Mechanisms of Drug Action and drug targets

Question 1

How do drugs typically work and what are the four key targets?

Answer 1

They work by binding to proteins.

- ion channels; enzymes; carrier molecules; receptors

Question 2

Where could drugs target at the level of synthesis?

Answer 2

Choline transporter (uptake of choline)- rate limitng step
Choline acetyl transferase (ChAT)- synthesiszes ACh

Question 3

Where could drugs target at the level of release?

Answer 3

• Vesicular ACh transporter- transports ACh into vesicles
• Voltage dependent calcium channels. Prevent calcium release
• VAMPS- calcium sensitive cytoskeleton that holds vesicles
• vesicular fusion?

Question 4

What are the four main families of receptors?

Answer 4

Ligand gated ion channels
G protein coupled receptors
Tyrosine kinase/ cytokine receptors
Nuclear/steroid hormone receptors

Question 5

Ligand gated ion channels (surround a pore)

4 important properties

Answer 5

-Mediate fast transmission and are multi sub unit

4- activated in response to specific ligands; conduct ions through an otherwise impermeable membrane; they are highly selective among diff ions; multi subunits

Question 6

Nicotinic acetylcholine receptors

Answer 6

LGIC
2 ACh needed to bind
binds at an alpha interface
5 subunits with 4 transmembrane domains
the 2 domain is facing the pore, 'kinks' and will open on binding

Question 7

What are some examples of drugs acting on LGIC’s?

Answer 7

Benzodiazepenes on GABAa (inhibitory neurotransmitter) and barbiturates

Question 8

GPCRs structure

Answer 8

7 Transmembrane domains. monomeric.
Intercat with G proteins (are dimers)

eg Gs receptor activating adenylyl cyclase (cAMP) versus Gi which inhibit
Gq for phospholipase C increasing IP3 and DAG

Question 9

What are key GPCRS with Ach?

Answer 9

M2,M4- Galpha I

M1.M3,M5- GalphaQ

Question 10

What is the important pre synaptic receptor?

What can be said about pre synaptic receptors?

Answer 10

M2 can be targeted

• usually Gi linked; inhibition of calcium voltage sensitive channels occurs, so less neurotransmitter release. Are pharmacologically distinct, so can be targeted (eg blockage causing prolonged release, 10x more release)

normally stimulated by choline (agonist) after being broken down by AChE

Question 11

Tyrosine Kinase receptors

Answer 11

A membrane bound enzyme that transfer phosphate groups from ATP to tyrosine residues on intracellular traget proteins.
-mediate actions of cytokines and certain hormones like insulin

Question 12

VEGFR2

therapies might target angiogenesis (A and I)

Answer 12

Ligand stimulated for dimerisation
Autophosphorylation of tyrosine residues in cytoplasmic domain
associates with DH2 domain proteins
Causes endothelial survival, proliferation, migration, NO and PGI2 production, increase permeability

Question 13

What is the endothelial proliferation pathway?

Answer 13

Activation of PLCy by phosphorylation
this hydrolyses PIP2 to DAG + IP3
DAG activates PKC
PKC actiavtes ERK via Raf and MEK
ERK causes increased gene transcription

Question 14

How do drugs bind to receptors?

Answer 14

VDW
Hydrogen bonding
Ionic
reversible
Covalent (irreversible, not that common)

Question 15

What is the affinity constant and how is it defined?

The ability of the drug to bind to a receptor

Answer 15

Kd
Fractional occupancy= drug concentration/ Kd constant plus conc
essentially the affinity constant is the concentration of drug required to occupy 50% of the receptor.

A smaller affinity constant means less required, so higher affinity. (lower conc required at which it produces a given level of occupancy)

Note in exam question, if sigmoidal curve, log the x axis

Question 16

What is the relationship between affinity and biological response?

Answer 16

Concentration response cruves are not a good measure of affinity because not a proportional relationship between occupancy and response

• some tissues may need low occupancy to have full effect
• downstream reponses

Question 17

What values are important on a response curve

Answer 17

Effect (y) vs drug concentration

Can take EC50 and EMAX (maximum response acquired of that drug)!!!

Question 18

What is potency?

Answer 18

The EC50 is used to measure the potency of an agonist
The effective concentration that causes 50% of the max response.
The more potent agonist has a lower EC50

Question 19

What is efficacy?

ability of a rug to produce a response

Answer 19

the ability of a drug to bind to receptor and cause a change in the receptors action. measured by EMAX

Positive efficacy, binds to receptors to promote cellular response is agonism
Negative efficacy, binds to receptor to decrease receptor activity is INVERSE AGONISM
A drug with no efficacy will bind to receptor causing no activity- ANTAGONIST ( to prevent natural agonist binding)

Question 20

What are the different types of agonists?

Answer 20

Full agonists: elicit the max tissue response
Partial: less than maximum response

not related to receptor occupancy, related to conformation of receptor it induces

NB that partial agonists/efficacy can not produce maximal response even at 100 receptor occupancy

Also a drug may appear to be a full agonist in some tissues but partial in others

Question 21

Antagonists

Answer 21

A drug with NO efficacy but has affinity.
Binds to receptor but doesn’t activate or inactivate it.

defined by how they bind

Question 22

Reversible Competitive antagonists

Answer 22

Bind to the receptor in a reversible manner to compete directly with agonist binding

On a response conc cruve, adding some RCA’s will cause a right shift of the cruve, as they are preventing agonist binding.
However if you continue to increase agonist concentration, they will outcompete the antagonist and produce the regular affect

Question 23

Irreversible antagonism

Answer 23

Covalent binding to receptor, is not reversible.
Will reduce the number of available receptors available to the agonist.
Will cause a reduced EMAX on your response conc curve (lowers the gradient)

At low doses an IA looks like a RCA. You keep adding IA and the EMAX will then fall: This tells us that a low receptor occupancy is needed to produce maximum effect (eg nicotnic receptor)

Question 24

Why can most drigs only inhibit enzymes, and transporters?

Answer 24

Due to the structure. A ligand binding to any of these will cause a conformational change tat will negatively affect it. Would need something to bind not on the orthosteric binding site

Question 25

Inactivation of the receptor/ neurotrasnmission

two key mechansims

Answer 25

Acetylcholine esterase. Will degrade ACh to choline and acetate, where choline is recycled.

Need to remove to prevent constant stimulation.
Transport back into pre synaptic synapse (dopamine, serotonin)
Enzyme degradation

Question 26

AChE inhibitors

Answer 26

Will increase NT

Irreversible- organophophates, highly toxic and not good thanks joe

Reversible- medium and short duration.

• Used with people with myastheina gravis. Can use Mestinon (pyridostigmine bromide) , won’t cross blood brain barrier.
• Also with Alzheimers like tetrahydroaminoacridine and DONEZEPIL

Question 27

Serotonin transporters

note serotonin made by tryptophan

Answer 27

With serotonin, to be taken up, the serotonin transporter will take it back into the pre synaptic synapse.

Bind serotonin as well as sodium and chloride also.
Sodium is needed for 5-HT binding, then Cl- for reuptake (but not binding).
These products dissociate and then potassium is moved outside the cell, then dissociates

Question 28

SSRI’s

Answer 28

Selective serotonin reuptake inhibitors, used to increase serotonin in synapse.
Examples: fluoxetine, prozac