Drug Interactions with Receptors and Ion Channels (Concepts)

Question 1

What are the different drug targets?

Answer 1

1. Rceptors (40%)
2. Enzymes (30%)
3. Hormones (10%)
4. Ion channels (5%)
5. DNA (4%)
6. Nuclear receptors (3%)
7. Others (8%)

Question 2

What is a receptor?

Answer 2

Protein acting as a signal transucer for a number of different chemical messengers and activating a number of different intracellular chemical cascades in their presence.

Question 3

What are the different types of drug-receptor interactions (from strongest to weakest)?

Answer 3

1. Covalent bonding
2. Ionic bonding
3. Hydrogen bonding
4. Van der Waals interactions
5. Hydrophobic interactions

Question 4

What is the ‘zipper model’ of drug-receptor interaction?

Answer 4

1. Ligand initially forms one interaction with receptor, stabilising the structure of ligsnd.
2. It becomes more energitically favourable for further interactions to form between ligand and receptor.
3. More interactions form between ligand and receptor and the ligand ‘zips’ onto the receptor.

Question 5

What evidence may be used to support fact that a drug may not act on receptors (or specific drug target)?

Answer 5

1. Higher potency: Effective concentration of drug needs to be high in order to produce pharmacological effect.
2. Reduced biological specificity: Affects multiple tissues in very similar ways.
3. Reduced chemical specificity: Slight changes in chemical structure have little effect on function.
4. No antagonist: No other molecules can inhibit their action in an indirect manner.

Question 6

What is the difference between selectivity and specificity?

Answer 6

• Selectivity is when a drug has a higher preference for one receptor to others, but still acts on a broad range of receptors.
• Specificity is when a drug acts on one type of receptors alone and no others.
• Most drugs are highly selective, but there are few drugs that are 100% specific.

Question 7

What is the therapeutic index?

Answer 7

• Theapeutic index = Toxic dose ÷ Therapeutic dose
• High therapeutic index → Drug causes few side effects as toxic dose >> therapeutic dose
• Low therapeutic index → Drug causes lots of side effects as toxic dose ≥ therapeutic dose

Question 8

What is affinity and efficacy?

Answer 8

• Affinity is a measure of the tendency for a drug to bind to a specific receptor.
• Efficacy is a measure of the ability for a drug to induce a response once bound to a receptor.

Question 9

What is an agonist?

Answer 9

Molecule that has high affinity for receptor and some degree of efficacy as well (determining type of agonist)

Question 10

What an antagonist?

Answer 10

Molecule that has high affinity for a receptor but zero efficacy.

Question 11

What are the different type of antagonism?

Answer 11

• Competitive antagonism: Antagonist competes with receptor ligand for binding to active site.
• Non-competitive antagonism: Antagonist binds to allosteric site on receptor, inducing conformational change affecting ligand binding/receptor function.

Question 12

What are the mechanism by which a ligand-receptor system can be antagonised?

Answer 12

1. Inhibition of enzymes in synthesis or conversion from precursor to active molecule.
2. Inhibition of molecule binding to receptor by occupation of binding site (competitive inhibition).
3. Inhibition of molecule binding to receptor by occupation allosteric site away from binding site (non-competitive inhibition).
4. Inhibition of elements involved in intracellular downstream response cascade from receptor activation.

Question 13

What is the shape typical shape of a [ligand] - % response curve and log([ligand]) - % response curve?

Answer 13

• [ligand] - % response curve is hyperbolic in shape
• log([ligand]) - % response curve is sigmoidal in shape

Question 14

What change occurs to the sigmoid log [agonist] - % response curve under competitive antagonism?

Answer 14

• Shifts to right
• No change in maximum response

Question 15

What is the significance of a structure-activity relationship?

Answer 15

If multiple receptors respond to the same ligand, structure-response relationships can be used to ‘fingerprint’ each type of receptor as it is unique for each type.

Question 16

What is differentential antagonism?

Answer 16

• Different receptors that respond to the same ligand should be structurally different enough that they react to different antagonists
• Some receptors are inhibited by some antagonists while being ignored by others
• This can be used to identify receptors

Question 17

What is relationship between affinity constant (K_a), drug concentration ([D]) and % occupancy (α) = 50%?

Answer 17

When α=0.5, [D] = K_d = 1/K_a

Question 18

What is the method of a radioligand binding experiment?

Answer 18

1. Radioactively labelled ligands (with ³H or ¹²⁵I) are added to test tube with isolated receptors bound to liposomes.
2. Mixture left for sufficient time until equilibrum is achieved.
3. Receptors are isolated by centrifugation or filtration and are thoroughly washed so that any unbound ligand is removed.
4. Radioactivity present is counted using scintillator.

Question 19

How is specific binding achieved in a radioligand experiment?

Answer 19

Adding an excess of a non-radiolabelled ligand and repeating experiment should allow non-specific binding to be measured as the specific binding should be abolished. Subtracting non-specific binding from total binding allows specific binding to be achieved.

Question 20

What are the characteristics of specific and non-specific binding?

Answer 20

Specific binding:

1. Low capacity
2. High afinity

Non-specific binding:

1. High capacity
2. Low affinity

Question 21

What is the Scatchard plot and how can K₁ and B_max be obtained from it?

Answer 21

Gradient = -K₁

X-intersect = B_max

Question 22

What is the key asumption made when using the 2 ligand fraction occupation equation?

Answer 22

When response is the same as when only 1 ligand present, fraction occupancy of receptor is also the same.

Question 23

What is the dose ratio equation and what is it used for?

Answer 23

[D]₂/[D₁] = 1 + [A]K₂

Where:

[D]₁ = Dose of agonist needed to produce response x

[D]₂ = Dose of agonist needed to produce response x in presence of antagonist A

K₂ = Affinity constant of A

This equation is used to obtain K₂

Question 24

What assumption is made when using the “Inhibition of radioligand binding equation”?

Answer 24

Unlabelled ligand cannot distinguish between receptors bound and unbound to the radioligand. It is assumed that statistically, when half of radioligands bound have been displaced by unlabelled ligand, half of all receptors have been occupied, so fraction occupation (α) = 0.5.

Question 25

What is the purpose of the Schild plot?

Answer 25

• 1 + [A]K₂ is already in the form mx + c and so will produce a straight line with gradient K₂. However, one may still choose to plot Schild plot as this distributes points over wider range, giving higher accuracy especially when plotting by hand.
• It also gives other information such as:
  1. Slope > 1 can indicate positive cooperativity in binding of antagonist
  2. Slope < 1 can indicate negative cooperativity in binding of antagonist

Question 26

What is the function of spare receptors?

Answer 26

• Spare receptors increase the sensitivity of response to small amounts of stimulating ligand.
• Alternative would be to increase receptor affinity for ligand, but that would signigicantly increase dissociation time and speed of response can be terminated.

Question 27

What is a partial agonist?

Answer 27

A ligand with high affinity but low efficacy. Even at 100% occupancy of receptors, a sub-maximal response is produced.

Question 28

What is the pharmacological significance of partial agonists?

Answer 28

A partial agonist drug can be used to clamp a particular tissue response to a specific level. If response exceeds level due to excess presence of agonist, drug acts as competitive antagonist and decreases it. If response falls below level due to absence of agonist, drug acts as agonist to increase it.

Question 29

How can a Hill plot be interpreted?

Answer 29

Gradient = n_H

n_H = 1 - No cooperativity

n_H > 1 - Positive cooperativity

n_H < 1 - Negative cooperativity

n_H < 1 + non-linear line - Multiple non-interacting bindng sites per receptor