Toxicology 3.
toxicokinetics

Biotransformation:
Hepatic-first pass metabolism
Xenobiotics 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
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
Distribution:
Binding 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
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.

Elimination
Exretion





Biotransformation
Ecretion via urine
Excretion via the bile
Excretion via the lungs
Ecretion with mother’s milk, placenta, hair, saliva,
tears…etc,
Ultra-filtration, passive reabsorption, active tubular
secretion
Nefron
Table: Molecular mass and the route
of some biphenyls in rats
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)
Enterohepatic circulation
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
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
Intracellular receptor
Tyrosine kinase: a transmembrane receptor having enzyme activity
Receptor with enzyme activity triggering a chain reaction
G-protein coupled receptor
Ion chanel receptor
Toxicodynamics
Ethanol

Absorption: passive diffusion from
the whole length of the GI tract
Pow= 0,4898 (logPow= -0,31)
Metabolism of ethanol

Ethanol


Acetaldehyde


Alcohol dehydrogenase (SER, MFO)
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
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
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
Acute effects of ethanol

g/l
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.
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
Further chronic effects

Pancreas



Heart


Pacreatitis
Tumours
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…
Effects on the development

Low birth weight
Small head circumference
nervous system disorders
abnormalities of the hippocampus: learning
difficulties
Small cerebellum: motion developmental
disorders
Small corpus callosum, hyperactivity, impulsive
behavior
Some characteristics of the fetal
alcohol syndrome
Dose dependent fetal effects:
Strong drinking: teratogenic effects
Moderate drinking:neurotoxic effects

(fetal NOAEL can not be established)
Summary of the chronic effects of
ethanol




Toxic for several organs
Carcinogenic (I.A.R.C. Cat1A, )
Neurotoxic
Teratogenic
Causes of carcinogenesis

Physical (ionizing radiation)
Chemical (genotoxic and non-genotoxic
carcinogens)
Biological (viruses, bacteria, endoparasitic
insects, etc.)
Steps of chemical carcinogenesis
Step 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).
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.
Capacity of DNA repair in some organs



Liver: good
Kidney: intermediate
Brain: practically missing
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
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
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
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 microtumour
Blood vessels develop in the harmless
microtumour ( max.0,5 mm3) and the small
tumour starts to grow at an exponential rate.
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.
Tumour therapy




Surgical therapy
Radiotherapy
Drug therapy
Komplex therapy
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)
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, antiandrogens, 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
3

Drugs inhibiting vascularisation
Avastine, thalidomide

Anti-metastasis agents
Bisphosphanates: prevent stone metastasis from
breast and prostate tumours.
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.
Literature recommended
Niesink et al: Toxicology, Principles and
applications (1996.) CRC Press, LLC and Open
University of the Netherlands
ISBN: 0-8493-9232-2
Gyires Klára, Fürst Zsuzsanna: Farmakológia
(2007.) Medicina Könyvkiadó Rt., Budapest
ISBN: 978 963 226 137 9 (I kötet)