L9- Pharmacodynamics Receptors

Question 1

a) Define: Ligand
b) Define: Affinity
c) Define: intrinsic efficacy
d) Define: efficacy

Answer 1

a) Something that binds to a receptor e.g. drug, hormone, neurotransmitter
b) affinity can be defined as the strength of interaction with which a drug/ligand binds to its receptor.
- Higher affinity: stronger binding
c) The ability of a ligand to activate a receptor by causing a conformational change in the target protein
d) the ability of a ligand/drug to cause a measurable biological response

Question 2

Binding obeys the law of mass action. What does this mean?

Answer 2

• related to the concentrations of reactants and products
• follows equlibrium pattern
• reversible

Question 3

Define:

a) Agonist

b) Antagonist

Answer 3

A) A ligand that binds to a receptor and turns it on, initiating a biological response

b) A ligand that binds t a receptor and blocks it- preventing agonist from binding i.e. no receptor activation and no biological response

Question 4

For a ligand to bind to a receptor, what quality must it have?

Answer 4

Affinity for the receptor

Question 5

a) What do agonists require?

b) What do antagonists require?

Answer 5

a) Affinity, intrinsic efficacy and efficacy (along with cell/tissue dependent factors)
b) Only affinity

Question 6

What is clinical efficacy?

Answer 6

It is more of an indication of how well a treatment succeeds in achieving its aim.

e. g. does it lower BP?
- does it cure headache?

Question 7

How do we measure ligand binding?

Answer 7

• by binding of a radioligand (radioactively labelled ligand) to cells or membranes prepared from cells
• incubate radioligand and receptors and this will lead to binding
• seperate the bound and free radioligands, measure the bound ligands
• the more binding the larger the measureable signal

Question 8

a) What is Kd

b) What is Bmax?

Answer 8

a) It is a dissociation constant and it gives you the concentration of ligand required to occupy 50% of available receptors
- measures strength of interaction/affinity

b) the max binding sites- it gives you info about the receptor number

Question 9

What do the following mean:

a) High Kd
b) Low Kd

Answer 9

a) lower affinity of ligand to receptor (more likely to dissociate)
b) Higher affinity of ligand to receptor (less likely to dissociate)

Question 10

What would a receptor drug concentration graph look like if it was plotted in the following ways:

a) Linear
b) Logarithmic

Answer 10

a) Hyperbolic

b) Sigmoidal

Question 11

Often dose and concentration are used interchangeably. Why are they different?

Answer 11

• Concentration: precise concentration of a drug at site of action e.g. cells and tissues
• Dose: concentration at site of action generally unknown

Question 12

To plot drug vs response, response has to be a measureable change. Give some examples

Answer 12

• change in signalling pathway
• change in cell or tissue behaviour
  e. g. cell death (cancer)
• hormone or neurotransmitter release

Question 13

Why is affinity important clinically?

Answer 13

• High affinity allows binding at low concentrations of hormones, neurotransmitters and drugs.
• high affinity means we can administer small amts of drugs
• If someone comes in with a heroin overdose we can give them naloxone (high affinity antagonist of u-opoid receptors), this means it out competes opoid, knocking off heroin preventing it from binding

Question 14

What is:

a) Emax
b) EC50

Answer 14

a) the maximal response (above which more drug will not produce a response)
b) Effective concentration giving 50 percent of the maximal response
- measure of agonist POTENCY

Question 15

What is the difference in Kd and EC50?

Answer 15

• Kd: concentration of ligand required to occupy 50% of receptors: only depends on affinity
• EC50: concentration of ligand giving 50 percent of the maximal response: depends on affinity, intrinsic efficacy and cell/tissue dependent events

Question 16

What are cell/tissue dependent events?

Answer 16

• sequential steps driven by original signal from ligand binding w its target tat leads to the final step of a measured therapeutic response
  e. g. receptor number

Question 17

What are spare receptors and how do spare receptors influence agonist potency?

Answer 17

• receptors that exist in excess of those required to produce a full/maximal response
• ## Spare receptors increase sensitivity/ potency i.e. allows responses at low concentrations of agonists

Question 18

a) What is potency?

b) How is it measured?

Answer 18

a) Potency: the amount of drug required to produce an effect of given intensity
- he drug which can produce an effect at lower drug concentrations is “more potent”

b) EC50

Question 19

If a full response requires 10,000 activated receptors. What would a full response be like if the cell has:

a) 10,000 receptors/cell
b) 20,000 receptors/cell

Answer 19

a) Full response will require 100% occupancy
- the drug conc will have to be a LOT higher than kD

b) - full response only requires 50% occupancy and hence the drug conc will be equal to Kd

Question 20

Why are receptor numbers not fixed?

Answer 20

• vary with cell type
• tend to increase with low activity (up-regulation) –> can lead to super sensitivity
• tend to decrease with high activity (down-regulation) –> can lead to drug tolerance
• physiological, pathological or drug-induced changes

Question 21

Are all agonists equal at the same receptor?

Answer 21

No

- they can have different affinities and different efficacies

Question 22

What are partial agonists?

Answer 22

• Drugs that bind to a receptor, but cannot produce a maximal effect even with full occupancy of all the available binding sites in the tissue
• they have lower intrinsic activity as lower efficacy

Question 23

Compare Partial and full agonists

Answer 23

• Partial have higher affinity (lower kd) than full agonists
• they have lower intinsic activity and hence low efficacy
• can allow a controlled response and can work in the absence or low levels of ligand
• if high levels of full agonist they can act as an antagonist due to higher affinity and bind to receptor hence preventing full agonist from binding

Question 24

a) What can opioids such as heroin, morphine and fentanyl cause?
b) Why does this explain why partial agonists are used clinically.

Answer 24

a) Action through the u-opioid receptor (GPCR)
- pain relief
- recreational use –> euphoria
- Respiratory depression ( can lead to death)

b) - buprenorphine is an opiate with partial agonism
- it is used to treat pain in hospitals but does not lead to respiratory depression which is the maximal effect of full agonists
- can also be used in the drug addiction setting acting as an antagonist of heroin, occupying u-opioid receptors and limiting the response –> gradual withdrawal

Question 25

Explain how drug users can get withdrawal?

Answer 25

• sustained drug taking leads to tolerance–> reduced receptor numbers (down regulation) and hence need larger concs to get full effect
• when drug is withdrawn the endogenous ligands are now less effective: withdrawal symptoms

Question 26

What are the 3 modes of action of antagonists?

Answer 26

1. Reversible competitive antagonism
2. Irreversible competitive antagonism
3. Non-competitive antagonism (generally allosteric)

Question 27

Outline how reversible competitive antagonism works

Answer 27

• relies on dynamic equilibrium between agonists and antagonists (ligands) and receptors
• greater antagonist concentration = greater inhibition
• the inhibition is surmountable (i.e. can be overcome) if more agonist is added hence it is reversible

Question 28

What would increasing reversible competitive antagonist concentration do to the shape of the agonist-conc response curve?

Answer 28

• it would cause a parallel shift to the right

- this is because more agonist would be needed to overcome the inhibition of the reversible antagonist

Question 29

a) What is Ki

b) What is IC50?

Answer 29

a) antagonist affinity for receptor

b) Concentration of antagonist to give 50% inhibition

Question 30

What is an example of reversible competitive antagonism used clinically?

Answer 30

• Nalaxone: high affinity, competitive antagonist at u-opioid receptors
• competes effectively with other opioids e.g. heroin for receptors
• reversal of opioid mediated respiratory depression

Question 31

Outline how irreversible competitive antagonism works

Answer 31

• the antagonist dissociates slowly or not at all
• bonds are covalent i.e. not reversible
• Non-surmountable: cannot be overcome by increasing agonist concentrations

Question 32

What would increasing irreversible competitive antagonist concentration do to the shape of the agonist-conc response curve?

Answer 32

• it would cause a parallel shift to the right and eventually down as well
• shift to the right as increased agonist need to try and fill receptors (to compete)
• shift down as the antagonist fills spare receptors and the maximal response becomes suppressed

Question 33

Example of irreversible competitive antagonism clinically

Answer 33

• Phenoxybenzamine: irreversible a-1 adrenoceptor blocker used in hypertensive episodes in pheochromocytoma (tumor of adrenal gland tissue)
• will bind using covalent bonds irreversibly to a-1 receptors preventing adrenaline binding and cannot be outcompeted by more adrenaline

Question 34

What is:

a) Orthosteric site
b) allosteric site

Answer 34

a) site where the natural ligand will bind to activate receptor
b) binding site elsewhere- often agonists will bind here or molecules that reduce effects of agonists

Question 35

Outline how non-competitive antagonism works

Answer 35

• antagonist will bind to the allosteric site (away from the orthosteric site)
• no competition for access to the orthosteric site with the endogenous ligand
• also known as negative allosteric modulation

Question 36

Example of non-competitive antagonism clinically

Answer 36

• Maraviroc:
• negative allosteric modulator of chemokine receptor 5 used by HIV to enter cells
• used in AIDS treatment