L9- Pharmacodynamics Receptors Flashcards
a) Define: Ligand
b) Define: Affinity
c) Define: intrinsic efficacy
d) Define: efficacy
a) Something that binds to a receptor e.g. drug, hormone, neurotransmitter
b) affinity can be defined as the strength of interaction with which a drug/ligand binds to its receptor.
- Higher affinity: stronger binding
c) The ability of a ligand to activate a receptor by causing a conformational change in the target protein
d) the ability of a ligand/drug to cause a measurable biological response
Binding obeys the law of mass action. What does this mean?
- related to the concentrations of reactants and products
- follows equlibrium pattern
- reversible
Define:
a) Agonist
b) Antagonist
A) A ligand that binds to a receptor and turns it on, initiating a biological response
b) A ligand that binds t a receptor and blocks it- preventing agonist from binding i.e. no receptor activation and no biological response
For a ligand to bind to a receptor, what quality must it have?
Affinity for the receptor
a) What do agonists require?
b) What do antagonists require?
a) Affinity, intrinsic efficacy and efficacy (along with cell/tissue dependent factors)
b) Only affinity
What is clinical efficacy?
It is more of an indication of how well a treatment succeeds in achieving its aim.
e. g. does it lower BP?
- does it cure headache?
How do we measure ligand binding?
- by binding of a radioligand (radioactively labelled ligand) to cells or membranes prepared from cells
- incubate radioligand and receptors and this will lead to binding
- seperate the bound and free radioligands, measure the bound ligands
- the more binding the larger the measureable signal
a) What is Kd
b) What is Bmax?
a) It is a dissociation constant and it gives you the concentration of ligand required to occupy 50% of available receptors
- measures strength of interaction/affinity
b) the max binding sites- it gives you info about the receptor number
What do the following mean:
a) High Kd
b) Low Kd
a) lower affinity of ligand to receptor (more likely to dissociate)
b) Higher affinity of ligand to receptor (less likely to dissociate)
What would a receptor drug concentration graph look like if it was plotted in the following ways:
a) Linear
b) Logarithmic
a) Hyperbolic
b) Sigmoidal
Often dose and concentration are used interchangeably. Why are they different?
- Concentration: precise concentration of a drug at site of action e.g. cells and tissues
- Dose: concentration at site of action generally unknown
To plot drug vs response, response has to be a measureable change. Give some examples
- change in signalling pathway
- change in cell or tissue behaviour
e. g. cell death (cancer) - hormone or neurotransmitter release
Why is affinity important clinically?
- High affinity allows binding at low concentrations of hormones, neurotransmitters and drugs.
- high affinity means we can administer small amts of drugs
- If someone comes in with a heroin overdose we can give them naloxone (high affinity antagonist of u-opoid receptors), this means it out competes opoid, knocking off heroin preventing it from binding
What is:
a) Emax
b) EC50
a) the maximal response (above which more drug will not produce a response)
b) Effective concentration giving 50 percent of the maximal response
- measure of agonist POTENCY
What is the difference in Kd and EC50?
- Kd: concentration of ligand required to occupy 50% of receptors: only depends on affinity
- EC50: concentration of ligand giving 50 percent of the maximal response: depends on affinity, intrinsic efficacy and cell/tissue dependent events
What are cell/tissue dependent events?
- sequential steps driven by original signal from ligand binding w its target tat leads to the final step of a measured therapeutic response
e. g. receptor number
What are spare receptors and how do spare receptors influence agonist potency?
- receptors that exist in excess of those required to produce a full/maximal response
- ## Spare receptors increase sensitivity/ potency i.e. allows responses at low concentrations of agonists
a) What is potency?
b) How is it measured?
a) Potency: the amount of drug required to produce an effect of given intensity
- he drug which can produce an effect at lower drug concentrations is “more potent”
b) EC50
If a full response requires 10,000 activated receptors. What would a full response be like if the cell has:
a) 10,000 receptors/cell
b) 20,000 receptors/cell
a) Full response will require 100% occupancy
- the drug conc will have to be a LOT higher than kD
b) - full response only requires 50% occupancy and hence the drug conc will be equal to Kd
Why are receptor numbers not fixed?
- vary with cell type
- tend to increase with low activity (up-regulation) –> can lead to super sensitivity
- tend to decrease with high activity (down-regulation) –> can lead to drug tolerance
- physiological, pathological or drug-induced changes
Are all agonists equal at the same receptor?
No
- they can have different affinities and different efficacies
What are partial agonists?
- Drugs that bind to a receptor, but cannot produce a maximal effect even with full occupancy of all the available binding sites in the tissue
- they have lower intrinsic activity as lower efficacy
Compare Partial and full agonists
- Partial have higher affinity (lower kd) than full agonists
- they have lower intinsic activity and hence low efficacy
- can allow a controlled response and can work in the absence or low levels of ligand
- if high levels of full agonist they can act as an antagonist due to higher affinity and bind to receptor hence preventing full agonist from binding
a) What can opioids such as heroin, morphine and fentanyl cause?
b) Why does this explain why partial agonists are used clinically.
a) Action through the u-opioid receptor (GPCR)
- pain relief
- recreational use –> euphoria
- Respiratory depression ( can lead to death)
b) - buprenorphine is an opiate with partial agonism
- it is used to treat pain in hospitals but does not lead to respiratory depression which is the maximal effect of full agonists
- can also be used in the drug addiction setting acting as an antagonist of heroin, occupying u-opioid receptors and limiting the response –> gradual withdrawal
Explain how drug users can get withdrawal?
- sustained drug taking leads to tolerance–> reduced receptor numbers (down regulation) and hence need larger concs to get full effect
- when drug is withdrawn the endogenous ligands are now less effective: withdrawal symptoms
What are the 3 modes of action of antagonists?
- Reversible competitive antagonism
- Irreversible competitive antagonism
- Non-competitive antagonism (generally allosteric)
Outline how reversible competitive antagonism works
- relies on dynamic equilibrium between agonists and antagonists (ligands) and receptors
- greater antagonist concentration = greater inhibition
- the inhibition is surmountable (i.e. can be overcome) if more agonist is added hence it is reversible
What would increasing reversible competitive antagonist concentration do to the shape of the agonist-conc response curve?
- it would cause a parallel shift to the right
- this is because more agonist would be needed to overcome the inhibition of the reversible antagonist
a) What is Ki
b) What is IC50?
a) antagonist affinity for receptor
b) Concentration of antagonist to give 50% inhibition
What is an example of reversible competitive antagonism used clinically?
- Nalaxone: high affinity, competitive antagonist at u-opioid receptors
- competes effectively with other opioids e.g. heroin for receptors
- reversal of opioid mediated respiratory depression
Outline how irreversible competitive antagonism works
- the antagonist dissociates slowly or not at all
- bonds are covalent i.e. not reversible
- Non-surmountable: cannot be overcome by increasing agonist concentrations
What would increasing irreversible competitive antagonist concentration do to the shape of the agonist-conc response curve?
- it would cause a parallel shift to the right and eventually down as well
- shift to the right as increased agonist need to try and fill receptors (to compete)
- shift down as the antagonist fills spare receptors and the maximal response becomes suppressed
Example of irreversible competitive antagonism clinically
- Phenoxybenzamine: irreversible a-1 adrenoceptor blocker used in hypertensive episodes in pheochromocytoma (tumor of adrenal gland tissue)
- will bind using covalent bonds irreversibly to a-1 receptors preventing adrenaline binding and cannot be outcompeted by more adrenaline
What is:
a) Orthosteric site
b) allosteric site
a) site where the natural ligand will bind to activate receptor
b) binding site elsewhere- often agonists will bind here or molecules that reduce effects of agonists
Outline how non-competitive antagonism works
- antagonist will bind to the allosteric site (away from the orthosteric site)
- no competition for access to the orthosteric site with the endogenous ligand
- also known as negative allosteric modulation
Example of non-competitive antagonism clinically
- Maraviroc:
- negative allosteric modulator of chemokine receptor 5 used by HIV to enter cells
- used in AIDS treatment
