Introduction to toxicology

Introduction to Toxicology & Toxicodynamics

Content/Objectives

  • Purpose of Lecture: To cover fundamental aspects of toxicology which include:

    • Review terms used in toxicology.

    • Discuss modes of exposure and mechanisms of action.

    • Explore mechanisms of cell and tissue toxicity.

    • Review general management strategies (antidotes or treatments) of exposure to toxicants.

    • Discuss therapeutic issues when managing exposure.

Outcome/Expectations

  • By the end of the lecture, students should:

    • Be familiar with the science of toxicology and general terminologies.

    • Understand the modes of exposure to poisons/toxicants and their consequences.

    • Know the molecular and cellular mechanisms leading to toxic manifestations.

    • Recognize the organs affected by toxicants/poisons.

    • Grasp general treatment approaches following intoxication/poisoning.

Definition and Key Terminology

Toxicology

  • Toxicology: A branch of Pharmacology that focuses on the nature, effects, and detection of poisons (toxins) in biological systems.

  • It encompasses the study of adverse effects of chemicals or physical agents on living organisms.

Terminology

  • Poison/Toxin/Toxicant:

    • A chemical capable of interfering with the homeostasis of an organism or life processes of its cells by its inherent qualities, without requiring mechanical action, regardless of temperature.

    • Types of toxins:

    • Organic: Biological and plant-based sources.

    • Inorganic: Minerals, heavy metals.

    • Synthetic: Man-made chemicals.

    • Natural agents: Naturally occurring toxins.

  • Toxicosis: A state of being poisoned by a toxicant.

  • (Bio)toxins: Poisons originating from biological sources, including zootoxins, bacterial toxins (endotoxins and exotoxins), phytotoxins, and mycotoxins.

  • Xenobiotic: A term for foreign substances entering the body, which can produce beneficial effects (e.g., pharmaceuticals) or toxic effects (e.g., lead poisoning).

    • The principal that “All substances are poisons; it is the dose that makes the poison” (Paracelsus).

Dose-Response Relationship

Doses Examples

Substance

Non-toxic or Beneficial Dose

Toxic Dose

Lethal Dose

Alcohol

0.05% blood ethanol

0.1%

0.5%

Carbon Monoxide

<10% Hb bound

20-30%

>60%

Secobarbital

0.1 mg/dL

0.7 mg/dL

1 mg/dL

Aspirin

0.65 gm (2 tablets)

9.75 gm

34 gm

Paracetamol

1000 mg (2 tablets)

3500 mg

30,000 mg

Mechanisms of Toxicity

Types of Effects

  • Effects of toxicants can vary based on:

    • Rapidity of onset (acute vs. chronic)

    • Specific biological responses after exposure.

Target Organs

  • Organs most prone to toxic effects typically exhibit high blood flow. Main target organs include:

    • Lungs: Affected by inhaled toxins (pulmonary toxicity).

    • Liver: Common site for hepatotoxicity.

    • Kidneys: Often associated with nephrotoxicity.

    • Brain (CNS/PNS): Subject to neurotoxicity effects.

    • Immune System: Vulnerable to immunotoxicity.

    • Eyes and Ears: Prone to oculotoxicity and ototoxicity, respectively.

    • Genetic Systems: Genetic toxicity (genotoxicity), including mutations and carcinogenic effects.

Toxicity Manifestations

  • Toxicity is assessed through:

    • Dose of Exposure: Quantity of the toxicant.

    • Type of Damage: The specific organ or system affected.

    • Organ Reserve Capacity: The ability of an organ to withstand damage.

    • Innate Repair Ability: The natural ability of an organ to heal.

Branches of Toxicology

  • Environmental Toxicology: Examines the effects of chemicals on ecosystems.

  • Molecular Toxicology: Involves mechanisms of toxicant action on cellular activity and gene response.

  • Clinical Toxicology: Focuses on developing antidotes and treatment regimens for poisoning.

  • Veterinary Toxicology: Concerned with diagnosing and treating poisoning in animals.

  • Forensic Toxicology: Addresses medico-legal issues related to poison detection.

  • Industrial Toxicology: Focuses on workplace safety related to toxic exposure.

Pharmacology vs. Toxicology

  • Similarities: Both fields share principles regarding the kinetics and dynamics involved in substance effects in living organisms.

  • Toxicokinetics: Involves the study of absorption, distribution, storage, metabolism, and excretion of toxic substances.

  • Toxicodynamics: Covers mechanisms of action and dose-response relationships.

Sites of Access and Processes of Intoxication

  • Routes of Exposure:

    • Oral

    • Inhalation

    • Dermal Contact

    • Injection

Behavioral and Nutritional Toxicology

  • Behavioral Toxicology: Studies how toxins affect behavior in humans and animals.

  • Nutritional Toxicology: Investigates dietary impacts on toxicity expression and mechanisms.

Levels of Toxicology Studies

  • Carcinogenesis: The process leading to abnormal cell growth and cancer.

  • Teratogenesis: Developmental issues during fetal growth due to chemical exposure.

  • Mutagenesis: Effects on genetic material and susceptibility to such effects.

  • Organ Function: Examines how toxins affect specific organ systems (neurotoxicity, hepatotoxicity, etc.).

Measurement of Toxicity

  • Methods of Evaluation: Analytical techniques to assess toxicity levels in organisms and the environment.

  • Toxicology Testing: Includes both acute and chronic testing, utilizing measures such as LD50/TD50.

Levels of Concern in Toxicology

  • LOAEL: Lowest observed adverse effect level.

  • NOAEL: No observed adverse effect level.

  • DNEL: Derived no-effect level from NOAEL.

  • MTD: Maximum tolerated dose without adverse effects.

Toxicologic Pathology

  • Studies the morphological effects of toxic agents on cells and tissues.

Molecular and Cellular Mechanisms

Apoptosis

  • A programmed form of cell death enabling targeted removal of dysfunctional or unnecessary cells.

    • Mechanism: Regulated by enzyme activation (caspases) to degrade cellular components.

Necrosis

  • An uncontrolled and often harmful form of cell death resulting from cellular damage.

    • Characteristics include loss of membrane integrity, cell swelling, and inflammation.

Disruption of Cellular Function

  • Mechanisms leading to cell dysfunction include:

    • ATP depletion

    • Elevated intracellular Calcium ions

    • Overproduction of reactive oxygen/nitrogen species (ROS/RNS)

Consequences of Chemical Exposure

  • Chemicals affect cell function through various mechanisms including oxidative stress and inhibition of vital cellular processes.

Management Strategies for Intoxication

  • Antidotes: Use of specific agents to counteract poisoning effects.

  • Supportive Care: Includes pain management and life support interventions.

  • Enhanced Elimination: Strategies to promote expulsion of toxic agents from the body.

Special Populations in Toxicology

  • Relevant considerations for dosing in:

    • Pediatric patients

    • Geriatric patients

    • Pregnant and lactating individuals

References

  1. Modern Pharmacology by Craig and Stitzel.

  2. A Textbook of Modern Toxicology by Ernest Hodgson.

  3. PubMed/Medscape search updates.

  4. Other internet research resources (Google images).