1. Topic # 3: Drug receptor interactions and pharmacodynamics

2. Intended learning outcomes
Understand basic principles related to the relationship between drug concentration, response & receptor binding
Differentiate between agonists, antagonists, partial agonists and inverse agonist
Understand and differentiate between the different signaling mechanisms and the molecular mechanisms by which a drug acts
Understand different mechanisms of receptor regulation
Understand the relation between drug dose & clinical response
Understand the concept of therapeutic index

3. Receptors
The effects of most drugs result from their interaction with macromolecular components of the organism that influence important biochemical or physiologic processes (receptors or drug targets)
Drugs bind receptors (Drug-receptor complex) and initiate events leading to alterations in biochemical and/or biophysical activity of a cell, and consequently, the function of an organ (biological response which could be either therapeutic or toxic)

4. Macromolecular nature of drug receptors?
Most receptors are proteins, because the structures of polypeptides provide both the diversity and the necessary specificity of shape and electrical charge:
Regulatory proteins: mediate the actions of endogenous chemical signals e.g. neurotransmitters, autacoids, and hormones, as well as many drugs
Enzymes (e.g. dihydrofolate reductase ,the receptor for the antineoplastic drug methotrexate)
Transport proteins: (E.g. Na+/K+ ATPase, membrane receptor for digoxin)
Structural proteins: (E.g. tubulin, the receptor for colchicine)
Not all drugs exert their effects by interacting with a receptor (e.g. antacids neutralizes excess gastric acid, reducing the symptoms of heartburn)
1. Like beta blockers
However Ppi and H2 antagonsit on protiens

5. Receptor Theory
Receptors largely determine the quantitative relations between dose or concentration of drug and pharmacologic effects
Receptors are responsible for selectivity of drug action
Receptors as regulatory proteins and components of chemical signaling mechanisms that provide targets for important drugs and mediate the actions of pharmacologic agonists & antagonists
Receptors are key determinants of the therapeutic and toxic effects of drugs in patient
Receptors largely determine the quantitative relations between dose or concentration of drug and pharmacologic effects
The affinity for binding a drug determines the concentration of drug required to form a significant number of drug-receptor complexes, and the total number of receptors may limit the maximal effect a drug may produce
Binding laregly confined to 1 receptor or multiple receptors
High affinity of binding means small concentration will give effect
4. Toxic efect through non selctive binding or extention to pharamcological effect

6. Relationship between Drug Concentration, Response & Receptor Binding (Quantitative aspects of drug-receptor interaction)

7. Drug-receptor binding
The first step in drug action on specific receptors is the formation of a reversible drug-receptor complex, and initiate events leading to alterations in biochemical and/or biophysical activity of a cell, and consequently, the function of an organ (biological response)
D+ R D-R complex  Biological effect
Reversible equilibrium between free drug and drug-receptor complex

8. Drug-receptor binding
The formation of a reversible drug-receptor complex, the reactions being governed by the Law of Mass Action
D+ R D-R
Where [D] = concentration of free drug; [DR] = the concentration of bound drug; [RT] = the total concentration of receptors; kd = the equilibrium dissociation constant and represents the concentration of free drug at which half-maximal binding is observed k1 k2
[DR] [D]
[RT] Kd + [D]
In these systems, drug bound to receptors (B) relates to the
concentration of free (unbound) drug (C) as depicted in Figure 2–1 B
and as described by an analogous equation:
in which B max indicates the total concentration of receptor sites
(ie, sites bound to the drug at infinitely high concentrations of free
drug) and K d (the equilibrium dissociation constant) represents
the concentration of free drug at which half-maximal binding is
observed. This constant characterizes the receptor’s affinity for
binding the drug in a reciprocal fashion: If the K d is low, binding
affinity is high, and vice versa. The EC 50 and K d may be identical,
but need not be, as discussed below. (cocn with 50% occupancy and 50% EFFECT) =

9. Drug-receptor binding
Receptor occupancy =
[DR]
[Rtotal]
[D]
[D] + Kd =
[DR] / [Rtotal]
Second plot: logarithmic scale
First plot is called hyperbolic curve, the second one is called sigmoidal curve
Black..high afinity of binding….kd is very small…we need very low conc to binf half of receptos
Plooting log conc on x-axis change the plot from hyperbolic inot sigmoidal curve
In the sigmoidal..as the curve is shifted left…highr affinity of binidn
At equilibrium….occupany is related to conc…As cocn increases ..occupany is increased till we reach maxiumu occupancy…receptoir saturation…paltue aby any increase in cocn…no further increase in binding
Drug concentration
* Fractional occupancy ([DR] / [Rtotal])
* At equilibrium, receptor occupancy is related
to drug concentration

10. Relationship of binding to effect
The binding of a drug to its receptor initiates events that ultimately lead to a measurable biologic response
The magnitude of response is proportional to the amount of receptors bound/occupied according to the following equation:
Where [E] = is the effect observed at concentration [D], Emax is the maximal response that can be produced by the drug; and EC50 is the concentration of drug that produces 50% of maximal effect
Emax × [D] E =
Also conc vs effect can be hyperbolic and whne use log scale it is changed into sigmoidal
Kd=EC50 if we do not have spare receptors,
EC50 is much less than Kdif we have spare recptors. If no spare recptors..EC50=kd (when 4 recptors and 4 effectors)…read parce receptors down
[D] + EC50

11. Drug receptor interaction
The tendency of the a drug to bind to its receptor is governed by its affinity
The equilibrium dissociation constant (kd) characterizes the receptor affinity for binding the drug in a reciprocal fashion i.e. a high affinity means a small kd
The ability of a drug , once bound, to activate a receptor and generate response is denoted by its efficacy: (intrinsic activity); a drug with high efficacy eliciting, at some concentration, a full response
Efficacy..reaching full reponse at any conc
Potency reaching full reponse at lower caon than other drugs

12. Drug-receptor binding
In idealized or in vitro systems, the relation between drug concentration
and effect is described by a hyperbolic curve ( Figure 2–1 A)
according to the following equation: (next slide)
where E is the effect observed at concentration C, E max is the
maximal response that can be produced by the drug, and EC 50 is
the concentration of drug that produces 50% of maximal effect.
FIGURE 2–1 Relations between drug concentration and drug effect ( A ) or receptor-bound drug ( B ). The drug concentrations at which
effect or receptor occupancy is half-maximal are denoted by EC 50 and K d , respectively.

13. Drug receptor interaction
OCCUPATION
Governed by affinity
ACTIVATION
Governed by efficacy
K+1 D + R DR
RESPONSE
K-1 50 A B C
Dose (log scale) 50 A B C
Dose (log scale)
100
100
A has affinity higher than B and B higher than A
The lower the EC50…the higher the potency
All A b and C are efftice drug, bec they reached max response at some conc
But the higfghest potency was for A foloowed by B then c
% of Maximal effect/response
% of receptor bound
EC50 Kd

14. Compounds Have Different Affinities for the Same Receptor
1.00
Kd=0.5
Kd=1
kd=5
0.75
[DR]/RT
0.50
0.25
0.00
0.01
0.10
1.00
10.00
100.00
[D]
(concentration units)

15. Classification of a drug based on drug-receptor interactions
Occupation of a receptor by a drug molecule may (agonist) or may not (antagonist) result in the activation of the receptor
Agonist: Drug that bind to a receptor and produces a biologic response that mimic the response to the endogenous ligand. In general, an agonist has a strong affinity for its receptor and good efficacy
Antagonist: drug decrease/ prevent the actions of another drug/ agonist or endogenous ligand by binding to a receptor, compete with and prevent binding by other molecules

16. Receptor activation & confirmation
Receptor can exist in at least two conformational states, active (Ra), and inactive (Ri) that are in equilibrium
The extent to which the equilibrium is shifted toward the active state is determined by the relative affinity of the drug for the two conformations

17. The two-state model Drug (D) Ri Ra DRi DRa The relative affinity
Of the drug to either conformation will determine the effect of the drug Ri Ra
DRi
DRa
Figure 2.9 The two-state model. The receptor is shown in two conformational states, 'resting' (R) and 'activated' (R*), which exist in equilibrium. Normally, when no ligand is present, the equilibrium lies far to the left, and few receptors are found in the R* state. For constitutively active receptors, an appreciable proportion of receptors adopt the R* conformation in the absence of any ligand. Agonists have higher affinity for R* than for R, so shift the equilibrium towards R*. The greater the relative affinity for R* with respect to R, the greater the efficacy of the agonist. An inverse agonist has higher affinity for R than for R* and so shifts the equilibrium to the left. A 'neutral' antagonist has equal affinity for R and R* so does not by itself affect the conformational equilibrium but reduces by competition the binding of other ligands.

18. Agonists
Full agonist: has a high affinity for the Ra than for the Ri receptor confirmation, and a maximal effect is produced at high drug concentration
Partial agonist: produces a lower response at full receptor occupancy than do full agonists due to low intrinsic efficacy. Partial agonists produce concentration-effect curves that resemble those observed with full agonists in the presence of irreversible antagonist
The failure to produce a “full” maximal response is not due to decreased affinity for binding to receptors (WHY THEN??)
Due to low abiity to activate the receptor..low intrinsic efficay, even increasing conc will not increase effect

19. Agonists
Inverse agonist: drug with negative efficacy (i.e. opposite pharmacological effect). The characteristics of inverse agonists will be more apparent in systems that express relatively high basal activity (constitutive activity)
Treatment with an inverse agonist may be appropriate if:
receptor is over-expressed (mutation);
higher level of basal activity i.e. High Ra conformation
Examples: famotidine, losartan, and metoprolol

20. % Maximal Effect [D] PARTIAL AGONISTS - EFFICACY (concentration units)
Even though drugs may occupy the same # of receptors, the magnitude of their effects may differ.
Full Agonist
1.0
Partial agonist
0.8
0.6
Partial agonist
% Maximal Effect
0.4
Partial agonist has low intrinsic efficacy
Black and green has same EC50 but low intrinsic efficacy (Emax) for partial agonist
0.2
0.0
0.01
0.10
1.00
10.00
100.00
[D]
(concentration units)

21. Response Full agonist Partial agonist Inverse agonist Constitutive
activity
Response
Dose (log scale)
Inverse agonist

22. Antagonism
Antagonists are drugs that decrease the actions of another drug or endogenous ligand
Types of antagonism:
Competitive antagonism
Non-competitive antagonism
Chemical antagonism
Physiological/ functional
Competitive Antagonist (reversible, same binding site)
Non-competitive antagonism (allosteric [diff. binding site, or irreversible [same binding site])
Allosteric can be reversibly or irreversibly bound to the allosteric site, based on the type of binding.
Physiological/ functional
Chemical antagonism

23. 1. Competitive antagonism
Both the agonist and the antagonist bind to the same site on the receptor. The antagonist binds to the receptor without activating it, but in such a way to prevent the binding of the agonist
In the presence of a fixed concentration of agonist, increasing concentrations of a competitive antagonist progressively inhibit the agonist response; high antagonist concentrations prevent response completely. Conversely, sufficiently high concentrations of agonist can surmount the effect of a given concentration of the antagonist;
that is, the Emax for the agonist remains the same for any fixed concentration of antagonist. Because the antagonism is competitive, the presence of antagonist increases the agonist concentration required for a given degree of response, and so the agonist concentration-effect curve is shifted to the right.

24. 1. Competitive antagonism
Reversible competitive antagonism: a The 2 drugs compete with each other and at a given agonist concentration, the agonist occupancy will be reduced in the presence of the antagonist
Reversible competitive antagonist
+ antagonist
+ more
antagonist 50
Dose (log scale)
100
Agonist alone
Response
100 rcepotos should be occupied for max response…if give antgonist it will binf 30, we need to increase agonsit conc to compete and dissociated the antagoist…then 100 will be avasilbne agins for agonist but we need more concentration (EC50 increases in the presence of antagonist)
Propranolol and epinephrin

25. 1. Noncompetitive antagonism
Irreversible antagonism: the antagonist binds to the receptor (same binding site) in an irreversible or nearly irreversible fashion, either forming a covalent bond with the receptor or by binding so tightly (i.e. antagonist dissociate very slowly) that , for practical purposes, the receptor is unavailable for binding of agonist 50
Dose (log scale)
100
Agonist alone
+ antagonist
Plot of irrvesible antagonist+ agonist looks like partial agonist alone
Emax of the drug is reducded
EC50 not changed or very close
The actions of a noncompetitive antagonist are different because, once a receptor is bound by such a drug, agonists
cannot surmount the inhibitory effect irrespective of their concentration. In many cases, noncompetitive antagonists bind to
the receptor in an irreversible or nearly irreversible fashion, sometimes by forming a covalent bond with the receptor.
After occupancy of some proportion of receptors by such an antagonist, the number of remaining unoccupied receptors may be too low for the agonist (even at high concentrations) to elicit a response comparable to the previous maximal response
(Figure 2–3B). If spare receptors are present, however, a lower dose of an irreversible antagonist may leave enough
receptors unoccupied to allow achievement of maximum response to agonist, although a higher agonist concentration will
be required (Figure 2–2B and C; see Receptor-Effector Coupling & Spare Receptors).
Response
+ more
antagonist

26. II. Non-competitive antagonism
Allosteric antagonism: the antagonist binds to a site other than where the agonist binds
The antagonist block at some point the chain of events that leads to the production of a response by the agonist
The effect of a non-competitive antagonist on the dose–response curve for an agonist would be the same as the effect of an irreversible competitive antagonist
Example of non-competitive antagonists: calcium channel blockers (CCBs)
Allosteric modulators: alter the function of the receptor without inactivating the receptor
Ex. benzodiazepines…..
For example, benzodiazepines bind
noncompetitively to ion channels activated by the neurotransmitter
γ-aminobutyric acid (GABA), enhancing the net activating
effect of GABA on channel conductance
Act as irreversible competitve antagonist because it bind to diffrnet site than the agonist…..increasing agonist conc will not affect the dingin og the antagonsit

27. 2. Non-competitive (allosteric) antagonism

28. 2. Non-competitive (allosteric) antagonism
D……potentiation we needed llower C50, a
D can either increase efficay or affinity

29. FIGURE 1–3 Drugs may interact with receptors in several ways
FIGURE 1–3 Drugs may interact with receptors in several ways. The effects resulting from these interactions are diagrammed in the dose response
curves at the right. Drugs that alter the agonist ( A ) response may activate the agonist binding site, compete with the agonist (competitive inhibitors, B ), or act at separate (allosteric) sites, increasing ( C ) or decreasing ( D ) the response to the agonist. Allosteric activators ( C ) may
increase the efficacy of the agonist or its binding affinity. The curve shown reflects an increase in efficacy; an increase in affinity would result in a leftward shift of the curve.
Competitive inhibito…does not decrease efficacy but decrease afffinity…we need higher cocn of the agonist to get same max effect…shift to right.
Allosteric..we can not reach max effect..decrease efficay (as it keep bound)….but as it bind different site…does not affect affinity.

30. 3. Chemical antagonism
Refers to the situation where two substances combine and as a result the effect of the active drug is lost i.e. Makes the drug unavailable for interactions with receptors
Examples:
protamine and heparin,
dimercaprol and heavy metals
Some types of antagonism does not involve interaction of drug with a receptor
Example: protamine (+) can be used clinically to counteract the effects of heparin (-)
In this case, one drug acts as a chemical antagonist of the other simply by ionic binding
that makes the other drug unavailable for interactions with proteins involved in blood clotting
Dimercaprol, also called British anti-Lewisite (BAL), is a medication used to treat acute poisoning by arsenic, mercury, gold, and lead.[2] May also be used for antimony, thallium, or bismuth poisoning but the evidence is not good for these uses.[2][3] It is given by injection into a muscle.[2]
Arsenic and some other heavy metals act by chemically reacting with adjacent thiol residues on metabolic enzymes, creating a chelate complex that inhibits the affected enzyme's activity.[9] Dimercaprol competes with the thiol groups for binding the metal ion, which is then excreted in the urine.[

31. 4. Physiological antagonisim
Describes interaction of two drugs whose opposing actions in the body tend to cancel each other
In general, physiologic antagonist produces effects that are less specific and less easy to control that are the effect for receptor- specific antagonist
Examples:
histamine and epinephrine,
glucagon and propranolol
Histamine: …vasodilation
Epinephrine: vascoconstriciton
Glucagon: hyperglycemia
Proranolo: blcok glycogenolysis (through blocking b-2 receptors in the liver)

32. Spare Receptors
According to the receptor occupancy theory there must be a linear relationship between occupancy and response, the maximum response is occurring when all receptors are occupied.
However, the maximal response can be elicited by an agonist at a concentration that does not result in occupancy of the full complement of available receptors.
Under these circumstances receptors are said to be “spare” for a given pharmacologic response or there is a large receptor reserve
Phramcological spare: ex dose of BB in HTN is less than HF, for HTN the receptors said to be

33. How can the presence of spare receptors be determined?

34. Spare Receptors (Cont’d)
Experimentally, spare receptors may be demonstrated by using :
irreversible antagonists to prevent binding of agonist to a proportion of available receptors
and showing that high concentrations of agonist can still produce an undiminished maximal response
Higher cocn are needed to keep equilibrium between D+R and DR complex. In the presence of anatgonist, the probabliltiy of binding of the agonist is less.

35. Spare Receptors (cont’d)
The presence of spare receptors increases sensitivity to the agonist
……the likelihood of a D-R interaction increases in proportion to the number of receptors available;
The sensitivity (EC50) of a cell or tissue to a particular conc. of agonist depend on:
the affinity of the receptor for binding agonist (Kd)
but also on the total no. of receptors present compared with the number actually needed to elicit a maximal (degree of spareness)
Thus, the sensitivity of a cell or tissue to a particular
concentration of agonist depends not only on the affinity of the
receptor for binding the agonist (characterized by the K d ) but also
on the degree of spareness —the total number of receptors present
compared with the number actually needed to elicit a maximal
biologic response.
- Degree of sparness is higher…lower number of receotrs are required to get maximal response.

37. Spare receptors Reduced maximal effect
FIGURE 2–2 Logarithmic transformation of the dose axis and
experimental demonstration of spare receptors, using different concentrations
of an irreversible antagonist. Curve A shows agonist
response in the absence of antagonist. After treatment with a low
concentration of antagonist (curve B ), the curve is shifted to the
right. Maximal responsiveness is preserved, however, because the
remaining available receptors are still in excess of the number
required. In curve C, produced after treatment with a larger concentration
of antagonist, the available receptors are no longer “spare”;
instead, they are just sufficient to mediate an undiminished maximal
response. Still higher concentrations of antagonist (curves D and E )
reduce the number of available receptors to the point that maximal
response is diminished. The apparent EC 50 of the agonist in curves D
and E may approximate the K d that characterizes the binding affinity
of the agonist for the receptor. (at other curves,... EC50 lesss than Kd because maximal effect occurs at conc less than required for full occupancies but using antagonists chnaged the cocn required from the agonist and increased that till it reached Kd which is cocnstant)
(kd is the Kd for the initial binding
Logarithmic transformation of the dose axis and experimental demonstration of spare receptors,
using different concentrations of an irreversible antagonist. Curve A shows agonist response in the
absence of antagonist. After treatment with a low concentration of antagonist (curve B), the curve
is shifted to the right; maximal responsiveness is preserved, however, because the remaining
available receptors are still in excess of the number required. In curve C, produced after treatment
with a larger concentration of antagonist, the available receptors are no longer "spare"; instead,
they are just sufficient to mediate an undiminished maximal response. Still higher concentrations
of antagonist (curves D and E) reduce the number of available receptors to the point that maximal
response is diminished. The apparent EC50 of the agonist in curves D and E may approximate the
KD that characterizes the binding affinity of the agonist for the receptor.

38. Spare Receptors (cont’d)
If a large receptor reserve is present the EC50 will be lower than the the Kd
i.e. the concentration of drug required to give 50% of maximum response is lower than the concentration of drug required to occupy 50% of receptors

39. Signalling mechanisms: the molecular mechanisms by which a drug acts

40. Overview Receptors act as a signal detectors
When a receptor is occupied by an agonist, the resulting conformational change is only the first of many steps usually required to produce a pharmacologic response
Receptors signals their recognition of a bound ligand by initiating a series of reactions that ultimately result in specific intracellular response
Receptors elicit many different types of cellular effect. Some of them are of very rapid, intermediate, or long timescales

41. Five basic mechanisms of transmembrane signaling
A lipid-soluble ligand crosses membrane and acts on an intracellular receptor
A transmembrane receptor that binds and stimulates a protein tyrosine kinase
A transmembrane receptor protein whose intracellular enzymatic activity is allosterically regulated by a ligand that binds to a site on the protein's extracellular domain
A ligand-gated transmembrane ion channel that can be induced to open or close by binding of a ligand
A transmembrane receptor protein that stimulates a GTP- binding signal transducer protein (G protein), which in turn generates an intracellular second messenger

42. Known transmembrane signaling mechanisms: 1: A lipid-soluble chemical signal crosses the plasma membrane and acts on an intracellular receptor (which may be an enzyme or a regulator of gene transcription); 2: the signal binds to the extracellular domain of a transmembrane protein, thereby activating an enzymatic activity of its cytoplasmic domain; 3: the signal binds to the extracellular domain of a transmembrane receptor bound to a separate protein tyrosine kinase, which it activates; 4: the signal binds to and directly regulates the opening of an ion channel; 5: the signal binds to a cell-surface receptor linked to an effector enzyme by a G protein. (A, C, substrates; B, D, products; R, receptor; G, G protein; E, effector [enzyme or ion channel]; Y, tyrosine; P, phosphate.)

43. I. Intracellular Receptors for Lipid-Soluble Agents
These receptors generally regulate gene transcription
The ligands are sufficiently lipid soluble to cross the plasma membrane to interact with the intracellular receptor
Most receptors are located in the nucleus but some are actually located in the cytosol and migrate to the nuclear compartment when a ligand is present.
Example: receptors for steroids corticosteroids, mineralocorticoids, sex steroids, vitamin D), and thyroid hormone
Stimulates the transcription of genes in the nucleus by binding to specific DNA sequences near the gene whose expression is to be regulated
binding to specific DNA sequences near the gene : hormone response element
Aldosterone: regulate the expressino of Na epithelial channels on the apical membrane for NA and water reabsorption in the collecting duct

44. Mechanism of glucocorticoid action
Mechanism of glucocorticoid action. The glucocorticoid receptor polypeptide is schematically depicted as a protein with three distinct domains.
A heat-shock protein (hsp90), binds to the receptor in the absence of hormone and prevents folding into the active conformation of the receptor.
Binding of a hormone ligand (steroid) causes dissociation of the hsp90 stabilizer and permits conversion to the active configuration.

45. II. Ligand-Regulated Transmembrane Enzymes Including Receptor Tyrosine Kinase
A large & heterogeneous group of membrane receptors
Examples: insulin, epidermal growth factors (EGF), platelet-derived growth factor (PDGF), atrial natriuretic peptide (ANP), transforming growth factor-  (TGF-)
Receptors are polypeptides consisting of an extracellular ligand-binding domain and a cytoplasmic enzyme domain (with tyrosine kinase or guanylyl cyclase) linked by a single transmembrane helix
Also serine kinase

46. II. Ligand-Regulated Transmembrane Enzymes Including Receptor Tyrosine Kinase
Binding of ligand to the receptor's extracellular domain cause:
change in the receptor conformation….
…..receptor molecules bind to one another bringing together the tyrosine kinase domains that become enzymatically active…
…..the domains phosphorylate each other as well as additional downstream signaling proteins
E.g, insulin, uses a single class of receptors to trigger increased uptake of glucose and amino acids and to regulate metabolism of glycogen and triglycerides in the cell
Receptor fopr onsulin: insulin receptor

47. Upon binding of EGF (circle), the receptor converts from its inactive monomeric state (left) to an active dimeric state (right), in which two receptor polypeptides bind noncovalently. The cytoplasmic domains become phosphorylated (P) on specific tyrosine residues (Y) and their enzymatic activities are activated, catalyzing phosphorylation of different substrate proteins (S)…..modulation of different biochemical processes

48. Insulin receptor

49. III. Cytokine Receptors
Examples: receptors for growth hormone, erythropoietin, interferons, and other regulators of growth and differentiation
A separate protein kinase from the janus- Kinase (JAK) family, binds non-covalently to the receptor
i.e. the tyrosine activity is not intrinsic to the receptor molecule

50. Cytokine receptors, like receptor tyrosine kinases, have extracellular and intracellular domains and form dimers. However, after activation by an appropriate ligand, separate mobile protein tyrosine kinase molecules (JAK) are activated, resulting in phosphorylation of signal transducers and activation of transcription (STAT) molecules. STAT dimers then travel to the nucleus, where they regulate transcription.

51. IV. Ligand-Gated Channels
Receptors for several neurotransmitters form agonist-regulated ion-selective channels in the plasma membrane that convey their signals by altering the cell's membrane potential or ionic composition
Many of the most useful drugs in clinical medicine act by mimicking or blocking the actions of endogenous ligands that regulate the flow of ions though plasma membrane channels
Examples: receptors for acetylcholine, serotonin, - aminobutyric acid, & excitatory amino acids (glycine, aspartate, glutamate)

52. Figure 3-4 Structure of the nicotinic acetylcholine receptor (a typical ligand-gated ion channel) in side view (left) and plan view (right). The five receptor subunits (α2, β, γ, δ) form a cluster surrounding a central transmembrane pore, the lining of which is formed by the M2 helical segments of each subunit. These contain a preponderance of negatively charged amino acids, which makes the pore cation selective. There are two acetylcholine binding sites in the extracellular portion of the receptor, at the interface between the α and the adjoining subunits. When acetylcholine binds, the kinked α helices either straighten out or swing out of the way, thus opening the channel pore. (Based on Unwin 1993, 1995.)

53. IV. Ligand-Gated Channels (Cont’d)
Signal is transmited across the plasma membrane by increasing transmembrane conductance of the relevant ion and thereby altering the electrical potential across the membrane
Response to these receptors is very rapid, having duration of a few milliseconds

55. G-Proteins coupled receptors (GPCRs)
GPCRs are comprised of a single polypeptide chain that has seven membrane-spanning (transmembrane) regions
GPCRs couple to a family of heterotrimeric GTP-binding regulatory proteins termed G proteins (signal transducers)
The G protein heterotrimer is composed of three subunits: α, β, and γ

56. G-Proteins coupled receptors (GPCRs)
The activated G-protein changes the activity of an effector (E) element, usually an enzyme or ion channel
This effector then changes the concentration of the intracellular second messenger

57. Diagram of the human AT2 receptor including the seven transmembrane domains along with intra and extra-cellular loops and tails. Proposed amino acids to be mutagenized are highlighted and their single letter designations are adjacent.

59. G-Proteins coupled receptors (GPCRs)
Depending on the nature of the α-subunit, the active, GTP-bound form binds to and regulates effectors such as adenylyl cyclase (via Gs or Gi) or phospholipase C (via Gq )
G-protein
Effector
Effector substrate
Second messenger response
Response Gs
Adenylyl cyclase
ATP
Increase cAMP
Increase Ca+2 & enzyme activity Gi
Decrease cAMP
Decrease Ca+2 & enzyme activity, increase K+ efflux Gq
Phosphlipase C
Membrane lipids
Increase IP3 and DAG
Increase Ca+2 & protein kinase activity

60. Receptor regulation
Receptors not only initiate regulation of biochemical events and physiological function but also are themselves subject to many regulatory and homeostatic controls
Repeated or continuous administration of an agonist may lead to changes in the responsiveness of the receptor. Changes in responsiveness can occur over short times (minutes) and longer periods (hours)
Continued stimulation of cells with agonists generally results in short-term diminution of the receptor response subsequent exposure to the same concentration of drug, sometimes called desensitization (also referred to as adaptation, refractoriness, or down-regulation)

61. Receptor regulation (Cont’d)
Desensitization can be the result of temporary inaccessibility of the receptor to agonist or the result of fewer receptors synthesized and available at the cell surface (receptor down regulation) e.g. response to β-adrenoceptor agonist. Other reasons include exhaustion of mediators or increased metabolic degradation of the drug.

62. Receptor regulation (Cont’d)
Chronic administration of an antagonist results in up-regulation as the number and sensitivity of receptors increase in response to chronic blockade... i.e. The patient develops tolerance requiring higher doses of an antagonist to counteract the increasing receptor number

63. Relation between drug dose & clinical response

64. Overview
The magnitude of drug effect depends on the drug concentration at the receptor site, which in turn determined by the dose of drug administered and by factors of the drug pharmacokinetic profile
There is a graded dose-response relationship in individuals and a quantal dose-response relationship in a population

65. Dose & Response in Patients A. Graded dose-response relation
The response to a drug is a graded effect, meaning that the response is continuous and graded
Graded dose response curves are constructed by plotting the magnitude of the response against increasing doses of a drug (or log dose). It is characerized by:
Potency (EC50 or ED50): a measure of the amount of drug necessary to produce an effect of a given magnitude
Efficacy (Emax): greatest effect (Emax) an agonist can produce if the dose is taken to very high levels

66. EFFECT
POTENCY
EFFICACY
ED50
Maximal Effect
Log [Dose]

67. Graded Dose-response Relations (cont’d)
Limitations of the graded dose-response curve:
Graded dose-response curves may be impossible to construct if the pharmacologic response is an either-or (quantal) event, such as prevention of convulsions, arrhythmia, or death.
Graded dose-response curves obtained in a single patient may be limited in application to other patient (variability of severity of disease and patient responsiveness to drugs)

68. Dose & Response in Patients B. Quantal dose-response relation
Are population-based dose-response curves
It determines the dose of a drug required to produce a specific magnitude of effect in a large number of patients
Quantal effects can be determined by plotting the cumulative percentage of the population that show the response at each dose versus the log of the dose administered

69. Quantal Dose-Effect Distribution
# of
Subjects
Threshold Dose

70. Cumulative Dose-Effect Curve
Cumulative % of Subjects
Dose

72. Dose & Response in Patients B. Quantal dose-response relation (Cont’d)
The quantal dose-effect curve is often characterized by:
Median effective dose (ED50): the dose at which 50% of the individuals/population exhibit the specified desirable/therapeutic effect
Median toxic dose(TD50): the dose at which 50% of individuals/population exhibit a particular toxic effect

73. B. Quantal Dose-Response Curves
These values allow us to:
Compare potencies of drugs
Obtain index of selectivity of drug action (e.g. suppression of cough against analgesia for opioids drugs)
Estimating the margin of safety (i.e. therapeutic index)

74. Therapeutic index
It is the ratio of the dose that produces toxicity to the dose that produces a clinically desired or effective response in a population of individuals
In humans, the therapeutic index of a drug is determined using drug trials and accumulated clinical experience. These usually reveal a range of effective doses and a different (sometimes overlapping e.g. warfarin) range of toxic doses
TD50
Therapeutic Index =
ED50

75. Percentage of patients
Therapeutic Index
Therapeutic
Toxic
Percentage of patients
TD50
ED50
Dose

76. Therapeutic index
The clinically acceptable risk of toxicity depends critically on the severity of the disease:
Treatment of a simple headache, for example, with a narrow therapeutic index drug would be unacceptable
Treatment of lethal diseases, such as Hodgkin's lymphoma, the acceptable difference between therapetuic and toxic doses may be smaller

77. Therapeutic index
*Since pharmacodynamic variation in the population may be marked it is important to keep the following points in mind when interpreting the therapeutic index:
The concentration or dose of drug required to produce a therapeutic effect in most of the population usually will overlap the concentration required to produce toxicity in some of the population, even though the drug’s therapeutic index is large
The concentration–percent curves for efficacy and toxicity need not be parallel, adding yet another complexity to determination of the therapeutic index in patients.
Finally, no drug produces a single effect, and the therapeutic index for a drug will vary depending on the effect being measured

78. RANK ORDER OF EFFICACY:B C D
RESPONSE
ED50
RANK ORDER OF POTENCY:
RANK ORDER OF EFFICACY:

79. A 55-year-old woman with hypertension is to be treated with
a vasodilator drug. Drugs X and Y have the same mechanism
of action. Drug X in a dose of 5 mg produces the same
decrease in blood pressure as 500 mg of drug Y. Which of the
following statements best describes these results?
(A) Drug Y is less efficacious than drug X
(B) Drug X is about 100 times more potent than drug Y
(C) Toxicity of drug X is less than that of drug Y
(D) Drug X has a wider therapeutic window than drug Y
(E) Drug X will have a shorter duration of action than drug Y because less of drug X is present over the time course
of drug action

80. Graded and quantal dose-response curves are being used for
evaluation of a new antiasthmatic drug in the animal laboratory
and in clinical trials. Which of the following statements
best describes quantal dose-response curves?
(A) More precisely quantitated than graded dose-response
curves
(B) Obtainable from the study of intact subjects but not
from isolated tissue preparations
(C) Used to determine the maximal efficacy of the drug
(D) Used to determine the statistical variation (standard
deviation) of the maximal response to the drug
(E) Used to determine the variation in sensitivity of subjects
to the drug

81. A study was carried out in isolated, perfused animal hearts.
In the absence of other drugs, pindolol, a β-adrenoceptor
ligand, caused an increase in heart rate. In the presence of
highly effective β stimulants, however, pindolol caused a
dose-dependent, reversible decrease in heart rate. Which of
the following expressions best describes pindolol?
(A) A chemical antagonist
(B) An irreversible antagonist
(C) A partial agonist
(D) A physiologic antagonist
(E) A spare receptor agonist

82. Which of the following
statements about spare receptors is most correct?
(A) Spare receptors, in the absence of drug, are sequestered in
the cytoplasm
(B) Spare receptors may be detected by finding that the drug receptor
interaction lasts longer than the intracellular
effect
(C) Spare receptors influence the maximal efficacy of the
drug-receptor system
(D) Spare receptors activate the effector machinery of the cell
without the need for a drug
(E) Spare receptors may be detected by the finding that the
EC50 is smaller than the Kd for the agonist

83. Two cholesterol-lowering drugs, X and Y, were studied in a large group of patients, and the percentages of the group showing a specific therapeutic effect (35% reduction in lowdensity lipoprotein [LDL] cholesterol) were determined. The
results are shown in the following table.
Which of the following statements about these results is correct?
(A) Drug X is safer than drug Y
(B) Drug Y is more effective than drug X
(C) The 2 drugs act on the same receptors
(D) Drug X is less potent than drug Y
(E) The therapeutic index of drug Y is 10

84. Sugammadex is a new drug that reverses the action of
rocuronium and certain other skeletal muscle-relaxing agents.
It appears to interact directly with the rocuronium molecule
and not at all with the rocuronium receptor. Which of the
following terms best describes sugammadex?
(A) Chemical antagonist
(B) Noncompetitive antagonist
(C) Partial agonist
(D) Pharmacologic antagonist
(E) Physiologic antagonist

85. Each of the curves in the graph
below may be considered a concentration-effect curve or a
concentration-binding curve.
Which of the curves in the graph describes the percentage of
binding of a large dose of full agonist to its receptors as the
concentration of a partial agonist is increased from low to very
high levels?
(A) Curve 1
(B) Curve 2
(C) Curve 3
(D) Curve 4
(E) Curve 5

86. Which of the curves in the graph describes the percentage
effect observed when a large dose of full agonist is present
throughout the experiment and the concentration of a partial
agonist is increased from low to very high levels?
(A) Curve 1
(B) Curve 2
(C) Curve 3
(D) Curve 4
(E) Curve 5

87. Which of the curves in the graph describes the percentage of
binding of the partial agonist whose effect is shown by Curve 4 if
the system has many spare receptors?
(A) Curve 1
(B) Curve 2
(C) Curve 3
(D) Curve 4
(E) Curve 5