Lecture 1 - Principles of Toxicology
Lecture 1 - Principles of Toxicology
Lecture 1 - Principles of Toxicology
• Toxins-naturally poisons
– Venoms
– Exotoxins (diphtheria and tetrodotoxin)
– Plants (atropa belladonna)
– Gases (hydrogen sulfide)
• Toxicants-synthetic poisons
– Industrial solvents
– Medications
– Heavy metals*
Exposome
• https://www.cdc.gov/niosh/topics/exposome/
Regulatory Toxicology
Toxic Substances Control Act
• Certain substances are generally excluded from TSCA, including, among others,
food, drugs, cosmetics and pesticides.
• TSCA addresses the production, importation, use, and disposal of specific chemicals
including polychlorinated biphenyls (PCBs), asbestos, radon and lead-based paint.
• http://www2.epa.gov/laws-regulations/summary-toxic-substances-control-act
REGULATORY AGENCIES
• Risk
– the probability of an adverse outcome
• Risk assessment
– the systematic scientific characterization of potential adverse
health effects resulting from exposure to hazardous agents of
situations.
Objectives of Risk Assessment
• Hazard identification
• Dose-response assessment
• Time-response assessment
• Exposure assessment (route and source)
• Risk characterization
Hazard identification
• Organ/tissue sites of toxicity
– hepatic, renal, cardiovascular
• Type of Pathology
– carcinogenic, teratogenic, cytotoxic
• Structure of chemical
• Mechanism of toxicity
– Important for extrapolation such as animal to human
• Study Type
– Computational, in-vitro, preclinical animal study,
clinical study, epidemiological study, case report
Hazard Identification and Risk Prediction
• Structure
• Modeling
• C-statistic
• Machine learning
• In vitro assays
• In vivo studies
Risk Assessment:
Define Specific Exposure Circumstances
• Intensity (concentration)
• Duration (length of time)
• Acute/Chronic
• Recent/Remote
Measuring and Reporting Toxicity
• Acute Toxicity
– Acute toxicity is used to describe effects which appear promptly, or
within 24 hours of exposure
• Chronic Toxicity
– Chronic toxicity is the delayed effect of exposure to a hazardous
chemical substance.
– It is measured in experimental conditions after three months of either
continuous or occasional exposure.
• High acute toxicity does not necessarily equal high chronic
toxicity.
• Low acute toxicity does not necessarily equal low chronic
toxicity.
Benefit/Risk Analysis
• Therapeutic benefit v. side effects (adverse effects, toxic
effects)
• Adverse effect
– An adverse effect may be termed a "side effect", when judged to
be secondary to a main or therapeutic effect.
Morphine 1,500
Nicotine 1
Tubocurare 0.5
Dioxin 0.001
Chocolate
Essentially harmless to humans, toxic to many animals e.g.,
dogs
Venom
LD50 results may be misleading due to the physiological
differences between mice, rats, and humans e.g., venomous
snakes have venom adapted to incapacitate mice.
Dose Dependent Toxicity
Margin of Safety
• Low dose
extrapolation
Allometric Scaling
• Empirical approach where the exchange of drug dose is based
on normalization of dose to body surface area.
Defining Exposure Circumstances
• In utero
• Childhood
• Adulthood
• Comorbid with illness
Age and Risk Characterization
• Younger individuals are more sensitive to toxic
substances than older individuals.
• The difference can be primarily attributed to
differences in metabolism; specially to differences in
the functioning of the liver.
• Lead absorption rate is four to five times greater in
young individuals than in adults
• Cadmium absorption rate is 20 times greater than in
adults.
Risk Characterization of Special Populations
• Electrophilic alkylating agents are often very toxic, due to their ability to
alkylate DNA; alkylated DNA does not uncoil properly.
• Cholestasis (e.g., impaired bile formation and flow which can be due to
several factors including disruption of canicular contractility by chemicals
such as cyclosporin)
• Cancer
Biomarkers of Hepatic Dysfunction:
• Aspartate aminotransferase (AST) increased by necrosis of
hepatocytes
• Alanine aminotransferase (ALT) increased by necrosis of
hepatocytes
• Bilirubin (product of hemoglobin degradation; total (indirect)
and conjugated (direct)
• Alkaline phosphatase (dephosphorylation of compounds.
• Hy’s Law (Dr. Hyman Zimmerman):
– The drug causes hepatocellular injury with jaundice, generally defined as an
elevated ALT and AST by 3-fold or greater above the upper limit of
normal.
– Among subjects showing such aminotransferase elevations, they also have
elevation of their serum total bilirubin of greater than 2× the upper
limit of normal, without findings of impaired bile formation (defined as
serum alkaline phosphatase activity less than 2× the upper limit of normal).
In vitro Chromosome Aberration Assay
• Identifies clastogenic agents (clastogens) which cause large scale structural damage
(chromosome/chromatid aberrations) to chromosomes leading to leading to
sections of the chromosome being deleted, added, or rearranged.
• Clastogenic agents interfere with cell cycle and mitotic spindle apparatus function.
• Primary cells (human peripheral blood lymphocytes) or cell lines (Chinese Hamster
Ovary cells a.k.a. CHO) are used
• Assay has good sensitivity but poor specificity (positive for non-carcinogens)
• Micronucleus is the name given to the small nucleus that forms whenever a
chromosome or a fragment of a chromosome is not incorporated into one of the
daughter nuclei during cell division.
• Approximately 24 hours after the last dose, bone marrow or peripheral blood is
collected to determine the frequency of micronucleated polychromatic
erythrocytes (MN-PCEs) or micronucleated reticulocytes (MN-RETs),
respectively.
• The mouse lymphoma TK assay (MLA) is part of an in vitro battery of tests designed to
predict risk assessment prior to in vivo testing.
– L5178Y mouse lymphoma tk ( +/- ) cells
– Measuring resistance to the lethal nucleoside analogue triflurothymidine (TFT).
– Detects mutagenic and clastogenic events induced by TFT
• The test has the potential to detect mutagenic and clastogenic events at the thymidine
kinase (tk) locus of L5178Y mouse lymphoma tk ( +/- ) cells by measuring resistance to
the lethal nucleoside analogue triflurothymidine (TFT).
• When added to selective medium containing TFT, wild-type tk ( +/- ) cells die, but TFT
cannot be incorporated into the DNA of mutant tk ( -/- ) cells, which survive
– Large colonies are indicative of gene mutations.
– Small colonies are indicative of large deletions (chromosomal mutations).
– Dose response mutant frequency is expressed as the number of mutants per 10(6) viable
cells.
Assessment of Carcinogenic Risk
• Inhibition of acetylcholinesterase
– Organophosphates
– Carbamates
• Inhibition of mitochondrial electron transport
chain function
– Paraquat
– Rotenone
• Prevent the closure of sodium channels
– Organochlorines (Lindane, dieldrin, DDT)
– Pyrethriods
Classes of Insecticides
and their Toxicity
• DDT (dichlorodiphenyltrichloroethane)
• Methoxychlor
• Lindane
• Aldrin
• Dieldrin
• Endosulfan
Organophosphates
T2
Hydrogen Sulfide - H2S
• Cellular hypoxia:
– Inhibits mitochondrial cytochrome-c oxidase
– Leads to histotoxic hypoxia and metabolic acidosis
Clinical Effects
Concentration
Effect
(ppm)
0.13 - 30 rotten egg odor
30 odor becomes sickeningly sweet
100-150 nose and eye irritation, gas eye
150 olfactory nerve paralysis
keratocojunctivitis, coughing, sore
250-500 throat, chest tightness, pulmonary
edema
headache, loss of consciousness,
500-1000
coma, convulsions
> 1000 Respiratory paralysis, death
Toxic Effects of Metal
Compounds
• Environmental, occupational, medical
exposures
• Risk of toxicity dependent on:
– molecular form of metal
– exposure route
– duration and dose
– host genotype and phenotype
Carcinogenic Metals
Inorganic Arsenic Skin, lung
methylated form
Beryllium Lung
Chromium Lung
hexavalent form
Nickel Nasal, laryngeal,
subsulfide lung
Mees Lines
• Acute exposure
– Abdominal cramps
– Constipation
– Nausea and vomiting
• Chronic exposure
– Alopecia
– Hyperkeratosis
– Neuropathy
Lead Toxicology
1976-1980
1988-1994
% Childhood Blood Pb > 10 ug/dL
by Ethnicity and Survey Period
Change in Flint MI Water Source and
Increase in Elevated Blood Pb
• Use of Hg in research
– See case report on fatality
https://en.wikipedia.org/wiki/Karen_Wetterhahn