antagonism. Schild analysis

Question 1

what are the 5 types of antagonists?

Answer 1

chemical
A chemical that binds to the agonist, preventing the agonist from having an effect
An example would be adding a ‘chelator’ to help with heavy metal poisoning, preventing the heavy metal from interacting with anything in the body

pharmacokinetic
Metabolism - e.g. reducing the amount of drug absorbed, metabolised or change how much is excreted

physiological
two drugs working on different receptors, whose action in the body counteract each other

non-competitive
a drug that blocks some step in the process of receptor activation and response, e.g. a second messenger, of another drug (so it doesn’t compete for the receptor)

competitive
Drugs that compete with an agonist for the binding site of the receptor

Question 2

give examples of drugs that have varying methods of pharmacokinetic antagonism

Answer 2

Absorption - An example would be opiates, and how they decrease gut motility, reducing absorption of other drugs at the gut

Metabolism - Warfarin is an example of why this concept is important. If prescribed with certain antibiotics, the antibiotic may stimulate the metabolism (breakdown) of warfarin

Question 3

how do competitive antagonists work?

Answer 3

They stabilise the receptor in a conformation that prevents any signalling

Question 4

how do you measure a competitive antagonist’s properties?

Answer 4

you can measure its affinity with a ligand-binding assay

OR

perform Schild analysis (more later)

Question 5

what is involved in Schild analysis? how do you calculate KD?

Answer 5

1. make conc.response curves for an agonist, then again several times with increasing conc. of antagonist of interest added
2. calculate dose ratio - a quantification of the shift in conc/response curve as a result of adding a competitive antagonist
3. make a Schild plot - x-axis being log[antagonist], y axis being log(dose ratio - 1)
4. analysis - the x-intercept gives us a ‘pA2’ value (remove negative first)
   rearranging the Schild equation tells us that taking the inverse log of -pA2 is equal to KD

KD = inverse log of X-intercept

Question 6

how do you calculate dose ratio?

Answer 6

at EC50,

DR = conc. of agonist in presence of antagonist

over

conc. of agonist in absence of antagonist

this is performed for each conc. of antagonist you added

Question 7

what is dose ratio?

Answer 7

this is a quantification of the shift in
conc/response curve as a result of adding a competitive antagonist. its kind of like ‘How much more agonist do you need to add in the presence of an antagonist to get the same size response you would get if the antagonist was absent’

Question 8

what kind of drugs can you do a Schild analysis for?

Answer 8

not just competitive antagonists, you can also do it for partial agonists because these act as competitive antagonists when added with a full agonist

note - the only difference will be when the partial agonist is applied with the full, there will be a slight inc. in initial response when compared to the full agonist applied alone

Question 9

what is an irreversible competitive antagonist?

how do you tell the difference between reversible and irreversible?

Answer 9

When a covalent bond forms between antagonist and receptor

just wash the tissue, this will not remove irreversible antagonists

Question 10

how do irreversible competitive antagonists highlight the use of receptor reserves?

Answer 10

Time dependent - more and more receptors OVER TIME will become occupied and unresponsive due to the irreversible antagonists. max response possible decreases as more receptors get occupied by antagonist irreversibly

but due to receptor reserve max response may be possible earlier on even as occupancy of an agonist lowers due to more irreversible binding of the antagonist

Question 11

what is desensitisation (as in specifically what can happen x2)?

Answer 11

very common in GCPRs

Time dependent - more and more receptors OVER TIME will become occupied and unresponsive due to the irreversible antagonists

Post translational modification of receptor leading to uncoupling of the receptor from it’s downstream effectors
Example is phosphorylation

Question 12

what is tachyphylaxis (x3 ways that it happens)?

Answer 12

• Exhaustion of the mediators that are responsible for causing the response
• Increased metabolic degradation or extrusion (forced removal) of the drug
• physiological adaptation making the receptor less responsive