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INTRODUCTION

What is Toxicology?

The science of poisons.

The study of the adverse effects of chemicals or physical agents on living organisms.

TOXICOLOGY is the study of the qualitative and quantitative effects of chemicals on living systems. Then what is it? Toxicology is the study of interaction of materials (drugs, chemicals, foods, polymers, pesticides, etc.) with a biological system and its responses.

ObjectivesUpon successful completion of this module you will be able to:

Define toxicology Know the history of toxicology Identify the six applied areas of toxicology

Explain ways in which toxicology is relevant to our daily lives Outline the basic principles of toxicology

IntroductionWhich of the following news headlines are concerned with issues related to toxicology?

a. Arsenic Widespread in Bangladesh Water

b. Sarin Gas Attack on the Tokyo Subway

c. Ukrainian President Yuschenko Poisoned by Dioxin

d. Gas Leak Accident in Bhopal: An Indian Tragedy

e. All of the above

The correct answer is "Option E. All of the above."

News has always had a penchant for the sensational and toxicology often fills the bill. There would likely have been a considerable buzz even in ancient times about events such as the sentencing of Socrates to drink hemlock.

When not bombarded by high impact news stories, we have to contend with personal worries about toxic substances:

What are the side effects of this drug?

Should I be picking these mushrooms?

Is it safe to be jogging outside with the ozone level this high?

This is the human part of the story.

Toxicology is the science behind the stories, not always as dramatic, but possibly as fascinating, and certainly necessary in determining how certain chemicals can harm us under particular conditions, and seeking ways to prevent or alleviate the harm.

Toxicology is relatively new as a distinct scientific discipline although many of its basic principles have been known for some time. This section will explore this history, describe how toxicology has changed over time, and offer a broad definition of this discipline. In addition, the six areas of applied toxicology will be described as well as how these areas are relevant to our daily lives.

Following this, the basic principle of toxicology - "the dose makes the poison" - will be explicated and the strengths and limitations of this principle will be explored. This exploration will include discussions of the different types of dose, how they are measured and how dose-response and dose-effect curves are constructed and used, especially in assessing risks to human health.

This module will provide the reader with an overview of toxicology and provide the context for the more detailed materials that follow.

This module includes the following sections:

1. Objectives

2. Introduction

3. What Is Toxicology?

4. Toxicology And Our Daily Lives

5. Determining Toxicity

What Is Toxicology?

History: Poisoning Highlights

Poisoning and the knowledge of poisons have a long and colorful history although the science of toxicology has only recently come into existence as a distinct discipline. Even the cave dwellers had some knowledge of the adverse effects of a variety of naturally occurring substances, knowledge that they used in hunting and in warfare.

Famous early victims of plant and animal poisons were the Greek philosopher Socrates and the Egyptian Queen Cleopatra.

Socrates was forced to drink Hemlock for corrupting the youth of Athens.

Cleopatra committed suicide through the bite of an asp, a poisonous snake.

As time progressed, toxicological knowledge and its applications expanded. Indeed, poisoning became institutionalized in a number of places, and some governments utilized poisons for state executions, a practice that continues in some jurisdictions, via means such as lethal injections.

The European Renaissance was a time notorious for poisonings and poisoners. Born in 15th century Italy, Cesare and Lucrezia Borgia used a concoction of chemicals to assassinate their political rivals. Their potion La Cantarella, may have included arsenic, copper, and phosphorus.

Unintentional poisoning has always been with us. Through the science of toxicology we now better understand these risks and work to avoid harm to human health. The naturally occurring element lead was used in the Roman era to line vessels and as a pottery glaze, as well as in cosmetics. In the 19th century in the United States paint manufacturers began to use lead as a pigment, although even in 1786, Benjamin Franklin outlined the hazardous effects of lead on the body in a letter to Benjamin Vaughan, a friend. Though banned in paint today, society needs to be ever vigilant in protecting children (who are particularly susceptible to the effects of lead on the brain and nervous system) from exposure to older flaking paint chips. Lead was also used in gasoline to prevent engine knocking. Because of bans of these uses and intensive public health efforts, lead concentration in urban children has decreased in the past several decades. Studies have demonstrated a correlation between minimal lead exposure and higher cognitive function.

Workers, because they tend to be exposed to higher levels of chemicals than the general population, are in danger of being unwittingly poisoned at rates higher than the general population. Asbestos, as an example, was widely known in antiquity but use increased as a result of the industrial revolution. It has been used in textiles, building materials, insulation, and brake linings. Capable of causing severe lung damage, including asbestosis and mesothelioma, asbestos is now strictly regulated. Today, we are not only concerned about workers exposed to traditional industrial chemicals, but also to those used in the electronics industry, as well as bio- and nano-engineered products.

Chemical and biological warfare date to antiquity. The mythological account of Paris slaying Achilles with a poisoned arrow in the heel has a basis in the way some battles were conducted at the time. Fast forward to 1914, when poison gas was used in a more systemized and large-scale fashion by the Germans in World War I. In 1988, Iraqi government troops attacked the Kurdish town of Halabja with chemical bombs involving multiple chemical agents including mustard gas and the neurotoxic agents, sarin, tabun and VX.

Another broad sphere of poisoning relates to toxicological disasters. While some may be natural (e.g., sulfur and other toxic gases emitted from volcanoes), others are related to industrial mishaps. Love Canal is an iconic example. The vicinity of the Canal was used by Hooker Chemicals as a dumping ground for numerous hazardous substances, a large population was put at risk, and hundreds of families living in homes on top of the dumpsite needed to evacuate. And in 1984, in Bhopal, India, a release of the chemical methyl isocyanate resulted in many thousands of deaths and many more injuries.

A more recent example of political poisoning is the remarkable survival (albeit chloracne scarred) of the Ukrainian president Viktor Yushchenko, after alleged poisoning with dioxin, and, possibly endotoxin, prior to the 2004 elections.

INCLUDEPICTURE "C:\\Users\\Eva Marie\\Documents\\Intro Toxicology_files\\ukranian_president.jpg" \* MERGEFORMATINET In 2006, in a case with overtones of espionage, Alexander Litvinenko, a former Russian spy, was fatally poisoned with radioactive polonium-210. The radioactive isotope was allegedly added to tea he drank at a London hotel. 2007 and 2008 have seen increasing incidents of product contamination, often via international trade. Pet foods, for example, have been found to contain melamine, an organic compound used to make a variety of plastic products. Although there is some doubt about the toxicity of melamine to dogs and cats in the doses to which they were exposed, it is an industrial chemical which should not have been added to the food, and its effects may have been exacerbated by other food ingredients. Toys have been discovered with lead and, in one instance, with a glue which, when ingested, metabolizes to GHB, the date rape drug. Recalls followed swiftly.

INCLUDEPICTURE "C:\\Users\\Eva Marie\\Documents\\Intro Toxicology_files\\2_3_catanddog.jpg" \* MERGEFORMATINET

INCLUDEPICTURE "C:\\Users\\Eva Marie\\Documents\\Intro Toxicology_files\\2_3radioactivemug.jpg" \* MERGEFORMATINET Thus toxicology, in its scope, casts a broad net, encompassing hazardous effects of chemicals (including drugs, industrial chemicals and pesticides), biological agents, also known as toxins (e.g., poisonous plants and venomous animals) and physical agents (e.g., radiation, noise). It has been newsworthy since ancient times and will continue to be a subject of fear and fascination, as well as the important source of information protecting humans, other animals, and the environment from dangerous exposures.

History: Toxicology ResearchDuring periods of intellectual ferment in Europe, scholars began more systematic studies of poisons and their effect. Two noteworthy examples, products of the 15th Century Renaissance, and the 18th Century Age of Enlightenment respectively, were the alchemist Paracelsus (born in Einsiedeln, now a city in Switzerland) and the Spanish physician Orfila.

Paracelsus (1493-1541) identified the specific chemical components of plants and animals that were responsible for their toxic properties. He also was able to show that varying the amount of the poison affected the severity of the effects.

Orfila (1787-1853), who is often referred to as the father of toxicology, was the first to establish a systematic correlation between the chemical properties and biological effects of poisons. Using autopsy results, he was able to link the presence of particular poisons with specific damage to tissues and organs.

During the 19th century, there was a proliferation of textbooks dealing with toxicology in relation to forensic medicine, in which scientific tools and principles are used to investigate crimes and accidents.

In part, this was due to the great advances in chemical analysis, allowing for a more precise determination of the amounts of toxicants in body tissues and fluids (from which study analytical toxicology is derived).

Toxicology developed as a modern science during the 20th century, particularly after the Second World War, at least partly in response to the rapid development and production of many new drugs and industrial chemicals.

Thus, toxicology, ancient in practice, came to be known simplistically as the science of poisons.

As the understanding of the working of living organisms became increasingly sophisticated, and a true scientific basis evolved, it has become clear that this definition is not adequate.

In light of this, the U.S. Society of Toxicology uses the following definition:

"Toxicology is the study of the adverse physicochemical effects of chemical, physical or biological agents on living organisms and the ecosystem, including the prevention and amelioration of such adverse effects."

Examples of such agents include cyanide (chemical), radiation (physical) and snake venom (biological).

The effects on organisms can occur at multiple levels, including the molecular and the organ levels.

Research and ApplicationToxicological research is an exciting field of study utilizing and integrating principles developed in a variety of disciplines including chemistry, biochemistry, physiology, pathology, biology, genetics and pharmacology. These inputs are reflected in the names of the areas of toxicological research such as biochemical toxicology, pathotoxicology, toxicogenomics, pharmacokinetics, and pharmacodynamics. Combining the understanding gained through these disciplines, toxicologists are able to characterize the disposition of agents in living organisms, the types of adverse effects that may be produced after exposure, the mechanisms of action behind these effects and the impacts of possible interactions among agents.

Toxicological studies may be carried out by scientists with training in these diverse disciplines. For example:

Biochemists, who study the fates of agents in living organisms and the mechanisms by which they exert their effects;

Geneticists, who investigate the effects of agents on genetic material and the impact of genetic variation on responses to toxic insults; and

Epidemiologists, who study populations exposed to such agents to look for possible connections between exposures and adverse health outcomes.

The ultimate objective of the combined research of such toxicological specialists is to determine how an organism is affected by exposure to an agent.

This includes an understanding of:

How the agent moves throughout the organism;

How it may be changed by interacting with living cells and tissues;

What parts of the organism are affected by its presence; and

The health outcomes of this exposure.

The more thorough this understanding, the more accurately toxicologists can predict what will happen when different types of organisms, particularly humans, are exposed to agents in the ambient environment, the workplace, or via exposure to food and drugs.

Today, one way of looking at toxicology divides it into six applied areas - clinical, forensic, analytical, environmental, occupational, and regulatory.

Clinical toxicology, the diagnosis and treatment of human poisoning, and forensic toxicology, the medical-legal aspects of clinical poisoning, are discussed further in a later slide. The related discipline of analytical toxicology is concerned with the identification and quantification of toxic chemicals in biological materials.

Environmental and occupational toxicology are self-explanatory in that they deal with toxic hazards in the environment and in the workplace, respectively. Regulatory toxicology focuses on laws and regulations and their enforcement, an important component of toxicology. Risk assessment, covered in a later module, is often considered a part of regulatory toxicology.

All of these branches of toxicology rely on the same basic science to achieve their goals, and are not all mutually exclusive. Thus, poisoning at the workplace would encompass aspects of both clinical and occupational toxicology.

Regulatory toxicology (i.e., the regulation of potentially toxic substances) has recently been a major driving force in toxicological research.

Much of the support for toxicology is predicated on the idea that increased knowledge will lead to better management of potentially toxic agents, actions which, in turn, will result in improved public health.

Accurate predictions of effects of chemicals on humans depend upon scientific studies. Most toxicological studies are empirical in nature, and are performed on experimental animals (in vivo) or in vitro test systems (i.e., cell culture or other systems used to mimic the results in part of a living organism).

Since the results are often used for regulatory purposes, the goal of such studies is to predict effects in humans.

To achieve this goal, scientists need to understand the differences between experimental animals and humans in the way that they process foreign chemicals (xenobiotics) and physical agents, as well as the applicability to humans of results obtained in vitro.

Toxicology and Our Daily LivesToxicology was relevant historically because knowledge of plant and animal poisons served various ends, some beneficial to society and others not.

In particular, this knowledge could be used for hunting large game, defense against enemies and understanding which plants and animals were safe to eat and which should be avoided.

Toxicology is just as important to us today. Our improved health status and increase in life expectancy are due in part to advances in pharmacology. Toxicology research helps ensure that the beneficial effects of therapeutic agents are not outweighed by unwanted side effects. There are about 100,000 chemicals in commercial use and 1,000 new ones added every year.

We have modern toxicology to thank for the safety of our food and drinking water, consumer products, and other industrial chemicals we use or to which we are exposed. Toxicology knowledge helps in the prevention of adverse effects. More specialized branches of toxicology, such as toxicogenomics and nanotoxicology, increasingly have a significant bearing on our daily lives.

Food ToxicologyDespite great advances made by toxicology in assuring that our food is uncontaminated, it is still important to know what products are safe to eat and in what quantities. In addition to concerns about naturally occurring substances in foods, food toxicologists also investigate the safety of other components of food that have been added deliberately or accidentally.

Deliberate additions include a variety of natural and synthetic additives and artificial substitutes for naturally grown food components. These include sweeteners, color and texture additives, fat substitutes and preservatives.

Accidental contaminants are generally synthetic or natural environmental contaminants in the food chain, such as polychlorinated biphenyls (PCBs) and methyl mercury, found in fish, microbial toxins such as produced by E. coli in contaminated food, and fungal toxins (like aflatoxins) which may contaminate grains.

INCLUDEPICTURE "C:\\Users\\Eva Marie\\Documents\\Intro Toxicology_files\\3_3_mushrooms.jpg" \* MERGEFORMATINET Recently scientists have been investigating and debating the safety of Genetically Modified Organisms (GMOs ) as food products.

Safety of PharmaceuticalsToxicological research is critical in the development and production of pharmaceuticals.

At the beginning of the drug discovery process, toxicity tests help to determine which potential pharmaceuticals are likely to be safe enough for humans and thus warrant further development.

All along the development process, additional testing is performed to ensure that the final product will not only be efficacious but also free of unreasonable side effects. For each and every pharmaceutical, prescription or over-the-counter, safety evaluation studies are performed. Safety evaluation studies often include experimental animals and clinical trials involving humans. This scrutiny includes multi-year studies of possible chronic effects, including cancer.

Toxicology of Industrial Chemicals and Consumer ProductsBefore new industrial and consumer products can be developed and marketed, toxicological research is needed to ensure that these can be used safely as intended.

This involves investigations of possible adverse effects on humans due to exposure to these products in the workplace, home, or in the general environment, and possible dangers to other animals, such as wildlife.

For agents that have been on the market for some time and are already out in the environment, toxicology can help to determine whether or not remediation of sites contaminated by these agents is needed and, if so, how thorough the clean-up must be to protect the local population and the environment.

Clinical and Forensic ToxicologyToxicology continues to be important in the clinical and forensic settings.

In the clinical setting, toxicologists assist in making diagnoses of possible agent-induced harm in individuals exposed occupationally or environmentally.

Clinical toxicologists are also concerned with drug toxicity. Prescription and non-prescription drugs, just like other chemicals, may cause adverse effects dependent upon many factors, including the dose, individual age, hypersensitivity, etc. Drugs of abuse, typically taken for pleasurable effects or sometimes for suicidal purposes, may pose threats ranging from mild to severe.

Forensic toxicologists can help to determine the possible effects of agents on behavior (e.g., alcohol on driving ability), and also assist in determining the cause of death in individuals who may have suffered from intoxication by chemical or physical agents.

ToxicogenomicsIn the new millennium, advances in toxicology are leading to additional and refined uses of toxicology.

Understanding the genetic basis of responses to chemical and physical exposures can help to predict which individuals will respond best, and with the least side effects, to particular pharmaceutical agents and also to predict which individuals will be most at risk from specific occupational and environmental exposures.

INCLUDEPICTURE "C:\\Users\\Eva Marie\\Documents\\Intro Toxicology_files\\3_7_doctorandbaby.jpg" \* MERGEFORMATINET This new area of study, known as toxicogenomics, will provide the opportunity for a more customized approach to individual health.

NanotoxicologyThe prefix, nano, implying a billionth of an amount, has been appropriated for nanotechnology, the science of extremely small materials. An increasing number of consumer and other products are now made of nanoscale substances. The corresponding study of the safety and potential hazards of nanoparticles and nanotubules is known as nanotoxicology.

INCLUDEPICTURE "C:\\Users\\Eva Marie\\Documents\\Intro Toxicology_files\\3_8_nanotubes.jpg" \* MERGEFORMATINET Many nanomaterials are initially formed from nanoparticles which can produce aerosols and colloidal suspensions. As such they can readily be inhaled, ingested, or potentially absorbed through the skin.

Although our knowledge of the toxic activity of these very small particles is scant, we can, and often do, use ultrafine particles as models of nanoparticulate behavior. Studies to date have indicated that ultrafine particles are more toxic on a mass for mass basis than their larger counterparts. Moreover, ultrafine particles have been shown to penetrate the skin and translocate from the respiratory system to other locations in the body.

Although nanotechnology promises major advances in the fields of physics, electronics, chemistry, and medicine, workers in nanotechnology industries often face exposure to unknown levels of nanoparticulates with unique sizes, shapes, and activities. Hence, research aimed at defining the potential toxicity of these particulates is needed as are effective monitoring and surveillance techniques and adequate protective equipment.

Short cut HISTORY OF TOXICOLOGY Throughout the ages, toxicological science has provided information that has shaped and guided society.I. Antiquity & Early history:referred to as ancient history covers the period from the founding of modern civilization in Mesopotamia, around 3500 BCE, to the start of the Middle Ages. Knew of human and animal poisons Search for an acceptable diet unfortunate outcome

Categorized and studied poisons Royal "tasters"

1. Socrates (470-399 B.C.):Tried and convicted of corrupting the youth sentenced to death. Most celebrated poisoning victim.

Executed by poison hemlock.

Active principle: coniine.

2. Cleopatra: conducted numerous experiments with poisons effects amusement Ended taking her own life with the aid of a serpent asp3. Evil Wind: Mazuku pocket of oxygen poor air (CO2 odorless asphyxiation)II. Middle Ages: AKA The Dark Ages, roughly refers to the time between the fall of Ancient Rome in the 5th Century until to the European Renaissance and cultural rebirth beginning around the 14th century.

15th Century Europe: Italians developed poisoning into an art form

Poisoning became a normal hazard of life.....

1. Venice's "Councilof Ten" (City Council)

Put out poisoning contracts on political enemies.

Council transactions: detailed records with name of victim, contractor, type and amount of poison given, results

2. Borgia: prominent family who practiced "applied toxicology"

Cesare, Lucretia and others.

Killed husbands, wives, lovers, political opponents, churchmen.

3. Black Death or Bubonic Plague: bacterial disease spread by rodents that wiped out around 1/3 of Europes population (nearly 25 million) in the mid 14th century.

A pandemic Little Ice AgeIII. Early Modern: spans the time from the Renaissance, beginning around the 14th century, to the Industrial Revolution, which began in the 18th century in some parts of the world.

1. Fowlers Solution: Potassium Arsenite 1786 1936 general tonic

Some say Charles Darwin used it often

Malaria, chorea and syphilis2. Catherine de Medici (1519-1589)

Wife of Henry II of France, mother of three French kings, ruler of France.

Early "experimental toxicologist".

Poisoned poor and sick street people under guise of "feeding" and assistance".

Killed political enemies for hire....

Documented signs and symptoms.

3. Paracelsus full name: Philippus Aurelius Theophastrus Bombastus von Hohenheim-Paracelsus (1493-1514) FATHER OF TOXICOLOGY Instrumental in logical development of toxicology as science.

Developed concept of "dose".

Action as a result of chemical entity -- toxicon.

4. Orfila (1787-1853) Spaniard--personal physician to Louis XVIII

"The FOUNDER of Modern Toxicology"

Developed toxicology into a science.

Compiled chemical and biological information on most known poisons.

Proposed the necessity of chemical analysis to prove cause-and-effect.5. Christian Hahnemann: founded HOMEOPATHIC MEDICINE

IV. Modern: generally referred to as the period from the Industrial Revolution of the 18th Century to the end of World War II.

Analytical methods developed for As, Hg and miscellaneous alkaloids Organic chemistry makes giant advances

V. Post Modern: stretches from World War II to the Present.

1. Love Canal: named after the late 18th century entrepreneur William T. Love

2. Minamata, Japan

3. Thalidomide

MAJOR DRIVING FORCES FOR THE RECENT EXPANSION & DEVELOPMENT OF THE SCIENTIFIC BASIS AND PRACTICE OF TOXICOLOGY

1. Exponential increase in the number of synthetically produced industrial chemicals

2. Major increase in the number and nature of new drugs, pharmaceutical preparations, tissue implantable materials and medical devices.

3. Increase in the number and types of pesticides & other substances used in agriculture and the food industry.

4. Mandatory testing and regulation of chemicals used commercially, domestically & medically.

5. Enhanced public awareness of potential adverse effects from xenobiotics (non naturally occurring) to man, animals and the environment.6. Litigation, principally as a consequence of occupational related illness unrecognized or poorly documented product safety concerns (including drugs) and environmental harm.

SCOPE OF TOXICOLOGY

WHAT DO TOXICOLOGISTS STUDY?Toxicology has become a science that builds on and uses knowledge developed in other related medical sciences, such as physiology, biochemistry, pathology, pharmacology, medicine, and epidemiology, to name only a few. Given its broad and diverse nature, toxicology is also a science where a number of areas of specialization have evolved as a result of the different applications of toxicological information that exist within society today. It might be argued, however, that the professional activities of all toxicologists fall into three main areas of endeavor: descriptive toxicology, research/mechanistic toxicology, and applied toxicology.

Descriptive toxicologists are scientists whose work focuses on the toxicity testing of chemicals. This work is done primarily at commercial and governmental toxicity testing laboratories, and the studies performed at these facilities are designed to generate basic toxicity information that can be used to identify the various organ toxicities (hazards) that the test agent is capable of inducing under a wide range of exposure conditions. A thorough descriptive toxicological analysis would identify all possible acute and chronic toxicities, including the genotoxic, reproductive, teratogenic (developmental), and carcinogenic potential of the test agent. It would also identify important metabolites of the chemical that are generated as the body attempts to break down and eliminate the chemical, as well as analyze the manner in which the chemical is absorbed into the body, distributed throughout the body and appropriate doseresponse test data are generated for those toxicities of greatest concern during the completion of the descriptive studies so that the relative safety of any given exposure or dose level that humans might typically encounter can be determined.

Basic research or mechanistic toxicologists are scientists who study the chemical or agent in depth for the purpose of gaining an understanding of how the chemical or agent initiates those biochemical or physiological changes within the cell or tissue that result in the toxicity (adverse effect). They identify the critical biological processes within the organism that must be affected by the chemical to produce the toxic properties that are ultimately observed. Or, to state it another way, the goal of mechanistic studies is to understand the specific biological reactions (i.e., the adverse chain of events) within the affected organism that ultimately result in the toxicity under investigation. These experiments may be performed at the molecular, biochemical, cellular, or tissue level of the affected organism, and thus incorporate and apply the knowledge of a number of many other related scientific disciplines within the biological and medical sciences (e.g., physiology, biochemistry, genetics, molecular biology). Mechanistic studies ultimately are the bridge of knowledge that connects functional observations made during descriptive toxicological studies to the extrapolations of doseresponse information that is used as the basis of risk assessment and exposure guideline development (e.g., occupational health guidelines or governmental regulations) by applied toxicologists.

Applied toxicologists are scientists concerned with the use of chemicals in a real world or non laboratory setting. For example, one goal of applied toxicologists is to control the use of the chemical in a manner that limits the probable human exposure level to one in which the dose any individual might receive is a safe one. Toxicologists who work in this area of toxicology, whether they work for a state or federal agency, a company, or as consultants, use descriptive and mechanistic toxicity studies to develop some identifiable measure of the safe dose of the chemical. The process wherebyaccumulated by various tissues and organs, and then ultimately excreted from the body. Hopefully, this safe dose or level of exposure is derived is generally referred to as the area of risk assessment.Divisions of Toxicology

1. DESCRIPTIVE TOXICOLOGY

Concerned directly with toxicity testing which provides information for safety evaluation and regulatory requirements.

Toxicity in animals evaluate the risk posed to humans and environment

Hypothesis generation mechanistic toxicology

Risk assessment regulatory toxicology

2. MECHANISTIC TOXICOLOGY

Concerned in identifying and understanding the cellular, biochemical and molecular mechanisms by which chemicals exert toxic effects on living organisms.

Risk assessment regulatory toxicology useful in demonstrating that an adverse outcome observed in animals is relevant to humans.

Examples: Thalidomide, 6 mercaptopurine, organophosphates, saccharin

Advent of molecular biology and genomics tools that explore exactly how humans may differ from laboratory animals.

3. REGULATORY TOXICOLOGY(industry and government setting) Responsible 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.

FDA, EPA, OSHA

Deciphers and analyzes toxicological data for risk estimation.

Solvent vapor thresholds in industry.

Safe level for human drugs.

Safe level of heavy metals in water.

Safe levels of pesticides.

Graphical representation of the interconnections between different

areas of toxicology.Some specialty areas (subdivisions) of Toxicology:

1. Clinical Toxicology: (hospital setting)

Involve physicians/individuals who received specialized training in emergency medicine and poison management

Deal with emergencies such as overdoses, poisonings, attempted suicides.

Compound identification and quantification.

Sign and symptom management.

Emergency care--home poisoning.

Poison control antidotes and regimens2. Forensic Toxicology: (Medical Examiners office) hybrid of analytical chemistry and fundamental toxicological principles

Medical-legal aspects of poisonings.

Identification and quantification of poisons.

Establish relationship between tissue residual level and probable cause of death.

3. Industrial Toxicology:

Estimation of worker safety based on 8 hr work day, 40 hr work week....

Engineering of safety measures.

Air sampling, worker sampling.

4. Environmental Toxicology:

Effects of compounds on water, wildlife.

Movement of chemicals in the environment--soil, air, water.

Residual life of chemicals in the environment.

Impacts of toxic substances on population dynamics in an ecosystem. One View

On earth creatures shall be seen who are constantly killing one another. Their wickedness shall be limitless; their violence shall destroy the worlds vast forests; and even after they have been sated, they shall in no wise suspend their desire to spread carnage, tribulations, and banishment among all living beings. Their overreaching pride shall impel them to lift themselves toward heaven. Nothing shall remain on earth, or under the earth, or in the water, that shall not be hunted down and slain, and what is in one country, dragged away into another; and their bodies shall become the tomb and the thoroughfare for all living things they have ruinedThe fertile earth, following the law of growth, will eventually lose the water hidden in her breast, and this water, passing the through the cold and rarified air, will be forced to end in the element of fire. Then the surface of the earth will be burned, and that will be the end of all terrestrial nature.

Leonardo Da Vinci, 1452-15195. Biochemical and Molecular Toxicology:

Determining mode of action of chemicals at the molecular level mechanisms by which toxicants modulate cell growth and differentiation and cells response to toxicants at the level of the gene. Effect of chemicals on DNA, cancer genes.

6. Product development Toxicology: (Corporate setting)

Service and preclinical toxicology for product development.

Evaluation of full toxic potential of chemicals destined for drug use.

Establish safe doses for people.

7. Genetic Toxicology Assesses the effects of chemical on the DNA and on the genetic processes of the living cells

8. Developmental Toxicology

Encompasses the study of pharmacokinetics, pharmacodynamics, pathogenesis and outcomes following exposure to agents or conditions leading to abnormal development. Examples: structural malformations, growth retardation, functional malformation and death.Within applied toxicology a number of subspecialties occur. These are: forensic toxicology, clinical toxicology, environmental toxicology, and occupational toxicology. Forensic toxicology is that unique combination of analytical chemistry, pharmacology, and toxicology concerned with the medical and legal aspects of drugs and poisons; it is concerned with the determination of which chemicals are present and responsible in exposure situations of abuse, overdose, poisoning, and death that become of interest to the police, medical examiners, and coroners. Clinical toxicology specializes in ways to treat poisoned individuals and focuses on determining and understanding the toxic effects of medicines and simple over-the-counter (nonprescription) drugs. Environmental toxicology is the subdiscipline concerned with those chemical exposure situations found in our general living environment. These exposures may stem from the agricultural application of chemicals (e.g., pesticides, growth regulators, fertilizers), the release of chemicals during modern-day living (e.g., chemicals released by household products), regulated and unintentional industrial discharges into air or waterways (e.g., spills, stack emissions, NPDES discharges, etc.), and various nonpoint emission sources (e.g., the combustion byproducts of cars). This specialty largely focuses on those chemical exposures referred to as environmental contamination or pollution. Within this area there may be even further subspecialization (e.g., ecotoxicology, aquatic toxicology, mammalian toxicology, avian toxicology). Occupational toxicology is the subdiscipline concerned with the chemical exposures and diseases found in the workplace.

Regardless of the specialization within toxicology, or the types of toxicities of major interest to the toxicologist, essentially every toxicologist performs one or both of the two basic functions of toxicology, which are to (1) examine the nature of the adverse effects produced by a chemical or physical agent (hazard identification function) and (2) assess the probability of these toxicities occurring under specific conditions of exposure (risk assessment function). Ultimately, the goal and basic purpose of toxicology is to understand the toxic properties of a chemical so that these adverse effects can be prevented by the development of appropriate handling or exposure guidelines.

BASIC PRINCIPLES OF TOXICOLOGYDetermining Toxicity

The Dose Makes the PoisonThe basic principle that governs toxicology was first stated by Paracelsus about 500 years ago. In his words:

"All substances are poisons; there are none which is not a poison. The right dose differentiates a poison and a remedy."

This is a very powerful principle since it lays out the fundamental challenge of all toxicological research - to determine the doses at which specific agents cause adverse effects on living things.

It is also important as a counterbalance to the popular idea that agents can be divided into those that are poisons or "toxic" agents, and those that are not. All agents are toxic; it is only the degree and type of toxicity that differ from one agent to another.

What is "Dose"?To understand this principle, it is important to first define 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.

For example, a physician may prescribe a dose of 10 milligrams once each day. However, hidden within this amount is the concept that dose really represents the amount of agent per unit (e.g., kilogram) of body weight. When physicians decide on a prescribed dose, they take into account the weight of the individual receiving the medication. A heavier person, for example, may require a greater dose to achieve the same effect that a lesser dose would have on a lighter person.

In toxicology, it is common for the dose to be explicitly expressed in terms of body weight and, often, in terms of time as well. Thus, the dose may be given as 10 milligrams per kilogram of body weight (10 mg/kg) or even 10 milligrams per kilogram of body weight per day (10 mg/kg/day). This latter dose given to a person weighing 70 kilograms, for example, would result in a total dose of 700 mg (70 x 10) over the course of a day.

One should also keep in mind that medications are chemicals designed for positive effects, but at the right dose. Thus a baby aspirin tablet may not cure an adult of a headache, while two ordinary aspirins are more likely to do the job. One hundred aspirins taken all together will likely result in toxic effects.

In addition to the somewhat different ways of expressing dose, there are different kinds of doses that can be important toxicologically.

For example, dose can refer to the amount of a substance to which an individual or population is exposed. This definition is generally applied in cases of occupational and environmental exposures.

In the experimental situation, the dose to which animals are exposed is known as the administered dose. Dose can also be defined as the amount absorbed into the organism, also known as the internal dose. This definition reflects the idea that only the amount that is absorbed is available to cause harm at sites in the body distant from the site at which the agent makes contact with the individual.

As toxicological knowledge grew more sophisticated, it became possible to define dose in yet other ways that better reflect the connection between dose and effect. One example is the target organ dose or the amount that reaches the site(s) at which the adverse effects occur. This is also known as the biologically effective dose.

It is always important to note that the extent and nature of adverse effects, for a given agent, may vary, dependent upon the dose and route of exposure. Major routes of exposure include ingestion, inhalation or skin contact (dermal).

Effects are also dependent upon the age and sex of the exposed individual, as well as other characteristics of this individual, including underlying disease, nutritional status, and history of previous exposures.

In addition, the time course and duration of the dose administration or exposure are important variables. A single large dose given all at once is likely to have quite a different impact than the same total dose given in small amounts over a long period of time.

Also, the spacing between doses given over long periods of time can be critical in determining whether or not adverse effects will occur.

Public interpretation of dose and effect, frequently fueled by media sensationalism, may fail to take into account all the variables affecting dose. Unless this is done, however, reaching conclusions about a chemical's or product's toxicity is purely speculative.

"All substances are poisons; there are none which is not a poison. The right dose differentiates a poison and a remedy."

The original German statement, "Alle Dinge sind Gift und nichts ist ohne Gift; allein die Dosis macht, dass ein Ding kein Gift ist," is sometimes summarized and translated as "The dose makes the poison." This is a good starting point, but really not as simple as it sounds.

So, while the basic principle expressed so eloquently by Paracelsus governs the practice of toxicology, it is important to understand that applying this principle is difficult and requires an appreciation of all of the factors that influence responses to a given dose and all of the ways that dose may be defined. All too often, conclusions about dose and effect are made without consideration for these issues and such conclusions should be carefully scrutinized before they are accepted.

Thus, dose entails many variables, and the ultimate extent of its effects is closely entwined with the route of exposure. To summarize, the variables which must be taken into account in making a full determination of the consequences of dose include:

Dose Amount - A measure of the magnitude of the dose.

Dose Frequency - How often exposure occurs, e.g., daily, weekly, five days out of seven, etc.

Dose Duration - Over how long a total period of time dose exposure occur, e.g., a week, a month, a year, a lifetime.

Subject Variability (Natural) - Individual characteristics such as age, sex, body weight, ethnic background, and genetics.

Subject Variability (Health Status) - Whether any pre-existing health conditions, such as asthma, diabetes, or hypertension, may affect susceptibility to an agent.

Route of Exposure - The way in which the person is exposed. The three most common routes of exposure are ingestion, inhalation and skin contact.

Later modules will consider how toxicity (of which dose is one attribute) together with extent of exposure determines risk.Which of the following does not affect toxicity?

a. Whether the agent is inhaled or ingested

b. Whether the exposed organism is male or female

c. Whether the agent is synthetic or naturally occurring

d. Whether the exposure is continuous or sporadic

e. Whether the exposed organism is a child or an adult

The correct answer is "Option C. The toxicity of an agent does not depend on its source."

How is Dose Measured?Depending on the situation, measuring dose can be a straightforward process or a very cumbersome one.

The best setting in which to accurately measure dose is the laboratory, where the conditions can be controlled.

When doing research on experimental animals, it is possible to present them every day with food and/or water containing exactly the same amount of the agent being tested.

Using measures of the amount ingested, it is possible to fairly accurately calculate the total amount of the agent the animal is exposed to each day, and thus the daily dose.

At the other extreme, measuring the doses to which individual humans are exposed in their everyday environment is much more difficult.

The amount of a particular agent ingested is likely to vary from day to day and measures of the concentration of the agent in the media of interest (e.g., air or water), are unlikely to be available on a daily basis.

In this case, it is only possible to estimate the doses to which individuals are exposed using whatever environmental measurements are available and combining them with general information about human behavior (e.g., amount of food or water consumed each day).

Because of the lack of appropriate data, these dose estimates are often quite uncertain; the fewer the data, the greater the degree of uncertainty.

Additional complexity is involved in real world exposures to multiple chemicals, especially at the same time. Consider, for example, the exposure of the population in the vicinity of the 9/11 World Trade Center disaster.

Measuring the absorbed dose is more difficult than quantitating the exposure dose since it requires information about the way that different animals absorb agents through various routes of exposure (e.g., inhalation, dermal absorption) and under differing conditions. For example, absorption through a young male rat's skin might be very different from the dose delivered through drinking water in an aged female monkey.

Information about absorption is collected through laboratory experiments, generally performed on a limited number of animals. Because of ethical and other considerations, such laboratory studies are typically performed on rodents and rarely on humans. As a result, there is a level of uncertainty in extrapolating the effects of absorbed dose from laboratory animal studies to humans.

The most difficult dose to quantitate is the target organ dose or biologically effective dose (e.g., the dose that actually reaches the liver) since this generally cannot be directly measured. To make such measurements in animals generally requires invasive procedures that could alter the response of the organism, while to make such measurements in humans would be unethical.

Thus, target organ doses are usually calculated using information about the distribution and fate of the agent in the organism.

This information can be gathered from studies on the amounts of an agent that are absorbed, excreted, stored and freely circulating in an organism.

Often this type of information is not available or is incomplete so that it is difficult to calculate target organ doses with confidence, even in organisms which have been extensively studied (e.g., rodents). Estimation of the target organ dose is much more difficult in humans since the needed information is generally even less available.

Which type of dose is most relevant for determining the toxicity of and agent?

a. Administered dose

b. Absorbed dose

c. Target organ dose

d. None of the above

The correct answer is "Option C. Target organ dose. This is the dose at the site at which toxicity occurs so it is the most relevant for assessing the relationship between dose and effect."

Dose-Effect and Dose-ResponseSince the basic question in toxicology is how dose is related to toxicity, most toxicology studies are designed to investigate how living creatures react as doses vary incrementally, from low to high levels. In studies on experimental animals, different groups of animals are administered, or exposed to, different daily doses.

Then, at various intervals, the animals are examined for the presence or absence of effects. These effects may be behavioral changes, alterations in the compositions of body tissues or fluids (e.g., blood, serum, urine), or structural changes in parts of the organism.

Since examination of internal organs requires invasive procedures that can have an impact on the effects observed, examinations of effects on these organs are generally done during autopsy after studies are completed.

Researchers are developing exciting new ways to assess effects in living animals, including techniques such as Magnetic Resonance Imaging (MRI), ultrasound, Positron Emission Tomography (PET), and optical imaging. In the latter, biolumenescing genes are inserted into the genetic material of animals and detection of light indicates functioning of certain biological pathways.

The data are then plotted on a graph that shows the dose on one axis and the response or effects on the other.

In a typical dose-response graph, the dose is plotted against the number or proportion of animals exhibiting a particular response.

For a long time, death was the response of choice for assessing short term (acute) toxicity, and the toxicology literature contains many citations listing such lethal doses of assorted agents for a variety of laboratory animals.

In most cases, the dose that is lethal to 50% of the exposed animals (LD50) is the value that is published.

Although LD50 values are widespread in the scientific literature and still used and useful, concerns over animal welfare and the development of more technically sophisticated tools have led to other approaches for assessing toxicity.

On the other hand, dose-effect comparisons can be depicted in graphs, charts, or tables which plot dose against the degree of response (i.e., the severity of the effects). Thus, a low dose may cause no effects, a higher dose, limited effects, a still higher dose, serious outcomes, and, at a high enough dose, death.

A common real world scenario that illustrates this type of dose-effect relationship is the sequence of events that can occur as a result of human alcohol consumption.

In this case, a low consumption does not result in observable effects but increasing amounts of alcohol lead to increasingly severe symptoms including incoordination and unconsciousness, and sometimes even death.

INCLUDEPICTURE "C:\\Users\\Eva Marie\\Documents\\Intro Toxicology_files\\6_4_alcohol_observedeffect_chart.jpg" \* MERGEFORMATINET While the alcohol example in the preceding slide illustrates the dose-effect relationship for a short term (acute) exposure, dose-effect relationships are also commonly used for assessing long term (chronic) effects.

When used to characterize chronic toxicity for chemicals that do not cause cancer, the dose-effect relationship in laboratory animals exposed over long periods of time is examined to determine the highest dose at which no observable adverse effect is seen (NOAEL) or the lowest dose at which an adverse effect is observed (LOAEL).

This single value on the curve is then extrapolated to humans to estimate the maximum exposures that are likely to be without adverse effect. Making this assessment often requires judgment because there may be subtle distinctions between normal variation and an adverse effect.

Note: The Y-axis in dose-response curves does not always measure the same effect. Earlier we saw a curve where lethality was the parameter under consideration as the dose (X-axis) was increased. In this generic graph, though, the response, still measured as a percentage, may represent various effects

BASIC TOXICOLOGY TERMINOLOGY

Toxic having the characteristic of producing an undesirable or adverse health effect.

Toxicity any toxic (adverse) effect that a chemical or physical agent might produce within a livingorganism.

Toxicology the science that deals with the study of the adverse effects (toxicities) chemicals orphysical agents may produce in living organisms under specific conditions of exposure. It is a science that attempts to qualitatively identify all the hazards (i.e., organ toxicities) associated with a substance, as well as to quantitatively determine the exposure conditions under which those hazards/toxicities are induced. Toxicology is the science that experimentally investigates the occurrence, nature, incidence, mechanism, and risk factors for the adverse effects of toxic substances.

Exposure to cause an adverse effect, a toxicant must first come in contact with an organism. The means by which an organism comes in contact with the substance is the route of exposure (e.g., in the air, water, soil, food, medication) for that chemical.Dose the total amount of a toxicant administered to an organism at specific time intervals. The quantity can be further defined in terms of quantity per unit body weight or per body surface area.Internal/absorbed dose the actual quantity of a toxicant that is absorbed into the organism and distributed systemically throughout the body.Delivered/effective/target organ dose the amount of toxicant reaching the organ (known as the target organ) that is adversely affected by the toxicant.Acute exposure exposure over a brief period of time (generally less than 24 h). Often it is considered to be a single exposure (or dose) but may consist of repeated exposures within a short time period.Subacute exposure resembles acute exposure except that the exposure duration is greater, fromSubchronic exposure exposures repeated or spread over an intermediate time range. For animal testing, this time range is generally considered to be 13 months.Chronic exposure exposures (either repeated or continuous) over a long (greater than 3 months) period of time. With animal testing this exposure often continues for the majority of the experimental animals life, and within occupational settings it is generally considered to be for a number of years.Acute toxicity an adverse or undesirable effect that is manifested within a relatively short time interval ranging from almost immediately to within several days following exposure (or dosing). An example would be chemical asphyxiation from exposure to a high concentration of carbon monoxide (CO).Chronic toxicity a permanent or lasting adverse effect that is manifested after exposure to a toxicant. An example would be the development of silicosis following a long-term exposure to silica in workplaces such as foundries.Local toxicity an adverse or undesirable effect that is manifested at the toxicants site of contact with the organism. Examples include an acids ability to cause burning of the eyes, upper respiratory tract irritation, and skin burns.Systemic toxicity an adverse or undesirable effect that can be seen throughout the organism or in an organ with selective vulnerability distant from the point of entry of the toxicant (i.e., toxicant requires absorption and distribution within the organism to produce the toxic effect). Examples would be adverse effects on the kidney or central nervous system resulting from the chronic ingestion of mercury.Reversible toxicity an adverse or undesirable effect that can be reversed once exposure is stopped. Reversibility of toxicity depends on a number of factors, including the extent of exposure (time and amount of toxicant) and the ability of the affected tissue to repair or regenerate. An example includes hepatic toxicity from acute acetaminophen exposure and liver regeneration.

Delayed or latent toxicity an adverse or undesirable effect appearing long after the initiation and/or cessation of exposure to the toxicant. An example is cervical cancer during adulthood resulting from in utero exposure to diethylstilbestrol (DES).Allergic reaction a reaction to a toxicant caused by an altered state of the normal immune response. The outcome of the exposure can be immediate (anaphylaxis) or delayed (cell-mediated).Idiosyncratic reaction a response to a toxicant occurring at exposure levels much lower than those generally required to cause the same effect in most individuals within the population. This response is genetically determined, and a good example would be sensitivity to nitrates due to deficiency in NADH (reduced-form nicotinamide adenine dinucleotide phosphate)methemoglobinreductase.Mechanism of toxicity the necessary biologic interactions by which a toxicant exerts its toxic effect on an organism. An example is carbon monoxide (CO) asphyxiation due to the binding of CO to hemoglobin, thus preventing the transport of oxygen within the blood.Toxicant any substance that causes a harmful (or adverse) effect when in contact with a living organism at a sufficiently high concentration.Toxin any toxicant produced by an organism (floral or faunal, including bacteria); that is, naturally produced toxicants. An example would be the pyrethrins, which are natural pesticides produced by pyrethrum flowers (i.e., certain chrysanthemums) that serve as the model for the man made insecticide class pyrethroids.Hazard the qualitative nature of the adverse or undesirable effect (i.e., the type of adverse effect) resulting from exposure to a particular toxicant or physical agent. For example, asphyxiation is the hazard from acute exposures to carbon monoxide (CO).Safety the measure or mathematical probability that a specific exposure situation or dose will not produce a toxic effect.Risk the measure or probability that a specific exposure situation or dose will produce a toxic effect.Risk assessment the process by which the potential (or probability of) adverse health effects of exposure are characterized.

A toxic agent is anything that can produce an adverse biological effect. It may be chemical, physical, or biological in form. For example, toxic agents may be chemical (such as cyanide), physical (such as radiation) and biological (such as snake venom).

A distinction is made for diseases due to biological organisms. Those organisms that invade and multiply within the organism and produce their effects by biological activity are not classified as toxic agents. An example of this is a virus that damages cell membranes resulting in cell death.

If the invading organisms excrete chemicals which are the basis for toxicity, the excreted substances are known as biological toxins. The organisms in this case are referred to as toxic organisms. An example is tetanus. Tetanus is caused by a bacterium, Clostridium tetani. The bacteria C. tetani itself does not cause disease by invading and destroying cells. Rather, it is a toxin that is excreted by the bacteria that travels to the nervous system (a neurotoxin) that produces the disease.

A toxic substance is simply a material which has toxic properties. It may be a discrete toxic chemical or a mixture of toxic chemicals. For example, lead chromate, asbestos, and gasoline are all toxic substances. Lead chromate is a discrete toxic chemical. Asbestos is a toxic material that does not consist of an exact chemical composition but a variety of fibers and minerals. Gasoline is also a toxic substance rather than a toxic chemical in that it contains a mixture of many chemicals. Toxic substances may not always have a constant composition. For example, the composition of gasoline varies with octane level, manufacturer, time of season, etc.

Toxic substances may be organic or inorganic in composition

Toxic substances may be systemic toxins or organ toxins.

A systemic toxin is one that affects the entire body or many organs rather than a specific site. For example, potassium cyanide is a systemic toxicant in that it affects virtually every cell and organ in the body by interfering with the cell's ability to utilize oxygen.

Toxicants may also affect only specific tissues or organs while not producing damage to the body as a whole. These specific sites are known as the target organs or target tissues. Benzene is a specific organ toxin in that it is primarily toxic to the blood-forming tissues. Lead is also a specific organ toxin; however, it has three target organs (central nervous system, kidney, and hematopoietic system).

A toxicant may affect a specific type of tissue (such as connective tissue) that is present in several organs. The toxic site is then referred to as the target tissue.

There are many types of cells in the body and they can be classified in several ways.basic structure (e.g., cuboidal cells)tissue type (e.g., hepatocytes of the liver)germinal cells (e.g., ova and sperm)somatic cells (e.g., non-reproductive cells of the body)

Germ cells are those cells that are involved in the reproductive process and can give rise to a new organism. They have only a single set of chromosomes peculiar to a specific sex. Male germ cells give rise to sperm and female germ cells develop into ova. Toxicity to germ cells can cause effects on the developing fetus (such as birth defects, abortions).Somatic cells are all body cells except the reproductive germ cells. They have two sets (or pairs) of chromosomes. Toxicity to somatic cells causes a variety of toxic effects to the exposed individual (such as dermatitis, death, and cancer).

Dose by definition is the amount of a substance administered at one time. However, other parameters are needed to characterize the exposure to xenobiotics. The most important are the number of doses, frequency, and total time period of the treatment.For example:650 mg Tylenol as a single dose500 mg Penicillin every 8 hours for 10 days10 mg DDT per day for 90 days

There are numerous types of doses, e.g., exposure dose, absorbed dose, administered dose and total dose.

Fractionating a total dose usually decreases the probability that the total dose will cause toxicity. The reason for this is that the body often can repair the effect of each subtoxic dose if sufficient time passes before receiving the next dose. In such a case, the total dose, harmful if received all at once, is non-toxic when administered over a period of time. For example, 30 mg of strychnine swallowed at one time could be fatal to an adult whereas 3 mg of strychnine swallowed each day for ten days would not be fatal.

The units used in toxicology are basically the same as those used in medicine. The gram is the standard unit. However, most exposures will be smaller quantities and thus the milligram (mg) is commonly used. For example, the common adult dose of Tylenol is 650 milligrams.

The clinical and toxic effects of a dose must be related to age and body size. For example, 650 mg is the adult dose of Tylenol. That would be quite toxic to young children, and thus Children's Tylenol tablets contain only 80 mg. A better means to allow for comparison of effectiveness and toxicity is the amount of a substance administered on a body weight basis. A common dose measurement is mg/kg which stands for mg of substance per kg of body weight.

Another important aspect is the time over which the dose is administered. This is especially important for exposures of several days or for chronic exposures. The commonly used time unit is one day and thus, the usual dosage unit is mg/kg/day.

Since some xenobiotics are toxic in much smaller quantities than the milligram, smaller fractions of the gram are used, such as microgram (g). Other units are shown below:

Environmental exposure units are expressed as the amount of a xenobiotic in a unit of the media.mg/liter (mg/l) for liquidsmg/gram (mg/g) for solidsmg/cubic meter (mg/m3) for air

Smaller units are used as needed, e.g., g/ml. Other commonly used dose units for substances in media are parts per million (ppm), parts per billion (ppb) and parts per trillion (ppt).

Dose Response The dose-response relationship is a fundamental and essential concept in toxicology. It correlates exposures and the spectrum of induced effects. Generally, the higher the dose, the more severe the response. The dose-response relationship is based on observed data from experimental animal, human clinical, or cell studies.Knowledge of the dose-response relationship:establishes causality that the chemical has in fact induced the observed effectsestablishes the lowest dose where an induced effect occurs - the threshold effectdetermines the rate at which injury builds up - the slope for the dose response.

Within a population, the majority of responses to a toxicant are similar; however, a wide variance of responses may be encountered, some individuals are susceptible and others resistant. As demonstrated above, a graph of the individual responses can be depicted as a bell-shaped standard distribution curve.

Dose responses are commonly presented as mean + 1 S.D. (standard deviation), which incorporates 68% of the individuals. The variance may also be presented as two standard deviations, which incorporates 95% of the responses. A large standard deviation indicates great variability of response. For example, a response of 15+8 mg indicates considerably more variability than 15+2 mg.

The dose-response curve normally takes the form of a sigmoid curve. It conforms to a smooth curve as close as possible to the individual data points. For most effects, small doses are not toxic. The point at which toxicity first appears is known as the threshold dose level. From that point, the curve increases with higher dose levels. 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.

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.

Dose Estimates of Toxic Effects (LD, EC, TD)Dose-response curves are used to derive dose estimates of chemical substances. A common dose estimate for acute toxicity is the LD50 (Lethal Dose 50%). This is a statistically derived dose at which 50% of the individuals will be expected to die. The figure below illustrates how an LD50 of 20 mg is derived.

Other dose estimates also may be used. LD0 represents the dose at which no individuals are expected to die. This is just below the threshold for lethality. LD10 refers to the dose at which 10% of the individuals will die.

For inhalation toxicity, air concentrations are used for exposure values. Thus, the LC50 is utilized which stands for Lethal Concentration 50%, the calculated concentration of a gas lethal to 50% of a group. Occasionally LC0 and LC10 are also used.

Effective Doses (EDs) are used to indicate the effectiveness of a substance. Normally, 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 usual terms are:

Toxic Doses (TDs) are utilized to indicate doses that cause adverse toxic effects. The usual dose estimates are listed below:

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.

Therapeutic Index and Margin of SafetyThe Therapeutic Index (TI) is used to compare the therapeutically effective dose to the toxic dose. The TI is a statement of relative safety of a drug. It is the ratio of the dose producing toxicity to the dose needed to produce the desired therapeutic response. The common method used to derive the TI is to use the 50% dose-response points. 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 as to safety, depending on the slope of the dose-response curves for therapeutic and lethal effects. To overcome this deficiency, toxicologists often use another term to denote the safety of a drug - the Margin of Safety (MOS).

The MOS is usually calculated as 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 use caution in prescribing a drug in which the MOS is less than 1.

Due to differences in slopes and threshold doses, low doses may be effective without producing toxicity. Although more patients may benefit from higher doses, this is offset by the probability that toxicity or death will occur. The relationship between the Effective Dose response and the Toxic Dose response is illustrated above.

Knowledge of the slope is important in comparing the toxicity of various substances. For some toxicants a small increase in dose causes a large increase in response (toxicant A, steep slope). For other toxicants a much larger increase in dose is required to cause the same increase in response (toxicant B, shallow slope).

NOAEL and LOAELTwo terms often encountered are No Observed Adverse Effect Level (NOAEL) and Low Observed Adverse Effect Level (LOAEL). They are the actual data points from human clinical or experimental animal studies.

Sometimes the terms No Observed Effect Level (NOEL) and Lowest Observed Effect Level (LOEL) may also be found in the literature. NOELs and LOELs do not necessarily imply toxic or harmful effects and may be used to describe beneficial effects of chemicals as well.

The NOAEL, LOAEL, NOEL, and LOEL have great importance in the conduct of risk assessments.

Toxic EffectsToxicity is complex with many influencing factors; dosage is the most important. Xenobiotics cause many types of toxicity by a variety of mechanisms. Some chemicals are themselves toxic. Others must be metabolized (chemically changed within the body) before they cause toxicity.

Many xenobiotics distribute in the body and often affect only specific target organs. Others, however, can damage any cell or tissue that they contact. The target organs that are affected may vary depending on dosage and route of exposure. For example, the target for a chemical after acute exposure may be the nervous system, but after chronic exposure the liver.

Toxicity can result from adverse cellular, biochemical, or macromolecular changes. Examples are:cell replacement, such as fibrosisdamage to an enzyme systemdisruption of protein synthesisproduction of reactive chemicals in cellsDNA damage

Some xenobiotics may also act indirectly by:modification of an essential biochemical functioninterference with nutritionalteration of a physiological mechanism

Systemic Toxic EffectsToxic effects are generally categorized according to the site of the toxic effect. In some cases, the effect may occur at only one site. This site is referred to as the specific target organ. In other cases, toxic effects may occur at multiple sites. This is referred as systemic toxicity.Following are types of systemic toxicity:

Acute ToxicitySubchronic ToxicityChronic ToxicityCarcinogenicityDevelopmental ToxicityGenetic Toxicity (somatic cells)Acute ToxicityAcute toxicity occurs almost immediately (hours/days) after an exposure. An acute exposure is usually a 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 India.Many people die each year from inhaling carbon monoxide from faulty heaters.

Non-lethal acute effects may also occur, e.g., convulsions and respiratory irritation.Subchronic ToxicitySubchronic toxicity 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.

Chronic ToxicityChronic toxicity represents cumulative damage to specific organ systems and takes many months or years to become a recognizable clinical disease. Damage due to subclinical individual exposures may go unnoticed. With repeated exposures or long-term continual exposure, the damage from these subclinical exposures slowly builds-up (cumulative damage) until the damage exceeds the threshold for chronic toxicity. Ultimately, the damage becomes so severe that the organ can no longer function normally and a variety of chronic toxic effects may result.Examples of chronic toxic affects are:cirrhosis in alcoholics who have ingested ethanol for several yearschronic kidney disease in workmen with several years exposure to leadchronic bronchitis in long-term cigarette smokerspulmonary fibrosis in coal miners (black lung disease)CarcinogenicityCarcinogenicity is a complex multistage process of abnormal cell growth and differentiation which can lead to cancer. At least two stages are recognized. They are initiation in which a normal cell undergoes irreversible changes and promotion in which initiated cells are stimulated to progress to cancer. Chemicals can act as initiators or promoters.

The initial neoplastic transformation results from the mutation of the cellular genes that control normal cell functions. The 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.

Developmental ToxicityDevelopmental 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:

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.

Genetic ToxicityGenetic 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. There are three types of genetic change:

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) or cell death (e.g. teratogenesis) in the exposed person.

Factors Influencing ToxicityThe toxicity of a substance depends on the following:

form and innate chemical activity dosage, especially dose-time relationshipexposure routespeciesagesexability to be absorbedmetabolismdistribution within the bodyexcretionpresence of other chemicals

The form of a substance may have a profound impact on its toxicity especially for metallic elements. For example, the toxicity of mercury vapor differs greatly from methyl mercury. Another example is chromium. Cr3+ is relatively nontoxic whereas Cr6+ causes skin or nasal corrosion and lung cancer.

The innate chemical activity of substances also varies greatly. Some can quickly damage cells causing immediate cell death. Others slowly interfere only with a cell's function. For example: hydrogen cyanide binds to cytochrome oxidase resulting in cellular hypoxia and rapid death nicotine binds to cholinergic receptors in the CNS altering nerve conduction and inducing gradual onset of paralysis

The dosage is the most important and critical factor in determining if a substance will be an acute or a chronic toxicant. Virtually all chemicals can be acute toxicants if sufficiently large doses are administered. Often the toxic mechanisms and target organs are different for acute and chronic toxicity. Examples are:

Exposure route is important in determining toxicity. Some chemicals may be highly toxic by one route but not by others. Two major reasons are differences in absorption and distribution within the body. For example: 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

Frequently there are different target organs for different routes of exposure.

Toxic responses can vary substantially depending on the species. Most species differences are attributable to differences in metabolism. Others may be due to anatomical or physiological differences. For example, rats cannot vomit and expel toxicants before they are absorbed or cause severe irritation, whereas humans and dogs are capable of vomiting.

Selective toxicity refers to species differences in toxicity between two species simultaneously exposed. This is the basis for the effectiveness of pesticides and drugs. Examples are:An insecticide is lethal to insects but relatively nontoxic to animalsantibiotics are selectively toxic to microorganisms while virtually nontoxic to humans

Age may be important in determining the response to toxicants. Some chemicals are more toxic to infants or the elderly than to young adults. For example:Parathion is more toxic to young animalsnitrosamines are more carcinogenic to newborn or young animals

Although uncommon, toxic responses can vary depending on sex. Examples are: male rats are 10 times more sensitive than females to liver damage from DDT

female rats are twice as sensitive to parathion as male ratsThe ability to be absorbed is essential for systemic toxicity to occur. Some chemicals are readily absorbed and others poorly absorbed. For example, nearly all alcohols are readily absorbed when ingested, whereas there is virtually no absorption for most polymers. The rates and extent of absorption may vary greatly depending on the form of the chemical and the route of exposure. For example: ethanol is readily absorbed from the gastrointestinal tract but poorly absorbed through the skin

organic mercury is readily absorbed from the gastrointestinal tract; inorganic lead sulfate is not

Metabolism, also known as biotransformation, is a major factor in determining toxicity. The products of metabolism are known as metabolites. There are two types of metabolism - detoxification and bioactivation. Detoxification is the process by which a xenobiotic is converted to a less toxic form. This is a natural defense mechanism of the organism. Generally the detoxification process converts lipid-soluble compounds to polar compounds. Bioactivation is the process by which a xenobiotic may be converted to more reactive or toxic forms.

The distribution of toxicants and toxic metabolites throughout the body ultimately determines the sites where toxicity occurs. A major determinant of whether or not a toxicant will damage cells is its lipid solubility. If a toxicant is lipid-soluble it readily penetrates cell membranes. Many toxicants are stored in the body. Fat tissue, liver, kidney, and bone are the most common storage depots. Blood serves as the main avenue for distribution. Lymph also distributes some materials.

The site and rate of excretion is another major factor affecting the toxicity of a xenobiotic. 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.

A large volume of blood serum is filtered through the kidney. Lipid-soluble toxicants are reabsorbed and concentrated in kidney cells. Impaired kidney function causes slower elimination of toxicants and increases their toxic potential.

The presence of other chemicals may decrease toxicity (antagonism), add to toxicity (additivity), or increase toxicity (synergism or potentiation) of some xenobiotics. For example: alcohol may enhance the effect of many antihistamines and sedatives

antidotes function by antagonizing the toxicity of a poison (atropine counteracts poisoning by organophosphate insecticides)Chemical InteractionsHumans are normally exposed to several chemicals at one time rather than to an individual chemical. Medical treatment and environment exposure generally consists of multiple exposures. 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 chemicals

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 chemicalsNormally, the toxicity of a specific chemical is determined by the study of animals exposed to only one chemical. Toxicity testing of mixtures is rarely conducted since it is usually impossible to predict the possible combinations of chemicals that will be present in multiple-chemical exposures.

Xenobiotics administered or received simultaneously may act independently of each other. However, in many cases, the presence of one chemical may drastically affect the response to another chemical. The toxicity of a combination of chemicals may be less or it may be more than would be predicted from the known effects of each individual chemical. The effect that one chemical has on the toxic effect of another chemical is known as an interaction.

Types of InteractionsThere are four basic types of interactions. Each is based on the expected effects caused by the individual chemicals.The types of interactions are:

This table quantitatively illustrates the percent of the population affected by individual exposure to chemical A and chemical B as well as exposure to the combination of chemical A and chemical B. It also gives the specific type of interaction:

The interactions described can be categorized by their chemical or biological mechanisms as follows: chemical reactions between chemicals

modifications in absorption, metabolism, or excretion

reactions at binding sites and receptors

physiological changes

AdditivityAdditivity is the most common type of drug interaction. Examples of chemical or drug additivity reactions are: Two central nervous system (CNS) depressants taken at the same time, a tranquilizer and alcohol, often cause depression equal to the sum of that caused by each drug.

Organophosphate insecticides interfere with nerve conduction. The toxicity of the combination of two organophosphate insecticides is equal to the sum of the toxicity of each. Chlorinated insecticides and halogenated solvents both produce liver toxicity. The hepatotoxicity of an insecticide formulation containing both is equivalent to the sum of the hepatotoxicity of each.

AntagonismAntagonism is often a desirable effect in toxicology and is the basis for most antidotes. Examples include:

PotentiationPotentiation occurs when a chemical that does not have a specific toxic effect makes another chemical more toxic. Examples are:The hepatotoxicity of carbon tetrachloride is greatly enhanced by the presence of isopropanol. Such exposure may occur in the workplace.Normally, warfarin (a widely used anticoagulant in cardiac disease) is bound to plasma albumin so that only 2% of the warfarin is active. Drugs which compete for binding sites on albumin increase the level of free warfarin to 4% causing fatal hemorrhage.

SynergismSynergism can have serious health effects. With synergism, exposure to a chemical may drastically increase the effect of another chemical. Examples are: Exposure to both cigarette smoke and radon results in a significantly greater risk for lung cancer than the sum of the risks of each.

The combination of exposure to asbestos and cigarette smoke results in a significantly greater risk for lung cancer than the sum of the risks of each.

The hepatotoxicity of a combination of ethanol and carbon tetrachloride is much greater than the sum of the hepatotoxicity of each.

Different types of interactions can occur at different target sites for the same combination of two chemicals. For example, chlorinated insecticides and halogenated solvents (which are often used together in insecticide formulations) can produce liver toxicity with the interaction being additive.

The same combination of chemicals produces a different type of interaction on the central nervous system. Chlorinated insecticides stimulate the central nervous system whereas halogenated solvents cause depression of the nervous system. The effect of simultaneous exposure is an antagonistic interaction.

What is Toxicokinetics?

Toxicokinetics is essentially the study of "how a substance gets into the body and what happens to it in the body".

Four processes are involved in toxicokinetics.

The study of the kinetics (movement) of chemicals was originally conducted with pharmaceuticals and thus the term pharmacokinetics became commonly used. In addition, toxicology studies were initially conducted with drugs. However, the science of toxicology has evolved to include environmental and occupational chemicals as well as drugs. Toxicokinetics is thus the appropriate term for the study of the kinetics of all toxic substances.

Frequently the terms toxicokinetics, pharmacokinetics, or disposition may be found in the literature to have the same meaning. Disposition is often used in place of toxicokinetics to describe the time-course of movement of chemicals through the body (that is, how does the body dispose of a xenobiotic?).

The disposition of a toxicant along with its' biological reactivity are the factors that determine the severity of toxicity that results when a xenobiotic enters the body. Specific aspects of disposition of greatest importance are:

duration and concentration of substance at the portal of entry

rate and amount that can be absorbed

distribution in the body and concentration at specific body sites

efficiency of biotransformation and nature of the metabolites

the ability of the substance or it's metabolites to pass throu