Agonists and inverse agonists

What are inverse agonists?

Bind to constitutively active receptors to reduce their activity, producing an opposite effect to an agonist.

What occurs to receptors under normal physiological conditions?

Low spontaneous activity (not 0).

What occurs to receptors during disease?

Enhanced spontaneous activity (above baseline activity).

What is [R*]?

Spontaneously activate and constitutively expressed receptors.

How do inverse agonists affect constitutively active receptors?

• Inverse agonists bind to these constitutively active receptors to reduce their activity.

• Produce an opposite effect to an agonist (negative efficacy/receptor deactivation/reduced activation).

What is the efficacy of agonists?

Have positive efficacy (cause receptor activation).

What is the efficacy of antagonists?

Have zero efficacy and do not affect baseline receptor activation level.

How do antagonists affect inverse agonist?

The action of an inverse agonist can be blocked by an antagonist (graph shifts to the right).

What are some clinical examples of agonists and antagonists?

• Adrenaline (epinephrine) - Hormone (endogenous ligand) produced by adrenal glands to help prepare for fight or flight response (stres/danger). Causes airway dilation,  to provide uncles with oxygen. Causes blood vessels contraction. Acts on a lot of receptors in the body (heart and lungs, and is a drug target. 

• Heart - Stimulation (agonism) of B1 receptors leads to increased blood pressure. For treating hypertension, B1 receptor agonists (beta blockers) such as atenolol. 

• Lungs - Stimulation (agonism) of B2 receptors lead to bronchial relaxation, increased breathing. For treating asthma, B2 are stimulated by agonists such as salbutamol.

What is an example of a partial agonist?

• Dopamine.

• Endogenous ligand, drug target. 

• Neurotransmitters produced by the brain, influences your mood and feelings (reward, motivation, pleasure).

• Acts on many excerpts in the body mainly in the central nervous system.

• increases the nerve stimulation (neuronal firing).

What is a clinical example of an inverse agonist?

• Histamine.

• Endogenous ligand, drug target. 

• Chemical messengers released by immune cells (inflammatory response) neurotransmitters, modulates neuronal activity. 

• Released by and acts on many cells in the CNS and in the peripheral tissues.

• acts on the following receptors which are constitutively active - H1, H2, H3, H4.

What is the importance of inverse agonists?

• Greater degree of control over receptor function (agonists, antagonists, inverse agonists), it is expected that this will translate into better treatment of diseases. 

• Physiological/therapeutic relevance - Treat diseases caused by enhanced constitutive receptor activity.

What is terazosin?

Helps lower BP (treats hypertension by relaxing blood vessels so blood passes through them easily). It is an antagonist at the A1 receptor (found on vascular smooth muscles, mediate vasoconstriction, GPCRs).

What is adrenaline?

Acts at both A and B adrenoceptors (vasoconstriction and vasodilation respectively).

What occurs to receptor occupancy when concentration = dissociation constant?

When given a concentration equal to its dissociation constant, 50% of the receptors will be occupied. This is a fundamental fact of the equilibrium dissociation constant.

How does affinity affect receptor occupance?

The greater their affinity, the less the drug will be required to occupy 50% of the receptors. Drugs with high affinity will achieve greater receptor occupancy at low concentrations.

What is drug potency?

Receptors saturate with drug at concentrations above its dissociation constant.

What is the equation for receptor occupancy (P_AR)?

(P_AR) - [D]

[D] +K_D

What are the types of agonists?

Can be full, partial or inverse.

What are the type of antagonists?

Can be competitive, non competitive, reversible and irreversible.

What are competitive antagonists?

• Competes with the agonist for the same receptor site. 

• Can block the function of the agonist.

• Interaction between the antagonist and the receptor is reversible. 

• Inhibitory effects can be surmounted by addition of a higher concentration of agonist. 

• Restore tissue function.

What are noncompetitive antagonists?

• Do not compete with the agonist for the same receptor site. 

• Reduce the maximum effect of the agonist.

• Interaction between the antagonist and the receptor is irreversible. 

• Inhibitory effects can not be surmounted by addition of a higher concentration of agonist. 

• Can not restore tissue function.

What are reversible competitive antagonists?

• Antagonists compete with the agonist for the same receptor site.

• Antagonists binding can block the function of the agonists. 

• The interaction between the antagonist and the receptor is reversible.

• This has an equation.

What is the equation for agonist concentration?

Agonist concentration = Restore receptor occupancy by the agonist = Restore tissue response.

What is the effect of a competitive antagonist on an agonist?

• The curve shifts to the right (parallel shift, towards higher concentration, no difference in slope).

• Maximal response stays the same (100%).

• higher agonist concentrations are required to produce the same effect (EC₅₀ increases).

• EC₅₀ increases as antagonist concentration increases.

What is a clinical example of a competitive antagonist on an agonist?

Reversible binding to B adrenoceptors. 

• Agonist - Isoprenaline, increases heart rate and contractility (treas bradycardia).

• Antagonists - Propranolol, slows down heart rate (treats hypertension).

What is the concentration ratio (r)?

The factor by which the agonist concentration needs to increase to restore a given response in the prescience of the antagonist.

What occurs when an antagonist is present?

When an antagonist is present, 100x more agonist is needed to produce the same response.There is a linear relationship between the concentration ratio (r) and the concentration of the antagonist. This can be plotted on Schild’s plot.

What is Schild’s plot?

• Concentration ratio (r) - The factor by which the agonist concentration needs to increase to restore a given response in the presence of the antagonist. 

• There is a linear relationship between r and the concentration of the antagonist. 

• Y axis - Is log(antagonist concentration.

• X axis - Is log(concentration ratio - 1).

• Antagonist equilibrium constant (K_B) - Molar concentration of the antagonist that makes it necessary to increase the concentration of the agonist in order to elicit the original response obtained. 

• Antagonists potency (pA₂) - -Log (K_B).

What is irreversible competitive antagonists?

• Antagonists compete with the agonist for the same receptor site.

• Antagonists binding with a receptor is governed by covalent bonds. 

• The interaction between the antagonist and the receptor is irreversible.

• The antagonist dissociates very slowly or not at all from the receptor.

• increasing the concentration of the agonist does not change receptor occupancy (antagonist stays bound to the receptor).

• Maximal response is reduced and dos is maximum agonist receptor occupancy.

• Increasing the agonist concentration does not restore the receptor occupancy by the agonist nor does it restore the biological response. 

• The higher the concentration of the antagonist, the lower the agonist receptor occupancy and response.

What are the receptor binding sites?

• Site 1 - Agonist binding.

• Site 2 - Antagonist binding.

• These are non-competitive antagonists, and do not compete for the same receptor site.

What is the affect of non-competitive antagonist on an agonist?

• Bind to different receptor sites.

• Agonists and antagonists can be bound to the same receptor at the same time.

• Antagonist induces change to the receptor such that, agonist affinity decreases and agonist efficacy decreases.

• Irreversible antagonism - Increasing the concentration of the agonist does not change receptor occupancy by the antagonist.

• Maximal response is reduced.

What occurs when a non-competitive antagonist binds irreversibly?

• When a non-competitive antagonist binds irreversibly (covalent bonds), it reduces the binding of the agonist. 

• The primary effect of a non-competitive antagonist is a reduction in the maximal effect produced by the agonist (reduced Emax). The slope may also be reduced. 

• As antagonist concentration increases, E_Max is reduced further. 

• EC₅₀ may or may not be altered.

• In contrast to a competitive antagonist, the effect of a noncompetitive antagonist cannot be reversed by simply increasing the concentration of the agonist.

What is IC₅₀?

• IC₅₀ - The concentration of the antagonist (Inhibitory Concentration) that is required to reduce the agonist invoked response by 50%.

• Antagonists are described by the extent of their inhibitory effect. 

• A smaller IC50 implies a more effective antagonist.

What are allosteric facilitators?

• Bind to a different receptor site than the agonist.

• Induce changes to the receptor such that agonist affinity increases and agonist efficacy increases.

• Have no effect of their own, but increase the response generated by the agonist.

• Agonist concentration response curve shifts to the left (increase potency).

What is the clinical relevance of these concepts?

• It is important for a pharmacist to understand how drugs act in the body.

• Evaluating drug-receptor interactions enables us to predict the desired and undesired effects of drugs. Such as action of the drug on the target receptor (drug specificity and efficacy) and action of the drug on other receptors (off-target effects / side-effects / toxicity).

• Prediction of clinical doses (potency) and evaluating whether a drug is behaving as an agonist or antagonist can enable us to monitor clinical outcomes and drug behaviour. Emax EC50 (agonist), IC50 (antagonist).

• Prediction of interactions of various drugs when given together - Drugs with similar actions (two agonists or two antagonists) and drugs with opposite effects (mixture of agonists and antagonists).

• So that we can ultimately closely monitor and balance therapeutic outcomes with unwanted side-effects and contraindications.