Download Concepts and Terminology Toxicology

Concepts and Terminology
Toxicology
Is the study of poisons, including their chemical properties
and biological effects.
Toxicant an alternative term for poison.
Toxin: A poison that originates from biological processes
also called a biotoxin. Examples; Mycotoxins (fungal
toxins) and zootoxins (animal toxins). Many plants are also
known to be toxic when consumed by specific types of
animals.
Toxicity: The quantity or amount of a poison that causes a
toxic effect.
Toxicosis: A disease state that results from exposure to a
poison.
Toxicology versus pharmacology
Pharmacology is the study of chemicals (drugs) used at doses
to achieve therapeutic (beneficial) effects on an organism.
Toxicology is the study of chemicals (toxicants) that produce a
harmful (detrimental) effect on an organism.
Dose: The amount of toxicant that is received per animal.
Dosage: The amount of toxicant per unit of animal mass or
weight. It can also be expressed as the amount of toxicant
per unit of mass or weight per unit of time. For examples, a
dog could receive a dosage of chemical at the rate of 2
mg/kg/day. When conducting traditional acute, subacute,
subchronic, or chronic studies, the length and frequency of
exposure are also noted. For examples, rats may receive a
chemical dosage of 2.5 mg/kg/day for 2 years.
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Route of exposure:
The most common routes of exposure are inhalation, oral,
and dermal, with some variations for each. Less frequently
used routes of exposure include rectal, sublingual,
subcutaneous, and intramuscular.
Threshold dose:
The highest dose of a toxicant at which toxic effects are not
observed.
Lethal dose (LD) or median lethal dose (MLD) (LD50):
The dose that will kill 50% of a group of animals during
some period of observation in acute toxicity study.
Lethal concentration (LC) or minimal toxic dose:
is the lowest concentration of a chemical or drug in a matrix
(usually feed or water) that causes death.
Effective dose (ED).The dose of drug or toxicant or
therapeutic agent that produces some desired effect in 50%
of a population.
Therapeutic index (TI).
Defined by the equation :
TI =
LD50
ED50
the TI is a unitless estimate that characterizes the relative
safety of a drug or chemical. The larger the TI, the more
“safe” a chemical is relative to another with a smaller TI. For
example, if chemical X has an LD50 of 1000 mg/kg and an
ED50 of 10 mg/kg, the TI would be 100 (the mg/kg units
cancel). Compare this to chemical Y, which has an LD50 of
50 mg/kg and an ED50 of 40 mg/kg. The TI of chemical Y is
1.25, a much less safe chemical when compared with
chemical X.
Standard safety margin (SSM) or margin of safety
(MoS).
Defined by the equation:
LD1
SSM =
ED99
the SSM, like the TI, is a unitless estimate that characterizes
the relative safety of a drug or chemical, but much more
conservative data are used. The larger the SSM, the more
safe the chemical tends to be relative to other chemicals
with smaller SSMs.
Exposure duration.
The length of time an animal is exposed to a drug or
chemical. In general, there are four subgroups:
- Acute: Exposure to a single or multiple doses during a 24-
hour period. The LD50 is often determined during acute
exposure studies.
- Subacute: Exposure to multiple doses of a toxicant for
greater than 24 hours but for as
long as 30 days.
- Subchronic: Exposure lasting from 1 to 3 months.
- Chronic: Exposure for 3 months or longer.
Hazard (risk):
a chemical or drug will cause harm under certain
conditions.
Toxic effect : damage effect to certain biological system or
process caused by poison or drug in high dose.
Side effect : secondary predicted undesired effect that
accompanied the therapeutic effect.
Adverse effect: unpredicted undesired effect caused by drug
used at recommended dose ex. Allergy of penicillin.
The dose-response relationship
The result of exposure to the dose is any measurable 
quantifiable, or observable indicator. The response depends
on the quantity and route of chemical exposure or
administration within a given period. Two types of dose–
response relationships exist, depending on
the numbers of subjects and doses tested.
Graded Dose–Response
The graded dose–response describes the relationship of an
individual test subject or system to increasing and/or
continuous doses of a chemical.
Graded dose–response curve for caffeine HCl
chloramphenicol HCl(●) atropine sulfate , and phenol
Quantal Dose–Response
The quantal dose–response is determined by the
distribution of responses to increasing doses in animals of
test subjects or systems. This relationship is generally
classified as an “all-or-none effect” in which the test
system or organisms are quantified as either responders or
non-responders. A typical quantal dose–response curve is
illustrated in Figure by the LD50 (lethal dose 50%)
distribution.
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Quantal dose–response curve showing experimental derivation and graphic
estimation of LD50
Three general assumptions must be considered when
evaluating the dose-response relationship:
1. The chemical interacts with a molecular or receptor site to
produce a response.
2. The production of the response, or the degree of response,
is correlated to the
concentration of the chemical at that receptor site.
3. The concentration of the chemical at the receptor site is
related to the dose of chemical received.
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Toxicokinetics
Toxicant Exposure
Entrance to Body
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Ingestion
Skin
Inhalation
Absorption into Blood Stream and
Distribution to Body Tissues and Organs
Toxicity
Storage
Metabolism
Excretion
xenobiotic (foreign compound): they are substance which not
enter any biological process or used as a source of energy or
nutrition such as heavy mental
Absorption
Defines how much of a chemical passes into the body
over a period of time. Different routes of exposure
produce different absorption patterns, which can vary
both within a species (intraspecies variation) and
between different species (interspecies variation). For
a xenobiotic to exert a toxic effect, it must reach its
site of actions. It must reach to the body by crossing
any number of body membranes (e.g., skin, lung,
gastrointestinal tract, and red blood cell membranes).
Composition of these membranes varies, resulting in
various levels of resistance to penetration. For
example, the skin more resistance to penetration than
the lung alveolar surface.
Absorption can be described in terms of bioavailability (F),
which is the quantity or percentage portion of the total
chemical that is absorbed and available to be processed
(DME) by the animal. In the case of intravenous
administration, F = 100% because all of the xenobiotic
enters the animal.
Inhalation, oral, and dermal are the three usual routes of
exposure to xenobiotics.
Inhalation (pulmonary)
Inhalation exposure to chemicals occurs when the chemical
is dissolved in the ambient air inhaled by the animal. The
chemical first reaches the nasal passages, when some
absorption can take place before it enters the trachea,
bronchi, and finally the alveoli, the chemical can cross the
very thin alveolar wall and enter the blood
Oral (gastrointestinal)
Chemicals can enter the gastrointestinal tract in either
contaminated food or water sources. Depending on the
physicochemical properties of the chemical. For example,
some chemicals are unstable in the stomach’s acidic
environment and can be destroyed to varying degrees,
resulting in decreased absorption. On the other chemicals
are readily absorbed from the stomach and enter the small
intestine, absorption through the intestinal mucosa and then
into the blood. Portal circulation delivers them to the liver,
a major metabolic organ of the body.
Dermal (percutaneous)
Three key events must occur for percutaneous absorption to
take place.
 First, the chemical must be soluble in the vehicle (solvent)
that is applied to the skin.
 Second, it must be able to penetrate the thick keratin layer
of the epidermis.
 Third, it must make its way through the lower cells of the
epidermis and into a blood vessel.
Xenobiotics can pass through body membranes by either
passive transport or active transport.
1. Passive transport. Passive transport requires no energy
expenditure on the body’s part to transport the xenobiotic
across a cell membrane. Passive transport occurs via two
mechanisms: simple diffusion and filtration.
a. Simple diffusion.
Simple diffusion depends on both the lipid solubility and the
size of the molecule. In biological matrices, most xenobiotics
exist in a solution as either an ionized or un-ionized form. Unionized (uncharged) molecules have greater lipid solubility than
the ionized forms. Xenobiotic will penetrate a body membrane,
three facts must be known:
(1) whether the xenobiotic is a weak acid or a weak base
(2) the pH of the biological matrix
(3) the association constant of the xenobiotic (or pKa, the pH at
which 50% of the xenobiotic is ionized and 50% is un-ionized).
Once this information is known, the Henderson-Hasselbalch
equation for either a weak acid or a weak base can be
applied.
For a weak acid:
[un-ionized]
pKa – pH = log
[ionized]
For a weak base:
[ionized]
pKa – pH = log
[un-ionized]
The higher the ratio of un-ionized:ionized, the greater the
potential for absorption across a lipid membrane.
b. Filtration. Filtration relies on the potency of a membrane.
When water flows in bulk across a porous membrane, any
solute that is small enough to pass through the pores
flows with it.
2. Active transport. Active transport mechanisms usually
require an energy expenditure on the body’s part to transport
the xenobiotic across a cell membrane.
a. Active transport active transport moves xenobiotics against
concentration
gradients.
b. Facilitated transport expends energy on xenobiotic
transport, but it does not occur against a concentration
gradient.
c. Pinocytosis is another type of active transport mechanism that
involves the ability of cells to engulf small masses of
xenobiotic and carry it through the cell membrane.
Distribution
Once absorbed across one of the body’s barriers, the chemical
enters the blood so that it can be distributed to the body’s
organs and tissues. The chemical leaves the blood and enters
the tissues at varying rates, depending on a number of factors:
(1) rate of blood flow (generally, the higher the blood flow, the
more potential distribution to the organ)
(2) the ability of the chemical to traverse the capillary
endothelial wall
(3) the physiochemical properties of the chemical, such as lipid
solubility.
The extent of distribution within an animal can be described
by the volume of distribution (Vd), which is a proportion
between the amount of a chemical found in the blood to the
total amount of drug in the body at any given time. The
equation is
Vd = AC(t) / CB
AC(t ) is the total amount of the chemical in the body at
time
CB is the concentration of the chemical in the blood .
Examining the Vd equation, the higher the numerator or the
lower the denominator, the higher the Vd. Restated, the
higher the total chemical in the body or the lower the
concentration in the blood, the higher the Vd. Therefore,
the higher the Vd, the higher the distribution from the
blood to the tissues.
Metabolism (biotransformation)
Metabolism of chemicals varies, ranging from simple
hydrolysis, to glutathione conjugation, to no metabolism at
all. The liver possesses the most metabolism capacity,
regardless of species, other organs such as the kidneys,
gastrointestinal tract, skin, heart, and brain also have
considerable metabolic capabilities.
The metabolism of a xenobiotic usually occurs in several
steps. As stated earlier, a key component of metabolism is to
convert the xenobiotic into a water-soluble form.
so it can be excreted from the body. metabolic conversion
can be categorized into two steps or phases.
Phase I
metabolism converts a polar, lipophilic xenobiotics into
more polar and more hydrophilic metabolites via liberation
of functional groups that can be used during phase II. Phase
I metabolism uses a wide assortment of reactions that
processes the xenobiotic via hydrolysis, oxidation, or
reduction pathways.
Phase II
conjugates either the xenobiotic itself or its metabolite
formed during phase I metabolism with a functional group
that results in a multifold increase in water solubility.
Excretion
usually occurs via the kidney (urine), gastrointestinal tract
(feces), or lungs (exhalation of volatile chemicals);
however, other excretory mechanisms do exist (e.g., tears,
sweat, skin exfoliation). Not all xenobiotics are completely
absorbed, particularly via oral exposure. If absorption is
less than 100%, the xenobiotic can continue down the
gastrointestinal tract and either be metabolized by gut
microbes or be passed unmetabolized out of the body via
feces, some xenobiotics may be metabolized, excreted via
the bile.
Other non renal routes of excretion include milk,
cerebrospinal fluid, sweat, and saliva. Determining the sum
of clearance pathways can be defined by the following
equation:
CLB = CLR + CLNR
CLB is the total amount of xenobiotic and its metabolites
CLR is the amount cleared via the urine
CLNR is the sum of all non renal pathways.
Treatment
Antidotes
Antidotes are therapeutic agents that have a specific action
against the activity or effect of a toxicant. Antidotes can be
broadly classified as chemical or pharmacologic antidotes.
Chemical antidotes specifically interact with or neutralize
toxicants. For example, metal chelators such as calcium
disodium edetate (CaNa2EDTA) or succimer combine with
metals to form soluble metal-chelator complexes that are
subsequently eliminated via the kidneys.
Pharmacologic antidotes neutralize or antagonize toxicant
effects. Such antidotes can prevent formation of toxic
metabolites (fomepizole), compete with or block the action
of a toxicant at a receptor site (naloxone), or help restore
normal function (N-acetylcysteine).
The use of some antidotes in food animals can result in food
safety concerns. Because of these concerns, extended
withdrawal times have been established for ammonium
salts for treatment of copper intoxicated sheep (30 days)
and for methylene blue for treatment of nitrate/nitrite
intoxication in ruminants (180 days).
Calcitonin
Calcitonin is widely recommended for the treatment of
cholecalciferol (vitamin D)-induced hypercalcemia. The
recommended protocol is to administer 4 to 6 IU every 6
hours for up to 3 weeks, the side effects such as anorexia,
anaphylaxis, and emesis.
Pamidronate
It is efficacious for the treatment of hypercalcemia
associated with several human diseases and in dogs, needs
to be administered less frequently than calcitonin.
Fomepizole
Fomepizole has replaced ethanol as the antidote of choice
for treating ethylene glycol–intoxicated dogs. It is a better
inhibitor of alcohol dehydrogenase than ethanol, and its
without side effects. Don’t used in cats because of the less
effective inhibition of alcohol dehydrogenase in cats.
Succimer
Succimer is the chelator of choice for the treatment of
lead intoxicated small companion animals.
N-Acetylcysteine (NAC)
It is antidotal for acetaminophen intoxication in
humans, which are not readily available for guiding
veterinary therapy. the first dose of NAC should be
administered within 8 hours of exposure. It can be
administered either orally or intravenously. Oral
administration has not been associated with adverse
effects in humans, whereas intravenous administration
has resulted in urticaria, anaphylactoid reactions, and,
rarely, death.
Enhanced elimination
Various methods of increasing the elimination of absorbed
toxicants have been advocated.
Active charcoal
AC, may provide a gastrointestinal “dialysis” effect. In
studies using dogs, multiple doses of AC given orally
enhanced the elimination of IV administered theophylline.
It was hypothesized that theophylline, passing from the
circulation into the lumen of the gastrointestinal tract, was
adsorbed, thus preventing its reabsorption.
Forced diuresis
Forced diuresis via the IV administration of sodiumcontaining solutions is often recommended to hasten the
elimination of many toxicants via the kidneys.
Ion-trapping
Facilitating the removal of absorbed toxicants via the urine by
ion-trapping may be indicated in several specific situations.
For example, alkalinization of the urine to a pH of 7 or
greater with sodium bicarbonate has been shown to enhance
the urinary elimination of weak acids such as ethylene glycol,
salicylates and Phenobarbital. The administration of
ammonium chloride to acidify the urine (pH of 5.5 to 6.5)
may enhance the elimination of weak bases such as
amphetamine and strychnine
Peritoneal dialysis
Peritoneal dialysis has also been advocated to enhance the
elimination of water-soluble, low-molecular-weight, lithium,
salicylate, and theophylline. Other methods for hastening
elimination of an absorbed toxicant, such as charcoal
hemoperfusion and hemodialysis, are less practical and less
available in veterinary medicine.
Diagnostic Toxicology
Diagnosis depends heavily on a systematic approach that
includes sample collection and handling. Such cases require
piecing together a “diagnostic puzzle” that includes a
complete case history, clinical signs, clinicopathologic
findings, postmortem findings, results from chemical
analyses, and occasionally bioassay findings.
History
The primary goal of the history is to identify possible
sources of a toxicant. Sources are found by examining the
environment, reviewing management practices, and
recording the movement and fate of animals, feed, water,
and bedding.
Recent medication history should be recorded, including
information about the substances given, time of
administration, amount used, reason for use, and individuals
treating the animal.
Sample collection and storage
Samples for toxicology testing fall into three general
categories: environmental, antemortem, and postmortem.
Toxicology Samples are often best collected and held until
other testing can be completed because results from other tests
(e.g., histology, bacteriology) can give information about the
organ that is affected and perhaps the poison itself. All
samples should be labeled regarding the date, case, source,
description, and the clinician taking the sample. Fresh samples
should be frozen, which should be kept cool and dry
Environmental samples
Many toxicants are not detectable in tissues. Thus,
environmental samples (e.g., source materials, feeds) are
critical to diagnosis in a poisoning case as well as being
important for identifying and removing the source of a poison.
Feed samples
For example, mold toxins (e.g., aflatoxin in grain or nitrate from
weeds) in a lot of hay occur in “hot spots,” or isolated portions
of a lot or bag. Samples are individually labeled. Sample sizes
vary, but the optimal size in most instances is about 500 g of
material. Moist samples should be frozen. Feeds to be kept dry
should be stored in a cool, dry environment in containers.
Water samples
Water samples are obtained at the source (well, canal, pond), in
transit (piping, tankers), in storage tanks, and at the site of
exposure. Some chemicals are toxic in water even when present
in very low concentrations. Glass preserving jars are useful for
water samples. Plastic and metal containers should be avoided.
 - Metal and salt samples can be covered with plastic wrap and
then the lid;
 - Organic chemical samples should be covered with foil and
then the lid.
Plant samples
Many plant poisonings still rely on plant identification for
diagnosis. Pastures should be examined, with plants being
identified as the investigation proceeds. The amount of the
weeds in the bale, stack, or lot should be estimated, and then
the weeds can be sent to a laboratory for identification if
needed. Diagnosis of plant poisoning also requires evidence
of consumption of a plant by the animal.
Antemortem samples
The toxicology case involves sudden onset of disease or death
in a number of animals. Other cases may involve onset of
signs, or perhaps merely a decrease in production. The next
step in investigating a toxicology case is to examine the
affected animals. Clinical effects of toxicants vary, depending
on the organs involved, route of exposure, species and host
characteristics (e.g., age, past history, environment, use).
For example, some poisons may lead to acute signs in 
multiple animals in a short time after a single exposure.
Conversely, toxicants that have a longer period from
exposure to onset of signs have incidence rates spread over
longer periods of time. For example, clinical pyrrolizidine
alkaloid poisoning may not appear until after weeks of
exposure.
- Whole blood and serum samples are used for a variety of
tests. Urine is tested for drugs and some plant toxins.
- Testing of ingesta and feces is valuable to determine exposure
to a variety of toxicants. The value of these matrices lies in
the likelihood that toxicants may be present in high enough
levels to be detected when compared with other body fluids
and substances. For example, many compounds are diluted
and metabolized after absorption. Thus, a compound is at its
highest concentration in the rumen or stomach content.
- Biopsies are invasive and are done only when other
diagnostic options are limited. Liver is the most common
tissue that is biopsied for toxicology analysis.
- Histology may help identify organ specificity of disease
Postmortem samples
A toxicology case frequently depends on the postmortem
examination for a diagnosis. Although practitioners are
trained to perform a necropsy to identify disease and
traumatic conditions. The diagnostic laboratories usually
have board certified pathologists with specialized training in
accepted procedures for investigating this type of case. If the
case is accepted, a complete necropsy should be done, not a
partial, “keyhole” necropsy.
- Photographs of the animal and findings may be helpful in
assessing the case.
- The urine should be sampled using a syringe. The urine is put
into a plastic or glass container; it is not left in the syringe.
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- Samples of tissue should be fixed in formalin for histology
testing.
Some toxicants cause nonspecific lesions. For example,
hemorrhagic gastroenteritis may result from a variety of
toxicants such as arsenic or even salt poisoning
Analytical toxicology
Many poisons can be identified in environmental samples or
tissues. Toxicology test methods range from simple
visualization (e.g., identification of plant parts) to modern
analytical chemistry methods.
- Analyses for metals are done using spectroscopy.
Spectroscopy is rapid and accurate, allowing for analysis of
lead in liquids (e.g., blood) within a few hours.
- Tissue analysis requires additional time for digestion of the
sample to free the metals before analysis.
- Analyses for organic compounds such as some plant
poisons, drugs, or pesticides are done using chromatography.
- Chromatography is separation of compounds based on
characteristic chemical properties in liquid (high
performance liquid chromatography), solid (thin layer
chromatography), or gas (gas chromatography).