Drug Receptor Interactions Flashcards
What are receptors ? Also briefly describe their role.
Receptors are specialised, localised proteins whose role it is to recognise stimulants and translate this event into an activation of the cell
Describe the main general properties of interaction between the drug and receptor
- Loose
- Freely reversible
- Does not involve strong chemical bonds
Identify the regulatory protein families which are commonly drug targets and are covered by the term receptor:
- Enzymes
- Carrier molecules (transporters)
- Ion channels
- Neurotransmitter, hormone or local hormone receptors
Is the active site of a receptor specific for one drug only ?
No, may fit more than one chemical substance
How similar are agonists and antagonists, chemically ?
a) Both bind same receptor, so must be chemical similarities
b) One activates the receptor, other does not: Must also be chemical differences
What is the law of mass action ?
“The rate of a chemical reaction is proportional to the product of the concentration of the reactants”
How do forward and backward rates compare at equilibrium ?
They are equal
What happens to the forward and backward rate of a reaction at equilibrium if more reactant is added ?
Forward rate will increase (i.e. be more than the backward rate) until a new eq is reached
When the law of mass of action is applied to pharmacology, what does the following stand for ?
(D) + (R) ⇌ (DR) ⇌ (DR)*
K1 and K2
α and β
What does the first part of the equation stand for ?
What does the second part stand for ?
[D] is the concentration of drug
[R] is the concentration of receptors
[DR] is the concentration of occupied receptors
K1 is the rate constant for associations
K2 is the rate constant for dissociations
α is the rate constant of receptor inactivation
β is the rate constant of receptor activation
First part is occupancy
Second part is activation
What is the formula for rate of associations ?
for rate of dissociations
rate of associations: (K1.[D].[R])
rate of dissociations: (K2.[DR])
What is affinity in the context of pharmacology ?
(KD or KA) is a measure of the concentration range over which a drug binds to its receptor
How may we experimentally determine the affinity of a receptor for a drug ?
- Label the drug
- Incubate the drug at various concentrations, with the receptor
- Allow dynamic equilibrium to be established
- Filtrate bound receptor-drug complex from unbound drugs
- Must take non-specific binding into account (binding in areas other than active site): add unlabelled drug, which will therefore bind to active site. Only signal coming from labeled drugs is from the ones non-specifically bound (gives measure of non-specific binding)
- Subtract non-specific binding from total binding to obtain specific binding
What is the shape of the graph obtained when graphing Quantity of Specifically Bound Drug vs Concentration ?
Hyperbolic
What information does the log scale graph of specifically bound drug vs concentration (log scale) give you ?
Maximum Binding
Affinity
What is the equilibrium dissociation constant ?
KD, concentration of drug required to occupy 50% of the receptors at equilibrium
Is the KD the same for all drugs ?
No
Can you measure affinity for a mixed population of drugs and/or receptors ? Why or why not ?
No, because KD is a measure of the affinity of any one drug for a receptor.
What is fractional occupancy ?
The fraction of all receptors that are bound to ligand.
What is the formula for fractional occupancy ?
Fractional Occupancy = [DR]/totalR = DR/[R]+[DR]
Given the unknowns we have, including the concentration of unoccupied receptor (so cannot determine [DR], so use HILL LANGMUIR equation:
Fractional occupancy = [D] / ([D] + KD)
where,
[D] is the concentration of drug
KD is the equilibrium dissociation constant of the drug
In the theoretical relationship between occupancy and ligand concentration, what would be the shape of the graph using:
- a linear concentration scale
- a log concentration scale
- a linear concentration scale: rectangular hyperbola
- a log concentration scale: symmetrical sigmoid curve
At which value of fractional occupancy is KD found ?
At 50% (because KD is the concentration of drug required to occupy 50% of the receptors at equilibrium)
What is pD2 ? What is its usage ?
pD2 = -log10(KD)
Conventional to express the equilibrium dissociation constant (KD) as -log10 of the KD. This is called the pD2 for an agonist.
Will affinity be larger for an agonist with a higher or lower pD2 ?
Higher (because -log10 of a small number is larger than -log10 of a larger number)
What is efficacy ?
Ability of the drug to generate/initiate a stimulus once bound to its receptor
Why is efficacy not the same as affinity ?
Because the maximum response of a tissue can be obtained by occupying less than 100% of the receptors
What kind of a relationship is there between drug concentration and the magnitude of the response obtained ?
Systematic relationship
What is the main conclusion of the occupancy theory ?
Response α fraction of receptors occupied –> Response = % of receptors occupied –> Response = [DR]/[RT]
What is the actual formula for response ?
Response = f(Stimulus) Stimulus = [DR]/[RT] Response = f([DR]/[RT])
If < 100% of receptors are required to evoke a maximum response, what are these “extra” receptors called ?
SPARE RECEPTORS
What is the difference between an efficacious drug and a less efficacious one ?
A very efficacious drugs will require to occupy fewer receptors to give a response than less efficacious ones
What is the function of a partial agonist ?
↓ the response to a full agonist because some receptors will be occupied by partial agonist molecules, giving a smaller response than if all the receptors were occupied by full agonist molecules
What is a competitive antagonist ?
Drug which interacts/binds reversibly with receptors to form a complex but this complex does not evoke a response
[D] + [R] + [A] = ? Where [D] is the drug [R] is a receptor [A] is a competitive antagonist
[D] + [R] + [A] = [DR] + [AR]
Where
Only [DR] give rise to a response
[AR] are receptors which are occupied but not activated
What is the measure of affinity of the antagonist to the receptor ?
Ka = Equilibrium dissociation constant for an antagonist
What is the effect of a competitive antagonist on Log Dose-response curve ?
- Displaced to the right
- Maximum response to agonist remains the same
- “Linear” sections are parallel
What is the Schild equation ? What does it show ?
r = ([A]/KA) + 1
Calculates the ratio by which the drug must be increased to overcome competition by the competitive antagonist.
What is the Equilibrium dissociation constant for an antagonist (Ka) (in normal words) ?
Concentration of antagonist that makes it necessary to add twice as much agonist to produce a response as would be needed in the absence of the antagonist
What is the pA2 of a drug ?
Measure of affinity of competitive antagonist for a specific receptor.
What is the formula for pA2 ?
pA2 = -log10(KA)
Can partial agonists ever produce a 100% response ? A full agonist ?
No
Yes
How do orthosteric/allosteric inhibitors work ?
Antagonist blocks access of the agonist to its binding site through steric hindrance.
Alter conformation of receptor, making it difficult to bind to active site
How do orthosteric/allosteric inhibitors work ?
Antagonist blocks access of the agonist to its binding site through steric hindrance.
Bind to binding site and alter conformation of active site on the receptor, making it difficult to bind to active site
What are the main groups of osthosteric/allosteric inhibitors ?
Non competitive inhibitors
What is an allosteric site ? Can they modify the affinity ? efficacy ?
- Regulatory site, not ‘active’ binding sites
- Can modify the affinity or efficacy of a receptor’s endogenous ligand or drug (may either potentiate or inhibit)
How does a Irreversible Competitive Antagonist affect the Log Dose-response curves ?
Slope decreased
Maximum response to agonist decreased
How do non competitive inhibitors work ?
Antagonist binds to a site that is independent of the active site.
Do non competitive inhibitors affect efficacy and/or affinity ?
They can affect either efficacy or affinity (if affect efficacy, also affect affinity. May also affect just affinity)
How do non competitive inhibitors affect Log Dose-response curves ?
Negative affinity modulation: Push it to the right but same maximum response
Negative efficacy modulation: Decreased maximum response
Do irreversible competitive inhibitors affect efficacy and/or affinity ?
They affect efficacy AND affinity at once ???
Do reversible competitive inhibitors affect efficacy and/or affinity ?
They affect affinity only
What is the premise behind homeopathy ?
Dilution and potency of homoeopathic medicines is inversely proportional
What is the difference between Ka and Kd ?
Ka = Equilibrium dissociation constant for an antagonist Kd = Equilibrium dissociation constant, concentration of drug required to occupy 50% of the receptors at equilibrium
Ka is for antagonist, Kd is for the drug
What are some formulas which involve Ka, and Kd respectively ?
pA2 = -log10(KA) pD2 = -log10(KD)
How is a full agonist going to change the shape of a Log Dose-response curve for occupancy (both the full and partial agonists being included) ?
Full agonist: Increased affinity, increased efficacy
Partial agonist: Decreased affinity, decreased effiacy
What is the mechanism of action of inverse agonists ?
Binds with the constitutively active receptors at the active site, stabilize them and shifts receptor equilibrium towards the inactive state, reducing the level of basal activity ie: negative efficacy or negative intrinsic activity.
Why are inverse agonists not simply considered antagonists ?
Because it can evoke a response (unlike antagonists) even if response is to reduce level of receptor activation
What happens if you add an inverse agonist to a system with just an agonist (no antagonist) ?
Get dampening of the response
What happens if you add a competitive antagonist in the presence of an inverse agonist, according to a Log Dose-response curve ?
A shift to the right of the curve
Can the Schild equation (calculating r = Ratio by which [D] must be increased to overcome competition by [A]) be used for inverse agonists ?
Absolutely yes
