GENERAL PRINCIPLES OF TOXICOLOGY Laboratory of toxicology Department of Pharmacology and Toxicology College of Pharmacy, University of Baghdad 2015
TOXICOLOGY - PRACTICAL SYLLABUS 2015 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
SCIENCE OF TOXICOLOGY 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
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 Chronic tests In which two species, one rodent and one non rodent are dosed daily for six months 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
DIFFERENT AREAS OF 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
ROUTE AND SITE OF EXPOSURE 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 OF ADMINISTRATION
DURATION AND FREQUENCY OF EXPOSURE 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
SPECTRUM OF UNDESIRED EFFECTS 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
IMMEDIATE VERSUS DELAYED TOXICITY 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
REVERSIBLE VERSUS IRREVERSIBLE TOXIC 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 Irreversible: cancer, necrosis Reversible: Neostigmine reversible inhibitor of cholinesterase
LOCAL VERSUS SYSTEMIC TOXICITY 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
INTERACTION OF CHEMICALS 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
ANTAGONISM 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
TOLERANCE 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
DOSE RESPONSE 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 RELATIONSHIP
DOSE-RESPONSE RELATIONSHIP
WHAT CAN BE LEARNED FROM A DOSE- RESPONSE CURVE? 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?
TOXICITY RATING FOR HUMANS (70 KG BODY WEIGHT) Rating/Class Dose Practically nontoxic Slightly toxic Moderately toxic Very toxic Extremely toxic Super toxic >15 g/kg 5 15 g/kg 0.5 5 g/kg 50 500 mg/kg 5 50 mg/kg <5 mg/kg
VARIATION IN TOXIC RESPONSES 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