileptic effect persists after dose stoppedPoor correlation between conc and effect

Protein binding 90-95%Dose dependent (free fraction varies with concentration)Displaced from protein by fatty acids (meals), aspirin

Side effects: GI, drowsiness, tremor, hepatotoxicity (in young children), haematological. teratogenicMonitoring generally unnecessary

Valproate - psychiatric indications

Acute mania Acute depression ?Prophylaxis of bipolar and schizoaffective disorder

Valproate in mania(Bowden et al., 1996)

-120

-100

-80

-60

-40

-20

0

20

40

60

80

0 20 40 60 80 100 120 140 160 180

Valproate, mg/L

% change in mania

scale

AminoglycosidesPoor oral absorption – give parenterallyNo metabolism or protein binding – renal excretionHalf-life 2-4h (much longer in renal failure)Toxicity – nephrotoxicity and ototoxicityMonitoring essential for control/toxicityExtended dose interval

High peaks, low troughs7 mg/kg gentamicin

Use TDM to determine interval, not dose (e.g. Hartford nomogram)

Not applicable to children (?), pregnancy, ascites, endocarditis, CF, burns, neutropenia, CrCl < 20 mL/min

Extended interval aminoglycosides:dosing (Hartford nomogram)

0

5

10

15

20

25

30

35

6.00 14.00Time post dose (h)

Conc.(mg/L)

6 10 14

Every 48h

Every 36h

Every 24h

DigoxinSamples must be at least 6h post-doseOther factors (e.g. K+) influence response/toxicityMonitoring not indicated in most patientsMonitor if:

Response poor? Toxic? Stop drug

0

0.5

1

1.5

2

2.5

0

C/Css

5 10 15 20 25Hours post dose

Digoxin post-dose(Nicholson, 1980)

LithiumHalf-life 10-35 h (longer in elderly/poor renal function)Not protein boundExcreted in urine Side effects – thirst, polyuria, nephrogenic DI, hypothyroidism, renal impairment, comaMonitor at 12h post-doseTarget range @ 12h 0.4-0.8 mmol/L (up to 1.2 in acute Rx)Individualise frequency of monitoringInteractions – dec clearance: thiazides, NSAIDsMonitoring essential at start of Rx, advisable at intervals thereafter, especially if ill/pregnant or if other drugs changed(especially diuretics)

TheophyllineShort half-life (3-13h: 4h in smokers, 24-30 h in neonates) –trough measurementsHepatic metabolismProtein binding 50-65% (less in babies)Side effects: CNS, GI, cardiacTarget range 10-20 mg/L (55-110 umol/L) (lower in babies)

Interactions: erythromycin dec clearance, enzyme inducers (phenytoin, CBZ) inc clearance Monitoring useful to identify undertreated patients, adjust dosage and confirm toxicityCompliance often erratic

Anti-retroviral drugsDrug class Examples Action

Nucleoside/nucleotide reverse transcription inhibitors (NRTIs)

ZidovudineEmtricitabineTenofovir

Inhibit RT by mimicking bases and terminating chain

Non-nucleoside RT inhibitors (NNRTIs)

EfavirenzNevirapine

Inhibit RT by binding adjacent to active site

Protease inhibitors (PIs)

Ritonavir Bind to protease and block HIV core maturation

Entry and fusion inhibitors

MaravirocEnfuvirtide

Prevent HIV from entering cells

Integrase inhibitors Raltegravir Block insertion of human proviral DNA into host DNA

Anti-retroviral drugsDrug class Examples Action

Nucleoside/nucleotide reverse transcription inhibitors (NRTIs)

ZidovudineEmtricitabineTenofovir

Inhibit RT by mimicking bases and terminating chain

Non-nucleoside RT inhibitors (NNRTIs)

EfavirenzNevirapine

Inhibit RT by binding adjacent to active site

Protease inhibitors (PIs)

Ritonavir Bind to protease and block HIV core maturation

Entry and fusion inhibitors

MaravirocEnfuvirtide

Prevent HIV from entering cells

Integrase inhibitors Raltegravir Block insertion of human proviral DNA into host DNA

TDM used

Essentials for effective TDMRational indication for assayAppropriate sampleAccurate analysisCorrect interpretationNecessary action taken

TDM - questionsPatient not responding to therapy

Could this be due to inadequate plasma concentration?Why is plasma concentration inadequate?

Poor compliance?Inappropriate dosage?Rapid metabolism?Malabsorption?

Could patient’s symptoms be caused by drug toxicity?

SampleHas steady-state been reached?

(>4 to 5 half-lives after dose change)

? Appropriate timing after last dose

InterpretationConcept of the “Therapeutic Range”

a guide to aim at!

“Therapeutic decisions should never be based solely on the drug concentration in the serum”

(Koch-Weser, 1972)

Pharmacogenetics

Links differences in gene structure (polymorphisms) to drug metabolism and response

(Genotype) (Phenotype)

Genetic Drug metabolism

variation & response

“One of the most striking things about modern medicines is how often they fail to work”

Goldstein

NEJM 6.2.2003

Variation between individuals

Pharmacogenomics and pharmacogenetics

PharmacogeneticsRefers to the study of inherited differences in drug metabolism and responseSingle gene/phenotype

PharmacogenomicsRefers to the general study of genes determining drug behaviourMultiple genes/phenotypes

(In practice, the two terms are used interchangeably!)

Possible pharmacokinetic consequences of polymorphisms

• Decreased first-pass effect• greater bioavailability• higher peak Cp

• Reduced parent drug elimination• longer half-life• more/fewer active metabolites

• Altered concentration-effect relationship between poor and good metabolisers

Applications of pharmacogenomics

Drug developmentMore targeted, more powerful drugs

Individualization of therapyThe right drug and the right dose for every patient

Predicting ADRs (& reducing mortality)Defining susceptibilityIdentifying potential addictionReducing cost of health care

1Positive

Response, Works as intended

2No Response, Choose new

drug

Adverse drug reaction

3

(Michael Murphy, Gentris)

Use of PGx:

Use of PGx

• Don’t treat non-responders (stratification)

• Don’t treat those most susceptible to toxicity (stratification)

• Adjust dose to maximise efficacy while avoiding toxicity

Estimated cost of ADRs

US: $100 billion per year….(Ingelman-Sundberg, 2001)

NHS: £2 billion per year(Compass Thinktank, 2008)

Can PGx reduce ADRs?

Systematic review (Phillips KA et al, JAMA 2001; 286: 2270-9)

- Identified 27 drugs frequently cited in ADRs(among top 200 used in US);

- 59% of these metabolized by at least 1 enzyme with variant alleles known to cause poor metabolism

- Conversely, only 7 - 22% of randomly selected drugs are known to be metabolized by enzymes with genetic variability

Clinical applications of pharmacogeneticinformation

• Anti-coagulation• Warfarin

• Psychiatry• Tricyclic anti-

depressants• Atomoxetine

• Oncology• Thiopurines• 5-fluorouracil• Herceptin

(Her-2/neu)• Tamoxifen• KRAS

• Cardiovascular• Statins

• Pain control• Codeine, methadone

• Epilepsy• Phenytoin

• Risk analysis• ADR’s• Disease

• Etc..

Enzymes of drug metabolism

• Phase 1 (oxidative) - SERCYP 1A2 CYP2B CYP 2C9CYP 2C19 CYP 2D6 CYP 2E1CYP 3A4 NADPH-quinone oxidoreductase

• Phase 2 (conjugative) - cytosolGlutathione S-transferaseN-acetyltransferaseUDP-glucuronosyltransferaseSulphotransferases

Enzymes of drug metabolism(showing polymorphisms in humans)

• Phase 1 (oxidative) - SERCYP 1A2 CYP2B CYP 2C9CYP 2C19 CYP 2D6 CYP 2E1CYP 3A4 NADPH-quinone oxidoreductase

• Phase 2 (conjugative) - cytosolGlutathione S-transferaseN-acetyltransferaseUDP-glucuronosyltransferaseSulphotransferases

CYP enzymes have different, sometimes overlapping, substrate specificity

CYP2D6 debrisoquinetricyclicsantipsychoticsSSRI’santi-arrhythmicsanti-hypertensivesmorphine derivs

CYP2C19barbituratestricyclicssedatives

CYP3A4 tricyclicsantipsychoticsSSRI’sSedatives

CYP1A2 tricyclicsantipsychotics SSRI’sSedatives

CYP2C9anti-epilepticsanticoagulants

Nomenclature

Cytochrome P450 2 D 6 *4

Superfamily

Family

Subfamily

Isoenzyme

Allele variant

CYP 2C9 polymorphism

• 9 exons; 490 amino acids

144 359 360CYP2C9*1 Arg Ile AspCYP2C9*2 Cys Ile AspCYP2C9*3 Arg Leu AspCYP2C9*4 Arg Thr AspCYP2C9*5 Arg Ile Glu

CYP2C9 polymorphism

• CYP2C9*2 15% of Caucasians7% of Asians5-fold lower warfarinclearance

• CYP2C9*3 7% of Caucasians3% of Asians25-fold lower warfarinclearance

Warfarin

Narrow therapeutic indexMultiple clinically important drug interactionsErratic safety profile10-fold variability around dose, target INR and side-effectsPolymorphisms account for 30-50% of the variability in dosing

Warfarin PGx testing

• CYP2C9• Metabolism of S-warfarin• *2 and *3 deficiency alleles

• VKORC1 (vitamin K epoxide reductase complex subunit 1)

• Target of warfarin inhibition• Converts vit K epoxide to quinone (vit K

recycling)• Deficiency (1639 G>A) means less warfarin

target and smaller dose requirement

CYP2D6Most extensively characterised polymorphic

drug-metabolising enzyme

• more than 75 allelic variants described• more than 15 encode an inactive enzyme or

no enzyme at all • other alleles encode enzyme with reduced,

normal or increased enzyme activity

Phenotyping

• Phenotyping for the metabolism of psychoactive drugs results in 4 categories• Poor metabolizers (PM)• Intermediate metabolizers (IM)• Extensive metabolizers (EM)• Ultrarapid metabolizers (UM)

(up to 13 copies of gene)

Prevalence of CYP 2D6 phenotypes

W European ChinesePM 10% 1%IM 40%EM 49%UM 1%

0.1

1

Plasmaconc/dose(nmol/L/mg)

0 5 10 15 20 25 30 35Time (h)

Haloperidol by debrisoquine phenotype

EM

PM

(Bertilsson, 1992)

Cancer biomarkers for Rx (Milone, 2012)

Drug Disease Marker associations

Tamoxifen Breast ca CYP2D6

Irinotecan Colon ca UGTIA1*28

5-FU Colon ca DPDY, TYMS

EGFR-specific TKIs(erlotinib)

Non-small cell lung ca EGFR mutationsEGFR gene copy noKRAS mutations

EGFR-specific Ab(cetuximab)

Colon caNon-small cell lung ca

EGFR gene copy noKRAS mutations

Azathioprine, 6-MP ALL, IBD TPMT

Methotrexate Lymphoma Methotrexate conc

Busulfan Myeloablation for BMT Busulfan conc/AUC

Tamoxifen – survival by genotype

Kiyotani et al, J Clin Oncol 2010; 28: 1287-93

CYP2D6 ABCC2

Thiopurine drugs

AzathioprineProdrug for 6-mercaptopurine

Azathioprine 6-MP + imidazole

• Breakthrough in organ Tx• Extensive use as steroid-sparing agent

6-mercaptopurine

N

N

HS

NH

N

6-mercaptopurine

6-methyl-mercaptopurine

6-thiouric acid

Xanthineoxidase

TPMT

6-thioinosine 5’-monophosphate

TPMT

6-methyl thioinosine 5’-MP

6-thioxanthosine 5’-MP

6-thioguanine nucleotides

IMPdehydrogenase

GMPsynthetase

HGPRT

0

2

4

6

8

10

12

% ofsubjects

0 5 10 20RBC TPMT activity, U/mL

TPMT Genetic Polymorphism(Weinshilboum & Sladek, 1980)

15

10

100

1000

10 4

0 5 10 15 20TPMT activity, U/mL rbc

Adult dermatology patients on azathioprine(Lennard)

6 TGN (pmol/ 8 x 108 rbcs)

0

20

40

60

80

100

Relapse-freesurvival (%)

0 20 40 60 80 100

50Time (months)

Children with ALL on 6-MP

100

29 9

288

Rbc 6-TGNabove median

Rbc 6-TGNbelow median

(Lennard, 1989)

0

2

4

6

8

10

12

% of subjects

0 5 10 20RBC TPMT activity, U/mL

TPMT Genetic Polymorphism

15

Severe marrow suppression

Toxicity at full dose

Low risk

Risk of therapeutic failure

TPMT genetics

• TPMT coded by 27 kb gene• 10 exons, located on 6p22.3• Wild-type is TPMT*1• Very low activity associated with 8 variants,

most common = TPMT*3 (55-70%)• 2 point mutations

• G460-A (Ala 154 - Thr)• A719-G (Tyr 240 - Cys)

= *3B= *3C ) =*3A

TPMT – genotype and phenotype(Yates et al., Ann Intern Med 1997; 126: 608-14)

Phenotype GenotypeHigh activity (21) *1/*1 (21)

Intermediate activity (21) *1/*1 (1)*1/*2 (1)*1/*3A (18)*1/*3C (1)

Deficient (7) *2/*2 (2)*2/*3A (1)*3A/*3A (3)*3A/*3C (1)

TPMT - summary

• TPMT phenotype/genotype can prospectively identify• Deficient patients at risk of life-

threatening toxicity• Heterozygotes who will respond well to

low-dose aza therapy• Patients with high TPMT activity who will

need high doses from the start (phenotype only)

KRASGene present in colorectal cancer tumoursImportant role in cell growth and tumour developmentGene can be mutated or normal in colorectal ca. cellsIf KRAS is mutated, then anti-EGFR therapies such as cetuximab are not effective and should not be used KRAS gene mutations occur in about 40% of colorectal ca. patients Patients diagnosed with metastatic colon ca. should be tested for KRAS mutation status to determine eligibility for anti-EGFR Rx

KRAS in colorectal ca

NEJM 2004; 351:2827-31

• 62 y.o man with CLL• 3 day h/o fatigue, dyspnoea, fever,

cough• On valproate 1500 mg/day for

epilepsy• Bilateral lower lobe pneumonia• Rx antibiotics* + codeine 25mg tds

* ceftriaxone, clarithromycin, voriconazole

• Day 4: rapid deterioration of consciousness → unresponsive

• pO2 7.4 kPa; pC02 10.6 kPa. Ventilated• Transfer to ITU: GCS 6• Valproate 63 mg/L• Naloxone 0.4 mg iv x2 resulted in dramatic

improvement in conscious level.

Drug levels

• Plasma codeine 114 ug/L(expected level in pt with extensive CYP2D6 metabolism = 13-75)

• Plasma morphine 80 ug/L(expected level in pt with extensive CYP2D6 metabolism = 1-4)

CYP 2D6 genotyping

• 3 or more functional CYP2D6 alleles • Ultrarapid metabolism• Confirmed by phenotyping with

dextromethorphan

Codeine metabolism

Codeine Codeine-6-glucuronide

Norcodeine

CYP3A4

MorphineMorphine-6-glucuronide

Morphine-3-glucuronideCYP2D6

X

Strattera (atomoxetine HCl)

-prescribing information

Clopidogrel (Plavix®): FDA 2010

Effectiveness of Plavix is dependent on its activation to an active metabolite by the cytochrome P450 (CYP) system, principally CYP2C19 …Plavix at recommended doses forms less metabolite and has a smaller effect on platelet function in patients who are CYP2C19 poor metabolizers. Poor metabolizers with acute coronary syndrome or undergoing percutaneous coronary intervention given Plavixat recommended doses exhibit higher cardiovascular event rates than do patients with normal CYP2C19 function.Tests are available to identify a patient's CYP2C19 genotype; these tests can be used as an aid in determining therapeutic strategyConsider alternate treatment or treatment strategies in patients identified as CYP2C19 poor metabolizers

Challenges for PGx

EthicsCostsUtilizationRegulatory

NACB consensus document, 2010

http://www.aacc.org/members/nacb/lmpg/pages/default.aspx

• “If it were not for the great variability among individuals, medicine might as well be a science and not an art”

Sir William Osler, 1892