Mechanisms of Drug Action and drug targets Flashcards
How do drugs typically work and what are the four key targets?
They work by binding to proteins.
- ion channels; enzymes; carrier molecules; receptors
Where could drugs target at the level of synthesis?
Choline transporter (uptake of choline)- rate limitng step Choline acetyl transferase (ChAT)- synthesiszes ACh
Where could drugs target at the level of release?
- Vesicular ACh transporter- transports ACh into vesicles
- Voltage dependent calcium channels. Prevent calcium release
- VAMPS- calcium sensitive cytoskeleton that holds vesicles
- vesicular fusion?
What are the four main families of receptors?
Ligand gated ion channels
G protein coupled receptors
Tyrosine kinase/ cytokine receptors
Nuclear/steroid hormone receptors
Ligand gated ion channels (surround a pore)
4 important properties
-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
Nicotinic acetylcholine receptors
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
What are some examples of drugs acting on LGIC’s?
Benzodiazepenes on GABAa (inhibitory neurotransmitter) and barbiturates
GPCRs structure
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
What are key GPCRS with Ach?
M2,M4- Galpha I
M1.M3,M5- GalphaQ
What is the important pre synaptic receptor?
What can be said about pre synaptic receptors?
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
Tyrosine Kinase receptors
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
VEGFR2
therapies might target angiogenesis (A and I)
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
What is the endothelial proliferation pathway?
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
How do drugs bind to receptors?
VDW Hydrogen bonding Ionic reversible Covalent (irreversible, not that common)
What is the affinity constant and how is it defined?
The ability of the drug to bind to a receptor
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
What is the relationship between affinity and biological response?
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
What values are important on a response curve
Effect (y) vs drug concentration
Can take EC50 and EMAX (maximum response acquired of that drug)!!!
What is potency?
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
What is efficacy?
ability of a rug to produce a response
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)
What are the different types of agonists?
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
Antagonists
A drug with NO efficacy but has affinity.
Binds to receptor but doesn’t activate or inactivate it.
defined by how they bind
Reversible Competitive antagonists
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
Irreversible antagonism
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)
Why can most drigs only inhibit enzymes, and transporters?
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
Inactivation of the receptor/ neurotrasnmission
two key mechansims
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
AChE inhibitors
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
Serotonin transporters
note serotonin made by tryptophan
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
SSRI’s
Selective serotonin reuptake inhibitors, used to increase serotonin in synapse.
Examples: fluoxetine, prozac
