PLH - 419
General Principles of Toxicology
Toxicology is the study of the adverse effects of
chemicals on living
organisms.
Toxicology also includes the study of harmful
effects caused by physical phenomena, such as
radiation and noise.
The purposes of toxicology are protection of
organisms and biological systems from
deleterious effects of toxicants and development
of selective toxicants.
Historical Development of Toxicology
It is one of the oldest practical sciences which began with early cave dwellers who recognized poisonous plants
and animals and used their extracts for hunting or in warfare.
By 1500 BC, hemlock, opium, arrow poisons, and certain metals were used to poison enemies or for executions
(Notable poisoning victims include Socrates, Cleopatra, and Claudius)
.
The Death of Socrates, 1787 Jacques-Louis David
By the time certain concepts fundamental to toxicology began to take shape especially
by the studies of Paracelsus (~1500AD) and Orfila (~1800 AD).
Paracelsus (1493 -1541):
His famous words were :
"All substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a
remedy."
1-He determined that specific chemicals were actually responsible for the toxicity of a plant or animal
poison.
2-He also documented that the body's response to those chemicals depended on the dose received
Orfila (founder of toxicology -19th century)
Spanish physician who first correlated between the chemical and biological properties of poisons.)
The 20th century is marked by an advanced level of understanding of
toxicology especially after the the widespread use of anaesthetics and
disinfectants. DNA (the molecule of life) and various biochemicals
that maintain body functions were discovered.
The discovery of radioactivity and the vitamins, led to the use of the first
large-scale bioassays (multiple animal studies) to determine whether
those new chemicals were beneficial or harmful to laboratory animals.
The 1960s started with the tragic of thalidomide incident, in which several
thousand children were born with serious birth defects. Attempts to
understand the effects of chemicals on the embryo and fetus and on the
environment as a whole gained momentum. New legislation was
passed, and new journals were founded.
The variety of potential adverse effects and the diversity of chemicals in
the environment make toxicology a very broad science. Therefore,
toxicologists often specialize in one area of toxicology.
A toxicologist is trained to examine the nature of those effects (including
their cellular, biochemical and molecular mechanisms of action) and
assess the probability of their occurrence.
Branches of toxicology
Toxicology is multidisciplinary as it entails:
1-Descriptive toxicology
It is concerned directly with toxicity testing, which provides information for safety evaluation and
regulatory requirements. The concern may be limited to effects on humans, as in the case of
drugs and food additives, or to potential effects on fish, birds and plants, as well as other
factors that might disturb the balance of the ecosystem.
2-Mechanistic toxicology
It s concerned with identifying and understanding the mechanisms by which chemicals exert
toxic effects on living organisms. In risk assessment, mechanistic data may be very useful in
demonstrating that an adverse outcome observed in laboratory animals is or is not directly
relevant to humans. Mechanistic data are also useful in the design and production of safer
alternative chemicals and in rational therapy for chemical poisoning and treatment of disease.
An understanding of the mechanisms of toxic action contributes to the knowledge of basic
physiology, pharmacology, cell biology and biochemistry.
3-Regulatory toxicology
has the responsibility for deciding, on the basis of data provided by descriptive and mechanistic
toxicologists, whether a drug or another chemical poses a sufficiently low risk to be marketed
for a stated purpose. Regulatory toxicologists also assist in the establishment of standards for
the amount of chemicals permitted in ambient air, industrial atmospheres and drinking water,
often integrating scientific information from basic descriptive and mechanistic toxicology
studies with the principles and approaches used for risk assessment.
4-Occupational (Industrial) toxicology is concerned with the protection of
workers from toxic substances and makes their work environment safe.
5-Environmental toxicology focuses on the impact of chemical pollutants in the
environment on biological organisms, most commonly on nonhuman
organisms such as fish, birds and terrestrial animals.
6-Ecotoxicology is a specialized area within environmental toxicology that
focuses more specifically on the impact of toxic substances on population at
the community and/or ecosystem level.
7-Forensic toxicology is concerned primarily with the medico-legal aspects of the
harmful effects of chemicals on humans and animals, in establishing causes of
death and in determining their circumstances in a postmortem investigation.
8-Analytical toxicology is a specialized area to identify the toxicant through
analysis of body fluids, stomach content, excrement, skin, or suspected
containers.
9-Clinical toxicology is concerned with disease caused by or uniquely associated
with toxic substances. Efforts are directed at treating patients poisoned with
drugs or other chemicals and at the development of new techniques to treat
those intoxications. Clinical toxicology deals with emergency cases such as
overdoses, poisonings, attempted suicides by: emergency care for patients,
management of sign and symptom, identification and quantification of the
drug ,poisons, chemicals…etc
Toxicity
Toxicity is the degree to which a substance can
harm humans or animal
Different xenobiotics cause many types of
toxicity by a variety of mechanisms. So, we
have to take an idea about:
-Different types of toxic agents
-Different type of toxicity
-Different mechanisms of toxic response
Toxic Agents
Toxic agent: is anything that can produce an adverse biological effect.
The most common terms used to describe a toxic agent are toxicant, toxin,
poison.
Classification of Tpoxic Agents
Toxic agents can be classified according to their nature: Chemicals:(as
alcohols, phenols & heavy metals, Physical (radiation),and Biological
(Snake & scorpion venoms).
Toxic agents can be also classified in terms of their target organs (liver,
kidney), use (pesticide, solvent), and effects (cancer, mutation).
Toxic agents may also be classified in terms of their physical state (gas, dust,
liquid), their chemical stability (explosive, flammable), general chemical
structure (aromatic amine, halogenated hydrocarbon) or poisoning potential
(extremely toxic, very toxic, slightly toxic).
Classification of toxic agents on the basis of their biochemical mechanisms of
action (e.g. alkylating agent, methaemoglobin producer) is usually more
informative than classification by general terms such as irritants and
corrosives.
Types of Toxicity
1-Systemic Toxicity :
Toxicity may occur at multiple sites. This is referred as systemic toxicity.
The following are types of systemic toxicity
a-Acute Toxicity:
It occurs almost immediately (hours/days) after an exposure to single dose or a
series of doses received within a 24 hour period. Death is a major concern in
cases of acute exposures. Examples are:
-In 1989, 5,000 people died and 30,000 were permanently disabled due to
exposure to methyl isocyanate from an industrial accident in Bhopal, India.
-Many people die each year from inhaling carbon monoxide from faulty
heaters.
b-Subchronic Toxicity (reversible)
It results from repeated exposure for several weeks or months. This is a common
human exposure pattern for some pharmaceuticals and environmental agents.
Examples are:
-Ingestion of coumadin tablets (blood thinners) for several weeks as a
treatment for venous thrombosis can cause internal bleeding.
-Workplace exposure to lead over a period of several weeks can result in
anemia.
c-Chronic Toxicity (irreversible) :
It is a cumulative damage to specific organ or system and it takes many months or years to
become a recognizable clinical disease. This damage is so severe that the organ can no
longer function normally (irreversible) and a variety of chronic toxic effects may result.
Examples are:
-Cirrhosis in alcoholics who have ingested ethanol for several years
-Chronic bronchitis in long-term cigarette smokers
-Pulmonary fibrosis in coal miners (black lung disease)
d-Carcinogenicity:
Carcinogenicity is a complex multistage process of abnormal cell growth and differentiation
which can lead to cancer.
e-Developmental Toxicity:
Developmental Toxicity result from toxicant exposure to either parent before conception or
to the mother and her developing embryo-fetus.
f-Genetic Toxicity:
Genetic Toxicity results from damage to DNA and altered genetic expression. This process
is known as mutagenesis. The genetic change is referred to as a mutation and the agent
causing the change as a mutagen.
2-Organ Specific Toxicity :
Blood and Cardiovascular Toxicity
Hypoxia due to carbon monoxide binding of hemoglobin preventing transport of oxygen
Hepatotoxicity
CCl4……..metabolized by HME…….CCl3 (causes lipid peroxidation in liver & lead to liver
necrosis.)
Nephrotoxicity
kidney damage and nephritis are produced by poisons such as phenol and sulphonamides.
Mercury & gentamycin are nepherotoxic.
Neurotoxicity
Antimony and arsenic
Organophosphorus compounds (insecticides)………damage to sensory fibers.
Respiratory Toxicity
Aluminum…..emphysema……inflated lung …….fibrosis(aluminosis).
Dermal Toxicity
dermal irritation due to skin exposure to gasoline
dermal corrosion due to skin exposure to sodium hydroxide
skin cancer due to ingestion of arsenic or skin exposure to UV light
Cardiovascular system Toxicity
Myocardial depression is produced by compounds such as
quinine and quinidine. Digitalis and strophanthus toxicity
may lead to arrhythmia and ventricular fibrillation. Arise in
blood pressure is produced by sympathomimetic agents
such as ephedrine. Hypotension is caused by reserpine and
nitrites.
Skeletal muscleToxicity:
Muscle paralysis is produced by lead, curare and flaxedil.
Blood changes; anaemia is produced by benzene and aniline.
Haemolysis may be due to saponins. Leukopenia is caused
by benzene.
Eye Toxicity
acids and strong alkalis may cause severe corneal corrosion
corticosteroids may cause cataracts
methanol (wood alcohol) may damage the optic nerve leading
to blindness
Mechanism of Cellular Injury
Transported,
dispersed,
and
possibly altered
Toxin
emitted
Ingested
Contacts
human
Metabolized
and/or
stored
Physiological
chain of
events
Reaches
an organ
Mechanism of Cellular Injury
When the cell is exposed to an injurious agent or stress, a sequence of
events follows that is loosely termed cell injury. Cell injury is
reversible up to a certain point.
If the stimulus persists or is severe enough from the beginning, the
cell reaches a point of no return and suffers irreversible cell injury
and ultimately cell death. Cell death, is the ultimate result of cell
injury.
There are two principal patterns of cell death:
Necrosis (occurs after ischemia and chemical injury it is always
pathologic).
Apoptosis (occurs when a cell dies through activation of an internally
controlled suicide program). It is designed to eliminate unwanted
cells during embryogenesis and in various physiologic processes
and certain pathologic conditions.
Mechanism of Cellular Injury
Toxicity can result from adverse cellular, biochemical, or macromolecular
changes. Examples are:
1-Alteration of a cell membrane permeability:
Toxic agents could change cell membrane permeability through interaction with
its component as;
a-SH-containing proteins
Heavy metals as As or Hg react with them……… change in protein structure
………
change membrane permeability.
b-Lipids
-Free radicals attack fatty acids in the lipid layer of biological membrane causing
lipid peroxidation , these peroxides are toxic to the cell and alter membrane
permeability.
E.g.: CCl4……..metabolized by HME……CCl3 (Trichloromethyl radical causes
lipid peroxidation and finally lead to liver necrosis.)
-This is why antioxidants should be used frequently by humans where it act as a
protective measure against many diseases(e.g.) Vit. E & Vit. C.
c-Na-K ATPase pump
Many toxicants can inhibit these pumps which are essential for transport of major
amino acids and calcium across the cell membrane.
E.g.: Hg, Cu, Pb , As and alcohol .
2-Chang in enzyme activity:
a-Inhibition
E.g.1: Carbamate esters (insecticides) reversibly inhibits cholineserase leading to increase in
A.Ch. Level
Toxicity (SLUD are the most
characteristic symptoms of toxicity).
E.g.2: Cyanide
inhibits cytochrome oxidase enzyme
no aerobic respiration
and finally cell death.
b-Activation
E.g.: Barbiturates induce hepatic microsomal enzymes
increase the conversion of some
non carcinogenic agents (in cigarette smoke) into carcinogenic ones.
3-Interferance with co-enzymes:
E.g.: CN- binds to essential metals as Fe3+ needed for the activity of cyochrome oxidase.
4-Modification of carriers:
E.g.1: CO binds with hemoglobin instead of O2 (affinity to Hb to CO is 210 times that for
O2)……… carboxyhemoglobin…….hypoxia……death.
E.g.2: Nitrates ,aspirin and sulfonamides oxidize Fe2+ in Hb into Fe3+
Hb
MeHb (methemoglobin) which can not
carry oxygen
NADPH-dependent
Hypoxia
MeHb reductase & Vit. C
5-Formation of reactive metabolites:
E.g.: Benzo(α)pyrene metabolized by HME
pyrene
Non-carcinogenic
In cigarette smoke
epoxide-7,8- dihydrodibenzo(α)
Carcinogenic
6-Reactions causing depletion of GSH:
Glutathione (GSH) is an antioxidant which protects the cell from the harmful
effect of oxidants. Reduction of GSH level into 20-30% causes impairment of
cell defense mechanism .
E.g.: N-acetyl-P-benzoquinone imine (NABQI) ,a toxic metabolite of paracetamol
it is conjugated with GSH
depletion of reduced form of GSH leading to
NABQI (Strong electrophilic agent) attack liver tissues causing liver necrosis.
-We can increase the level of GSH or overcome its depletion by; methionin (a
precursor of GSH) & N-acetylcysteine (contains –SH).
7- Action on nucleic acids:
E.g.: SO2 (air pollutant) + H2O
HSO3(causing damage to DNA &
mutation).
E.g.: Benzidine
Metabolism by HME
N-hydroxybenzidine.
Non-carcinogenic
Mutagenic & Carcinogenic
In cigarette smoke
8- Disruption of protein synthesis:
Some toxicants either increase or decrease protein synthesis leading to cellular
injury.E.g.: Ricin is a poison found naturally in castor beans. It works by
getting inside the cells of a person's body and preventing the cells from making
the proteins they need. Without the proteins, cells die. Eventually this is
harmful to the whole body, and may cause death.
9-Lysosomal changes:
a-Toxicants which causes labialization of lysosomal enzymes:
E.g.: Hg , Cu , silica , nicotine , bee venom , hypervitaminosis A , monosodium ureate
crystals deposited in gout increase lysosomal membrane permeability
release of
hydrolases cell death.
b-Toxicants which causes stabilization of lysosomal enzymes:
E.g.: Corticosteroids causes indirect toxicity by decreasing the response of the body
defense mechanism .
Factors Influencing Toxicity
There are many factors which can enhance, increase or decrease toxicity. These
factors are divided into :
I-Factors related to the host:
A-The species
-Selective toxicity: refers to species differences in toxicity response between two
species simultaneously exposed.
Rats cannot vomit and expel toxicants before they cause severe irritation, whereas
humans and dogs are capable of vomiting.
B-Sex:
.Men traditionally weigh more than women. Therefore, doses of a chemical in a male would be
expected to produce lower blood and tissue levels than the same in females, simply because of the
male's larger blood volume and greater tissue mass which dilute the chemical.
· For substances that are injected intramuscularly, lower blood levels can be expected with those drugs
in individuals (usually men) with a greater muscle mass.
· Also, drugs with a high lipid coefficient that normally partition into fat may produce different
toxicological responses in different sexes, based on the individual's ratio of body fat/total weight.
–
For example, some studies report that a sex-related difference exists for absorption of
erythromycin, resulting in less of the drug being absorbed by women, following oral
administration.
C-Age:
-In geriatric patients the toxic effects of an injected drug may be reduced because of a generalized
physiological reduction of blood supply into tissues. Similarly, the toxic response from an
orally ingested drug or chemical may also be reduced, because once absorbed less of the
substance will be delivered to a particular tissue site.
Elderly people may have a greater incidence of debilitating diseases (e.g., hepatic, renal, and
cardiovascular) which may further reduce their ability to detoxify, excrete, or distribute
the drug or chemical.
Some chemicals are more toxic to infants or the elderly than to adults.
Example:
1)-Bounded bilirubin with p.p.+Sulfonamides
replacement from P.P. binding sites.
conjugated with glucoronyl transferase
Free bilirubin
excreted in adults
(Low activity of GT + Immature B.B.B in neonates)
Kernikterus (in newborn)
2)-Nitates(in well , s water)
due to stomach pH is high in newborn
Nitrite (oxidant)
Oxidation
Hb
MetHb
MetHb reductase
(v.weak in newborns)
3)-Chloramphenicol conjugation by GT is low in neonates
accumulation of it ,and it
oxidizes Hb into MetHb
Grey baby syndrome (hypoxia, cyanosis , collapse and
death).
D-Genetics:
I-Pharmacogentices (Idiosyncratic reaction ): An odd response to a given normal dose of a
drug on hereditary bases
1) Succinylcholine apnea in individuals deficient in pseudo cholinesterase ….?
2) Individuals deficient in glucose-6-phosphate dehydrogenase suffer from hemolytic
anemia upon using sulfa drugs , aspirin or naphthalene (oxidants)
glucose-6-phosphate
+
NADP
G6PD
6-phosphogluconic acid
In case of G6PD deficiency
2 GSH (protect RBCs from hemolysis by oxidants)
GSH reductase
+
NADPH
NADPH
GSSG
GSH
Oxidants attack RBCs
II-Toxicogentices: An odd response to a given toxicant on hereditary basis
* smoking causes emphysema in certain individuals deficient in α1- antitrypsin.
III-Hypersensitivity (allergic reactions):
e.g. Some people suffer from an anaphylactic reaction when given penicillin.
hemolysis
Pharmacogentices (Idiosyncratic reaction)
E-Dietary factors
1) Heavy metal absorption is influenced by diet.:
-Calcium, iron, fats, and protein are all reported to enhance lead absorption.
-Deficiency of calcium, iron, or protein, on the other hand, enhances cadmium absorption.
2)Low protein in diet :P.P. level decrease
free drug
toxicity . Also, a low dietary
protein intake may result in a decreased level of hepatic microsomal enzymes and, thus,
the metabolic processes may proceed less readily.
3)Food containing tyramine as old cheese ,salted dried fish ,banana, Beer, Canned figs,
Chicken liver ,Chocolate, Sherry & wines increase MAOIs (e.g., pargyline, phenelzine)
toxicity which is severe symptoms of hypertensive crisis and even death may occur.
4) Calcium in milk, which may bind to tetracyclines, and thus reduce its absorption.
5) Foods rich in pyridoxine may significantly lower the pharmacological action of levodopa
6) Fatty foods, on the other hand, enhance griseofulvin absorption
F. Occupation
Individuals working in industries where organic compounds such as chlorinated
hydrocarbon pesticides or volatile substances are used may have an enhanced ability
to metabolize drugs and chemicals. The reason for this is related to the chemical's
presence in the environment, which may have enhanced the worker's liver microsomal
enzyme activity. His expected reaction to a toxic dose of any subsequent chemical,
normally detoxified by the liver microsomal system, would be less than normal.
G-Health factors:
1)Acidosis: insulin activity decrease leading to hyperglycemia
2)Hypertensive patients may respond more intensely to chemicals that have
sympathomimetic activity.
3)Opiates and other chemicals that cause respiratory depression are more
hazardous in persons with head injuries.
4)Asthma: patient is more liable to the effect of air pollutants as SO2.
5)Kidney & liver diseases: toxicity of many drugs increase.
H. Living Conditions
The living conditions of an individual could be an extremely important factor to
consider. At present, we should consider that factors such as crowding in living
conditions, noise, and social pressures are important areas for research.
End of factors related to Host
II-Factors releated to the poison:
a-Dose
Anything can be toxic if enough is taken (and, conversely, even the most toxic of
substances may not be harmful when low concentrations are taken).
Also, the number of doses, frequency, and total time of the exposure are also very
important .
Example:
b-Routes of exposure
-Routes of exposure can influence the time of onset, intensity, and duration of the
toxic effects.
-The route of administration may also predict the degree of toxicity and possibly
the target systems which will most readily be affected.
-Ingested chemicals, when absorbed from the intestine, distribute first to the liver
and may be immediately detoxified .
-Inhaled toxicants immediately enter the general blood circulation and can
distribute throughout the body prior to being detoxified by the liver.
Inhalation › Intravenous › Interapertoneal › Intramuscular › Subcutaneous ›
c-Chemical structure
The physicochemical composition of the toxicant can sometimes be helpful in predicting
the risk involved in exposure to a particular compound.
-Silica (amorphous)
has little effect on health
after it is heated
silica (crystalline)
serious lung damage.
-Cr3+ is relatively nontoxic whereas Cr6+ causes skin or nasal corrosion and lung cancer.
d-Composition and formulation: ( mainly › absorption)
-Concentration .
-Lipid solubility.
In general, solids are less easily swallowed than liquids, and bulky solids are more difficult
to consume than light, more fluffy compounds, especially for a small child
-Chemicals in liquid form are more toxic than those in solid form
-Coloring agents as tartazin yellow cause allergy.
-Micronization increase toxicity.
-Vehicles as alcohols increase CNS depressant action of hypnotics.
-Impurities; some herbicide as 2,4,5-trichlorophenoxy acetic acid may contain the toxic
impurity DIOXIN which is mutagenic ,teratogenic and carcinogenic.
-pH of the preparation (high acidity or alkalinity) cause local sever effect.
-Low stability of the compound + bad storage condition increase toxicity as food
contaminants aflatoxin ( it is a product of certain molds)
-The particle size: Only particles having a small diameter (1 m or less) will effectively
reach the alveoli and be available for pulmonary absorption. Larger particles may be
deposited on the walls of the throat and trachea to produce irritation.
F-Temperature
-The response of a biological system to a toxic agent decreases as the
environmental temperature is lowered ( but the duration of the overall
response may be prolonged)·
The reasons for these are:
a. Decreased rate of absorption occurring in the colder environment;
b. Lowered rate of metabolic degradation and excretion.
Example
1-Atropine-like compounds may produce significantly greater toxicity in a warm
environment than in a colder one. Because anticholinergic agents inhibit
sweating, the body temperature becomes elevated because the absence of
perspiration prevents cooling of the body; so the toxic effects are from
hyperthermia.
2-Alternatively, drugs such as chlorpromazine that suppress the body's thermal
regulatory center may be more toxic at certain temperatures.
.
End of factors related to Toxicant
III-Toxicokinetic factors:
i-Factors affecting absorption:
* GIT :
Gastric content:
-Empty stomach has higher emptying rate Toxicity.
-Carbonated beverages increase G.E.
Toxicity.
-A full stomach with proteins & fats Toxicity.
Secretion : Pepsin & HCl digest peptide poisons .
GIT flora: migration of intestinal flora into the stomach in newborns due to
their high gastric pH ,this flora can convert nitrates into nitrite,
oxidizing Hb into metHb Hypoxia.
*Skin (Thickness & Keratin layer protect the skin ):
-Newborn (thin delicate skin).
-Lipophilicity of insecticides.
Toxicity
-Cutting , abrasions & dryness of skin
*Pulmonary:
-Conc. Of toxicants in air.
-Solubility of the toxin in blood & tissues.
-Respiration rate.
-Exposure time.
ii-Factors affecting metabolism (biotransformation):
There are two types of metabolism :detoxification and bioactivation. Detoxification is the
process by which a xenobiotic is converted to a less toxic form. Bioactivation is the
process by which a xenobiotic may be converted to more reactive or toxic forms.
Examples of chemicals known to be metabolized to more toxic compounds are;
Acetanilid, Acetaminophen, Aniline, Carbon tetrachloride, Chloral hydrate, Codeine,
Ethylene glycol, Isopropanol, Methanol, 2-Naphthylamine, Parathion, Pyridine and
Sulfanilamid.
iii-Factors affecting distribution:
Main mechanisms opposing distribution of the toxicants are:
a-Plasma proteins:
-Bilirubin in neonates???.
-High affinity for binding to P.P. may cause toxicity due to
drug interaction as sulfonamide displace tolbutamide from
P.P. binding site causing hypoglycemia.
b-Storage:
-DDT is stored in fat tissues and upon short term diet
,mobilization of fats occur leading to release of DDT and
toxicity.
-Fluoride bind to calcium in bones flurosis.
-Pb is stored in bones (non toxic to it) , osteoporosis mobilize
Pb leading to toxicity.
c-Special barriers (B.B.B.):
-Mainly depends on lipid solubility
iv-Factors affecting excretion:
-The kidney is the primary excretory organ, followed by the gastrointestinal tract,
and the lungs (for gases). Xenobiotics may also be excreted in sweat, tears, and
milk.
-Impaired cardiac , kidney or liver function causes slower elimination of toxicants
and increases their toxic potential.
End of Toxicokintics factors
And Factors related to poison
IV-Chemical Interactions
Humans are normally exposed to several chemicals at one time rather
than to an individual chemical.
Examples are:
-Hospital patients on the average receive 6 drugs daily
-Home influenza treatment consists of aspirin, antihistamines, and cough syrup taken
simultaneously
-Drinking water may contain small amounts of pesticides, heavy metals, solvents, and other
organic chemical
-Air often contains mixtures of hundreds of chemicals such as automobile exhaust and
cigarette smoke
-Gasoline vapor at service stations is a mixture of 40-50 chemicals
-There are four basic types of interactions.
Additivity
-A tranquilizer and alcohol, often cause depression equal to the sum of that
caused by each drug.
-Chlorinated insecticides and halogenated solvents (which are often used
together in insecticide formulations) can produce liver toxicity with the
interaction being additive.
P.S. : this same combination of chemicals produces a different type of interaction on
CNS. Chlorinated insecticides stimulate CNS whereas halogenated solvents cause
its depression . So, the effect of simultaneous exposure on CNS is an antagonistic
interaction.
Potentiation
It occurs when a chemical that does not have a specific toxic effect makes
another chemical more toxic.
Example:
-Warfarin (a widely used anticoagulant in cardiac disease) is bound to plasma
albumin so that only 2% of the warfarin is active (FREE). Drugs which
compete for binding sites on albumin increase the level of free warfarin to 4%
causing fatal hemorrhage.
-Phenobarbitone pre-treatment induces toxicity of paracetamol
-Proniazid “MAOI” induces CVS toxicity by tyramine.
Antagonism
It is often a desirable effect in toxicology and it is the basis for most antidotes.
Examples include:
Synergism
-Exposure to both cigarette smoke and asbestos results in a significantly greater
risk for lung cancer
-The hepatotoxicity of a combination of ethanol and carbon tetrachloride is
much greater than the sum of the hepatotoxicity of each.
End of Factors affecting Toxicity
Sources of information on safety
1. Experimental studies
Experimental toxicology is a branch of toxicology, which deals with
toxicity studies in experimental animals to evaluate the safety of a
new chemical (drugs, food additives, pesticides and industrial chemicals).
2. Controlled clinical studies
Drug is tested on small number (50–60) of healthy volunteers in a
controlled dose for specified time
3. Epidemiological studies
Thalidomide: its teratogenic activity was discovered in the 1960s.
Sulphonamide elixir (1930): its vehicle was ethylene glycol (metabolize
to oxalic acid) that resulted in titanic convulsion
Goals of toxicity studies
· Predict the toxicity of chemical in human
· Give information about mechanism of toxicity
· Give information about toxicity of dosage used in humans
· Toxicity studies indicate the therapeutic index which gives
information about safety.
Experimental toxicity studies
Advantages of Experimental toxicity tests
We can perform different experimental protocols using
different routes of administration. And from this we can
determine the following doses:
1. No-effect dose: it is the maximum dose that produces no
observable toxic effect on the animals.
2. Minimal toxic dose: it is the dose that produces the least
toxicity
3. Median lethal dose (LD50 & LC50): This is the dose that
kills 50% of the animals.
Disadvantages of toxicity tests using experimental animals
1-We can not extrapolate results obtained from experimental studies to
humans: e.g.: insulin in experimental animals is toxic and causes
teratogenic effects in pregnant animals, but in pregnant women it shows
no teratogenic effects.
2-Toxicity related to genetic factors can not be detected in animals: e.g.:
Sulphonamides and aspirin in glucose-6-PO4 dehydrogenase deficient
patients.
3-Side effects of low occurrence appear only in humans but not in animal
(because it needs large scale & more time).
Characteristics of ideal animal species
Non expensive
Available
Easily breaded
Of short gestation period
Of short life span for multigenerational life studies
Physiology should be similar to human physiology
 There is a priority to carry out toxicity tests according to the exposure of
humans
A. Industrial chemicals: People subjected to industrial chemicals are liable to
Accidental toxicity by inhalation or by oral route. So acute toxicity tests by
inhalation & oral route should be carried out
Carcinogenicity, mutagenicity & reproduction toxicity tests
In case of biocides we should carry out acute, subacute, subchronic and chronic
toxicity tests in addition to carcinogenicity, mutagenicity, and reproduction toxicity
test.
B. Food additives
All types of toxicity tests should be performed
C. Cosmetics:
Dermal toxicity : Local allergy
Oral toxicity: if taken orally by mistake in children
Cosmetics carry label not tested on animals this is because they are tested on:
Isolated eyes (obtained from slaughterhouse)
Chorio-allantoin membrane: it is obtained from eggs before hatching and it is very rich
in blood vessels.
General toxicity studies
1-Acute toxicity tests
A. Toxicometric studies
These studies utilize dose response curve to determine LD50 and/or
LC50
The dose-response correlates exposures and the spectrum of induced
effects.
-Generally, the higher the dose, the more severe the response.
LD50: Median lethal dose i.e.: It is the dose of a compound that causes
50% mortality in a population
LC50: Median lethal concentration (inhaled drugs).
Factors affecting LD50
Species, age, sex, body weight, general health condition, strain, diet, nutritional status
& number of animals used in the test
Route of administration (oral route differ from parental route)
Environmental conditions (lab conditions) i.e. intra & inter laboratory conditions
Experimental procedure, stress, dosage formulation
We cannot say that LD50 of drug X is 50 mg/Kg absolutely but we must specify the
route of administration e.g. oral LD50 of drug X is 50 mg/Kg.
Importance of LD50:
In spite of the many variables affecting the LD50 determination, many governmental
agencies still regard the LD50 as the sole measurement of the acute toxicity of all
materials.
Many agencies classify chemicals according to LD50 of drugs given orally to rats into
the following groups.
Super toxic chemicals: LD50 > 5mg/kg
Extremely toxic chemicals: LD50 = 5 – 50 mg/kg
Very toxic chemicals: LD50 = 50 – 500 mg/kg
Moderately toxic chemicals: LD50 = 0.5 – 5 g/kg
Slightly toxic chemical: LD50 = 5 – 15 g/kg
Practically non- toxic compound: LD50 > 15g/kg
Other dose estimates also may be determined:
Threshold dose level :The point at which toxicity first appears ,and below which no toxic
effect occur.
-In the hypothetical curve above, no toxicity occurs at 10 mg whereas at 35 mg 100% of the
individuals experience toxic effects.
-A threshold for toxic effects occurs at the point where the body's ability to detoxify a
xenobiotic or repair toxic injury has been exceeded. For most organs there is a reserve
capacity so that loss of some organ function does not cause decreased performance. For
example, the development of cirrhosis in the liver may not result in a clinical effect until
over 50% of the liver has been replaced by fibrous tissue.
-Dose-response curves are also used to derive dose estimates of chemical
substances: EDs & TDs.
Toxic Doses (TDs) : the doses that cause adverse toxic effects.
Effective Doses (EDs) are used to indicate the effectiveness of a
Substance. Normally, an effective dose refers to a beneficial effect (relief of pain).
It might also stand for a harmful effect (paralysis). Thus the specific endpoint
must be indicated.
-The knowledge of the effective and toxic dose levels aides the toxicologist and
clinician in determining the relative safety of pharmaceuticals.
-As shown above, two dose-response curves are presented for the same drug, one
for effectiveness and the other for toxicity. In this case, a dose that is 50-75%
effective does not cause toxicity whereas a 90% effective dose may result in a
small amount of toxicity.
The Therapeutic Index (TI) is used to compare the therapeutically effective dose to
the toxic dose
The Therapeutic Index (TI) It is the ratio of the dose producing 50% toxicity to the
dose needed to produce the 50% therapeutic response.
The TI is a statement of relative safety of a drug.
- For example, if the LD50 is 200 and the ED50 is 20 mg, the TI would be 10
(200/20). A clinician would consider a drug safer if it had a TI of 10 than if it
had a TI of 3.
-The use of the ED50 and LD50 doses to derive the TI may be misleading to safety,
depending on the slope of the dose-response curves for therapeutic and lethal
effects.
-Knowledge of the shape and slope of the dose-response curve is extremely
important in predicting the toxicity of a substance at specific dose levels.
-Major differences among toxicants may exist not only in the point at which the
threshold is reached but also in the percent of population responding per unit
change in dose (i.e., the slope).
-As illustrated above, Toxicant A has a higher threshold but a steeper slope than
Toxicant B.
To overcome this deficiency, toxicologists often use another term to
determine the safety of a drug .
The Margin of Safety (MOS).
It is the ratio of the dose that is just within the lethal range (LD01) to
the dose that is 99% effective (ED99). The MOS = LD01/ED99.
-A physician must take care when prescribing a drug with MOS less
than 1.
NOAEL and LOAEL:
-No Observed Adversed Effect Level (NOAEL)
-Lowest Observed Adverse Effect Level (LOAEL)
They do not necessarily imply toxic or harmful effects ,and may be used to describe
beneficial effects of chemicals as well.
Disadvantages of Toxicometrics
-Needs large number of animals of at least 3 different species
-LD50 is not constant, it is affected by many factors (age, sex, species environmental conditions
& route of administration)
Toxicity of chemicals due to genetic abnormalities cannot be detected.
B. Dermal (skin) toxicity studies
-It provide information on the adverse effects resulting from a dermal
application of a single dose of a test substance.
-The acute dermal test also provides the initial toxicity data for regulatory
purposes, labeling, classification & subsequent subchronic & chronic
dermal toxicity studies.
Eye irritation
It can be defined as reversible inflammatory changes in the eye and its
surrounding mucus membranes following direct exposure to a material on
the surface of the anterior portion of the eye.
Draize test
It is a simple and generalized test developed to study eye irritation in
rabbits.
It is used as the animal test to identify human eye irritants.
The Draize test can adequately identify most of the moderate to severe
human eye irritants
Pyramidal single dose test
This test is carried out on dogs. Large number of dogs ≈ 100 are given a
single daily X dose of a compound under test (X mg/Kg)
At the end of the day, the number of dogs which died & those which
survived is observed.
The procedure is continued, till all dogs die, then plotting is done.
Example: 1st day 20 died & 80 living
At the second day, double the X dose for living dogs i.e. 2 X mg/kg. This
resulted in 20 died & 60 lived i.e. the dose starts small and then
increases.
Uses of pyramidal single dose test
It helps in studying the mechanism of drug toxicity.
Used to determine any pathological changes by examination of the animal
after death.
The effect of the drug on all body organs can be examined..
Biochemical tests can be performed on living animals (hematology, and
detection of different biotransformation and excretion product, and
determination of t½ of the compound).
2-Prolonged toxicity studies (Repeated Dose Toxicity Studies)
A. Sub-acute toxicity studies
-They predict any cumulative effect of the drug & play a role in the safety
assessment of pharmaceuticals, pesticides, food additives, and other
chemicals.
In this test, at least 3 species of animals are used: rats, mice and rabbits.
Compound under test is given daily in 3 dose levels (Maximum tolerated dose,
Therapeutic dose and Dose in between) for 2 – 4 weeks.
Animals are observed for different parameters: physiological, clinical and chemical
tests, behaviour, CNS & autonomic profiles.
The no-effect dose is determined & used it in the following tests.
If oral route is intended, these doses are mixed with food or water.
In addition, there is a fourth group of animals: control group it is used to test the
vehicle of the drug.
Animals are observed during the period of test for: mood symptoms, CNS effects,
reflexes (corneal, righting) & autonomic responses.
At the end of the test, animals are subjected to the these tests and then are killed.
Hematological studies: hemoglobin, RBCs, WBCs, platelets
Clinical chemistry studies: serum creatinine, ALT, AST
Histopathological studies: for different organs (spinal cord, heart, kidney, muscle)
B.Subchronic Toxicity Test
It is performed for compounds administered for a long period.
The compound is given for 90 days by the intended route.
Animals are observed all over the specified time.
For dead animals autopsy is performed by taking samples from
different organs to be examined, chemically, microscopically &
macroscopically
C. Chronic Toxicity Test
It is performed for compounds administered for a long period.
The compound is given for more than 90 days by the intended route.
D. Life – Span Toxicity Test
The same previous procedures are applied but treatment with
chemicals starts after weaning of offsprings (litters).
Administration of the chemical is continued till death of animals.
When animals die spontaneously, the same parameters are
determined.
Specific Toxicity Studies
Reproductive Toxicity
It involves toxicant damage to either the male or female
reproductive system.
Toxic effects may cause:
-Decreased libido and impotence
-Infertility
-Interrupted pregnancy (abortion, fetal death, or premature
delivery)
-Infant death or childhood morbidity
-Altered sex ratio and multiple births
-Chromosome abnormalities and birth defects
-Childhood cancer
Developmental Toxicity
Developmental Toxicity pertains to adverse toxic effects to the developing
embryo or fetus. This can result from toxicant exposure to either parent
before conception or to the mother and her developing embryo-fetus.
The three basic types of developmental toxicity are:
Embryolethality
Failure to conceive, spontaneous abortion
Embryotoxicity
Growth retardation or delayed growth of
specific organ systems
Teratogenicity
Irreversible conditions that leave permanent
birth defects in live offspring (cleft palate,
missing limbs)
Chemicals cause developmental toxicity by two methods.
They can act directly on cells of the embryo causing cell death or cell damage,
leading to abnormal organ development.
A chemical might also induce a mutation in a parent's germ cell, which is transmitted
to the fertilized ovum. Some mutated fertilized ova develop into abnormal
embryos.
Carcinogenicity
It is a complex multistage process of abnormal cell growth and differentiation which
can lead to cancer.
The initial neoplastic transformation results from the mutation of the cellular genes
that control normal cell functions.
Mutation may lead to abnormal cell growth. It may involve loss of suppresser genes
that usually restrict abnormal cell growth. Many other factors are involved (e.g.,
growth factors, immune suppression, and hormones).
A tumor (neoplasm) is simply an uncontrolled growth of cells.
Benign tumors grow at the site of origin; do not invade adjacent tissues or
metastasize; and generally are treatable.
Malignant tumors (cancer) invade adjacent tissues or migrate to distant sites
(metastasis). They are more difficult to treat and often cause death.
Mutation
Mutation results from any change in DNA structure.
If the mutation occurs in a germ cell the effect is heritable. There is no effect on the
exposed person; rather the effect is passed on to future generations.
If the mutation occurs in a somatic cell, it can cause altered cell growth (e.g. cancer)