Principles of Toxicology Applications to Carcinogenesis

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Principles of Toxicology - Applications to
Carcinogenesis

• Mary Ann Smith, Ph.D.

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The Toxicological Paradigm
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Fate of Toxic Substances
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Sequence of Events Lead to Toxicity

• Toxicity results from a sequence of events that
  can be characterized mechanistically
• In carcinogenesis this sequence is the
  initiation, promotion, progression pathway

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The ADME Paradigm is one Framework for the
Sequence
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Factors Affecting Occurrence of Toxic Response

• Chemical physical properties of the agent.
• Exposure situation.
• Metabolism by the system
• Susceptibility of the biologic system

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Route Site of Exposure

• Major routes
• GI tract (ingestion)
• Lungs (inhalation)
• Skin (topical, percutaneous, dermal)
• Parenteral (iv, ip)
• Onset of response
• Ivgtinhalation gtipgtscgtimgt intradermalgtpogtdermal

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Factors to consider

• Vehicle
• Concentration of agent in vehicle
• Total volume of vehicle
• Properties of vehicle
• Rate of exposure

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Duration Frequency of Exposure

• Acute exposures
• Repeated exposures
• Most relevant for cancer studies

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Frequency of exposure can affect range of toxic
effects
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Chronic Toxic Effects

• Accumulation
• Irreversible interactions
• Recovery interval vs exposure frequency

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Dose-Response

• General Issues
• Individual vs population
• Whole animal vs target organ
• Multiple sites of action / mechanisms
• Shape of dose-response curve
• Essential vs non-nutritive nutrients
• Concept of threshold

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Dose-Response
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Assumptions in Dose-Response

• Response related to agent administered
• Magnitude of response related to dose
• Molecular or receptor site for interaction
• Response degree of response related to
  concentration _at_ receptor site
• Concentration _at_ receptor site related to dose

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Quantifiable method of measuring precision in
expressing toxicity

• Choosing appropriate endpoints
• Relevant to toxic effect
• Not always easy

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LD50

• Statistically derived single dose of a substance
  that is expected to cause death in 50 of animals
  tested
• Not a biological constant
• Affected by many variables
• Can gain useful information

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Dose-Response
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Variation in Toxic Responses

• Selective Toxicity
• Utilized in agriculture, chemotherapy
• Capitalize on differences in structure,
  biochemistry, physiology, metabolism
• Species Differences
• Useful in defining mechanisms
• Relevance of animal to human data
• Transgenics
• Individual Differences
• Genetic polymorphisms

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Purpose of Biotransformation

• Mechanism to convert compounds from a chemical
  structure that favors absorption to one that
  favors elimination (lipophilic ? more
  hydrophilic)

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Consequences of Biotransformation

• Termination of effect
• Pharmacologic vs toxicologic
• Bioactivation
• Reactive intermediate formation

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Phase I Metabolism

• Introduction of a polar group that can be
  conjugated in Phase II reactions.
• Carried out by microsomal monooxygenations,
  cytosolic mitochondrial oxidations,
  cooxidations by prostaglandin synthetase rxns,
  reductions, hydrolyses, and epoxide hydration.
• Cytochrome P450- dependent reactions.

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Phase II Metabolism

• Conjugation reactions
• Glucuronidation, sulfation, methylation,
  glucosidation, acylation, glutathione
  conjugation.
• Further enhance the water solubility of the
  compound.
• In some instances conjugation reactions can lead
  to toxicity when the conjugate is cleaved in an
  organ such as the kidney.
• Conjugation reactions are saturable.

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Biotransformation in the Context of the
Multistage Model of Carcinogenesis

• Initiation
• Rapid, irreversible alteration in genetic
  material that primes cell for neoplastic
  development.
• Electrophilic compound or metabolically activated
  to electrophile
• Covalent binding of electrophile to DNA
• Cell may remain dormant until exposed to promoter

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Electrophilic Theory

• Elizabeth James Miller
• Hypothesized that carcinogens were metabolically
  activated to reactive electrophiles capable of
  binding to DNA
• Parent carcinogen
• Proximate carcinogen
• Ultimate carcinogen

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Electrophilic Theory

• Theory further refined
• If DNA adduct not repaired before daughter cell
  replicates, then mutation can be permanently
  fixed in daughter cell.
• Could represent initiating event

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PAHs and Biotransformation

• PAHs important class of compounds w/ respect to
  biotransformation and carcinogenesis

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PAHs
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Diol expoxide formation
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Diol expoxide formation
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Interactions of Electrophilic Intermediates with
DNA
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Reactive Electrophile Binding

• Nucleophilic oxygen and nitrogen atoms in DNA
  bases
• Proteins, peptides, RNA
• Water is the most abundant nucleophile
• Reactive oxygen species generation

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Nucleophilic Binding Sites
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Consequences of DNA / Xenobiotic Interaction
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Utility of Understanding Biotransformation in
Relation to Carcinogenesis

• Identify paths for injury
• Possibly alter pathways of metabolism
• Biomarkers
• Exposure
• Susceptibility
• Effect

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Factors impacting biotransformation

• Genetics
• Polymorphisms in biotransformation enzymes
• Hormonal influences
• Concomitant xenobiotic exposures
• Induction / inhibition of biotransformation
  enzymes
• General health of the individual
• Hepatic function / renal function