SESSION 6 Flashcards

1
Q

Define a ligand

A

A molecule that binds to another molecules

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2
Q

Define affinity

A

The degree to which a substance tends to combine with another

To bind to a receptor a ligand must have affinity for the receptor

Higher affinity -> stronger binding

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3
Q

Define partial agonist

A

Drugs that bind to/ and activate a given receptor, but have only partial efficacy at the receptor relative to a full agonist

All receptors occupied- insufficient efficacy fro maximal response
Low intrinsic efficacy

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4
Q

Define intrinsic activity/ efficacy

A

The ability of a ligand to cause a response (receptor activation)
Efficacy is governed by intrinsic efficacy
The number of receptors, influence the response

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5
Q

Define functional antagonist

A

An antagonist may act as a completely separate receptor, initiating effects that are functionally the opposite of the agonist

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6
Q

Define reversible competitive antagonism

A

a receptor antagonist that binds to a receptor but does not activate the receptor.
The antagonist will compete with available agonist for receptor binding sites on the same receptor
- the inhibition is surmountable

Relies on dynamic equilibrium between ligands and receptors

Cause a parallel shift to the right of the agonist concentration- response curve

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7
Q

Define irreversible competitive antagonism

A

A type of antagonist that binds permanently to a receptor, either by forming a covalent bond to the active site that does not dissociate or dissociates too slowly

Non surmountable
- with increased antagonist or increased time more receptors are blocked

Cause a parallel shift to the gist of the agonist concentration- response curve
And at high concentrations suppress the maximal response

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8
Q

Define non- competitive antagonism

A

Non- competitive antagonism is when the antagonist binds to an allosteric site of the receptor

No competition for binding site- reduce orthosteric ligand affinity and/ or efficacy

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9
Q

Define agonist

A

A substance which initiates a physiological response when combined with a receptor

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10
Q

Define antagonist

A

A substance which interferes with or inhibits the physiological action of another

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11
Q

What is drug action determined by?

A

The concentration of drug molecules around receptors

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12
Q

What is the significance of molarity?

A

An international unit of measure- allowing easy comparison

LEARN MOLE CONCENTRATION TABLE

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13
Q

What is drug- receptor interactions governed by?

A

Binding governed by association and dissociation

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14
Q

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

A

Agonists:

  • have intrinsic efficacy- activate the receptors
  • have efficacy- cause a measurable response

Antagonists:

  • only have affinity
  • no efficacy as the receptor is not activated
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15
Q

How do we measure drug -receptor interactions by binding?

A
  • bind radioactively labelled ligands to cells

- these radio ligands enable us to track and measure the drug- receptor interactions

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16
Q

Define Bmax

A

Bmax is the maximum binding capacity- all the receptors are saturated, adding more ligand will have no further effect

17
Q

Define Kd

A

Kd is the dissociation constant
Kd is the concentration of ligand required to occupy 50% of the available receptors
Indicated the strength of interaction

Kd- index of affinity
Lower value = higher affinity

18
Q

Is affinity important for ligand?

A

High affinity allows binding at low concentrations

As a result sensible seized tablet can be given

19
Q

Define Emax

A

Emax is the effective maximum response achievable from an applied or dosed agent

20
Q

Define EC50

A

Effective concentration giving 50% of the maximal response
This is a measure of agonist potency
It depends on both affinity and intrinsic efficacy

21
Q

Define potency

A

The generation of a measurable response to efficacy and affinity
Affinity + efficacy = potency
Same potency could occur with different combinations of affinity and efficacy

22
Q

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

A

1) ligand binding to the receptor- affinity
2) receptor activation - intrinsic efficacy
3) things to happen to generate measurable response

2&3 are efficacy

23
Q

Define concentration

A

Known concentration of drug at site of action

E.g. In the cells or tissues

24
Q

Define dose

A

Concentration at site of action unknown

E.g. Dose to a patient in mg

25
Q

Using asthma as an example explain the clinical relevance of receptor selectivity

A

Asthma results in the airways contracting
It is treated by adrenaline- attaches to B2- adrenoreceptors (GPCR)- causing the airways to relax

Problem:
It is hard to create a drug that is specific to B2 - adrenoreceptors
As B1 receptors are very similar and result in increased contraction of the heart

26
Q

How do we achieve selectivity in treating asthma?

A

Salbutamol
Small difference in affinity but large difference in efficacy so doesn’t activate b1

Salmeterol:

  • No selective efficacy
  • Selectively based on affinity
  • B2 affinity is a lot higher

Salbutamol cannot be given to people with heart problems as it isn’t;t selective enough
May result in angina- sipped up of the heart

27
Q

Define spare receptors

A

The situation in which maximum tissue response occurs when not al the receptors of the tissue are occupied by the drug

28
Q

What factors other than the number of receptors limit response?

A
  • a muscle can only contract so much

- a gland can only secrete so much

29
Q

Why are spare receptors important?

A

Spare receptors increase sensitivity
- allow response at low concentrations of agonist

10% occupancy of muscarinic receptors –> 90% of receptors are spare

30
Q

What is the relevance of altered receptor number ?

A

Changing receptor number changes agonist potency
- and can affect the maximal response

Tend to increase with low activity (increase regulation)
Tend to decrease with high activity (down regulation - result in tolerance with reference to drugs)

31
Q

Define full agonist

A

Full agonists have affinity for/ and activate a receptor, producing full efficacy at that receptor

Often endogenous ligands

32
Q

What is the relevance of partial agonists?

A
  • allow a more controlled response
  • work in the absence of low levels of endogenous ligands
  • act as antagonist if high levels of full agonist
33
Q

Give an example of a partial agonist

A

Buprenorphine- higher affinity but lower efficacy than morphine

Advantageous to morphine in some clinical settings:

  • pain control
  • less respiratory depression

If a heroin addict is placed on buprenorphine they may become ill as a result of withdrawal symptoms
Buprenorphine will inhibit the effect of heroin, ie. partial agonist

34
Q

Are partial agonists always partial agonists?

A

NO:

  • increasing receptor number can change partial agonists to full agonists
  • partial agonist still has low intrinsic efficacy at each receptor
  • BUT sufficient receptors to generate a full response
35
Q

Define IC50

A

IC50- index of antagonist potency

Determined by strength of stimulus (ie. agonist)

36
Q

Give an example of an irreversible competitive antagonist that is used clinically

A

Phenoxybenzamine- non- selective irreversible a1- adrenoceptor blocker used in hypertension episodes

Pheochromocytoma - tumour of adrenal chromaffin cells
Resulting in excessive adrenaline
Phenoxybenzamine prevents vasoconstriction by a1- adrenoreceptors

37
Q

Give an example of a non- competitive antagonist that is used clinically

A

Maraviroc:

  • negative allosteric modulator of chemokine receptor 5
  • used in AIDs treatment
  • usually used by HIV to enter the cells