Pharmacodynamics Flashcards
How do endogenous and exogenous ligands exert their effects?
By binding to targets
Usually proteins
(some exceptions antimicrobial/antitumour bind to DNA)
Molecular targets for drugs (5)
GPCR Other e.g. Enzymes, DNA, Integrins, transporters Ion channels Nuclear receptors Kinases
What are gpcr regulated by? (5)
Light, odorants, hormones, neurotransmitters, ions
What receptors do not have identified ligands?
What do they act as?
Orphan receptors - potential drug targets
Most GPCR’s are
Olfactory receptors (smell)
Examples of Therapeutic ligands targeting GPCRs (3)
Bisoprolol fumarate (hypertension, angina - B1 antagonist)
Salbutamol (Asthma, B2 agonist)
Co-codamol (u-opioid receptor, pain relief)
Top prescribed drug targeting protein (not a GPCR ligand)
Atorvastatin (HMG CoA reductase inhibitor - lower cholesterol)
What determines drug action?
Concentration (molarity) of drug molecules surrounding receptor
What is molarity?
How do you work it out?
Moles per litre of solution
g/L divided by Molecular weight
g/L =
Molecular weight x molarity
Different useful prefixes for concentrations (5)
Molar Millimolar (10^-3) mM Micromolar (10^-6) µM Nanomolar (10^-9) nM Picomolar (10^-12) pM
Why is molarity important?
Can have less molecules than another chemical but same concentration
Why do we need to consider drug concentrations in molarity?
Concentration of drug molecules around receptor is critical in determining action
What is binding governed by?
How do most drugs bind to receptors?
Association and disassociation (most bind reversibly)
What law does binding obey?
Law of mass action
velocity of reaction depends on concentration of reactants
How do drugs work?
Agonist - activate receptor
Antagonist - Block binding of endogenous agonist
How do ligands bind to receptors?
Must have affinity
Binding is governed by affinity
(high = stronger binding)
What do antagonists do?
They do not cause receptor to do anything:
JUST BLOCK AGONIST from binding
What is receptor activation governed by?
Intrinsic efficacy
What is intrinsic efficacy?
Ability of a agonist to generate active form of receptor via conformational change
Active form = R*
How is an active receptor represented?
R*
What do agonists have to have to be effective? Vs what antagonists have to have?
Agonists need affinity and efficacy (have to activate)
Antagonists need affinity ONLY
What happens after intrinsic efficacy?
Cell/tissue dependent factors to evoke response
Lock and key affinity and efficacy
Affinity is key fitting in lock
Efficacy is key being able to turn lock and unlock door
What determines overall efficacy (NOT intrinsic)
Intrinsic efficacy and cell/tissue dependent factors
Pharmacological efficacy vs clinical efficacy
Pharmacological: intrinsic efficacy and cell/tissue dependent factors
Clinical: how well does treatment succeed? ie Does it lower blood pressure
How do we measure binding?
Radioactively labelled ligand
Incubate radioligand and receptors
Separate bound and free and measure BOUND
Binding graph
proportion of bound receptors over drug concentration
Bmax - max binding capacity
Kd - disassociation constant
What is Kd?
Disassociation constant
Concentration of ligand required to occupy 50% of receptors
Index of affinity
Low value = high affinity as not much drug needed for 50% binding
How else can affinity be determined?
Ka
How is affinity important for ligands?
Naloxone is used in drug overdose (heroin/morphine)
Naloxone has higher affinity to µ-opioid receptor
Out competes opioid = no respiratory depression/death
Problems with affinity
If drug has high affinity (eg fentanyl) drives opioid crisis as there is no longer ligand to out compete
How are drug concentrations usually represented on graphs?
Logarithmic (not linear) eg -11 Log10 = 10^-11
Scale = -11 –> -7 (Log10)
= 10^-11 –> 10^-7
What can response be measured as?
Change in signalling pathway
Change in cell/tissue behaviour
requires drug efficacy
Response graph
Response % over drug concentration
EC50
Emax
What is EC50?
Effective concentration giving 50% of the maximal response
DRUG POTENCY
What does potency depend on?
Affinity and intrinsic efficacy AND cell/tissue specific components
What is concentration vs dose?
Concentration - known concentration of drug at site of action
Dose - concentration at site is usually UNKNOWN
Functional antagonism example
B2 adrenoreceptors targeted for asthma treatment
Binding causes relaxation of bronchioles
Where else do B adrenoreceptors work? Why is this a problem?
B1 adrenoreceptors in heart - cause increase HR and contractility when stimulated
Need selective/specific activation of B2 receptors if treating asthma to minimise side effects
Salbutamol explained
Poor selectivity affinity for B2 receptor
BUT very good B2 selective efficacy
so get selective response (based on efficacy and route of administration)
Salmeterol explained
Really good selective affinity for B2
No selective efficacy
so get selective response too (based on affinity)
What things are fixed in ligand receptor combination?
Affinity and intrinsic efficacy are FIXED
What things are not fixed in ligand receptor combination?
Potency is VARIABLE
Cell/tissue dependent factors effect
What relationship do we see between affinity and potency?
Often only need 50% binding for 100% response due to ‘spare receptors’
Response is limited by other factors within tissue (muscles can only contract so much)
Graph binding and response?
Response shifted left as not needing 100% binding to get 100% response
Where do we often see spare receptors?
Tyrosine kinase
G protein coupled receptor
Why do spare receptors exist?
amplification in signal transduction
response is limited by post receptor event
INCREASE SENSITIVITY/POTENCY
How do spare receptors increase sensitivity/potency?
Allow responses when there is only minimal agonist
(eg if full response requires 10,000 activated receptors, a cell with 20,000 receptors only requires 50% occupancy for full response)
What does receptor number influence?
Agonist sensitivity and potency
AND MAXIMAL RESPONSE
(more receptors = more sensitive)
What happens if you have too little receptors?
100% occupancy but insufficient receptors to illicit 100% response
Receptor numbers are not fixed, what do they vary with?
Cell type
Increase with low activity (up-regulation, more sensitive)
Decrease with high activity (down regulation, less sensitive)
Analogy down regulation receptors
Receptor number decreases when highly stimulated by drugs for example
This is what contributes to tolerance and withdrawal
Are all agonists equal at same receptor?
NO
Different affinities and efficacies
What are partial agonists vs full agonists?
Full agonists will elicit full response (+/- spare receptors)
Partial agonists will have no spare receptors but only illicit 50% response
EC50 and Kd full agonist and partial agonist
Full agonist:
EC50 < or equal to Kd
(half receptors bound is greater than receptors needed to illicit 50% response - spare receptors)
Partial agonist
EC50 SIMILAR or equal Kd
What does height of graph show on response graph?
Intrinsic activity (full vs partial response)
Partial agonist = lower intrinsic activity as lower efficacy
Why are partial agonists good?
Allow for a more controlled response
Work if there is low levels (or none) of endogenous ligand
Act as antagonist if high levels of full agonist
Use of partial agonists
Opioids:
Provide pain relief but high levels can result in respiratory depression (100% response)
Partial agonists (eg buprenorphine/methadone) occupies receptors and limits danger response (only 50% response)
Used to treat addiction
What are partial agonists often referred to as?
Mixed agonist/antagonist
How can antagonism be achieved?
Functional antagonism (asthma treatment stimulating B2 receptors) Antagonist AT ITS RECEPTOR (using ligand)
3 Receptor antagonists
Reversible competitive antagonism
Irreversible competitive antagonism
Non-competitive antagonism
Reversible competitive antagonism
Relies on dynamic equilibrium between ligands and receptors
More antagonists = out completes agonists (greater inhibition)
What is IC50?
Concentration of antagonist giving 50% inhibition
What does IC50 give an indication of?
Some antagonist affinity
But this also is influenced by antagonist and strength of stimulus (agonist)
What type of inhibition is competitive?
Surmountable
Adding more agonist can overcome antagonists actions
What does reversible competitive antagonists cause?
Parallel shift to right of agonist response curve (Maximal response is just achieved at higher concentrations)
Example of reversible competitive inhibition
Naloxone - out competes heroin
Irreversible competitive antagonist
With increased antagonists or time more and more receptors are permanently blocked by ligand (glued on)
NOT SURMOUNTABLE
Graph of irreversible competitive agonist
Decrease in size of response even when increasing antagonist even if increasing concentration of agonist
SUPPRESS MAXIMAL response
Example of irreversible competitive agonist
Phaechromocytoma = tumour of chromaffin
Lots of excreted adrenaline
acts on A1 adrenoreceptors
Irreversible competitor must be used as otherwise VERY high concentrations of competitor would have to be used
Clopidogrel antagonist example
Irreversible binds
P2Y12 antagonist
Reduced platelet activation so reduces risk of thrombosis
Where do non competitive antagonists bind?
allosteric site
What can allosteric sites provide binding sites for?
Agonists (potential drug targets)
Molecules that enhance or reduce effects of agonists (non competitive antagonists)
= negative allosteric modulation
What effect do non competitive antagonists have?
Similar to irreversible competitive antagonism
Need additional experiments to distinguish
Allosteric compounds example
Maraviroc
Negative allosteric modulator of chemokine receptor 5
Usually used by HIV to enter cells
Allosteric antagonist prevents HIV from entering
