Pharmacodynamics

Question 1

Drugs exert effects by binding to a target. What are targets?

Answer 1

TARGETS are mostly proteins - exceptions eg. some antimicrobial & antitumour drugs bind DNA
E.g. ion channels, GPCRs, kinases, nuclear receptors

Question 2

Give an example of a drug which targets a gpcr

Answer 2

Adalimumab - antibody bonds TNFa - rheumatoid arthritis

Salmeterol - b2 adrenoceptor agonist - asthma

Question 3

What is a ligand

Answer 3

ligand - something that binds to a receptor eg. drug, hormone, neurotransmitter
- applicable to targets other than GPCRs

Question 4

What is critical in determining drug action

Answer 4

Concentration of drug molecules around receptors
Therefore we need to consider drug concentrations in molarity
If the MWt of 2different drugs is different, if the conc in mg/L is the same the molarity will be very different

Question 5

Give an equation for molarity

Answer 5

Molecular weight x Molarity = concentration (g/L)

Molarity = conc/MWt

Question 6

Do most drugs bind reversibly or irreversibly?

Answer 6

Reversibly i.e. binding governed by association and dissociation

Question 7

Define agonist and antagonist

Answer 7

Most drugs either 1) block the binding of an endogenous agonist (antagonist) OR 2) activate a receptor (agonist)

Question 8

What is binding governed by?

Answer 8

To do anything drugs must bind to the receptor
To bind they must have affinity for the receptor
Binding is governed by affinity
Higher affinity = stronger binding

Question 9

What is the difference between intrinsic efficacy and efficacy?

Answer 9

Intrinsic efficacy = ability to activate receptor
Efficacy = the ability of the ligand to cause a response - efficacy governed by intrinsic efficacy plus other things that influence the response

Question 10

What is R*?

Answer 10

Activated receptor

Question 11

Do antagonists have efficacy?

Answer 11

No - only affinity

AGONISTS have intrinsic efficacy and efficacy

Question 12

What is clinical efficacy

Answer 12

Clinical efficacy is a more a measure of how well a treatment succeeds in achieving its aim. Does it lower blood pressure? Does it cure a headache?

Question 13

How do we measure binding?

Answer 13

Often by binding of a radioactively labelled ligand (radioligand) to cells or membranes prepared from cells
Many cells/many receptors (e.g. 10,000-100,000 per cell)
Incubate radioligand and receptors - binding occurs
Separate bound and free, usually measure bound

Question 14

How can we quantify binding graphically? What is the shape of the graph?

Answer 14

Plot proportion of receptors bound against conc of drug (usually logarithmic)

rectangular hyperbola if not logarithmic
Sigmoidal if logarithmic

Log of a number = power by which 10 has to be raised to get that number - but remember ligand concs at receptors usually «1M

Question 15

What is the Bmax?

Answer 15

Maximum binding capacity - gives info on receptor number

Question 16

What is the Kd?

Answer 16

The concentration of ligand required to occupy 50% o the available receptors
It is the index of AFFINITY (strength of interaction)
LOWER value = HIGHER affinity i.e. K
is actually the reciprocal of affinity

Question 17

How I.D affinity important for ligand

Answer 17

High affinity allows binding at low concentrations

Question 18

What could a response be?

Answer 18

Change in signalling pathway
Change in cell or tissue behaviour e.g. contraction

Response requires efficacy

Question 19

What is the shape of a concentration response curve?

Answer 19

Rectangular hyperbola if drug conc is linear

Sigmoidal is drug conc is logarithmic

Question 20

What is Emax?

Answer 20

Maximu effect

Question 21

What is EC50?

Answer 21

Defective concentration giving 50% of maximal response

Question 22

What is EC50 a measure of and what does it depend on?

Answer 22

Measure of agonist potency
Depends on both affinity AND intrinsic efficacy PLUS cell/tissue specific components that allow something to happen - including the NUMBER OF RECEPTORS

Question 23

Define concentration and dose

Answer 23

Concentration – known concentration of drug at site of action – e.g. in cells and tissues Dose - concentration at site of action unknown – e.g. dose to a patient in mg or mg/kg
Terms often used interchangeably

Question 24

How is asthma treated and what side effects can this have

Answer 24

Therapeutic target:
b2-adrenoceptors – relaxation – agonist for treatment activates the receptor (agonist) but provides functional antagonism of contraction
b2-adrenoceptors in airways smooth muscle
but other b-adrenoceptors elsewhere eg. b1 in heart - increase force and rate
need selective/specific activation of b2-adrenoceptors (in the airways) to treat asthma

Question 25

How do we achieve selectivity?

Answer 25

Eg salbutamol - Administered directly to lungs to conc around b2 in lungs is high compared to around b1 in heart Should target b2 but also targets b1 which are mainly present in the heart 20-fold selectivity is not good - but good intrinsic efficacy Salmeterol - much higher selectivity for b2 but has higher kd so a lot more needed

Question 26

What are the problems with giving salbutamol and salmeterol?

Answer 26

Salmeterol has high selectivity for b2 but has bad solubility so cant give intravenously Sambutamol is suitable but has an effect on the heart - speeds up heart bc of b1 adrenoceptors - so bad for ppl with angina Need for drugs with enhanced selectivity - affinity and/or efficacy

Question 27

Are affinity, intrinsic efficacy and potency variable or fixed?

Answer 27

``` Strength of interaction (affinity) - FIXED Active conformation (intrinsic efficacy) -FIXED Potency - VARIABLE - cell/tissue dependent factors addict efficacy ```

Question 28

Explain spare receptors

Answer 28

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 I.e. in some cases <100% occupancy = 100% response This can be seen on a graph of binding occupancy against drug conc Extra receptors = SPARE

Question 29

Where are spare receptors often found?

Answer 29

Often seen when receptors catalytically active eg. tyrosine kinase or G-protein coupled receptors Exist because of: • amplification in the signal transduction pathway • response limited by a post-receptor event

Question 30

Give an example of spare receptors

Answer 30

In the smooth muscle of the lungs Para nerves -> ACh -> M3 mAChRs 10% occupancy = max contraction

Question 31

Why have spare receptors?

Answer 31

Increase sensitivity - allow responses at low conc of agonist An example to illustrate this point: - full response requires 10,000 activated receptors/cell If no spare receptors (ie. 10,000 receptors/cell) full response requires 100% occupancy— more than Kd conc of drug needed - e.g. if Kd=1nM, requires 10-100 nM with spare receptors (eg. 20,000 receptors/cell) only 50% occupancy required for full response - requires Kd conc of drug - e.g. if Kd = 1nM, requires 1nM

Question 32

How can agonist potency be changed?

Answer 32

Changing receptor number changes agonist potency and can affect maximal response As receptor number increases, less conc of agonist needed for full response (curve shifts to left on conc-response graph) E.g. if double the amount of receptors needed, 50% occupancy (Kd) = full response If not enough receptors - 100% occupancy does not give full response

Question 33

Can receptor numbers change?

Answer 33

Receptor numbers are not fixed: • tend to increase with low activity (up-regulation) • tend to decrease with high activity (down-regulation) - for drugs this can contribute to tolerance and withdrawal symptoms Changes are continuous and could be physiological, pathological or drug-induced

Question 34

Do all agonists elicit maximal responses in same assay?

Answer 34

No. Full agonist e.g. endogenous ligand - EC50=Kd (spare receptors) Partial agonist - EC50 approx = Kd - no spare receptors - all receptors occupied - insufficient intrinsic efficacy for maximal response

Question 35

Describe the potency and activity of partial agonists

Answer 35

Partial agonist much more potent - high affinity but not good at dissociating Partial agonists have lower intrinsic activity as lower efficacy than full agonists (usually lower intrinsic efficacy) I.e curve on conc-response graph is lower and shifted to left

Question 36

What is the relevance of partial agonists as drugs?

Answer 36

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 Example- Opioids: • Pain relief • Recreational use (eg. heroin) - euphoria • BUT respiratory depression – can lead to death Action primarily through μ-opioid receptor (GPCR)

Question 37

What are the differences between morphine and buprenorphine?

Answer 37

buprenorphine – higher affinity (ie. lower Kd) but lower efficacy (inability to produce full response) than buprenorphine can be advantageous to morphine in some clinical settings e.g. pain control – adequate pain control, less respiratory depression

Question 38

How can partial agonists be used to treat hopped addiction?`

Answer 38

Heroin is a full agonist Buprenorphine has a higher affinity • Buprenorphine will inhibit the effect of heroin: ie. a partial agonist can provide antagonism • Partial agonists sometimes referred to as a mixed agonist/antagonist Addict will take heroin - no effect as receptors bound with partial agonist Buprenorphine now occupy opioid receptors but not giving a full effect – hence withdrawal symptoms.

Question 39

What is withdrawal syndrome

Answer 39

1. Withdrawal or abstinence syndrome generally opposite to acute drug effects – contributes to continued drug taking Continued drug-taking leads to tolerance (eg. reduced receptor numbers and/or reduced post-receptor signalling). When drug is withdrawn, the endogenous ligands are now less effective – hence the withdrawal symptoms. Buprenorphine now occupy opioid receptors but not giving a full effect – hence withdrawal symptoms.

Question 40

Name 3 types of antagonism

Answer 40

Reversible competitive - commonest and most important Irreversible Non-competitive (generally allosteric)

Question 41

Describe reversible competitive antagonism

Answer 41

Competitive antagonists compete with agonists for binding – the inhibition is SURMOUNTABLE Reversible competitive antagonism relies on a dynamic equilibrium between ligands and receptors Greater conc of antagonist = greater inhibition

Question 42

What is IC50?

Answer 42

Concentration f antagonist giving 50% inhibition | IC50 – index of antagonist potency determined by strength of stimulus (i.e. [agonist])

Question 43

What is KB?

Answer 43

Kd used to describe antagonist affinity (see earlier) | KB when derived pharmacologically (50% occupancy - reciprocal of affinity)

Question 44

How do reversible competitive antagonists shift the curve of an agonist concentration response curve?

Answer 44

Reversible competitive antagonists cause a parallel shift to the right of the agonist concentration-response curve Agonist has to compete with antagonist - more needed so curve shifts to the right

Question 45

Give an example of a reversible competitive inhibitor

Answer 45

Naloxone is a high affinity, competitive antagonist at μ-opioid receptors. Why might such a compound be useful clinically? Reversal of opioid-mediated respiratory depression - high affinity means it will compete effectively with other opioids (e.g. heroin) for receptors

Question 46

Describe irreversible competitive antagonism

Answer 46

Irreversible competitive antagonism occurs when the antagonist dissociates slowly or not at all With increased [antagonist] or increased time more receptors are blocked by antagonist – NON-SURMOUNTABLE

Question 47

How do irreversible competitive antagonists shift the agonist concentration-response curve?

Answer 47

Irreversible competitive antagonists cause a parallel shift to the right of the agonist concentration-response curve and at higher concentrations suppress the maximal response Spare receptors - spare disappear if they become occupied by antagonists - fall in max response

Question 48

Give an example o an irreversible competitive antagonist

Answer 48

e.g. phenoxybenzamine – non-selective irreversible a1-adrenoceptor blocker used in hypertension episodes in pheochromocytoma Drug stops vasoconstriction as adrenaline/NA cannot bind to a1 adrenoceptors

Question 49

What s the orthosteric site?

Answer 49

Site of binding of endogenous ligand

Question 50

Describe non-competitive antagonism

Answer 50

Bind to allosteric site Allosteric sites Provide binding sites for: • agonists (potential novel drug targets!) • molecules that enhance or reduce effects of agonists no competition for binding site - reduce orthosteric ligand affinity and/or efficacy

Question 51

What is another name for non-competitive antagonism?

Answer 51

negative allosteric modulation)

Question 52

What are the effects of non-competitive antagonism?

Answer 52

effect on pharmacology: - similar to irreversible competitive antagonism - need additional experiments to distinguish

Question 53

Give an example of an allosteric compound

Answer 53

Allosteric compounds for GPCRs just emerging in the clinic – inhibitors and activators Maraviroc - Negative allosteric modulator (NAM) of chemokine receptor 5 (CCR5) Used by HIV to enter cells - Used in AIDS. - Virus attaches to receptor - receptor undergoes conformational change- taken into the cell taking virus with it Drug chanegs shape of part which virus binds to Allosteric compounds more established in other areas eg. ion channels and enzymes – positive AND negative effects