Lecture 1: Drug receptor interaction (pharmacodynamics) Flashcards
What is pharmacology?
The study of drugs
What is a drug?
A substance used as a medicine to treat a disease
A substance used to prevent disease
A substance used to diagnose disease
A substance used with the intent of producing a change within the body
Pharmacodynamic processes
Receptor and signal transduction
The actions of the drug on the body
What is a receptor?
A protein molecule in the cell that interacts with drugs (aka ligands) and initiates a chain of events causing some form of cellular response
What is a ligand?
A substance that forms a complex with receptors including drugs, hormones and neurotransmitters
Location of receptors
Cell membrane, cytoplasm, or nucleus
Structure of receptors
Proteins
Function of receptors
Bind to ligands -> activates or inhibits post-receptor signalling (signal transduction cascade) -> triggers biological responses
Significance of receptors
Transduces a signal from outside cell to inside
Four receptor families
G protein coupled receptors
Ligand gated ion channels
Enzyme linked receptors
Intracellular receptors
G protein coupled receptors (GPCRs) general info
Biggest family of receptors (30% of drugs act on members of this family)
Most common site of drug action
Structure of G protein coupled receptors
7 transmembrane domains
External domain: ligand binding
What are the G protein subunits?
Alpha, beta, gamma
G protein alpha subunit iso forms?
Gas (stimulatory), Gai (inhibitory), Gq
Function of alpha subunit of G protein
Binds GTP and GDP
Function of Beta-gamma subunit of G protein
Inhibits alpha subunit
What occurs once the ligand binds to the G protein coupled receptor?
Ligand binds -> receptor conformation change -> receptor binds to G protein -> Cellular effectors (enzyme, protein, ion channel) -> second messenger
Effectors of G proteins
Adenyl cyclase (Gas and Gai), phospholipase C (Gaq)
Second messengers of G proteins
cAMP (Gas, Gai)
IP3, DAG (Gaq)
What happens when Gas is activated?
1) Adenyl cyclase is stimulated
2) AC converts ATP to cAMP
3) cAMP activates protein kinase A
4) PKA phosphorylates target proteins
(see figure)
What happens when Gai is activated?
Inhibits adenyl cyclase and downstream effects
What happens when Gaq is stimulated?
1) Gaq activates phospholipase C (PLC)
2) PLC hydrolyzes PIP2 (membrane phospholipid) into DAG and IP3
3) IP3 stimulates release of Ca2+ from ER
4) Ca2+ and DAG stimulate protein kinase C
5) protein kinase C phosphorylates target proteins
(see figure)
What does activation of GPCRs do?
Increases or decreases production of second messengers (depending on which G protein is activated)
Examples of GPCRs
Muscarinic receptors (M1-M5) - acetylcholine, drugs for parasympathetic nervous system
Adrenic receptors (alpha, beta receptors) - norepinephrine, epinephrine, drugs for sympathetic nervous system
Dopamine receptors (D1-D5) - Dopamine, antipsychotics
Serotonin (5-HT) receptors - serotonin, antipsychotics
Opioid receptors - endorphins, morphine, other analgesics
Why do ions move across a ligand-gated ion channel?
Asymmetrical distribution of ions
Electric potential is different across cell membrane
Where are Ligand gated ion channels abundant?
On excitable cells (neurons and muscle cells)
Resting membrane potential of nerve cell and smooth muscle cell
Nerve cell: -70 mV
smooth muscle: -50 mV
Structure of ligand gated ion channels?
Various subunits
Extracellular domain binds to ligand
Regulation of ligand gated ion channels
Ligand binding causes conformational change in the receptor
Channel opens, ion moves across membrane
Selectivity of LGIC
Different ion channels for different ions
Direction of movement across LGIC
Determined by electrochemical gradient (influx or efflux)
Which ions will move into the cell when their LGIC open?
Na+, Ca2+, Cl-
see figure
Which ions will move out of the cell when their LGIC open?
K+
Nicotinic acetylcholine (ACh) receptor
Ligand gated Na+ channel
Muscle contraction
Drugs: succinylcholine
Glutamate N-methyl-D-aspartate (NMDA) receptor
Ligand gated Ca2+ channel
Long-term potentiation (learning and memory)
Drug: memantine, ketamine
Gamma-Aminobutyric acid (GABA) receptor
Ligand gated Cl- channel
Central nervous system depression
Drugs: benzodiazepines
LGIC vs VGIC
VGIC respond to changes in electrical membrane potential
LGIC respond to ligand binding
LGIC and VGIC in nervous system
VGIC transmit signals INSIDE a neuron (electrical)
LGIC transmit signals BETWEEN neurons (chemical)
Similarities between Ion channels and ion pumps
Located in cell membrane
Transmembrane proteins
What happens in Ion pumps?
Ions move across a membrane AGAINST their concentration gradient
Uses ATP
Re-establishes ion gradients
What happens in Ion channels?
Ions move down their concentration gradients
Types of enzyme-linked receptors
Cell membrane enzyme-linked receptors
Intracellular enzyme-linked receptors
Type of cell membrane enzyme linked receptor
Tyrosine kinase receptors
Examples of tyrosine kinase receptors
Nerve growth factor (NGF) receptor
Brain derived neurotrophic factor (BDNF) receptor
Epidermal growth factor (EGF) receptor
Platelet-derived growth factor (PDGF) receptor
Insuline receptor
Cytokine receptor
Structure of tyrosine kinase receptors
Spans the membrane
Many form dimers or multi-subunit complexes
Extracellular domain binds ligand
Intracellular domain has cytosolic enzyme activity (induces tyrosine phosRphorylation)
Regulation and function of tyrosine kinase receptors
1) Binding of ligand to receptor subunits -> conformational changes
2) Form dimers
3) Kinases are converted from inactive to active forms
4) Tyrosine receptor auto-phosphorylation
5) Recruit many protein targets
Important biological functions controlled by tyrosine kinase receptors
Metabolism, growth and
differentiation
Examples of receptor tyrosine kinases that act as growth factor receptors
Imatinib (Gleevec -> tyrosine kinase -> chronic myeloid leukemia
Interleukin-2 (Proleukin) -> tyrosine kinase -> cancers (malignant melanoma, renal cell cancer)
Example of intracellular enzyme-linked receptor
Soluble guanylyl cyclase (GC)
In cytoplasm
Structure of Guanylyl cyclase
Forms a heterodimer composed of an α- and a β-subunit
Contains a regulatory domain (RD), a coiled- coil domain (CCD) and a cyclase domain (CD)
(see figure)
Regulation and function of guanylyl cyclase
1) Is activated by nitric oxide (NO) - can cross membrane
2) GC converts GTP to cGMP
3) cGMP activates protein kinase G which c uses smooth muscle vasodilation
3) Phosphodiesterase (PDE) covers cGMP to GMP
Examples of guanylyl cyclase
Nitroglycerin (glyceryl trinitrate) -> guanylyl
cyclase -> treats angina
Sildenafil (Viagra) ->protects cGMP from phosphodiesterase -> treats hypertension and erectile dysfunction
Structure of intracellular (nuclear) receptors
Ligand binding domain and DNA binding domain
Usually located in the nucleus
Regulation and function of intracellular (nuclear) receptors
Receptor ligands are lipid soluble
The ligand must diffuse into the cell to
interact with nuclear receptor which is cytosol or nuclear
ligand-receptor complex translocates to nucleus
Activated receptor binds to promotor region of gene -> acts as transcription factor
Regulate gene expression
Examples of intracellular nuclear receptors
Steroid receptors (cortisone, estrogen, progesterone, testosterone)
Non-steroid nuclear receptors (retinoid acid, vitamin D, thyroid hormone)
Duration of action of ion channels
Milliseconds
Duration of action of G protein coupled receptors
Seconds to minutes
Duration of action of enzyme-linked receptors
Guanylyl cyclase: seconds to minutes
Receptor tyrosine kinases: minutes to hours
Duration of action of Intracellular nuclear receptors
Hours to days
Which ion has the greatest difference between intracellular and extracellular
Ca2+
Duration of effect of activated receptors (relative)
Intracellular (nuclear) receptors > Enzyme-linked receptors > G-protein- coupled receptors > Ligand-gated ion channels
What is Bmax?
The maximal specific binding of a ligand to receptor
Indicates the total concentration of receptor sites
(see figure)
Kd
Equilibrium dissociation constant between ligand and receptor
Represents the concentration of drug at which half-maximal binding (50%) is observed
Affinity
The ability of the drug to bind to a receptor (the concept is not related to response)
Affinity describes the strength of binding between a ligand and its receptor – how attractive the receptor is to the drug
What determines the affinity of a drug for its receptor?
Affinity is inversely proportional to Kd
(The higher Kd is, the higher the concentration of drug needs to be for half the sites to be filled, which means the sites have a lower affinity for the drug)
Selectivity
The degree to which a drug acts on a given site relative to other sites.
Describes preference for one receptor over another
Refers to the affinity of a drug for the “desired” target relative to its affinity for “non-desired” targets.
Example: The Kd of a drug for receptor A is lower than for receptor B. Which receptor does the drug have more affinity and more selectivity for?
More selective and more affinity for A
Emax
the maximal effect induced by a drug (full agonist)
see figure
EC50
the concentration of drug producing an effect that is 50 percent of the maximum
Potency of a drug
a measure of the amount of drug required to produce an effect of given magnitude.
Determined by EC50
The higher EC50, the lower potency the drug has
(see figure)
Efficacy
Measure of the ability of a drug to elicit a biological response by agonist
Determined by Emax (higher Emax, higher efficacy)
(see figure)
What efficacy is most therapeutically beneficial?
A drug with greater efficacy
More important than potency
What is an agonist?
Agents that can bind to a receptor and elicit a biologic response.
usually mimics the action on the original endogenous ligand on the receptor
Types of agonists
Full agonists, partial agonists, inverse agonists
see figure
What can block an agonist?
Antagonist
What is a full agonist?
A drug binds to a receptor and produces a maximal biologic response that mimics the response to the endogenous ligand.
Good efficacy
Example of a full agonist
phenylephrine – α1-adrenoceptor
What is a partial agonist?
Have affinity for the receptor but have low efficacy
Binding site is the same with a full agonist
What happens when a partial agonist is administered alone?
Activates the receptor, but less than full agonist
What happens when a partial agonist is administered in the presence of a full agonist?
Partial agonist reduces the effects of the full agonist
Example of partial agonist
Aripiprazole
What is an inverse agonist?
Have affinity for the receptor but have negative effect (negative efficacy)
Reverse the constitutive activity of receptors and exert the opposite pharmacological effect of receptor agonists.
What is an antagonist
A drug has affinity for the receptor but has no efficacy
Can bind to a receptor, but fails to produce a response
An agent that can decrease actions of agonist or endogenous ligand.
Antagonist used alone
No biological response
Partial agonist used alone
Biological response less than full agonist
Partial agonist used with full agonist
Reduces biological response
Antagonist used with full agonist
Reduces biological response
Antagonist used with partial agonist
No biological response
Mechanism of competitive antagonist
Bind to the same site on the receptor as the agonist
Prevent an agonist from binding to its receptor
Increasing the concentration of the agonist to the receptor will tend to overcome the inhibition.
Dose-response curve of drug in presence of antagonist
Curve shifts to right (p.37 of notes)
Increase EC50 of agonist
Emax and Efficacy are the same
Types of Irreversible antagonists
Orthosteric
Allosteric
Orthosteric irreversible antagonists
Bind covalently or with very high affinity to the active site of the receptor -> reduces the amount of receptors available to the agonist
Example: Naloxazone
Allosteric irreversible antagonists
Bind to a site other than the agonist binding site -> prevents the receptor from being activated even when the agonist is attached to the active site.
Example: Strychnine
What happens to Efficacy? EC50? Potency? when irreversible antagonists are used
Emax and Efficacy: decrease
EC50, potency: less effect
Competitive antagonist vs irreversible antagonist
see figure
What is an adverse effect?
An undesired harmful effect resulting from a medication
Types of adverse effects
Too much therapeutic effect (overdose), i.e. benzodiazepine
Poor tissue selectivity, i.e. antihistamines (become drowsy)
Poor receptor selectivity, i.e. tricyclic
antidepressant (dry mouth etc)
Drug interactions: i.e. benzodiazepine and alcohol
How can we measure drug safety?
Therapeutic index
How to determine therapeutic index (TI)?
TI = TD50/ED50
What is TD50?
The drug dose that produces a toxic effect or adverse effect in 50% of patients taking the drug
ED50
The drug dose that produces a therapeutic or desired response in 50% of patients taking the drug.
If TI is high…
Therapeutic window is wide -> safety is high, less adverse effects, and vice versa
A drug with high affinity and high selectivity has a high therapeutic index.
See figure
How is the TI determined?
Drug trials and accumulated clinical experience.
Hyporeactive
A lower response to a drug than is usual among the population.
Hyperreactive
A higher response to a drug than is usual among the population.
Idiosyncratic
Individuals exhibit an unusual drug response
Tachyphylaxis
an acute rapid loss of response to a
drug.
receptors are still present on the cell surface but are unresponsive to the ligand
Tolerance
a decreased response to a drug when the drug is taken repeatedly.
receptors are down-regulated in the presence of continual stimulation;
or receptor undergoes endocytosis
Receptor desensitization
a mechanism that reduces
the receptor response to an agonist.
Tolerance and Tachyphylaxis are examples
What is drug development?
process to discover new candidates
Clinical trials are done in…
Humans
Phase 1 of clinical trial
in a small number (20–100) of healthy volunteers.
Screening for safety
Find the maximum tolerated dose, is designed to prevent severe toxicity
Phase 2 of clinical trial
In a modest number of patients (100–200)
Identify the therapeutic dose and study the efficacy of drugs.
Phase 3 of clinical trial
In a larger numbers of patients (usually thousands)
Further establish and confirm safety and efficacy.
Phase 4 of clinical trial
in a large numbers of patients (Post-marketing studies)
Monitor the safety of the new drug under actual conditions of use
Safety studies during sales.
