Pharmacokinetics (distribution)
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Questions and Answers

What primarily contributes to the reversible tissue binding of drugs?

  • Drug metabolism
  • Kidney filtration
  • Plasma protein levels
  • Cellular constituents (correct)

    • What is the primary organ responsible for drug metabolism?

  • Liver (correct)
  • Kidney
  • Lungs
  • Bile

    • How does high plasma protein binding affect a drug's volume of distribution?

  • It decreases the volume of distribution. (correct)
  • It does not affect the volume of distribution.
  • It increases the volume of distribution.
  • It changes the clearance rate.

    • What does the volume of distribution (Vd) relate to in pharmacokinetics?

    <p>Amount of drug to the concentration in blood.</p> Signup and view all the answers

    Which of the following statements about volume of distribution (Vd) is correct?

    <p>Vd can vastly exceed any physical volume in the body.</p> Signup and view all the answers

    What major role do the kidneys play in drug elimination?

    <p>Excreting drugs from the body.</p> Signup and view all the answers

    The concentration of quinacrine in the liver compared to that in the blood can be described as:

    <p>Several thousand-fold higher in the liver.</p> Signup and view all the answers

    What distinguishes the two-compartment model from the one compartment model in drug pharmacokinetics?

    <p>It describes drugs distributed between two distinct areas.</p> Signup and view all the answers

    In the context of pharmacokinetics, what do the central compartment and peripheral compartment represent?

    <p>Regions with different rates of drug distribution and elimination.</p> Signup and view all the answers

    What does drug clearance (CL) generally refer to in pharmacokinetics?

    <p>The elimination of drug from the body.</p> Signup and view all the answers

    What role does tissue binding play in pharmacokinetics?

    <p>It can act as a reservoir for the drug.</p> Signup and view all the answers

    Which of the following organs is typically associated with the central compartment in the two-compartment model?

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

    Which factor is considered when determining the AUC (Area Under the Curve) in a concentration-time profile?

    <p>The drug’s clearance rate from the body.</p> Signup and view all the answers

    Why might a two-compartment model be preferred over a one compartment model for certain drugs?

    <p>It accounts for complex drug distribution patterns.</p> Signup and view all the answers

    Which tissues are grouped together into the peripheral compartment in the two-compartment model?

    <p>Fat and muscle</p> Signup and view all the answers

    What is one limitation of using the one-compartment model in pharmacokinetics?

    <p>It assumes immediate drug distribution throughout the body.</p> Signup and view all the answers

    In pharmacokinetics, what does the term 'concentration' refer to in the concentration-time profile?

    <p>The amount of drug present in a specific volume of plasma.</p> Signup and view all the answers

    Which aspect of drug pharmacokinetics is primarily illustrated by the AUC in the concentration-time graph?

    <p>The total drug exposure over a specific time frame.</p> Signup and view all the answers

    What is a primary characteristic of the two-compartment model?

    <p>Consists of a fast distribution phase and a slow elimination phase</p> Signup and view all the answers

    Which factor does NOT affect the distribution of a drug?

    <p>Age of the patient</p> Signup and view all the answers

    What form of a drug is able to permeate biological membranes?

    <p>Only the unionized and unbound form</p> Signup and view all the answers

    To which plasma protein do acidic drugs, such as salicylates, primarily bind?

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

    What characteristic of protein binding makes it significant in pharmacokinetics?

    <p>It is saturable at high concentrations</p> Signup and view all the answers

    How does tissue binding impact drug concentration in tissues?

    <p>May lead to concentrations higher than extracellular fluids</p> Signup and view all the answers

    In a multi-compartment model, how does it differ from the two-compartment model?

    <p>Involves more than two compartments</p> Signup and view all the answers

    What happens to the percent bound of most drugs when they are within their therapeutic plasma concentration range?

    <p>It is generally constant</p> Signup and view all the answers

    What role does permeability of tissue membranes play in drug distribution?

    <p>It affects the rate and extent of drug distribution</p> Signup and view all the answers

    Which statement about protein binding is true concerning its impact on drug distribution?

    <p>Binding reduces the amount of free drug available</p> Signup and view all the answers

    What is the significance of half-life (t1/2) in pharmacokinetics?

    <p>It indicates the time required to decay 50% from any concentration after administration.</p> Signup and view all the answers

    Steady State Concentration (Css) is approximately achieved after how many half-lives?

    <p>Four half-lives</p> Signup and view all the answers

    The elimination constant (KE) is most closely related to which pharmacokinetic property?

    <p>Half-life (t1/2)</p> Signup and view all the answers

    What is the volume of distribution (Vd) for digoxin?

    <p>500 L</p> Signup and view all the answers

    Which of the following statements about half-life is NOT true?

    <p>Half-life is the time taken for a drug to reach steady state.</p> Signup and view all the answers

    Which factor does NOT affect drug clearance?

    <p>Drug formulation</p> Signup and view all the answers

    Which of the following factors is NOT influencing the time course of a drug in the body?

    <p>Route of administration</p> Signup and view all the answers

    What does a drug's clearance (CL) measure?

    <p>The volume of plasma cleared of the drug per unit time.</p> Signup and view all the answers

    Which drug is classified as a high-extraction drug due to its rapid clearance?

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

    What does a high volume of distribution indicate about a drug?

    <p>It is largely distributed in body tissues.</p> Signup and view all the answers

    What is the significance of half-life (t1/2) in pharmacokinetics?

    <p>It indicates the efficiency of drug clearance from plasma.</p> Signup and view all the answers

    Which condition is associated with capacity-limited elimination of a drug?

    <p>Ethanol consumption</p> Signup and view all the answers

    The measure of the body’s efficiency in eliminating a drug is known as:

    <p>Clearance (CL)</p> Signup and view all the answers

    An increase in which factor would likely enhance the clearance of a high-extraction drug?

    <p>Increased organ blood flow</p> Signup and view all the answers

    What does a plasma volume of 2.8 L indicate about drug distribution?

    <p>It is the amount of drug in the circulatory system.</p> Signup and view all the answers

    What type of kinetic process does drug decay typically follow in the context of half-life?

    <p>First-order kinetics</p> Signup and view all the answers

    Study Notes

    One-Compartment Model

    • The concentration-time profile (or course) of a drug following an intravenous bolus injection can be represented by a one-compartment model.
    • Areas under the concentration-time curve (AUC) are used to calculate various pharmacokinetic parameters that reflect how a drug is absorbed, distributed, metabolized, and excreted.

    Two-Compartment Model

    • Some drugs are better described with a two-compartment model. This model divides the body into a central compartment (blood) and a peripheral compartment (tissues like fat, muscle, liver, etc.)
    • The central compartment is where the drug is distributed rapidly, while the peripheral compartment represents areas with slower distribution.

    Multi-Compartment Model

    • When more than two compartments are necessary to accurately describe the pharmacokinetic profile of a drug.
    • Less commonly used.

    Drug Distribution

    • The extent and rate of drug distribution are impacted by several factors:
      • The physicochemical nature of the drug (e.g., its solubility, size, ionization)
      • Organ perfusion rate (blood flow to organs)
      • Permeability of tissue membranes (how easily the drug can cross cell membranes)
      • Plasma protein binding (the extent to which the drug binds to proteins in the blood)
      • Tissue binding (the extent to which the drug binds to proteins and other molecules in tissues)

    Drug Distribution Principles

    • Only the free (unbound) form of the drug can permeate across biological membranes.

    Protein Binding

    • Many drugs bind to plasma proteins:
      • Acidic drugs bind to albumin (e.g., salicylates)
      • Basic drugs bind to α1-acid glycoprotein (e.g., propranolol)
    • Protein binding is typically reversible and saturable.
      • At high drug concentrations, protein binding may become saturated.
    • For most drugs, the therapeutic range is well below the saturation point, so the percent bound remains relatively constant.

    Tissue Binding

    • Many drugs exhibit higher concentrations in tissues compared to extracellular fluids and blood.
    • For example, quinacrine (an antimalarial) can be found in much higher concentrations in the liver than in the blood.
    • Tissue binding can involve cellular components like proteins, phospholipids, and nuclear proteins.
    • Tissue binding is typically reversible and can act as a drug reservoir.

    Distribution Principles

    • Drugs with high plasma protein binding often have a small volume of distribution.
    • Drugs with high tissue binding usually have a large volume of distribution.

    Elimination

    • Elimination involves removing drugs from the body.
    • Two primary processes are responsible for elimination:
      • Metabolism: primarily carried out by the liver
      • Excretion: primarily performed by the kidneys, but other routes like bile are also possible

    Important PK Parameters

    • Volume of distribution (Vd)
    • Clearance (CL)
    • Half-life (t1/2)

    Volume of Distribution (Vd)

    • Vd is a calculated volume that relates the amount of drug in the body to the concentration in blood or plasma.
    • It's an apparent volume, not a real physiological volume.
    • The Vd can exceed any physical volume in the body because it represents the hypothetical volume needed to contain the drug homogeneously at the blood or plasma concentration.

    Volume of Distribution Examples

    • Digoxin: Vd = 500 L
    • Chloroquine: Vd = 13,000 L
    • Gentamicin: Vd = 18 L
    • Drugs with large Vd values are not distributed evenly throughout the body.

    Clearance (CL)

    • Clearance is a measure of the body's efficiency at eliminating a drug.
    • Clearance is the most important factor determining drug concentrations.
    • It reflects the rate of drug elimination in relation to the drug concentration.
    • Clearance is additive, meaning various organs can contribute to overall clearance.

    Factors Affecting Clearance

    • Dose:
      • Some drugs, like ethanol, exhibit capacity-limited elimination, meaning clearance may be slowed at high doses.
    • Organ blood flow:
      • High-extraction drugs (e.g., lidocaine) are rapidly cleared by organs with high blood flow like the liver.
    • Intrinsic function of the liver or kidneys:
      • Reduced organ function can lower clearance.
    • The unbound fraction of drug:
      • The free (unbound) portion of the drug is available for clearance.
      • High protein binding can decrease clearance.

    Protein Binding and Clearance

    • High protein binding can decrease clearance by reducing the amount of free drug available for metabolism and excretion.

    Half-life (t1/2)

    • Half-life (t1/2) is the time required for the amount of drug in the body to decrease by 50% during elimination or during a constant IV infusion.
    • Half-life is useful for determining how long it takes for drug concentrations to reach 50% of their initial value or to reach 50% of steady-state concentration.

    Half-life (t1/2) and Steady State

    • For repeated oral administration, steady-state concentration (Css) is typically achieved after approximately four half-lives.

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    Pharmacokinetics (Distribution) PDF

    Description

    This quiz covers the various compartment models used in pharmacokinetics, including the one-compartment, two-compartment, and multi-compartment models. You'll explore how these models help in understanding the distribution of drugs within the body. Test your knowledge of drug absorption, distribution, metabolism, and excretion through this detailed overview.

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