1.2 Concepts in toxicology

1.2 Concepts in toxicology : 

1.2 Concepts in toxicology A background and some terms used in toxicology

Slide 2: 

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Advance Reading : 

Advance Reading Hughes, pp 19-27 (VISTA) Shibamoto, pp 19-33 NLM Toxicology Tutor, Introduction, Toxic Effects, Interactions, and Toxicity Testing Methods http://sis.nlm.nih.gov/enviro/toxtutor/Tox1/a11.htm http://sis.nlm.nih.gov/enviro/toxtutor/Tox1/a31.htm http://sis.nlm.nih.gov/enviro/toxtutor/Tox1/a41.htm http://sis.nlm.nih.gov/enviro/toxtutor/Tox1/a51.htm 3

More advance reading : 

More advance reading Foundation for biomedical research http://www.fbresearch.org/ Alternatives to animal testing http://altweb.jhsph.edu/ Americans for medical progress http://www.amprogress.org/site/c.jrLUK0PDLoF/b.913145/k.4502/Americans_for_Medical_Progress.htm 4

Key words : 

Key words Absorption Acute toxicity Biotransformation Chronic toxicity Delayed toxicity Distribution Excretion, elimination Immediate toxicity CNS = central nervous system GIT = gastro-intestinal tract in vitro in vivo Linear dose sequence Local toxicity sequence Storage Systemic toxicity Toxicity testing Toxicodynamics Toxicokinetics  Toxicosis 5

Learning Objectives : 

Learning Objectives Define toxicology and toxicity Discuss different types of toxic responses Explain how toxicants are classified Describe the phases of toxicosis Explain how concomitant exposure influences toxicity Develop an introductory understanding of toxicity testing 6

Toxicology : 

Toxicology The science that deal with the adverse effects of chemicals on living systems. Classifications. Descriptive toxicology. What? Mechanistic toxicology. Why? Analytical toxicology. How much? 7

Definition of Toxicity : 

Definition of Toxicity Toxicity: The degree to which a substance can harm humans or animals. Toxicity can be acute, sub-chronic, or chronic 8

Toxicity testing1: Select a test organism : 

Toxicity testing1: Select a test organism Plants or animals Algae, bacteria, mice, rabbits, primates Not humans. Why? Where do human data come from? In vivo uses a whole organism In vitro uses tissue cultures, cells. Cheaper than in vivo and fewer ethical problems How do these studies relate to humans? 9

Toxicity testing 2: The response : 

Toxicity testing 2: The response Response clinically observable (the classical method) Response observable on a molecular level (arguable) Response quantifiable. For example: Changes in cell number Cell morphology Biochemical products Number of tumours Changes in sleep patterns Changes in growth and development Death 10

Toxicity testing 3: Duration of test : 

Toxicity testing 3: Duration of test Select an exposure period A few seconds? E.g. eye irritants Years? E.g. reproductive studies 11

Toxicity testing 4: The dose : 

Toxicity testing 4: The dose How much do you use? Related to body weight for in vivo tests Concentration in in vitro tests What do you expect to happen? How toxic is the substance? What doses are relevant to likely exposures in real life? 12

Toxicity testing 5: Cost : 

Toxicity testing 5: Cost Toxicity testing is very expensive 13

Acute Toxicity : 

Acute Toxicity Involves harmful effects in an organism through a single or short-term exposure Death of Socrates Jacques-Louis David http://www.bc.edu/bc_org/avp/cas/his/CoreArt/art/neocl_dav_soc.html 14

Sub-chronic Toxicity : 

Sub-chronic Toxicity The ability of a toxic substance to cause effects for more than one year but less than the lifetime of the exposed organism 15

Chronic Toxicity : 

Chronic Toxicity The ability of a substance or mixture of substances to cause harmful effects over an extended period, usually upon repeated or continuous exposure, sometimes lasting for the entire life of the exposed organism. www.osh.dol.govt.nz/kidz/gore/lead.shtml Chronic lead poisoning 16

Specialty Areas in Toxicology : 

Specialty Areas in Toxicology Target Organ/System. Neurotoxicology, Genetic Toxicology, Reproductive Toxicology, Immunotoxicology, Endocrine Toxicology. Target Species/Systems. Aquatic Toxicology,Environmental Toxicology, Food ToxicologyWildlife Toxicology, Veterinary Toxicology. Selected Responses. Teratology, Carcinogenesis 17

Applied Toxicology : 

Applied Toxicology Occupational toxicology. Clinical toxicology. Toxic induced diseases and antidotes. Forensic toxicology. Determining causes of death. Regulatory toxicology. Risk assessment from descriptive tests Developmental toxicology. New chemicals and uses. 18

Classification of Toxicants 1 : 

Classification of Toxicants 1 Target organ. Hepatotoxin, neurotoxin. Intended use. Pesticide, herbicide, insecticide, solvent. Source. Natural, synthetic. Special effect. Carcinogen, mutagen, endocrine disruptor. 19

Classification of Toxicants, 2 : 

Classification of Toxicants, 2 Physical state. Gas, liquid, solid. Toxicity. Extremely, slightly. Chemical composition. Heavy metal, organophosphate. Mechanism of action. Anticholinergic, cholinesterase inhibitor, uncoupler of Krebs cycle. http://aenews.wsu.edu/Feb03AENews/Feb03AENews.htm 20

Types of Toxic Responses 1 : 

Types of Toxic Responses 1 When? Immediate Minutes to hours after a single exposure. Delayed Days to years after exposure. Some both. Where? Local –Effect at site of contact. –GIT, lungs, skin. •Systemic –Effect distant from exposure site. –CNS, kidney, lungs. •Some both. 21

Types of Toxic Responses, 2 : 

Types of Toxic Responses, 2 Reversible vs. Irreversible Largely determined by Tissue involved, length of exposure and magnitude of toxic insult. Reversible - rapidly regenerating tissue. Liver, intestinal mucosa,blood cells. Irreversible CNS (central nervous system) damage,carcinogenesis, mutagenesis, teratogenesis 22

Organic-aqueous partition : 

Organic-aqueous partition To determine whether a chemical prefers solvent to water, the chemical is shaken with a mixture of solvent and water. The layers are separated and the concentration in each phase determined. What would you need to know before this would work? 23

Bioavaliability:Partition coefficient : 

Bioavaliability:Partition coefficient Compounds in a mixture are separated by their differential solubility in two immiscible solvents (one is usually water). The differential solubility is often expressed quantitatively as a partition coefficient, Kp. The partition coefficient is an equilibrium constant describing how much of a compound will be in each of the two immiscible solvents used in the extraction process. Kp=Csolvent 1/Csolvent 2 What chemical concept does this coefficient measure? 24

Partition coefficients : 

Partition coefficients Water and ether are immiscible solvents and so will form two layers in a container. If a mixture of anthracene and NaCl is shaken up in a mixture of water and ether what is predominantly found in the ether? What is predominantly found in the water? The solubility in water of compound A is 10g/100mL. The solubility in ether of compound A is 1g/100mL. What is the partition coefficient (Kether/water) for compound A? 25

Phospholipids and cell membranes : 

Phospholipids and cell membranes Glycerophospholipids are the main component of cell membranes They form a lipid bi-layer : hydrophobic tails are buried together, and polar head groups are exposed to water. The membrane is an effective barrier to polar molecules (water, ions) into and out of the cell http://www.chemistry.sc.chula.ac.th/bsac/GEN%2032%20-%20proteins%20&%20lipids.pdf 26

Bioavailability : 

Bioavailability Octanol-Water partition coefficient, Kow Kow = [T]octanol / [T]water [T] = concentration of the toxin An empirical solubility term that can be used to assess trans-membrane movement potential. Kow = 102 to 103 indicates a good chemical for absorption(Log Kow = 2 to 3). OK lipid solubility and OK water solubility. 27

Three Phases of Toxicology : 

Three Phases of Toxicology Exposure phase. Toxicokinetic phase. Absorption. Distribution – movement and storage. Metabolism. Excretion. Toxicodynamic phase. Mechanisms by which toxicants affect cells. 28

Exposure Phase: 1 : 

Exposure Phase: 1 We are constantly exposed to toxins especially in plants They determine who eats who Potatoes and solanine 29

Exposure Phase: 2 : 

Exposure Phase: 2 Bioavailability. The fraction of a dose available for absorption. This is the same concept you learned about in Nutrition Main factors. Time and frequency of exposure, e.g. acute, sub-chronic Route of administration. Animal: oral, lung, skin, injection. Plant: roots, leaves. 30

Exposure Phase: 3 : 

Exposure Phase: 3 Host related factors – how strong is the individual? Pre-absorption metabolism. Dose. Physical and chemical form of the toxicant. Particle size, solubility. 31

Absorption Phase: 1 : 

Absorption Phase: 1 Comparative aspects. Cellular to organism. Membrane morphology. Lipoprotein bilayer. Physiochemical processes that govern trans-membrane movement. Lipid-water solubility, Kow Ionization (pKa), functional groups Molecular size and conformation. 32

Absorption Phase: 2 : 

Absorption Phase: 2 Trans-membrane movement. Simple diffusion – Fick’s Law of Diffusion. Filtration – aqueous pores. Carrier mediated. Sites of Absorption. Animals – GIT, dermal, lung. Plants – stomatal pores, cuticle, roots. Insects – pore canals, oral. Fish – gills, GIT, dermal. 33

Distribution Phase: 1 : 

Distribution Phase: 1 Four fates. Site of toxic action, storage, metabolism, excretion. How it occurs. Animals – blood, lymph. Plants – xylem and/or phloem. Barriers of toxicological significance. Blood/brain. Placental (maternal - fœtal). Mammary (blood - milk). 34

Distribution Phase: 2 : 

Distribution Phase: 2 Factors affecting distribution. Affinity of tissues for the xenobiotic (e.g. fat and DDT). Blood flow, protein binding. Route of administration, rate of metabolism. Toxin storage in the body Redistribution. Enterohepatic recirculation. 35

Metabolism Phase : 

Metabolism Phase Phase I – Bioconversion. Factors affecting toxicity and metabolism. Environmental, genetic… Phase II – Conjugation. “Grease to salt” adding sugar, polar residues to a non-polar chemical to enhance water-solubility Dioxin is very lipophilicand requires conjugation to make it soluble enough for excretion http://www.buddycom.com/molecule/tcdd/name.html 36

Factors Influencing Toxicity : 

Factors Influencing Toxicity Concomitant Exposure Additive 2 + 2 = 4 2 organophosphate compounds leading to cholinesterase inhibition. Synergistic 2 + 2 = 10 CCl4 with ethanol leading to hepatotoxicity. Potentiation 2 + 0 = 6 Isopropanol with CCl4 as a tumour promoter. Antagonism 2 +2 = 0 BAL (British anti-Lewisite) against heavy metals Atropine against cholinesterase Antidotes 37

Excretion : 

Excretion Toxicological significance. Renal excretion – water-soluble. Non-renal excretion. Biliary, expiration, skin Comparative aspects. Animals, plants. 38

Toxicodynamics: dose response relationships : 

Toxicodynamics: dose response relationships Toxic chemical Key interaction Critical Target Modified Critical Target (Key lesion) Progression Overt BiologicalResponse or Effect e.g A nephrotoxin Key lesion progresses to renal failure 39

Intrinsic Activity : 

Intrinsic Activity Intrinsic activity: response Agonist - substances with intrinsic activity, e.g. O2 Antagonist - substances that work against agonist, e.g. CO T + R = TR (the response) Drugs are typically reversible Toxicants are typically non-reversible 40

Oxygen Transport Toxicants : 

Oxygen Transport Toxicants Methaemoglobin formation (Fe2+ to Fe3+). Nitrate, nitrite. Naphthalene. Chlorate. Acetominophen. O2 competition at Fe2+ CO, carbon monoxide. CN-, cyanide. http://www.bio.davidson.edu/Courses/Molbio/MolStudents/spring2005/Heiner/heme.jpg 41

O2 Transport, Haemoglobin : 

O2 Transport, Haemoglobin Conjugated protein 3-D ribbon structure of the haemoglobin molecule. Each of the 4 globulin chains is represented in a different colour. The haem molecule is shown in red. http://www.bio.davidson.edu/Courses/Molbio/MolStudents/spring2005/Heiner/heme.jpg 42

Toxicity rating – oral human dose : 

Toxicity rating – oral human dose We are very rarely exposed to extremely toxic or supertoxic materials Use these doses as reference for toxicants you meet in this course 43

Spectrum of toxic dose : 

Spectrum of toxic dose Agent LD50 (mg/kg) Ethanol 10,000 NaCl 4000 Ferrous sulphate 1,500 Morphine sulphate 900 Phenobarbitol 150 DDT 100 Picrotoxin 5 Strychnine sulphate 2 Nicotine 1 D-Turbocuranine 0.1 Dioxin (TCDD) 0.001 Botulinum toxin 0.00001 What classes do these agents belong in? 44

Acknowledgement : 

Acknowledgement Dr Greg Möller University of Idaho 45