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Pharmacology – notes from Rang and Dale
Binding of drug molecules to cells
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Most drug targets are protein molecules
A drug will not work unless it is bound
There are 4 main kinds of regulatory proteins commonly involved as primary drug targets;
o Receptors
o Enzymes
o Carrier molecules (transporters)
o Ion channels
Receptors:
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Recognition molecule for a chemical mediator
When the drug molecule binds to a receptor, a chain of reactions is initiated
o Agonist: activate the ‘receptor’
o Antagonist: May combine at the same site without causing activation and thereby
blocks the effect of an agonist on that receptor
Drug targets: other macromolecules (other than enzymes) with which drugs interact to
produce their effects
Specificity is reciprocal – individual classes of drugs bind only with certain targets and
individual targets recognise only certain classes of drugs
>>>>> HOWEVER>>>>>
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If a drug is to be considered useful, it needs to show a high degree of binding site specificity
 conversely, proteins that function as drug targets will recognise only ligands of a certain
type
In general, the lower the potency of a drug and the higher the dose needed, the more likely
it is that sites of activation other than the primary one, will assume significance 
associated with the appearance of unwanted side affects  Therefore, no drug is
completely specific
Affinity: The tendency of a drug to bind to the receptors
Efficacy: the tendency of the drug to activate the receptor (once bound)
Therefore  Agonists would have a high efficacy, whilst antagonists should have an efficacy of 0.
The binding of drugs to receptors;
(Interesting point: receptors tend to increase in number over the course of a few days if the relevant
hormone or transmitter is absent or scarce, whilst it will decrease in number if it is in excess 
adaption to drugs, of they are administered continually)
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Concentration-effect or dose-response curves are often plotted in order to estimate maximal
response a drug can produce (Emax) and the concentration or dose needed to produce 50% of
the maximal response (EC50 or ED50). Binding can be measured directly, however, this is a
more common method of charting a drugs binding effectiveness
An example of a comparative Dose-Response Curve:
Drug X has greater biologic activity per dosing
equivalent and is thus more potent than drug Y or Z.
Drugs X and Z have equal efficacy, indicated by their
maximal attainable response (ceiling effect). Drug Y is
more potent than drug Z, but its maximal efficacy is
lower.
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Concentration-effect curves cannot be used to chart the affinity of agonist drugs because the
physiological response produced is not directly proportional to occupancy (eg. Heart
medication – many other factors in play such as vasoconstriction/dilation and heart rate etc)
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These curves are not considered overly reliable, as the concentration of the drug at the
receptor site may vary significantly from in the organ bath
Partial Agonists and the Concept of Efficacy
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Difference between full (capable of producing the maximum response) and partial (can
produce only a submaximal response) agonists lie in the relationship between occupancy
and response
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Partial agonists cannot produce a maximal response even at 100% occupancy
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Differences in transductor function and the density of receptors in different tissues can
result in the same agonists, acting on the same receptor, appearing as a full agonist in one
tissue and as a partial agonist in another  the relative potencies of two agonists may be
different in different tissues, even though the receptor is the same
Constitutive Receptor Activation and Inverse Agonists
Constitutive Activation: activation in the absence of any ligand (can be in a disease state or
experimentally created)
Inverse Agonist: a drug capable of reducing the constitutive activation (have negative efficacy)
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Most normal receptors have a preference for an inactive state and there would be no
practical difference between an inverse agonist and a competitive antagonist
The two-state receptor model
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Occupies receptors can switch from a resting state (R) to an activated state (R*). R* is
favoured by binding of an agonist molecule (and not an antagonist)
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For pure antagonist solutions, R will stay.
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Not all receptors in a drug-free environment will be at R, but there may be a mixture of both
(constitutive activation)
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The problem with this theory is that there is more than one active and inactive conformation
that they may adopt
Spare Receptors
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The mechanism linking the response to receptor occupancy has a substantial reserve
capacity – receptor reserve / spare receptors (common with drugs that elicit smooth muscle
contraction)
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This just indicates that the pool of receptors is larger than the number needed to evoke a full
response
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This actually means that a given number of agonist-receptor complexes, corresponding to a
given level of biological response, can be reached with a lower concentration of hormone or
neurotransmitter than would be the case if fewer receptors were provided  economy of
hormone or transmitter secretion is therefore achieved at the expense of providing more
receptors
Drug Antagonism
Drug antagonism is where the presence of one drug diminishes or completely abolishes the effect of
another (competitive antagonism is also an example of this).
Includes;
 Chemical antagonism
 Pharmacokinetic antagonism
 Antagonism by receptor block
 Non-competitive antagonism (i.e. block of receptor – effector linkage)
 Physiological antagonism
Chemical Antagonism: Refers to the uncommon situation where 2 substances combine in a solution
and the effect of the active drug is lost (eg. Use of chelating agents which bind to heavy metals to
decrease their toxicity)
Pharmacokinetic Antagonism: situation in which the antagonist effectively reduces the
concentration of the active drug at its site of action. Can happen in a variety of ways including;
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Rate of metabolic degradation of active drug may be increased (i.e. hepatic acceleration of a
drug increased)
Rate of absorption form the GIT can be reduced
Rate of renal excretion can be increased
Antagonism by Receptor Block:
2 important mechanisms;
1. Reversible competitive antagonism
2. Irreversible, or non-equilibrium, competitive antagonism
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Competitive antagonism describes the situation whereby a drug binds selectively to a
particular type of receptor without activating it, but in such a way to prevent the binding of
the agonist (receptor can only bind with one at a time)
Reversible Competitive Antagonism:
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Agonist occupancy at a given concentration, will be reduced by the presence of the
antagonist, however if the concentration of the agonist is raised, agonist occupancy can be
restores  this antagonism is referred to as surmountable
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Dose ratio: the ratio by which the agonist concentration needs to be increased in the
presence of an antagonist in order to restore a given level of response  dose ratio would
increase linearly with the concentration of the antagonist and the slope of this line is a
measure of the affinity of an antagonist for the receptor
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In effect, this means that the agonist is able to displace the antagonist molecules from the
receptors (although they are incapable of evicting a bound antagonist molecule)  this
occurs because by occupying a proportion of the vacant receptors, the agonist reduces the
rate of association of the agonist molecules  the rate of dissociation temporarily exceeds
that of association and the overall antagonist occupancy falls
Irreversible/ non-equilibrium Competitive Antagonism:
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Occurs when the antagonist dissociates very slowly, or not at all from the receptors, the
result being that there is no change in antagonist occupancy when the agonist concentration
is increased
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Irreversible competitive antagonism occurs with drugs that possess reactive groups that
form covalent bonds with the receptor.
(The difference of irreversible and reversible competitive antagonism is not always clear – in the
case of spare receptors – if the agonist occupancy in order to create maximal response is very
small (1%), then it is possible to irreversibly block 99% of the receptors without reducing the
maximal response.)
Non-Competitive Antagonism
Definition: The antagonist blocks at some point in the chain of events that which leads to the
production of a response by the agonist. The effect will be to reduce the slope and maximum of the
agonist log concentration- response curve.
Physiological Antagonism: term used to describe the interaction of two drugs whose opposing
actions in the body cancel each other out.
Desensitisation and Tachyphylaxis
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Terms used to describe the phenomenon whereby the effect of a drug diminishes when it is
given continuously or repeatedly. This develops in the course of a few minutes
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Tolerance is usually used to describe the more gradual decrease in responsiveness to a drug,
taking days or weeks to develop
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Refractoriness: word used to talk about loss of therapeutic efficacy
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Drug Resistance: term used to describe loss in effectiveness of antimicrobial or antitumour
drugs.
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Many factors can contribute to the phenomenon listed above. These include;
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Change in receptors
 Among receptors directly coupled to ion channels, desensitisation is often
rapid and pronounced
 State is caused by the conformational change in the receptor, resulting in a
tight binding of the agonist without the opening of the ion channel.
 Phosphorylation of the intracellular regions of the receptor protein is a
second, slower mechanism by which ion channels become desensitised
 Phosphorylation of the receptors also interferes with its ability to initiate
second messenger cascades, although it can still bind to the agonist
 Takes a few minutes to form, and will recover at a similar rate when the
agonist is removed
Loss of receptors
 Prolonged exposure to an agonist often leads to a decrease in the number of
receptors at the cell surface (internalisation of the receptors)
 Numbers can fall rapidly, but will take longer to recover (i.e. 8hrs and
several days fro recovery)
 Receptors are taken into cell be endocytosis and also depends on receptor
phosphorylation
 Common for hormone receptors
Exhaustion of mediators
 Desensitisation can be associated with depletion of an intermediate
substance (rarer)
Increased metabolic degradation of the drug
 After repeated administration, there can be a lower plasma concentration of
the drug due to increased metabolic degradation
Physiological adaptation
 Drug effect can be nullified by a homeostatic response – this is very common
 An example can be where side effects such as nausea or sleepiness decrease
with continued administration of the drug
 Long-term delayed responses result from changes in gene expression
Active extrusion of drug from cells
Quantitative Aspects of Drug-Receptor Interactions
The binding reaction: Binding of drugs to receptors necessarily obeys the Law of Mass Action (the
rate of chemical reaction is proportional to the product of concentrations of the reactants)
Example from book:
A piece of heart muscle contains a total number of receptors, Ntot, for an agonist such as
adrenaline. When the tissue is exposed to the adrenaline at concentration XA, and allowed to
come to equilibrium, a certain number NA, of the receptors will become occupied and the
number of vacant receptors will be reduced to Ntot - NA. Normally, the number of adrenaline
molecules will greatly exceed Ntot and therefore the binding reaction will not dramatically
reduce XA. The magnitude of the response will be related to the number of receptors
occupied.
Much more complicated stuff on page 20.
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The higher the affinity of the drug for the receptor, the lower the concentration at which it
produces a given level of occupancy
Binding when more than one drug is present
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The same principle applies when 2 or more drugs are competing for the same receptor –
each has the effect of reducing the apparent affinity of the other
The nature of drug effects
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Direct effects of drugs on cellular function generally leads to secondary, delayed effects,
which are often relevant both in terms of therapeutic treatment and harmful effects
Balancing Benefit and Risk (from chapter 6, Rang and Dale, pp, 95-7)
Therapeutic Index is designed to indicate a margin of safety for the use of a drug.
It takes into account the minimum effective dose and the maximum tolerated dose (of lethal dose)
of a drug. However, in order to recognise that individuals may vary widely in their tolerance for a
drug, it works on calculating the median of each of these values to work out the drugs relative
safety.
Therefore, the definition of the Therapeutic Index is as follows;
Therapeutic Index = LD50/ED50
(LD = Lethal Dose, ED = Effective Dose)
Therefore, the greater the ratio between LD50 and ED50, the safer the drug. Drugs must be >1.0 to be
a therapeutic agent, whilst <2.0 is typically a toxic substance at the therapeutic dose.
The limitations of the therapeutic index are;
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LD50 does not reflect toxicity and unwanted side effects that do not lead to death
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ED50 is often not definable as it would depend on what measure of effectiveness is used. For
example, would the effectiveness of aspirin be measured according to its affect on a mild
headache or as its use as an antiheumatic drug?
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Neither of the terms take into account individual attributes – i.e. some relatively “harmless”
drugs would be extremely toxic for an asthmatic.