Cont’d from series 3
Q. Enumerate at least four group of factors that can affect toxicity
- 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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