M&R session 8: drugs and receptors

Question 1

What is the relationship between concentration and molarity?

Answer 1

Drugs at equal molar concentrations have the same concentration of drug molecules, but drugs at equivalent concentration by weight may not.
molarity (M)= concentration (g/L)/molecular weight

Drugs usually in concentrations of uM or nM units:
uM=10^-6 M
nm=10^-9 M

Question 2

What is the difference between antagonists and agonists?

Answer 2

ANTAGONISTS: block the binding of an endogenous agonist. They only possess affinity: unable to convert receptor into an active conformation

AGONISTS: activate a receptor, governed by intrinsic efficacy, then things must happen to generate response. They have affinity, intrinsic efficacy (can activate receptor) and efficacy (can generate a measurable response)

Question 3

What is intrinsic efficacy?

Answer 3

How good an agonist is at generating the active state of the enzyme

Question 4

What is efficacy and what does it determine?

Answer 4

The ability of a ligand to cause a response. Governed by the intrinsic efficacy plus other things that influence the response

Question 5

How might affinity (binding) be measured?

Answer 5

1. Binding a radioligand to cells or membranes (to find KD)

2. Pharmacologically to find KA (KD equivalent)

Question 6

What is Bmax and KD?

Answer 6

Bmax: the maximum binding capacity, so gives information about receptor number. Highest point reached on graph as proportion of bound receptors

KD: the dissociation constant. The concentration of ligand at 50% binding of the available receptors. Measure of affinity: lower KD=higher affinity. Read off graph for [drug] where there is 50% of receptors bound

Question 7

What does a high KD value indicate?

Answer 7

Low affinity

Question 8

What is a logarithm?

Answer 8

An exponent by which a fixed base value has to be raised to give a particular number

Question 9

What is IC50?

Answer 9

The inhibiting concentration giving 50% of the maximum inhibition. Used in inhibitory drugs

Question 10

What could a response of a drug be?

Answer 10

1. Change in a signalling pathway

2. Change in a cell or tissue behaviour

Question 11

What do the values of Emax and EC50 refer to, and where are they found on a concentration-response curve?

Answer 11

Emax: maximum effect (highest point on curve)

EC50: effective concentration giving 50% of the maximal response. Describes the POTENCY of the agonist (depends on both affinity and intrinsic efficacy, in addition to the cell/tissue-specific components). Find the drug concentration at 50% of maximum response

Question 12

What is required for a ligand to have potency (generate a measurable response)?

Answer 12

1. Affinity (ligand binding to effector)
2. Intrinsic efficacy (receptor activation)
3. “Things” to generate a measurable response, e.g. no. receptors

2+3 constitute efficacy

Question 13

The same potency could occur with different combinations of affinity and efficacy

Answer 13

E.g. drugs with identical affinites (same binding occupancy curve) could have different efficacies (one having a faster response than the other)

Question 14

What is the difference between concentration and dose>

Answer 14

Terms often used interchangeably, but:

• CONCENTRATION: known concentration of drug at site of action e.g. in cells and tissues
• DOSE: concentration at a site of action that is unknown, e.g. the dose given to a patient in mg/kg

Question 15

What factors are considered in the study of a drug?

Answer 15

Potency: affinity (does it bind?) and efficacy (does it do what its supposed to? zero/low=antagonist or antagonist; high=agonist)

Selectivity: are there off-target effects?

Pharmokinetics: how does the body metabolise it?

Physiochemical properties: solubility, pH, stability, crystallinity

Question 16

Describe the therapeutic products used to treat asthma

Answer 16

SALBUTAMOL: agonist to relieve symptoms quickly when needed by reversing bronchoconstriction. 20-fold B2 selective (over B1; as B2 has lower Kd) which is poor selectivity but the affinity is increased by B2-selective efficacy and the route of administration (inhaled)

SALMETEROL: long acting agonist to relieve symptoms. 3455-fold B2 selective (good), but no selective efficacy (so selectivity is based on the affinity)

Question 17

Problems with asthma drugs?

Answer 17

Salmeterol is insoluble

Salbutamol is also a beta 1 adrenoceptor agonist so speeds up heart rate

Need for drugs with enhanced selectivity: affinity and/or efficacy

Question 18

Explain the concept of spare receptors

Answer 18

Can have

Question 19

What is the effect of changing receptor number?

Answer 19

Changes agonist potency and can affect the maximal response

Question 20

Receptor numbers are not fixed: how do they change?

Answer 20

Increase with low activity (up regulation)-can lead to hypersensitivity reactions
Decrease with high activity (down regulation)-can lead to tolerance/tachyphylaxis

Both of these changes in receptor number can be due to physiological, pathological or drug-induced changes

Question 21

Describe the differences between full and partial agonists

Answer 21

Full agonists: EC50

Question 22

Why might a partial agonist have a high potency despite having a low maximal effect?

Answer 22

Potency of partial agonist is the concentration to produce half of IT’S maximum effect (usually sub-maximal for the tissue), so even though maximal effect is low, potency could still be high if the drug binds with high affinity

Question 23

How could a partial agonist have a better clinical effect than a full agonist?

Answer 23

If only a small proportion of receptors need to be activated to produce a clinical effect , then a partial agonist with a higher potency could produce a larger response at a given low concentration of drug than a full agonist at a lower potency

Question 24

Describe why buprenorphine can be advantageous over morphine in certain clinical settings

Answer 24

Buprenorphine has a higher affinity than morphine (lower Kd), but a lower efficacy (inability to produce a full response). This can be advantageous for pain control, as morphine acts more strongly at u-opioid GPCRs, causing respiratory depression so buprenorphine has less effect

Question 25

Why does injecting buprenorphine instead of heroin (diamorphine) produce undesirable effects?

Answer 25

Heroin is a full agonist at the u-opioid receptor; if inject buprenorphine becomes very ill due to the withwarawl effects and partial agonism. The bunprenorphine is usually a partial agonist but in absence of heroin acts as an antagonist so occupies receptors, limiting response

Question 26

How can opioid addiction be treated?

Answer 26

Buprenorphine can enable gradual withdrawal and produce the use of other illicit opioids

Question 27

Why are partial agonists not always partial agonists?

Answer 27

It is compound and system-dependent. Increasing receptor number can change a partial agonist into a full agonist. The partial agonist still has low intrinsic efficacy at each receptor, but sufficient receptors to generate a full response. Although they have lower efficacy than full agonists, it is possible for FA with identical intrinsic activities to have different efficacies

Question 28

Difference between clinical and pharmacological meaning of efficacy?

Answer 28

In clinical terms it is often used to describe how good a drug is at producing a response

Question 29

Describe reversible competitive antagonism

Answer 29

• needs dynamic equilibrium between L and R
• greater [antagonist]=greater inhibition
• IC50: index of antagonist potency, determined by strength of stimulus ([agonist])
• Kd: to describe antagonist affinity
• KB: when derived pharmacologically
• compete with agonists for binding: inhibition is SURMOUNTABLE (can be overcome with increased agonist)
• cause a parallel shift to the right of the agonist concentration-response curve

Question 30

Action of naloxone

Answer 30

A reversible competitive antagonist which has high affinity at u-opioid receptors. Can be used to reverse opioid-mediated respiratory depression as its high affinity means it will compete effectively with other opioids e.g. heroin for receptors

Question 31

Describe irreversible competitive antagonism

Answer 31

• antagonist dissociates slowly/not at all
• more [antagonist] or more time: more receptors are blocked. Therefore is NON-SURMOUNTABLE
• cause parallel shift to the right of the agonist C-R curve
• at higher concentrations suppress the maximal response, as spare receptors are filled by antagonist so there are insufficient receptors for a full response

Question 32

Action of phenoxybenzamine

Answer 32

Non-selective irreversible alpha 1 adrenoceptor antagonist used for hypertension episodes in pheochromocytoma. Prevents the action of adrenaline which would cause excess vasoconstriction

Question 33

Describe non-competitive antagonism

Answer 33

• Endogenous ligand binds to the orthosteric site, antagonist binds elsewhere (ALLOSTERIC sites)
• provide binding for agonists and non-competitive antagonists
• effects similar to irreversible competitive antagonism, need additional experiments to distinguish

Question 34

Describe allosteric regulation of GPCRs

Answer 34

Higher receptor subtype selectivity, non-competitive, often require orthosteric ligand. E.g. cinacalet: positive allosteric modulator of the calcium signalling receptor; stimulates calcitonin release from C cells of thyroid so used in hyperparathyroidism

Question 35

What is tachyphylaxis?

Answer 35

Aka desensitisation

Increased exposure to an agonist causes receptor downregulation

Question 36

What is supersensitivity?

Answer 36

Over-expression of a receptor to counter the chronic administration of antagonist. If the antagonist treatment is then removed, there are now more unoccupied receptors available to the endogenous agonist and supersensitivity results.

Question 37

What mechanisms may be responsible for receptor desensitisation?

Answer 37

1. Change in receptor properties: change to desensitised conformation due to prolonged agonist binding, receptor phosphorylation or binding of inhibitor protein
2. Loss of receptors: reversible internalisation of receptors by RME (can be recycled), irreversible loss by endocytosis to lysosomes

Question 38

What do homologous and heterologous receptor desensitisation mean?

Answer 38

Homologous desensitisation: the process by which only the signal from the stimulated receptor is reduced, signalling through other receptors is not affected

Heterologous desensitisation: the process where receptors for other agonists become less effective, even though only one has been continuously stimulated. Likely to involve negative feedback on a signalling pathway common to both the stimulated receptor and others. Receptor phosphorylation is commonly associated §

Question 39

What might be the effect of increased breakdown of the agonist on the response of a drug?

Answer 39

Tissue desensitisation

Question 40

What is the effect of receptor recycling in receptor REsensitisation?

Answer 40

Restores the levels of receptor in the plasma membrane

Question 41

Why can receptor supersensitivity to beta blockers cause problems if a patient suddenly stops taking it?

Answer 41

May suddenly increase heart rate and contraction, increasing O2 consumption and potentially causing MI