Receptor Pharmacology
Lecture 1: General Pharmacological Principles
Pharmacology: interactions of chemical substances (drugs) with the living organism.
Drug: chemical substance of known structure which when administered to a living organism, produces a biological effect.
Chemical applied to a physiological system that affects its function in a specific way.
Pharmacodynamics: how does drug affect the body?
Pharmacokinetics: how does the body affect the drug?
One of the basic principles of pharmacology is that drug molecules must exert some chemical influence on one or more cell
constituents in order to produce a pharmacological response. Drug molecules must be bound in order to do its work.
Specificity of drug actions:
For a drug to be useful as either a therapeutic or a scientific tool, it must act selectively on particular cells and tissues: In
other words, it must show a high degree of binding site specificity.
‘Non-specific’:
- Biological effect at relatively high drug concentrations (e.g. antacids, adsorbents and osmotic agents). Require
high concentrations!!
‘Specific’:
- Biological effect at relatively low drug concentrations.
- Chemical and biological specificity. Most drugs are specific.
Most drugs act on target proteins with high affinity to the drug.
EXAM: What is specificity?
Drug actions
Four main kinds of regulatory protein are commonly involved as primary drug targets. Drugs act on target proteins:
Receptors, Ion channels, Enzymes and Carrier molecules (transporters).
- Receptors: Target molecules whose function is to recognize and respond to specific endogenous chemical signals,
such as hormones, neurotransmitters and inflammatory mediators.
They are molecules that are expressed on the cell, sometimes on cell membrane or inside the cell. They
recognize a ligand and translate the presence or the ligand to communication.
Reciprocal specificity of receptor-drug interaction:
Individual classes of drugs bind to discrete receptors, while individual receptors recognize only discrete classes of drugs.
High specificity by high affinity.
However, no drugs are completely specific in their action! In many cases, side effects may occur at relatively high drug
concentrations, due to binding to other targets (receptors) with lower affinity.
- Off target side effect occurs when drug is not 100% specific! A 100% specific drug is not possible. At high
concentration the drug can bind to other receptors causing side effects.
- On target side effect occurs bc receptor has multiple functions: e.g. block β receptors, bp benefits for it, but when
you have asthma, β blocker causes bronchoconstriction!!
Drug-Receptor Interactions
Occupation of a receptor by a drug molecule may or may not result in activation of the receptor.
By activation, we mean that the receptor is affected by the bound molecule in such a way as to
alter the receptor’s behaviour and elicit a tissue response.
Agonists and Antagonists
If a drug binds to the receptor without causing activation and thereby prevents the agonist from
binding it is a receptor antagonist. The tendency of a drug to bind to the receptor is governed by
its affinity, whereas the tendency for it to activate the receptor is denoted by its efficacy.
Drugs of high potency generally have a high affinity for the receptors and thus occupy a
significant proportion of the receptors, even at low conc.
Agonist: binds to receptor and activates it. Receptor occupation leads to biological response
Antagonist: ligand does bind to the receptor but in this case, there is no response. Receptor occupation does not lead to
response and prevents the effect of an agonist, mostly by preventing it from binding.
Partial agonists: drugs with intermediate levels of efficacy, such that even when 100% of the receptors are occupied, the
tissue response is submaximal.
Full agonists: efficacy is sufficient that they can elicit a maximal tissue response.
We’re better in designing drugs that are antagonists.
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,Molecular level of receptor →
A receptor is mobile and moves all the time. The difference between inactive and active state is extremely small. Especially
at the extracellular site where the ligand binds to.
A small conformation is already enough sufficient to cause a biological response.
Agonist versus Antagonist
Example: ACh induces bp drop intravenously. If you apply high dose of ACh a steeper drop,
more conc and thus higher effect and longer effect. In presence of atropine, the ACh no longer
has a functional effect.
Agonist: Acetylcholine – Antagonist: Atropine
Drug-Receptor binding
Hill-Langmuir equation is derivative of Law of mass action describes how two
molecules interact with each other, can be enzyme – substrate, ligand –
receptor. The relationship between agonist occupied receptor (AR complex)
with the free receptor and free drug. → Utilize this model to describe
relationship between ligand concentration and ligand binding.
What if a drug which has high association constant and low dissociation
constant? → you only need a low concentration for the drug to bind to the
receptor: high affinity. And vice versa.
Suppose that you have a drug: measuring binding of the drug to a receptor. If
you apply the drug in higher and higher concentration: you’ll find that more
drug will bind → Describe the conc of the drug and the binding of the drug is
called the fractional occupancy PA (the agonist receptor-complex divided by
sum to free receptor and the receptor complex) = What proportion of the receptor is bound?
Ka is K-1 / K+1 → which is the dissociation constant / association constant.
Ka is the binding constant which describes how well a compound binds to the receptor. A very important constant which
describes the affinity of the drug. Ka is a constant which can also be described as Kb when talking about an antagonist.
• Suppose that conc of the drug is equal to the Ka. Then the equation will be Xa / Xa+Ka, which means 50% of the
drug will be bound.
If Ka is high, the affinity is low which means you need high amounts of drug to get the 50% binding.
The point at which there is 50% bound, is at the steepest part of the log scale curve.
Relationship between ligand binding to receptor is determined by conc of your compound and by the Ka which is the
affinity of the compound. The lower the binding constant, the less you need to modify the system. The lower the conc of
drug that you apply, the more likely you get a specific interaction and therefore produces little side effects → The most
POTENT drug.
Receptor binding measurement
e.g. drug can choose between two receptors. For example, the α and β receptor. Drug binds to the α receptor with Ka = 10
nM and bind to the β receptor with Ka = 10 microM. This is a α receptor selective drug! Ka describes the amount of drug
you need to reach 50% binding. You need a lower concentration of drug to reach the Ka. The lowest Ka has highest
selectivity → EXAM!!!
Compound A binds to receptor with Ka = 1 × 10-6 M and compound B binds to the same receptor with Ka = 1 × 10-7 M.
Which drug is more potent? Compound B. → The lower the Ka, the more potent your compound.
How to experimentally determine the Ka and maximum binding? → by radioligand assays → Scatchard analyse
Radioligand binding assay are assays where two competing ligands that bind to the same receptor. 1 is radioactive labeled
and the other is not. In Red: radioactive label.
β-adrenoceptor binding by [3H]-cyanopindolol in cardiac membranes.
Suppose this one is in cyanopindolol (β-blocker). You take a preparation from the heart; the compound will bind to those
receptors on the heart. Therefore, if you increase the conc if cyanopindolol, you will see more and more ligand binding to
the receptors. Unfortunately, not all of this is specific. So, any ligand will always bind preferentially to its specific receptor,
but some of it (especially at high conc) will bind non-specifically.
What you can do is to repeat this assay in presence of very very high conc of a different β-blocker which is NOT
radiolabeled. Non-specific binding is measured in presence of saturating conc of the non-radiolabeled agonist, which
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,prevents the radioactive ligand from binding to the β-adrenoceptors. The difference between these two lines represent the
specific binding (C). (D) shows the specific binding plotted against the concentration on log scale.
Radioligand binding → can be used to determine the Ka and
maximum binding.
Concentration-effect curves
Binding is not the same as effect. Because an antagonist can bind to a receptor but produce no effect.
We have to separate these two terms! We measure the outcome.
The more drug is bound, the more effect it has → similar effect as binding.
EC50 is the conc of drug that leads to 50% of its own effect. The EC50 is related to the Ka (how much
drug is bound). There is maximum effect which is 100% (same for drug A and B) the maximal biological
effect that a drug can achieve is called efficacy. The concentration at which 50% of the effect is
achieved is called potency. Efficacy and Potency are independent from each other.
Drug A requires a lower concentration to yield effect compared to drug B.
Occupation theory: effect (EA) proportional to occupancy (NA) (over-simplification)
Efficacy is not the same for every ligand. In reality, the constant efficacy (described as epsilon) is between 0 – 1. In
case of full agonist which maximally activates a receptor system, epsilon = 1. In case the maximal effect is NOT
achieved, we call this a partial agonist. When epsilon = 0.5, it will reach 50% effect.
So how to determine the EC% effective concentration. So, Ka = 50% binding. Here we use a different term: EC50 =
drug concentration where you see 50% of its OWN effect.
- Drug A and B both have the same efficacy, but A is more potent
than B. they both have an EC50 of 50%.
- The EC50 concentration is determined as the half maximal
effect. Therefore, EC50 for drug C is 50% × 0.5 = 25%.
- Drug B and C both have equally potency, but drug C is less
efficacious.
pD2 = negative log of EC50. If EC50 is 1 microM, the pD2 is 6.
Intrinsic efficacy and efficacy and epsilon ε are the same.
Ka is only used to describe binding, the lower the better – bc the more potent your drug will be.
EC50 is the concentration that reaches 50% of its OWN maximal effect.
In most cases 50% receptor binding yields 50% effect. If activated for 50%, it will generate half maximal effect.
Efficacy = epsilon (0 – 1): the higher the number the more efficacious your drug is.
Partial agonist: receptor doesn’t have two states, but partial agonist has intermediate state which doesn’t lead to full
activation. Induce receptor conformation that is NOT fully capable to create downstream signaling factors.
Ion channel is either open or closed. But receptor can be activated in different gradients!! The intermediate states mean
that receptors are partially activated.
Occupancy-response relationships
Exception where 50% receptor binding does not yield 50% effect → when there are more receptors than needed to
achieve maximal effect. If there are 1000 receptors on the heart, and you need only 100 to achieve maximal effect →
Receptor Reserve / Spare Receptor.
If only 10% of receptor population is needed to activate the system maximally.
EC50 = Ka in case of no receptor reserve.
EC50 < Ka in case of receptor reserve, bc you need only a partial binding to achieve maximal effect. You achieve maximal
effect at a lower conc.
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, Receptor reserve means there are more receptors than you need: suppose the heart has enormous receptor reserve. You
need 100 receptors to achieve it. But heart expresses 1000. The drug binds 500 receptors at Ka. But you need only a 100 for
maximal effect. So, 100 receptors bound is a point before the Ka, this already leads to maximal effect.
Principle of receptor reserve: more binding site than required to achieve maximal effect. Impact of receptor reserve on
agonist is: EC50 < Ka which means you reach 50% effect before you reach 50% binding. If there is receptor reserve, the
EC50 is lower than the Ka, but only for the full agonist!! Not for the partial agonist, this one does not benefit from the
receptor reserve.
Example 2: suppose we have two drugs which both bind to β receptor. One drug is partial agonist and the other a full
agonist. Now we have two systems: the heart and the lungs. The β receptor is overexpressed in the heart and not in the
lung. The lung expresses 100 receptors. Drug A is full agonist with ε = 1 and drug B is partial agonist with ε = 0.1
The heart expresses 1000 receptors: drug A will become more potent compared to in the lung, due to presence of receptor
reserve. However, Drug B will not become more potent, bc it will only activate the system by 10%: so, there are 1000
receptors, you need 100 to reach maximum effect. A hundred can be activated due to epsilon of 0.1. The partial Agonist B
will not benefit from potency but will behave as a full agonist.
What does receptor reserve do in case of a full agonist? It shifts the curve to
the left; more potent. The heart is much more sensitive to adrenalin due to
receptor reserve → If you inject adrenalin in circulation, the first system
responding is the heart → the reason is bc receptors are expressed on
different extends.
However, in case of a partial agonist; receptor reserve will make your agonist
more efficacious, bc you need only a part of receptors activated to achieve
maximal effect.
Theoretical occupancy and response curves for full and partial agonists →
Partial agonism
Relationship between agonist structure and agonist effect. Response as function of concentration of 4
different compounds. The compound has epsilon of 0.8. if we take a compound which is bulkier (ethyl) the
efficacy reduces to 0.4. A bulkier molecule fits less and less into binding site. The ability of a drug to bind is
specifically related to the chemical structure. By modifying the chemical structure of the ligand, we can
change the intrinsic activity and form a partial agonist from a fully agonist. Or even an antagonist from an
agonist as it still binds to the binding pocket but no longer activates it.
Antagonist classification
We can subdivide the antagonists into the ones that actually bind to the receptor: Receptor Antagonists and the ones that
don’t: Non-receptor Antagonists.
The receptor antagonists can be classified into two groups: (1) receptor antagonists which
bind at the active site and (2) those that bind somewhere else.
Active site binding: antagonists interferes directly with the agonist: prevents agonist from
binding.
Allosteric binding: agonist and antagonist bind at completely diff sites at molecule.
The non-receptor antagonists can be divided in chemical, pharmacokinetic antagonist ad
physiological antagonists.
Reversible competitive antagonism
This is the biggest group of antagonists. Key characteristics:
- Drug binds to receptor to prevent the agonist from binding → Classical
antagonist.
- Can be competed in high conc of agonists by competing out the
antagonist from the receptor and making the receptor available. Bc of
presence of antagonist, agonist has more difficulty to bind to the
receptor: competitive. If the dose of agonist is high enough you will
compete the antagonist out.
- The EC50 drops (potency drops) but Emax (efficacy) stays the same. → you see a right shift whereas maximal
effect is stable. Maximal effect will always be achieved – potency drops but efficacy stays the same.
- Characterized by a rightward shift without drop in efficacy, bc agonist competes out the antagonist.
e.g. adrenaline in the heart. Blue line is the adrenaline itself. We start adding β-blockers before adding adrenaline. The β-
receptors are blocked, and adrenaline can no longer bind to it. The dose response curve shifts to the right: depending on
concentration of antagonist you apply (the more you apply, the more it shifts to the right).
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