Laboratory of toxicology

Department of Pharmacology and Toxicology

College of Pharmacy, University of Baghdad

2015

GENERAL PRINCIPLES OF TOXICOLOGY

General Principles of Toxicology

Determination of LD50

Nicotine poisoning

Guidelines for Seminar Presentations

Seminar CCl4

Ethanol and methanol

Cyanide and carbon monoxide

Organophosphorus and Organochlorine

TOXICOLOGY - PRACTICAL SYLLABUS 2015

Toxicology is the study of the adverse effects of chemicals on living organisms

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

Toxicology testing (safety testing, toxicity testing) Conducted to determine the degree to which a substance

can damage a living or non-living organisms

Routinely performed by pharmaceutical manufacturers in the investigation of a new drug

It is often conducted by researchers to comply with governing regulations

SCIENCE OF TOXICOLOGY

Chronic tests In which two species, one rodent

and one non rodent are dosed daily for six months

TOXICOLOGY TESTING

Acute toxicity tests in which a single dose is used in each animal on one occasion only for the determination of gross behavior and

LD50 or median lethal dose

Sub acute tests In which animals (usually rats) are dosed

daily, starting at around expected therapeutic level and increasing stepwise every two to three days until toxic signs

are observed

TOXICOLOGY

• Mechanistic toxicology •Identifies the cellular, biochemical, and molecular mechanisms by

which chemicals exert toxic effects on living organisms

•Organophosphate pesticides inhibit AChE

• Descriptive toxicology •Concerned directly with toxicity testing, which provides

information for safety evaluation and regulatory requirements

•Acute oral toxicity test, LD50 levels

• 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

•Warnings, cautions,…etc

DIFFERENT AREAS OF TOXICOLOGY

The major routes (pathways) by which toxic agents gain access to the body are:

Gastrointestinal tract (ingestion)

Lungs (inhalation)

Skin (topical, percutaneous, or dermal)

Other parenteral (other than intestinal canal) routes

Toxic agents generally produce the greatest effect and the most rapid response when they are introduced directly into the bloodstream (the intravenous route)

ROUTE AND SITE OF EXPOSURE

ROUTE OF ADMINISTRATION

Toxicologists usually divide the exposure of experimental animals to chemicals into four categories:

Acute exposure is defined as exposure to a chemical for less than 24 h

Subacute exposure refers to repeated exposure to a chemical for 1 month or less

Subchronic for 1 to 3 months

Chronic for more than 3 months

DURATION AND FREQUENCY OF EXPOSURE

Allergic Reactions Chemical allergy is an immunologically mediated adverse

reaction to a chemical resulting from previous sensitization to that chemical or to a structurally similar ones

Most chemicals and their metabolic products are not sufficiently large to be recognized by the immune system as foreign substances and thus must first combine with an endogenous protein to form an antigen (or immunogen). This kind of molecule is called a hapten. The hapten-protein complex (antigen) is then capable of eliciting the formation of antibodies

Chemical idiosyncrasy Genetically determined abnormal reactivity to a chemical The response observed is usually qualitatively similar to that

observed in all individuals but may take the form of extreme sensitivity to low doses or extreme insensitivity to high doses of the chemical

SPECTRUM OF UNDESIRED EFFECTS

Immediate toxic effects occur or develop rapidly after a single administration of a substance, whereas delayed toxic effects occur after the lapse of some period of time

Carcinogenic effects of chemicals usually have long latency periods, often 20 to 30 years after the initial exposure, before tumors are observed in humans

IMMEDIATE VERSUS DELAYED TOXICITY

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

Irreversible: cancer, necrosis

Reversible: Neostigmine reversible inhibitor of cholinesterase

REVERSIBLE VERSUS IRREVERSIBLE TOXIC EFFECTS

Local effects

Occur at the site of first contact between the biological system and the toxicant

Target organs: hepatotoxicity

Systemic effects

Require absorption and distribution of a toxicant from its entry point to a distant site at which deleterious effects are produced

CNS depression; multiple organic toxicities

LOCAL VERSUS SYSTEMIC TOXICITY

Additive effect When the combined effect of two chemicals is equal to the sum of

the effects of each agent given alone (example: 2 + 3 = 5)

Synergistic effect When the combined effects of two chemicals are much greater

than the sum of the effects of each agent given alone (example: 2 + 2 = 20).

Both carbon tetrachloride and ethanol are hepatotoxics, but together they produce much more liver injury than the mathematical sum of their individual effects at a given dose

Potentiation When one substance does not have a toxic effect on a certain

organ or system but when added to another chemical makes that chemical much more toxic (example: 0 + 2 = 10)

Isopropanol is not hepatotoxic, but when it is administered with carbon tetrachloride, the hepatotoxicity of carbon tetrachloride is much greater than is the case when it is given alone

INTERACTION OF CHEMICALS

Functional antagonism When two chemicals counterbalance each other by producing opposite

effects on the same physiologic function Severe barbiturate intoxication Vs norepinephrine

Chemical antagonism or inactivation Chemical reaction between two compounds that produces a less toxic

product Chelators of metal ions decrease metal toxicity

Dispositional antagonism Absorption, biotransformation, distribution, or excretion of a chemical is

altered so that the concentration and/or duration of the chemical at the target organ are altered

The prevention of absorption of a toxicant by ipecac or charcoal, increased activity of metabolizing enzymes with enzyme inducers, and the increased excretion of a chemical caused by diuretics

Receptor antagonism When two chemicals that bind to the same receptor produce less effect

when given together than the addition of their separate effects (example: 4 + 6 = 8) or when one chemical antagonizes the effect of the second chemical (example: 0 + 4 = 1)

Often are termed blockers, beta blockers

ANTAGONISM

Tolerance is a state of decreased responsiveness to a toxic effect of a chemical resulting from prior exposure to that chemical or to a structurally related chemical

Two major mechanisms are responsible for tolerance:

One is due to a decreased amount of toxicant reaching the site where the toxic effect is produced (dispositional tolerance)

The other is due to reduced responsiveness of a tissue to the chemical

TOLERANCE

The characteristics of exposure and the spectrum of effects come together in a correlative relationship customarily referred to as the dose-response relationship

Two types

Individual to varying doses of a chemical, Response of an individual organism to varying doses of a chemical (e.g. enzyme activity, blood pressure)

Population of individuals

DOSE RESPONSE

DOSE-RESPONSE RELATIONSHIP

DOSE-RESPONSE RELATIONSHIP

LD50 – Median Lethal Dose, quantity of the chemical that is estimated to be fatal to 50% of the organisms LD50 values are the standard

for comparison of acute toxicity between chemical compounds and between species

TD50 – Median Toxic Dose

ED50 – Median Effective Dose

LC50 – Median Lethal Concentration

WHAT CAN BE LEARNED FROM A DOSE-RESPONSE CURVE?

NOAEL Value – No Observed Adverse Effect Level, The highest dose of a chemical that, in a given toxicity test, causes no observable effect in test animals The NOAEL for the most sensitive test species and the most

sensitive indicator of toxicity is usually employed for regulatory purposes

LOAEL Value – Lowest Observed Adverse Effect Level, The lowest dose of a chemical that, in a given toxicity test, does cause an observable effect in test animals

WHAT CAN BE LEARNED FROM A DOSE-RESPONSE CURVE?

Therapeutic Index = LD50 / ED50

Margin of Safety = LD1 / ED99

WHAT CAN BE LEARNED FROM A DOSE-RESPONSE CURVE?

WHAT CAN BE LEARNED FROM A DOSE-RESPONSE CURVE?

Rating/Class Dose

Practically nontoxic >15 g/kg Slightly toxic 5–15 g/kg

Moderately toxic 0.5–5 g/kg

Very toxic 50–500 mg/kg

Extremely toxic 5–50 mg/kg Super toxic <5 mg/kg

TOXICITY RATING FOR HUMANS (70 KG BODY WEIGHT)

Selectively toxicity A chemical produces injury to one kind of living without

harming another of life

Pesticides for agriculture are less toxic to the plant but harmful to fungi

Species differences toxicity and Individual difference in response

VARIATION IN TOXIC RESPONSES