pharmacodynamics

Pharmacodynamics

The study of the biochemical, physiologic, and molecular effects of drugs

What are applications of pharmacodynamics?

Designing Dosing Regimens

Ceiling effect

Understanding Variability and interactions

Signal transduction

Biochemical mechanism responsible for “transmitting” extracellular signals inside the cell, which lead to activation of target proteins that control metabolic pathways or regulate gene expression

(Cells receive signals via receptors, then relay and amplify these signals inside cell through molecular cascades (pathways) to produce a specific response

How do cells respond to the environment?

Signal → reception → amplification → transduction → response(s)

What are outcomes/purposes to signal transduction?

Protein conformational changes

Covalent protein modifications (PTM)

Altered rates of gene expression

What are the stages of signal transduction?

1. Signal molecule (first messengers, ligand) travels to the cell

2. First messengers binds to a receptor protein and initiates a conformational change in the receptor: activate or deactivate protein, initiate cascade events to relay messages, change activity, gene expression, or function

3. Receptor protein stimulates signalling proteins

4. Second messengers amplify these signals inside signal: free to diffuse, cross talk bnetween pathways exists

5. Second messengers bind to additional signalling proteins

6. Signal is propagated often by a protein kinase cascade

7. Target proteins are affected (activated, inhibited): transcription factors, metabolic enzymes, cytoskeleton proteins, transport proteins

8. Signal is terminated - phosphates

What are examples of conformational changes in receptors?

Membrane receptors stimulus and transfer info across membrane

Most molecules are too polar or too large to cross the membrane, so the stimulus does not enter without membrane receptors

Fat soluble or non-polar molecules (steroid hormones) diffuse across membranes, bind to intracellular receptors

Generally, receptors are membrane proteins with extra- and intracellular domains (except intracellular receptors)

How does solubility determine receptor type?

Lipid-soluble ligands - hydrophobic and can readily diffuse across the cell membrane’s lipid bilayer

Water-soluble ligands - hydrophilic and cannot easily cross the lipid cell membrane

What are examples of water-soluble ligands?

Polypeptide: insulin, amine: epinephrine

What are examples of lipid-soluble ligands?

Steroid: cortisol

Amine: thyroxine

First messengers/ligands

Extracellular signaling molecules that typically bind to receptors on the cell surface rather than directly crossing the hydrophobic cell membrane

How do first messengers help with signal transduction?

First messengers binding to receptors initiates signal transduction - converts the external message into internal signals using second messengers and kinase cascades to amplify the effect within the cell

Why are receptors important in pharmacology?

Drug action

Specificity

Mechanism of action

Therapeutic targets

Personalised medicine

Receptors

Specialised protein, often on a cell surface or inside the cell, that binds specific signaling molecules (ligands) to initiate a biological response

Drug receptors

Protein macromolecules that receive chemical signals (ligands like drugs) to trigger cell responses, with drugs acting as agonists (activating) or antagonists (blocking) to produce therapeutic effects (like pain relief) or side effects, based on their specific binding to these targets

Agonists

Drugs that activate receptors, mimicking the action of natural ligands

Agonists example

Morphine on opioid receptors

Antagonists

Drugs that bind to receptors but do not activate them

Instead, they block the action of natural ligands or agonists from acting

Antagonists example

B-blockers are antagonists that inhibit the action of adrenaline (epinephrine) on B-adrenergic receptors, reducing heart rate and blood pressure

Antagonism

Binds to the orthosteric (agonist) site but causes no activation of the receptor, blocking the effect of the agonist

Antagonism effect on efficacy

Zero

Antagonism key feature

Prevent the agonist from binding

Partial agonism

Binds to the receptor and causes some activation, but cannot elicit a maximal tissue response, even when occupying 100% of available receptors

Partial agonism effect on efficacy

Intermediate

Partial agonism key feature

Acts as an antagonist by blocking the binding site against full agonists

Inverse agonism

Binds to a constitutively active receptor (a receptor active even without a ligand) and shifts the equilibrium toward the inactive (resting) state

Inverse agonism effect of efficacy

Negative

Inverse agonism key feature

Reduces the basal level of receptor activation

What is the inhibition at the receptor level like?

(Highest) full agonist → partial agonist → antagonist → inverse agonist (lowest)

What is the competitive binding like for inhibition at receptor level?

Reversible competitive antagonism

Irreversible competitive antagonism

Reversible competitive antagonism

The antagonist dissociates rapidly, meaning a sufficiently high concentration of agonist can overcome (or surmount) the block to restore the maximal response - most common form

Irreversible competitive antagonism

The antagonist dissociates very slowly or forms covalent bonds with the receptor, making the block insurmountable by increasing the agonist concentration - this lowers the maximum attainable response

4 main types of receptors in drug action

1. Channel-linked receptors / ionotropic

2. G-protein-coupled receptors / metabotropic

3. Enzyme-linked receptors (kinase linked)

4. Intracellular receptors (nuclear receptors)

How do ionotropic receptors work?

Ligand binds to receptor → induces conformational change → Form ligand-ion channels that open or close in response to ligand binding → allow flow of specific ions to flow across membrane → electrical signal → rapid changes in cell’s membrane potential → cellular responses

What are ionotropic receptors for?

Rapid transmission of signals in NS

Mediate fast synaptic transmission, making them essential for processes that require quick responses - muscle contraction, sensory perception

Ligand-gated ion channel

A type of ion channel that opens when a signal
molecule (ligand) binds to an extracellular receptor region
of the channel protein. This changes the conformation of
the channel protein and hence opens the channel

Examples of ionotropic receptors

Nicotine acetylcholine receptors

GABA-Areceptors

Glutamate receptors

Example of nicotinic receptor as ionotropic receptor

Nicotine receptor is a cation channel → opens when it binds acetylcholine → conformation change upon binding which opens channel and allows passage of cations → conducts Na+ in and K+ out of cells → depolarisation → selective for cations due to negatively charged amino acids which line the pore → open channel conducts 10^7 ions/second → normal opening time is 1-2 msec

GPCR structure

Span the cell membrane 7 times (seven-transmembrane receptors)

GPCR mechanism of action

Ligand binds to GPCR → conformational change → activates G-protein → triggers cascade of intercellular events: GDP exchanged for GTP, Gb and Gy subunits dissociate → Ga subunit activates target effector enzymes (AC, phospholipase C) → converts ATP to cAMP → generates 2nd messengers → GTPase activity of Ga subunit increased when it binds to target protein 1 → signal terminates: hydrolysis of GTP bound to GDP + re association of GB and Gy

GPCR function

wide range of physiological processes: sensory perception, immune responses, and neurotransmission

Modulate various cellular activities: gene expression, enzyme activity, ion channel function

Examples of metabotropic receptors

Adrenergic receptors - receptors respond to adrenaline (epinephrine) and noradrenaline (norepinephrine) and are involved fight or flight response

Muscarinic acetylcholine receptors: role in parasympathetic NC, affecting functions like heart rate and digestion

Primary effectors in GPCR signalling

Ardently Cyclaase: enzyme responsible for cAMP formation (ATP → cAMP + diphosphate)

Phospholipase C (PLC): catalyses hydrolysis of PIP into IP3 and DAG - in Ca2+ contraction

Ion channels: Ca+ and K+

Rho-associated coiled-coil containing kinase (ROCK) - system that controls the activity of many signalling pathways, controlling cell growth, proliferation, smooth muscle contraction

A-receptor

Causes contraction (inotropy and chonotropy)

B-receptors

Linked to adenylyl cyclase

GPCR general function

Upon ligand binding, they activate G-proteins, which then trigger various intracellular pathways involving secondary messengers like cAMP or IP3

Enzyme linked receptors structure

Have a single transmembrane domain. The extracellular domain binds to ligands, while the intracellular domain has enzymatic activity or is associated with an enzyme

Enzyme-linked receptors function

Role in various cellular processes - growth, differentiation, and metabolism

Enzyme-linked receptors mechanism of action

Ligand binding → conformational changes → activate their intrinsic enzymatic activity or associated enzymes → phosphorylation of specific tyrosine residues on receptor itself or on downstream signalling proteins → initiate a cascade of intracellular signalling events

Types of enzyme-linked receptors

Receptor tyrosine kinases (RTKs)

Receptor serine/threonine kinases

Tyrosine-kinase associated receptors

Receptor guanylyl cyclases

Receptor tyrosine phosphatases

Receptor tyrosine kinases (RTKs)

contains intrinsic tyrosine kinase activity, involved in cell growth, survival and differentiation e.g. epidermal growth factor receptor (EGFR) epidermal growth factor, insulin receptor

Receptor serine/threonine kinases

Contains intrinsic serine/threonine kinase activity

Involved in regulation of cell proliferation and differentiation e.g. transforming growth factor-beta (TGF-B) receptors

Tyrosine-kinase associated receptors

Receptors that associate with proteins that have tryrosine kinase activity e.g. cytokine receptors such as interleukin-6 (IL-6)

Receptor guanylyl cyclase

Contain intrinsic cyclase activity

Convert GTP to cyclic GMP (cGMP), which acts as a secondary messenger e.g. atrial natriuretic peptide (ANP)

Receptor tyrosine phosphatases

integral membrane proteins with an extracellular
domain that can bind ligands and an intracellular domain with tyrosine phosphatase activity;
essential for maintaining cellular homeostasis and are involved in various physiological and
pathological processes e.g. CD45 involved in regulating T and B cell receptor signalling in the
immune response

Protein kinase receptors

An enzyme that can transfer a phosphate group from ATP to a protein

3 types of protein kinase receptors

Receptor tyrosine kinase

Receptor serine/threonine kinases

Cytokine receptors

Protein kinase receptors mechanism

Involves dimerization leading to autophosphorylation of tyrosine residues → activates various acceptor proteins

What are protein kinase receptors for?

Some hormones (insulin and leptin)

Growth factors

Cytokines

Tyrosinekinase receptors mechanism

Involves dimerization leading to autophosphorylation of tyrosine residues → activates various acceptor proteins → phosphorylation of domains promotes binding of downstream signalling proteins to receptor via SH2 domains

Tyrosine kinase receptors control

Cell division

Growth and differentiation

Inflammation and immune responses

Tyrosine kinase receptors regions

All have three regions:
1. N-terminal extracellular ligand
binding domain
2. Single transmembrane helix
3. Cytosolic domain with tyrosine
kinase activity

Intracellular receptors general function

Located within cell (cytoplasm or nucleus and interact with lipophilic ligands that can cross the cell membrane (steroid hormones)

Intracellular receptors ligand binding

Bind to lipophilic ligands (steroid, thyroid hormones, certain vitamins (e.g. vit D) → ligands can diffuse through cell membrane due to their lipid-soluble nature

Intracellular receptors mechanism of action

Ligand binding → conformational change: allows them to act as transcription factors → bind to specific DNA sequences (directly influence gene expression) + regulate the transcription of target genes and influencing protein synthesis

Intracellular receptors gene regulation

By acting as transcription factors, intracellular receptors directly influence gene expression, leading to changes in cellular function and behaviour

Intracellular receptors long term effects

Cell function - alterations in growth, differentiation and metabolism

Nuclear receptor structure

N-terminal domain: AF1 (activation function 1) site which binds cell-specific transcription factors that modify the properties of the receptor

Core domain with 2 ‘zinc fingers’ which recognise and bind to DNA

Flexible hinge region - allows receptor to dimerise with other NRs

C-terminal domain - contains ligand-binding module and is specific to each class of receptor

Nuclear receptors examples

Glucocorticoid receptors: bind to glucocorticoids (class of steroid hormones involved in regulating metabolism, immune response and stress)

Oestrogen receptors: bind to oestrogen - hormone critical for reproductive and sexual development mainly in females

Thyroid hormones receptors - bind to thyroid hormones - play role in regulating metabolism, growth and development

How are drugs classified?

Based on their mechanism of action at multiple levels:

Molecular level - classified as receptor agonists, antagonists, enzyme inhibitors or ion channel modulators

Cellular level - reflect their effect on signalling pathways such as increasing or decreasing second messengers like cAMP

Tissue or organ level - based on physiological effects such as bronchodilators or anti hypertensives

Multi-level classification - reflect complexity of drug action and helps guide therapeutic use and drug development