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Toxicity:- describes the degree to which a substance is poisonous or can cause injury. The
toxicity depends on a variety of factors: dose, duration and route of exposure, shape and structure
of the chemical itself, and individual animal factors.
What is Toxic? This term relates to poisonous or deadly effects on the body by inhalation
(breathing), ingestion (eating), or absorption, or by direct contact with a chemical.
What is a Toxin? The term “toxin” usually is used when talking about toxic substances
produced naturally. A toxin is any poisonous substance of microbial (bacteria or other tiny plants
or animals), vegetable, or synthetic chemical origin that reacts with specific cellular components
to kill cells, alter growth or development, or kill the organism.
What is a Toxic Symptom? This term includes any feeling or sign indicating the presence of a
poison in the system.
What are Toxic Effects? This term refers to the health effects that occur due to exposure to a
toxic substance; also known as a poisonous effect on the body.
What is Selective Toxicity? “Selective toxicity” means that a chemical will produce injury to
one kind of living matter without harming another form of life, even though the two may exist
close together.
How Does Toxicity Develop? Before toxicity can develop, a substance must come into contact
with a body surface such as skin, eye or mucosa of the digestive or respiratory tract. The dose of
the chemical, or the amount one comes into contact with, is important when discussing how
“toxic” a substance can be.
What is dose-response? Dose-response is a relationship between exposure and health effect can
be established by measuring the response relative to an increasing dose. This relationship is
important in determining the toxicity of a particular substance. It relies on the concept that a
dose, or a time of exposure (to a chemical, drug, or toxic substance), will cause an effect
(response) on the exposed organism. Usually, the larger or more intense the dose, the greater the
response, or the effect. This is the meaning behind the statement “the dose makes the poison.”
What is the threshold dose? Given the idea of a dose-response, there should be a dose or
exposure level below which the harmful or adverse effects of a substance are not seen in a
population. That dose is referred to as the ‘threshold dose’. This dose is also referred to as the no
observed adverse effect level (NOAEL), or the no effect level (NEL). These terms are often used
by toxicologists when discussing the relationship between exposure and dose. However, for
substances causing cancer (carcinogens), no safe level of exposure exists, since any exposure
could result in cancer.
What is meant by ‘individual susceptibility?’ This term describes the differences in types of
responses to hazardous substances, between people. Each person is unique (because our genes
differ in approximately 0.1% of the three billion base pairs that comprise the animal genome),
and because of that, there may be great differences in the response to exposure. Exposure in one
person may have no effect, while a second person may become seriously ill, and a third may
develop cancer or toxicity symptoms.
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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What are “toxicokinetics” and “toxicodynamics?” Toxicokinetics describes the fate of toxic
compound in the body. The measurement of the time course of absorption, distribution,
biotransformation, and excretion of toxic compounds (sometimes referred to as
pharmacokinetics).
Toxicodynamics describes the determination and quantification of the sequence of events at the
cellular and molecular levels leading to a toxic response to an environmental agent (sometimes
referred to as pharmacodynamics).
What are “toxicogenetics and toxicogenomics?” Toxicogenetics describes consideration of
stable and heritable alterations in the genome that are able to influence the relative susceptibility
of an individual (or group of individuals) to the adverse health effects that may result from
exposure to an exogenous material. Toxicogenomics, on the other hand, describes analysis of
gene-expression changes induced in a biological system by exposure to a xenobiotic. The two
disciplines are linked; polymorphisms that alter biological function may change the spectrum of
genes regulated in response to a toxicant. In this way, toxicogenetic differences can underpin
variations in toxicogenomic response.
Sub-disciplines of Toxicology (The field of toxicology can be further divided into the
following subdiscipline):
1- Environmental Toxicology:- is concerned with the study of chemicals that contaminate
food, water, soil, or the atmosphere. It also deals with toxic substances that enter bodies
of waters such as lakes, streams, rivers, and oceans. This sub-discipline addresses the
question of how various plants, animals, and animals are affected by exposure to toxic
substances.
2- Occupational (Industrial) Toxicology:- is concerned with health effects from exposure
to chemicals in the workplace. This field grew out of a need to protect workers from toxic
substances and to make their work environment safe. Occupational diseases caused by
industrial chemicals account for an estimated 50,000 to 70,000 deaths, and 350,000 new
cases of illness each year in the United States.
3- 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.
4- Food Toxicology:- is involved in delivering a safe and edible supply of food to the
consumer. During processing, a number of substances may be added to food to make it
look, taste, or smell better. Fats, oils, sugars, starches and other substances may be added
to change the texture and taste of food. All of these additives are studied to determine if
and at what amount, they may produce adverse effects. A second area of interest includes
food allergies. Almost 30% of the American people have some food allergy. For
example, many people have trouble digesting milk, and are lactose intolerant. In addition,
toxic substances such as pesticides may be applied to a food crop in the field, while lead,
arsenic, and cadmium are naturally present in soil and water, and may be absorbed by
plants. Toxicologists must determine the acceptable daily intake level for those
substances.
5- Clinical Toxicology:- is concerned with diseases and illnesses associated with short term
or long term exposure to toxic chemicals. Clinical toxicologists include emergency room
physicians who must be familiar with the symptoms associated with exposure to a wide
variety of toxic substances in order to administer the appropriate treatment.
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6- Descriptive Toxicology:- is concerned with gathering toxicological information from
animal experimentation. These types of experiments are used to establish how much of a
chemical would cause illness or death.
7- Forensic Toxicology:- is used to help establish cause and effect relationships between
exposure to a drug or chemical and the toxic or lethal effects that result from that
exposure.
8- Analytical toxicology:- identifies the toxicant through analysis of body fluids (e.g. urine,
blood, bile, oral fluid, vitreous humor or cerebrospinal fluid) and tissues (brain cortex,
liver, heart, kidney, fat tissues), stomach content, and/or drug paraphernalia. Hair is
mostly used to detect long-term exposure to drugs of abuse. Assessment of drugs effects
on behavior and performance, and evaluation of toxic effects, can be made only after
determination of the concentrations of drugs and of their active metabolites in blood.
Urine is the matrix of choice for drug detection and identification. Oral fluid will become
the matrix of choice for roadside testing of drugs impairing driving capability.
9- Mechanistic Toxicology:- makes observations on how toxic substances cause their
effects. The effects of exposure can depend on a number of factors, including the size of
the molecule, the specific tissue type or cellular components affected, whether the
substance is easily dissolved in water or fatty tissues, all of which are important when
trying to determine the way a toxic substance causes harm, and whether effects seen in
animals can be expected in animals.
Classification of Toxic Agents (Toxic substances are classified into the following):
1. Heavy Metals:- Metals differ from other toxic substances in that they are neither created
nor destroyed by animals. Their use by animals plays an important role in determining
their potential for health effects. Their effect on health could occur through at least two
mechanisms: first, by increasing the presence of heavy metals in air, water, soil, and
food, and second, by changing the structure of the chemical. For example, chromium III
can be converted to or from chromium VI, the more toxic form of the metal.
2. Solvents and Vapors:- Nearly everyone is exposed to solvents. Occupational exposures
can range from the use of “white-out” by administrative personnel, to the use of
chemicals by technicians in a nail salon. When a solvent evaporates, the vapors may also
pose a threat to the exposed population.
3. Radiation and Radioactive Materials:- Radiation is the release and propagation of
energy in space or through a material medium in the form of waves, the transfer of heat
or light by waves of energy, or the stream of particles from a nuclear reactor.
4. Dioxin/Furans:- Dioxin, (or TCDD) was originally discovered as a contaminant in the
herbicide Agent Orange. Dioxin is also a by-product of chlorine processing in paper
producing industries.
5. Pesticides:- The EPA defines pesticide as any substance or mixture of substances
intended to prevent, destroy, repel, or mitigate any pest. Pesticides may also be described
as any physical, chemical, or biological agent that will kill an undesirable plant or animal
pest.
6. Microbial toxins:- Bacteria, fungi and algae are the microorganisms typically associated
with microbial toxin production. Cholera toxin produced by Vibrio cholerae is the
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virulence factor responsible for the massive secretory diarrhea seen in Asiatic cholera (5
million cases each year).
7. Mushroom toxins:- Several mushroom species (e.g. Amanita phalloides, A. virosa),
produce a family of cyclic octapeptides called amanitins. Symptoms of intoxication
appear at the end of a latent period of 6-48 hours during which the patient shows no
symptoms. Death in 50-90% of the cases from progressive and irreversible liver, kidney,
cardiac damage may happen 6-8 days after ingestion.
8. Plant Toxins:- Different portions of a plant may contain different concentrations of
chemicals. Some chemicals made by plants can be lethal. For example, taxol, used in
chemotherapy to kill cancer cells, is produced by a species of the yew plant. One of the
main toxic proteins is "ricin" from the seeds of castor bean plant. Perhaps, ingestion of
just one milligram of ricin can kill an adult. The main alkaloid of Aconitum plants is
aconitine, a highly toxic diterpenoid alkaloid. Ingestion of a few grams of roots may
result in death occuring from ventricular arrhythmias, which are most likely to occur
within the first 24 hours.
9. Animal Toxins:- These toxins can result from venomous or poisonous animal releases.
Venomous animals are usually defined as those that are capable of producing a poison in
a highly developed gland or group of cells, and can deliver that toxin through biting or
stinging. Poisonous animals are generally regarded as those whose tissues, either in part
or in their whole, are toxic. Batrachotoxins are extremely potent cardiotoxic and
neurotoxic steroidal alkaloids found in skin secretions from certain species of frogs
(poison dart frogs). The most toxic frog is very likely the golden poison frog, Phyllobates
terribilis.
Subcategories of Toxic Substance Classifications (All of these substances may also be
further classified according to their):
1- Effect on target organs (liver, kidney, hematopoietic system),
2- Use (pesticide, solvent, food additive),
3- Source of the agent (animal and plant toxins),
4- Effects (cancer mutation, liver injury),
5- Physical state (gas, dust, liquid),
6- Labeling requirements (explosive, flammable, oxidizer),
7- Chemistry (aromatic amine, halogenated hydrocarbon), or
8- Poisoning potential (extremely toxic, very toxic, slightly toxic)
Toxicokinetics - the study of the time course of toxicant absorption, distribution, metabolism, and
excretion
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PRINCIPLES OF TOXICOKINETIC STUDIES
I-ABSORPTION
The rate and extent of absorption of the administered substance can be estimated by various methods,
with and without reference groups (i.e., a test group in which the substance is administered via another
route that ensures complete availability of the dose). These methods include:
(a) determination of the amount of test substance and/or metabolites in urine, bile, feces, and exhaled
air, and that remaining in the carcass;
(b) comparison of a biological response (e.g., acute toxicity studies) between test and control and/or
reference groups;
(c) comparison of the amount of dose excreted really in test and reference groups; or
(d) determination of the area under the plasma steady state curve of the test substance and/or
metabolites and comparison with data from a reference group.
Factors influencing the rate and extent of absorption of a chemical
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1. Properties of (the Morphology and dimension of the absorbing organism body surface, perfusion
of the absorbing area, distribution and elimination processes, general factors (e.g., nutritional
status, age, disease).
2. Characteristics of Relative molecular mass the chemical -Physical state
- conformation
- aggregation
- dispersion
Charge
- acid or base characteristics
Stability
Reactivity
Solubility in various solvents
3. Characteristics of Dose/concentration, duration of contact with exposure the absorbing surface
4. Exogenous factors Formulation
- vehicle
- additives
Interaction with other toxic chemicals
Physical conditions (e.g., temperature,
radiation)
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Routes of Exposure of toxicants
1. Dermal route for dermal exposure must take into account species differences and micro-lesions
that appear after shaving in furbearing species. If a definite area of skin or even the whole
animal is exposed to a chemical, the solvent used, water uptake by skin, etc., can influence the
absorption rate, much more than the physical and chemical properties of the substance under
consideration. After various lengths of exposure, blood samples can be assayed or the appearance
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of the chemical and of its metabolites can be monitored in urine. The skin can also be analyzed
after washing and cleaning. Precautions should be taken to avoid inhalation exposure.
2. The intravenous (iv) route
The intravenous (iv) route introduces the chemical in solution directly into the blood stream, avoiding
the process of absorption.
3. Intraperitoneal (ip) administration
In general, after intraperitoneal (ip) administration of a chemical, absorption is facilitated by the large
surface of the peritoneal cavity. The chemical mainly enters the liver by the portal circulation; thus, first
pass effects must be considered.
4. Intramuscular (im) administration
In general, the chemical is readily absorbed after intramuscular (im) administration, because of the
good perfusion of muscular tissue, but it must also pass several membranes.
5. Subcutaneous (sc) administration
After subcutaneous (sc) administration, absorption is relatively slow. Changes in the perfusion by
vasoactive compounds (e.g., vasoconstriction by sympathomimetics, vasodilatation by local anaesthetics)
as well as peculiarities in the formulation (solvent, microcrystals, etc.) can strongly influence the rate of
absorption.
Factors Affecting GI Absorption
Disintegration of dosage form and dissolution of particles
Chemical stability of chemical in gastric and intestinal juices and enzymes
Rate of gastric emptying
Motility and mixing in GI tract
Presence and type of food
II-DISTRIBUTION
Distribution is the processes by which an absorbed substance and/or its metabolites circulate and
partition within the body.
Two approaches can be used for the analysis of distributio patterns:
(a) the qualitative approach using information obtained by whole-body autoradiographic techniques; and
(b) the quantitative approach using information obtained by sacrificing animals at different times after
exposure and determining the concentration, and amount of, the test substance and/or metabolites in
tissues and organs.
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Invasive Methods
1- Auto radiographic methods
Auto radiographic methods have been developed to study the distribution of toxic chemicals after
labeling with radioactive isotopes (
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H,
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C,
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S, etc.) with alpha-, beta-, and gamma-radiation, in the
whole body of experimental animals. By killing the animals at different times after administration or by
taking biopsies, the time curves of distribution in the various tissues and organs as well as the
distribution patterns at equilibrium can be obtained. Autoradiographic methods can also be used for
distribution studies at the tissue and cell levels.
2- Radiometric methods
After administration of a radiolabelled chemical, blood and tissue samples are taken at intervals, either
after killing the animals or by obtaining biopsies, and the activities determined by different techniques,
depending on the radiolabel and radiation type. In this way, time distribution curves as well as
distribution patterns can be obtained.
3. Chemical methods
If sensitive and specific chemical detection methods are available (section 2), distribution-time curves
and patterns can be studied after killing the animals or by taking biopsies at different intervals after
administration.
Non-Invasive Methods
Non-invasive methods are desirable in studying the distribution of toxic chemicals in expensive non-
rodents, and especially in man. Limited information can be obtained through the detection of toxic
chemicals and their metabolites in saliva, breath, and urine (using radiometric, chemical, or stable isotope
methods), as well as through whole-body scanning, after administration of radiolabelled substances.
However, developments in this field, such as positron imaging, X-ray fluorescence, neutron activation,
and magnetic pneumography, deserve special attention. They include positron imaging, X-ray
fluorescence, neutron activation, and magnetic pneumography.
Blood Brain Barrier – characteristics:
1. No pores in endothelial membrane
2. Transporter in endothelial cells
3. Glial cells surround endothelial cells
4. Less protein concentration in interstitial fluid
BINDING
The following physical methods for studying chemical-protein binding can be carried out only in very
special cases:
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(a) ultraviolet and visible absorption spectroscopy of free and bound chemical;
(b) fluorescence spectroscopy;
(c) optical rotatory dispersion and dichroism; or
(d) nuclear magnetic resonance.
III- METABOLISM
In this section, metabolism refers to the process or processes by which an administered xenobiotic
chemical is structurally altered in the body by either enzymatic or nonenzymatic reactions. In this
context, the terms biotransformation and metabolic transformation are used interchangeably with
metabolism. Xenobiotic denotes a relatively small (relative molecular mass < 1000), non-nutrient
chemical that is foreign to the species in which biotransformation is being studied, though certain
compounds biosynthesized by some species (e.g., alkaloids, glycosides) are
xenobiotics in others.
The major role of biotransformation is to convert poorly excretable lipophilic compounds to more polar
entities that can be readily excreted in the urine and/or the bile. In the absence of metabolism, such
xenobiotics accumulate in the mammalian body, increasing the potential for a toxic response. Examples
of such compounds are certain polychlorinated biphenyl (PCB) and polychlorinated dibenzofuran (PCDF)
congeners. On the other hand, biotransformation is less likely in xenobiotics that have high water/oil
partition ratios (hydrophilic compounds), which are rapidly excreted in urine.
Two or more sequential enzymatic reactions are routinely required to convert lipophilic xenobiotics to
metabolites that are efficiently excreted. Classified the pathways involved into phase I and phase II
reactions. Oxidation, reduction, and hydrolysis are termed phase I reactions, whereas conjugation and
synthesis are phase II reactions. Normally, one or more phase I reactions precede phase II metabolism.
Initially, xenobiotic metabolism was associated with detoxication. However, it is now known that both
phase I and phase II reactions function in metabolic activation processes as well. Many different types of
compounds are converted to their ultimate toxic chemical species during metabolism; a few of the best
studied examples include acetaminophen, 2-acetylaminofluorene, aflatoxin B1, benzopyrene, carbon
tetrachloride, diethyl nitrosamine, dimethylnitrosamine, and 4-ipomeanol.
VI- EXCRETION
Toxic chemicals are eliminated from the body by various routes. The relative importance of the
excretion processes depends on the physical and chemical proporties of the compound and its various
metabolites. The mechanisms by which a chemical passes through a biomembrane can be classified into
2 general types:
(a) diffusion or filtration of the substance, in which the cell membrane does not require energy to carry
out the process; and
(b) carrier-mediated transport of the chemical through the membrane, in which energy-dependent and
-independent processes can be involved.
Sites of Excretion
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1- Kidney
The kidney is the most efficient organ for the elimination of most toxic chemicals from the body. It
receives about 25% of the cardiac output, 20% of which is filtered at the glomeruli. The excretion
processes involved are passive glomerular filtration, tubular reabsorption, and active tubular secretion.
2- Liver-biliary excretion
In general, lower relative molecular mass anionic and cationic compounds are excreted through the
kidneys, whereas biliary excretion is an important excretion route for many compounds with
comparatively high relative molecular mass (approximately 300 - 700). The metabolites formed in the
liver may be excreted directly into the bile without entering the blood-stream. The biliary excretion of
compounds is influenced not only by hepatic function, but also by blood flow.
It has been suggested that there are 2 types of bile formation: bile salt-dependent and bile salt-
independent. Over 200 toxic chemicals and/or their metabolites have been detected in the bile. The
biliary excretion of toxic chemicals varies considerably among species, including human beings, and is
generally high in the dog and the rat.. The bile-to-plasma concentration ratios also vary markedly from
compound to compound.
3- Other excretory sites (saliva, milk, tears, and sweat)
The excretion of toxic chemicals in biological fluids such as saliva, milk, tears, and sweat is minor
compared with renal excretion. However, these fluids are quite important in studies of toxicokinetics,
because they can be monitored for xenobiotics and their metabolites. Concentrations of toxic chemicals
in saliva generally reflect the free fractions of the chemical in plasma and can be determined by non-
invasive techniques. Because of pH differences between saliva (pH 6.7 - 6.9) and plasma, organic bases
such as nicotine, theobromine, and caffeine tend to be concentrated in the saliva, whereas organic acids
such as salicylic acid diffuse into the saliva less readily