INTRODUCTION TO TOXICOLOGY THE DOSE-RESPONSE AND...

INTRODUCTION TO TOXICOLOGY THE DOSE-RESPONSE AND BASIC

CONCEPTS. MARINA GOUMENOU

Chemist - Toxicologist

BSc, MSc, PhD, ERT

EUROPEAN FOOD SAFETY AUTHORITY

DISCLAIMER

This presentation contains the view of the author, and does not represent an official view of European Food Safety Authority

Dr. Marina, Goumenou, Scope and ethical principles of toxicology and 3R, EUROTOX Course of Basic Toxicology, Zagreb, Croatioa, 4/4/2016

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PRESENTATION OVERVIEW

1 • Basic terms

2 • Fields of Toxicology

3 • The dose and the dose – response

4 • The Risk Assessment

5 • Toxicity Testing

4/4/2016 Introduction to Toxicology, the Dose-Response and Basic Concepts, EUROTOX Course of Basic Toxicology, Dr. Marina Goumenou

WHAT IS TOXICOLOGY?

“The science of poisons”

“The study of how natural or man-made poisons cause undesirable effects in living organisms”

“The study of the adverse effect of chemical or physical agents on living organisms”


WHAT IS ADVERSITY?

“A change in the morphology, physiology, growth, development, reproduction or lifespan of an organism, system or (sub)population that results in an impairment of functional capacity, an impairment in the capacity to compensate for additional stress or an increase in susceptibility to other influences” (IPCS/WHO, 2009)

Adverse effects may occur in many forms, ranging from immediate death to subtle changes not realized until months or years later. They may occur at various levels within the body, such as an organ, a type of cell, or a specific biochemical. Knowledge of how toxic agents damage the body has progressed along with medical knowledge. It is now known that various observable changes in anatomy or body functions actually result from previously unrecognized changes in specific biochemicals in the body


PARACELSUS (~1500AD)

"All substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy“

• Specific chemicals were actually responsible for the toxicity of a plant or animal poison.

• The body's response to those chemicals depended on the dose received.

• Small doses of a substance might be harmless or beneficial whereas larger doses could be toxic.


THE DOSE MAKES THE POISON


TOXICANT, TOXIN, OR POISON?

A toxic agent is anything that can produce an adverse biological effect. It may be chemical, physical, or biological in form. For example, toxic agents may be chemical (such as cyanide), physical (such as radiation) and biological (such as snake venom).


HOW DOES TOXICITY DEVELOP?

The toxicity pathway concept as proposed by the National Academies. The horizontal line depicts a biological process, or signaling pathway, that generates a normal biologic function from an input. When a xenobiotic chemical is introduced (vertical arrows descending toward the pathway), interaction of the chemical with some molecular target perturbs the pathway and leads to early cellular changes. At low levels of exposure (low internal dose), homeostatic mechanisms generate adaptive responses that maintain or restore the function of the pathway. At high doses, these adaptive responses are overcome, leading to cell injury and, eventually, to morbidity and mortality. Adapted from NRC (2007).


WHAT IS A “SENSITIVE SUB-POPULATION?

” A sensitive sub-population describes those persons who are more at risk from illness due to exposure to hazardous substances than the average, healthy person. These persons usually include the very young, the chronically ill, and the very old. It may also include pregnant women and women of childbearing age. Depending on the type of contaminant, other factors (e.g., age, weight, lifestyle, sex) could be used to describe the population.


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FIELDS OF TOXICOLOGY (1)

Environmental Toxicology

• How various plants, animals, and humans are affected by exposure to toxic substances. • Study of chemicals that contaminate food, water, soil, or the atmosphere. • Toxic substances that enter bodies of waters such as lakes, streams, rivers, and oceans.

Occupational Toxicology

• Study of health effects from exposure to chemicals in the workplace.

• High exposure in specific chemicals (e.g. operator exposure)



Regulatory Toxicology

• Gathers and evaluates existing toxicological information to establish concentration-based standards of “safe” exposure. • The standard is the level of a chemical that a person can be exposed to without any

harmful health effects.

Food Toxicology

•Safe and edible supply of food to the consumer. determine the acceptable daily intake level for those substances. •Additives and processing aids, pesticides, heavy metals, contaminants, novel foods •Food allergies.



Clinical Toxicology

•Diseases and illnesses associated with short term or long term exposure to toxic chemicals. •Poisonings

Descriptive Toxicology

• The science of toxicity testing for safety evaluation and regulatory requirements

• Information from animal experimentation.



Forensic Toxicology

•Establish cause and effect relationships between exposure to a drug or chemical and the toxic or lethal effects that result from that exposure.

Analytical Toxicology

• Analytical toxicology identifies the toxicant through analysis of body fluids, stomach content, excrement, or skin.



Mechanistic Toxicology

•Study of the molecular, bio-molecular and cellular basis of toxic effects

Mixtures

Toxicology

• Study of mixtures toxicity and interactions of the components.

• Specific requirements in Risk Assessment


WHAT IS A DOSE IN TOXICOLOGY?

• The actual amount of a chemical that enters the body.

• Acute (short) exposure: a very short period of time, usually 24 hours

• Chronic (long-term) exposure: long periods of time such as weeks, months, or years.


BUT….

WHAT IS THE DOSE-RESPONSE RELATIONSHIP?

Dose-response relationship is a relationship between exposure and health effect, that can be established by measuring the response relative to an increasing dose.

The dose-response curve is the mathematical curve that expresses this relationship.

The dose-response relationship is based on observed data from experimental animal, human clinical, or cell studies


HOW A DRC LOOKS LIKE?

Usual assumption: Monotonic DRCs!

However there are Non-Monotonic DRCs!



HOW A DRC LOOKS LIKE?

WHAT A DRC SERVES?

• Establishes causality that the chemical has in fact induced the observed effects

• Establishes the lowest dose where an induced effect occurs

• Determines the rate at which injury builds up - the slope for the dose response.


WHAT IS THE THRESHOLD DOSE?

The dose below which (adverse) effect is not seen

• No observed adverse effect level (NOAEL), or the no observed effect level (NOEL).

• No all effects have a threshold!!! Genotoxicity


Response

Actual data

Approximate sensitivity limit

Dose

Threshold No threshold

A threshold for toxic effects occurs at the point where the body's ability to detoxify a xenobiotic or repair toxic injury has been exceeded. For most organs there is a reserve capacity so that loss of some organ function does not cause decreased performance. For example, the development of cirrhosis in the liver may not result in a clinical effect until over 50% of the liver has been replaced by fibrous tissue.

Knowledge of the shape and slope of the dose-response curve is extremely important in predicting the toxicity of a substance at specific dose levels. Major differences among toxicants may exist not only in the point at which the threshold is reached but also in the percent of population responding per unit change in dose (i.e., the slope). As illustrated above, Toxicant A has a higher threshold but a steeper slope than Toxicant B.


WHAT A DRC SERVES?


A, B, C, D: Options for low-dose extrapolation

(below observed response level)

E: Point of departure (POD)

F: No observed adverse effect level (NOAEL)

G: Lowest observed adverse effect level (LOAEL)

H, I: Mid and high dose response

T: Threshold dose

ED10 (EC10)

ED50 (EC50)

etc...

WHAT IS THE THRESHOLD DOSE?


BMD10 = 10% increase versus control

BMDL10 = lower one-sided 95%

confidence interval for BMD10

What is the Benchmark Dose (BMD)?

The dose that corresponds to a specific change in an adverse response compared to the response in unexposed subjects, and the lower 95% confidence limit is termed the benchmark dose level (BMDL).


What is Hormesis?

Implying one molecule may cause adversity

Radiation, genotoxicans and maybe endocrine disruptors

Examples of divergence of opinion on whether to assess as threshold or no threshold carcinogen: naphthalene,formaldehyde, ethylbenzene

4/4/2016

NO THRESHOLD EFFECTS?

Introduction to Toxicology, the Dose-Response and Basic Concepts, EUROTOX Course of Basic Toxicology, Dr. Marina Goumenou


• In silico - QSAR, structure alerts, selective binding to specific receptors

• In vitro - prokaryotic/eukaryotic, cell/organ culture

• In vivo

- Single dose toxicity (also skin and eye irritation) - Repeat dose toxicity (oral, dermal etc) - Genotoxicity (in vitro, in vivo) - Carcinogenicity (rodent lifetime assay) - Reproduction (fertility, developmental tox) - Other; sensitization, immunotoxicity, neurotoxicity

Hazard identification

DEFINITION OF RISK ASSESSMENT

• Many definitions!!!!

• But keep in mind:

Risk Assessment is a scientific/ mathematical discipline which is a substantive, changing and controversial field


Risk Management:

Putting the elements together


TERMINOLOGY OF RISK ASSESSMENT

http://www.efsa.europa.eu/sites/default/files/scientific_output/files/main_documents/2664.pdf


Risk assessment approaches of CAC, OIE and IPPC organisations cover in principle the same main questions:

• What can cause an adverse effect?

• How can it cause an adverse effect?

• What is the probability of an adverse effect occurring (i.e. what is the risk)?

• What are the consequences?

• What are the prerequisites for an adverse effect to indeed occur?

CAC: Codex Alimentarius Commission of the FAO/WHO

OIE: The World Organisation for Animal Health (Office International des Epizooties)

IPPC: The relevant international and regional organisations operating within the framework of the International Plant Protection Convention

TERMINOLOGY OF RISK ASSESSMENT


RISK ASSESSMENT - CAC A scientifically based process consisting of the following steps: (i) hazard identification, (ii) hazard characterization, (iii) exposure assessment, and (iv) risk characterization. (CAC, 2011)

Qualitative Risk Assessment: A risk assessment based on data which, while forming an inadequate basis for numerical risk estimations, nonetheless, when conditioned by prior expert knowledge and identification of attendant uncertainties permits risk ranking or separation into descriptive categories of risk. (FAO/WHO, 2008) Quantitative Risk Assessment: A risk assessment that provides numerical expressions of risk and indication of the attendant uncertainties (FAO/WHO, 2008)


RISK ASSESSMENT - OIE & IPPC OIE Evaluation of the likelihood and the biological and economic consequences of entry, establishment and spread of a hazard within the territory of an importing country (OIE, 2011) IPPC Pest risk assessment (for quarantine pests): Evaluation of the probability of the introduction and spread of a pest and the magnitude of the associated potential economic consequences (IPPC, 2011b) Pest risk assessment (for regulated non quarantine pests): Evaluation of the probability that a pest in plants for planting affects the intended use of those plants with an economically unacceptable impact (see (IPPC, 2011b)


RISK ASSESSMENT: TWO ROADS

Qualitative

• virtually the same thing as “hazard evaluation” step of “Quantitative” Risk Assessment

• is the material harmful to humans under any circumstances

• Codified by agencies, especially for cancer

Quantitative

• A formal process with four steps

• Ends with a mathematical estimation of actual risk, usually quantified as deaths per 1,000,000 per year or less.


(in many instances the three further steps are not taken)

Examples: EPA, IARC Cancer Monographs

“HAZARD EVALUATION” IS THE EQUIVALENT OF QUALITATIVE RISK ASSESSMENT.


TYPES OF STUDIES AVAILABLE FOR HAZARD EVALUATION

• BEST: Human Evidence (Epidemiology)

• Next best: Whole animal studies (toxicology; animals exposed to known dose and allowed to live to times of sacrifice or natural death)

• Other:

• In-vitro studies (studies on cells in culture)

• Structure-function relationship study and similar

• Identification of active compounds in metabolism


EXPOSURE ASSESSMENT

• Who will be exposed?

• From what source(s)?

• How much of the chemical do people inhale/ ingest ?

• In what period of time?

• To what exactly?

• Do exist interactions (e.g. smoking)


EXPOSURE ASSESSMENT

• Do exist adequate data for exposure assessment?

• Select the type of assessment based on the available data

• Screening

• Deterministic

• Probabilistic

• Model the exposure if possible

• How many do we want to include? E.g. 95%?


3 main ways:

1) derivation of a health-based guidance value, such as an ADI, TDI or acute reference dose (ARfD);

2) estimation of the margin of exposure (MOE) between a defined point on the dose–response curve and the level of human exposure; and

3) quantification of the magnitude of the risk at specified levels of human exposure.

http://apps.who.int/iris/bitstream/10665/44065/8/WHO_EHC_240_8_eng_Chapter5.pdf

RISK CHARACTERIZATION


Calculation of the health-based guidance value (HBGV) can be described as follows:

HBGV = POD / Ufs

where POD is a point of departure synonymously with Reference Point, RP (e.g. NOAEL) and UF is the uncertainty factor, a term often used synonymously with safety factor.

http://apps.who.int/iris/bitstream/10665/44065/8/WHO_EHC_240_8_eng_Chapter5.pdf



Indication of the margin between recommended or actual intakes and the RP

• Reference points (RPs) in toxicology studies used to calculate a safe level for human intake: NOAEL or BMD

• Description of the scientific uncertainties associated with the RP estimates

• Indication of the degree of scientific confidence that can be placed in these estimates

• where RP cannot be established an indication should be given on the highest level of intake where there is reasonable confidence in data on the absence of adverse effects

• Health-based guidance value (e.g. TDI) = NOAEL / UF

• UF = Uncertainty Factor (interspecies X intraspecies X other elements)



RISK ASSESSMENT VS RISK MANAGEMENT


BASIC BIBLIOGRAPHY!

• Casarett and Doull's Toxicology (C. Klaassen, M. Amdur, and J. Doull, eds.)

• Principles and Methods of Toxicology (A. W. Hayes, ed.)

• Basic Environmental Toxicology (L. Cockerham and B. Shane, eds.)

• Industrial Toxicology (P. Williams and J. Burson, eds.)


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Thank you for your attention

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