Cont’d from series 3
Q. Enumerate at least four group of factors that can
affect toxicity
(i) Host factors (factors related to subject)
Factors related to toxicant or
associated with xenobiotics
Environmental factors
Individual or non-individual factors
Q. Discuss briefly host related factors that affect
toxicity. Give suitable examples
Size: Large individuals can tolerate a larger dose
than individuals of small size. The metabolism and activity is proportional to
the surface area of the body.
Age:Young animals or the human infants are uniquely
susceptible to chemicals that are relatively safer at a later period of life.
The difference in response during early life is a consequence of the relative
inefficiency of various metabolic and excretory pathways, For example,
deficiency of glucuronyl transferase activity results in an enhanced toxicity
of chemicals which are dependent upon of detoxification.
Species, breeds and Strains:Differences in strain of
animals also induce a variation in response of chemical agents and such
differences have been detected in acute toxicity measurement of various inbred
strains of mice. Since man is a remarkably heterogeneous species, the rate of
metabolism of any compound may differ greatly from person to person. For example, hemolysis is observed in certain
individuals during the administration of drugs like aspirin and sulphonamides
because some individuals are deficient in the metabolic enzyme Glucose-6-phosphate
dehydrogenase (G6PD).
Similarly, rabbit can survive even after eating Atropa
belladonna (belladonna leaves) because they contain the enzyme
atropinase, which destroys atropine.
Likewise, some breeds of animals are more susceptible to
the toxic effects of chemicals For example, in Koolies, ivermectin easily
crosses blood brain barrier and causes neurological symptom. Mink (species)
animal is highly sensitive for polychlorinated biphenyls (PCB) than other
species.
Greyhounds are more susceptible to the toxic effects
barbiturates (used as anesthetics) because barbiturates mainly distribute to
adipose tissue. Since, greyhounds, have little body fat. resulting in higher
circulating concentration of barbiturates causing toxicity.
Sex: The sex of an animal often has an influence on
the toxicity of chemical agent. Variation in toxicity due to sex is well known,
the chemical agents or drugs must be used with special care during pregnancy
because they could lead to teratogenic effects in females, differences are shown to be under
direct endocrine influence.. During lactation, it is important to remember that
some chemicals or drugs may be excreted in milk, and may even act on the
offspring. Thus, it is desirable to measure acute toxicity on both male and
female animals of any species.
Q. What are the factors associated with toxicants that
affect the outcome of response?
a) physical state and chemical
properties of the toxicant
b) routes and rates of toxicant
administration
c) previous or coincident
exposure to other drugs/ chemicals (drug-chemical interactions)
d) tolerance of individuals
Q. How do physical state
and chemical properties of toxicants affect toxicity?
The physical state
and chemical properties of the toxicant such as
(i) solubility in water, (ii) solubility
in vegetable oils, (iii) the suspending medium, (iv) the chemical stability of
the chemical agent, (v) the particle size, (vi) rates of disintegration of
formulations of chemicals, (vii) the crystal form, and (viii) the grittiness of
inert substances given in bulk amounts.
For example, fine particles are more readily absorbed
than coarse ones (in the case of poisons bearing irritating properties e.g.,
α-naphthylthiourea, zinc phosphide). Solvents and other substances included in
commercial preparations may also affect the overall toxicity of the active
principle(s). Non-polar solvents may considerably increase the absorption rate
of lipophilic poisons, especially when considering the exposure by the dermal
route.
Q. How do routes and rate of administration of
chemicals affect the toxicity?
Generally, toxicity is the
highest by the route that carries the compound to the blood stream most
rapidly. For most xenobiotics, parenteral routes of exposure entail a more
prompt and complete bioavailability than the oral one and therefore often
result in a lower LD50 .
Q. How does previous or coincident exposure to other
chemicals (Drug–Drug Interactions) affects the toxicity?
A variety of chemicals (drugs, plant toxins, pesticides,
environmental pollutants) are capable of increasing (enzyme inducers) or decreasing (enzyme
inhibitors) the expression and the activity
of hepatic and extrahepatic phase I and phase II enzyme systems participating
in the biotransformation reactions hence modulating the toxicity of several
xenobiotics. Thus administration of two or more chemicals of different
structures when administered simultaneously may lead to additive effect, ‘summation’, or negative summation, or ‘antagonism’ or ‘potentiation’.
Q. How does repeated administration of drug affect the
response of a drug?
It is well known that the
toxic reaction of an animal to a given dose of a drug may decrease, remain
unchanged, or increase on subsequent administration of that dose. A decrease in
toxic response is usually called ‘tolerance’, and an increase ‘hyper
susceptibility’. The enzyme induction, or the
increased activity of enzymes concerned with detoxification and elimination of
drug is a common mechanism for the development of tolerance to a drug on
repeated administration.
For example, repeated
administration of Chlorpromazine depresses the Central Nervous System (CNS) of
normal albino rats and lessens their loco motor activity.
Q. How do feed and feeding affect
the results of toxicity?
The composition of the feed or
food can affect the results of toxicity tests. For example high fat diets can
sensitize animals to, while high carbohydrate and high protein diets provide
protection from, the hepatotoxic effects of
chloroform.
Q. Discuss in brief environmental conditions that effect
toxicity
The environment can affect the
toxic response to chemicals given to animals or human beings. There are three
basic factors in the environment of laboratory animals used in toxicity
testing, namely:
The presence of other species of animals, usually human
being.
The presence of other animals of the same species.
Physical environment.
Several physical factors such as light, temperature,
relative humidity, etc., can influence the LD50 of several chemicals.
For
example, high ambient temperatures are reported to enhance the toxicity of
chlorophenols and nitrophenols that cause an increased production of heat by
uncoupling mitochondrial oxidative phosphorylation. Conversely, cold
temperatures are predisposing factors for α-chloralose, a rodenticide/avicide
formerly used as an anesthetic agent, which may induce a life-threatening
hypothermia especially in poisoned cats by acting on hypothalamic
thermo-receptors.
Q. How do changes in the
internal environment affect toxicity?
Several physiological factors,
such as physical activity, stress conditions, hormonal state of animals, and
degenerative changes in internal organs, are known to influence the toxicity of
any compound. For example, some compounds may induce increased synthesis of
liver micromosal enzymes and influence the metabolism of another.
Inhibition of drug or chemical agent metabolism, displacement of protein
binding of a chemical or inhibition of its renal clearance can also be
accomplished by chemical agents.
Q. How do habitually used drugs
affect the sensitivity of man to toxic doses of chemicals?
The habitual use of certain psychoactive drugs, and particularly excessive use, of these chemicals could
affect the sensitivity of man to toxic doses of drugs and other chemicals.
Q. What is idiosyncratic
reaction?
Occasionally toxicity peculiar
to an individual or which appears in a few persons but not in general
population has been observed. Patients
with a deficiency of glucose-6-phosphate dehydrogenase, for example, develop
hemolysis after ingesting certain drugs or foods. Drugs that are known to cause
this type of idiosyncratic reaction include troglitazone, valproate,
amiodarone, ketoconazole, disulfiram and isoniazid. However, some involvement
of allergic mechanism cannot be ruled out.
Q. Describe time-effect relationship of a toxicant?
The relationship between dose and response is usually established when the chemical/ drug
effect at a particular dose has reached a maximum or a steady level (Fig 1.17).
The chemical effects do not develop instantaneously or continue indefinitely;
they change with time. Thus, the magnitude of a chemical effect at any given
moment is a function not only of the dose but also of the amount of time
elapsed since the chemical made contact with the reactive tissues. This curve
represents several important features (there are three distinct phases and a
fourth phase that may be present or pronounced with some chemicals while absent
with others. These include:
Time of onset of action (Ta)
Time to peak effect (Tb)
Duration of action (Tc)
Residual effects (Td)
Phase I: Time of onset of action (Ta): Following
the administration of a chemical agent to a system, there is a delay in time
before the first signs of chemical effects are manifested. The lag in onset is of finite time, but for
some chemicals the delay may be so short that it gives the appearance of an
instantaneous action. There are various reasons responsible for the chemical
effect to reach an observable level.
Phase II: Time to peak
effect (Tb): The maximum response will occur when the
most resistant cell has been affected to its maximum or when the chemical has
reached the most inaccessible cells of the responsive tissue.
Phase III: Duration of
action (Tc): The duration of action extends from the
moment of onset of perceptible effects to the time when an action can no longer
be measured. It will depend upon the rate at which it is metabolized, altered
or otherwise inactivated or removed from the body.
Phase IV: Residual
effects (Td): Even after its primary actions are
terminated many chemicals are known to exert a residual action. It is not
always possible to determine whether the residual effect is caused by a
persistence of minute quantities of the chemical or by persistence of
subliminal effects.
Q. What is
dose relationship?
The dose–response relationship, or exposure–response relationship,
describes the change in effect on an organism caused by
differing levels of exposure (or doses) to a stressor (usually a chemical) after a certain exposure time. This may apply to individuals (e.g.: a
small amount has no significant effect, a large amount is fatal), or to
populations (e.g.: how many people or organisms are affected at different
levels of exposure).
Q. What are different types of dose-response relationships?
Dose response relationship is of two types:
a) graded
or gradual
b) quantal
(all-or-none) such as death.
Q. What is a graded or gradual dose-response relationship?
The individual dose-response relationship, which
describes the response of an individual
organism to varying doses of a chemical, often referred to as a “graded”
response because the measured effect is continuous over
a range of doses.
Explanation: This type of relationship is useful in
measuring the incremental responses of a compound and can be seen in an
individual organism e.g. contraction of small intestine produced by carbachol,
convulsions produced by strychnine and inhibition of cholinesterase (ChE)
produced by organophosphorus (OP) insecticides. This type of relationship is
useful in studying efficiency of therapeutic drugs or toxic symptoms produced
by a toxicant(s). A typical dose-response curve in which the percentage of
organisms or systems responding to a chemical is plotted against the dose.
Q. What are the assumptions of graded dose-response
relationship?
The
graded dose-response relationship is based on following presumptions:
The pharmacological/toxicological effect is a result of
the known drug/toxicant.
There is a molecular or receptor site(s) with which the
drug/toxicant interacts to produce the response.
The production of a response and the degree of response
are related to the concentration of the drug/toxicant at the molecular or
receptor site.
The concentration of the drug/toxicant at the molecular
or receptor site in turn, is related to the administered dose of the
agent.
The effect of drug/toxicant is proportional to the
fractions of molecular or receptor site occupied by the agent; therefore, by
increasing or decreasing the dose, the response also increases or decreases,
respectively. The maximal effect occurs when the drug /toxicant occupy all
molecular or receptor sites.
The
logarithmic transformation of dose is often employed for the dose-response
relationship because:
It permits display of a wide range of doses on a single
graph.
It facilitates visual and mathematical comparisons
between dose-response curve for different agents or for different responses to
a single agent.
Log-dose plots usually provide a more linear
representation of data.
Q. What is a quantal or all-or-none dose-response
relationship?
Quantal dose-response relationship is one involving an all-or-none
response i.e. on increasing the dose of a compound, the response is either
produced or not. This relationship is seen with certain responses that follows
all-or-non phenomenon and can’t be graded e.g. death.
Explanation: In toxicology, quantal dose-response relationship
is extensively used for the calculation of lethal dose because in it we observe
only mortality. The quantal dose-response relationship is always seen in a
population because the assumption is made that individual responds to maximal
possible or not at all. Both are graphs from the same set of experimental data. The log-dose
scale results in a more linear representation of the data, and is more
desirable since we will use the linear, portion of the curve (from
approximately 16 to 84%) to calculate toxic
potency.
Expanation
The graph of a quantal dose-response relationship does
not show the intensity of effect, but rather the frequency with which any dose
produces the all-or-none phenomenon. A widely used
statistical approach for estimating the response of a population to a toxic
exposure is the “Effective Dose” (ED) or “Lethal
Dose” (LD). Generally, the mid-point, or 50 %, response level
is used, giving rise to the“ED50 ” or LD50 value. However,
any response level, such as an ED01, ED10 or ED30
or LD01, LD10 or LD30 could be chosen. A
graphical representation of an approximate ED50 is shown in Fig 1.20.
Please note that these responses may be mortality (LD) or effective dose (ED).
Q. What is the shape of quantal dose-response curve?
In
quantal dose-response, the log-dose response curve is sigmoid in character. The
sigmoid curve has a relatively linear portion between 16 and 84 % (Fig 1.19),
which is used to determine the slope of the curve. A small portion of
population at left and right sides of the curve respond to low and high doses
and constitute hyper-reactive (hypersensitive) and hypo-reactive (hyposensitive) groups respectively. If a
compound produces its effect at very low dosage, the individual is said to be
hyper-reactive or hypersensitive, if the same effect is produced by the
compound at unusually large doses, the individual is said to be hypo-reactive
or hyposensitive.
Q. What is hormesis dose response phenomenon?
In toxicology, hormesis is a dose response phenomenon characterized by a low dose
stimulation, high dose inhibition, resulting in either a J-shaped or an inverted
U-shaped dose response.
Q. What is U-shaped dose
response curve?
Sometime dose-response curves
do not follow typical sigmoidal dose-response
curve and U-shaped dose-response curves are
observed. For examples essential metals and vitamins show U-shaped curves. At
low dose, adverse effects are observed since there is a deficiency of these
nutrients to maintain homeostasis. As dose increases, homeostasis is achieved,
and the bottom of the U-shaped dose-response curve is reached. As dose
increases to surpass the amount required to maintain homeostasis, overdose
toxicity can ensue. Thus, adverse effects are seen at both low and high dose.
Q. What do you mean by lethal dose-50 (LD50) and median lethal dose (MLD)?
Lethal dose-50 (LD50 ), also called median lethal dose
(MLD), is the dose that is lethal to 50 % of animals exposed to a given
toxicant under defined conditions.
Explanation: The LD50
value is the common way of expressing the
acute toxicity and may not pertain to the severity of clinical signs observed
of the characteristic changes caused by the toxicant but depend only on the
lethality produced by the toxicant. Though recently some toxicological
organization and Government regulatory agencies have greatly reduced reliance
on the LD50 (in order to reduce
the number of animals needed for study), yet it is still considered an
important index to assess the toxicity of chemicals.
The LD50 value is obtained by plotting the percentage
of individuals succumbing to a given dose of lethal chemical as ordinate
against the dose of the compound used as abscissa. In this way one obtains as S
shaped curve as shown Fig 1.19. The shape of the curve indicates the degree of
variation. The LD50 is
obtained from the curve by drawing a horizontal line from the 50 % mortality
point on the ordinate where it intersects the curve. At the point of
intersection, vertical line is drawn and this line intersects at the LD50 point. This dose is designated as LD50. The same data from the sigmoid curve or a bell shaped curve will form a straight
line when transformed into probit units (Fig 1.20). These values are statistically obtained and
represent the best estimation of the dose required to kill 50 % of the animals.
The information with respect to the lethal dose for 95% or for 5% of the
animals can also be derived by a similar procedure.
Q. Describe different variables of dose-response curve
The dose-response curve has four characteristic variables
a) efficacy
b) potency
c) slope
d) biological
variation.
Q. What is efficacy?
The maximal effect or response produced by an agent is
called its maximal efficacy or efficacy .
Q. What is potency?
Potency is the dose of drug / toxicant required to produce a specific effect of
given intensity as compared to a standard reference. Itis a comparative rather than an absolute expression of drug activity. Drug potency depends on both affinity and efficacy. The more potent compound is on the left
because less compound is needed to produce an equivalent response compared to
the compound depicted on the right.
Q. What is the difference
between potency vs. efficacy?
From their
relative positions along the x-axis, compound "A" is more potent than
compound "B." Both "A" and "B" also reach
maximum efficacy since their effects both reach the limit of response.
Q. What is slope?
The slope of a
dose-response curve gives the relationship between the receptor / target site
and the agent.
Q. What is biological variation?
Biological variation or variance can be defined as the
appearance of differences in the magnitude of response among individuals in the
same population given the same dose of a compound.
Q. What is a margin of safety?
The margin
of safety of a drug is the ratio of LD1/ED99.
The farther apart these curves are the wider the margin of safety.
For example.
safety
margin =LD1 / ED99
where, LD1 = Dose that is lethal for 1% of the
population; ED99=Dose that is effective for 99% of the population.
Safety margin is a more conservative estimate than
therapeutic index as values are derived from extremes of the respective
dose-response curves.
Q. What is the difference between therapeutic index and
margin of safety?
The therapeutic index is the ratio of the TD50 (or
LD50) and the ED50 ; the margin of safety (a more conservative estimate) is the
ratio of the LD1 and
the ED99.
Therapeutic
index (TI) =LD50 / ED50
where, LD50 =
Dose that is lethal for 50 % of the population; ED50 = Dose that is
effective for 50 % of the population.
Therapeutic
index measure is commonly used for evaluating the safety and usefulness of
therapeutic agents. The higher the index, the safer is the drug.
Q. What is a therapeutic ratio?
Therapeutic ratio may be
defined as the ratio of the lethal dose -25 (LD25) and the effective
dose-75 (ED75).
Therapeutic
ratio (TR) = LD25 / ED75
where, LD25 = Dose that is lethal for 25% of
the population; ED75 = Dose that is effective for 75% of the
population.
Therapeutic
ratio is considered a better index of safety of a compound as it includes
steepness of curve also. In toxicity cases, a flatter curve is considered more
toxic or hyper-reactive groups are at a much more risk than hypo-reactive or
normal group. Shallower curves usually have low therapeutic ratios.
Q. What is a chronicity factor?
Chronicity factor is the ratio
of the acute LD50 (one
dose) to chronic LD50 doses.
Chronicity
factor= acute LD50 /chronic LD50
Chronicity
factor is used to assess the cumulative action of a toxicant. Compounds with
cumulative effects have a higher chronicity factor.
Q. What is a risk ratio?
The ratio between the inherent toxicity and
the exposure level gives the risk ratio. Risk ratio indicates the risk of a
compound. Substances of higher inherent toxicity may pose little risk as access
of exposure of individuals to such agents is limited. Compounds of low toxicity
may be dangerous if used extensively.
Q. What do you understand by interaction with receptors?
Many toxicants/xenobiotics exert their effects by
interacting with specific receptors in the body. This xenobiotic-receptor
interaction leads to a change in the macromolecule, which in turn triggers a
sequence of events resulting in a response of the tissue or organ. The intensity of response produced by a
toxicant /xenobiotic depends on its intrinsic activity, which in turn depends on the
chemical structure of the compound.
Q. What is a drug affinity?
Affinity is the ability of a xenobiotic to
combine with its receptors. A ligand of low affinity requires a higher
concentration to produce the same effect than ligand of high affinity. Agonists, partial agonist, antagonist and inverse agonist have same or similar affinity for the
receptor.
Q. What do you mean by intrinsic activity?
Intrinsic activity is defined
as a proportionately constant ability of the agonist to activate the receptor as compared to the maximally active compound
in the series being studied. It is maximum of unity for full agonist and minimum
or zero for antagonist.
Q. What is an agonist?
Agonist (full agonist) is an agent that
interacts with a specific cellular constituent (i.e. receptor) and elicits an
observable positive response.
Q. What is a partial agonist?
Partial agonist
(PA) is an agent that acts on the same receptor as other agonists in a group of
endogenous ligands or xenobiotics, but regard less of its dose, it can’t
produce the same maximal biological response as a full agonist.
Q. What is an antagonism/antagonistic effect:
When the combined effect of two compounds given together is lesser in magnitude
to sum of the effects of each compound given alone, or when one compound having
no effect of its own decreases or inhibits the effect of other compound, the
interaction is called antagonism and the effect produced us called antagonistic
effect. The toxic effect of a
chemical, A, agonist, can be reduced when given with
another chemical, B, the antagonist. Antagonists, are often used
as antidotes.
Q. What are possible mechanism of antagonism?
There are several
mechanisms of antagonism:
1) functional
antagonism: simple counterbalancing of the toxic effect (caffeine and
phenobarbital);
2) chemical
antagonism: antagonist reacts with the toxin to reduce toxicity
(dimercaprol chelates toxic heavy metals such as;
3) receptor antagonism: antagonist
binds to receptor, (atropine with organophosphate insecticides);
4) dispositional
antagonism: fate of the toxin is altered (cholestyramine can prevent
absorption of organic chemicals by binding with them).
Q. What
is an inverse agonist?
Inverse agonist is a compound that interacts
with the same receptor as the agonist, but it produces a response just opposite
to that of the agonist.
Q. What
will be the response if two drugs/xenobiotics are used simultaneously?
When two or more xenobiotics are used together, the pharmacological/toxicological
response is not necessarily the same of two agents used individually. This is
because one agent may interfere with the action of another agent called
xenobiotic/drug interaction.
Q. What
is an addition/additive effect?
When the combined
effect of two compounds given together is equal in magnitude to sum of the
effects of each compound given alone, the interaction is called addition and the effect produced is called additive
effect.In this case no specific interactions occur.
1 + 1 =
2
Q. What
is potentiation/potentiative effect?
When one compound having no effect of its own increases
the effect of another compound the interaction is called potentiation and the
effect produced is called potentiative effect.
Explanation:
A dose of a compound A is toxic to animals in vivo. Another chemical B is not
toxic when given at doses several orders of magnitude higher but when the two
are given together the toxic response is greater than that of the given dose of
A alone. This means the compound B has a potentiative effect on compound A.
This is known as potentiation.
Q. What
is synergism/synergistic effect?
The combined
effect of the administration of two compounds may be greater than the sum
of the two effects; this is called synergism. The synergist
piperonyl butoxide is added to some insecticides to greatly increase their toxicity
to insects.
For example:
1+ 1 = more than two
Q. What is the
difference between synergism and potentiation?
The difference
between the two concepts is that synergism is the interaction of two or more
substances, while potentiation is about a singular substance and how it may act
when in a synergy relationship.
Q. Why
are toxins often selective to tissues, give suitable examples?
Toxins are
often selective to certain tissues because of the following reasons:
Preferential
accumulation: toxicant may accumulate in only certain tissues, and cause
toxicity to that particular tissue. For example, Cd in kidney, paraquat in
lung.
Selective
metabolic activation: enzymes needed to convert a compound to the active
form may be present in highest quantities in a particular organ. For
example, CCl4, nitrosamines in liver.
Characteristics
of tissue repair: some tissues may be protected from toxicity by actively
repairing toxic damage; some tissues may be susceptible because they lack sufficient
repair capabilities e.g. nitrosamines in liver.
Specific
receptors and/or functions: toxicant may interact with receptors in
a given tissue. For example, curare: a receptor-specific neuromuscular
blocker.
Physiological
sensitivity: the nervous system is extremely sensitive to agents that
block utilization of oxygen. For example, nitrite: oxidizes hemoglobin (methemoglobinemia)
and cyanide: inhibits cytochrome oxidase (cells not able
to utilize oxygen), barbiturates: interfere with sensors for oxygen and
carbon dioxide content in blood.
Q. What are the main target
organs most frequently affected by toxicants?
a) Central nervous system
a) Central nervous system
b) Circulatory system (blood, blood-forming system)
c). Visceral organs (liver, kidneys, lung)
d) Muscle and Bone
Q. Why effect
or response is observed after administration of any chemical? Give primary
assumptions.
Primary assumptions include:
There is
a molecular site (or receptor) with which the chemical interacts to
produce a response.
Production of
response is related to the concentration of the compound at the
active site.
The
concentration of the compound at the active site is related to
the dose administered.
To be cont’d
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