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Samenvatting Medical Pharmacology
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MEDICINES/DRUGSPharmacology is the explanation of the actions of pharmaca (= biologically active substances, including
toxins) in the human (or animal) body. Pharmacodynamics is about what the drug does with the body
and pharmacokinetics is about what the body does with the drug. Pharmacotherapy is the application of
drugs for treatment of diseased humans (or animals) in a responsible, effective and safe manner.
pharmacology is the study of what drugs do and how they do it. A drug is a
chemical that is usually used to treat diseases. Drugs are intended to have a
selective action, but ideal is seldom achieved. There is always a risk of adverse
effects as well as a benefit connected with using any drug. A knowledge of
pharmacology is essential for using drugs effectively in therapy.
The concentration of a certain drug in the blood plasma must be between the
minimum effective concentration and the minimum toxic concentration to have an effect. This is also
called the therapeutic window. The effects a certain drug can have are a primary effect and side effects.
The primary effect is the effect for which the compound is administered. The side effects are the
adverse/unwanted effects. The distinction between primary effect and side effect is determined by the
aim for which the medicine is administered.
A placebo is a preparation without any pharmacologically active substances. A nocebo is a placebo that
induces unwanted side effects. A placebo may have a therapeutic effect, which is called the placebo
effect. The expectations of the patient determine the strength of the placebo effect. The contribution of
the placebo effect to the overall effects of a drug are investigated in “randomized, placebo-controlled
clinical trials”. When taking a drug, changes are high to get nauseous. So, when taking a placebo, the
nocebo effect causes the patient to become nauseous, even though there are no active substances
involved.
PHARMACODYNAMICS
Pharmacodynamics is defined as what the drug does with the body. The action of the drug can be
determined on 4 different levels:
1. An effect on system function (system) affects integrated systems, including linked systems
(e.g. Nervous system, cardiovascular system)
2. An effect on tissue function (tissue) affects electrogenesis, contraction, secretion, metabolic
activity, proliferation
3. Transduction (cellular) affects the biochemicals linked to the drug target (e.g. Ion channel,
enzyme, g-proteins)
4. Interaction with the drug’s molecular target (molecular) affects the drug target (e.g. Receptor,
ion channel, enzyme, carrier molecule)
The main targets for drug action are receptors, ion channels, enzymes and transporters.
Receptors Enzymes
Ion channels Transporters
,Targets of drugs
Each drug target results in a different effect. For the receptor part, there are 4 different receptor families
on which the drug can act.
Ionotropic receptors Metabotropic receptors
Kinase-linked receptors Nuclear receptors
type 1 receptors are present in cell membranes and contain ion channels, depending on their subtypes.
So, despite of being a receptor, it is also an ion channel. By binding of drugs to the binding domain, the
activity status of the ion channels is immediately altered. Either they open, or they close. Type 2
receptors are also expressed inside membranes. Once drugs bind to the binding domains, g-protein
(cytoplasmic side) are directed towards the receptors and their activity is altered. By doing this, a signal is
transduced, using to alteration and activity of second messenger systems. Type 3 receptors are also
enzymes, that contain a catalytic domain (usually kinase for phosphorylation) as part of the protein
molecule. When a drug binds to the binding domain, the catalytic activity is altered. It can be either
stimulated or blocked. Type 4 receptors are located inside the cell nucleus. In inactive state, they are in
the cytosol and upon binding of a ligand, they move into the nucleus and interact with dna to modify
transcription.
A drug is an exogenous chemical compound that modifies the functioning of a physiological system in a
selective manner. Pharmacology reserves the term receptor to describe a protein that “recognizes” an
endogenous chemical compounds, thereby causing a cellular effect. Receptor proteins show a
characteristic selectivity. Different receptor proteins “recognize” only certain endogenous compounds
and drugs. A drug (usually) doesn’t have a specific effect. Depending on the dose, a drug may mediate
multiple effects via interaction with different (sub)types of receptor proteins.
, Dose-response relations
certain pharmacological characteristics can be concluded from the dose-
response curves that are made for each type of drug. From a certain
concentration on the x-axis, responses start to occur. When increasing
the concentration, the effect also increases until a certain maximum. For
example, the effect starts at a concentration of 3 x 10-7 m and the
maximum effect is reached at a concentration of 10-3 m. This means that
the 50% effect concentration (ec50) lies around 3 x 10-5 m. These
concentrations are logarithms, which can be reversed: - log 3 x 10-7 = + 7
– log 3 = 6.5. This is needed to calculate, for example, the affinity constant of a drug (pd2). The affinity of
a drug is the tendency to bind to receptors. The efficacy is the relationship between the receptor
occupancy and the ability to initiate a response at the molecular, cellular, tissue or system level. The
intrinsic activity is the capacity of a single drug-receptor complex to evoke a response. So, the intrinsic
activity is the affinity of the drug-receptor complex to activate a
signalling cascade.
There are 2 types of drugs, agonists and antagonists. The agonists
bind to receptors, leading to agonist-receptor complexes. The
agonist will occupy a certain number of receptors that is available
for binding. This occupation (number of receptors that is available
and bound by the agonist) is governed by the affinity of the agonist.
The agonist also has a tendency to withdraw from the receptor, which in the end determines the affinity.
An agonist doesn’t only bind to a receptor, but it activates the receptor by changing the conformation.
This generates a response. The extend of this activation is governed by the efficacy of the agonist. The
antagonists also bind to receptors, forming antagonist-receptor complexes. Again, the affinity governs
the occupation of the receptors. However, the antagonist doesn’t alter the activity of the receptor. So,
there is no response observed and antagonists generally have an efficacy of 0. The receptor-theory
explains how a drug interacts with the human body:
The agonist binds in a reversible manner to its receptor
The agonist has a very high affinity for its receptor
Agonist concentration is not altered as a consequence of binding to its receptor
Agonist efficacy is proportionate to the occupancy grade of its receptor at increasing drug
concentrations (occupancy postulate)
The efficacy of an agonist (e) of an agonist a, with intrinsic activity α, that interacts with receptor r is
represented by: Ea / Emax = α / (1 + kDa / a). The agonist affinity is the logarithm of the association
constant = log 1 / kDa = - log kDa. This - log kDa = - log [a]50 = pd2.
Potency and efficacy
Potency Efficacy
Drug a has a higher potency compared to drug b, because drug a requires a lower dose to reach 50% of
its maximal effect. There is no difference between the maximal effect rate of both drugs. In the next
graph, drug a has a higher efficacy than drug b, but both drugs are equally potent. Drug a is called a full
agonist and drug b is considered a partial agonist, since it doesn’t reach its full potential.
Lock/key model of drug-receptor interaction
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