Membranes and Receptors - Session 8.2
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Questions and Answers

What does affinity describe in the context of drug-receptor interactions?

  • The duration of the drug's effect
  • The ability of a drug to produce a response
  • The strength of drug binding with a receptor (correct)
  • The concentration of the drug in the body

    • Which statement is accurate regarding antagonists?

  • They do not interact with receptors.
  • They have both affinity and efficacy.
  • They bind to receptors and produce a response.
  • They prevent receptor activation by agonists. (correct)

    • How does drug response relate to receptor sites in the body?

  • It is directly proportional to the number of receptor sites bound. (correct)
  • It is determined entirely by the efficacy of the drug.
  • It generally varies inversely with the number of bound sites.
  • It is unrelated to the number of receptor sites bound.

    • What is meant by the term 'efficacy' in pharmacology?

    <p>The ability of a drug-bound receptor to produce a physiological response</p> Signup and view all the answers

    In what way do most drugs interact with their receptors?

    <p>By binding reversibly with receptors</p> Signup and view all the answers

    Which type of drug interacts with a non-endogenous site on a receptor?

    <p>Acceptor</p> Signup and view all the answers

    What principle governs the binding of drugs to receptors?

    <p>The law of mass action</p> Signup and view all the answers

    What is achieved when an agonist interacts with spare receptors?

    <p>Emax at subBmax</p> Signup and view all the answers

    Which statement correctly describes partial agonists?

    <p>They bind to receptors but have limited efficacy.</p> Signup and view all the answers

    Which statement best describes a partial agonist?

    <p>Its effectiveness can vary depending on tissue type.</p> Signup and view all the answers

    What does the term EC50 refer to in pharmacology?

    <p>Effective concentration producing 50% of Emax</p> Signup and view all the answers

    What role do spare receptors play in heart failure regarding drug efficacy?

    <p>They help maintain Emax despite receptor loss.</p> Signup and view all the answers

    Which of the following is not a type of antagonist?

    <p>Inversely Competitive Antagonism</p> Signup and view all the answers

    How does potency relate to drug receptors?

    <p>It is influenced by both affinity and efficacy.</p> Signup and view all the answers

    What defines irreversible competitive antagonism?

    <p>It permanently occupies the active site of receptors.</p> Signup and view all the answers

    In context of spare receptors, which physiological response is enhanced?

    <p>Efficient O2 exchange during low adrenaline stimulation</p> Signup and view all the answers

    Study Notes

    Membranes and Receptors - Session 8.2

    • Drugs and Receptors: Session is about drug-receptor interactions. Dr. Zena Hasan Sahib, PhD in Pharmacology and Therapeutics, is the instructor.

    • Aims and Objectives:

      • The lecture aims to teach drug-receptor interactions for better therapeutic decisions in clinical practice.
      • It presents the quantitative relationship between drug concentrations and responses.
      • Students will learn about agonist and antagonist drug action.
      • Key terms like affinity, efficacy, potency, agonist, antagonist, and partial agonist will be defined.

    Objective 1: Drugs and their Targets

    • Medical Pharmacology: The study of how chemicals (drugs) interact with the human body.

    • Drug Targets: Drugs usually affect targets, primarily proteins. Some drugs may interact with DNA.

    Pharmacodynamics and Pharmacokinetics

    • Pharmaceutics: Drug formulation for specific administration routes.

    • Pharmacodynamics: Interactions between drugs and their targets(eg receptors) (D+T)

      • Outcomes include efficacy, Emax, E, Bmax, B, potency, TD50, and TI.

    • Pharmacokinetics: Drug interactions with body tissues, excluding target interactions (D+B).

      • Outcomes include ADME (absorption, distribution, metabolism, and elimination)

    Receptor Interactions

    • Drug + Target Interactions: Drugs frequently interact with receptors, which are proteins with sites for binding endogenous ligands (like steroid receptors).
    • Receptor definition: A protein containing a site for binding endogenous ligands like steroid receptors.
    • Acceptor definition: If a drug binds to a non-endogenous ligand site, this is called an acceptor-e.g., enzyme blocker.

    Receptor Types

    • Agonist/Inverse Agonist/Antagonist: (See detailed explanation in the objective section)
    • Ion Channels:
      • Direct mechanisms: Affecting ion channel opening/closing.
      • No effect on endogenous mediators.
    • Enzymes:
      • Inhibitors, false substrates, prodrugs
    • Transporters:
      • Inhibitors, false substrates
      • Abnormal compound formation, and prodrug activation.

    Objective 2: Drug Concentration Relevance

    • Mole Concept: 1 mole contains 6 x 1023 particles
    • Molar Solution: A 1 molar solution contains 1 mole in 1 litre.
    • Calculating Grams per Liter: Molecular Weight (MWt) x Molarity (M).
    • Example (Acetylcholine): Acetylcholine (ACh) has a molecular weight of 146.21. A 1 M solution contains 146.21 g/L.

    Drug-Receptor Interactions Details

    • Reversible Binding: Most drugs bind to receptors reversibly.
    • Mass Action Law: Drug binding depends on reactants and products' concentrations.

    Objective 3: Affinity, Efficacy, and Potency

    • (See following detailed explanation for these concepts)

    Drug Concentration and Drug Response

    • Proportional Response: The drug response is generally proportional to the number of receptor sites bound by the drug.
    • Receptor Distribution: Receptor targets exist in varied tissues throughout the body.

    Graphs and Figures

    • Various graphs illustrate different aspects of drug-receptor interactions, including concentration-response curves. Key points such as Bmax, B50, Emax, and E50 are marked on the graphs.

    Detailed Explanation of Affinity, Efficacy, and Potency

    • Affinity: Measures the strength of drug binding to the receptor.
    • Efficacy: A drug-bound receptor's ability to induce a response.
    • Agonist: Possesses both affinity and efficacy.
    • Mechanism: Drug (A) binds to receptor (R), causing activation (AR*) and a response.

    Agonists and Antagonists

    • Agonist: Drugs that bind to receptors and cause a response, possessing both affinity and efficacy.
    • Antagonist: Drugs binding to receptors without triggering a response, possessing only affinity; they block agonist effects.
    • Quantitative Relationship: Binding force (B) to the receptor, drug dose (Kd) to occupy 50% of receptors, efficacy as a response (E) and potency as measured by the needed drug dose is measured to achieve 50% of maximum effect(EC50).

    Practical Determination of Key Values

    • Occupancy Estimation: Measuring receptor binding, typically using radioactive labels, to determine Bmax (maximum binding) and B50 (drug concentration for 50% receptor occupancy).
    • Efficacy Estimation: Assessing response (e.g. muscle strain in grams as a response to Ach concentration) to determine E and Emax (maximum response).

    Spare Receptors Concept

    • Spare Receptors: Unoccupied receptors exist when the agonist achieves its maximum effect (Emax) occuring at below maximum receptor occupancy (subBmax). It's the opposite of a partial agonist.
    • Clinical Significance: Spare receptors provide hormonal/energy economy, efficient physiological response (e.g., O2 exchange during rest with adrenaline, maintaining drug efficacy even with receptor reduction).

    Relationship between Efficacy, Potency, and Occupancy

    • EC50 (effective concentration/dose): Drug concentration or dose causing 50% of the maximal effect, measuring potency.
    • Kd: Drug concentration to occupy 50% of receptors, measuring binding affinity.
    • Spare receptors: In some cases, 100% response can happen even with less than 100% receptor occupancy.

    Influence of Receptor Number

    • Reduced Receptors: with fewer receptors, higher drug concentrations are needed to occupy the same number of receptors. This affects both potency and maximal response.

    Concentration/Dose Response Curves

    • Emax: Maximum response.
    • EC50: Effective concentration or dose producing 50% of the maximum response; measures potency.
    • Potency Comparisons: Comparing the EC50 values allows for potency comparisons among drugs.

    Partial Agonists

    • Partial Agonist Effect: Some drugs cannot elicit a maximal response, even with full receptor occupancy. These are called partial agonists.
    • Intrinsic Activity: Partial agonists exhibit reduced intrinsic activity, producing a lower maximum response compared to a full agonist.
    • Relationship to Full Agonists & Ka: A partial agonist's EC50 equals its Kd and it can be more or less potent than a full agonist.

    Types of Antagonists

    • Reversible Competitive Antagonism: Common form; high agonist concentration can overcome antagonist effect, causing a rightward shift on the concentration-response curve.

      • IC50: Concentration of antagonist causing 50% inhibition.
      • Kd: Concentration of antagonist needed for 50% occupancy.

    • Irreversible Competitive Antagonism: Antagonist binds irreversibly, causing a rightward shift in the concentration-response curve and a diminished maximal response.

    • Non-Competitive Antagonism: Antagonist binds to a different site on the receptor, preventing activation regardless of agonist concentration; reduces maximal response.

    Antagonist Strength and Comparison

    • IC50: Concentration of an antagonist that causes 50% inhibition of a response. Antagonist strength and potency are assessed based on causing a rightward shift of the concentration-response curve for the agonist in question, measured by log of drug concentration.

    Agonist and Antagonist Characteristics

    • Agonist, Partial Agonist, Inverse Agonist, and Antagonist identification: Characteristics described for each class of drugs to identify potency, efficacy, or intrinsic activity (maximal response). Receptor occupancy required to distinguish full agonists from partial agonists, based on concentration-response curve shapes, is also described.

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    Description

    This session focuses on drug-receptor interactions essential for therapeutic decision-making in clinical practice. Students will explore the quantitative relationship between drug concentrations and responses, alongside key concepts such as agonists and antagonists. Dr. Zena Hasan Sahib will guide the discussion on medical pharmacology and drug targets.

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