Mammalian Toxicology: History & Principles Lecture 1, Fall 2006

History and Scope of Toxicology: How we got here  Toxicology: The study of adverse effects of xenobiotics.  Xenobiotics: From the Greek xeno (ξένο) for “foreign” and bios (βίος) for “life”.  This discipline actually has its roots in the ancient art of poisoning.  Now its scope is much broader.

History and Scope of Toxicology: Different Branches  Biomedical: –Mechanisms of actions –Effects of exposure –Understanding biological responses through model toxic compounds  Public Health: –Recognition and identification of hazards –Occupational exposure –Development and use of pesticides

History and Scope of Toxicology: Different Branches  Regulatory: –Development of exposure standards –Detection methods  Environmental: –Chemical effects on plants, animals & ecosystems  Clinical: –Development of antidotes & treatments –Recognition of exposure

History and Scope of Toxicology: How we got here  Toxicology, like other disciplines, is a mixture of science, art & creative thinking –Science: The observational and data-gathering phase.

History and Scope of Toxicology: How we got here –Art: Utilization of the data to predict outcomes in humans based on in vitro and in vivo studies. –Creative Thinking: Determining the next hypothesis and how to design experiments to actually answer the questions posed.

History and Scope of Toxicology: How we got here  It is important to note that facts are different from predictions.  Facts have been proven; predictions are based on probabilities. They don’t have equal value, in terms of scientific weight.  Toxicologists need to be careful when talking to the public to make sure they don’t confuse the two!

History of Toxicology—Antiquity  Humans have a long history of using poisons –Hemlock (Greek capital punishment) –Aconite (Chinese poison arrows)  Milestones –Dioscorides—Greek physician who classified poisons for Nero. He included descriptions and drawings. This was a standard text for 1600 years!

History of Toxicology—Antiquity  Toxicology during this time, however, mainly focused on poisoning (suicide, state- sanctioned & personal usage…)  This knowledge also lead to antidotes. –Emetics (εμμετικός)—Agent to induce vomiting following poisonings

History of Toxicology—Antiquity  King Mithridates VI of Pontus— Experimented on criminals and himself! –He would drink a poison cock-tail (36 ingredients!) to prevent political enemies from being able to poison him. –When ultimately captured, he had to resort to using his sword!

History of Toxicology—Antiquity  Poisonings were so rampant in Rome, a law was enacted in 82BC. It made poisoning illegal, and later extended to careless dispensers of drugs (an early regulatory effort!)

History of Toxicology—Middle Ages  Maimonides—Concept of bioavailability: Based on the forms of toxicants, or what one eats/drinks before ingestion, the chemical can be more or less readily available in the body. –Milk, butter and cream could delay intestinal absorption (due to the fat content) –Full stomach also delays absorption

History of Toxicology—Middle Ages  The poisoner, in Renaissance Italy, was an integral part of society. –Toffana—Woman who sold arsenic- laced cosmetics –Hieronyma Spara—Provided ‘services’ to local young soon-to-be widows.

History of Toxicology—Middle Ages  Catherine de Medici—Systematic study of the effects of poisons in the sick and poor to make sure the correct concoction was delivered to her ‘customers’. –Noted the following:  Rapidity of the toxic response (onset of action)  Effectiveness of the compound (potency)  Degree and specificity of response (site of action)  Complaints of victims (clinical signs and symptoms)

History of Toxicology—Age of Enlightenment  The age of Paracelsus ( )—Responsible for the most famous saying in all of toxicology:  All substances are poisons; there is none which is not a poison. The right dose differentiates a poison from a remedy.

History of Toxicology—Age of Enlightenment  Paracelsus focused on the importance of the ‘toxicon’—a primary toxic agent and a single chemical entity.  This was in contrast to previous schools of thought that included the concept of mixtures.

History of Toxicology—Age of Enlightenment  Fundamental contributions: –Experimentation is essential in the examination of responses. –There is a difference between the therapeutic and toxic properties. –The above are not easily determined, except by dose. –It is possible to ascertain a degree of specificity of chemicals and their therapeutic or toxic effects.

History of Toxicology—Age of Enlightenment  Seminal texts: –On the Miners’ Sickness and other Diseases of Miners (1567) by Paracelsus  Included treatment and prevention strategies –Discourse on the Diseases of Workers (1700) by Bernardino Ramazzini  Set the standard for occupational medicine.  Also included information about miners, midwives, printers, weavers and potters.

History of Toxicology—Age of Enlightenment  Major developments: –1775—Role of soot in scrotal cancer in chimney sweeps (due to polyaromatic hydrocarbons) –1825—Synthesis of phosgene and mustard gas (chemical warfare) –1880—Boom in organic chemical synthesis led to over 10,000 new compounds (no industry testing for toxicity)

History of Toxicology—Age of Enlightenment  Major developments: –Orfila ( ): Introduced the use of autopsy material to toxicology to provide legal proof of poisoning. –Magendie ( ): Detailed the absorption and distribution of various compounds in the body.

Modern Toxicology  Toxicologists must understand aspects of biology, chemistry and metabolism. –They tend to function as detectives who must utilize many clues.  Initial growth in the field spurred by need to explain deaths occurring after administration of ether, chloroform and carbonic acid in iatrogenic deaths.  Iatrogenic: From the Greek iatros (ιατρός) for doctor

Modern Toxicology  1890s-1900s –Discovery of vital amines (vitamins) led to the wide-spread usage of bioassays to determine whether these new chemicals were beneficial. –Development of neurotoxicity field due to the production of bootleg liquor by- products (methanol & lead). –Toxicology of metals due to the production of ‘the bomb’.

Modern Toxicology  Post World War II –Discovery of organophosphates (OPs) as cholinesterase inhibitors. –Today used as non-bioaccumulating pesticides  Production of quinine as an antimalarial. –Based on derivative of chincona bark –First use of non-human primates  Discovery of mixed-function oxidases (MFOs) –Prelude to latter work on P450s

Modern Toxicology  Two major discoveries (1948): –Paper chromatography for chemical separation. –Use of blood and urine for testing presence of various chemical metabolites (biomarkers).

Modern Toxicology  Formalization of the experimental program for the testing of food, drug and cosmetic safety in –Updated by the FDA in –Basically states that any chemical found to be carcinogenic in lab animals or humans cannot be added to the US food supply.

Modern Toxicology  Toxicology and Applied Pharmacology started around 1958—First journal dedicated to toxicology.  Textbook of Toxicology published in  Society of Toxicology (SOT) founded in 1965.

Modern Toxicology  Major events in the 1960s: –Thalidomide babies –Silent Spring by Rachel Carson –Equipment for detecting parts per billion (ppb) –Genetic assays for point mutations (Ames assay)

Modern Toxicology  In the 1970s: –“Discovery” of Love Canal as a major dumping site for toxic chemicals. –Push for toxicology to consider exposures to complex mixtures.  Recall that this shift had already taken place once—move from mixtures to the toxicon. Now a movement back to mixtures.

Currently  Now a unique and separate discipline –Offered at many graduate schools –“Surprisingly, courses in toxicology are now being offered in several liberal arts undergraduate schools as part of their biology and chemistry curricula.” (p 10)

Possible Short Research Topics

Review  Toxicology: The study of adverse effects of chemicals on living organisms.  Toxicologist: A person trained to examine the nature of toxic effects. –Including cellular, biochemical, and molecular mechanisms of action. –Assess the probability of their occurrence.

New Concept   Risk assessment: A quantitative estimates of the potential effects on human health. – –Also includes the environmental significance of various types of toxicant exposures – –For example: Pesticide residues on food, contaminants in drinking water.

So Many Possibilities!   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, and then sub-specialize. – –Neurotoxicity of metals or organo-metallic pesticides…

Broad Areas of Toxicology  Mechanistic Toxicology: – Concerned with identifying and understanding the cellular, biochemical, and molecular mechanisms by which chemicals exert toxic effects on living organisms.

Mechanistic Toxicology  Demonstrating cause and effect  Identifying responses in animals that may not actually affect humans  Designing and producing safer chemicals  Understanding individual genetic responses to certain drugs

Mechanistic Toxicology  Can also be useful in identifying responses in animals that may not actually affect humans. –Accumulation of high levels of saccharin in urine in rats.

Mechanistic Toxicology  Design and produce safer chemicals –Thalidomide was used as a sedative for pregnant women. Usage lead to devastating birth defects. –Mechanistic studies demonstrated that thalidomide may prevent blood vessel formation (angiogenesis). –Perhaps can be used to treat cancers.

Mechanistic Toxicology  A basic understanding of how certain toxicants work has also lead to greater understanding of general metabolism and biology. –Use of tetrodotoxin (TTX) and botulinum toxin to understand neuronal membrane potential and axonal firing.

Mechanistic Toxicology  In some instances, knowledge of the metabolism of xenobiotics may lead to “designer drugs” or “designer treatments”. –A small portion of the population lacks the proteins to detoxify 6- mercaptopurine (a chemothera- peutic). –This may lead to serious side-effects of treatment. Genetic testing can determine if an individual is at risk.

Broad Areas of Toxicology   Descriptive Toxicology: – –Concerned directly with toxicity testing, which provides information for safety evaluation and regulatory requirements. – –Tests are designed to determine if a specific chemical poses a real risk to humans or the environment.

Descriptive Toxicology  Although concern may be limited to humans, this is not necessary. –Industrial toxicologists may be interested in whether a pesticide will affect any non-target organisms.

Descriptive Toxicology  Although a descriptive toxicologist is not foremost interested in mechanism, the studies do provide clues for mechanism of action.

Broad Areas of Toxicology  Regulatory Toxicology: Focuses on deciding, based on 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.

Areas for Regulatory Toxicologists  FDA—Food and Drug Admin  EPA—Environmental Protection Agency  OSHA—Occupational Safety and Health Administration  DOT—Department of Transportation

Regulatory Toxicology  Food & Drug Administration (FDA): Responsible for allowing drugs, cosmetics and food additives to be sold in the market according to the Federal Food, Drug and Cosmetic Act (FDCA). –Interesting to note that herbal supplements and ‘natural’ products escape regulation because they are not strictly ‘additives’.

Regulatory Toxicology  Environmental Protection Agency (EPA) regulates chemicals that fall under the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA), Safe Drinking Water Act and the Clean Air Act (among others…) –Provides direction and financial support to clean up toxic sites.

Regulatory Toxicology  Occupational Safety and Health Administration (OSHA) helps monitor safety in the workplace. Also provides rules and regulations for guidance.

Regulatory Toxicology  Department of Transportation (DOT) ensures that materials shipped in interstate commerce are labeled and packaged appropriately.

Regulatory Toxicology   Role also includes involvement in the establishment of standards for the amount of chemicals permitted in ambient air, industrial atmospheres, and drinking water. – –This often demands the integration of scientific information from basic descriptive and mechanistic toxicology studies with principles and approaches used for risk assessment.

Other Areas in Toxicology  Forensic Toxicology: Concerned with the medical aspects of the harmful effects of chemicals.  Clinical Toxicology: Medical area concerned with diseases caused by, or unique to, toxic substances (poison control).  Environmental Toxicology: Focuses on the impact of pollutants on the environment and biological organisms.

Getting Started—A Toxic Dose  Recall Paracelsus’ ( ) dictum for toxicologists:  All substances are poisons; there is none which is not a poison. The right dose differentiates a poison from a remedy.

A Toxic Dose  There is no single toxic dose for all compounds.  Some chemicals are harmful at kg/kg doses, while other will kill at μg/kg levels.

A Toxic Dose

Important Definition  LC 50 : The concentration of a toxin or toxicant needed to produce death (lethality) in 50% of the treated population. –LC 50 is the most common expression of this, but one can also see LC n.  EC 50 : The dose needed to produce an effect in 50%...  IC 50 : The dose needed to inhibit an end-point in 50% …

Important Notice  Measures of acute lethality may not accurately reflect the full spectrum of toxicity, or hazard, associated with exposure to a chemical. –Some chemicals with low acute toxicity may have carcinogenic effects at doses that produce no evidence of acute toxicity.

Implications  It is important to know what the definition means. The LC 50 is often a measure of acute (short- term) toxicity. –But one can get very, very sick without dying. Does this mean the compound is not toxic? –No…it only means that dose won’t kill you!

Next time… Chapter 2 continued