Pharmacodynamics: Receptor Theory

Question 1

How do drugs have an effect in the body?

Answer 1

They bind to a target. The majority of targets are proteins (with some exceptions of some antimicrobial drugs that bind to DNA)

Question 2

What different molecular targets can small molecule drugs interact with?

Answer 2

GPCRs (a third of drug targets)
Ion channels 
Kinases
Nuclear receptors 
Other (eg enzymes, DNA, integrins, transporters)

Question 3

How many different types of GPCR are there in the body?

Answer 3

About 800 different types of GPCR in the body. Almost everything you do is regulated by GPCRs. About half of the GPCR receptors are olfactory receptors (smell). The pathophysiological roles of these receptors are often unknown or not fully defined.

Question 4

What are orphan receptors?

Answer 4

Orphan receptors are GPCRs that have been identified but, the ligand they binds to it has not. These represent potential drug targets.

Question 5

Want are we interested as GPCRs as drug targets?

Answer 5

They have a proven history in therapeutics

Onto 50/400 non olfactory GPCRs are used as drug targets so there is a lot of potential.

Question 6

What is a ligand?

Answer 6

Something that binds to a receptor. Can be endogenous eg hormones or neurotransmitter or exogenous eg drugs.

Question 7

Why are pharmacologists interested in drug receptor interactions?

Answer 7

Help us to better understand physiology and pathology
Help us to understand drug action
Informs clinical decisions
Allows development of new drugs

Question 8

How much (in terms of moles) is 
Millimolar?
Micromolar?
Nanomolar?
Picomolar?

Answer 8

Millimolar: 10-3M
Micromolar: 10-6M
Nanomolar: 10-9M
Picomolar: 10-12M

Drug concentrations (or ligands) are often in the micromolar, nanomolar or picomolar range.

Question 9

Why do we need to consider drug concentrations in molarity?

Answer 9

Because, although the concentrations of two different things may be the same, the Moles will be different as they have different Mr. (This is because 1M of a substance contains 6x10^23 molecules (Avogadros constant).

This is important because the concentration (no. of molecules) of drug molecules around receptors is critical in determining drug action therefore, we need to consider molarity.

Question 10

How do drugs act with receptors?

Answer 10

Most drugs bind reversibly to receptors to form a receptor-ligand complex. This means binding is governed by the rate of association and dissociation.

Question 11

What are the different consequences of a ligand-receptor complex forming?

Answer 11

Block the binding of endogenous agonists (antagonist)-They only have affinity and no intrinsic efficacy.
Activate a receptor (agonist). -They have intrinsic efficacy and efficacy.

Question 12

What must a ligand have for a receptor to be able to bind to it?

Answer 12

To bind to a receptor, a ligand must have affinity for the receptor. The stronger the affinity, the stronger the binding.

Question 13

What is intrinsic efficacy ?

Answer 13

The ability of a ligand to bind and activate a receptor (R*). After this, things can then occur.

Question 14

What is efficacy?

Answer 14

The ability of a ligand to cause a response.

The efficacy is governed by the intrinsic efficacy plus other things (cell / tissues dependant factors) that influence the response.

Question 15

What is the difference between pharmacological efficacy and clinical efficacy?

Answer 15

Pharmacological efficacy is what the drug will do to the body (eg dilate blood vessels) whereas clinical efficacy is how the treatment succeeds in achieving its aim (eg decrease BP).

Question 16

How do we measure binding?

Answer 16

We often do this by binding a radioactively labelled ligand (radioligand) to cells or membranes prepared from cells that express the receptor we are interested in (10,000-100,000 receptors per cell).
We then incubate the radioligand and receptors to cause binding. (The higher the conc of ligand, the higher the binding)
We then separate the bound and free radioligand and measure the bound.
We then quantify it by graphing it.
Use the graph to calculate binding maximum (Bmax) to work out how many receptors are present in out preparation.
We also use the graph to work out 50% occupancy (concentration of drug when 50% of receptor have ligands). This is called Kd.

Question 17

What is Kd?

Answer 17

Kd is the dissociation constant. It is the concentration of ligand required to occupy 50% of the available receptors.
It is a measure of affinity. The lower the Kd, the higher the higher the affinity.

Kd is the reciprocal of affinity.

Question 18

Is affinity important for ligands?

Answer 18

Does it bind to receptor? How well does it bind? High affinity allows binding at low concentration so, high affinity is preferable!(can give smaller tablets).

Question 19

What scale do we use to discuss drug concentration?

Answer 19

We use a logarithmic scale

Question 20

What different responses can dugs cause?

Answer 20

A response requires drug efficacy (the drug needs to be an agonist)

The response could be:

• Change in signalling pathway
• Change in cell or tissue behaviour (eg contraction)

Question 21

What does a concentration response curve look like?

Answer 21

A rectangular hyperbola.

Question 22

What is EC50?

Answer 22

This is the effective concentration which gives 50% of the maximal response.

Question 23

What does EC50 depend on?

Answer 23

This depends on affinity, intrinsic efficacy, cell/tissue specific components and overall efficacy.

Question 24

What is the difference between concentration and dose`/

Answer 24

Concentration = known concentration of drug at site of action eg in cells and tissues 
Dose = Concentration at site of action is unknown. eg dose to a patient in mg or mg/kg.

Question 25

What is asthma?

Answer 25

Asthma is a chronic inflammatory disease. There is also reversible airflow obstruction and bronchospasm.

Question 26

What pathological mechanisms mediate contractions of bronchioles in asthma?

Answer 26

Can use B2- adrenoreceptors to cause relaxation (It is na agonist for treatment and it activates the receptor (agonist) but, it provides functional antagonism of contraction.

Question 27

How do we achieve selectivity for activation of B2-adrenoreceptors in the airways but not B1-adrenoreceptors in the heart?

Answer 27

Salbutamol- Binds to B2 receptor in the lungs and causes relaxation. It has 20 fold higher affinity for B2 than for B1. When bind to B2 receptor, good intrinsic efficacy and high conc of drug as administered by asthma pump straight into the lungs.

Salmeterol - much longer acting than salbutamol. Binds to B2 with much higher affinity (3500 fold) for B2 than B1 receptors. But, no selective efficacy (selectivity is only based on affinity).

Selectivity can be achieved via affinity or efficacy or a mixture of both.

Question 28

What is the difference between selectivity and specificity?

Answer 28

Selectivity - Preference of binding to one protein or drug target but, often interact with other targets as well.
Specificity - Only interact with one molecular target (extremely unusual).

Question 29

Why can you not give sameterol IV?

Answer 29

Because it is insoluble

Question 30

What side effects come when you give salbutamol IV?

Answer 30

It binds to the B1-adrenoreceptors and causes the heart to speed up, particularly in patients with angina.

We need drugs with enhanced selectivity -affinity and/or efficacy.

Question 31

How is potency measured?

Answer 31

Potency is measured using the EC50 (the effective concentration required to give 50% of the maximal response).
It depends on both affinity and intrinsic efficacy and the cell/tissue dependant factors inc. NUMBER of receptors.

Question 32

What factors can limit the response?

Answer 32

No. of receptors
But the response is often controlled or limited by other factors than this. eg a muscle can only contract so much or a gland can only secrete so much.

Question 33

What different relationships can you get between binding and response? Why?

Answer 33

You can get 100% occupancy = 100% response. But, often see that a lot less occupancy (eg 50%) can generate 100% response due to ‘spare receptors’.

Question 34

Where are spare receptors often seen?

Answer 34

Spare receptors are often seen when receptors are catalytically active, eg tyrosine kinase or GPCRs

Question 35

How does contraction in the smooth muscle of the airways contract?

Answer 35

Parasympathetic nerves cause the release of acetylcholine. These bind to Muscarinic (M3) GPCRs which, combined with other mechanisms, cause contraction.
But, only 10% occupancy of the muscarinic receptors is needed for maximal contraction.

Question 36

Why does a cell make spare receptors?

Answer 36

Spare receptors increase sensitivity of a tissue. They allow responses to occur at a very low conc of agonist.

Question 37

What effect does changing the receptor number have?

Answer 37

Changing the receptor number changes the agonist potency and can effect the maximal response. More receptors, more potent so need less for maximal response

Question 38

Is receptor number fixed?

Answer 38

NO!

Receptor number tend to increase with low activity (up-regulation) and decrease (down regulation) with high activity.

Question 39

What can reduce sensitivity of drugs?

Answer 39

Down regulation. This is when the number of receptor decrease due to high activity.

Question 40

What is the difference between a full agonist and a partial agonist?

Answer 40

A full agonist produces a 100% response (like an endogenous ligand would). therefore, its Kd is bigger than EC50 and it does have spare receptors.
However, with a partial agonist, EC50 is roughly equal to Kd and there are no spare receptors.This means that all receptors are occupied and that there is insufficient intrinsic efficacy for maximal response (does not ‘turn receptor on’ as well as full agonist).

Question 41

What does maximal response indicate?

Answer 41

Maximal response indicates intrinsic activity.

Partial agonists have lower intrinsic activity as they have a lower efficacy than full agonists.

Question 42

Why do partial agonists make good drugs?

Answer 42

• They allow a more controlled response.
• Work in the absence or in low levels of endogenous ligands.
• Can act as antagonists if high levels of full agonist.

Question 43

Why would beprenorphine be advantageous to morphine in some clinical settings?

Answer 43

Buprenorphine has a high affinity (lower Kd) but a lower efficacy (inability to produce a full response) than morphine.
It would be advantageous because, it could provide adequate pain control with less respiratory depression (side effects).
Their properties mean it can also be used to treat heroin (opioid) addiction because it will inhibit the effects of heroin (act as a mixed agonist, antagonist as higher affinity that heroin to receptors)

Question 44

What are the three different types of antagonists?

Answer 44

Antagonists block the effects of agonists (prevent receptor activation by agonists). There are three types:

1. Reversible competitive antagonist -most common and most important in therapeutics
2. Irreversible competitive antagonism
3. Non-competitive antagonism -generally allosteric and it can even work post receptor.

Question 45

How does reversible competitive antagonism work?

Answer 45

This relies on the dynamic equilibrium between ligands and receptors.
The higher the conc. of antagonists, greater the inhibition -This is because it will outcompete all the agonists.

Question 46

What is IC50?

Answer 46

This is the concentration of antagonist that gives us 50% inhibition. (inhibitory concentration 50).
This is a measure of antagonist potency. It is determined by the strength of the stimulus.

Question 47

Can you overcome inhibition from competitive antagonists?

Answer 47

Yes! Because if you add enough agonists, they will outcompete the antagonist.
Reversible competitive antagonists cause a parallel shift to the right of the agonist conc. response curve.

Question 48

What is naloxone?

Answer 48

Naloxone is a high affinity competitive antagonist at u-opioid receptors. This is useful because it will reverse opioid mediated respiratory depression by effectively competing (outcompeting) other opioids (heroin) for receptors.

Question 49

How does irreversible competitive antagonism work?

Answer 49

This is when antagonists bind to the receptor (sometimes covalently) and does not dissociate or dissociates very slowly.
This is NOT SURMOUNTABLE because if the irreversible binding. With increased antagonist conc. or increase time more receptors are blocked by antagonists.

Question 50

What effect do irreversible competitive antagonists have on the concentration response curve of the agonist?

Answer 50

They cause a parallel shift to the right IF there are spare receptors.
However, if there are insufficient receptors, there is a decrease in the full response (full response will be suppressed).

Question 51

What is phenoxybenzamine?

Answer 51

This is a drug that binds irreversibly to the a1-adrenoreceptor. This is used in hypertension episodes of pheochromocytoma because it prevents the excessive adrenaline which is released by the adrenal chromaffin cells from binding and causing vasoconstriction.

Question 52

How does non-competitive antagonism work?

Answer 52

This is when a ligand binds to an allosteric site and change the shape of the binding site to reduce the effect of the agonist.
This is non-surmountable and is also known as negative allosteric modulation.
These antagonists have a similar effect to irreversible competitive antagonists and you need to conduct additional tests to distinguish between the two.