Week 1/2 Flashcards
Drug
Any chemical substance, other than a nutrient, that when
administered to the body, produces a biological effect
Endogenous Substance
NOT a drug
Exogenous substance (including endogenous substances
ADMINISTERED into the body)
A drug
Pharmacodynamics
what the drug does to the body and how (molecular
interactions between a drug and its target)
Pharmacokinetics
what the body does to the drug (ADME; Metabolism, Absorption, Distribution, Elimination)
‘Pharmaceutics’
preparation of a drug into an appropriate form for dosing into the
body
Toxicity/Toxicology
the adverse/unwanted effects of drugs (everything, if given in a
large enough dose, is toxic)
How do drugs work?
They bind to different structures or ‘targets’ in the body in a
concentration/dose dependent manner
‘More drug = more biological response’
Agonist effects
promote a biological response
Antagonist effects
inhibit biological response
Inverse agonist effects
promote the opposite effect to the
normal biological response
Drug targets:
Receptors
Ion Channels
Transporters
Enzymes
Rite
What factors are important for binding to occur
charge, shape, hydrophobic or
hydrophilic domains (regions) in the drug and
receptor
Types of agonist-activated receptors and their speed of response
Fastest to slowest:
1. Ligand-gated ion channels (ionotropic receptors)
2. G-protein coupled receptors (metabotropic)
3. Kinase-linked receptors
4. Nuclear receptors
Ligand gated ion channels
Agonist binds to the receptor which causes a conformational change in the receptor that opens it up to allow ions (eg. K+, Na+) to pass through the membrane
* located on cell membrane and agonists bind on the extracellular side
* Induce VERY rapid cellular effects (milliseconds – no second messengers)
* Most commonly located on conductive or excitable tissues eg. nerves, heart and neuromuscular junctions
* usually receptors on which fast neurotransmitters act
* eg. nicotinic acetylcholine receptor, GABA and glutamate receptors
Can be composed of 3, 4 or 5 subunits
E.g. nicotinic acetylcholine (Ach) receptor
* Assembled from 5 subunits to form a pentamer (2 x α units, 1 x β unit, 1 x γ
unit, 1 x δ unit)
* Contains 2 Ach binding sites on the borders between the two α subunits
and their neighbours
* Both binding sites must be bound to Ach to open the channel
* Each subunit crosses/spans the membrane 4 times (20 transmembrane
helixes)
G-protein coupled receptors (GPCR)
- composed of a single subunit that has 7 transmembrane domains (7 α helixes).
N terminus on extracellular domain, C terminus intracellular - Approx 70% drugs target GPCRs and 2% of all human genes are GPCRs
- Examples include: dopamine, adrenergic, 5-HT, opiate, purine, peptide and chemoreceptors.
- Evoke a fast response (in seconds)
The G-protein
The work horse of the GPRC
* A heterotrimer (contains 3 subunits – α, β and γ)
* Normally bound to GDP, but binds to GTP when the receptor is activated
* When all 3 subunits are combined the protein is inactive or in its resting state.
* When the α-subunit (which contains the GDP) is released from the trimer, it is active and free to activate an effector molecule (which triggers a second messenger system)
* G-proteins move around in the cell, and can interact with many GPCRs (they are promiscuous)
Mechanism of action
diagram memorise
Kinase linked receptors
Single transmembrane spanning domains on the cell membrane
* Activated by a wide range of protein mediators (eg. growth factors & cytokines)
* Generally involved in slow cellular processes (take hours) involved in eg. cell
growth and differentiation (in tissue repair, inflammation, apoptosis)
*
2 agonists bind to receptor = receptors dimerise = phosphorylation = effect
Mechanism of action 2
memorise
Nuclear receptors
Located mainly in the cell cytosol (some are located in the nucleus, depending
on the receptor class)
* Are commonly receptors for hormones (oestrogens, progesterones, androgens,
glucocorticoids) and also fat soluble vitamins
* Generally, once an agonist binds receptor binds to another agonist-bound
receptor (dimerization) dimer translocates into the nucleus binds to DNA
effects gene transcription cellular response
Ion channels
Gateways in cell membranes that selectively allow the passage of ions
Ligand gated:
More accurately described as receptors
Activated by ‘drug’ or agonist binding
Voltage gated:
Opened or closed in response to
transmembrane potential (electrical signals)
Enzymes
Proteins that catalyse chemical changes in ligands or drug molecules
Can be destructive (catabolic) or productive (anabolic)
* Drugs normally inhibit the in vivo action of enzymes
* Eg. Cytochrome P450s metabolise drugs; proteases degrade proteins.
Transporters
- ions and polar drugs need transporters to get across membrane
- process requires ATP
Agonist
- promotes a receptor mediated biological response (promotes normal response).
- Eg. Hormones, neurotransmitters, drugs….
- Also called ligands if endogenous agonists
Inverse agonist
- promotes a receptor mediated biological response (opposite to the normal physiological response)
Agonists and Receptors
Generally, the binding of an agonist causes small changes in the receptors shape which can then lead to the activation of ‘second messenger’ systems (called
‘effectors’)
Binding of an agonist to its receptor is a reversible process
Steps from receptor-agonist binding to a physiological response
- Receptor-agonist interactions involve physical contact
- Bound agonist (or drug) remains in contact with the receptor for only a very
short time (so relies upon a high enough local concentration to ensure that
agonist is always bound to the receptor) - Agonists only bind to receptors at the active site that have the correct
shape - Binding of an agonist to the receptor must change the receptor in some
way to elicit a physiological response (activation of second messenger
system) - Agonist-receptor interactions are usually reversible
Agonist-receptor interactions are concentration dependant
Affinity and efficacy
Affinity: How readily an agonist binds to its receptor and stays bound.
Efficacy: the tendency of a drug, once bound to the receptor, to activate the
receptor and cause a physiological response (how effectively does the drug induce the tissue to respond MAXIMALLY).
Affinity (drug binding)
- binding affinity is measured as KD – the ‘reciprocal affinity constant’, or ‘equilibrium dissociation constant’
- KD are in concentration (in M or molar)
- KD is roughly the concentration of drug at which 50% of the available receptors are bound to drug/agonist
- A bigger KD means a smaller affinity for the receptor, a smaller KD means
a higher affinity for the receptor
Measuring drug activity or ‘efficacy’
There’s a direct correlation between the concentration of a drug and its
ability to exert a physiological response.
* ‘efficacy’ is easier to measure than ‘affinity’, so we normally measure
the efficacy of a drug instead. These responses can include:
* Tissue contraction or relaxation (eg. organ bath experiments)
* Blood flow (eg. vasodilation)
* Enzyme activation
* This is evaluated by changing the concentration of the drug and
measuring how this changes the physiological effect (like in your virtual
ileum or organ bath pracs)
* At the higher concentrations of drug we observe a plateau in
physiological response (ie. all receptor binding sites are bound to drug
and no further increase in response can be observed).
* The maximal physiological response that the tissue can exert (when all
receptor sites are bound to agonist/drug) is termed Emax or 100% Effect.
All other responses (at lower drug concentrations) are reported as a %
of this maximal response.
Partial agonists vs inverse agonists
Partial agonists only have part efficacy (limited %Emax)
Inverse agonists cause tissue response to have the opposite effect
Eg. An agonist may cause tissue contraction, an inverse agonist would cause the tissue to relax
- Requires the tissue to have a baseline level of ‘response’
Spare receptors
refer to a phenomenon where a cellular response can be achieved with only a fraction of the total available receptors. In other words, a drug or signaling molecule can produce a maximal effect even when it doesn’t bind to all the receptors on a cell’s surface.
- suggests that a significant portion of receptors can remain unoccupied, and the cell can still respond to a stimulus because the signalling pathway is highly efficient.
exemption for spare receptors
The biological response of agonist drugs is almost proportional to the fraction of
receptors that are bound to the drug (eg. KD and EC50 are almost the same) in the following systems:
* Smooth muscle relaxation
* Hormone secretion
Constitutive receptor activation
- where a cell’s receptor is constantly and spontaneously active, even in the absence of its normal activating ligand (molecule that binds to the receptor to trigger a response).
- can result from genetic mutations, structural changes in the receptor, or other factors that cause the receptor to be in an “on” state.
Agonists have a higher affinity for ‘active state’ receptors and therefore promote MORE receptor activation (eg. more muscle contraction than usual)
Inverse Agonists have a higher affinity for ‘inactive state’ receptors and therefore promote LESS receptor activation (eg. more muscle relaxation than usual)
Desensitisation
reduced responsiveness of a receptor to a drug that occurs over a few minutes
Tolerance
reduced responsiveness of a receptor to a drug that occurs slowly over prolonged use
Resistance
loss of effectiveness of chemotherapeutic drugs (anticancer/anti-microbial drugs)
Mechanisms by which receptor desensitisation and tolerance occur
- Change in receptors (eg. shape)
- Translocation of receptors away from (e.g.) the cell surface
- Run out of mediators
- More accelerated metabolism of the drug
- Physiological adaption (homeostatic regulation in the body to counter the physiological effect of the drug. E.g. increased baseline blood pressure in response to long term treatment with a BP lowering drug)
Chemical antagonist
Where one drug binds to another drug in solution and prevents it from
binding to its receptor
Clinical example:
Heparin = negatively charged polysaccharide used as an anticoagulant during surgery. It
works by binding to a number of heparin binding proteins.
Protamine = given after surgery to ‘mop up’ left over heparin and prevent it from binding to
heparin binding proteins, to prevent bleeding and blood loss after surgery
Pharmacokinetic antagonist
Where one drug acts to reduce the concentration of another drug at its binding site, limiting the number of drug molecules that can bind receptors
Clinical example:
Phenobarbital is an anticonvulsant used to treat seizures and also upregulates cytochrome P450
enzymes (increases their expression in the liver)
These P450 enzymes can then metabolise drugs such as warfarin (an anticoagulant, like heparin)
Physiological antagonism
Where one drug binds to a different and unrelated receptor to another drug to produce an effect that is the opposite to the second drug
Clinical example:
* Glucocorticoid drugs (eg. cortisone) bind to the glucocorticoid receptor blood glucose
* Insulin binds to the insulin receptor promotes glucose uptake into cells blood glucose
Pharmacological antagonism
Where one drug directly competes with an agonist drug for receptor binding, but does not stimulate a physiological response.
- Three types of pharmacological antagonism:
- Reversible competitive antagonism (antagonist can come off of receptor)
- Irreversible competitive antagonism (antagonist sticks and doesn’t let go)
- Non-competitive antagonism (antagonist binds to a different part of the receptor)
Competitive antagonism
- similar in structure to agonists,
but they do not allow the promotion of second messenger signalling - The agonist and the antagonist compete in a concentration dependant manner for binding to the receptors ‘active’ binding site (as in the binding site that promotes second messenger signalling)
‘Reversible’ competitive antagonism
- Binding of antagonist is reversible
- Time antagonist stays bound
relates to its affinity (KD) - Emax is the same as agonist alone
- EC50 is larger than agonist alone
‘Irreversible’ competitive antagonism
- antagonist sticks and doesn’t let go (or leaves very slowly)
- These types of antagonists often have reactive groups that form covalent bonds with the receptor
- Since they only decrease the number of available receptor sites, they usually reduce efficacy (Emax), but the EC50 stays the same.
Non-competitive antagonism
antagonist binds to a different part of the receptor
Importance of drugs as allosteric modulators?
- Some receptor classes have a high degree of orthosteric binding site similarity, making it hard to develop drugs that just target one specific receptor.
- Targeting the allosteric site increases drug target selectivity
