Developmental Toxicology

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Developmental Toxicology

• Organ Systems Toxicology
• Dr. Mary Alice Smith

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Developmental Toxicology

• any chemical administered under appropriate
  conditions of dose and time of development can
  cause some disturbances in embryonic development
  in some laboratory species. Karnofsky, 1965

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• Congenital malformations are the leading cause of
  infant mortality in the US

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Developmental Toxicology

• Includes study of pharmacokinetics, mechanisms,
  pathogenesis, and outcome after exposure to
  agents or conditions that lead to abnormal
  development
• Teratology - the study of birth defects (some
  limit to structural)

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Human birth defects

• Less than 1/2 of conceptions result in birth of a
  normal healthy infant
• postimplantation pregnancy loss 31
• major birth defects at birth 2-3
• major birth defects recognized at 1 yr 6-7
• minor birth defects 14
• low birth weight 7
• infant mortality (before 1 yr) 1.4
• abnormal neurologic function 16-17

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Human Developmental ToxicantsClasses and examples

• Radiation - therapeutic atomic fallout
• Infections - rubella syphilis, L. monocytogenes
• Maternal - alcoholism folic acid deficiency
• Drugs/Chemicals - cigarette smoke DES metals
  thalidomide anticancer drugs

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Developmental Toxicology

• Cell proliferation - division and growth
• Induction growth factors (NGF, EGF, etc)
• Differentiation endoderm, mesoderm, ectoderm
• Migration neural plate, notochord
• Cell growth - organs
• Cell death- form structures
• Final Structure normal or abnormal
• After Chernoff and Jones, 1983

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Wilsons General Principles of Teratology

• Susceptibility depends on genotype of conceptus
  and how it interacts with environment.
• susceptibility varies with developmental stage at
  time of exposure.
• teratogenic agents act by specific mechanisms to
  initiate pathogenesis.
• Deviant development includes death, malformation,
  growth retardation and functional deficit.
• Defects increase in frequency and degree as dose
  increases.

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Teratogenic Exposure and Genotype

• Interplay between environmental and genetic
  factors
• Socioeconomic, geographic, ethnic and maternal
  health
• Genotype of fetus (ex - strains differences in
  mice and rats)

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Critical periods of sensitivity

• Gametogenesis - forming haploid germ cells
  vulnerable to toxicants
• preimplantation period - increase in cell number
  somewhat resistant, probably results in death of
  embryo
• gastrulation - formation of ectoderm, mesoderm
  and endoderm somewhat sensitive to toxicants

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Critical periods of sensitivity- cont

• organogenesis - formation of organs very
  sensitive to toxicants
• fetal period - growth and differentiation, all
  organs present but not fully developed sensitive
  endpoints are growth and functional maturation.

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Animal models for developmental toxicity studies

• Differences in gestational development in animal
  models.
• Is not just a matter of scale (ex 3rd
  trimester in humans does not necessarily compare
  to last 1/3 of pregnancy for mice).
• Choice of animal model and time of treatment is
  very important.

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Teratogenic doses of valproic acid in several
species
Not detected
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Dose response threshold concept

• Major effects at time of birth are lethality,
  malformations and growth retardation.
• May represent continuum of increasing toxicity.
• Assume threshold because of growth potential,
  cellular homeostatic mechanisms and maternal
  metabolic defenses

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Mechanisms of Developmental Toxicity

• Cell death - programmed cell death or apoptosis
  necrosis
• proliferation
• damage to DNA during cell cycle

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Apoptosis

• gene-directed cell death
• Occurs in both normal and abnormal development

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Apoptosis

• Counterbalance to mitosis
• Helps give shape
• Cells die on schedule
• Ex digits versus webbed

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Apoptosis

• Induction of apoptosis
• Oxidative stress
• Hepatotoxins often produce both necrosis and
  apoptosis
• Ex retinoid acid (Vit A)
• Measurement of apoptosis

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Measurement of apoptosis

• Light microscopy and histological staining
• DNA cleavage DNA ladder patterns, PCR
  amplification
• Flow cytometry
• immunohistochemistry

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From Casarett Doulls Toxicology, 5th ed
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Methods for studying mechanisms

• Targeted gene disruption - gene knockout.
• genetic constructs with inducible promoter
  attached to gene of interest.
• Microarrays

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Maternal and developmental toxicity

• Maternal toxicity can result in birth defect
• genetics
• disease
• nutrition
• stress
• placental toxicity

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CASE STUDY THALIDOMIDE

• Introduced in 1956 as sedative/hypnotic used as
  sleep aid and for nausea and vomiting in
  pregnancy.
• Oral dose 50 to 200 mg/day as much as 14 g had
  been taken with suicidal intent and patients had
  recovered.
• (cont)

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Cont

• 1949-1959 no cases of phocomelia were seen at
  University Pediatric Clinic in Hamburg.
• 1959 1 case reported.
• 1960 30 cases reported.
• 1961 154 cases reported.
• (cont)

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Thalidomide Victims of Canada

• http//www.thalidomide.ca/en/index.html

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Cont

• Affected children had amelia (absence of limbs)
  or phocomelia (preaxial reduction of long bones
  of limbs), usually affected arms more than legs
  both right and left sides although to different
  degrees.
• 1961 Lenz in Germany and McBride in Australia
  identified thalidomide as agent.
• (cont)

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Cont

• 1961 drug withdrawn by August 1962 epidemic
  subsided.
• Never approved for use by FDA and US not
  affected.
• 10,000 infants were deformed by thalidomide.
• (cont)

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Cont

• Congenital abnormalities could not be produced in
  rats originally, but certain strains of white
  rabbits were sensitive eventually a thalidomide
  induced syndrome was produced in monkeys.
• Critical period of limb malformations found to be
  the 6th and 7th weeks of pregnancy.
• (cont)

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Cont

• 70 of mothers with affected children had taken
  thalidomide during the first 3 months of
  pregnancy.

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CASE STUDY SYNTHETIC RETINOIDS ACCUTANE

• Used to treat cystic acne
• Of 57 pregnant women exposed during 1st
  trimester, there were
• 9 spontaneous abortions
• 1 still birth
• 10 malformed live
• 37 no evidence of malformation
• (cont)

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Cont

• Rats and mice need an order of magnitude higher
  dose to produce effects
• Despite warning still have pregnancies with women
  on Accutane. Should drug be withdrawn?

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• Daston (1993) summarized two approaches for
  establishing the existence of a threshold. The
  first, which is exemplified by a large teratology
  study of 2,4,5-T (Nelson and Holson, 1978),
  suggest that no study is capable of evaluating
  the dose response at low response rates (805
  litters per dose would be necessary to detect the
  relatively high rate of a 5 percent increase in
  resorptions).

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Tests for Developmental Toxicants

• Both animal tests and epidemiology are necessary
  to protect public health.
• Animal tests identify the no observed adverse
  effect level (NOAEL)
• NOAEL highest dose that does not produce a
  significant increase in adverse effects.
• Multigenerational tests animals exposed to test
  agent over one or more generations

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Tests for Developmental Toxicants

• Childrens health
• Diets are different E. sakazakii in infant
  formula
• Hand to mouth activity
• Neurobehavioral effects
• Cancer