M&R S8 - Drugs and Receptors

Question 1

What is a drug target?

Answer 1

Something a drug binds to to exert its effect

Majority are proteins

Some bind to DNA (E.g. Anti-microbial and anti-tumour drugs)

Question 2

Give the different types of drug targets and percentage of drugs that bind to that target type

Answer 2

Enzymes (47%)
GPCRs (30%)
Ion channels (7%)
Transporters (4%)
Nuclear hormone receptors (4%)
Other receptors (4%)
Integrins (1%)
Miscellaneous (2%)

Question 3

What is the function of agonist drugs?

What is the function of antagonist drugs?

Answer 3

To mimic endogenous ligands

To block endogenous ligands from binding to their target

Question 4

What is the relevance of concentration of drug molecules around receptors?

Answer 4

Critical to determining drug action

Question 5

What is the difference between two solutions of equivalent molar concentration and two solutions of equivalent concentration by weight?

Answer 5

Solutions of equivalent molar concentration have the same number of molecules

Solutions of equivalent concentration by weight may not

Question 6

What is the definition of Affinity?

Answer 6

Likelihood of a ligand binding to its target

Question 7

What is the definition of efficacy?

Answer 7

The likelihood a drug bound to its target activates the target

Question 8

What is the difference between agonists and antagonists in terms of efficacy and affinity?

Answer 8

Agonists:
- Have both an affinity and efficacy

Antagonist:

• Have Affinity
• Not efficacy (as binding doesn’t result in activation)

Question 9

What factors does the ability of an agonist to cause a response in a cell or tissue depend on?

What do these factors combined equate to?

Answer 9

Intrinsic Efficacy:
- The ability to produce an active form of the receptor

Other cell/tissue dependent factors

These factors combined reflect efficacy

Question 10

Do most drugs bind reversibly or irreversibly to targets?

Answer 10

Reversibly

Question 11

What governs the amount of drug bound to receptors at any given time?

Answer 11

Association and dissociation rates

Binding obeys the law of mass action:
- It is related to the concentrations of reactants and products

Question 12

Explain the term ‘KD’

On what type of graph might it be seen?

Answer 12

KD is the dissociation constant

It is used as a measure of drug affinity

It gives the [drug] required to reach 50% receptor occupancy

Lower KD = Higher affinity

Seen in graphs plotting [drug] against receptor occupancy

Question 13

How is information about drug binding obtained?

Answer 13

Using radioactive versions of the drug in question

‘Radioligands’

Question 14

In graphs that plot [drug] against other factors, how is [drug] commonly expressed?

Answer 14

Logarithmically

Question 15

What is meant by the term Bmax?

On what type of graph would it be found?

Answer 15

Bmax is the maximum binding capacity

This gives information about the number of receptors available for binding to ligands

Found on graphs plotting [drug] against receptor occupancy

Question 16

What is the difference between a concentration response curve and a dose response curve?

What do the axis show in each graph?

Answer 16

Concentration response curves used in measuring the response in tissues/cells

Dose response curve used in measuring the response of the whole animal

X axis = [drug]
Y axis = Response (%)

Question 17

What is the Emax?

In what graphs does an Emax appear?

Answer 17

Emax is the maximum response of a cell/tissue/organism to a drug

Emax appears on concentration response curves and dose response curves

Question 18

What is EC50?

Answer 18

The effective [drug] giving 50% of maximum response (Emax)

EC50 is also a measure of potency (a combination of both affinity and efficacy + number of receptors)

Question 19

What is IC50?

Answer 19

The [antagonist] that give 50% of maximal inhibition

Question 20

Will two agonists with identical Emax have identical efficacy?

Explain your answer

Answer 20

Not necessarily

Affinity may differ between the two agonists

This means the relationship between occupancy and response will be different

One may be more able to convert binding into function

Question 21

Discuss how affinity and efficacy affect salbutamol

Answer 21

Salbutamol is a B2 adrenoagonist

Kd = 20uM for B1
Kd = 1uM for B2

Affinity is 20x higher for B2

Salbutamol also has has B2 selective-efficacy (efficacy is higher for B2 than B1)

Along with route of administration (oral spray), this helps prevent unwanted side effects through activation of B1 receptors

Question 22

Compare Salmeterol to salbutamol

Answer 22

Salmeterol is a longer lasting B2 adrenoagonist as compared to salbutamol

Kd = 1900nM for B1
Kd = 0.55nM for B2

Therefore salmeterol affinity is 3,455x higher for B2

This is much better than the 20x higher affinity for B2 shown by salbutamol

Salmeterol shows no selective efficacy

Question 23

Explain the issues surrounding the treatment of an acutely ill severely asthmatic patient with either salmeterol or salbutamol

Assume the patient is unable to use an inhaler

Describe an ideal drug in this situation

Answer 23

Salmeterol is preferable, as there is relatively lower B1 affinity, however it is insoluble, so cannot be given via IV

Salbutamol has relatively high B1 affinity as compared to B2 affinity leading to increased side effects such as increased heart rate

An ideal drug in this situation would have relatively little B1 activation (as in salmeterol) but would be soluble so as to be given by IV

Question 24

Briefly explain the concept of spare receptors

Answer 24

Often the case that less than 100% occupancy will give 100% response

i.e. EC50 < Kd

Some tissues must therefore have more receptors than are required to produce maximum response, these are spare receptors (receptor reserve)

Question 25

What influences the relationship between receptor occupancy and response?

Answer 25

The transduction system and amplification by secondary messengers

Properties of the tissue

Question 26

What is the function of spare receptors?

Answer 26

Increase sensitivity of the tissue, allowing for responses at low concentrations of agonist

To put it another way, it is easier to obtain maximal response when not all receptors need to be filled (think about the shape of binding curves)

Question 27

How might a cell react to having lower or higher than normal activity of a specific receptor type?

Answer 27

The number of receptors in a cell is not fixed

Number can increase (up-regulation) in response to low activity and increase sensitivity, raising activity

Number can decrease *down-regulation) in response to high activity and decreases sensitivity, lowering activity

Question 28

In what 3 conditions is maximal response of a receptor not achieved?

Answer 28

The [drug] is too low to reach occupancy needed for Emax

There are not enough receptors available to reach Emax

The [drug] is only a partial agonist

Question 29

What is a partial agonist?

Answer 29

Some drugs cannot reach Emax even with maximum receptor occupancy reached

This is a partial agonist

Question 30

Describe the relationship between EC50 and Kd of a partial agonist

Explain your answer

Answer 30

EC50 = Kd

Half of maximal response (EC50) of a partial agonist is always obtained by 50% receptor occupancy (Kd) as there are no spare receptors when Emax/Bmax is reached

Question 31

Are partial agonists less potent than full agonists?

Answer 31

No, partial agonists can be less or more potent than full agonists

Question 32

What are opioids used for?

What is one major side effect?

How type of drugs are opioids?

Answer 32

Used for:

• Pain relief
• Recreationally

Side effect:
- Respiratory depression - can lead to death

Drug type:
- u-opioid receptor (GPCR) agonists

Question 33

Compare morphine and buprenorphine

Why might buprenorphine be advantageous compared to morphine?

Answer 33

Morphine is a full agonist
Buprenorphine is a partial agonist

Morphine has a lower affinity but higher efficacy

Buprenorphine may be advantageous in some situations:
- If pain is not too great then it may provide adequate pain relief with less respiratory depression

Question 34

Carl the heroin addict injects buprenorphine, why does he become ill?

Answer 34

Buprenorphine is not having the maximal response that his body is used to and would satisfy his cravings

He begins to withdraw, becoming ill

Question 35

When might a partial agonist become a full agonist?

Answer 35

If more receptors are available then more of the partial agonist can bind and a signal for maximum response can be achieved and the partial agonist now acts like a full agonist

Note: you would still need a greater concentration of the partial agonist than a full agonist to achieve maximal response

Question 36

What are the 3 types of antagonist?

Answer 36

Reversible competitive antagonism

Irreversible competitive antagonism

Non-competitive antagonism

Question 37

Describe reversible competitive anatagonism

Answer 37

Most common form in therapeutics

Relies on a dynamic equilibrium between ligands and receptors

Introducing reversible antagonists shifts the [agonist]/response curve to the right

Doesn’t effect Emax

Antagonism can be overcome by increasing [agonist] (shifts the curve back to the left)

Question 38

Describe irreversible competitive antagonism

Answer 38

Occurs when the antagonist dissociates slowly or not at all

Shifts the [agonist]/response curve to the right

No loss in Emax until all spare receptors are filled

Then Emax is lowered as available receptors for the agonist to bind is reduced below the level required for maximal response

The inhibition cannot be overcome by increasing [agonist]

Question 39

Describe non-competitive antagonism

Answer 39

The allosteric binding of an antagonist to a receptor not at the ligand binding site

Question 40

Give an example of a drug that is a reversible competitive antagonist

Include it’s use and explain why it is suitable

Answer 40

Naxolone

High affinity u-opioid antagonist

Used to reverse opioid induced respiratory depression

The high affinity means it competes effectively with other opioids for receptors

Question 41

Give an example of a drug that is an irreversible competitive antagonist

Include its use

Answer 41

Phenoxybenzamine

Non-selective irreversible A1 adrenoceptor antagonist

Used in hypertension episodes in pheochromocytoma (which causes excessive noradrenaline/adrenaline release)

Question 42

Give an example of a drug that is a competitive antagonist

Answer 42

Ketamine

NDMA receptor antagonist

NDMA-R is an ion channel, ketamine closes the ion channel causing analgesia (at low concentrations)

Question 43

What is receptor desensitisation?

Answer 43

Receptor desensitisation is a reduction in tissue response to an agonist following exposure to that agonist

Many mechanisms might be responsible

Question 44

Give some of the possible mechanisms of receptor desensitisation

Answer 44

Binding to an agonist/antagonist
Irreversible agonist binding
-ve feedback
Decrease in receptor numbers expressed due to repeated activation

Question 45

What is homologous desensitisation?

Answer 45

The desensitisation of one type of receptor in response to prolonged exposure to high levels of agonist

Question 46

What is heterologous desensitisation

Answer 46

The desensitisation of a broad category of receptors in response to prolonged activation of one type of receptor in that category

Question 47

What other mechanisms may underlie a tissue becoming desensitised to a drug?

Answer 47

Pharmacokinetic tolerance

Drug is metabolised faster as expression of relevant enzymes is increased

Question 48

An agonist is applied to a tissue, then applied again a few minutes later and a smaller tissue response is seen, this ‘tolerance’ only persists for a few minutes

Give a likely mechanism

Answer 48

Agonist binds covalently to the receptor

This prevents more agonist binding until the agonist dissociates

When second round of agonist is applied some of the original application won’t yet have dissociated and will limit tissue response

Question 49

Agonist is applied to a tissue and then the tissue is washed, a second application of agonist is done immediately after washing

What difference will there be in tissue response?

Answer 49

Almost none

Short term desensitisation due to receptors being blocked by agonist still bound is negated by washing

Question 50

By what 3 mechanisms can GPCRs become desensitised?

Give a short description of each

Answer 50

Modification of receptor by phosphorylation:

• Receptor is phosphorylated preventing the G proteins from binding
• Fast acting

Reversible receptor internalisation:
- Receptors on the membrane are taken into the cell and into endosomes before being recycled back to the membrane

Down-regulation:
- Expression of the gene(s) encoding for receptor is reduced so less receptor is available for agonists to bind to

Question 51

Which mechanism of GPCR desensitisation is most commonly associated with heterologous desensitisation?

Answer 51

Phosphorylation modification of receptor

Enzymes which phosphorylate the receptor can be non specific and desensitise other GPCR types

Question 52

Which mechanism of GPCR desensitisation is most likely to underlie drug tolerance?

Answer 52

Long term drug use will trigger down-regulation of the relevant receptors, which will cause the receptor desensitisation

Question 53

What is receptor supersensitivity?

Suggest how B-adrenoceptor supersensitivity might be induced

Answer 53

The over-expression of receptors due to prolonged reduction in tissue response

Treatment with B-adrenoceptor antagonists will reduce tissue response to endogenous ligands and the receptor expression will be up-regulated

If treatment with antagonists is stopped there will be excess receptors and receptor supersensitivity can occur

Question 54

How can termination of treatment with B-adrenoceptor antagonists cause a heart attack?

Answer 54

Receptor supersensitivity occurs due to treatment

Lack of antagonist causes adrenaline and noradrenaline to produce a greater tissue response

BP and HR rise, this can lead to ischaemia and subsequent myocardial infarction

Question 55

How can receptor recycling resensitise a membrane?

Answer 55

Receptors that have been desensitised can be replaced from receptor reserves in the endosomes

Addition of receptors from the endosome to the cell membrane can therefore restore sensitivity and correct tissue response