Exam IV: Pharmacodynamics Flashcards
Pharmacodynamics
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
Importance of Receptors: Dose vs. Effect
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
Importance of Receptors: Selectivity
Receptors are responsible for selectivity of drug action
Drug structures determine affinity for different classes of receptors
Non-selective drugs can cause side-effects
Importance of Receptors: Agonist vs. Antagonist
Receptors mediate actions of pharmacologic agonists and antagonists
Drugs binding to receptors can activate or interfere with normal physiological processes
Receptors: Occupancy Theory
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
Occupancy Theory Issues
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?
Lock and Key vs. Induced Fit
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
Spare Receptor Concept
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
Spare Receptor Concept Examples
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
Affinity, Efficacy, Potency
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%)
Drug-Response Relationship
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
- 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
Receptor Antagonists: Competitive vs. Non-Competitive
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
Competitive Antagonist
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
Non-Competitive Antagonist
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
Functional/ Physiological Antagonist
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
Partial Agonist
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
Partial Agonist vs. Full Agonist + Antagonist
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
Inverse Agonist
Some unoccupied receptors have intrinsic activity (baseline activity level)
Inverse agonists bind and abrogate (eliminate) the intrinsic activity
Quantal Dose-Response Curves
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
Therapeutic Index
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
Therapeutic Index: Changing Slope
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
Margin of Safety
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
Factors that Affect Dose-Response Curves
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
Receptor-Effector Coupling
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:
- Ligand Binding
- Activation of Effector System
Mechanisms of Membrane Signaling
- Lipid soluble ligand and intracellular receptors
- Transmembrane receptor with intrinsic enzyme activity
- Transmembrane receptor without intrinsic enzyme activity
- Ligand-gated ion channels
- G-protein coupled receptors
Lipid Soluble Ligands and Intracellular Receptors
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
Intracellular Receptors and Chaperone Proteins
Intracellular receptors contain:
- Ligand-binding domain
- DNA-binding domain
- 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
Lipid Soluble Ligands & Reactions
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
Nitric Oxide
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)
Transmembrane Receptors with Intrinsic Enzymatic Activity
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
Transmembrane Receptors with Intrinsic Enzymatic Activity: Pathway Steps
- The receptor is inactive in its monomeric form
- Upon ligand binding, receptors dimerize, bringing the enzymes (kinases) to close proximity
- Close proximity of enzymes lead to phosphorylation of the same receptor (auto-phosphorylation) or adjacent receptor
- Phosphorylated enzymes in turn phosphorylate and activate adjacent protein targets
Transmembrane Receptors with Intrinsic Enzymatic Activity: Termination of Signal
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:
- Ligand dissociation and degradation
- Receptor endocytosis for proteolytic degradation- leads to receptor downregulation (reduced receptor number due to digestion, need to produce new receptors for cell reactivation)
Transmembrane Receptors Without Intrinsic Enzymatic Activity
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
Transmembrane Receptors Without Intrinsic Enzymatic Activity: JAK Phosphorylation
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
Transmembrane Receptors Without Intrinsic Enzymatic Activity: Effects
Effects usually take minutes to hours to see
JAK-STAT phosphorylation amplifies signal
Termination of signal occurs in two ways:
- Ligand dissociation and degradation
- Receptor endocytosis for proteolytic degradation- leads to receptor downregulation and receptors need to be resynthesized since old ones were broken down
Ligand-Gated Ion Channels
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
Ligand-Gated Ion Channels: ACh
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
Ligand-Gated Ion Channels: Regulation
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
G-Protein Coupled Receptors
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
G-Protein Coupled Receptors: Amplification of Signals
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
G-Protein Coupled Receptors: Recycling of G Proteins
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
G-Protein Coupled Receptors: Desensitization
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:
- β-arrestin prevents binding of G-protein
- Internalization of GPCR for recycling
Types of G Proteins
There are four types of G-proteins and response depends on which G-proteins get activated
Gs, Gi, and Gq are the important ones
Types of G Proteins: Second Messenger Pathways
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)
Second Messenger Pathways: cAMP Activation and Inhibition
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
Second Messenger Pathways: Gq Proteins
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
Second Messenger Pathways: Termination of 2nd Messengers
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
Drugs That Do Not Fit the Drug-Receptor Model
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
