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Summary Molecular and Cellular Toxicology (XM_0113)
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This document consists of a summary of the course Molecular and Cellular Toxicology, corresponding to the Master Drug Discovery Sciences.
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Molecular and Cellular Toxicology, Daniëlle BandLecture 1. Introduction
An other word for toxic is “poison”. This is a substance that is capable of causing the illness of death of
a living organism when introduced or absorbed. Paracelsus is the godfather of toxicology. He stated
that “everything is toxic (nothing is without toxicity) and the dose alone makes the poison”. The dose is
a concentration that can be taken over time.
There are different sources of toxins: (1) natural, like radiation, metals, or light, (2) biological, like
bacteria, plants, fungus, or animals, and (3) synthetic, like industrial waste products, manufacturing,
cosmetic, or drugs and its metabolites.
Besides the different sources of toxins, there are also different exposure routs: (1) inhalation, via the
lung, mouth or oesophagus, (2) dermal, via the skin, (3) ingestion, via the stomach and intestine, (4)
injection (intravenous or intraperitoneal), via all internal organs.
The safety (therapeutic) window is the concentration range
between efficacious response and toxic responses. This
therapeutic window is shown in a pharmacokinetic curve.
This is the description of the concentration of a compound
in plasma (or serum or whole blood) with respect to time,
based on a limited number of plasma samples—as a
measure for exposure—within a toxicologic study. In other
words, what the body does with a drug. The aim of such a
curve is to describe systemic exposure in animals and its
relationship to dose level and the time course of toxicity. A
pharmacokinetic curve can include a curve of one single dose, or of multiple doses. Various parameters
can be obtained including the AUC, 𝐶𝑚𝑎𝑥 , 𝑇𝑚𝑎𝑥 , F, and 𝑡1/2.
The benefit (efficacy) of taking a drug needs to be stronger or as strong as the risk (toxicity) of taking
that drug. You also need to look at how many patients are effected and how severe the effect is
(incidence vs severity). If more people are saved than died the benefit is still higher than the risk.
Drug discovery is costly and slow. It takes approximately 15
years and 1 billion euros to get a compound on the market.
The high rate of lead compound attrition is due to (1)
appearance of (unpredicted) toxicity at any stage, and (2) lack
of (sustained) efficacy, especially in clinical phase (II and III).
Thalidomide
Thalidomide is developed by the a German pharmaceutical company and was released in 1957 as a
sedative. It was widely used to prevent off label morning sickness. In the late 1950s and early 1960s,
more than 10 000 children in 46 countries were born with deformities. Several countries never approved
the drug, or restricted its used and it was withdrawn in 1961.
Thalidomide is a derivate of glutamate. It is lipid soluble which causes it to readily enters the placenta.
The mode of teratogenic effects are not fully elucidated, but it likely involves oxidative stress, protein
induction and wnt signaling antagonism.
The Kefauver-Harris Drug Amendments Act in 1962, legislators tightened restrictions surrounding the
surveillance and approval process for drugs to be sold in the U.S. For any drug that might be taken by a
pregnant woman, there will be emphasis on testing in pregnant animals of several species (Reproductive
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Toxicology). This is because Thalidomide showed to be a potent teratogen in zebrafish, chickens, rabbits
and primates, but not in mice and rats.
Thalidomide was reintroduced for therapy for a leprosy complication (erythema nodosum leprosum) in
the 1970s. The World Health Organization (WHO) does not recommend the use of thalidomide in
leprosy. The is because experience has shown that it is virtually impossible to develop and implement a
fool-proof surveillance mechanism to combat misuse of the drug. Today, a number of thalidomide
babies continue to be born each year reflecting regulatory insufficiency and widespread use under
inadequate supervision.
Measurement of toxicity
Toxicity can be measured with the LD50 (Lethal Dose 50) test.
This is a test to determine the single dose of a chemical that
would kill half of the animals exposed to it. The LD50 test is
performed at different doses. If all animals are dead at LD100
and you can calculate the LD50. No Observable Adverse Effect
Level (NOAEL) and Minimal Lethal Dose (MLD) can also be
observed from the graph same graph. The comparative toxicity index is used to compared the relative
toxicity, based on the LD50, of compounds for the first time. This gives instant appeal to government
regulators, so much so that variants of the LD50 remain the most prevalent animal tests even to this
day.
Pro’s of LD50 Cons of LD50
o Comparative index to compare toxic level of o In itself no other toxicity information
compounds o Sometimes stupid (when not toxic)
o Can compare species sensitivity o Cruel (animals suffer literally to
o Acquire an NOAEL and minimum lethal level death)
Toxicity tests are designed to generate data concerning the adverse effects of a substance on human or
animal health, or the environment. It is based on specific endpoints, like eye irritation, skin corrosion
and cancer. Toxicity tests are general tests where severe ill health or death are used over certain
exposure times to generate a NOAEL (i.e. less than minimal observed adverse affect level). There are a
lot of toxicity tests like reproductive toxicity, neurotoxicity, genetic toxicity, acute toxicity, etc.
The Organization for Economic Co-operative and Development (OECD) is an intergovernmental
organization with 38 member countries, founded in 1961 to stimulate economic progress and world
trade. A forum and its members are countries which describe themselves as committed to democracy
and the market economy, providing a platform to compare policy experiences, seek answers to common
problems, identify good practices and coordinate domestic and international policies of its members.
Acute oral toxicity
Acute oral toxicity testing is the bases for hazard classification and labeling of chemicals information for
comparison of toxicity and dose-response among chemicals. It may also provide information about the
mode of toxic action of a substance and is used to standardize biological products and can serve to
establish dosing levels for repeated dose studies. Acute oral toxicity testing is based on the median
lethal dose (LD50) value and is mostly performed in rats. Compounds that are used for these tests must
be absorbed by the body and distributed by the circulation to sites in the body where it or its metabolites
can exert toxic effects. Usually a singe dose of a substance is administered and the animals are observed
up to 14 days after.
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Chronic tests
For chronic tests there are different OECD guidelines that mainly include repeated doses for a certain
amount of days. The typical information that is collected is ADME data, body weight, hormones, etc.
Reproductive/Developmental toxicity
Reproductive and developmental toxicity tests are animal tests that evaluate the effects of prenatal
exposure on pregnant animals and their offspring. It is usually performed with female rats and rabbits.
The test substance is administered orally, the pregnant animals are killed just prior to delivery, and the
fetuses are examined for toxic effects. There are different types of reproductive toxicity tests. (1) A one-
generation reproduction toxicity study in rats or mice is used to evaluate toxic effects on male and
female reproduction. Males and females are dosed orally before mating, and females during pregnancy.
The F1 generation is then also dosed to adulthood. (2) A two-generation reproduction toxicity study
continues dosing with the test substance to the second generation offspring. The assays that are
performed in reproductive toxicity studies are weighing, sperm parameter, oestrus cycle parameters,
and offspring parameters. A properly conducted reproductive toxicity test should provide a satisfactory
estimation of a no-effect level and an understanding of adverse effects on reproduction, parturition,
lactation, postnatal development including growth and sexual development.
Modalities: types of toxicity, levels, molecular underpinning and biomarkers
There are different levels of mortality: (1) individual level, (2) organ level, (3) cellular and subcellular
level, and (4) molecular level. The combination of a cell and a molecular substance leads to toxicity inside
a cell. This shows that a molecular compound needs to interact with cellular compounds to cause
toxicity.
The exposure to a certain substance can lead to a
dose of that substance in the tissue and a biological
interaction. Under depletion of the innate defense
mechanism this causes cellular perturbation and an
adaptive response (transcriptional reprogramming).
This can lead to morbidity and mortality, or to a
stressed phenotype and sub-optimal function
(allostasis = achieving stability through change).
Nuclear receptors are a large family of 48 members, responsible for sensing (e.g. steroid and thyroid
hormones). They regulate the expression of specific genes involved in development, homeostasis, and
nutrient metabolism. These receptors can form homo/heterodimers with a ligand and DNA binding
domains. Nuclear receptors are very important in xenobiotic-metabolism, toxicity (e.g. oxidative stress),
transport, and endocrine disruption.
Toxicology is critical for understanding the biology and molecular pathways. By blocking a specific
pathway one can understand its use much better. Most drugs have been released and are used without
the knowledge of how they work.
A method to assess the hazard of a chemical that does not use whole animals can be in vitro and/or in
silico approaches. This follows the reductionist approach in which higher order systems are removed. In
vitro approaches can use cell lines, primary cells, or Induced Pluripotent Stem Cells (IPSC). IPSCs are a
type of pluripotent stem cell derived from adult somatic cells. They have been reprogrammed through
inducing genes and factors to be pluripotent. iPSCs are similar to embryonic stem (ES) cells in many
aspects.
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Lecture 2. General introduction Signal Transduction, Ubiquitination
and Transporters
Cells tell us where it hurts, and this is signal transduction.
They register if something is not comfortable and they
adapt to it. Basic events mechanism in cells: (1) first line
defense (drug metabolism: basal expression of genes),
(2) second line defense (antioxidant response: induction
of genes), (3) third line defense (apoptosis: suicide of the
cell). Afterwards it will lead to necrosis which is
something you want to avoid.
There are sensor proteins that can detect that something is wrong. They
affect certain effector proteins that lead to a certain stress response.
Based on what is wrong, the cell choses a certain pathway. The net
effect is eventually cell survival or cell death. There are tools that can be
used to investigate the pathways that are activated after certain types
of cell stress. When tagging proteins with a fluorescent label, the
differences in protein levels can be investigated. When, for example, in
the case of oxidative stress, a GFP is hooked to a specific gene and when
the GFP turns green they know that this gene is expressed and that oxidative stress has taken place.
Signal transduction
Reactive Oxygen Species (ROS) is induced by
extracellular influences (e.g. chemotherapy, UV,
radiation), or intracellular influences (e.g. peroxidase,
catalase, glutathione, SOD). This results in the activation of various transcription factors that are
modulated by ROS.
Signal transduction starts with an extracellular molecule that binds to
a receptor. This leads to activation of a cascade of cellular signaling
proteins which activates various effector proteins. These effector
proteins can cause an altered metabolism, altered gene expression or
altered cell shape or movement. This overall leads to an altered cell
behavior. In cell signaling there three layers of signaling: (1)
sensors/reception (receptors on the membrane or in the cytosol
activated by signal molecules), (2) transducers (signal transduction
pathway) and (3) effectors/response (activation of cellular response).
Drugs may activate, inhibit or disrupt the transfer of a signal at different points withing the transduction
pathway. The consequences depend on the nature of the drug and its target and the cellular
composition of the cell and the growth conditions (e.g. serum). Standard operating conditions for
experiments are essential. This is because cells might interact with each other and the way they do this
will change the buildup of the cell, the presence of certain receptors, the downstream signaling. How
the cells are cultured are very important, and a difference in how cells are cultured will change the
response to a drug or chemical.
Whether a signal can be send into the nucleus to generate a response depends on the availability of
molecular components (e.g. receptors, kinases) forming the transduction pathway and the effectors
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