Session 2- General Principles Of Drug Action :molecular Aspects Flashcards
What is an ideal drug consist of ?
Small molecule
High affinity and selectivity for desired target
Few adverse effects
Low production cost
Can be administered in a single daily oral dose
Drug bind interaction from weakest to strongest ?
From weakest to strongest: Van der waals/ hydrophobic Hydrogen bonds Ionic bonds Disulphide bonds
Definition of affinity and efficacy ?
Affinity describes the strength of a drug interaction with receptor
Efficacy describes the strength of a drug bound receptor to produce a response
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Agonist has affinity and efficacy whereas antagonist has affinity but no efficacy
What does low and high Kd mean ?
Low Kd = high affinity
high Kd= low affinity
What does dissociation constant mean (Kd)
dissociation constant describes the affinity between a protein and a ligand. A small dissociation constant indicates a more tightly bound ligand
What does Bmax mean ?
Total number of receptors in a particular tissue
Occupation theory ?
The idea that a response emerges from a receptor only when it is occupied by an appropriate ligand (drug)
What does ic50 mean ?
Half-maximal inhibitory concentration (IC50) is the most widely used and informative measure of a drug’s efficacy. It indicates how much drug is needed to inhibit a biological process by half, thus providing a measure of potency of an antagonist drug in pharmacological research
What is KI?
Inhibitor constant . Indication of how potent an inhibitor is
What is the Chen’s Prusoff equation ?
The Cheng-Prusoff equation defines the theoretical relation- ship between the measured IC50 for a competitive inhibitor of given Ki,
What is Ec50?
Effective concentration .
The concentration of a agonist required to produce half maximal response .
A measure of potency.
(Low ec50 = high potency )
Efficacy ?
Ability of drug bound receptor to produce a response
Some agonists acting on the same receptor illicit different maximal responses at maximal receptor occupancy
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Partial agonist ?
partial agonist is an agonist which is unable to induce maximal activation of a receptor population, regardless of the amount of drug applied
What is a competitive antagonist ?
A competitive antagonist binds to the same site as the agonist but does not activate it, thus blocks the agonist’s action.
Features of competitive antagonist ?
Reversible
Can be overcome by high agonist
Straightforward to analyse quantitatively using a schild plot
Non competitive antagonist?
Cannot be overcome by high agonist
Can Occur when a competitive antagonist binds irreversibly
Can occur from binding of antagonist to a site distinct from agonist binding site - allosteric effect
Allosteric effect ?
The binding of a ligand to one site on a protein molecule in such a way that the properties of another site on the same protein are affected
What happens if you increase non competitive antagonist ?
Initially increase EC50
What is propranolol?
It is a beta blocker. It slows down your heart rate, Propranolol can tackle the physical symptoms of anxiety and help you to feel calmer. Propranolol also inhibits the effects of the stress hormone noradrenaline, which further combats these physical anxiety symptoms.
Non competitive antagonist
What is isoprenaline?
Isoprenaline is a non-selective β-adrenergic agonist. It has positive inotropic and chronotropic effects, increasing cardiac output by increasing the heart rate and cardiac contractility. Isoprenaline also decreases diastolic blood pressure by lowering peripheral vascular resistance.
Is isoprenaline an agonist or antagonist ?
Isoprenaline is a non-selective beta adrenergic receptor agonist used in a number of indications for the heart, as well as bronchospasm in anesthesia.
Effect of propanolol on isoprenaline ?
Propranolol almost completely blocked the effects of all doses of isoprenaline on heart rate,
What is Methysergide?
Methysergide is used to prevent vascular headaches (e.g., migraines) and to reduce their severity
Mixture antagonist?
In pharmacology the term agonist-antagonist or mixed agonist/antagonist is used to refer to a drug which under some conditions behaves as an agonist (a substance that fully activates the receptor that it binds to) while under other conditions, behaves as an antagonist
Physiological antagonism ?
Physiological agonism describes the action of a substance which ultimately produces the same effects in the body as another substance
Inverse agonism ?
In pharmacology, an inverse agonist is a drug that binds to the same receptor as an agonist but induces a pharmacological response opposite to that of the agonist.
Full agonist Partial agonist Antagonist Partial inverse agonist Full inverse agonist
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What drugs act as 7TMRs?
Salbutamol (b2 adrenoceptor agonist)
Morphine (opiate receptor agonist)
Valsartan (antagonist)
Info about 7TMRs
Have many functions - stimuli examples include : hormones, ions, lights, proteases .
Function- neurotransmission, cell growth, vision
Over 30% of prescribed drugs act as 7TMRs such as ;
Salbutamol (used for asthma)
Morphine (used as an analgesic)
Valsartan ( used for hypertension)
7 TMR structure ? (Check word)
Seven membrane spanning alpha helices
Extracellular N terminus, intracellular C terminus
Helices connected by 3 intracellular and 3 extracellular loops
Intracellular loops response for initiation and regulation of down-streaming signalling.
7TMRs family examples
Examples ;
1a- epinephrine and dopamine
1b- chemokine and thrombin
1c- thyroid stimulating hormone
How do 7TMR signal ?
They signal .. they have a receptor at the membrane , this binds to the extracellular agonist.
This agonist bound receptor must interact with a G-protein which present on the inner leaflet of the plasma membrane
The G protein the interacts with an effector enzymes .
(The G protein and effector are both response to translate agonist binding event into generation of intracellular signal)
Receptor-Gprotein-Effector
G proteins
Their jobs is to control activity of effectors after activation by GPCRs
They are heterotrimeric (3 diff subunits) alpha, beta and gamma.
Key function of alpha subunits is - binds GDP or GTP and displays GTPase activity critical for function
G proteins cycle between GDP bounds inactive state and GTP bound active state (essential for G protein to function properly)
How does G protein activation cycle work ?
In the basal state the G protein is present as an intact heterotrimer on the alpha subunit in a GDP bound state.
Then when an agonist bound receptor that changes the conformation of the receptor and then can interact with a GDP bound heterotrimeric G protein 
On contacting the receptor , it is able to catalyse GDP release from the heterotrimer. this is an amplification step in the pathway.
Then once GDP is released this allows GTP to bind to the G protein because it’s present in a higher concentration in the cell and this binds to alpha subunit .
the G protein then dissociates into GTP bound alpha subunit and a beta gamma complex. Aswell as that happening the agonist dissociates from the receptor once the G protein is activated.
And after a certain length of time the GTP on the alpha subunit will be hydrolysed to GDP through the intrinsic ATPase activity on the alpha subunit.
GDP alpha and beta gamma dissociate from the effectors And then recombine to form heterotrimer
Then back to basal state . G protein available for another cycle of activation
7 TMR-activated 2nd messenger pathway.
A receptor activates G protein. This can then control activity of effector enzymes. This regulates levels of a second messenger. This then is able to regulate bind and activate the activity of the second messenger dependant protein kinase. This can then phosphorylate specific substrates of the cell and change the activity
Function of Gs, Gi and Gq?
Gs- stimulates adenylyl Cyclase and calcium channels
Gi- inhibit adenylyl cyclase but activate potassium channels
Gq- activate phospholipids C
Check cyclic amp pathway on word
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Check example of CAMP/PKA regulation (salbutamol)
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Benefits and adverse effects of beta 2 agonist receptors ?
Benefits- Activates adenylyl Cyclase, generation of CAMP and activation of PKA triggers bronchodilation
Adverse effects - can become phosphorylated and bind arresting proteins . This turning off adenylyl Cyclase . Overall stimulating inflammation of the airways
Flip
Cyclic AMP activated cAMP dependant protein kinase to mediate its effects
What pathway is the MOA of clinically important drugs ?
Cyclic AMP/PKA pathway
E.g B2 adrenorecptors agonisg bronchodilators for asthma and COPD
Info about receptor tyrosine kinases ?
Comprise ligand binding domain-single TM helix-cytoplasmic domain
Possess protein tyrosine kinase activity in their cytoplasmic domain
Kinase activity stimulated by ligands
Ligands for RTKs are proteins examples include : insulin, epidermal growth factor, platelet derived growth factor
RTKs stimulate important cellular events ;
Cell proliferation, survival, differentiation, migration, and metabolism
How is RTK is activated by dimerisation ? (By ligands )
1) In the basal state it’s inactive , no ligand, no tyrosine kinase activity
2) upon binding of an agonist ligand . This promotes a dimerisation of the receptor .
This has two effects ; so a conformational change allows ATP to bind to each tyrosine kinase domain.
And allows one of the ATP-bound tyrosine kinases to undergo transphosphorylation
RTK is active
3) phosphotyrosine function as docking sites for binding of intracellular signalling proteins which then relay and amplify downstream signal
What do phosphotyrosine serve as docking sites for ?
SH2 domains present on specific intracellular signalling molecules .
RAS CYCLE ?
RAS cycle switches between inactive and active state
GDP is inactive
GTP is active
GDP—> GTP (via guanine nucleotide exchange factors e.g SOS)
GTP —> GDP (via GTPase activating proteins GAPS)
RAS controls the MAP kinase pathway —> proliferation
How is RAS activated by RTKS ?
At the basal state RAS is inactive at the membrane in its GDP bound off state .
RTK is activated.
Phosphorylated TYRs act as docking sites for SH2 domain on an adaptor protein called Grb2
Grb2 interacts with a Ras guanine nucleotide exchange factor (SOS)
By bringing sos close to the membrane allows release of GDP thus allowing GTP to bind to GTP-bound ras = active
This onward signal leads to activation of MAPK pathway
RTK activation leads to MAPkinase ?
First step- At the plasma membrane GTP-bound ras recruits a MAP kinase kinase kinase called RAF to the plasma membrane
RAF activates a map kinase kinase called MEK.
MEK activates MAP kinase
MAP kinase can then phosphorylate and alter activity or multiple enzymes and transcription factors to stimulate cell proliferation.
How to target kinases in cancer ?
- We can target receptors
- Use therapeutic antibodies
- Bind to receptor to prevent activation
- Can use small molecule inhibitors on the receptor tyrosine kinase activity
- inhibit signalling pathway downstream of activated RTK
What is HER2 and what does it do?
HER2 is an RTK and is over expressed by 20-30% in breast cancers. It is constitutively active and inappropriate amplification of mitogenic signalling can result in aggressive tumour growth increased risk of metastasis .
Treatments for HER2
Herceptin /trastuzumab= monoclonal Ab which binds and inactivates HER2
Lapatinib= HER2-selective Tyr kinase inhibitor
MOA for Herceptin?
Herceptin works by attaching itself to the HER2 receptors on the surface of breast cancer cells and blocking them from receiving growth signals. Herceptin is an example of an immune targeted therapy.
How does trastuzumab inhibit HER2 activity?
Trastuzumab reduces signaling mediated by HER2 through the phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) cascades. Reduced downstream signaling through these pathways induces the cyclin-dependent kinase inhibitor p27kip1, which promotes cell-cycle arrest and apoptosis
Where is mutated B-Raf a present in ?
It is present in 70% of metastatic melanomas
Val600 GLU mutation mimics the effect of GTP-RAS binding
It generates constitutively active B-Raf that cannot be turned it.
This is a potential drug target for melanomas
What is vemurafenib?
Orally available , selective inhibitor of VAl600 GLU B-raf.
Developed to selectively inhibit mutationally active BRAF
Approved for treatment of metastatic melanoma
MOA sildenafil
sildenafil ultimately prevents or minimizes the breakdown of cyclic guanosine monophosphate (cGMP) by inhibiting cGMP specific phosphodiesterase type 5 (PDE5). The result of doing so allows cGMP present in both the penis and pulmonary vasculature to elicit smooth muscle relaxation and vasodilation that subsequently facilitates relief in pulmonary arterial hypertension and the increased flow of blood into the spongy erectile tissue of the penis that consequently allows it to grow in size and become erect and rigid.
PDE5 is a substance produced in the lungs and other parts of the body that breaks down another substance called cyclic guanosine monophosphate (GMP). Cyclic GMP causes the blood vessels (arteries) to relax and widen. Sildenafil decreases the activity of PDE5, so that more cyclic GMP is available for the blood vessels inside the lungs. This leads to relaxation, or widening, of those vessels. Relaxing and widening of the blood vessels in the lungs decreases the pulmonary blood pressure to the heart and improves its function. This reduces blood pressure in the lungs which generally results in the ability to be more active.
What is sildenafil
A PDE5 inhibitor
Molecular structure of sildenafil mimics that of cyclic guanosine monophosphate (cGMP)- this means it can bind to PDE5
PDE5 is an enzyme which degrades cGMP which causes contraction of vascular smooth muscle and in
turn reduces blood flow.
When PDE5 is inhibited, accumulation of cGMP leads to increased cGMP-dependent protein kinase activity > phosphorylates multiple targets in the smooth muscle cell
This phosphorylation leads to decreased intracellular calcium, increased efflux of potassium and
deactivation of MLCK > smooth muscle relaxation
