Pharmacodynamics Notes

Pharmacodynamics: Understanding Drug Action

Suggested Use of Notes

Study (patho)physiology to understand pharmacology.
Integrate physiological systems and pharmacology for rational pharmacotherapy decisions.
Research unclear concepts or contact instructors.
Use notes with narrated presentations and learning activities.

Pharmacodynamics Overview

Study of the relationship between drug concentration and pharmacological effects.
"What the drug does to the body."
- Aspects of drug reactions.
- Receptor interactions.
- Mechanisms of action.
- Mechanisms of toxicity.
- Dose-response relationships.

Drug Targets

Drugs must bind to a cellular component to exert an effect.
Most drug targets are proteinaceous, exceptions exist (e.g., calcium salts in bone).
Binding sites on drug targets allow molecules to attach.

Binding Sites

Orthosteric Sites

Active site of target where the endogenous ligand binds.
Orthosteric binding of ligand influences receptor activity.
Facilitator results in cellular effect.
Inhibitor prevents cellular effect.
Many drugs bind to orthosteric sites, mimicking or preventing endogenous ligand binding.

Allosteric Sites

Separate binding site causing conformational changes in the orthosteric site.
Allosteric modulators: Binding changes orthosteric site's shape and affinity for endogenous ligand.
Influences efficacy of endogenous ligand.
Modulator does not elicit effect on its own.
Facilitator amplifies orthosteric ligand's effect.
Inhibitor diminishes orthosteric ligand's effect.

Main Regulatory Proteins as Drug Targets

Receptors
Ion channels
Enzymes
Carrier molecules and transporters

Receptors

Protein molecules that respond to external stimuli to produce a cellular effect through messenger systems.
Four primary types or superfamilies of receptors:
- Ligand-gated ion channels (ionotropic receptors)
- G-protein coupled receptors (GPCRs, or metabotropic receptors)
- Kinase-linked and related receptors
- Nuclear receptors

Ionotropic Receptors: Location and Structure

Water-filled ion channel that opens upon ligand-binding.
Extracellular ligand-binding domain.
Transmembrane α-helix.
Receptor found in membrane.

Ionotropic Receptors: Structure

Centralised pore-lining helices, kinked to close channel.
Pentameric structure consists of 5 subunits (α, β, γ, or δ).
Each subunit consists of 4 helices.
Central channel or pore allows ion transfer when open.

Ionotropic Receptors: Binding Domains and Functions

Ligand-binding sites are extracellular, forming between specific subunit groups.
Allows for wide range of ligand specificities and affinities.
Functions:
- Ions are secondary messengers.
- Membrane voltage changes: Depolarisation (net positive) or Hyperpolarisation (net negative).

Ionotropic Receptors: Resting Phase

Ions (hydrophilic) cannot cross the membrane.
Kinked α-helix closes channel in resting phase.
Water-filled channel present.

Ionotropic Receptors: Signal Transduction

Ligands bind to ligand-binding site.
Ligand-binding induces conformational change, straightening α-helix.
Channel opens and ions flow through.
Ions produce cellular effect.

Time of response: milliseconds.

GPCRs: Location and Structure

Receptor activity mediated through G-protein that is activated upon ligand-binding.
Extracellular ligand-binding domain.
Intracellular G-protein-association domain associates with unattached G-protein.
7 transmembrane helices.

GPCRs: Structure

G-protein consists of an α-, β- and γ- subunit.
Guanine nucleotide-binding site on α-subunit.
α-subunit possesses GTPase activity (hydrolysis of GTP to GDP).
β- and γ-subunits remain complexed.

GPCRs: Binding Domains and Functions

Ligand-binding sites are extracellular.
Functions:
- Variety of cellular effects depending on G-protein and effector proteins (channels or enzymes).
- One of the most targeted drug targets in pharmacology.

GPCRs: Resting Phase

GDP bound to α-subunit.

GPCRs: Signal Transduction

Ligand-binding changes conformation, increasing affinity for G-protein.
G-protein binds to G-protein association domain and exchanges GDP for GTP.
G-protein dissociates into α-subunit and βγ-complex.
α-subunit (primary modulator) binds to effector protein.
Binding increases GTPase activity.
Energy activates effector protein which produces cellular effect.
Once energy expended, G-protein released and returns to resting state.

Time of response: seconds.

Kinase-Linked Receptors: Location and Structure

Ligand-binding sites: Large extracellular binding site.
Functions: Variety of cellular effects depending on receptors targeted.
Receptor found in membrane.
Single large transmembrane helix monomer.

Kinase-Linked Receptors: Resting State

Two separate monomers in close proximity.
Extracellular ligand-binding domains.
Intracellular kinase domains containing enzymatic activity.

Kinase-Linked Receptors: Signal Transduction

Ligand-binding results in dimerization of adjacent monomers (association).
Dimerization activates kinase domains.
Kinase domains result in auto-phosphorylation of tyrosine residues.
SH2-domain proteins bind to phosphotyrosine- residues, producing cellular effect.
Phosphatases dephosphorylate tyrosine- residues, terminating the signal.

Time of response: hours.

Nuclear Receptors: Location and Structure

Location of receptor: In cytosol or nucleus.
Co-factors often required for activation.
Functions: Alters genetic expression of various proteins (inhibition or induction).
Binding domain, DNA-binding domain (zinc fingers), and hinge.

Nuclear Receptors: Resting State

Nuclear receptor present intracellularly in cytosol or nucleus.
Cellular membrane prevents entry of compounds that are not lipophilic or transportable.
Time of response occurs within hours to days.

Nuclear Receptors: Signal Transduction

Ligand binding translocates nuclear receptor to nucleus (if not yet there).
Nuclear receptor binds to hormone response element.
Genetic expression of key proteins inhibited or induced.

Time of response: hours to days.

Receptor Modulation by Drugs

Simulates endogenous ligand’s activity (-mimetic).
Prevents endogenous ligand’s activity (-lytic).
Modulation of endogenous ligand’s activity.

Ion Channels

Water-filled ion channel that opens upon different stimuli.
Ion transfer across membrane.
Ions cannot cross unaided due to hydrophilicity.
Rate and direction of transfer controlled by electrochemical gradient of each specific ion.
Ion channels as drug targets: blocks transfer of molecules or modulation of opening/closure.

Ion Channels: Characterisation

Selectivity for ion species (size of the pore, nature of channel lining).
Gating properties (stimulus that controls opening).

Ion Channels: Selectivity

Cation channels (positively-charged ions: Na$^+$, Ca$^{2+}$, K$^+$).
Anion channels (negatively-charged ions: Cl$^-$).

Ion Channels: Gating Properties

Voltage-gated (triggered by altered cell membrane voltages).
Ligand-gated (extracellular ligand-binding).

Enzymes

Catalytic molecules that accelerate metabolic conversion of chemical entities.

Enzymes: Drug Activity

Enzymatic functions: Detoxification, activation of pro-drugs, formation of toxic metabolites.
Enzymes as drug targets: Inhibition to reduce transmitter/substrate degradation or metabolite synthesis.

Enzymes: Allosteric Activation

Drugs can bind to an allosteric site on the enzyme, causing a conformational change that enhances the enzyme's activity.
This can increase the enzyme's efficiency in catalyzing the reaction.
Example: AMP-activated protein kinase (AMPK) can be activated by specific compounds like AICAR, which enhances energy metabolism.

Enzymes: Enzyme Induction

Some drugs can stimulate the expression of specific enzymes by upregulating the gene responsible for encoding the enzyme.
This results in higher enzyme concentrations.
Example: Corticosteroids can induce the production of certain liver enzymes like cytochrome P450 enzymes, increasing the metabolism of various substances.

Enzymes: Coenzyme or Cofactor Provision

Certain drugs may provide or enhance the availability of coenzymes or cofactors needed for enzyme activity.
Coenzymes are essential for the catalytic activity of enzymes.
Example: Folic acid supplements increase the activity of enzymes involved in DNA synthesis by providing the necessary cofactors.

Enzymes: Inhibition of Negative Regulators

Some drugs inhibit molecules or proteins that normally act as inhibitors of enzymes, indirectly increasing the enzyme's activity.
Example: Nitrate drugs increase the activity of guanylate cyclase by inhibiting phosphodiesterases, leading to increased cyclic GMP and smoother muscle relaxation.

Enzymes: Competitive Inhibition

A drug competes with the substrate for the enzyme's active site.
By binding to the active site, the drug prevents the substrate from binding, thus decreasing enzyme activity.
Example: Methotrexate acts as a competitive inhibitor of dihydrofolate reductase, which is involved in folate metabolism, thereby reducing enzyme activity and slowing cell division.

Enzymes: Non-Competitive Inhibition

A drug binds to a site other than the active site (an allosteric site), changing the enzyme's shape and reducing its activity, regardless of the concentration of the substrate.
Example: Allopurinol inhibits xanthine oxidase, an enzyme involved in uric acid production, thus decreasing its activity and treating gout.

Enzymes: Uncompetitive Inhibition

The drug binds only to the enzyme-substrate complex, making the enzyme less effective at catalyzing the reaction.
Example: Lithium inhibits certain enzymes in the phosphoinositide signaling pathway, which can be used to treat mood disorders by reducing enzyme activity in the brain.

Enzymes: Covalent Modification

Some drugs irreversibly bind to enzymes, permanently altering their structure and reducing their activity.
This is often used to control enzymes that play a role in pathological conditions.
Example: Aspirin irreversibly inhibits cyclooxygenase (COX), thereby reducing the production of prostaglandins, which are involved in inflammation.

Enzymes: Enzyme Degradation

Drugs can stimulate the degradation of enzymes, reducing their concentration and activity in the body.
Example: Protease inhibitors (used in HIV treatment) inhibit enzymes necessary for viral replication, decreasing enzyme activity and viral load.

Enzymes: Feedback Inhibition

Drugs can mimic or enhance the action of natural feedback inhibitors, decreasing enzyme activity.
Example: Statins inhibit HMG-CoA reductase, a key enzyme in cholesterol synthesis, by mimicking a feedback inhibitor and reducing cholesterol production.

Enzymes: Diseases of High Metabolite Levels

Enzyme inhibition decreases product synthesis which is in excess.
Example: Allopurinol decreases uric acid production, thus no hyperuricaemia occurs (treatment of gout).

Enzymes: Diseases of Low Transmitter Levels

Some diseases occur due to low concentrations of transmitters.
Example: Selegiline inhibits monoamine oxidase to increase serotonin levels in depressed individuals.

Transporters and Carriers

Protein molecules that facilitate transfer of ions, small molecules or macromolecules across cell membrane.
Polar or hydrophilic molecules cannot cross membrane unaided.
Requires facilitator to aid transport, but is saturable.
Can be energy-dependent or –independent.
Transporters as drug targets:
- Inhibition of transporter function.
- Blocking transporter structure.

Transporters: Types

Uniporter: Single molecule type.
Antiporter: One molecule type exchanged for another.
Symporter: Two or more molecule types transported.

Targets for Drug Action

Mimetic of endogenous ligand: Same activity.
Blockers: Prevent transfer or binding.
Allosteric modulators: Influence activity positively or negatively.

Dose-Response Relationship

Direct relationship between concentration/dosage (x-axis logarithmic scale) and response/effect (y-axis)
As concentration/doses increases, effect increases

Dose-response curves plateaus of tapered effect is observed

If large enough concentration range is used, a bottom and top plateau of tapered effect is observed
Bottom plateau: Concentration not high enough to elicit effect
Top plateau: Effect saturated, will not increase with higher concentrations

Dose-response curves : Biological effect

Biological effect of a drug is dependent on it’s activity, and thus will differ between compounds
$E_{max}$ Maximal efficacy of drug’s specific activity
$E_{max}$ is not necessarily 100%

Dose-response curves information

Concentration at which 50% of effect occurs $EC_{50}$
Absolute where 50% of maximum effect occurs
Relative where 50% between highest and lowest plateau occurs
Parameters depend on variables used
Efficacy or Inhibitory or Lethal or Toxic (E/I/L/T)
Concentration or Dosage (C/D)

Dose-response curves (potency)

Potency: Measure of drug amount needed to elicit specific efficacy
Low amount achieves efficacy
High amount achieves efficacy
Potency typically measured relative to other drugs or a central point

Therapeutic Index (TI)

Measure of drug’s safety as garnered from the animal model.
TI = \frac{TD{50}}{ED{50}}
- $TD_{50}$ = median toxic dose
- $ED_{50}$ = median effective dose
Not a perfect parameter, but suggests safety in the general population.
Narrow TI drugs are dangerous.
Broad TI drugs are considered safer.

Therapeutic Index (TI) (broad/large therapeutic index drugs)

Toxicity appears at high concentrations, far above what is needed in the population
Considered safe or unlikely to cause harm

Therapeutic Index (TI) (narrow/small therapeutic index drugs)

Toxicity appears within range necessary to treat the majority of the population
Considered more dangerous or likely to cause harm

Ligand-Receptor Binding

Occupation and Affinity

Occupation: Binding of the drug to a receptor.
Controlled by affinity.
Affinity: Ability of drug to bind to receptor, higher affinity allows for greater occupation.
Affinity determined by forces at play: van der Waals (weakest) to covalent (strongest).

Activation and Efficacy

Activation: Eliciting a cellular response.
Controlled by efficacy.
Efficacy/Intrinsic activity: Tendency to activate the receptor based on drug-receptor complexes.

Ligand Types: Agonists

Ligands for receptor that result in its activation, thus possesses affinity and efficacy
Full agonist elicits maximum efficacy (1) if enough receptors occupied
Super agonist elicits supramaximal efficacy (>1) relative to endogenous ligand
Partial agonist elicits sub- maximal efficacy (between 0 and 1) even at full occupancy

Ligand Types: Antagonists

Ligands for receptor that possess affinity, but do not activate the receptor
Silent antagonist does not possess efficacy (0) even at full occupation

Co-administration of agonists (full agonist and partial agonist)

Full and partial agonist both bind receptors and activate the system within their efficacy spectrum
Additive Agonists’ efficacy is summated

Over-administration of agonists (full agonist overdose)

Full agonists activate drug targets fully and overburden system*
Physiological system hyper-activated
Leads to complications

Constitutive Activity

Some receptors active without bound ligand.
Conformational differences between active and inactive states.
States typically in equilibrium to maintain homeostasis.
Large disequilibrium may result in disease.

Inverse Agonists

Inverse agonists (partial or full) reduce constitutive activity by shifting active receptors to inactive conformation
Agonists (partial or full) active receptor by shifting inactive receptors to active conformations
Diminishes the receptor’s baseline activity by shifting it to an inactive state (“negative efficacy”)

Over-administration of agonists (partial agonist as antagonist)

Partial agonist (at high doses) displaces full agonist
System still activated, but sub-maximally, thus symptoms decrease
Low-efficacy partial agonist can thus be an antagonist (e.g. buprenorphine [partial agonist] counteracts morphine [full agonist])

Selectivity and Cross-Reactivity

Drug targets have specific conformations that allows for recognition of individual drugs or ligands.
Selectivity: Ability to bind only specific receptors or drug targets.
Cross-reactivity: Drug acts as ligand to numerous drug targets.
Affinity and efficacy depends on the receptor type.
*Produces differential effects if selectivity is lost, most often unwanted effects

Drug Antagonism

Effect of one drug is diminished or abolished by another.
Types: chemical, pharmacokinetic, non-competitive and competitive.

Chemical Antagonism

Two substances combine in solution to form a product with no affinity for drug targets
Reduces toxicity of a toxic product

Pharmacokinetic Antagonism

Antagonist alters plasma concentration by changing any ADME pharmacokinetic parameters
Example: rifampicin increases oral contraceptive metabolism by inducing CYP450, thus antagonising its contraception

Competitive Antagonism

Competition between antagonist and agonist to bind at the same binding site
High levels of one ligand will displace the other, or natural dissociation can occur
May be reversible or irreversible
Prevents or reverses activity of drugs (e.g. overdoses)

Reversible Competitive Antagonism

Antagonist can be displaced by another or dissociate from freely
Increasing concentration of antagonist shifts dose-response curve to the right (potency decreased)
Higher levels of agonist required to displace antagonist and maintain efficacy (as potency is decreased)

Irreversible Competitive Antagonism

Antagonist cannot be displaced or dissociate from receptor
Increasing concentration of antagonist shifts dose- response curve down (efficacy decreased)
Increasing concentration of agonist does not displace antagonist or improve efficacy

Non-Competitive Antagonism

Drug antagonises agonist by binding to site different than active site (no competition), reducing activation chain of events
Non-competitive antagonism – allosteric antagonism
Allosteric inhibitor binds to allosteric binding site
Agonist cannot bind to the altered binding site

Physiological Antagonism

Drugs counteract one another’s effects by activating different physiological systems
Doesn’t matter that agonist is activating target, secondary processes or chain of events interrupted
Minoxidil and Adrenaline example

Alterations to a Pharmacodynamic Profile

Diminished Drug Effect

Drug’s activity may decrease with continuous administering

Tachyphylaxis or desensitization: Acute reaction, sometimes within minutes
Tolerance: Gradual reaction, days to weeks
Refractoriness: Resistant to treatment, May be inherent or develop over time.

Possible Explanations

Change in receptors: Conformational change or phosphorylation of receptor prevents further activity
Translocation of receptors: Chronic agonist exposure decreases receptor expression
Exhaustion of receptors: Essential intermediate substance depleted
Altered drug metabolism: Increased or decreased expression of enzymes
Physiological adaptation: Shift of homeostatic regulators alters bodies physiological response to drug

Patient Factors

Patient factors may influence pharmacodynamic profile: Physiological Pathological, Genetic, Environment, Interactions