Presentation on theme: "Human risk assessment perspectives for high risk conditions Jean Lou Dorne Institute of Human Nutrition University of Southampton, UK." — Presentation transcript: Slide 1 Slide 2 Human risk assessment perspectives for high risk conditions Jean Lou Dorne Institute of Human Nutrition University of Southampton, UK Slide 3 Resveratrol Lycopene Allyl sulphides (Allicin…) Isothiocyanates, Sulphorafane Isoflavones Vitamine C, limonene Slide 4 Slide 5 Slide 6 PARACELSUS (1493-1541) All things are toxic and there is nothing without poisonous qualities: it is only the dose which makes something a poison Pharmaco/Toxicokinetics How the chemical is eliminated from the body or activated into a toxic species (ADME) Pharmaco/Toxicodynamics How the chemical exerts its pharmacological effect/ toxicity Target receptor/cell/organ Slide 7 RISK ASSESSMENT METHODS LOW - DOSE EXTRAPOLATION RISK ASSOCIATED WITH THE KNOWN INTAKE QUANTITATIVE RISK ASSESSMENT NO THRESHOLDTHRESHOLD NOAEL AND SAFETY FACTORS INTAKE WITH NO APPRECIABLE EFFECTS eg ADI NON - QUANTITATIVE RISK ASSESSMENT Slide 8 ADI (mg/kg/day) = NOAEL(mg/kg) / 100 Derivation of the Acceptable Daily Intake (ADI)
Slide 9 KINETICSDYNAMICSKINETICSDYNAMICS SPECIES DIFFERENCES HUMAN VARIABILITY Extrapolation from group of test animals to average human and from average humans to potentially sensitive sub-populations 10 The use of uncertainty or safety factors (UFs) Slide 10 Chemical specific adjustment factors can replace the default uncertainty factors (WHO, 2001; IPCS, 2006) 100 - FOLD UNCERTAINTY FACTOR INTER-SPECIES DIFFERENCES 10 - FOLD INTER-INDIVIDUAL DIFFERENCES 10 - FOLD TOXICO- DYNAMIC 10 0.4 2.5 TOXICO- KINETIC 10 0.6 4.0 TOXICO- DYNAMIC 10 0.5 3.2 TOXICO- KINETIC 10 0.5 3.2 Slide 11 Towards a more flexible framework Data-derived or Pathway-related Uncertainty factors or general default Data-derived or process related Uncertainty factors or general default Interspecies differences Human variability ToxicokineticsToxicodynamics UFs for main routes of metabolism in test species and humans – intermediate option between default factor and chemical specific adjustment factors Adapted from Dorne and Renwick, 2005 Toxicol Sci 86, 20-26 Slide 12 Phase I enzymes Cytochrome P-450, ADH, Esterases % of Pharmaceuticals Metabolized by Individual Cytochrome P450s in man P4502D6 P4501A2 P4502A6 P4503A P4502C9 P4502C19 P4502E1 Phase II enzymes Conjugation reactions Glucuronidation Sulphation N-acetylation (Polymorphic) Amino acid conjugation Renal excretion CYP2C9, CYP2C19, CYP2D6* Polymorphic (Extensive and Poor metabolisers, EMs and PMs) *Caucasian 8% PMs 92% EMs Major Routes of chemical metabolism and excretion Slide 13 Beta Blockers Bufuralol Propafenone Metoprolol propranolol Carvedilol Antiarrhythmics Encainide S-mexiletine Analgesics Dextromethorphan Codeine Tramadol Antidepressants Fluoxetine Paroxetine Amitriptylline Desipramine Imipramine Venlafaxine Pesticides Chlorpyrifos Diazinon Methoxychlor Adapted from Dorne et al., 2002 FCT 41, 1633-1656 CYP2D6 Substrates Antipsychotics Risperidone Haloperidol Slide 14 Introducing metabolic and toxicokinetic data into risk assessment Slide 15
Aims Aims Quantify human variability in kinetics for major metabolic routes Markers of chronic exposure (plasma Clearance) Markers of acute exposure (plasma peak concentration Cmax) Prefer the oral route (gut + liver): relevance to environmental contaminants Comparison to the IV route (liver) Identify susceptible subgroups of the population Derive pathway-related uncertainty factors for each subgroup Slide 16 Methods Methods Literature searches Medline, Toxline and EMBASE (1966-current) Compounds metabolised by single route (complete oral absorption, >60% of dose) In vitro metabolism data (cell line, liver microsomes): metabolic route In vivo excretion data: HPLC detects parent compound and metabolites In vivo pharmacokinetic studies for human subgroups Slide 17 Meta-analysis of studies reporting PK parameters for each compound/ parameter/ subgroup of the population: Mean, SD and CV N (normal distribution) transform to geometric mean and GSD, CV LN (lognormal distribution) Derive Coefficient of variation (CV) for each compound/parameter and pool CVs to get overall value for metabolic route (pathway-related variability) Derive Pathway-related uncertainty factors (to cover 95, 97.5 and 99th centiles) using CV and magnitude of difference in internal dose (clearance or Cmax) between healthy adults and subgroups Methods II Methods II Slide 18 Results Results Database for >200 compounds HPLC method for the detection of parent compound and metabolites In vitro metabolism of compound inter-species and human In vivo metabolism data (% excretion for compound and each metabolite HPLC data) Kinetic studies for each compound (> 2500 studies) Subgroups of the human population (healthy adults, genetic polymorphism, interethnic differences, neonates, children and the elderly) Slide 19 Monomorphic pathways Pathway-related UFs below the kinetic default factor (3.2) Low variability in healthy adults (<30%), exception of CYP3A4 : role of gut CYP3A4, P- glycoprotein, polymorphism Pathway n compounds n CVPathway-related UFs (99 th ) CYP1A24379302.0 CYP3A4121381462.7 Glucuronidation15906292.0 Renal excretion 6444211.6 Healthy adults Slide 20 Pathway n compounds n CVPathway-related UFs (99 th ) CYP2C19 (EM)256603.8 CYP2C19 (PM)2212052 CYP2D6 (EM)9192665.8 CYP2D6 (PM)7742926 Polymorphic pathways Variability for polymorphic pathways larger than for monomorphic pathways Large difference in internal dose between EMs and PMs for CYP2D6 (9- fold) and CYP2C19 (12-fold) Pathway-related uncertainty factors above the current kinetic default factor (3.2)
Slide 21 Exponential relationships between ratio EM/PM and % CYP2D6 metabolism Ratio EM/PM 0 20 40 60 80 020406080100 % CYP2D6 metabolism in EMs PMs covered by pathway- related UFs for substrates with up to 25% (dose) of CYP2D6 metabolism in EMs Quantitative involvement of dose handling on kinetic differences: CYP2D6 Slide 22 Quantitative involvement of dose handling on kinetic differences: CYP2C19 PMs covered by UFs for substrates with up to 20-25% (dose) of CYP2C19 metabolism in EMs. Slide 23 Results: Subgroups of the population Interethnic differences Less variability in Asian vs Caucasian for CYP2D6 and CYP2C19 (+ different frequencies of phenotypes) Pathway- related uncertainty factors above kinetic default for CYP2C19 and NAT metabolism Historically smaller database for non-Caucasian subjects: Modern man : mixture of ethnic groups and more so in the future ! Ex relationship for CYP2C19 and ratio EMs/PMs in Asian healthy adults (R 2 =0.87) : Slope 100% metabolism via CYP2C19 gives a ratio of 30 (80 in Caucasian !) Slide 24 Children and neonates Potential susceptible subgroups of the population: -Immaturity of phase I, phase II and renal excretion (particularly for neonates) -Quantify differences in internal dose from in vivo PK database -Provide pathway-related UFs for these subgroups -Identify datagaps Slide 25 Neonates The most susceptible subgroup for all pathways with data: immaturity of phase I, II metabolism and renal excretion. No reliable data available for polymorphic pathways. PathwayNcnCVRatioPathway-related UFs GM95th99th CYP1A22251356.21114 CYP3A4235653.08.112 Glucuronidation494503.98.612 Glycine Conjugation11016192528 Renal excretion7656321.72.83.4 All data from the IV route Slide 26 Children Limited data-Susceptible subgroup for both polymorphic CYP2C19 and CYP2D6 PathwayNcnCVRatioPathway-related UFs GM95 th 99th CYP1A2*1195340.821.41.8 CYP2C19125861.65.49.0 CYP2D611731404.02245 CYP3A4316450.701.41.8 Glucuronidation5131230.861.31.5 Renal Excretion*6126300.701.21.5 * IV data (all other data PO route)
Slide 27 Polymorphism in metabolism and Children and neonates: Examples Fluoxetine and paroxetine metabolised largely via CYP2D6 and other CYP isoforms (CYP2C9, CYP3A4 and CYP2C19) Large inter-individual differences in kinetics in healthy adults and children: up to 10-18-fold variation in clearance in healthy adults PMs (including 2 PM children) Holden, C. Prozac Treatment of Newborn Mice Raises Anxiety. Science. 2004 Oct 29;306(5697):792. Ibuprofen and indomethacin in preterm neonates : up to 10-fold difference decrease in clearance : immature CYP2C9, glucuronidation and renal excretion. Lansoprazole (CYP2C19-CYP3A4): 1 neonate and 1 infant PM (3- and 7-fold decrease in clearance) Slide 28 Predicting human variability in toxicokinetics using Monte Carlo modelling Slide 29 Latin hypercube sampling: variant of Monte Carlo (random), stratified sampling throughout the distribution. Compounds handled by multiple pathways : predict variability and uncertainty factors for healthy adults, children and neonates. Combine distributions describing pathway –related variability and quantitative metabolism data. Compare Simulated data and published kinetic data. Slide 30 Poor metabolisers, neonates and children : -GM ratio of internal dose (mean) compared to healthy adults and pathway-specific variability (GSD) for each pathway. -Neonates and children: ideally use metabolism data but often not available: liver microsome / in vitro and/or healthy adult data -Polymorphic pathways : Combine distribution for EM and PM using frequency of EM and PMs ( for CYP2D6 7.4% PM in Caucasian) Slide 31 Non-phenotyped healthy adults: Uncertainty factors (99 th centile) PublishedSimulated Slide 32 Phenotyped healthy adults: Uncertainty factors (99 th centile) CYP2D6 EMs CYP2D6 PMs Combined EMs and PMs Slide 33
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