WIXLIB

Chapter 4The Dose-ResponseRelationship

Introduction The need to establish some criteria to base the relative safety ofchemicals is essential. Several methods are used to obtain data in order to establish safelevels and provide information about the relative toxicity ofchemicals.

Acute Toxicological Studies (1) Information concerning the toxicity of a substance is obtainedprimarily from either acute or chronic toxicity studies. Acute toxicity studies are the most commonly performed studies forobtaining information on the effects of chemical exposure. They areshort-term, relatively inexpensive tests with death of the test animalbeing the useful observed effect. Information obtained from acute toxicity tests can be used to– determine the relative toxicity of different chemicals comparing the respective LD50values, which are the doses that prove to be lethal for 50 percent of the test animals;– identify the target organs (heart, liver, kidneys, etc.) that are affected as a result ofexposure; and– determine the appropriate doses for long term, chronic studies.

Acute Toxicological Studies (2) To perform acute toxicity studies the test animal population is dividedinto several groups, with an equal number of individuals (usually 10 to50) in each group. One group of test animals –referred to as thecontrol group – is exposed to all the same experimental conditionsexcept exposure to the toxic substance. The groups are observed for 14 days, and the number of deaths in eachgroup is recorded.

Acute Toxicological Studies (3) LC50 is a measure of chemical toxicity resulting from inhalation. LCmeans “lethal concentration” and the subscript has the same meaning asdescribed previous for LD. The unit of measure for the LC50 is usuallyexpressed as part per million (ppm).

Chronic Toxicological Studies (1) Chronic toxicity tests may be performed over a period of months, years,or the lifetime of the test animal. Doses of the toxic substance areselected to assure that most of the animals will survive the entire timethe study is performed. Different species of test animals may be more or less sensitive to thesame toxic chemical. Males and females of the same species may respond differently to thesame substance. Although acute and chronic studies provide useful information inevaluating chemical toxicity, it is important to understand that they arenot truly representative of the environment to which people are exposedin everyday life.

Chronic Toxicological Studies (2) Characteristics of animal studies–––––A single doseA single route of exposureThe number of animals exposed is small.The genetic make-up of the population is not very diverse.Only individuals in good health and/or of the same sex are selected. In reality– Several different routes– Exposure to more than one chemical at a time– Greater genetic variability in a larger population – the range ofresponses to a given chemicals more diverse.– The collective interactive effect of all these factors on toxicitymakes it difficult to establish a cause-effect relationship.

Dose-Response Curves (1) The major purpose for performing acute and chronic toxicity studies isto establish a cause-effect relationship between exposure to a toxicsubstance and an observed effect in order to determine a safe exposurelevel. A curve can be drawn that illustrates the relationship between the doseadministered and the observed response. This curve is referred to asthe dose-response curve. A dose-response curve can be developed form most chemicals. Fromthese curves the threshold level and the relative toxicity of chemicals canbe obtained to help establish safe levels of chemical exposure.

Dose-Response Curves (2) The threshold is the dose below which no effect is detectedor above which an effect is first observed. The threshold information is useful information inextrapolating animal data to humans and calculating whatmay be considered a safe human dose for a given toxicsubstance. The threshold dose (ThD0.0) is measured as mg/kg/day. Itis assumed that humans are as sensitive as the test animalused. To determine the equivalent dose in man the ThD0.0is multiplied by the average weight of a man, which isconsidered to be 70 kg.

Dose-Response Curves (3) The calculation used to determine the safe human dose(SHD) is as follows:ThD 0.0 70 (kg)SHD Amount mg/day of toxic substanceSFWhereSHDThD0.070 KgSF Safe Human Dose Threshold dose at which no effect is observed. Average weight of a man Safety factor (ranges from 10 to 1000, which variesaccording to the type of test and data used toobtain the ThD0.0.

Terminology Associated withDose-Response Curves (1) No observed effect level (NOEL)– The highest tested dose of a substance that has beenreported to have no harmful (adverse) health effects onpeople or animals. No observed adverse effect level (NOAEL)– It denotes the level of exposure of an organism, found byexperiment or observation, at which there is nobiologically or statistically significant (e.g. alteration ofmorphology, functional capacity, growth, developmentor life span) increase in the frequency or severity of anyadverse effects in the exposed population whencompared to its appropriate control.

Terminology Associated withDose-Response Curves (2) lowest-observed-effect-level (LOEL)– Lowest concentration or amount of a substance, foundby experiment or observation, that causes any alterationin morphology, functional capacity, growth, development,or life span of target organisms distinguishable fromnormal (control) organisms of the same species andstrain under the same defined conditions of exposure lowest-observed-adverse-effect-level (LOAEL)– Lowest concentration or amount of a substance, foundby experiment or observation, which causes an adversealteration of morphology, functional capacity, growth,development, or life span of a target organismdistinguishable from normal (control) organisms of thesame species and strain under defined conditions ofexposure.

Terminology Associated withDose-Response Curves (3) Frank-effect level (FEL)– Level of exposure that produces unmistakable andirreversible effects (such as impairment or mortality)at statistically significant increased severity orfrequency.– The level where overt effects are observed is referred toas the Frank-effect level.

Threshold Dose Dose below which no adverse effects are observable in apopulation of exposed individuals. Threshold dose approximated by a NOAEL (No ObservedAdverse Effect Level)

Non-threshold Effects Thresholds are assumed not to exist for genotoxiccarcinogens

Safe Dose for Threshold Effect NOAEL approach– Identify the adverse effect that occurs at the lowest dose– Determine the NOAEL or LOAEL for that endpoint– Divide NOAEL/LOAEL by uncertainty factors

Acceptable Daily Intake ADI estimated (maximum) amount of an agent, expressedon a body mass basis, to which a subject may be exposedover his lifetime without appreciable health risk (also TDI,tolerable daily intake) ADI derived from NOAEL by the use of uncertaintyfactors UFs (or safety factors SFs)

Reference Dose (RfD) RfD is an estimate of the daily exposure that is likelyto be without appreciable health effect even ifcontinued exposure occurs over a lifetime.

Uncertainty Factors Default values:UFinterspecies 10 , UFhuman variability 10

Additional Uncertainty Factors Sometimes applied:– UFLOAEL-NOAEL 3 or 10– UFsubchronic-chronic 3 or 10– UF database insufficiences up to 10 MF, modifying factor for professional judgement: upto 10 RfD NOAEL (or LOAEL) / UF1 UF2 UF3 MF

Benchmark Dose Approach (1) NOAEL approach uses only single points, shape ofdose-response is ignored. Bench mark dose (BMD) is calculated from the curvefitted to the dose-reponse data, so all information isused. BMD can only be used when available data aresuitable for modelling. Not replacement for NOAEL, but additional tool

Benchmark Dose Approach (2)1. A mathematical model is applied to theexperimental data to produce a dose-response curveof best fit.2. By statistical calculation an upper 95% confidencelimit of the curve is determined3. The Benchmark Response is defined as 10% (or 5%,or 1%).4. The Benchmark Dose corresponds to the benchmark response on the upper confidence limit curve.

Benchmark Dose Approach (3)

Non-threshold? Absence of a detectable effect at low doses is eitherbecause the dose is below a threshold or because theresponse is below the level that can be detected by thetest sytem.

Threshold or Non-threshold Presence of a threshold cannot be proven fromexperimental data. Conclusions about existence of a threshold are basedon biological plausibility and expert judgement. Genotoxic effects (DNA dammage leading to cancer)are thought to be possible at any level of exposure, sono threshold.

Low-dose Extrapolation Dose-response curve is use to extrapolate to doses lowerthan then the experimental region.

Calculated Risk or Safe Dose Extrapolation may be used to calculate:– the risk associated with a known intake.– intake associated with a de minimis risk, e.g. anincreased life time risk of developing cancer of 1in 106 (‘virtually safe dose’, VSD).

Linear Low-dose Extrapolation Assumption: linear relationship between dose andbiological response

Linear Extrapolation from LED10 LED10– low confidence limit of dose with response of 10%

From LED10 to VSD If LED10 57 mg/kg/day (response 10% or 0,1) then VSD (virtual safe dose, response 1 in 106 or 10-6) 57 x 10-6 / 0,1 5,7 x 10-4 mg/kg/day

ALARA Approach For genotoxic carcinogens (non-threshold effects)also the ALARA-approach may be followed:exposure to be reduced to as low as reasonablyachievable (ALARA) or as low as reasonablypractical (ALARP)

Relative Toxicity (1) A comparison of the dose-response curves for two or more differentchemicals – administered to the same type of test animal – can be usedto determine the relative toxicity of the chemicals. The LD50 for chemical A is less than the LD50 for chemical B.Therefore, chemical A is considered more toxic than chemical B.

Relative Toxicity (2) Chemical A is more toxic than chemical B at the higherdoses, but at the lower doses chemical B is more toxic thanchemical A.

Relative Toxicity (3) Relative toxicity also be evaluated by comparing the slopeof each dose-response curve. A steep dose-response curveis indicative of a chemical that is rapidly and extensivelyabsorbed and slowly detoxified: therefore, the chemical ismore toxic.

Variables Influencing Toxicity These factors include:–––––––––the dose-time relationship,the route of exposure,the physical and chemical properties of the toxic substance,the chemical interactions,the interspecies and intraspecies variability,genetics,age,sex, andhealth of the individual collectively. The toxicity of a given substance is the net result of thecomplex interaction between all of these various factors.

Dose-Time Relationship Not only is the dose of a toxic substance important indetermining toxicity, but so is the frequency and length ofexposure. The rapid elimination of the substance from the bloodstream decreases the probability of an adverse effect. Gradual accumulation increases the probability that anadverse effect will occur.– Symptoms of acute exposure to sublethal quantities of chlorinatedsolvents, such as chloroform or carbon tetrachloride, are primarilyassociated with the nervous system, such as excitability, dizzinessand narcosis.– Chronic exposure to these solvents is primarily associated withliver damage.

Routes of Exposure LD50 values for various organochlorine andorganophosphate pesticides resulting from ingestion andskin exposure. The different absorption rates for various regions of theskin in the human male