Workbook Qs

Drug targets are often ________ in nature.

Protein (e.g. a receptor / enzyme / carrier protein)

The affinity of a drug for its receptor is usually analysed using what

Radioligand binding assays where the drug is incubated in a subcellular fraction containing the receptors of interest

The binding of drugs to receptors can often be measured directly using what

Drug molecules labelled with 1 or more radioactive atoms (³H, ¹⁴C or ¹²⁵I)

Incubate samples of the tissue with various concs on the radioactive drug until equilibrium is reached.

The radiolabelled drug bound to target receptors is measured.

If we know the Kd for a ligand (the concentration of the ligand where 50% of the receptors are bound), we can calculate the proportion of receptors occupied at a given ligand concentration using the following equation

How is Kd read off a graph

Find 50% occupancy and go across & down

If a drug has a Kd value of 1 x 10^-9 M, calculate the receptor occupancy when [D] (concentration) is 3.0 x 10^-9 M

0.75

Find the Kd value

1 x 10^-9 M

Construct a receptor occupancy curve for Drug A and calculate the Kd and Bmax using the graph.

Drug Concentration (M)	Receptor Occupancy (DR/DRT)
1 x 10-10	0.00
3 x 10-10	0.01
1 x 10-9	0.05
3 x 10-9	0.13
1 x 10-8	0.33
3 x 10-8	0.60
1 x 10-7	0.83
3 x 10-7	0.94
1 x 10-6	0.98
3 x 10-6	0.99
1 x 10-5	1.00

From the curve the Kd value is 3 x 10^-8 M. whilst the Bmax is the maximal point of the curve, in this case equating to 1.00.

How do affinity & Kd relate

High affinity = low Kd

This graph allows us to determine whether Drug A is an agonist or an antagonist

True/False

False

Which of the following statements is true of the Bmax values shown on the graph

• Drugs A and B have the same Bmax value

• All the drugs have the same Bmax value

• Drugs B, C and D have a lower Bmax than Drug A

Drugs A and B have the same Bmax value

All the drugs have the same Bmax value

Which drug has the highest affinity

Drug A (lowest Kd)

Drug A is an agonist. Drugs B and C are antagonists. Identify the nature of their antagonism, i.e. competitive/non-competitive

Drug B = competitive antagonist - Increasing concentrations of Drug A will fully remove all of this antagonism

Drug C= non-competitive antagonist, the antagonism can only be partially overcome by increasing concentrations of Drug A

How do you tell the difference between a drug occupancy or concentration/response curve

In drug occupancy, the y-axis is receptor occupancy, whilst in a concentration/response curve, it is Biological Response.

Name the 3 types of agonist & 2 types of antagonist

Agonist: full, partial or inverse

Antagonist: competitive, noncompetitive

When are responses plotted as a concentration–response curve vs a dose–response curve

Concentration–response curve: When dealing with an in vitro or isolated organ preparation

Dose–response curve: When it is derived in vivo, i.e. in laboratory animals or human subjects

How would you plot this on a graph

Concentration in organ bath (M)	Acetylcholine (ACh)		Drug A
	Response (mm)	(%Emax)	Response (mm)	(%Emax)
1 x 10-9	0	0
3 x 10-9	1	2
1 x 10-8	5	8	0	0
3 x 10-8	22	35	2	3
1 x 10-7	42	68	16	26
3 x 10-7	51	82	38	61
1 x 10-6	58	94	50	81
3 x 10-6	61	98	57	92
1 x 10-5	62	100	61	98
3 x 10-5	62	100	62	100
1 x 10-4			62	100

The Emax for Drugs A and C is _____

100%

The Emax for Drug B is ______

60

Does a high therapeutic index mean that a drug is more/less safe

Give 3 examples

High TI = more safe (some available over the counter)

E.g. H2 blockers, proton pump inhibitors (PPIs) & pencillin antibacterials

Give some examples of low TI drugs

These drugs require great care in ensuring that the dose does not produce harmful effects, such as digoxin, warfarin, anticancer drugs & barbiturates

What figures do we need to calculate therapeutic index

ED50 and TD50 for a drug

How would this look on a graph

Drug Dose (mg)	Log Drug Dose	Therapeutic response (%)	Toxic response (%)
1	0	0
2	0.3	22	0
3	0.5	45	2
4	0.6	65	9
8	0.9	92	42
16	1.2	98	82
32	1.5	100	97
64	1.8		100