FACTORS ALTERING TOXICITY OF A COMPOUND

FACTORS ALTERING TOXICITY OF A COMPOUND

            For a toxic effect to occur, the toxic agent or its metabolites must reach an appropriate site of action at a sufficient high concentration and for sufficient time to initiate a toxic change. Several factors affect the toxicity of a compound. These are closely related to the factors modifying the drug responses. Toxicosis potential is usually determined more by the multitude of related factors than by actual toxicity of the toxicant. Exposure-related, biologic, or chemical factors regulate absorption, metabolism, and elimination, and thus, influence observed clinical consequences. There are numerous factors which may change response to the toxicants. These factors, either singly or in combination, influence the outcome of toxicity due to a toxicant. These factors can be broadly divided into four major groups:

I. Factors related to the toxicant.

II. Factors related to the exposure conditions.

III. Factors related to the subject.

IV. Factors related to the environmental conditions.

I. Factors Related to the Toxicant

            Source of the Compound: In case of plant toxicities, the intoxication is related to the part of the plant ingested. Generally seeds or roots of the plants have higher concentration of the toxic princi­ples and hence more toxic than the other parts. The state of growth also influences the toxicity due to plants. Young shoots of sorghum are toxic, but not the fully grown sorghum plants. Tender pods of Acacia leucophloea are more toxic than the mature pods. In both the cases the variation is due to level of hydrocyanic acid.  Similarly the toxicity due to a plant varies according to the locality. Wildly grown plants are more toxic than the cultivated plants. Season also alters the level of toxic principles in the plants. A. leucophloea has higher levels of hydrocyanic acid in summer than in winter. Faulty storage conditions also convert a nontoxic food to a most toxic food (mold infestation of seed cakes, silage or sweet clover). Certain plants become less toxic following drying and storage for long periods.

            Nature of the Compound: The toxicity of an ingested toxicant depends upon its physical state. A compound in solid state is less toxic than its powdered form. Finely powdered forms are more toxic than the coarse powders (arsenical dips). The differences in toxicity are due to variation in the rate of dissociation and absorption. If the toxicant is in liquid form, the nature of vehicle influences the toxicity. Dermal application of insecticides in aqueous medium may by nontoxic (negligible absorption); whereas, their, oily solutions cause dermal toxicity (increased absorption). The chemical nature of a toxicant determines solubility, which in turn influences absorption. Nonpolar or lipid-soluble substances tend to be more readily absorbed than polar or ionized substances.

            The toxicity is also related to the chemical nature of a compound. Trivalent arsenicals are more toxic than pentavelent arsenicals. Barium carbonate is highly toxic (high solubility) as compared to barium sulfate (insoluble). Carbon monoxide is more toxic than carbon dioxide. Methanol is a highly potent toxicant as compared to ethanol.

            Toxicity of a compound differs due to its inherent proper­ties, i.e. structure and affinity for the target site. There is of course, some tendency for compounds of similar chemical nature to resemble each other in toxicity but this resemblance is more likely to be qualita­tive than quantitative. For example, organophosphates produce a similar clinical picture but difenphos does so only at a dosage over 10,000 greater than that required by TEPP.

            Physico-chemical properties of toxicant:

a.Lipid solubility: Chemicals with high lipid solubility are more readily absorbed through the lipid-protein matrix of the cell membrane than those without, so are more toxic.

b.Degree of ionization: Compounds containing groups ionized at physiologic pH are usually more water soluble and less likely to be absorbed across membranes, e.g. amines, carboxyls, phos­phates, sulphates. Ammonium ion (NH₄⁺) is absorbed less across rumen wall than ammonia (NH₃).

c.Polarity: Non-polar compounds are more readily absorbed across membranes than polar compounds.

d.Molecular weight: Compounds of low molecular weight are more readily absorbed than large complex molecules.

e.Proteins binding: Binding of toxicants to physiologic proteins (e.g. albumin) can restrict the availability of active drug by limiting its passage across membranes. Such binding may also retard excre­tion by reducing the glomerular filtration of the toxicant.

            Composition of sample: Impurities or contaminants present in com­mercial products can modify the response of the biologic system to the toxicant. For example, herbicide, 2, 4, 5-T use has declined be­cause of contamination by dioxins produced during its manufacturing which are highly hazardous.

            The vehicle or carrier of the toxic compound also affects its availability for absorption. Toxicity of a compound is markedly influenced by formulation which can either increase or decrease toxicity by altering the chemical nature of the agent or by affecting the solubility which influences the absorption of the toxic agent into the biologic system. Formulations of wettable powders, suspensions and emulsions can affect the retention rate on hair and skin or the absorption rate from the GIT. For example, plant pesticide formula­tions may be dangerous if used on animals because of the retention properties of the pesticide, which allows excessive pesticide accumula­tion on animal hair or skin, Organochlorine insecticides (BHC, DDT) are more toxic if given in oily vehicle due to quick absorption.

            Isomers, including optical isomers, vary in toxicity. For example, the γ isomer of hexachlorocyclohexane (lindane) is more toxic than other isomers.

            Nature and type of dosage form: Apart from the toxicant itself, toxicity often depends to a great extent on the dosage form of the toxicant. The more complex a dosage form, greater is the number of rate-limiting steps. As a general rule, the bioavailability of a toxicant from various dosage forms is in the following order: solutions > emulsions> suspensions > capsules > tablets > sustained released products.

            Adjuvants are formulation factors used to alter the toxicologic effect of the active ingredient (eg, piperonyl butoxide enhances the insecticidal activity of pyrethrins).

            Binding agents, enteric coating, and sustained-release preparations influence absorption of the active ingredient. As absorption is delayed, toxicity decreases. Flavoring agents affect palatability and thus the amount ingested.

            Active metabolites: Some toxicants form active metabolites after biotransformation. This often leads to more and prolonged toxicity. For example, malathion after biotransformation forms malaoxon, which is a potent inhibitor of cholinesterase enzyme.

            Rate of disintegration and dissolution: Faster the rate of disintegra­tion (from bigger to smaller particles) and rate of dissolution (from solid to liquid phase) of a toxicant, more will be its absorption and toxicity.

             Dose of the compound: The intensity of effects due to a toxicant are directly related to its dose, which the animals get exposed. Higher the level of exposure, intensity will be more. Drugs, which are used for the relief of suffering will become harmful if administered at high doses. Even the dietary essential component salt causes toxicity if ingested at higher levels. The toxicity of a compound is expressed by finding out its median lethal dose (LD50). This dose and toxicity are inversely related. Higher the LD50, lesser the toxicity.

            Type of Exposure: A least acutely toxic compound may cause severe adverse effects on the health of the animals, when they get exposed to its subtoxic doses repeatedly over a prolonged period. This is more so in case of toxicants which have a tendency to accumulate in the body tissues (cumulative poisons: lead, DDT, warfarin, copper etc.). In such cases the ill effects will not be evident immediately (carcinogenecity, radiation injury etc.).

            When an animal is exposed simultaneously to two or more toxicants, the toxicity of one compound is altered by the other. This interaction results in either decrease in toxicity (antagonism) or increase in toxicity (potentiation). Copper and molybdenum are mutually antagonistic. DDT lowers the toxicity of parathion. EPN potentiates the toxicity of malathion. Yellow phosphorus becomes extremely toxic when administered as oily solution.

II. Factors Related to the Exposure Conditions

            Dose is the primary concern; however, the exact intake of a toxicant is seldom known. Duration and frequency of exposure are important. The route of exposure affects absorption, translocation, and perhaps metabolic pathways. Exposure of a toxicant relative to periods of stress or food intake may also be a factor. After ingestion of some toxicants, emesis may occur if the stomach is empty, but if partly filled, the toxicant is retained and toxicosis can occur.

            Dose/Amount of toxicant: Control of dose/dosage is the basis of safety measures in the use of chemicals. A sufficiently large dose of an or­dinarily harmless material (e.g. sodium chloride) is fatal, but a small dose of a potent poison is without any effect and even some are used as therapeutic agents (e.g. arsenic, atropine).

            Route and site of exposure: The major routes by which toxicants gain access to the body are through GIT (oral), lungs (inhalation), skin (topical) and through parenteral administration. Compounds are usually more toxic by oral route than dermal route. But some or­ganophosphrus insecticides arc quickly absorbed by dermal route and less by intestine. Elemental mercury is virtually non-toxic by oral route, but highly toxic when its vapour are inhaled.

            Rate of administration: With few exceptions, rapid intravenous ad­ministration of drugs is more toxic than when given by slow infusion. For example diacetylmonoxime administration by rapid I.V. injection produces severe side effects, but same when given slowly is well tolerated.

            Volume and concentration: Toxicity of agents given by oral route tends to be more when given in dilute solution. But opposite situation occurs with agents whose toxic effects are due to local irritation. Parenteral administration of hypo- or hyper-tonic solutions are more toxic than isotonic solutions. For example, LD50 of normal saline (isotonic) by I. V. route is 68 ml/kg in rats, whereas, the same of hypotonic solution is 1 ml/kg. Problems associated with large volume is excessive hydration, impaired renal clearance or vehicle toxicity.

            Time of administration/exposure: Changes in the susceptibility of biologic systems to toxic agents may be related to the time of ad­ministration or diurnal factors. Most of the diurnal variations are re­lated to the eating and sleeping habits of the test species. For example, in nocturnal animals such as rats, there is more food in the stomach in the morning than in the afternoon. So absorption of toxic agent is variable depending on the time of its exposure. In human beings, hypnotics are more effective if taken in the evening, as darkness itself has sedative action.

            Duration and frequency of exposure: It is difficult to predict the dura­tion which is toxic for a chemical because it varies from compound to compound and individual to individual. A single dose of an agent that produces an immediate severe effect might produce less than half of the above effect when given in 2 divided doses. For some agents, toxic effects of acute exposure are quite different from those produced by chronic exposure. For example, some OP insecticides (monocrotophos) in acute toxicity produce characteristic anti­cholinesterase toxicity symptoms but same on repeated exposure produce delayed neuropathy. Similarly, some agents are more toxic on repeated administration due to their cumulative toxic effect (copper) than after the single exposure.     Schedule of dosage: A bolus (i.e., one large dose by gavage) may produce poisoning with a much lower dosage than toxicant diluted in food or diet and taken by voluntary consumption. Similarly, an oral dose when given empty stomach is usually absorbed over a brief period to produce more toxicity than the same dose administered in full stomach. However, cyanogenetic glycosides are more toxic in full stomach.

            Interaction of compounds: In broad sense, when two or more toxic agents are administered simultaneously or within short duration, they may affect each other's toxic actions.

a. Potentiation: Many organochlorine insecticides and fumigants (CCI₄) are much more likely to injure the liver, if alcohol is con­sumed at the same time.

b. Addition: If two drugs having same mechanism of action are given together then effect may be additive, e.g. organophosphorus in­secticides.

c. Antagonism: This occurs when two agents neutralise one another's action. For example, there are less chances of OP in­secticides toxicity, if atropine is already present in the animal's body. Similarly, tannins and proteins in the GI tract can complex with toxicants and reduce their absorption.

            Palatability: Palatability may increase voluntary intake of toxicants and thus, their toxicity. For example, flavoured and paraffin-embedded rodenticides are used to enhance palatability. This increases dosage uptake by rodents and thus their mortality.

III. Factors Related to the Subject:

            Species and strains: Various species and strains within species react differently to a particular toxicant because of variations in absorption, metabolism, or elimination. There is wide biologic diversity among different species in the rate and pattern of metabolism to detoxify a compound. In addition, differences in absorption, excretion, plasma protein binding, tissue distribution and response at the receptor sites have been postulated. Functional differences in species may also affect the likelihood of toxicosis, e.g., species unable to vomit can be intoxicated with a lower dose of some agents.

            Belladonna or atropine is toxic to most species, but is nontoxic to rabbits due to presence of liver enzyme atropinase which hydrolyses it.

            Strychnine is relatively less toxic to birds as compared to dog.

            Monogastrics are less susceptible to cyanogenetic plant poisoning than the ruminants.

            Dogs vomit after taking red squill and escape from mortality, but rodents cannot vomit (due to absence of vomiting mechanism) and die.

            The type and condition of the digestive tract also influences the orally ingested toxicants (dog is more susceptible than the ruminant).

            Albino Norway rats are more susceptible to nor­bormide (oral LD50, 4.3 mg/kg) than wild strain of Norway rats (oral LD50, 12 mg/kg), while the same toxicant is non-toxic to dogs, cats, sheep, and monkeys.

            Glucuronide conjugation is less developed in cat, so is highly susceptible to poisoning by agents such as phenol.

            Type of digestive tract: Ruminants can store large volume of ingesta, potentially prolonging the absorption of toxicant. On the other hand, microbial action in the rumen can reduce toxicity by metabolizing some poisons to less active forms (e.g. some OP insecticides). Urea is more toxic to ruminants than simple stomach animals due to presence of enzyme urease in ingested plants which rapidly releases ammonia.

            Metabolism: Biotransformation (metabolism) also plays an important role in altering the toxicity of a compound. In general through metabolism most of the toxicants are inactivated or made nontoxic (detoxication) and/or converted to easily excretable metabolites (cyanide to thiocyanate, formation of glucuronides etc.). However, a parent nontoxic compound is also converted to a potent toxic metabolite (malathion to malaxon, parathion to paraxon, fluoroacetate to fluorocitrate, urea to ammonia etc.). The process by which a nontoxic compound is metabolized to a potent toxic metabolite in the animal’s body is called “lethal synthesis”.    Among the ruminants, the rumen microflora also alters the toxicity of an ingested compound. The drug metabolizing enzymes of microflora cause both activation (nitrate to nitrite, urea to ammonia etc.) and inactivation (parathion to aminoparathion) of the ingested toxicants.

            Age of the Animal: Age and size of the animal are primary factors in toxicosis. Metabolism and translocation of xenobiotic agents are compromised by the underdeveloped microsomal enzyme system in young animals. Generally young animals are more susceptible to the toxicants than the fully grown or adult animals. In young stock, especially during early weeks of life, the metabolic or detoxication and excretory mechanisms are not developed to full capacity as compared to the adults. The young ruminants behave like monogastrics. Similarly, the old animals are more susceptible to the toxicants than the normal middle-aged healthy animals. In old age the mechanisms of detoxication and excretion will be defective. For example, calves and lambs are markedly more susceptible than adult cattle or sheep to sprays or dips of chlordane, dieldrin and lindane.

            Membrane permeability and hepatic and renal clearance capabilities vary with age, species, and health.

            Size of the Animal: As the factors modifying drug responses, the amount of the toxicant required to produce toxicity is also related to the body size of the animal. Large sized animals require exposure to high doses. Similarly, obese or fatty animals require more quantity of the fat-soluble toxicants than the lean animals (DDT, barbiturates). In case of large ruminants, the voluminous rumino-reticular contents also inactivate the ingested toxicants. An adult bullock requires a large oral dose of the toxicant as compared to a small built adult cow.

            Weight: In toxicological studies, it is customary to adjust the dose on the basis of body weight. But animals having a large body weight may receive disproportionate increase of toxicant. For example, in pigs, anaesthetics are required in higher dosage due to their storage in body fat. This increases susceptibility of pigs to anaesthetics.

            The amount of toxicant required to cause pathology is generally correlated to body weight, but with greater body weight, a disproportionate increase in toxicity (per unit body weight) of a compound often occurs. Body surface area may correlate more closely with the toxic dose. No measurement parameter is consistent for every situation.

            Nutritional and dietary factors, hormonal and health status, organ pathology, stress, and sex all affect toxicosis.

            Nutritional status of the animal: Nutritional factors may directly affect the toxicant (i.e., by altering absorption) or indirectly affect the metabolic processes or availability of receptor sites. The copper-molybdenum-sulfate interaction is an example of both. Starvation for few hours will reduce blood glucose and produce changes in the activity of several of the drug metabolizing enzymes. Protein deficiency for longer period results in lesser percentage of microsomal enzyme activity and thus increases the toxicity of a variety of pesticides and other agents. For example, the acute toxicity of captan (fungicide) is many folds more in rats maintained on protein deficient diet  (oral LD50 480 mg/kg) as compared to those maintained on normal protein diet (oral LD50 12,500 mg/kg.). Deficiency of any essential trace element is injurious in itself. For example, vitamins deficiency, especially the an­tioxidants Vit. E and Vit. C can result in increased damage from the free radicals.

            Hormonal status of animal: In addition to pregnancy and sex, there are a number of other hormonally dependent situations that may in­fluence the toxicity of various agents. For example, hyperthyroidism and hyperinsulinism may alter the susceptibility of animals to toxic agents. Adrenalectomised rats are generally more susceptible to toxicants.

            Idiosyncrasy: It is an unusual type of response by body tissues in the presence of a xenobiotic. The cause is often an abnormal pattern of metabolism or detoxification resulting in an abnormal, usually exces­sive reaction to chemical. For example, parenteral penicillin ad­ministration produces fatal anaphylactic shock in some human beings and guinea pigs.

            Sex of the Animal: Male and female animals of the same strain and species usually exhibit only slight differences in susceptibility to toxic agents. Generally females are more sensitive to the action of toxicants or drugs than the males. The male resistance may be due to presence of testosterone. The female animals pretreated with the male sex hormone also exhibit relatively less susceptibility to poisons as compared to the untreated female animals. The examples of sex differences in toxicity include higher susceptibility of females to parathion, EPN, red squill and potash as compared to the males. Therefore, in order to assess the acute toxicity of a compound its LD 50 is determined in both the sexes or by taking equal number of both male and female animals per each dose. Female rats are 2 times more susceptible to red squill than male rats. Male mice are highly susceptible to nephrotoxic effect of chloroform but females show virtually no effect. Sex related differences in toxicity to agents are mostly seen where enzymatic biotransformation of agent is influenced by sex hormones.

            General Status of the animal: Generally weak, debilitated or emaciated animals readily succumb to the action of poisons because their general resistance to adverse conditions is low also due to defective detoxication and excretion processes as compared to the healthy animals.

            Starved animals also have lowered capacity to sustain the intoxication (low hepatic glycogen stores).

            Malnutrition and infestation with parasites are also important factors in lowering resistance to poisons, and may account for some of the discrepancies between the results of toxicity experiments in the laboratory and observations in the field.

            Pregnancy and lactation: Pregnancy and lactation cause marked hor­monal and metabolic changes, thus affecting toxicity of some agents. Pregnancy has been shown to increase the susceptibility of mice to some pesticides. Development of placenta enhances metabolism of some xenobiotics. For example, anticoagulants are more toxic to preg­nant animals. Lactation may enhance the excretion of some lipophilic toxicants (e.g. DDT, polychlorinated biphenyls).

            Emotional status of the animal: Toxicity of drugs like amphetamine and other CNS stimulants can be increased by crowding (number of animals/cage), size of cage, bedding and handling of animal. Anything that disturbs the animal (i.e. noise), may influence its physiological reaction and thus possible change in reactions to foreign chemicals.

            Genetic status of the animal: Genetic makeup of the subject often markedly influences the response to a toxicant. For example, individuals having low level of pseudocholinesterase show prolonged muscular relaxation and apnoea produced by succinylcholine. Individuals with Glucose-6- phosphate dehydrogenase deficiency are more susceptible to the haemolytic effect of drugs like sulfonamides, aspirin, acetanilide, etc.

            Tolerance: Failure of the animal to react to the ordinary dose of an agent may be seen after continuous intake of that agent in small doses over long period, e.g. opium, alcohol, some OP insecticides etc.

            Individual differences: Individual variations are common in all types of toxicological tests. Within a population, some animals are hyper­sensitive and some are hyposensitive to toxic agents. For example, acute oral toxicity of some pesticides shows great variation in their lethal dose for rats in a population,

            Disease and biological conditions: Liver diseases may reduce the ac­tivity of microsomal enzymes, thus altering biotransformation and ac­tion of drugs and toxicants. Liver diseases can also reduce synthesis of protective binding molecules (glutathione) allowing increased ef­fects of toxicants. For example, hepatitis prolongs the action of anaesthesia due to which ultra-short acting barbiturates become toxic. Due to this reason long acting barbiturates are contraindicated in hepatic diseases. Hepatic and renal disease states greatly enhance the toxicity of a compound.

            The GI disease conditions alter the toxicity of an orally ingested compound. Constipation increases the toxicity (enhanced absorption) and diarrhoea decr­eases the toxicity (reduced absorption).

            Similarly animals with dermal wounds suffer from toxicity when sprayed with insecticides for the control of ectoparasites.     

IV. Factors Related to the Environmental Conditions: Environmental factors, such as temperature, humidity, and barometric pressure, affect rates of consumption and even the occurrence of some toxic agents.

            Many mycotoxins and poisonous plants are correlated with seasonal or climatic changes. For example, the ischemic effects of ergot toxicosis are more often seen during the winter cold, and plant nitrate levels are affected by rainfall amounts.

            Temperature: Environmental temperature has been demonstrated to influence toxic response in animals by affecting absorption, storage, metabolism and elimination. Low environmental temperature enhan­ces biotransformation of xenobiotics, possibly metabolic activity in­creases to keep the body warm. Oxidative uncouplers increase body temperature, and thus their toxic effect is more in hot weathers. Der­mal toxicity of insecticides may be greater in hot weather as more blood is diverted to skin (so rapid absorption) to affect cooling. Or­ganochlorine and pyrethroid insecticides show negative correlation with environmental temperature (more toxic in low temperature and vice-versa).

            Atmospheric pressure and altitude: Changes in response to toxicants are generally associated with changes in environmental oxygen tension. Carbon monoxide, barbiturates and cyanide are more toxic in low oxygen conditions. Similarly, pressure resulting from altitude may be a factor in the toxicity of compounds especially those affecting cardio­respiratory functions. Greater toxicities of red squill, strychnine and digitalis are seen at high altitudes as compared to low altitudes.

            Light and other radiations: Radiation exposure is known to affect blood tissue barriers, modify enzyme systems and produce disturbances in the normal excretory patterns of numerous species. Whole body irradiation has been shown to produce a dose-dependent decrease in the pseudocholinesterase activity in the ilia of intestine of rodents thus changing response to acetylcholine and physostigmine. Toxicity of atropine increases if intoxicated animal is exposed to direct sunlight due to its mydriatic and cyloplegic effects on eye.

            Relative humidity: Relative humidity might influence the reaction of an animal to a toxicant especially those exposed by dermal route. For example, dermal absorption of some organophosphorus insecticides (parathion) is more in hot and humid environment.

            Environmental pollution: Chemicals in the environment are capable of influencing

toxic responses in a variety of ways by altering biotransformation (induction or inhibition of hepatic microsomal en­zymes). Affinity for receptors, absorption, distribution and excretion. Similarly noise pollution may aggravate the toxicity of agents (e.g. strychnine),

which produce seizures and hyperesthesia.