LECTURE 1
GENERAL
PRINCIPLES OF
TOXICOLOGY
objectives
■ On completion of this session students should be able to:
i.
Discuss the general principles of toxicological evaluation of drugs
ii. Describe the various characteristics of toxic effects
iii. Describe the features of possible interaction between chemicals
iv. Explain the principle of the dose-response curve
v. Describe the various animal toxicological test used in drug screening
vi. Discuss the various phases of drug development
Introduction
Paracelsus (1493-1541)
Swiss German Philosopher
“All substances are poisons: There is none which is not a poison. The right
dose differentiates a poison and a remedy”
definitions
■ Toxicology is a multi-disciplinary science used to address a
variety of questions related to the nature, effects and detection
of poisons.
– Examines adverse effects ranging from short term (acute) to
long-term (chronic)
– Assess the probability of hazards by analyzing adverse effects
– Predict effects on individuals, populations and ecosystems
Disciplines/branches
Classification of toxic
agents
■ Toxic Agent: substance with poisonous/deadly effects
■ Xenobiotic: a foreign chemical substance found in an organism that is not
naturally produced by or expected to be present within.
■ Toxin: toxic substances produced by biological systems (produced
naturally), such as plants, animals, bacteria and fungi.
■ Toxicant: substances that are produced by or are by-products of human
activities. Toxins which are not naturally occurring
– Example: Dioxin (2,3-7,8-tetrachlorodibenzo-p-dioxin, TCDD), byproduct of certain chlorinated chemicals
Classification of toxic agents
CLASSIFICATION
EXAMPLES
Heavy Metals
Lead, Mercury, Cadmium Renal dysfunction, CNS
damage, Endocrine damage
Carbon monoxide, ozone Tissue damage,
etc.
inflammation
Cesium 137, Cobalt 60,
Cataract, cancer, DNA
Uranium 238 etc.
damage, sterility etc.
Solvents and Vapours
Radioactive Material and
Radiation
Pesticides, Herbicides,
Insecticides
Plant Toxins
Animal Toxins
Organochlorines,
Organophosphates,
Paraquat
Alkaloids, Terpenes,
Resins, hypoglycin etc.
Snake venom etc.
EFFECTS
Carcinogenic effects etc.
Cell Death, depression of
CNS,
Internal bleeding
Classification of toxic agents:
subcategories
■ Effects on target organs (Liver, Kidneys, hematopoietic system)
■ Effects (cancer, mutation, liver injury)
■ Chemical Stability or reactivity (explosive, flammable, oxidizer)
■ Chemistry (aromatic amine, halogenated hydrocarbon)
■ Biochemical Mechanism: alkylating agent, sulfhydryl inhibition,
methemoglobin producer)
Toxic agents: encounters
Toxic agents: encounters
Toxic agents: encounters
Risk, hazard and exposure
explained
Characteristics of toxic
exposure
■ A toxic response is dependent on the:\
– Chemical and physical properties of the agent
– Exposure situation
– Metabolism of the agent by the system
– Concentration of the active form of the substance at the target
site
– Susceptibility of the biological system/organism
Characteristics of Toxic Exposure:
route and site of exposure
■ Approximate decreasing order
of effectiveness:
– Inhalation
– Intraperitoneal
– Subcutaneous
– Intramuscular
– Intradermal
– Oral
– Dermal
■ For example, the water and
lipid solubility characteristics
of a chemical affect its
absorption across the;
■ Rate and site of absorption
(organ) may also determine the
rate of metabolism and excretion
of the chemical.
Characteristics of toxic exposure:
duration and frequency
The exposure of organisms to toxic agents can be divided into 4 categories
■ Acute- Exposure to a chemical for
less than 24 hours, usually a single
administration, also refers to cases
of repeated exposures within the
24 hour period for slightly toxic or
seemingly non-toxic agents
■ Subacute- Repeated exposure to a
chemical for 1 month or less
■ Subchronic- Repeated
exposure for 1-3 months
■ Chronic- Repeated exposure
for more than 3 months
Characteristics of toxic exposure:
side effects vs. toxic effects
■ Side Effects
Occur at the normal dosage
levels of the drug/substance
(undesirable but
unavoidable)
■ Toxic Effects
– Adverse effects usually seen at higher
doses
– Usually irreversible but may be treated
– May be related to the main
pharmacological action
– Bleeding with Warfarin (Coumadin®)
– May be unrelated to the main
physiological action
– Liver toxicity from Paracetamol
(Panadol® and Tylenol®)
Spectrum of undesired effects
Allergic Reaction
■ Chemical allergy is an immunoadverse
logically mediated reaction to a chemical
resulting from previous sensitization to that
chemical or to a structurally similar one.
■ Most chemicals and their metabolic products
are not sufficiently large to be recognized by
the immune system as a foreign substance and
thus must first combine with an endogenous
protein to form an antigen (or immunogen).
■ A molecule that must combine with an
endogenous protein to elicit an allergic
reaction is called a hapten.
■ Idiosyncratic Reaction
■ determined abnormal
reactivity to a chemical.
– Extreme sensitivity to low
doses or extreme
insensitivity to high doses
Spectrum of undesired effects:
Immediate vs. delayed toxicity
■ Immediate toxic effects can be
defined as those that occur or
develop rapidly after a single
administration of a substance.
■ Delayed toxic effects are those
that occur after the lapse of
some time.
– Carcinogenic effects of
chemicals usually have a
long latency period, often
20 to 30 years after the
initial exposure, before
tumors are observed in
humans.
Spectrum of undesired effects:
reversible vs. irreversible effects
■ If a chemical produces
pathological injury to a tissue, the
ability of that tissue to regenerate
largely determines whether the
effect is reversible or irreversible.
■ However, injury to the CNS is
largely irreversible because
differentiated cells of the CNS
cannot divide and be replaced.
■
■ Carcinogenic and teratogenic
effects of chemicals, once they
occur, are usually considered
irreversible toxic effects.
■ Thus, for a tissue such as liver,
which has a high ability to
regenerate, most injuries are
reversible.
Spectrum of undesired effects:
local vs. systemic toxicity
■ Local effects are those that occur at
the site of first contact between the
biological system and the toxic agent.
■ For example, chlorine gas reacts with
lung tissue at the site of contact,
causing damage and swelling of the
tissue, with possibly fatal
consequences, even though very little
of the chemical is absorbed into the
bloodstream.
■ Systemic effects require
absorption and
distribution of a toxicant
from its entry point to a
distant site, at which
deleterious effects are
produced.
Chemical interaction
■ Chemical interactions are known
to occur by a number of
mechanisms, such as;
– Alterations in absorption
– Protein binding, and
– The biotransformation and
excretion of one or both of the
interacting toxic agents.
■ Chemical interactions can be
increased or decreased in one of
four ways;
– Additive
– Synergistic
– Potentiation
– Antagonistic
Variations in toxic response:
Selective toxicity
■ Selective toxicity means that a chemical produces injury to one kind
of living matter without harming another form of life even though
the two may exist in intimate contact.
– The living matter that is injured is termed the uneconomic form
(or undesirable),
– The matter protected is called the economic form (or desirable).
■ Selectivity usually resulting from differences in the absorption,
biotransformation, or excretion of the toxicant.
Variations in toxic response:
Species differences
■ There are many reasons for selective toxicity among different
species. Even among phylogenetically similar species (e.g. rats,
mice, guinea pigs, and hamsters), large differences in response may
occur.
■ Identifying the mechanistic basis for species differences in response
to chemicals is an important part of toxicology.
Variations in toxic response:
individual differences
■ Even within a species, large inter-individual differences in
response to a chemical can occur because of subtle genetic
differences.
■ Hereditary differences in a single gene that occur in more than
1% of the population are referred to as genetic polymorphism.
Dose response relationships
■ What is "Dose"?
– The dose of a chemical or physical agent is the amount of that
agent that comes into contact with a living organism or some
part of a living organism.
■ The type of dose most familiar to the average person is that
associated with medicines.
Dose response relationships
Response changes as a function of
dose
■ Plot of the effect of a chemical based
on the dose:
– The drug dose is increasing but
there is little response.
– The drug must reach a desired
concentration before the response
increases significantly.
– Until there is no further response
produced
Median effective concentration
(MEC/ed50
■ The minimum concentration of a chemical in plasma required to produce
the therapeutic effect.
■ It is the dose of the chemical that is therapeutically effective in 50% of
the population.
■ It reflects the concentration of chemical at the receptor site to elicit the
desired toxicological response.
■ The concentration below the MEC is referred to as the sub-therapeutic
level.
Median toxic concentration
(mtc/td50)
■ The concentration of chemical in plasma at and above which adverse or
unwanted effects are triggered.
■ It is the dose of the chemical that causes a toxic response in 50% of the
population.
■ Concentration of chemical above the MTC is said to be in the toxic level .
■ The MTC may also be called the maximum safe concentration (MSC).
Median lethal concentration
(ld50)
■ Statistically derived concentration
of a substance in an environmental
medium expected to kill 50% of
organisms in a given population
under a defined set of conditions.
Measurement of toxicity –
therapeutic index (TI)
Therapeutic index (ti)
■ The “Therapeutic Index” also
called the “Therapeutic
Window” is comparison of the
amount of a therapeutic agents
that elicits a therapeutic effect to
the amount which causes
toxicity
The drug development process
■ Drug development is the
process by which new
pharmaceutical drugs are
introduced to the market once
a lead compound has been
identified through the process
of drug discovery.
The drug development process
■ Step 1: Discovery &
development
■ Step 2: Preclinical research
■ Step 3: Clinical research
■ Step 4: FDA review
■ Step 5: FDA post-marketing
safety monitoring.
Descriptive animal
toxicity tests
■ Testing for the safety or efficacy of a substance or product accounts for a
major use of animals as defined in this assessment, most of which are rats
and mice.
■ Of these, probably the largest portion are used in developing drugs.
– A significant portion are also used to test other substances—
pesticides, industrial chemicals, and consumer products—to assess
possible toxicity and to establish conditions under which they can be
used safely.
■ Testing methodologies are standardized to control experimental variables,
thus allowing results to be easily compared.
Descriptive animal
toxicity tests
■ Pharmacokinetics
– Pharmacokinetic studies provide information about;
■ The mechanisms of absorption
■ A substance’s distribution among the various body compartments
■ About metabolism and elimination.
– They facilitate the interpretation of results from other tests and their
extrapolation to humans because the distribution and elimination of a
foreign substance will often explain its toxicity or lack thereof.
Animal toxicity tests
■ 2 main principles
– Effects produced by a compound in laboratory animals, when
properly qualified, are applicable to humans
– Exposure of experimental animals to chemicals in high doses is a
necessary and valid method of discovering possible hazards in
humans.
Descriptive animal toxicity
tests
■ Acute Toxicity Testing
■ Skin and Eye Irritation
■ Sensitization
■ Sub-Acute
■ Sub-Chronic
■ Chronic
■ Developmental and Reproductive Toxicity
■ Mutagenicity
■ Oncogenicity Bioassays
■ Neurotoxicity Assessment
■ Immunotoxicity Assessment
Genetic toxicity
■ Genetic toxicity involves the assessment of the deleterious effects of
chemicals or physical agents on hereditary material and related
genetic processes of living cells.
■ By altering the integrity and function of DNA at the gene or
chromosomal level, the damage can lead to heritable mutations
ultimately resulting in genetic disorders, congenital defects or
cancer.
Genetic toxicity
■ Toxicogenomics tools are
important for research aimed at
identifying the mechanisms and
mode of action of toxic substances.
■ Genomics
– Characterization of a section
or all of the genome of an
organism
■ Transcriptomics
– Characterization of most or all
of the mRNAs or transcriptome,
expressed in a given cell/tissue
■ Proteomics
– Characterization of most or
all of the proteins expressed
in a given cell/tissue
■ Metabonomics
– Characterization of most or
all of the small molecules in
a cell/ tissue, including
substrates, products, and
co-factors of enzyme
reactions.