Genetic polymorphism

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Genetic polymorphism drug interactions in pain
management

• Prof Ian Whyte, FRACP, FRCPE
• Calvary Mater Newcastle
• University of Newcastle

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Napoleon Bonaparte (1769 1821)

• Medicine is a collection of uncertain
  prescriptions, the results of which, taken
  collectively, are more fatal than useful to
  mankind

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Variability in drug response

• Common and multifactorial
• environment, genes, disease, other drugs
• absorption, distribution, metabolism, excretion
• Optimise dosage regimen for each individual
  patient

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(No Transcript)
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Drug metabolism

• Analgesics
• need to get into the brain to work
• hydrophobic (fat soluble)
• Elimination
• hydrophilic (water soluble)
• Enzymatic conversion
• liver
• intestinal wall

6
Drug metabolising enzymes

• Phase I (oxidating enzymes)
• reductases, oxidases, hydrolases
• Phase II (conjugating enzymes)
• transferases
• glucuronidase, sulphatase, acetylases, methylases
• Transmembrane transporters
• P-glycoprotein (P-gp)

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Cytochrome P-450 enzymes

• Superfamily of microsomal drug-metabolising
  enzymes (Phase I)
• Biosynthesis and degradation
• steroids, lipids, vitamins
• Metabolism of chemicals in our diet and the
  environment
• medications

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CYPs

• Classified by amino acid similarities
• family number
• subfamily letter
• number for each gene within the subfamily
• asterisk followed by a number (and letter) for
  each genetic (allelic) variant
• allele 1 is the normal function gene (wild
  allele)
• CYP2D61a gene encodes wild-type protein CYP2D6.1
• http//www.imm.ki.se/CYPalleles/

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Genetic polymorphism

• Greek
• poly different and morph form
• Differences in gene expression
• frequency gt 1 of the population
• Many enzymes
• drug metabolism
• drug transporters
• drug targets

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Significance

• Drug
• eliminated gt 50 by a polymorphic enzyme
• narrow therapeutic window
• activity depends on metabolite (pro-drug)
• Drug interactions
• interacting drug is inhibitor or inducer
• mimic genetic variability
• Phenotype
• different profile of enzyme activity

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Analgesic metabolism

• Main enzymes involved are
• CYP2C9, CYP2D6, CYP3A4
• can be inhibited and / or induced
• Amount of enzyme related to
• mix of non-functional, decreased function or
  fully functional alleles
• co-administration of inducers or inhibitors

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    1A2 2B6 2C9 2C19 2D6 2 E1 3A4
aceclofenac                
mefenamic acid                
alfentanil                
amitriptyline                
buprenorphine                
celecoxib                
citalopram                
clomipramine                
codeine                
dextromethorphan                
diclofenac                
dihydrocodeine                
escitalopram                
fentanyl                
fluoxetine                
flurbiprofen                
fluvoxamine                
hydrocodone                
ibuprofen                
imipramine                
indomethacin                
maprotiline                
meloxicam                
methadone                
mianserin                
naproxen                
nortriptyline                
oxycodone                
paracetamol                
paroxetine                
piroxicam                
sertraline                
tenoxicam                
tramadol                
trimipramine                
valdecoxib                
                 
                 
major metabolic pathway major metabolic pathway major metabolic pathway major metabolic pathway          
minor metabolic pathway minor metabolic pathway minor metabolic pathway minor metabolic pathway          
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CYP2C9 genotypes

• 6 known allelic variants
• In Caucasians
• CYP2C91, 2 and 3
• CYP2C91 (80 82) encodes normal (wild type)
  activity
• CYP2C92 (11) slightly reduced enzymatic
  activity
• CYP2C93 (7 to 9) 5 10-fold decreased enzyme
  activity
• Ethnic variability
• Ethiopia
• CYP2C92 is 4
• CYP2C93 is 2
• Far East
• CYP2C92 is 0
• CYP2C93 is 2

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CYP2C9 function

• Most substrates are weak acids
• NSAIDs
• ibuprofen, indomethacin, flurbiprofen, naproxen,
  diclofenac, piroxicam, lornoxicam, mefenamic
  acid, meloxicam, celecoxib
• Ibuprofen and celecoxib
• homozygous carriers of CYP2C93
• clearance is halved and half-life doubled
• No clinical correlates demonstrated

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CYP2D6 genotypes

• CYP2D6 polymorphism autosomal recessive
• almost 80 allelic variants
• Non-functional alleles
• CYP2D64
• CYP2D65
• CYP2D63
• Decreased function alleles
• CYP2D610
• CYP2D617
• Normal function (wild type) allele
• CYP2D61

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CYP2D6 phenotypes

• Poor metabolisers (PMs)
• homozygous for a non-functional allele
• CYP2D64 (20 25 Caucasians 70 90 PMs)
• CYP2D65 (5)
• CYP2D63 (2)
• complete enzyme deficiency
• 5 10 of Caucasians
• Ethnic variability
• PMs rare outside Caucasians
• Asians and Africans lt 2 non-functional alleles

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CYP2D6 phenotypes

• Intermediate metabolisers (IMs)
• homozygous for a decreased function allele
• CYP2D610
• CYP2D617
• decreased enzyme activity
• 10 15 of Caucasians
• Ethnic variability
• 50 of Asians are carriers of CYP2D610
• Extensive metabolisers (EMs)
• homozygous for the normal function allele
• CYP2D61
• 60 70 of Caucasians

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CYP2D6 phenotypes

• Ultra-rapid metabolisers (UMs)
• multiple (2 13) copies of normal function
  alleles
• 1 to 10 of Caucasians
• Ethnic variability
• Middle East (20)
• Ethiopia (up to 29)
• Europe
• North / South gradient
• Sweden (1 2)
• Germany (3.6)
• Switzerland (3.9)
• Spain (7 10)
• Sicily (10)

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CYP2D6 clinical implications

• Metabolism
• 25 of common drugs
• many opioids, most antidepressants
• Effect varies
• activity of parent compound
• activity of any metabolite
• UMs have increased elimination
• antidepressants
• standard doses can result in ineffective
  treatment
• PMs higher concentrations after standard doses
• increased efficacy but also toxicity
• dose adjustment is therefore essential

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CYP2D6 and codeine

• Bioactivation by CYP2D6
• codeine, tramadol, hydrocodone, oxycodone
• affects efficacy and toxicity
• Codeine is converted to morphine for analgesia
• EMs
• 10 of codeine is converted to morphine
• PMs
• none (0) is converted to morphine
• codeine is an ineffective analgesic
• UMs
• morphine production is increased
• severe intoxication with codeine at standard
  dosages
• death in a child
• UM mother breastfeeding while on codeine

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CYP2D6 and tramadol

• CYP2D6 activity important for
• analgesic effect
• side effect profile
• Tramadol
• low affinity for µ-opioid receptor
• O-desmethyl-tramadol gt 200-fold affinity
• inhibits reuptake of 5HT gt NA
• PMs
• unlike codeine tramadol retains activity
• opioid effect decreases but monoaminergic effect
  increases
• non-responders twice as frequent (46.7) as in
  EMs (21.6)
• increased risk of serotonin toxicity
• UMs
• no issues reported

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CYP2D6 and methadone

• Marked interindividual differences in steady
  state blood concentrations
• higher in PMs on maintenance
• over 70 of PMs had effective treatment
• 28 of PMs required doses gt 100 mg
• lower in UMs on maintenance
• 40 of UMs had effective treatment
• almost 50 of UMs required doses gt 100 mg

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CYP2D6 and opioid dependence

• PMs may be protected
• no PMs were found in those addicted to codeine
• 4 in patients never substance addicted
• 6.5 in those with other dependencies (alcohol,
  cocaine, amphetamines)
• Pharmacogenetic protection against oral codeine
  dependence
• odds ratio gt 7

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CYP2D6 and antidepressants

• Antidepressants used as co-analgesics
• over 25 of patients do not respond
• Most metabolised by CYP2D6
• 30 to 40 fold variation in plasma levels
• UM phenotype
• risk factor for therapeutic ineffectiveness
• PMs
• toxic effects at recommended doses

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CYP2D6 and antidepressants

• Clearance decreased in PMs
• amitriptyline, clomipramine, desipramine,
  imipramine, nortriptyline, trimipramine,
  paroxetine, citalopram, fluvoxamine, fluoxetine,
  venlafaxine
• Increased side effects in PMs
• desipramine
• only PMs had adverse reactions
• confusion, sedation, orthostatic hypotension
• venlafaxine
• cardiotoxicity
• palpitations, dyspnoea, arrhythmias
• twice as many PMs among patients reporting side
  effects

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CYP2D6 and antidepressants

• Effective dosing in depression
• depends on PM or UM status
• nortriptyline 10 to 500 mg/day
• amitriptyline 10 to 500 mg/day
• clomipramine 25 to 300 mg/day
• Chinese patients (majority IMs) need generally
  lower doses
• Dose recommendations
• PMs
• 50 to 80 dose reduction for tricyclic
  antidepressants
• 30 dose reduction for SSRIs
• UMs
• increase dose to 260 for desipramine
• 300 for mianserin
• 230 for nortriptyline

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CYP3A4

• CYP3A subfamily has a role in 45 to 60 of all
  drugs
• codeine, tramadol, buprenorphine, methadone,
  fentanyl, dextromethorphan
• 30-fold differences in expression of CYP3A exist
  in certain populations
• CYP3A subfamily consists of four enzymes
• CYP3A4, CYP3A5, CYP3A7, CYP3A43
• most important is CYP3A4
• Allelic variants of CYP3A4 are described
• none results in a significant change of enzyme
  activity

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CYPs and drug interactions

• Plasma levels of substrates may increase with
  co-administration of inhibitors
• potentially increased side effects
• Plasma levels of substrates may decrease with
  co-administration of inducers
• potentially less therapeutic effect

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CYP2C9

• Inhibitors of CYP2C9
• amiodarone, fluvastatin, fluconazole,
  phenylbutazone, sulphinpyrazone, sulphonamides
• potentially increased NSAID side effects
• Inducers of CYP2C9
• carbamazepine, phenobarbitone, ethanol
• potentially less NSAID therapeutic effect

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CYP2D6

• Inhibitors of CYP2D6
• antiarrhythmics (quinidine), neuroleptics
  (chlorpromazine, haloperidol, thioridazine,
  levopromazine), many antidepressants (paroxetine,
  fluoxetine)
• increase plasma concentrations
• inactivate pro-drugs (codeine)
• Inducers of CYP2D6
• None

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CYP3A4

• Inhibitors of CYP3A4
• grapefruit juice, macrolide antibiotics
  (erythromycin), some antidepressants
  (paroxetine), neuroleptics (olanzapine), protease
  inhibitors (ritonavir, indinavir, saquinavir),
  amiodarone
• increase methadone plasma levels
• toxicity (overdose)
• 4 5-fold reduction in metabolism
• fentanyl, alfentanil, sufentanil

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CYP3A4

• Inducers of CYP3A4
• rifampicin, carbamazepine, phenytoin
• decrease plasma levels of methadone
• symptoms of opioid withdrawal
• gt 3-fold increase in clearance of alfentanil
• unclear clinical significance

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    2C9 2D6 3A4         2C9 2D6 3A4
valproic acid             isoniazid        
amiodarone             itraconazole        
amprenavir             ketoconazole        
bupropion             levomepromazine        
celecoxib             losartan        
St Marys thistle (silibinin)             methadone        
chloroquine             metronidazole        
chlorpromazine             miconazole        
cimetidine             moclobemide        
ciprofloxacin             nateglinide        
citalopram             nefazodone        
clarithromycin             nelfinavir        
clomipramine             nifedipine        
clopidogrel             nitrendipine        
delavirdine             paroxetine        
desogestrel             phenylbutazone        
dihydralazine             phenytoin        
diltiazem             promethazine        
diphenhydramine             propafenone        
efavirenz             quinidine        
erythromycin             risperidone        
ethinyloestradiol             ritonavir        
flecainide             roxithromycin        
fluconazole             saquinavir        
fluoxetine             sertraline        
fluvastatin             simvastatin        
fluvoxamine             terbinafine        
gemfibrozil             thioridazine        
gestodene             tacrolimus        
grapefruit             valdecoxib        
halofantrine             venlafaxine        
haloperidol             verapamil        
imatinib             voriconazole        
indinavir             zafirlukast        
irbesartan                      
                       
                       
potent inhibitor                      
moderate inhibitor                      
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    2C9 3A4
aminoglutethimide      
amprenavir      
carbamazepine      
cyclophosphamide      
dexamethasone      
efavirenz      
ethanol      
felbamate      
ifosfamide      
meprobamate      
St Johns wort      
nevirapine      
oxcarbazepine      
phenobarbitone      
phenylbutazone      
phenytoin      
primidone      
rifabutin      
rifampicin      
ritonavir      
topiramate      
       
       
potent inducer      
moderate inducer      
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P-glycoprotein

• Transmembrane transport protein
• expels drugs out of cells
• decreases drug levels in the tissue
• 30 mutations
• Substrates
• loperamide, morphine, methadone, meperidine,
  hydromorphone, naloxone, naltrexone,
  pentazocine, some endorphins and enkephalins
• Decreased intestinal P-gp function
• increased amount absorbed
• increased plasma concentration
• Minor influence on brain bioavailability of
  morphine, methadone and fentanyl

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Phenotyping

• Characterises enzyme activity in an individual
  patient
• Test substrate given
• parent drug, metabolite in blood / urine
• metabolic ratio
• amount of unchanged parent drug / amount of
  metabolite

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Phenotyping

• Quick, simple, inexpensive and reproducible
• Must give a pharmacologically active substance
  for a diagnostic purpose
• may raise ethical questions
• Information on the phenotyping of specific groups
  is limited
• children, elderly, renal and liver disease

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Phenotyping availability

• CYP2C9
• 1 out of 507 (0.2)
• Hospital / University facility
• CYP2D6
• 6 out of 507 (1.2)
• Hospital (2), Hospital / University (2),
  University (2)
• CYP3A4
• None

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Genotyping (PCR)

• Advantages
• direct analysis of genetic mutations
• does not require a substrate drug
• not influenced by drugs or environmental factors
• performed once in a lifetime
• Disadvantages
• not commonly available
• cost and sensitivity varies with the CYP
• only detects currently described allelic variants
• not all mutations detected
• new allelic variants found on a regular basis
• may need to repeat the test

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Genotyping availability

• CYP2C9
• 5 out of 507 (1.0)
• commercial pathology laboratory (1), state
  government pathology service (1), university (2),
  university/hospital (1)
• CYP2D6
• 4 out of 507 (0.6)
• commercial pathology laboratory (1), state
  government pathology service (1),
  hospital/university (1), university (1)
• CYP3A4
• None

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GenesFX Health Pty. Ltd(http//www.genesfx.com)

• Individual gene tests
• CYP2C9 140
• CYP2D6 180
• CYP3A4/5 Not available
• DNADose 270
• CYP2D6, CYP2C9, CYP2C19, VKORC1
• "Personalised Drug-Specific report
• Dosage guidance for all drugs that GenesFX is
  informed about
• Suggestions of alternative drugs when appropriate
• Suggestions of drugs to avoid in the future

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Clinical utility

• May occasionally be justified retrospectively
• few cases of treatment failure or drug toxicity
• poor compliance vs fast metabolism
• excessive intake vs poor metabolism
• suspected drug addiction vs metabolic defect
• high intake of codeine
• Limited availability
• Dose recommendations are preliminary
• Efficacy and clinical utility remain to be
  validated
• No economic analysis
• tests needed to prevent one case of toxicity vs
  cost

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Conclusions

• Analgesics
• importance of individualisation of drug
  prescription
• most are metabolised by CYPs subject to genetic
  polymorphism
• may help explain some of the ineffectiveness or
  toxicity
• Detection of these polymorphisms could give us
  tools for
• optimising drug treatment
• anticipating therapeutic side effects and
  ineffective therapy
• identifying the right drug and the right dose
• predict the most effective and safest drug for
  each patient
• distinguish between rapid metabolism and drug
  abuse
• Cost / benefit analysis has not been done
• We are not there yet but
• there is real potential