Toxicology 3. toxicokinetics

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Toxicology 3.toxicokinetics

• Biotransformation
• Hepatic-first pass metabolismXenobiotics
  absorbed from the gastrointestinal tract
• Exceptions Mouth (Nitroglycerine, weak base, to
  be put under the tongue), Colon
• Extrahepatic metabolism
• Epithelium of the GI tract
• Skin
• lungs

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bioavailability

• Ratio of the dose reaching the systemic
  circulation (0-1)
• Bioavailability depends on oral absorption and
  the first pass metabolism.
• Concentration of the molecule in the blood after
  oral administration/ Concentration of the
  molecule in the blood after intravenous
  administration

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DistributionBinding to plasma proteins

• Plasma protein binding helps the distribution of
  a xenobiotic and prevents its excretion
• Reversible/irreversible
• affinity, Kd concentration of free xenobiotic
  x concentration of free binding site/
  concentration of occupied binding sites

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Species differences in binding to plasma proteins

• Thyroxine is binding to plasma proteins in
  humans. 2/3 of thyroxine molecules are present in
  the form of thyroglobulin, the remaining
  molecules bind to albumins or prealbumins.
• In rats thyroxine is dissolved in the blood
  without binding to proteins.
• Some xenobiotics induce metabolic enzymes and
  accelerate the elimination of thyroxine.
• In rats as the level of thyroxine is reduced,
  the thyroid gland tries to produce more thyroxine
  by cell proliferation. This can lead to tumours
  in the thyroid.
• In humans the same xenobiotic will not cause
  thyroid tumours, because of the protein binding
  of the thyroxine.

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EliminationExretion

• Biotransformation
• Ecretion via urine
• Excretion via the bile
• Excretion via the lungs
• Ecretion with mothers milk, placenta, hair,
  saliva, tearsetc,

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Ultra-filtration, passive re-absorption, active
tubular secretion Table Molecular mass and the
route of some biphenyls in rats
Nefron
Route of excretion() Route of excretion()
compound Molecular mass kidney Faeces
Biphenyl 154 80 20
4-monochloro-biphenyl 188 50 50
4,4-dichloro-biphenyl 223 34 66
2,4,5,2,5-pentachloro-biphenyl 326 11 89
2,3,6,2,3,6-hexachloro-biphenyl 361 1 99
Reference H. B. Mattheus in Introduction to
Biochemical Toxicology (1960)
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Enterohepatic circulation
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Effects depend on

• Dose
• Time period of dosing
• Other molecules present (induction, inhibitionj)
• Graded response- measured on a continouos scale
• Quantal response measured by counting responders
  in a group

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Receptor types

• Intracellular receptors
• Cell surface receptors
• Receptors with enzyme activity
• Receptors leading to a chain of reactions
• Receptors triggering a secondary messenger
• Ion chanels

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Intracellular receptor
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Tyrosine kinase a transmembrane receptor having
enzyme activity
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Receptor with enzyme activity triggering a chain
reaction
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G-protein coupled receptor
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Ion chanel receptor
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ToxicodynamicsEthanol

• Absorption passive diffusion from the whole
  length of the GI tract Pow 0,4898 (logPow
  -0,31)

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Metabolism of ethanol

• Ethanol
• Alcohol dehydrogenase (SER, MFO)
• Acetaldehyde
• Aldehyde dehydrogenase (2 isoforms, in cytosol or
  mitochondria)
• Acetic acid
• ACSS2 enzyme (Acetyl-coenzymeA synthetase S2,
  cytosol)
• Acetyl-coenzyme A
• Enzymes of the citrate cycle
• 3 H2O 2 CO2
• 
  Energy approximately 1300 kJ/mol

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Elimination of ethanol and its metabolites

• Ethanol urine, exhalation, sweat
• Acetic acid- urine
• Acetyl-coenzyme A- used for biosynthetic
  processes, biotransformation, like acetilation or
  as an energy source in the citrate cycle

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The elimination rate of ethanol

• Rate limiting step oxydation to acetaldehyde
• Reaction of 0 grade (KM 80 mg/l, but much higher
  plasma concentration is frequent)
• Elimination rate 10 g ethanol/hour, the
  elimination of ½ l wine takes 7 hours

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Acute effects of ethanol

• 
  g/l

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For later effects (hangover, intoxication
effects) mainly the acetaldehyde is responsible
and the free radicals it generates. Free radicals
cause oxidative stress and cell death.

• Two forms of aldehyde dehydrogenase are present
  in the cytosol or in mitochondria In the white
  population both forms are active while in 50 of
  Asiatic people the mitochondial enzyme is missing
  or has a very low activity.

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Effects of chronic ethanol exposure free
radicals cell damage

• Liver is the main target organ
• Chronic hepatitis
• Fatty liver (5-50 lipid content )
• Livercirrhosis
• Liver tumours

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Further chronic effects

• Pancreas
• Pacreatitis
• Tumours
• Heart
• The performance of the heart is reduced (Chronic
  cardiomyopathy)
• Nervous system
• Tremors, impairment of the sight and the memory
  willpower, impaired judgment, emotional lability,
  outbursts of anger

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Effects on the development

• Low birth weightSmall head circumferencenervous
  system disordersabnormalities of the
  hippocampus learning difficultiesSmall
  cerebellum motion developmental disordersSmall
  corpus callosum, hyperactivity, impulsive behavior

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Some characteristics of the fetal alcohol syndrome
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Dose dependent fetal effects

• Strong drinking teratogenic effects
• Moderate drinkingneurotoxic effects
• (fetal NOAEL can not be established)

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Summary of the chronic effects of ethanol

• Toxic for several organs
• Carcinogenic (I.A.R.C. Cat1A, )
• Neurotoxic
• Teratogenic

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Causes of carcinogenesis

• Physical (ionizing radiation)Chemical (genotoxic
  and non-genotoxic carcinogens)Biological
  (viruses, bacteria, endoparasitic insects, etc.)

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Steps of chemical carcinogenesisStep 1 Initiation

• Highly electrophilic structures, free radicals,
  organic cations bind covalently to special
  nucleophilic binding sites of the DNA and form
  DNA-adducts
• (some critical sites Guanine C8, N2, N3, O6,
  Timidine, Uracil O2, O4, N3 position).

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Step 2 Fixation

• If repair enzymes cut off the altered parts, then
  DNA polymerase synthetizes the missing part on
  the basis of the complementer strand ---the cell
  remains normal
• If the mutation takes place in an inactive part
  of the DNA, this does not change the
  functions-the--cell remains normal
• The mutation damages seriously the functions of
  the cell ---the cell dies, the organism remains
  healthy
• if the altered cell survives and divides-the
  mutation can be fixed.

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Capacity of DNA repair in some organs

• Liver good
• Kidney intermediate
• Brain practically missing

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Step 3 Promotion selective increase of the
iniciated cells helped by a repeated effect of a
promoter within a critical period of time

• Not a genetic effect
• The result is a microscopic neoplasia
• The promoter effect is dose dependent but there
  is a limit dose

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Some promoters

• Mitogens substances stimulating cell
  proliferation (endogenous mitogens, like
  estrogens or other hormones or exogenous
  mitogens.)
• Permanent presence of cytotoxic substances
• Persisting mechanical effects (irritation)
  causing cytotoxicity
• Blood loss in rats is promoting leukemia

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Inhibition of the carcinogenic process

• Inhibition of cell division
• Immune system (macrophags, limphokins)
• Reduced total energy intake
• Specific dietary components Vitamines A, C and E

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Step 4 Progression

• For further increase of iniciated cells often a
  new mutation is needed. New initiated cell types
  have to appear which produce angiogenesis factor,
  helping vascularisation of the micro-tumour
• Blood vessels develop in the harmless microtumour
  ( max.0,5 mm3) and the small tumour starts to
  grow at an exponential rate.

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Step 5 Metastases

• The tumour becomes invasive, the cancer cells
  disseminate through blood and lymph vessels and
  new, secunder tumours develop at different parts
  of the body.

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Tumour therapy

• Surgical therapy
• Radiotherapy
• Drug therapy
• Komplex therapy

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Types of anti-cancer drugs

• Drugs inhibiting DNA duplication
• Antimetabolites (Enzyme inhibitors, nucleic acid
  synthesis inhibitors methotrexate, antifolates,
  pirimidin antagonists, dezoxycyitidine analogs,
  purine antagonists
• DNA alkylating agents cyclophosphamide (they
  prevent cell proliferation, but might have
  serious effects)
• Topoizomerase I és II inhibitors ( they inhibit
  the integration of DNA chains)

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Agents that affect the regulation of cell
proliferation

• Mitotic spindle inhibitors Vinca alkaloids,
  taxanes, vinblastine, vinchristine, taxol,
  taxotere (inhibiting the development of the
  mitotic spindle)
• Hormones and hormone-like compounds, like
  progesterones, anti-estrogens, aromatase
  inhibitors, anti-androgens, which inhibit the
  growth of hormone dependent tumours.
• Cytokins (interferons, interleukins) inhance
  the immune response of the host
• Tyrosine kinase inhibitors they reduce the
  viability of tumour cells by inhibiting signal
  transduction
• Monoclonal antibodies they inactivate cell
  surface receptors

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3

• Drugs inhibiting vascularisation
• Avastine, thalidomide
• Anti-metastasis agents
• Bisphosphanates prevent stone metastasis
  from breast and prostate tumours.

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Drugs to improve the quality of life of the
patient

• Filgrastine. Enhances the production of white
  blood cells
• Erithropoetine alpha helps the proliferation of
  erithrocytes
• Mesna, Amifostine neutralise the reactive groups
  of drugs in the healthy tissues.

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Literature recommended

• Niesink et al Toxicology, Principles and
  applications (1996.) CRC Press, LLC and Open
  University of the NetherlandsISBN 0-8493-9232-2
• Gyires Klára, Fürst Zsuzsanna Farmakológia
  (2007.) Medicina Könyvkiadó Rt., BudapestISBN
  978 963 226 137 9 (I kötet)