Drug-receptor interactions

Question 1

What is the law of mass action?

Answer 1

The rate of a chemical reaction is proportional to the product of the concentration of the reactants

Question 2

What is the equilibrium dissociation constant (Kd)?

Answer 2

The concentration of drug required to occupy 50% of the receptors at equilibrium

Question 3

How can affinity be expressed?

Answer 3

• It is conventional to express equilibrium dissociation constant (Kd) as -log10 of the Kd
• This is called the pD2 for an agonist
• pD2 is the -log of [D] that occupies 50% of the receptors at equilibrium

Question 4

What is the equation for pD2?

Answer 4

pD2 = -log10(Kd)

Question 5

Explain efficacy

Answer 5

• The maximum response of a tissue can be obtained by occupying less than 100% of the receptors
• Different drugs have different capacities to initiate a response
• A maximum response is achieved when a particular stimulus is generated by receptor occupation

Question 6

What can partial agonists do?

Answer 6

Partial agonists can decrease the response to a full agonist because some receptors will be occupied by partial agonist molecules, giving a smaller response than if all the receptors were occupied by full agonist molecules

Question 7

What is a competitive antagonist?

Answer 7

A competitive antagonist may be regarded as a drug which interacts/ binds reversibly with receptors to form a complex but this complex does not evoke a response

Question 8

What does the Schild equation give us?

Answer 8

The ratio by which [D] must be increased to overcome competition by [A]

Question 9

What is the Schild equation?

Answer 9

r = ([A]/Ka) + 1

Question 10

What is an irreversible antagonist?

Answer 10

An antagonist that binds to the receptor but dissociates very slowly (if at all).
This means maximum response to agonist is decreased

Question 10

What is an irreversible antagonist?

Answer 10

An antagonist that binds to the receptor but dissociates very slowly (if at all).
This means maximum response to agonist is decreased

Question 11

What do allosteric binding sites do?

Answer 11

• Modify the affinity or efficacy of a receptor’s endogenous ligand or drug
• Can potentiate or inhibit

Question 12

What is a non-competitive antagonist?

Answer 12

• Non competitive antagonists are an example of allosteric modulation
• The antagonist blocks access of the agonist to its binding site through steric hindrance