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Receptor pharmacologySection 1: general pharmacological principles
Lecture 1
Pharmacology: interaction of chemical substances with the human body.
Non-specific: biological effect at high drug concentration
Specific: effect with low drug concentration, chemical and biological specificity > act on target
proteins with high affinity to the drug.
Drugs act on target proteins:
- Receptors: target molecules whose function is to recognize and respond to specific signals
such as hormones, neurotransmitter and inflammatory mediators > cell communication.
Reciprocal specificity to receptor-drug interaction.
On target side effect = other molecules with the same receptor also get affected
Off target side effect = molecules with other receptors get affected.
- Ion channels
- Enzymes
- Carriers (e.g. cocaine)
Agonist (A): receptor occupation leads to biological response > A + R = AR > AR* > response
Antagonist (B): receptor occupation does not lead to response and prevents the effect of an agonist,
mostly by preventing it from binding.
N: receptor
Fractional occupancy
Ka = concentration of ligand to
which the receptor occupation is exactly 50% > steepest part of the curve.
Receptor binding measurement: Scat chard plot > relation between specific
vs non-specific binding > relation between concentration and binding >
steepness determines Ka (thus receptor occupation).
Efficacy = the extent to which a drug can affect the system (0-1)
Potency = amount of drug required to produce an effect of given intensity (high potency = low EC50)
Occupation theory: effect (Ea) proportional to the occupancy (Na) (over-simplification)
EC50 = concentration where the efficacy is half, no matter how efficient the drug is!
-Log of EC50 = pD2
Receptor reserve: more receptors than needed, so less concentration relative to the
receptor is needed to fully excite the system, theoretical occupancy and response
curves, EC50 < Ka (only for full agonist). {Occupancy both drugs} = {EC50 full} and
{EC50 partial} intersect.
,(Reversible) competitive antagonism:
- Bind to receptor with no reaction
- More antagonist > more agonist needed > specific curve!
- Dose ration: relation between antagonist with and without agonist
- Schild plot: dose ration curves, pA2 = -log Ka
- Irreversible = non-competitive, sometimes bind at other site than
agonist (allosteric) > agonist can bind but no activation, loss of efficacy!
Partial agonism/antagonism:
- Partial agonist = partial antagonist
- The more full agonist, the more the partial agonist becomes antagonist
- Can be very useful as a drug
Non-competitive-like antagonism:
- Dissociation is very low
- Binding is not irreversible, but very slow
- Behaves like a non-competitive antagonist
Binding constants can give you information about the selectivity of a drug.
Non-competitive antagonist don’t always induce loss of efficacy.
Lecture 2
08-05-2020
Lower binding constant > lower concentration needed > more selective.
Partial agonist: occupies all receptors but does not activate them/ the system fully.
Intrinsic activity = maximal effect*100%
Inverse agonism: the receptor is (a bit) in the activated state
when no agonist is present > baseline activity > inverse agonist
presence lowers the activity. Can be blocked by antagonist.
➔ 2 state model
Antagonist has no activity in the absence of an (inverse) agonist,
has no preference in affinity for receptor state.
80% of G-protein coupled receptor antagonists are inverse agonist!
,Physiological/functional antagonism:
- activate the receptor that has the opposite effect
- e.g. M2 and beta2 receptors in lungs or parietal cells
➔ High concentrations are harder to physiological antagonise
Chemical antagonism:
- Non-receptor antagonist
- E.g. antibodies, antacids, chelators
- Can take place in absence of physiological system
Pharmacokinetic antagonism:
- Increased rate of metabolic degradation/elimination of another drug
- Decreased g-I absorption
- E.g. warfarin long halve live > rifampicin > helps eliminate warfarin
Desensitization/tachyphylaxis/tolerance: loss of receptor response
- Loss/change in receptors
- Exhaustion of mediators
- Enhanced drug metabolism
- Compensatory physiological mechanisms
- Extrusion of drugs form cells (out pumping)
Purple = response
Green = response after wash
Red = number of receptor
Yellow = number of receptors
Uncoupling: ligand binding > GRK > phosphorylation >
uncoupling > arrestin > endocytosis > receptor
recycling/degredation/new receptor synthesis due to gene
expression.
➔ Rapid and delayed drug response!
Types of target for drug activation:
- Receptors
- Ion channels
- Enzymes
- Transporters
, Lecture 3
Receptor types:
- Ligand-gated ion channels: ligand binding dependent > hyper-
/depolarization, very fast, ACh/nicotinic
- G-protein coupled: ligand binding > opening ion channel / second
messengers, quite fast, muscarinic/ACh
- Kinase-linked: ligand binding > protein phosphorylation > gene
transcription > protein synthesis, quite slow, cytokine receptors
- Nuclear receptor: goes inside the cell > ligand binding > gene
transcription, very slow, oestrogen
Structure of receptor types:
- Ligand-gated: 4/5 channels, extracellular binding domain
- G-protein coupled: 7 channels, extracellular of membrane binding
domain
- Kinase linked: 1 channel, extracellular binding domain, intracellular
catalytic domain
- Nuclear: DNA binding domain
Ligand-gated ion channels:
Nicotinic ACh receptor:
- 5 subunits
- Nm (muscle) and Nn (neuronal) subtypes
- Increased Na+ and K+ permeability
- 2 binding domains (need to both be occupied)
GABA/NMDA receptors: ligand binding receptors for learning and memory
G-protein coupled receptors: for hormones, neurotransmitters, mediators, e.g.
- 7 membrane channels, extracellular and membrane binding domain, intracellular domain
- Muscarinic ACh receptors: M1/2/3/4/5
- Adrenoceptors: α1/2, β1/2/3
Protease-activated receptors (PARs): blood clothing
Cleavage by thrombin extracellular N-terminus > terminus becomes active and a tethered agonist >
phosphorylation of intracellular terminus > receptor is desensitised (irreversible).
‘Shuttle model’
GDP = inactive state
GTP = active state > β-λ complex splits from alpha subunit
βλ heads to target cell 2
Alpha heads to target cell 1 > GTP hydrolyses to GDP
Specificity alpha subunit, beta-gamma subunits for all
GPCRs.
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