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Introduction & Definitions
What are the key factors that characterise the interaction of a drug with a receptor?
 Usually described using 'fractional occupancy' rate:
 Value between 0  1 is given depending on the fraction of receptors bound to the drug
 The value of 1 represents maximal saturation of binding sites (B_{max})
 Fractional occupancy forms the yaxis in an occupancy curve (receptor occupancy vs dose)
 Drugs of high affinity bind to a critical number of receptors at low concentration compared with drugs of low affinity which require a high concentration to bind to a critical number of receptors
 Affinity of a drug is described by it's K_{D}  the concentration of a drug at which 50% of the available receptors are occupied
 K_{D} best represented using an occupancy curve (receptor occupancy vs dose)
(Intrinsic Activity)
 Can be described using graded or quantal responses
 Graded response
 Value between 0  100% given depending on the effect on a particular system
 The value of 100% represents maximal effect of a drugreceptor complex (E_{max}) irrespective of drug concentration
 Quantal response:
 Value between 0  100% is given depending on the proportion of a specific population in which effect is produced
 Efficacy forms the yaxis in a doseresponse curve (efficacy vs dose)
 Drugs of high potency produce an effect bind at low concentration compared with drugs of low potency which require a high concentration to bind to a critical number of receptors
 Potency of a drug is described by it's ED_{50} or EC_{50} depending upon the use of graded or quantal response
 Graded response:
 EC_{50} describes the concentration of a drug that produces a specific response that is exactly halfway between baseline and maximum
 Quantal response:
 ED_{50} describe the dose of a drug that induces a specific response in exactly 50% of the population who take it
 EC_{50} / ED_{50} best represented using a doseresponse curve (efficacy vs dose)
Overview of Occupancy & Affinity
What are receptors, ligands and drugs?
How is drug receptordrug interaction modelled and explained mathematically?
 Drugreceptor interaction is explained by the ‘law of mass action’:
 Proposes that the rate of binding is directly proportional to the product of the concentrations of the reactants
 Can be demonstrated by the equation:
[R] is receptor
[DR] is drug–receptor complex
K_{on} is rate of forward reaction (assocation rate constant)
K_{off} is rate of backward reaction (dissociation rate constant)
What is the definition of receptor occupancy?
 Usually described using 'fractional occupancy' rate:
 Value between 0  1 is given depending on the fraction of receptors bound to the drug
 The value of 1 represents maximal saturation of binding sites (B_{max})
 Fractional occupancy forms the yaxis in an occupancy curve (receptor occupancy vs dose)
What is fractional occupancy?
 Fractional occupancy (r) is a way of describing the degree of receptor occupancy
 It is equal to the number of occupied receptor sites divided by the total receptor binding sites and can be demonstrated by the following:
Which factors determine receptor occupancy?
 Binding occurs when ligand and receptor collide (due to diffusion) with the correct orientation and sufficient energy:
 The rate of binding is a product of the concentration of drug, concentration of receptor and the rate of association (K_{on})
 This is demonstrated by the equation:
 Once binding has occurred, the ligand and receptor remain bound together for a random amount of time
 The rate of unbinding is a product of the concentration of drugreceptor complexes and the rate of dissociation (K_{off})
 This is demonstrated by the equation:
 The equilibrium dissociation constant k_{D} describes the relationship between the association and dissociation rate and thus the strength of drugreceptor binding
 It is a physical constant unique to each drugreceptor complex:
What is the definition of affinity?
 Drugs of high affinity bind to a critical number of receptors at low concentration compared with rugs of low affinity which require a high concentration to bind to a critical number of receptors
 Affinity of a drug is described by it's K_{D}  the concentration of a drug at which 50% of the available receptors are occupied
 K_{D} best represented using an occupancy curve (receptor occupancy vs dose)
How can the K_{D} be demonstrated mathematically?
 In equilibrium, the rates of drugreceptor association and dissociation will be equal
 Using equations described previously:
 In equilibrium, the rates of drugreceptor association and dissociation will be equal
 Using equations described previously:
Rearranges as:
{K_b}/{K_f} = \frac{[D]\cdot[R]}{[DR]}Substituted as:
{k_D} = \frac{[D]\cdot[R]}{[DR]}
 If a drug has a high affinity:
 The DR form will be favoured at equilibrium, hence the value of [D][R] will be small and that of [DR] will be high
 Therefore, the value of K_{D} will be small
 If a drug has low affinity :
 The D and R form will be favoured, hence the value of [DR] will be low
 Therefore, the value of K_{D} will be large
 The above equation can be considered at the point when a drug occupies exactly 50% of receptors in equilibrium (r=0.5)

 At this point, the number of free receptors [R] will equal that of occupied receptors [DR]
 Therefore, these cancel each other out, demonstrating that K_{D} is equal to the concentration of the drug at this time

How is the K_{D} defined?
 K_{D} is the uquilibrium dissociation constant describing the relationship between association and dissociation rates
 It can also be described as:
The drug concentration at which 50% of the maximum receptor population is occupied, usually expressed in units of mmol/L
What is the K_{D} used for?
 The K_{D} is a physiochemical constant – it the same for a given receptor and drug combination in any tissue, in any species
 It can therefore be used:
 To quantitatively compare the affinity of different drugs on the same receptor
 To identify an unknown receptor
How can the K_{D} be used to help determine fractional occupancy rate?
 The previous equation representing K_{D} can be rearranged:
 This can be substituted into the equation for fractional occupancy (r) and after simplification becomes:
Binding Curves
How is the relationship between concentration and receptor occupancy graphically displayed?
 Displayed using a binding curve which plots:
 Concentration of the drug on the xaxis
 Fractional occupancy on the yaxis
 Can be used to determine the KD of a drugreceptor system
 The xaxis is labelled with drug concentration (mmol/L). It is linear in nature
 The yaxis is labelled with fractional occupancy with a value 01
 The curve is a rectangular hyperbola passing through the origin and demonstrates how fractional occupancy varies with concentration of drug
 K_{D} can be demonstrated and is a marker of the affinity of the specific drug
Why is a semilog plot of the binding curve used?
 It is now customary to use a semilog curve to demonstrate fractional occupancy vs. dose:
 Drug concentration / dose plotted on a logarithmic scale
 Produces a sigmoidal curve
 Has the advantages of:
 Better assessment of the effects of low doses and for a wide range of doses on the same plot
 Easier comparison of different drugs on the same plot
How is a log binding curve graphically displayed?
 The xaxis is labelled with drug concentration (mmol/L). It is logarithmic in nature with each point 10x greater than the previous
 The yaxis is labelled with fractional occupancy with a value 01
 The curve sigmoid in shape with a linear middle portion and demonstrates how fractional occupancy varies with concentration of drug
 K_{D} can be demonstrated and is a marker of the affinity of the specific drug
How does the log binding curve change with differing affinity?
 The graph is a log binding curve with xaxis as drug concentration (mmol/L) in a logarithmic nature, and the yaxis as fractional occupancy
 The curves for each drug are sigmoid in nature
 A drug with increased affinity has a lower K_{D}, thus shifting the curve to the left
 A drug with decreased affinity has a higher K_{D}, thus shifting the curve to the right
Overview of Efficacy & Potency
What is the definition of efficacy?
(Intrinsic Activity)
 Can be described using graded or quantal responses
 Graded response
 Value between 0  100% given depending on the effect on a particular system
 The value of 100% represents maximal effect of a drugreceptor complex (Emax) irrespective of drug concentration
 Quantal response:
 Value between 0  100% is given depending on the proportion of a specific population in which effect is produced
 Efficacy forms the yaxis in a doseresponse curve (efficacy vs dose)
How can efficacy and potency be measured?
Can be measured by assessing graded or quantum response:
 Measures the efficacy of a receptoreffector in a particular system (e.g. tissue, animal or patient)
 Measures effect on a continuous scale
 Used to plot drug concentration vs. response on a graded doseresponse curve
 Examples of graded response include: effect of GTN on arterial pressure, streptomycin on protein synthesis etc.
 Measures the efficacy of a receptoreffector in a population, where the effect is binary on each individual (present or absent)
 Measures effect on a percentile scale, describing the size of population in which the effect is seen
 Used to plot drug dose vs outcome occurrence on a quantal doseresponse curve
 Examples include: effect of aspirin in reducing MI occurrence etc.
What is the definition of potency?
 Drugs of high potency produce an effect bind at low concentration compared with drugs of low potency which require a high concentration to bind to a critical number of receptors
 Potency of a drug is described by it's ED_{50} or EC_{50} depending upon the use of graded or quantal response
 Graded response:
 EC_{50} describes the concentration of a drug that produces a specific response that is exactly halfway between baseline and maximum
 Quantal response:
 ED_{50} describe the dose of a drug that induces a specific response in exactly 50% of the population who take it
 EC_{50} / ED_{50} best represented using a doseresponse curve (efficacy vs dose)
Which terms can be used to describe potency and how are they defined?
EC_{50}
(Median Effective Concentration)ED_{50}
(Median Effective Dose)What is the relationship between occupancy, efficacy, affinity and potency?
 Classical receptor theory suggests that the response seen will be proportional to the percentage of receptors occupied
 In this situation when the relationship between receptor occupancy and response is linear
 Occupancy is directly proportional to efficacy
 Affinity is directly proportional to potency, with K_{D} = EC_{50}
 If a simple doseoccupancy curve for a full agonist is plotted on the same axes as a dose–response curve they would be identical
 In this situation when the relationship between receptor occupancy and response is linear
 However, this relationship usually does not bear true:
 Receptor Reserves:
 It is often the case that only 5–10% occupancy is needed to produce a full response
 Indicates that ∼90% of receptors are not needed to elicit a maximum response and hence form the receptor reserve
 Results in doseresponse response lying to left of the binding curve in full agonist (EC_{50} less than K_{D})
 Partial agonism:
 Implied that at a full response can be produced with full receptor occupancy
 For a partial agonist, even at 100% occupancy a full response (similar to the full agonist) cannot be produced
 Spare receptors are not pooled or hidden; they are simply surplus to requirements
 Receptor Reserves:
DoseResponse Curve
What is a graded doseresponse curve and how is it plotted?
 The xaxis is labelled with drug concentration (mmol/L). It is linear in nature
 The yaxis is labelled % of maximum response
 The curve is a rectangular hyperbola passing through the origin and demonstrates how fractional occupancy varies with concentration of drug
 The graph is identical to a binding curve except for the yaxis
 EC_{50} can be demonstrated and is a marker of the potency of the specific drug
What is a quantal doseresponse curve and how is it plotted?
 The xaxis is labelled with drug dose (mg). It is linear in nature
 The yaxis is labelled % of population
 The curve is a rectangular hyperbola passing through the origin and demonstrates how fractional occupancy varies with concentration of drug
 The graph is again identical to a binding curve except for the yaxis
 ED_{50} can be demonstrated and is a marker of the potency of the specific drug
Why is a log doseresponse curve used and how is it plotted?
 The xaxis is labelled with drug dose (mg). It is logarithmic in nature with each point 10x greater than the previous
 The yaxis is labelled with % of population responding
 The curve sigmoid in shape with a linear middle portion and demonstrates how fractional occupancy varies with concentration of drug
 The graph is identical to a log binding curve except for the axis labels
 ED_{50} can be demonstrated and is a marker of the potency of the specific drug
How does the log doseresponse curve change with different potency of drugs?
 The graph is a log doseresponse curve with xaxis as drug dose (mg) in a logarithmic nature, and the yaxis as % of population responding
 The curves for each drug are sigmoid in nature
 A drug with increased potency has a lower ED_{50}, thus shifting the curve to the left
 A drug with decreased potency has a higher ED_{50}, thus shifting the curve to the right
Therapeutic Index
Besides the ED50 / EC50 which other markers of a drug’s potency can be described?
TD_{50}
(Median Toxic Dose)LD_{50}
(Median Lethal Dose)What is the therapeutic index?
 The therapeutic index is the ratio of the dose that produced toxicity to the dose that produces a clinically desired effect:
 It is a statement of the relative safety of a drug – the larger the TI, the safer the drug
 In animal models, the LD50 may be used instead of the TD50 when calculating the therapeutic index
How is therapeutic index determined?
 Therapeutic index is determined from quantal dose curves
 Both the concentrations of drugs producing desired and toxic clinical effects are plotted and ED50 / TD50 determined
What is the margin of safety?
 Because of differences in slopes and threshold doses, low doses of a drug may be effective but at higher concentrations cause toxicity in specific parts of the population without being effective
 The Margin of Safety (MOS) is used as a marker of drug safety to overcome limitations with the therapeutic index (demonstrated in the above graph)
 It uses the ratio of the toxic dose of a drug to 1% of the population (TD_{01}) to the dose that is 99% effective to the population (ED_{99})
Overview of Agonism
What is drug agonism?
The phenomena of a drug to selectively bind to a specific receptor and trigger a response
What types of agonism can drugs exhibit towards a receptor?
 Has affinity for receptors
 Able to produce a maximal response
 Morphine on MOP receptors
 Has affinity for receptors
 Produces a submaximal response
 Buprenorphine on MOP receptors
 Has affinity for receptors
 Produces no effect of its own
 Presence inhibits the action of other agonists at that receptor
 Atenolol on β receptors
 Has affinity for receptors
 Produces the opposite effect to the endogenous agonist
 Flumazenil on the GABA_{A} receptor
Agonism Types
How can the response to a full agonist be demonstrated on a log doseresponse curve?
 The graph is a log doseresponse curve with log drug dose on the xaxis and % of maximal effect on the yaxis
 The curve sigmoid in shape
 The height of the curve is the maximal effect which for a full agonist is 100%
 The curve always passes through the ED_{50}
 For a full agonist (A) this is always at 50% of maximal effect
 A full agonist of lower potency has a higher ED_{50}, thus shifting the curve to the right
How can the response to a partial agonist be demonstrated on a log doseresponse curve?
 The graph is a log doseresponse curve with log drug dose on the xaxis and % of maximal effect on the yaxis
 The curve sigmoid in shape
 The height of the curve is the maximal effect which for a partial agonist is lower than 100%
 The curve always passes through the ED_{50}
 which is at 50% of partial agonists maximal effect
 Partial agonist B has an ED_{50} which is the same dose as full agonist A and thus they have equal potency despite exhibiting a lower maximal effect
 Partial agonist C has an ED_{50} which is the same dose as full agonist A and thus they have equal potency despite exhibiting a lower maximal effecthigher than both drugs A&B and thus is less potent
How can the response to an antagonist be demonstrated on a log doseresponse curve?
 The graph is a log doseresponse curve with log drug dose on the xaxis and % of maximal effect on the yaxis
 An antagonist displays no effect regardless of the concentration and thus follows a horizontal linear path at 0% effect
How can the response to an inverse agonist be demonstrated on a log doseresponse curve?
 The graph is a log doseresponse curve with log drug dose on the xaxis
 However, the yaxis shows % response of maximal response and is drawn to show both a poitive and negative response
 The curve of an antagonist is sigmoid in shape and is a reflection of the curve of a full agonist
Antagonism Types
What is an antagonist?
A drug that reduces the action of another drug, generally an agonist
 Antagonists can act by a number of different mechanisms:
 Pharmacological antagonism: Interaction at the same receptor macromolecule as the agonist
 Chemical antagonism: Combination with the substance being antagonized
 Functional antagonism: Disruption to effect occurring at cellular sites distinct from the receptor mediating the agonist response
What is a functional antagonist?
 Describes disruption to the agonist effect occurring at cellular sites distinct from the receptor mediating the response
 Can include mechanisms such as:
 Indirect antagonism: competition for the binding site of an intermediate macromolecule that links the binding of the administered agonist to the effect observed
 Physiological antagonism: drug exerts an opposite physiological effect to that of the original agonist, usually through different receptors
Which characteristics can be used to describe a pharmacological antagonist?
 The antagonist and agonist compete for the same binding site
 Reduces the number of free receptor sites for agonist binding
 The antagonist binds to a separate site on the receptor to the agonist
 Can act to change the shape of the agonist binding site (allosteric modulation) or prevent receptor activation without effect on agonist binding
 Antagonist forms only shortlasting combinations with the receptor
 Antagonist form a stable covalent bond with the receptor
 The effect of the antagonist can be overcome by increasing concentrations of the agonist
 The maximum effect of the agonist is reduced by the antagonist and cannot be overcome by increasing concentrations
What are the main forms of pharmacological antagonists?
(Surmountable)
 The agonist and antagonist compete for the same binding site
 Only shortlasting combinations with the receptor are formed, so that equilibrium between agonist, antagonist, and receptors is reached
 Antagonism is surmountable as increased concentrations of agonist can outcompete the antagonist
(Insurmountable)
 The agonist and antagonist compete for the same binding site
 A stable covalent bond with the receptor is formed
 Antagonism becomes insurmountable at higher concentrations when no spare receptors remain as this cannot be overcome by increased concentrations of agonist to 'outcompete'
(Insurmountable)
 The antagonist binds to a different site on the than the agonist
 Antagonism is insurmountable as the agonist action is prevented when receptors are bound by the antagonist and cannot be overcome by increased concentrations
 Binding can be short lived (reversible) or permanent (irreversible) but the antagonism is insurmountable with either
How can the effect of the addition of a competitive reversible antagonist be demonstrated on a doseresponse curve?
 The graph is a log doseresponse curve with log drug dose on the xaxis and % of maximal effect on the yaxis
 A standard sigmoid curve represents a full agonist alone
 A second curve to the right of the first curve can be drawn to show the effect of the addition of a competitive reversible inhibitor
 The curve sits to the right due to decreased potency of the agonist in the presence of the antagonist
 This can be demonstrated by plotting the ED_{50} for both curves
How can the effect of the addition of a competitive irreversible antagonist be demonstrated on a doseresponse curve?
 The graph is a log doseresponse curve with log drug dose on the xaxis and % of maximal effect on the yaxis
 A standard sigmoid curve represents a full agonist alone
 At low doses a competitive irreversible antagonist is surmountable at high concentrations but at the expense of decreased potency of the agonist  represented by a curve to the right of the first curve
 At high doses the antagonistic effects become insurmountable despiute increasing agonist concentrations  represented by a curve to the right represeting reduced potency, and with a lower height representing reduced maximal effect
How can the effect of the addition of a noncompetitive antagonist be demonstrated on a doseresponse curve?
 The graph is a log doseresponse curve with log drug dose on the xaxis and % of maximal effect on the yaxis
 A standard sigmoid curve represents a full agonist alone
 A noncompetitive antagonist prevents antagonist binding and the effect is insurmountable
 This results in decreased potency and decreased maximal effect of the agonist  the curve is shifted down and to the right
 The curve is similar in shape to the effects of a partial agonist alone