Exam IV: Pharmacodynamics

Question 1

Pharmacodynamics

Answer 1

The study of drug effects on the body

Therapeutic and toxiceffects of drugs result fromthe interactions with physiological molecules in the body

The receptor concept isimportant for the development of drugs and for making therapeutic decisions in clinical practice

Drug + receptor (physiological molecules) = therapeutic and toxic effects

Question 2

Importance of Receptors: Dose vs. Effect

Answer 2

Receptors largely determine the quantitative relationship between dose (or concentration) of a drug and the pharmacological effect
Binding affinity determines concentration required to form drug-receptor complexes
Maximal effect of drug is limited by number of receptors

Less receptors have less responses
Affinity: how well the drug binds to the receptor; well bound = good effect/action and vice versa
Number of receptors within the body determines effect

Question 3

Importance of Receptors: Selectivity

Answer 3

Receptors are responsible for selectivity of drug action
Drug structures determine affinity for different classes of receptors
Non-selective drugs can cause side-effects

Question 4

Importance of Receptors: Agonist vs. Antagonist

Answer 4

Receptors mediate actions of pharmacologic agonists and antagonists
Drugs binding to receptors can activate or interfere with normal physiological processes

Question 5

Receptors: Occupancy Theory

Answer 5

Intensity of a drug’s response is proportional to the amount of receptors occupied by that drug

When you have a drug it will bind to receptors and form complex to activate receptors causing transduction of signals to cause effects in cells
K1 is the rate of association and K2 is the rate of disassociation aka binding and unbinding of drug to receptors
Kd= K2/K1 = affinity of the drug itself
Drug binding to receptor will activate effector molecules on receptor itself
Efficacy= maximal effect of drug binding to receptor

Question 6

Occupancy Theory Issues

Answer 6

Maximal response is achieved when all receptors have been occupied

Does not explain “potency”
When two drugs bind to the same receptor and bind maximally to all receptors, how does one drug achieve the same effect with a lesser dose?

Question 7

Lock and Key vs. Induced Fit

Answer 7

Depending on the receptor’s conformational change, a better fit will increase the drug-receptor binding affinity and drug efficacy
A better fit can also explain the difference in potency

Lock and Key: the shape of the drug MUST fit the receptor
Induced fit: drug is not 100% similar to receptor, just need to be mostly similar to activate the receptor via binding causing a conformational change to the shape of the drug itself
A drug has more affinity when closer to the receptor shape because less time is needed to conform to the shape
The better the fit the better the activation of the receptor because less time is required for the conformational change

Question 8

Spare Receptor Concept

Answer 8

Occupancy theory assumes that a maximal effect is achieved when all of the receptors are occupied
Physiologically, maximal effect can be obtained when only a fraction of the receptors are occupied
“Spare” receptors (EC50<Kd)
aka less receptors are bound to provide the same effect

Question 9

Spare Receptor Concept Examples

Answer 9

A cell with 4 receptors and 4 effectors. A high drug concentration is needed to “find and bind” those limited receptors to activate effectors for the drug response

A cell with many receptors and 4 effectors. Increased (spare) receptors increase sensitivity for drug binding. Therefore, less drugs are needed to activate effectors for the drug response

Question 10

Affinity, Efficacy, Potency

Answer 10

Kd = free drug concentration at which 50% of drugs are bound; affinity is inversely proportional to Kd; low Kd = high affinity
Bmax is 100% of receptors bound

EC50 = free drug concentration at which 50% of maximal effect is achieved; potency depends on affinity and efficacy (require two-drug comparison)
Emax = maximal response that can be produced by the drug (efficacy)

EC50 and Kd may be identical (occupancy theory) or different (spare receptors)
Normal: Kd=EC50 when 50% are bound = 50% of response
Spare receptor concept: 50% of the maximal response without binding to 50% of the receptors (less than 50%)

Question 11

Drug-Response Relationship

Answer 11

Graded dose-response curves:
1. Linear dose-response curve for two drugs- concentration of the drug varies widely; need large concentration to reach certain effects; sometimes have curves that are beyond the size of the graph.. Therefore linear is not helpful

2. Semilogarithmic dose-response curve for two drugs

EC50 determines potency
Drug A more potent than B because it takes less of A to reach 50% compared to drug B
Drug A = Drug B in efficacy (maximal response)
Potency ≠ Efficacy
We are interested as clinicians in the efficacy, not potency

Question 12

Receptor Antagonists: Competitive vs. Non-Competitive

Answer 12

A. Unbound inactive receptor

B. Receptor activated by agonist

C. Competitive ntagonist does not activate receptor but “competes” with the agonist for the binding site, reversible

D. Non-competitive antagonist binds to allosteric site to cause conformational change to the receptor and inhibits agonist activation of the receptor, highly irreversible

Question 13

Competitive Antagonist

Answer 13

Agonist alone can reach 100% of the response
Antagonist alone does not cause a response
Agonist + antagonist, more agonist is required to overcome the antagonism; due to competitive nature, changes agonist potency- increases EC50
Competitive antagonist causes a shift right on graft, but reaches 100% response anyway
Based on Occupancy Theory

Question 14

Non-Competitive Antagonist

Answer 14

Agonist alone can reach 100% of the response
Antagonist alone does not cause a response
Agonist + antagonist, no matter the concentration of the drug you add, it will not overcome the antagonist
Due to non-competitive nature, changes agonist efficacy (reduces)
Based on Occupancy Theory
Spare receptor model: reach efficacy with less receptors bound
Non-competitive antagonist causes a rightward shift followed by a downward shift
Non-competitive: changes efficacy because EC50 does not change, just won’t reach 100% efficacy/maximal response

Question 15

Functional/ Physiological Antagonist

Answer 15

Functional antagonism shows the same kinetic response as non-competitive antagonism
Although agonists can bind to allavailable receptors (and supposedly reach 100% maximal response),functional antagonism binds to another receptor and antagonizes theresponse
While agonist-receptor binding may reach 100%, effect is antagonized by a separate receptor and does not reach 100% efficacy

Example: If drug A itself can cause 100% effect on heart rate, but drug B 100% in opposite direction

Question 16

Partial Agonist

Answer 16

Partial agonist may be more or less potent than a full agonist; partial agonists do not reach 100% efficacy
Clinical relevance: can use partial agonists to blunt physiological response in diseased population

Eg. Use partial agonist in patient with exertion angina (heart attack)
Exercise increases heart rate, which causes a higher O2 demand > O2 supply leading to a heart attack
Partial agonist increase heart rate at baseline, but exercise will not increase heart rate rapidly to reduce incidence for heart attack; reduces efficacy

Question 17

Partial Agonist vs. Full Agonist + Antagonist

Answer 17

With antagonist + full agonist have drug tolerance causing upregulation and downregulation of the medication, and both together would cause an in between effect
Antagonists bind receptors too much and the body wants to fight back

Partial agonist: no drug tolerance with no upregulation or downreguation of medication
Because don’t reach 100% efficacy with partial agonist = no drug tolerance

Question 18

Inverse Agonist

Answer 18

Some unoccupied receptors have intrinsic activity (baseline activity level)
Inverse agonists bind and abrogate (eliminate) the intrinsic activity

Question 19

Quantal Dose-Response Curves

Answer 19

Quantal (all-or-none-response) dose-response curves

ED50-dose at which 50% of subjects exhibit a therapeutic response to a drug
TD50-dose at which 50% of subjects exhibit a toxic response to a drug
LD50-dose at which 50% of subjects die

Demonstrate average effect of a drug as a function of its concentration in a population of individuals and determine how safe a drug is

Question 20

Therapeutic Index

Answer 20

Effect and toxicity relationship is determined by the therapeutic index
Therapeutic Index = TD50/ED50
The higher the therapeutic index, the safer the drug is because there is a wider amount separating the ED50 from the TD50

Warfarin: narrow therapeutic index, meaning the drug is more dangerous to patients
Penicillin: wide therapeutic index, very safe

Question 21

Therapeutic Index: Changing Slope

Answer 21

Therapeutic index is not a good estimate of the safety of the drug because it doesn’t take into account the curves within the graph aka change in slope
Some drugs have steeper slopes

The flatter toxicity slope at lower concentration will affect more of the population aka more toxicity and side effects compared to the drug toxicity S curve and vice versa

Question 22

Margin of Safety

Answer 22

Marginal safety accommodates the change in slope, therefore fixing therapeutic index

Margin of Safety = LD1 / ED99
Get margin of safety for the drug effect curve at 99% response and compare it to lethality curve at 1% for the steep slop and S curve
The range of safety is between ED99 and the steep slope LD1 until the ED99 and the S curve LD1

Question 23

Factors that Affect Dose-Response Curves

Answer 23

Body Size/Weight-more tissue for drugs to move
Age- younger and older patients require less than normal adults due to differences in metabolism
Sex- women have more fat tissue
Route of Administration- determines bioavailability
Time of Administration- before or after meal
Pathological State- healthy vs. unhealthy individuals
Tolerance- greater tolerance require more drugs
Genetic Factors- differences in drug metabolism
Presence of other Drugs- drug-drug interactions

Question 24

Receptor-Effector Coupling

Answer 24

Receptor: macromolecule made of proteins that interact with endogenous ligand or exogenous drug to mediate a pharmacological or physiological effect

Effector: downstream molecules that transduce drug-receptor interaction to cellular effects

Two main functions of receptors:

1. Ligand Binding
2. Activation of Effector System

Question 25

Mechanisms of Membrane Signaling

Answer 25

1. Lipid soluble ligand and intracellular receptors
2. Transmembrane receptor with intrinsic enzyme activity
3. Transmembrane receptor without intrinsic enzyme activity
4. Ligand-gated ion channels
5. G-protein coupled receptors

Question 26

Lipid Soluble Ligands and Intracellular Receptors

Answer 26

There are receptors located in the cytosol and nucleus
Ligands have to be lipid soluble (lipophilic) to cross the phospholipid bilayer
Steroids (estrogen) and gases (nitric oxide)
Upon ligand binding, cytosolic or nuclear receptors are activated and cytosolic receptors translocate to the nucleus
The activated receptors bind to very specific DNA sequences determined by the response element, which is in the promoter region of many genes
This turns on transcription, translation, and protein synthesis
Once in the cytoplasm, some ligands will bind to cytoplasmic receptors or nuclear receptors, but no matter what must get to the nucleus to cause an effect to activate transcription
Need to be tightly regulated because you don’t want just anything getting in to change proteins/synthesize proteins you don’t want

Question 27

Intracellular Receptors and Chaperone Proteins

Answer 27

Intracellular receptors contain:

1. Ligand-binding domain
2. DNA-binding domain
3. Transcription-activating domain

Receptors are kept inactive by chaperone proteins (heat shock protein 90 [hsp90])
Upon ligand binding, hsp90 dissociates
DNA-binding domain binds to DNA
Transcription-activating domain activates transcription

Question 28

Lipid Soluble Ligands & Reactions

Answer 28

Effect is very slow (30 min~12 hours to see an effect)
Steroids are usually bound to a carrier protein in circulation
Upon binding to receptor, chaperone dissociates
Dimerization of receptors is required for activation
Upon binding to DNA, process leads to protein synthesis- response persists for hours to days after [agonist]=0
No simple relationship between plasma [drug] and effect

Question 29

Nitric Oxide

Answer 29

Nitric oxide (NO) is a bioactive gas that can rapidly cross the membrane
NO stimulates soluble guanylate cyclase, generates cyclic GMP and leads to vasodilation
cGMP is inactivated by phosphodiesterases/PDE5 (sildenafil blocks this enzyme)

Sildenafil: vasodilator Viagra which increases cGMP levels via inhibition of cGMP breakdown
Never give this drug with nitrates because causes too much of an increase of vasodilation = dangerous hypotension and die
Nitrates and NO effect two pathways (one inhibits breakdown and one increases cGMP)

Question 30

Transmembrane Receptors with Intrinsic Enzymatic Activity

Answer 30

These receptors contain intrinsic enzymatic activity
Ligands for these receptors are trophic hormones (act on another endocrine gland): EGF, TGFβ, insulin, PDGF, ANP
Span membrane once
Consist of extracellular ligand-binding domain and cytoplasmic enzyme domain
Enzyme domain may be tyrosine kinase, serine kinase, or guanylyl cyclase

Question 31

Transmembrane Receptors with Intrinsic Enzymatic Activity: Pathway Steps

Answer 31

1. The receptor is inactive in its monomeric form
2. Upon ligand binding, receptors dimerize, bringing the enzymes (kinases) to close proximity
3. Close proximity of enzymes lead to phosphorylation of the same receptor (auto-phosphorylation) or adjacent receptor
4. Phosphorylated enzymes in turn phosphorylate and activate adjacent protein targets

Question 32

Transmembrane Receptors with Intrinsic Enzymatic Activity: Termination of Signal

Answer 32

The phosphorylation can last 10-20 seconds
Since receptors can phosphorylate themselves, receptor remains active after ligand has disappeared
Activation of downstream pathways lead to amplification of signal

Termination of signal occurs in two ways:

1. Ligand dissociation and degradation
2. Receptor endocytosis for proteolytic degradation- leads to receptor downregulation (reduced receptor number due to digestion, need to produce new receptors for cell reactivation)

Question 33

Transmembrane Receptors Without Intrinsic Enzymatic Activity

Answer 33

These receptors do not contain intrinsic enzymatic activity
Receptors respond to peptide ligands that include growth hormone, cytokines, interferons
Span membrane once
Consist of extracellular ligand-binding domain
A separate protein kinase, Janus-kinase (JAK), binds non-covalently to the receptor
Upon ligand binding, receptor dimerizes and brings JAK to close proximity
This activates JAK and JAK phosphorylates the receptors

Question 34

Transmembrane Receptors Without Intrinsic Enzymatic Activity: JAK Phosphorylation

Answer 34

Phosphorylation of the dimerized receptors recruit a second signaling molecule called signal transducer activator of transcription (STAT)

When STAT binds to JAK, JAK phosphorylates STAT
After JAK phosphorylates STAT, STAT dissociates from the receptor as a dimer
STAT translocates to the nucleus, functions as a transcriptional activator, and causes transcriptional activation of a variety of genes

Question 35

Transmembrane Receptors Without Intrinsic Enzymatic Activity: Effects

Answer 35

Effects usually take minutes to hours to see
JAK-STAT phosphorylation amplifies signal

Termination of signal occurs in two ways:

1. Ligand dissociation and degradation
2. Receptor endocytosis for proteolytic degradation- leads to receptor downregulation and receptors need to be resynthesized since old ones were broken down

Question 36

Ligand-Gated Ion Channels

Answer 36

Channels are important for synaptic transmission
Ligand binding domain can be extracellular, inside the channel, or intracellular
Natural ligands are acetylcholine, serotonin, GABA, glutamate
Different ligands increase transmembrane conductance of different (and relevant) ions to alter membrane potential across the membrane

Brain requires fast transmission of signals for catching a ball or exercise or responding to an immediate stimulus
Depends on ligand and receptor to cause a function

Question 37

Ligand-Gated Ion Channels: ACh

Answer 37

Nicotinic acetylcholine (ACh) receptor is one of the best characterized
Consists of 5 subunits
α subunits contain ligand binding domain
When two ACh binds, conformational change occurs
Channel opens and allows sodium to cross the membrane and enter the cell
Channel opens for 2.4 milliseconds, effect is very rapid

Contains two alpha subunits which contain the ligand binding site so you need two ACh to bind to the subunits to activate the channel… NEED 2 ACh

Question 38

Ligand-Gated Ion Channels: Regulation

Answer 38

Regulated by phosphorylation and endocytosis
Although ACh receptors only assume two states (open and close), other ion channels can assume other states and become refractory or inactivated
During refractory period, the channel cannot be reactivated for a number of milliseconds
Some drugs utilize this state-depend binding to regulate channel action
Eg. Local anesthetic drugs

Question 39

G-Protein Coupled Receptors

Answer 39

Most abundant membrane protein
All G-protein coupled receptors (GPCR) cross the membrane 7 times, also called serpentine receptors
Amino terminus is extracellular and carboxy terminus is intracellular
These receptors couple to a trimeric G-protein composed of α (binds GDP), β, and γ subunit
The G proteins are usually bound to the GDP, and when ligand binds and elicits activation, GDP leaves and GTP binds so the subunits separate into the 1. alpha and the 2. beta/gamma complexes

Question 40

G-Protein Coupled Receptors: Amplification of Signals

Answer 40

One receptor can activate ~100 G-proteins
Kd < EC50, spare receptors!
G-protein activation is longer than ligand on-off rate
GPCR regulation: desensitization

A lot of signal amplification because 1 receptor activates 100 G proteins, which activate downstream proteins so the effect lasts longer and amplification of the signals

Question 41

G-Protein Coupled Receptors: Recycling of G Proteins

Answer 41

G proteins go through desensitization
G protein activation causes release of subunits so G protein can bind to GRK causing phosphorylation of intracellular portion of the receptor
Then arrestin:
1. Binds to G protein itself so it cannot go back to the G protein coupled receptor to limit effect and not allow anymore activation
2. Causes activation of endocytosis of G protein receptors so the ligands can no longer activate the receptors

Receptors are endocytosed into the cells and go through lysosome, which breaks them down
Proteolytic degradation occurs, so you need to re-synethesize the G protein receptors
Aka desensitization causes recycling of the receptors

Question 42

G-Protein Coupled Receptors: Desensitization

Answer 42

Desensitization:
Normally, GPCR mediates ligand response
After receptor activation and dissociation of G-protein, GPCR undergoes a conformational change in the 3rd intracellular loop with a new higher affinity
Allow association of GPCR kinase (GRK), which phosphorylates specific residues on the cytoplasmic tail
Phosphorylation recruits β-arrestin protein

Two consequences:

1. β-arrestin prevents binding of G-protein
2. Internalization of GPCR for recycling

Question 43

Types of G Proteins

Answer 43

There are four types of G-proteins and response depends on which G-proteins get activated
Gs, Gi, and Gq are the important ones

Question 44

Types of G Proteins: Second Messenger Pathways

Answer 44

Ligands bind to selective cell-surface receptors
The receptor triggers activation of a G-protein
G-protein changes activity of an effector element (enzyme or ion channel)
1. Adenylate cyclase activate cAMP (Gs)
2. Guanylate cyclase activate cGMP (NO)
3. Phospholipase C activate DAG, IP3 (Gq)
Effector changes 2nd messenger concentration and can amplify or dampen response
Effector molecules (adenylate cyclase, guanylate cyclase, and phospholipase c) are activated causing activation of second messagers (cAMP, cGMP, DAG, and IP3)

Question 45

Second Messenger Pathways: cAMP Activation and Inhibition

Answer 45

cAMP:
Activated by β adrenergics, glucagon, histamine (H2) on Gs
Inhibited by α2 adrenergics, muscarinics (M2, 4) on Gi

Gs: cAMP is the second messenger and activates protein kinase A
PKA contains two regulatory, so requires 2 cAMP to activate them= activate PKA which goes downstream to cause cellular responses

Gi: Gi proteins inhibit adenylate cyclase, reduced activity/formation of cAMP
Drugs that activate them: functional antagonists

Question 46

Second Messenger Pathways: Gq Proteins

Answer 46

Gq proteins: ligands binds to receptor to activate the G proteins, then activate phospholipase C, which causes breakdown of PIP2 into DAG and IP3
DAG activates protein kinase c (PKC)
IP3 goes into endoplasmic reticulum to release Ca2+ (second messenger) causing contraction of blood vessels
Alpha 1 adrenergics are on blood vessels

Calcium signaling and phophoinositides- activated by α1 adrenergics, muscarinics (M1, 3, 5), histamine (H1) on G

Question 47

Second Messenger Pathways: Termination of 2nd Messengers

Answer 47

Enzymes terminate 2nd messenger responses by breaking them down into inactive or less active form
cGMP is degraded by phosphodiesterases (inhibited by sildenafil)
cAMP is degraded by phosphodiesterases (inhibited by caffeine, theophylline)
IP3 is inactivated by dephosphorylation
DAG is phosphorylated to a less active messenger called phosphatidic acid by DAG kinase
Calcium is actively removed from the cytoplasm by calcium pumps

Question 48

Drugs That Do Not Fit the Drug-Receptor Model

Answer 48

Drugs do not necessarily have to bind to receptors to exert their actions:
Antimicrobials
Osmotic diuretics
Antacids- no receptors because just neutralize acid in the stomach