Linger pharmacodynamics DSA

Question 1

Receptor (Broad Definition):

Answer 1

In the broadest sense, the site of binding and initial effect for any drug is that drug’s receptor; enzymes, transport proteins, structural proteins, and even RNA and DNA, can act as drug receptors

Question 2

Receptor (Specific Definition):

Answer 2

regulatory proteins that evolved for receiving & sending chemical signals

• mediate the physiologic effects of endogenous ligands (e.g., neurotransmitters and hormones) via signaling pathways
• bind endogenous signal molecules, then transduce and amplify this binding reaction into an intracellular signal that changes cellular function;
• these same proteins bind to and mediate the effects of exogenous drugs

Question 3

What is the importance of receptors?

Answer 3

i) Receptors mediate the action of drugs
ii) The selectivity of drug action is dependent on receptors
iii) Receptors largely determine the quantitative relationships between the dose (or concentration) of drug and the physiologic response(s) (both therapeutic and toxic)

Question 4

Some examples of drugs that do not act via receptors

Answer 4

antacids, osmotic diuretics, and ethanol

Question 5

key points re: the binding reaction

Answer 5

i) driven purely by chemical/physical forces dictated by the properties of the drug and the potential macromolecular binding sites of cells and tissues
ii) factors: size, shape, charge, hydrophobicity, etc. of the drug and the complementary binding site
iii) Small changes in the structure of the drug can drastically change its binding characteristics

Question 6

several types of chemical interactions contributing to drug-receptor binding

Answer 6

(1) Van der Waals forces (weakest interaction)
(2) Hydrogen bonds
(3) Ionic interactions
(4) Covalent bonds

Question 7

covalent bonds and an example

Answer 7

strongest interactions that may result in irreversible binding;

e.g., penicillin irreversibly inhibits transpeptidases (aka, penicillin-binding proteins) by covalent bonding to a serine residue in the active site of the transpeptidase enzyme

Question 8

Consequences of drug-receptor binding (3 choices)

Answer 8

May…

(1) do nothing,
(2) lead to some undesirable effect, or
(3) initiate a sequence of biochemical reactions that ultimately results in a desirable therapeutic effect

Question 9

Intrinsic efficacy:

Answer 9

• the ability of the drug to elicit a physiologic effect after binding to and activating a receptor-effector system;
• independent of binding affinity (i.e., a drug may bind vary tightly, but may exhibit low intrinsic efficacy –> a minimal response)

Question 10

Constitutive activity

Answer 10

the level of physiologic response produced by the receptor-effector system in the absence of (endogenous or exogenous) agonist

Question 11

Agonist

Answer 11

a drug that binds to and alters the activity of a receptor, mimicking the endogenous ligand by directly or indirectly stimulating the same response typically produced by the endogenous ligand

Usually involves a conformational change in the receptor

Question 12

Full agonist

Answer 12

an agonist that activates the receptor-effector system to the maximum extent of the receptor system

Question 13

Partial agonist

Answer 13

an agonist that binds to and activates the receptor in the same way as a full agonist, but –> stimulation of the receptor-effector system to a submaximal level, even when the concentration of partial agonist is high enough to occupy all the receptors

(1) Failure to produce a maximal response is NOT due to reduced affinity for binding the receptor
(2) In the absence of a full agonist, partial agonists act like an agonist and increase the response
(3) In the presence of a full agonist, partial agonists act like competitive antagonists by reducing the response

Question 14

partial agonist vs competitive antagonists

Answer 14

Partial agonists and competitive antagonists may appear to have similar effects in the presence of endogenous agonist, BUT an important difference between them is their action in the absence of endogenous agonist:

• partial agonists mimic the endogenous agonist at a submaximal level
• competitive antagonists do not elicit a functional response in the absence of agonist

Question 15

Inverse agonists

Answer 15

a drug that binds to a receptor, but produces a physiologic response that is the opposite of the effect normally elicited by conventional agonists at the same receptor by inhibiting constitutive activity of the receptor (e.g., agonists of the GABA receptor cause sedation, but inverse agonists cause agitation and anxiety)

Question 16

Antagonist

Answer 16

a drug that binds to a receptor and prevent activation (or inactivation) of the receptor by both physiologic and pharmacologic agonists; also called “blockers” or “inhibitors”

Question 17

Competitive antagonists

Answer 17

bind to the receptor at the same site as agonists (the active site) in a competitive fashion; the resultant effect depends on the concentration and affinity of all molecules competing for binding to the site; e.g., antagonist binding can be overcome by increasing the agonist concentration/dose

Sometimes called neutral antagonism because, in the absence of agonist, there is neither an increase nor a decrease in the physiologic response elicited by the receptor-effector system

Question 18

Noncompetitive antagonism

Answer 18

can occur either via irreversible binding at the active site or by reversible binding at an allosteric site; inhibition by either type of binding interaction is called noncompetitive because it cannot be overcome by increasing the concentration of agonist

Question 19

Antagonist binding

Answer 19

(1) Binding to the active site: covalent binding of antagonist to the active site results in a high affinity drug-receptor interaction that is essentially irreversible (or pseudo-irrversible)
(2) Allosteric binding: when an antagonist binds a receptor at a site different than the agonist binding site, the interaction can be either reversible or irreversible because the inhibition of receptor activity is independent of agonist binding to the active site

Question 20

Nonreceptor antagonists:

Answer 20

some types of antagonism do not involve a drug-receptor interaction

• chemical antagonists
• physiologic antagonists

Question 21

Chemical antagonist

Answer 21

a drug that inhibits an agonist by modifying or sequestering it such that it is incapable of binding to and activating its receptor

Examples: protamine sequesters heparin; anti-TNF monoclonal antibodies (mAb) bind to soluble TNF and prevent binding to and activation of TNF receptors

Question 22

Physiologic antagonist

Answer 22

a drug that blocks a target either upstream or downstream of the agonist receptor or blocks a receptor in an opposing physiologic system

Examples: to treat bradycardia caused by increased release of acetylcholine from vagus nerve endings, the physician could use isoproterenol, a β-adrenoceptor agonist that increases heart rate by mimicking sympathetic stimulation of the heart; use of this physiologic antagonist would be less rational—and potentially more dangerous—than use of a receptor-specific antagonist such as atropine (a competitive antagonist at the M2 ACh receptors at which acetylcholine slows heart rate)

Question 23

Allosteric modulators

Answer 23

drugs that bind to the receptor at a site different than the agonist binding site

i) Effect may be inhibition or facilitation of agonist action
ii) Effects cannot be overcome by increasing the concentration of agonist

Question 24

Receptor Occupancy Theory

Answer 24

i) Binding of drug to receptors increases until all of the receptors are occupied
ii) Kd = the drug concentration at which half-maximal receptor binding occurs
iii) Drug effect increases until some maximal effect occurs
iv) EC50 = the drug concentration at which half-maximal effect occurs
v) Note that Kd and EC50 could be, but often are not, identical

Question 25

Ligand-Receptor Binding Curves and Dose-Response Curves

Answer 25

i) Drug-receptor binding and drug-effect relationships can be described by mathematical equations and plotted as hyperbolic curves
ii) X-axis = drug concentration or dose
iii) Y-axis = action/effect/response (Panel A below) OR receptor binding (Panel B below)
iv) Because response often increases rapidly over a narrow range of drug concentrations, it is often informative to transform the data to a semi-logarithmic plot, which produces a sigmoid curve with a linear portion (see Figure 3-2 below)
v) X-axis = LOG drug concentration or dose
vi) Y-axis = action/effect/response OR receptor binding
vii) This mathematical transformation expands the scale of the x-axis at low concentrations (where the effect is changing rapidly) and compresses it at high concentrations (where the effect is changing slowly); this mathematical maneuver has no other biologic or pharmacologic significance

Question 26

spare receptors

Answer 26

maximal physiologic effect is often achieved before 100% of the receptors are occupied by drug; many responses require occupation of only a small percentage of the total receptors to produce a full response

Question 27

What is known/ unkown in drug dose & clinical response tudies?

Answer 27

Remember, the actual concentration of drug is often known in experimental systems; conversely, the actual concentration of drug at a receptor site in the human body is almost never known; instead, the dose of drug administered is related to clinical responses

Question 28

Graded Dose-Response Curves

Answer 28

relate the administered dose to the observed response for clinical data, much like concentration-effect curves do for laboratory data

Question 29

Potency

Answer 29

refers to the amount of drug necessary to produce a particular effect; the concentration (EC50) or dose (ED50) of a drug required to produce 50% of that drug’s maximal effect; dependent on binding affinity (Kd) and the ability of drug-receptor interaction to produce a response; important for determining dosing strategy

(1) Drugs that fill their receptor binding sites at lower doses have higher affinity and are more potent; their curves lie to the left on the dose-response curve
(2) Generally, potency is much less important than efficacy; if a drug is less potent, a bigger dose can be given; but if the drug cannot obtain the desired effect (lack of efficacy), its use may be limited

Question 30

Maximal efficacy

Answer 30

the limit of the dose-response relation on the response axis; may depend on the mode of interaction with receptors (e.g., partial agonists), or the characteristics of the activated receptor-effector system; may be limited by the drug’s toxic effects; important for clinical decisions when a large response is needed

Question 31

Limitations of Graded Dose-Response Curves

Answer 31

(1) Can be generated for any action or effect of drug that is continuously variable (graded) from no response up to a maximal response (e.g., drug-dependent increase in heart rate)
(2) Cannot be constructed for binary (quantal) events (e.g., prevention of convulsions, death)
(3) Data from a single patient may not be applicable to other patients due to variability in severity of disease, responsiveness to drugs, etc.

Question 32

Quantal Dose-Effect Curves

Answer 32

relate the administered dose to the cumulative frequency distribution (i.e., proportion or percentage) of patients exhibiting a response of specified magnitude; these curves are used to establish the safety and efficacy of a drug

Question 33

Median effective dose (ED50):

Answer 33

the dose at which 50% of individuals exhibit the specified quantal response (note this is different from the meaning of ED50 in relation to graded dose-response curves)

Question 34

Median toxic dose (TD50)

Answer 34

the dose at which 50% of individuals exhibit a specified toxic effect; in animal studies, the dose at which 50% of the animals die is called the median lethal dose (LD50)

Question 35

Therapeutic Index

Answer 35

= TD50/ED50

(1) Often precisely defined in animal experiments, but rarely in humans
(2) The therapeutic window is of greater clinical value in humans

Question 36

Therapeutic Window

Answer 36

the dose range between which the minimal therapeutic response and the minimal toxic response are observed; Acceptability of risk depends on the severity of disease

Question 37

5 types of transmembrane signaling mechanisms

Answer 37

intracellular receptors for lipid-soluble agents
Ligand-Regulated Transmembrane Enzymes 
Enzyme-Linked Transmembrane Receptors 
Ligand- and Voltage-Gated Ion Channels 
G Protein-Coupled Receptors

Question 38

Intracellular Receptors for Lipid-Soluble

Answer 38

(1) Some endogenous ligands are capable of passive diffusion through lipid membranes
(2) MOA: gene transcription occurs after ligand binding stimulates a conformational change in the receptor resulting in binding to specific DNA sequences (often called response elements)

(3) Some examples of clinically relevant drugs
(a) Steroid hormones (e.g., corticosteroids, mineralocorticoids, sex steroids, etc.)
(b) Thyroid hormones
(4) Therapeutically important consequences
(a) Requirement for gene transcription results in a characteristic lag period of 0.5 to several hours prior to onset of action
(b) Effects persist for hours to days after drug is eliminated because turnover of newly synthesized proteins must occur

Question 39

Ligand-Regulated Transmembrane Enzymes

Answer 39

• contain an extracellular ligand-binding domain and an intracellular enzymatic domain, connected by a hydrophobic transmembrane domain
• The intracellular domain may contain either a tyrosine kinase, a serine/threonine kinase, a tyrosine phosphatase, or a guanylyl cyclase

Question 40

Tyrosine kinase (aka, Receptor Tyrosine Kinases, RTKs)

Answer 40

(i) Most common type of ligand-regulated transmembrane enzyme
(ii) Some examples of endogenous ligands that stimulate RTKs (NOT a comprehensive list): insulin, EGF, PDGF, VEGF, FGF, Trk
(iii) MOA: Ligand binding causes a conformational change resulting in receptor dimerization, activation of the intracellular kinase, and trans-phosphorylation of the receptor; receptor phosphorylation creates binging sites (SH2 domains) for signaling proteins such as Grb2, which activate downstream effector pathways; activated receptors also catalyze phosphorylation of tyrosine residues on a variety of downstream effector molecules, including pathways that culminate in activation of transcription factors; thus, a single type of activated RTK can modulate a number of different biological processes
(iv) Some examples of clinically relevant drugs that modulate RTKs (NOT a comprehensive list): insulin, trastuzumab & cetuximab (anti-HER2 and anti-EGFR mAbs, respectively), erlotinib & gefitinib (EGFR inhibitors), and imatinib (BCR-ABL, c-Kit, & PDGFR inhibitor)
(v) Therapeutically important consequence: intensity and duration of action are often limited by receptor down-regulation, a phenomenon whereby ligand binding induces endocytosis and degradation of the receptor

Question 41

Serine/Threonine kinase

Answer 41

(i) Example: TGF-β receptors
(ii) MOA: Ligand binding causes a conformational change resulting in receptor dimerization, activation of the intracellular kinase, and phosphorylation of the receptor; the activated receptor phosphorylates and activates effector proteins (e.g., activated TGF-β receptors phosphorylate Smad proteins, which then dissociate from the receptor, migrate to the nucleus, and interact with transcription factors to modulate gene expression)

Question 42

Guanylate cyclase

Answer 42

(i) Example: natriuretic peptide receptors (atrial natriuretic peptide, ANP; brain natriuretic peptide, BNP; C-type natriuretic peptide, CNP)
(ii) MOA: Ligand binding causes a conformational change in the receptor dimers that stimulates guanylate cyclase activity

Question 43

Enzyme-Linked Transmembrane Receptors

Answer 43

(1) Transmembrane receptors that contain an extracellular ligand-binding domain and an intracellular domain connected by a hydrophobic transmembrane domain, but lack intrinsic enzyme activity
(2) Classic example: cytokine receptors bound to cytoplasmic Janus kinases (JAKs)
(3) Some examples of endogenous ligands that stimulate cytokine-JAK receptors (NOT a comprehensive list): growth hormone, erythropoietin, interferons, prolactin
(4) MOA: Ligand binding cause a conformational change in the receptor that results in activation of the JAKs and phosphorylation of the cytokine receptor; these events lead to binding and phosphorylation of proteins called STATs (signal transducers and activators of transcription); activated STATs translocate to the nucleus where they regulate transcription
(a) Cytokine receptors are stably associated with JAKs, but other types of enzyme-linked receptors don’t associate with the cytoplasmic kinase until after ligand binding
(b) Some receptors exist as inactive dimers; others can be activated as monomers

Question 44

clinically relevant enzyme-linked transmembrane receptor examples

Answer 44

Some examples of clinically relevant compounds: ruxolitinib selectively inhibits JAK1 and JAK2, it is approved for treatment of myelofibrosis; tofacitinib selectively inhibits JAK1 and JAK3, it is approved for treatment of rheumatoid arthritis

Question 45

Ligand- and Voltage-Gated Ion Channels

Answer 45

(1) Large transmembrane proteins, often comprised of multiple subunits, that form a pore which selectively gates influx and efflux of ions
(2) Some examples of endogenous ligands that activate ligand-gated ion channels (NOT a comprehensive list): acetylcholine, GABA, glutamate, glycine, serotonin
(3) Some ligand-gated channels are activated by intracellular signaling molecules; these are structurally distinct from the conventional ligand-gated ion channels
(a) Examples: hyperpolarization and cAMP-gated (HCN) channel, IP3-sensitive Ca2+ channel expressed on ER membranes

Question 46

MOA of ligand- and voltage-gated ion channels

Answer 46

(a) Voltage-gated channels open or close in response to changes in membrane potential
(b) Ligand-gated channels open or close in response to ligand binding
(5) Some examples of clinically relevant compounds:
(a) Verapamil inhibits voltage-gated calcium channels in the heart
(b) Benzodiazepines are allosteric modulators of GABA-gated chloride channels

Question 47

G Protein-Coupled Receptors

Answer 47

(1) Large transmembrane proteins comprised of a single subunit with seven transmembrane domains; often called “serpentine” receptors because the polypeptide chain “snakes” through the membrane; the intracellular domains of GPCRs bind to heterotrimeric G proteins
(2) Some examples of endogenous ligands that activate GPCRs (NOT a comprehensive list): acetylcholine, catecholamines, GABA, glutamate, serotonin, peptide hormones and neurotransmitters, endocannabinoids
(3) MOA: Inactive receptors associate with GDP-bound G proteins; ligand binding causes a conformational change that results in exchange of GDP for GTP; the GTP-bound alpha subunit of the G protein is active and dissociates from the receptor complex to engage downstream effectors, usually an enzyme or ion channel; the beta/gamma subunit heterodimer also dissociates from the GPCR and can engage additional, different effectors; the G protein also has the ability to hydrolyze GTP to GDP, which inactivates the G protein and results in reassembly of the inactive GPCR complex; various subtypes of G proteins signal via distinct effector pathways

Question 48

Clinical relevance of GPCRs

Answer 48

(4) Some examples of clinically relevant compounds: beta-adrenergic receptor agonists (albuterol) and antagonists (propranolol); muscarinic cholinergic receptor agonists (pilocarpine) and antagonists (atropine); opioid receptor agonists (morphine, oxycodone); many, many others

(5) Therapeutically important consequences
- Signal transduction may be amplified and prolonged by G proteins remaining bound to GTP (i.e., active) well after ligand has dissociated from the GPCR

• GPCR-mediated responses diminish over seconds to minutes of agonist exposure due to receptor desensitization; receptors can typically be resensitized by washout of agonist for several to tens of minutes; these phenomena are likely due to endocytosis and recycling of receptors from and to the plasma membrane; chronic or prolonged agonist exposure can lead to receptor degradation and failure of resensitization

Question 49

Second Messengers

Answer 49

The primary signaling pathways regulated by GPCRs include stimulation (G alpha-s) or inhibition (G alpha-i) of adenylyl cyclase and activation of phospholipase C (PLC) by G alpha-q

Question 50

Gs

Answer 50

reeptor for Beta adrenergic amines, histamine, serotonin, glucagon, and other hormones.

Increases adenylyl cyclase, –> increases cAMP

Question 51

Gi

Answer 51

receptor for alpha 2 adrenergic amines, acetylcholine (muscarininc), opioids, serotonin, and many others

decreases adenylyl cyclase, –> decrease cAMP

also opens cardiac K+ channels –> decreased heart rate

Question 52

Gq

Answer 52

receptor for acetylcholine (muscarinis), bombesin, serotonin (5-HT2), and many others

increases phospholipase C–> increased IP3, diacylglycerol, cytoplasmic Ca2+

Question 53

ADME

Answer 53

absorption
distribution
metabolism
elimination