Fundamentals: Pharmacology - Pharmacodynamics

Question 1

Define pharmacodynamics

Answer 1

The actions of a drug on the body

Question 2

Define agonist

Answer 2

Ligand that binds to and activates the receptor to directly or indirectly bring about the effect

Question 3

Define antagonist

Answer 3

Ligand that by binding to a receptor, competes with and prevents binding by other molecules

Question 4

Define allosteric modulator

Answer 4

Ligand that binds to the same receptor molecule as an agonist but does not prevent agonist binding
I.e. binds to a different site (allosteric vs orthosteric)

Question 5

What is constitutive activity?

Answer 5

Thermodynamic considerations indicated that even in the absence of any agonist, some of the receptor pool must exist in the activated (rather than the inactivated) form some of the time and may produce the same physiologic effect as agonist-induced activity - this effect is termed constitutive activity

Question 6

What is the difference between full and partial agonists?

Answer 6

When administered at concentrations sufficient to saturate the receptor pool, full agonists activate receptor-effector systems to the maximum extent of which the system is capable
Partial agonists do not evoke as great a response a full agonists regardless of their concentration

Question 7

How do partial agonists sometimes act as antagonists?

Answer 7

By blocking access by full agonists to receptor sites

Question 8

What is neutral antagonism?

Answer 8

Conventional antagonist action which fixes the fractions of drug-bound inactivated and activated receptors in the same relative amounts as in the absence of any drug
In this situation, no change in activity will be observed, so the drug will appear to be without effect
However, the presence of the antagonist at the receptor site will block access of agonists to the receptor and prevent the usual agonist effect

Question 9

What is an inverse agonist?

Answer 9

Inverse agonists have a much stronger affinity for the inactivated than the activated receptor state and stabilise a large fraction in the Ri-D pool, thereby reducing any constitutive activity and resulting in effects that are the opposite of the effects produced by conventional agonists at that receptor

Question 10

Define receptor

Answer 10

Component of a cell or organism that interacts with a drug and initiates the chain of events leading to the drug’s observed effects

Question 11

What is an orphan receptor?

Answer 11

Identified receptors for which there is not yet a known natural ligand

Question 12

Outline 3 consequences of the receptor concept

Answer 12

1. Receptors largely determine the quantitative relations between dose or concentration of drug and pharmacologic effects (in part due to variance in receptor affinity, and in receptor availability/numbers)
2. Receptors are responsible for selectivity of drug action
3. Receptors mediate the actions of pharmacologic agonists and antagonists

Question 13

What is inert binding site? What is the pharmacological significance of inert binding sites?

Answer 13

A nonregulatory molecule capable of binding drug - no detectable change in function occurs with drug binding
However this does affect drug distribution

Question 14

1. Describe the general pattern of drug dose-response
2. Outline the relationship between drug effect, E, and concentration, C
3. What law does this relationship resemble?

Answer 14

1. Responses to low doses of a drug usually increase in direct proportion to dose
   However response increment decreases as dose increases
   Finally, doses may be reached at which no further increase in response can be achieved (this is the Emax)
2. This relation is traditionally described by a hyperbolic curve:
   E = (Emax x C) / (C + EC50)
3. Resembles the law of mass action describing the association between two molecules of a given affinity, suggesting that drug agonists act by binding to a distinct class of biologic molecules with a characteristic affinity for the drug

Question 15

What is EC50

Answer 15

Concentration of a drug that produces 50% of its maximal effect

Question 16

Outline the law of mass action (as represented by the relationship between drug bound to receptors, B, and concentration of unbound drug, C)

Answer 16

B = (Bmax x C) / (C + Kd)
In which Bmax indicates total concentration of receptor sites (i.e. sites bound to the drug at infinitely high concentrations of free drug) and Kd (the equilibrium dissociation constant) represents concentration of free drug at which half-maximal binding is observed

Question 17

What is the relationship between Kd and receptor affinity for drug binding?

Answer 17

If Kd is low, receptor affinity is high (i.e. a relatively low concentration of drug is required to produce half-maximal binding), and vice versa

Question 18

If you have lots of spare receptors, will the EC50 be lower, the same as, or higher than the Kd?

Answer 18

Lower
Don’t need much concentration of drug to achieve half-maximal response
Half receptors will not be occupied at the concentration required to achieve half-maximal response (as most are spare)

Question 19

What is the difference between competitive and noncompetitive antagonists?

Answer 19

Competitive antagonists progressively inhibit agonist response, such that high concentrations prevent response almost completely; however, sufficiently high agonist concentrations can surmount the effect
Agonists generally cannot overcome the inhibitory effect of noncompetitive antagonists irrespective of concentration (usually because noncompetitive antagonists bind irreversibly via covalent bonds) - the exception to this is in the presence of spare receptors

Question 20

What is a negative allosteric modulator? What is a positive allosteric modulator? Give an example of a positive allosteric modulator

Answer 20

Negative and positive allosteric modulators function by binding a site on the receptor protein separate from the agonist binding site (the classical, or orthosteric, site)
Negative allosteric modulators decrease receptor activity, and positive allosteric modulators potentiate receptor activity (e.g. benzodiazepines)

Question 21

Is phenoxybenzamine a competitive or a noncompetitive antagonist? Why does this make it well-suited to treat hypertension caused by phaeochromocytoma? What are the downsides of this mechanism clinically?

Answer 21

Noncompetitive
Blockade is maintained even when the tumour episodically releases very large amounts of catecholamines, as the receptors are irreversibly bound
In overdose, excess effects must be antagonised using a pressor agent that does not act via the irreversibly bound alpha-receptors

Question 22

What is the relevance of an antagonist being noncompetitive with relation to its duration of action?

Answer 22

For noncompetitive agonists, duration of action is more dependent on rate of turnover of receptor molecuoles than the antagonist’s own rate of elimination

Question 23

Explain chemical vs physiologic antagonism, and give an example of each

Answer 23

Chemical antagonist: reacts with the agonist, making it unavailable for other interactions (e.g. protamine is positively charged and ionically binds with negatively charged heparin)
Physiologic antagonist: produces the opposite physiologic effect of an agonist, but acts on a different endogenous regulatory pathway mediated by different receptors (e.g. insulin antagonising the hyperglycaemic effects of glucocorticoid)

Question 24

Describe the five basic mechanisms of transmembrane signaling

Answer 24

1. Lipid-soluble chemical signal crosses the plasma membrane and acts on an intracellular receptor (may be an enzyme or a regulator of gene transcription)
2. Signal binds to the extracellular domain of a transmembrane protein, thereby activating an enzymatic activity of its cytoplasmic domain (i.e. acts as an allosteric modulator)
3. Signal binds to the extracellular domain of a transmembrane receptor bound to a separate intracellular protein tyrosine kinase, which it activates
4. Signal binds to and directly regulates the opening or closing of an ion channel
5. Signal binds to a cell-surface receptor that stimulates a GTP-binding signal transducer protein (G protein), which in turn modulates production of an intracellular second messenger

Question 25

Give four examples of exogenous hormones that act via ligand-regulated transmembrane enzymes, and specify the receptor’s cytoplasmic domain in each instance

Answer 25

Insulin, EGF, PDGF: tyrosine kinase
TGF-B: serine and threonine kinase
ANP: guanylyl cyclase

Question 26

How does tyrosine kinase signaling function work?

Answer 26

When the agent binds the extracellular domain, it causes a conformational change resulting in two receptor molecules dimerising
This activates the tyrosine kinase enzyme activity in the cytoplasmic domain of the dimer, leading to phosphorylation of the receptor and downstream signaling proteins
Activated receptors catalyse phosphorylation of tyrosine residues on different target signaling proteins, thereby allowing a single type of activated receptor to modulate a number of biochemical processes

Question 27

How does down-regulation occur?

Answer 27

Ligand binding often induces accelerated endocytosis of receptors from the cell surface, followed by their degradation
When this process occurs at a faster rate than de novo receptor synthesis, the total number of cell-surface receptors is reduced (down-regulated), and the cell’s responsiveness to ligand is correspondingly diminished

Question 28

Does ligand binding and receptor endocytosis always result in receptor down-regulation?

Answer 28

No, e.g. with nerve growth factor where endocytotic vesicles travel to the cell body and transcribe genes related to cell survival

Question 29

Tyrosine kinase inhibitors are finding increased use in neoplastic disorders. Outline two possible mechanisms of action

Answer 29

Some of these inhibitors are monoclonal antibodies (e.g. cetuximab), which bind the receptor extracellular domain to interfere with growth factor binding
Some are membrane-permeant small molecule chemicals (e.g. gefitinib) which inhibit the receptor’s kinase activity in the cytoplasm

Question 30

Explain how cytokine receptors work

Answer 30

1. A separate protein tyrosine kinase from the Janus-kinase (JAK) family, binds noncovalently to the cystoplasmic domain of the receptor
2. Bound JAKs become activated and phosphorylate tyrosine residues on the receptor
3. Phosphorylated tyrosine residues on the cytoplasmic surface set in motion a complex signaling dance by binding another set of proteins called STATs (signal transducers and activators of transcription)
4. Bound STATs are phosphorylated by the JAKs and dimerise
5. STAT/STAT dimer dissociates from receptor and travels to the nucleus to regulate gene transcription

Question 31

What do cytokine receptors respond to? Give 3 examples

Answer 31

A heterogenous group of peptide ligands
Includes growth hormone, erythropoietin, and several kinds of interferon, as well as other regulators of growth and differentiation

Question 32

Give 4 examples of natural ligands of ion channels

Answer 32

Neurotransmitters: ACh, serotonin, GABA, glutamate

Question 33

Give four examples of protein structures that are common receptor targets, with examples of each

Answer 33

1. Regulatory proteins (most common): mediate actions of endogenous chemical signals, e.g. neurotransmitters, hormones, autocoids
2. Enzymes: most commonly activated but sometimes inhibited (e.g. dihydrofolate reductase inhibition by methotrexate)
3. Transport proteins (e.g. Na/K ATPase)
4. Structural proteins (e.g. tubulin, receptor for colchicine)

Question 34

Give an example of the spare receptor concept as it pertains to myocardial cells

Answer 34

Myocardial cells exhibit the same maximal inotropic response to catecholamines, even when 90% of receptors are blocked by irreversible antagonist

Question 35

What effect does the addition of competitive antagonists have on an agonist’s dose-response curve?

Answer 35

Shifts the curve to the right (i.e. increases EC50, the agonist concentration required for a given degree of response)

Question 36

Describe the Schild equation

Answer 36

Schild equation describes the relationship between agonist and antagonist concentration, and drug effect
Ci/C = 1 + I/Ki
Where Ki is the dissociation constant, and Ci is the agonist concentration required in presence of fixed antagonist concentration (I) to produce the same response as that produced by agonist in absence of antagonist (C)

Question 37

What are the two important clinical implications of the Schild equation?

Answer 37

1. Degree of inhibition caused by competitive antagonist is related to the antagonist concentration and can be very different from patient to patient due to difference in clearance
2. Clinical response to competitive antagonist depends on agonist concentration that is competing for binding

Question 38

What can the Schild equation used to be determine?

Answer 38

Ki, the dissociation constant of a competitive antagonist

Question 39

What effect does the addition of non-competitive antagonists have on an agonist’s dose-response curve?

Answer 39

Shifts the curve to the right (i.e. increases EC50, the agonist concentration required for a given degree of response) and decreases the maximal response Emax (unless there are spare receptors present in which case a sufficiently high dose of agonist may still be able to produce the maximal response)