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Genetic polymorphism & drug interactions in pain management Prof Ian Whyte, FRACP, FRCPE Calvary Mater Newcastle University of Newcastle Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Napoleon Bonaparte (1769 – 1821) “Medicine is a collection of uncertain prescriptions, the results of which, taken collectively, are more fatal than useful to mankind” Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Variability in drug response Common and multifactorial – environment, genes, disease, other drugs – absorption, distribution, metabolism, excretion Optimise dosage regimen for each individual patient Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Drug metabolism Analgesics – need to get into the brain to work – hydrophobic (fat soluble) Elimination – hydrophilic (water soluble) Enzymatic conversion – liver – intestinal wall Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Drug metabolising enzymes Phase I (oxidating enzymes) – reductases, oxidases, hydrolases Phase II (conjugating enzymes) – transferases l glucuronidase, sulphatase, acetylases, methylases Transmembrane transporters – P-glycoprotein (P-gp) Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Cytochrome P-450 enzymes Superfamily of microsomal drugmetabolising enzymes (Phase I) Biosynthesis and degradation – steroids, lipids, vitamins Metabolism of chemicals in our diet and the environment – medications Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 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) CYP2D6*1a gene encodes wild-type protein CYP2D6.1 http://www.imm.ki.se/CYPalleles/ Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Genetic polymorphism Greek – poly: different and morph: form Differences in gene expression – frequency > 1% of the population Many enzymes – drug metabolism – drug transporters – drug targets Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Significance Drug – eliminated > 50% by a polymorphic enzyme – narrow therapeutic window – activity depends on metabolite (pro-drug) Drug interactions – interacting drug is inhibitor or inducer l mimic genetic variability Phenotype – different profile of enzyme activity Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Analgesic metabolism Main enzymes involved are – CYP2C9, CYP2D6, CYP3A4 l 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 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 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 minor metabolic pathway Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 3A4 2E1 2D6 2C19 2C9 2B6 1A2 aceclofenac CYP2C9 genotypes 6 known allelic variants In Caucasians – CYP2C9*1, *2 and *3 l l l CYP2C9*1 (80 – 82%) encodes normal (wild type) activity CYP2C9*2 (11%) slightly reduced enzymatic activity CYP2C9*3 (7 to 9%) 5 – 10-fold decreased enzyme activity Ethnic variability – Ethiopia l l CYP2C9*2 is 4% CYP2C9*3 is 2% – Far East l l CYP2C9*2 is 0% CYP2C9*3 is 2% Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP2C9 function Most substrates are weak acids – NSAIDs l ibuprofen, indomethacin, flurbiprofen, naproxen, diclofenac, piroxicam, lornoxicam, mefenamic acid, meloxicam, celecoxib Ibuprofen and celecoxib – homozygous carriers of CYP2C9*3 l clearance is halved and half-life doubled No clinical correlates demonstrated Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP2D6 genotypes CYP2D6 polymorphism autosomal recessive – almost 80 allelic variants Non-functional alleles – CYP2D6*4 – CYP2D6*5 – CYP2D6*3 Decreased function alleles – CYP2D6*10 – CYP2D6*17 Normal function (wild type) allele – CYP2D6*1 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP2D6 phenotypes Poor metabolisers (PMs) – homozygous for a non-functional allele l l l CYP2D6*4 (20 – 25% Caucasians; 70 – 90% PMs) CYP2D6*5 (5%) CYP2D6*3 (2%) – complete enzyme deficiency l 5 – 10% of Caucasians Ethnic variability – PMs rare outside Caucasians – Asians and Africans < 2% non-functional alleles Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP2D6 phenotypes Intermediate metabolisers (IMs) – homozygous for a decreased function allele l l CYP2D6*10 CYP2D6*17 – decreased enzyme activity l 10 – 15% of Caucasians Ethnic variability – 50% of Asians are carriers of CYP2D6*10 Extensive metabolisers (EMs) – homozygous for the normal function allele l l CYP2D6*1 60 – 70% of Caucasians Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP2D6 phenotypes Ultra-rapid metabolisers (UMs) – multiple (2 – 13) copies of normal function alleles l 1 to 10% of Caucasians Ethnic variability – Middle East (20%) – Ethiopia (up to 29%) – Europe l North / South gradient – – – – – Sweden (1 – 2%) Germany (3.6%) Switzerland (3.9%) Spain (7 – 10%) Sicily (10%) Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP2D6 clinical implications Metabolism – 25% of common drugs l many opioids, most antidepressants Effect varies – activity of parent compound – activity of any metabolite UMs have increased elimination – antidepressants l standard doses can result in ineffective treatment PMs higher concentrations after standard doses – increased efficacy but also toxicity – dose adjustment is therefore essential Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP2D6 and codeine Bioactivation by CYP2D6 – codeine, tramadol, hydrocodone, oxycodone l affects efficacy and toxicity Codeine is converted to morphine for analgesia – EMs l 10% of codeine is converted to morphine – PMs l none (0%) is converted to morphine – codeine is an ineffective analgesic – UMs l morphine production is increased – severe intoxication with codeine at standard dosages – death in a child • UM mother breastfeeding while on codeine Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP2D6 and tramadol CYP2D6 activity important for – analgesic effect – side effect profile Tramadol – low affinity for μ-opioid receptor l O-desmethyl-tramadol > 200-fold affinity – inhibits reuptake of 5HT > NA PMs – unlike codeine – tramadol retains activity l l l 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 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP2D6 and methadone Marked interindividual differences in steady state blood concentrations – higher in PMs on maintenance l l over 70% of PMs had effective treatment 28% of PMs required doses > 100 mg – lower in UMs on maintenance l l 40% of UMs had effective treatment almost 50% of UMs required doses > 100 mg Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 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 > 7 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 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 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP2D6 and antidepressants Clearance decreased in PMs – amitriptyline, clomipramine, desipramine, imipramine, nortriptyline, trimipramine, paroxetine, citalopram, fluvoxamine, fluoxetine, venlafaxine Increased side effects in PMs – desipramine l only PMs had adverse reactions – confusion, sedation, orthostatic hypotension – venlafaxine l cardiotoxicity – palpitations, dyspnoea, arrhythmias – twice as many PMs among patients reporting side effects Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP2D6 and antidepressants Effective dosing in depression – depends on PM or UM status l l l l 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 l l 50 to 80% dose reduction for tricyclic antidepressants 30% dose reduction for SSRIs – UMs l l l increase dose to 260% for desipramine 300% for mianserin 230% for nortriptyline Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 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 l most important is CYP3A4 Allelic variants of CYP3A4 are described – none results in a significant change of enzyme activity Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 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 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 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 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 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 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP3A4 Inhibitors of CYP3A4 – grapefruit juice, macrolide antibiotics (erythromycin), some antidepressants (paroxetine), neuroleptics (olanzapine), protease inhibitors (ritonavir, indinavir, saquinavir), amiodarone – increase methadone plasma levels l toxicity (overdose) – 4 – 5-fold reduction in metabolism l fentanyl, alfentanil, sufentanil Clinical Toxicology & Pharmacology, Calvary Mater Newcastle CYP3A4 Inducers of CYP3A4 – rifampicin, carbamazepine, phenytoin – decrease plasma levels of methadone l symptoms of opioid withdrawal – > 3-fold increase in clearance of alfentanil – unclear clinical significance Clinical Toxicology & Pharmacology, Calvary Mater Newcastle isoniazid amiodarone itraconazole amprenavir ketoconazole bupropion levomepromazine celecoxib losartan St Mary’s 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 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 3A4 2D6 2C9 3A4 2D6 2C9 valproic acid amprenavir carbamazepine cyclophosphamide dexamethasone efavirenz ethanol felbamate ifosfamide meprobamate St John’s wort nevirapine oxcarbazepine phenobarbitone phenylbutazone phenytoin primidone rifabutin rifampicin ritonavir topiramate potent inducer moderate inducer Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 3A4 2C9 aminoglutethimide 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 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Phenotyping Characterises enzyme activity in an individual patient Test substrate given – parent drug, metabolite in blood / urine – metabolic ratio l amount of unchanged parent drug / amount of metabolite Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 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 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Phenotyping availability CYP2C9 – 1 out of 507 (0.2%) l Hospital / University facility CYP2D6 – 6 out of 507 (1.2%) l Hospital (2), Hospital / University (2), University (2) CYP3A4 – None Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 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 l not all mutations detected – new allelic variants found on a regular basis l may need to repeat the test Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Genotyping availability CYP2C9 – 5 out of 507 (1.0%) l commercial pathology laboratory (1), state government pathology service (1), university (2), university/hospital (1) CYP2D6 – 4 out of 507 (0.6%) l commercial pathology laboratory (1), state government pathology service (1), hospital/university (1), university (1) CYP3A4 – None Clinical Toxicology & Pharmacology, Calvary Mater Newcastle 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“ l l l Dosage guidance for all drugs that GenesFX is informed about Suggestions of alternative drugs when appropriate Suggestions of drugs to avoid in the future Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Clinical utility May occasionally be justified retrospectively – few cases of treatment failure or drug toxicity l l poor compliance vs fast metabolism excessive intake vs poor metabolism – suspected drug addiction vs metabolic defect l 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 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle Conclusions Analgesics – importance of individualisation of drug prescription – most are metabolised by CYPs subject to genetic polymorphism l may help explain some of the ineffectiveness or toxicity Detection of these polymorphisms could give us tools for – optimising drug treatment l l l 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 Clinical Toxicology & Pharmacology, Calvary Mater Newcastle
