basic pharmacodynamics and receptors

What is the difference between pharmacokinetics (PK) and pharmacodynamics (PD)?

PK describes drug concentrations over time; PD describes the biological effects produced by drug concentrations at sites of action.

What is the biophase, and why is it important?

The site of drug action where receptors encounter drug concentrations that determine the biological response.

What is a ligand, and what are examples of endogenous receptor ligands?

A molecule capable of binding a receptor; acetylcholine binds cholinergic receptors and epinephrine binds adrenergic receptors.

What are the four main features of drug-receptor interactions?

Specific/stereoselective binding, saturability, competition at shared orthosteric sites, and production of physiological responses.

Why is receptor binding saturable?

There is a finite population of receptors, so binding approaches a maximum as available sites become occupied.

Why can two stereoisomers of the same drug have different effects?

Receptors discriminate between molecular orientations; stereoisomers may have different affinity, PK, PD, and side effects.

Which propranolol stereoisomer has greater affinity for beta-adrenergic receptors?

(S)-propranolol, which decreases heart rate and contractility; (R)-propranolol has lower beta-receptor affinity and inhibits SOX18.

What is the difference between an orthosteric and an allosteric binding site?

The orthosteric site is the primary agonist-binding site; an allosteric site is a distinct location that can modify receptor activity.

Which molecular targets can drugs act upon?

Receptors, ligand-gated ion channels, enzymes, nuclear receptors, and nucleic acids; receptor classes include GPCRs and receptor tyrosine kinases.

What is the key difference between affinity and efficacy?

Affinity is the ability to bind a receptor; efficacy is the ability to activate it and generate a biological response.

Which pharmacological property distinguishes an agonist from an antagonist at the same receptor?

Intrinsic efficacy. Agonists activate receptors and produce responses; antagonists bind without activating receptors.

What happens to a receptor after an agonist binds?

The receptor changes conformation, allowing coupling to intracellular signaling proteins or pathways that produce a response.

How does activation of an ionotropic glutamate receptor produce neuronal excitation?

Glutamate binding opens a cation channel, allowing Na+ influx and producing excitatory postsynaptic potentials.

What is EC50?

The agonist concentration producing 50% of its own maximal effect on a graded concentration-response curve.

Two drugs reach the same maximal response, but Drug A has a lower EC50. Which drug is more potent?

Drug A; lower EC50 means less drug is required to produce a half-maximal response.

How do leftward and rightward shifts of a graded concentration-response curve relate to potency?

Leftward shift indicates greater potency and lower EC50; rightward shift indicates lower potency and higher EC50.

What is the difference between drug potency and efficacy?

Potency describes the concentration required for an effect; efficacy describes the magnitude of the maximal response.

Two agonists have similar EC50 values but different maximal responses. Which property differs?

Efficacy, because maximal response differs despite similar potency.

What distinguishes a full agonist from a partial agonist?

A full agonist can produce the maximal system response; a partial agonist produces a lower maximum even with extensive receptor occupancy.

Can a partial agonist be more potent than a full agonist?

Yes. Potency and efficacy are distinct; a partial agonist may have a lower EC50 but a lower maximal effect.

Why can full agonists produce maximal effects without occupying every receptor?

Intracellular signaling amplification allows maximal effects with only a fraction of receptors activated, reflecting spare receptors.

Why does EC50 not necessarily correlate with receptor-binding affinity?

Signal amplification and spare receptors allow large biological responses without proportional receptor occupancy.

What intracellular signaling pathway illustrates amplification of beta-adrenergic receptor activation?

Epinephrine activates beta-2 receptors, adenylate cyclase, cAMP-dependent protein kinase, and glycogen phosphorylase pathways, amplifying glycogenolysis.

Why can partial agonists produce limited effects despite binding many receptors?

They have lower intrinsic efficacy and a reduced ability to stabilize receptor conformations that activate signaling.

What defines a receptor antagonist?

A ligand with receptor affinity but no intrinsic efficacy that can prevent or reduce agonist responses.

What defines reversible competitive orthosteric antagonism?

An antagonist competes at the agonist's binding site, has no intrinsic efficacy, and can be overcome by sufficiently high agonist concentrations.

What happens to the agonist concentration-response curve when a reversible competitive antagonist is added?

It shifts right: EC50 increases and potency decreases, but the maximal response remains unchanged.

Why does increasing agonist concentration overcome reversible competitive antagonism?

Higher agonist concentrations increase competition for the shared receptor-binding site.

What defines noncompetitive antagonism?

Antagonism that cannot be overcome by excess agonist, often involving irreversible binding or interference with receptor signaling.

What happens to an agonist's maximal response during noncompetitive antagonism?

Maximal response decreases, reflecting reduced agonist efficacy.

A drug progressively lowers Emax despite increasing agonist concentrations. What mechanism is most likely?

Noncompetitive antagonism.

Can noncompetitive antagonists act at the same receptor site as agonists?

Yes. They may bind irreversibly at the orthosteric site or act at a different site or downstream signaling component.

How can competitive and noncompetitive antagonism be distinguished on concentration-response graphs?

Competitive antagonism produces a rightward shift with unchanged Emax; noncompetitive antagonism reduces Emax.

What is functional or physiological antagonism?

Two agonists activate different receptors but produce opposing physiological effects that may cancel each other.

How does functional antagonism differ from competitive antagonism?

Functional antagonism involves different receptors and opposing effects; competitive antagonism involves competition at the same receptor site.

How are antagonists clinically useful?

They reverse agonist/toxicant effects, such as naloxone reversing opioid effects, or block endogenous agonists, such as antihistamines opposing histamine.

Why combine medetomidine with vatinoxan?

Medetomidine provides central alpha-2 adrenergic effects, while peripheral alpha-2 antagonist vatinoxan reduces unwanted peripheral cardiovascular effects.

What does it mean that GPCRs can be promiscuous?

One receptor can couple to multiple G proteins or other intracellular signaling pathways depending on the agonist or cellular state.

What is a biased orthosteric agonist?

A ligand that stabilizes a receptor conformation preferentially activating certain signaling pathways rather than all available pathways.

Why might biased agonists have fewer adverse effects?

They may favor therapeutic signaling pathways while avoiding pathways associated with unwanted effects.

What is the biased adrenergic ligand example from the lecture?

Carvedilol, which is biased toward arrestin-associated signaling linked to cardioprotection.

What is a positive allosteric modulator (PAM)?

A ligand binding outside the orthosteric site that increases an endogenous agonist's potency or efficacy.

What is a negative allosteric modulator (NAM)?

A ligand binding outside the orthosteric site that decreases an endogenous agonist's potency or efficacy.

Can positive allosteric modulators act on receptors other than GPCRs?

Yes. Ionotropic receptors such as GABAA receptors can also be allosterically modulated.

How do barbiturates produce neural inhibition through GABAA receptors?

They positively modulate GABAA receptors, increasing GABA-mediated chloride flux and promoting neuronal inhibition and sedation.

How does cicutoxin from water hemlock affect GABAA receptors?

It negatively modulates GABAA receptors, decreasing chloride flux and neuronal inhibition, potentially causing seizures.

How does fipronil kill fleas and ticks?

It negatively modulates insect GABA receptors, disrupting inhibitory neurotransmission.

Why might allosteric modulators produce fewer adverse effects than direct orthosteric agonists?

Their effects depend on physiologically regulated endogenous ligand availability, potentially limiting excessive activation.

What is a biased allosteric modulator (BAM)?

An allosteric ligand that selectively increases or decreases particular signaling pathways coupled to a receptor.

Why might a BAM be useful when targeting a promiscuous GPCR?

It could restrict or favor selected signaling pathways, increasing therapeutic specificity and reducing side effects.

What is a bitopic ligand?

A ligand engineered to bind simultaneously to orthosteric and allosteric sites on the same receptor.

How do graded and quantal dose-response curves differ?

Graded curves measure response magnitude as concentration changes; quantal curves measure the proportion of animals exhibiting an all-or-none response at each dose.

Which clinical responses are examples of all-or-none drug effects?

Anesthetic, bacteriostatic, antiarrhythmic, and anticonvulsant responses.

What does ED50 represent on a quantal dose-response curve?

The median dose producing a specified effect in 50% of the patient population.

How do ED50, TD50, and LD50 differ?

ED50 produces the specified effect in 50%; TD50 produces toxicity in 50%; LD50 causes death in 50%.

Which animals respond at the lowest doses on a quantal dose-response curve?

The most drug-sensitive individuals in the population.

What is the therapeutic window?

The range of drug doses producing desired therapeutic effects without unacceptable adverse effects.

How is therapeutic index calculated?

TI = LD50 / ED50; it compares median lethal and effective doses.

How is the certain safety factor calculated?

Certain safety factor = TD1 / ED99; it compares the dose toxic to 1% of patients with the dose effective in 99%.

Why are drugs with narrow therapeutic windows clinically concerning?

Effective and toxic doses are close together, increasing the likelihood of adverse effects with relatively small dosing or exposure changes.