1. Relative potency of Drugs
Drug potency is related to efficacy
Relates the concentration of a drug required to produce a given response
Consider two drugs - DRUG A & DRUG B
These drugs act on the same receptors
But have different potencies

2. Log10 conc v response to show potency
Drug A
Drug B
150
100
Response (mm) 50 -8 -7 -6 -5 -4 -3
Log conc agonist (M)

3. Calculation of Potency
Which of these drugs is more potent?
Calculate EC50
Concentration required to give 50% of the maximum response
Rather “old fashioned term” pD2 = -log EC50

4. Obtaining EC50 values Response (mm) Log conc agonist (M) MAX 50% 150
100
Response (mm)
50% 50 -8 -7 -6 -5 -4 -3
Log conc agonist (M)

5. Calculation of potency ratio of drugs A and B
EC50 drug B = µM or 2.31 x 10-5 M
EC50 drug A = 0.18 µM or 1.8 x 10-7 M
Relative potency = EC50 drug B
EC50 drug A
= 23.07
0.18
drug A is 128 times more potent than drug B

6. Recommended Reading Potency Pharmacology, 5th Ed: Page 11
Basic and Clinical Pharmacology Online Book: Section I, Chapter 2, Signalling Mechanisms and Drug Action