Session 6

Question 1

1. The target site for drugs are mainly proteins , give examples of protein targets.
2. What are the exceptions?

Answer 1

1. R - receptor

I - ion channel

T - Transporters?

E - enzyme

2. some antimicrobial & antitumour drugs bind DNA

Question 2

Give examples of receptors which can be targeted

Answer 2

K - tyrosine kinase

I - ion channel

N - Nuclear hormone receptors

G - GPCRs

Question 3

State the prefixes for concentration

Answer 3

Question 4

How do you work out molarity?

Answer 4

Question 5

Why we need to consider drug concentrations in molarity

Answer 5

Question 6

1. Most drugs bind ? to receptors -binding governed by association AND dissociation
2. Most drugs either ?​

Answer 6

1. reversibly
2. • block the binding of an endogenous agonist (antagonist) OR
     - activate a receptor (agonist)

Question 7

1. To do anything they must BIND to the receptor. To bind to a receptor a ligand must have ? for the receptor

Answer 7

1. AFFINITY

Question 8

1. Binding governed by affinity higher affinity = ?
2. Does an antagonist have affinity?
3. An agonist has bound to its receptor due to its affinity for the receptor. What happens next?

Answer 8

1. stronger binding
2. Yes
3. Beyond the activated receptor THINGS have to happen to evoke a response – dependent on the response and the cell/tissue

The ability of a ligand to cause a response is an indication of the ligand’s EFFICACY

Efficacy governed by intrinsic efficacy PLUS other things that influence the response:

Question 9

1. Agonists have …
2. Antagonists have… • affinity ONLY

Answer 9

1. • affinity

• have intrinsic efficacy (ie. can activate the receptor)
• have efficacy (ie. cause a measurable response)

2. • affinity ONLY

Question 10

1. How do we measure drug-receptor interactions by binding?

Answer 10

Often by binding of a radioactively labelled ligand (radioligand) to cells or membranes prepared from cells

​

Question 11

1. What is B_max
2. What is K_d

Answer 11

1. Bmax (max. binding capacity – information about receptor number)
2. concentration of ligand required to occupy 50% of the available receptors

Kd (or KD) = dissociation constant

Kd= index of AFFINITY

LOWER value = HIGHER affinity
i.e. Kd = is actually the reciprocal of affinity

Question 12

Answer 12

Question 13

[Drug] – usually logarithmic not linear:

Answer 13

Question 14

Relation between [drug] and RESPONSE nb. response requires drug efficacy agonist

1. Response could be e.g. ?
2. What graph do we use to determine the relationship between [drug] and RESPONSE?
3. What is EC₅₀

Answer 14

1. • change in a signalling pathway

• change in cell or tissue behaviour (e.g. contraction)

2. Concentration-response curve
3. effective concentration giving 50% of the maximal response

Question 15

1. Concentration –
2. Dose –

Answer 15

1. known concentration of drug at site of action – e.g. in cells and tissues
2. concentration at site of action unknown – e.g. dose to a patient in mg or mg/kg

Question 16

What is EC₅₀ ?

Answer 16

POTENCY

effective concentration giving 50% of the maximal response

This is a measure of agonist POTENCY -
it depends on BOTH affinity and intrinsic efficacy (ie. ability to activate receptor)
PLUS cell/tissue-specific components (that allow something to happen)

Question 17

ie. for a ligand to have potency (generate a measurable response) requires:

Answer 17

Question 18

Note that the same potency could occur with ?

Answer 18

different combinations of affinity and efficacy

Question 19

What is the effect of adrenaline in asthmatics

Answer 19

Adrenaline (noradrenaline) -> B2 adrenoceptor agonists -> relaxation

HOWEVER

but other b-adrenoceptors elsewhere eg. b1 in heart – increase force & rate

need selective/specific activation of B₂ adrenoceptors (in the airways) to treat asthma

Question 20

Which agent is better to use for asthmatics and why. Salbutamol or salmeterol

Answer 20

Question 21

What is the problem with using salbutamol?

Answer 21

angina

b₁-adrenoceptor – speed up heart

Question 22

Potency depends on BOTH affinity & intrinsic efficacy PLUS cell/tissue-dependent factors including

Answer 22

the NUMBER of receptors

Question 23

How do cell/tissue dependent factors such as receptor number influence agonist potency?

Answer 23

often the response is controlled or limited by other factors eg.
• a muscle can only contract so much • a gland can only secrete so much

Question 24

Spare Receptors

1. Often seen when receptors catalytically active eg.
2. Exist because of:

Answer 24

1. tyrosine kinase or G-protein coupled receptors
2. • amplification in the signal transduction pathway

• response limited by a post-receptor event

Question 25

Give an example where signal amplification is seen

Answer 25

- Adrenaline - B - adrenoceptor - Gs protein - adenylyl cyclase - cAMP - PKA

Question 26

Why have spare receptors?

Answer 26

**increase sensitivity** – allow responses at low concentrations of agonist

Question 27

Changing receptor number changes?

Answer 27

agonist potency and can effect the maximal response

Question 28

Explain why receptor no’s are not fixed

Answer 28

* tend to increase with low activity (up-regulation) * tend to decrease with high activity (down-regulation) (for drugs this can contribute to tolerance/tachyphylaxis) physiological, pathological or drug-induced changes

Question 29

What does the statement Not all agonists elicit maximal responses in the same assay mean

Answer 29

Partial agonist

Question 30

1. Maximal response indicates ?

Answer 30

1. intrinsic activity

Question 31

Describe the intrinsic activity of partial agonists

Answer 31

Partial agonists have lower intrinsic activity as they have lower efficacy than full agonists (– usually lower intrinsic efficacy)

Question 32

Relevance of partial agonists: (3)

Answer 32

* Can allow a more **controlled response** * Work in the absence or low levels of (endogenous) ligand * Can act as **antagonist if high levels of full agonist**

Question 33

Opioids are clinically used for? Unfortunately are also used for? ADR? Action primarily through?

Answer 33

* Pain relief * Recreational use (eg. heroin) - euphoria * BUT respiratory depression – can lead to death * μ-opioid receptor (GPCR)

Question 34

Along with the graph explain how buprenorphine can be used clinically

Answer 34

buprenorphine – higher affinity (ie. lower K efficacy (inability to produce full response) than morphine buprenorphine can be advantageous to morphine in some clinical settings e.g. pain control – adequate pain control, less respiratory depression Partial agonists and the treatment of opioid addiction e.g. buprenorphine to enable gradual withdrawal and prevent use of other illicit opioids

Question 35

Heroin (diamorphine) addict (heroin - full agonist at μ-opioid receptor) Addiction related to physical and psychological dependence. The addict frequently injects heroin but injects a stolen narcotic instead of heroin – turns out to be buprenorphine. Immediately become very ill. Why?

Answer 35

1. Withdrawal or abstinence syndrome – generally opposite to acute drug effects - contributes to continued drug taking. 2. Partial agonism Withdrawal symptoms as buprenorphine antagonizes the effect of heroin – low efficacy at receptor

Question 36

Partial agonism is compound AND **_SYSTEM_** dependent. Increasing receptor number can change a partial agonist into a ?

Answer 36

full agonist Partial agonist still has low intrinsic efficacy at each receptor BUT - sufficient receptors to generate a full response.

Question 37

* Partial agonists have lower efficacy than full agonists * BUT full agonists with identical intrinsic activities may have different efficacies Draw a diagram to represent this information

Answer 37

Question 38

How would you describe the compounds in terms of agonism? Rank in terms of efficacy. Rank in terms of potency.

Answer 38

Question 39

1. What is an antagonist? 2. What are the three ways in which they can act?

Answer 39

1. Block the effects of agonists ie. prevent receptor activation by agonists 2. - Reversible competitive antagonism (commonest and most important in therapeutics) - Irreversible competitive antagonism - Non-competitive antagonism (generally allosteric or even post-receptor)

Question 40

1. What is IC₅₀ 2. Competitive antagonists compete with agonists for binding – the inhibition is ?

Answer 40

1. Concentration of antagonist giving 50% inhibition index of antagonist potency determined by strength of stimulus (i.e. [agonist]) Kd used to describe antagonist affinity KB when derived pharmacologically (50% occupancy - reciprocal of affinity) 2. SURMOUNTABLE

Question 41

How do reversible competitive antagonists effect the agonist concentration-response curve?

Answer 41

cause a parallel shift to the right

Question 42

Naloxone is a high affinity, competitive antagonist at μ-opioid receptors. Why might such a compound be useful clinically?

Answer 42

Reversal of opioid-mediated respiratory depression - high affinity means it will compete effectively with other opioids (e.g. heroin) for receptors

Question 43

1. Irreversible competitive antagonism occurs when the antagonist ? 2. Effects are? 3. How do Irreversible competitive antagonists effect the concentration-response curve

Answer 43

1. dissociates slowly or not at all 2. With increased [antagonist] or increased time more receptors are blocked by antagonist – NON-SURMOUNTABLE 3. Irreversible competitive antagonists cause a parallel shift to the right of the agonist concentration-response curve and at higher concentrations suppress the maximal response

Question 44

Give an example of a Irreversible competitive antagonists and where it is used clinically

Answer 44

Pheochromocytoma e.g. phenoxybenzamine – non-selective irreversible a -adrenoceptor blocker used in hypertension episodes in pheochromocytoma

Question 45

Allosteric sites Provide binding sites for:

Answer 45

Question 46

Allosteric compounds for GPCRs just emerging in the clinic Maraviroc - Negative allosteric modulator (NAM) of chemokine receptor 5 (CCR5) Used by HIV to enter cells. - Used in AIDS. Allosteric compounds more established in other areas eg. ion channels and enzymes