Pharmacology - Receptor Binding

Question 1

Dissociation constant (KD)

Answer 1

conc. of drug that occupies 50% of the receptors –> measure of affinity

Question 2

EC50

Answer 2

conc. of drug that gives 50% of maximum response

Question 3

CR

Answer 3

conc. ratio = agonist EC50 in presence of antagonist / agonist EC50 alone

remember to inverse log conc. to conc.

Question 4

Gaddum equation

Answer 4

for reversible competitive antagonists:

CR = (conc. of antagonist / KD) + 1

Question 5

Direct measurement of KD

Answer 5

1) radiolabel drug probe
(ex: replace H+ in salbutamol with radioactive H3)

2) radioligand binding of drug to receptor
3) measure amount of radioactivity bound to receptor
- can use fluorescent labels
- saturation binding analysis
- 50% binding –> KD

Question 6

Competition binding

Answer 6

affinity of unlabeled drug competing for radioligand, use for new drugs

1) add 1 conc. of known labeled radiolignad
2) increasing conc, of unlabeled drug –> decreasing radioactivity

Question 7

IC50

Answer 7

conc. of unlabeled drug that reduces binding of radiolabeled drug by 50%

radiolabeled drug with greater affinity / more radiolabelled drug –> increased IC50

Question 8

Chen-Prusoff Equation

Answer 8

Ki = IC50 / (1 + (radiolabeled drug conc. (RL) /KD of radiolabeled drug))

Question 9

What binding can tell us

Answer 9

• KD: affinity of compound for receptor
• Selectivity: affinity between receptors
• Bmax: number of receptors present
• Which receptor is present
• Whether there are multiple receptors
• k(off) rate: how long lasting the binding is

Question 10

Dissociation rates

Answer 10

K(on): L+R –> LR
K(off): LR –> L+R

ex: ipatropium (short-lasting) and tiotropium (long-lasting)

Question 11

What binding can’t tell us

Answer 11

compound agonist or antagonist?