Pharmacokinetics Key Parameters
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Questions and Answers

What does bioavailability measure?

  • The percentage of a drug that causes side effects.
  • The time it takes for a drug to be eliminated from the body.
  • The fraction of the administered dose that reaches systemic circulation. (correct)
  • The effectiveness of a drug in treating a specific condition.

Which of the following parameters are used to evaluate bioavailability?

  • Therapeutic index, absorption rate, and distribution half-life.
  • Rate of elimination, volume of distribution (Vd), and clearance (Cl).
  • Area under the serum concentration/time curve (AUC), maximum concentration (Cmax), and time to maximum concentration (Tmax). (correct)
  • Dosing frequency, peak plasma concentration, and average steady-state concentration.

How is bioavailability (F) calculated when comparing extravascular and intravenous doses?

  • F = (AUCPO/AUCIV)(DPO/DIV) (correct)
  • F = AUCPO / AUCIV while accounting for patient metabolism.
  • F = AUCIV / AUCPO
  • F = AUCPO - AUCIV divided by the volume of distribution.

What is required for a generic drug to be considered bioequivalent to a branded drug?

<p>It must produce a similar serum concentration/time profile to the branded drug. (A)</p> Signup and view all the answers

What happens to a portion of an orally administered drug that does not dissolve?

<p>It is eliminated in the stool. (C)</p> Signup and view all the answers

What happens to the free fraction of a drug when a second, highly protein-bound drug is administered concurrently?

<p>It increases due to displacement. (B)</p> Signup and view all the answers

If a drug is present in a total of 3 g and results in a concentration of 0.15 g/L, what is the volume of the tank?

<p>30 L (D)</p> Signup and view all the answers

How is the volume of distribution calculated when a drug is administered intravenously?

<p>By dividing the plasma concentration by the administered dose. (C)</p> Signup and view all the answers

What loading dose should be recommended for achieving a drug level of 2 μg/mL given a volume of distribution of 10 L?

<p>20 mg (A)</p> Signup and view all the answers

What effect does the elimination rate constant have on drug dosing if administered at a rate of 2 mg/hr?

<p>It predicts the time for the drug to reach steady state. (C)</p> Signup and view all the answers

Why is protein binding important in pharmacokinetics?

<p>It determines the amount of drug that is free and active. (B)</p> Signup and view all the answers

Which of the following statements about volume of distribution is true?

<p>It helps to determine how extensively a drug distributes in body tissues. (A)</p> Signup and view all the answers

What is the formula for hepatic clearance?

<p>ClH = LBF x ERH (D)</p> Signup and view all the answers

Which factor does not influence hepatic clearance?

<p>Renal blood flow (C)</p> Signup and view all the answers

Which statement is true concerning the clearance of a drug?

<p>Cl = ClH + ClR represents total clearance. (A)</p> Signup and view all the answers

What does intrinsic clearance (Cl'int) specifically represent?

<p>The inherent ability of the enzyme to metabolize the drug (A)</p> Signup and view all the answers

Which of the following contributes to an increase in hepatic clearance (ClH)?

<p>An increase in liver blood flow (C)</p> Signup and view all the answers

What do Vmax and Km represent in the context of drug metabolism?

<p>Vmax is the maximum rate of drug metabolism, and Km is the concentration at which half-maximal rate occurs. (B)</p> Signup and view all the answers

Which of the following formulas represents renal clearance?

<p>ClR = RBF x ERR (B)</p> Signup and view all the answers

What factor is represented by 'fB' in the formula for hepatic clearance?

<p>The unbound drug concentration in the bloodstream (C)</p> Signup and view all the answers

How does an increase in the renal excretion ratio (ERR) affect renal clearance (ClR)?

<p>It increases renal clearance. (A)</p> Signup and view all the answers

What is the primary reason clearance is considered the most important pharmacokinetic parameter?

<p>It determines the maintenance dose required for steady-state serum concentration. (A)</p> Signup and view all the answers

Which organ is primarily responsible for the Phase I metabolic reactions?

<p>Liver (B)</p> Signup and view all the answers

What does the extraction ratio (ER) measure?

<p>The ability of an organ to remove or extract drug from the blood. (A)</p> Signup and view all the answers

Which enzyme system is primarily responsible for Phase I drug metabolism?

<p>Cytochrome P-450 (CYP) enzyme system (A)</p> Signup and view all the answers

How is the maintenance dose (MD) expressed in pharmacology?

<p>mg/h or µg/min (D)</p> Signup and view all the answers

Which of the following is involved in Phase II reactions of drug metabolism?

<p>Formation of glucuronide or sulfate esters (A)</p> Signup and view all the answers

What is the formula for calculating clearance (Cl) based on organ function?

<p>Cl = Blood flow x Extraction ratio (ER) (D)</p> Signup and view all the answers

What happens to the metabolite generated in Phase I reactions?

<p>It is made more water-soluble for easier elimination. (A)</p> Signup and view all the answers

Which of the following best describes the function of the kidney in drug elimination?

<p>It filters and secretes drugs through the nephron. (B)</p> Signup and view all the answers

Flashcards

Bioavailability

The fraction of the administered drug dose that reaches the systemic circulation.

Absorption

The process by which a drug enters the bloodstream from the administration site.

AUC (Area Under the Curve)

The area under the curve of the drug concentration versus time graph, reflecting the total drug exposure in the body.

Cmax

The maximum concentration of a drug reached in the blood after administration.

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Tmax

The time it takes for the drug concentration to reach its peak.

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Volume of Distribution (Vd)

The volume of distribution (Vd) is a pharmacokinetic parameter that describes the apparent volume into which a drug distributes in the body after administration.

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Volume of Blood (VB)

The volume of blood (VB) represents the total volume of blood circulating within the body, typically around 5 liters for an average adult.

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Free Fraction (fB)

The free fraction (fB) indicates the proportion of a drug that is not bound to proteins in the plasma and therefore available for pharmacological action.

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Unbound Drug Concentration in Tissue (fT)

The unbound drug concentration in the tissue (fT) represents the amount of a drug that is not bound to proteins within the tissue cells and is available for interaction with receptors or other targets.

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Size of Tissues and Organs (VT)

The size of various tissues and organs of the body, measured in units of volume (VT), reflects the total space occupied by these components within the organism.

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Calculating Volume of Distribution (Vd)

The volume of distribution (Vd) is an important pharmacokinetic parameter that reflects how a drug is distributed throughout the body. It is calculated as Vd = (fB/fT) x VT + VB, where fB is the free fraction in the blood, fT is the unbound drug concentration in tissue, VT is the volume of tissues, and VB is the volume of blood.

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Drug Competition for Plasma Protein Binding

When two drugs highly bound to plasma proteins are given concurrently, they compete for binding sites, increasing the free fraction of the first drug. This can lead to an increase in its volume of distribution and potential drug interactions.

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Clearance (Cl)

The most important pharmacokinetic parameter that determines the dosage needed to maintain a desired drug level in the body.

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Clearance

The amount of drug that is eliminated from the body per unit time. It is a measure of how quickly a drug is removed from the bloodstream.

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Metabolism

The process of converting a drug into a more water-soluble form, making it easier to eliminate from the body.

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Liver

The primary organ responsible for drug metabolism. It contains enzymes that modify drug molecules.

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Cytochrome P-450 (CYP) enzyme system

A group of enzymes found in the liver that catalyze phase I reactions in drug metabolism. They oxidize drug molecules to make them more water-soluble.

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Hepatic Clearance (ClH)

The rate at which a drug is cleared from the body by the liver.

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Phase II reactions

A process that adds polar groups to drug molecules, further increasing their water solubility for excretion.

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Hepatic Extraction Ratio (ERH)

Represents the fraction of drug extracted from the liver blood flow during a single pass through the liver.

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Kidney

The primary organ responsible for excreting drugs and metabolites.

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Intrinsic Clearance (Cl'int)

The intrinsic ability of an enzyme to metabolize a drug.

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Elimination

The process of eliminating drugs and their metabolites from the body.

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Extraction ratio (ER)

The proportion of a drug that is removed from the blood or serum in a single pass through an organ.

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Unbound Fraction (fB)

The proportion of drug in the bloodstream that is not bound to proteins, and is therefore free to interact with its target.

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Km

The concentration of unbound drug at which the metabolic rate is half of Vmax.

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Renal Clearance (ClR)

The rate at which a drug is cleared from the body by the kidneys.

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Renal Extraction Ratio (ERR)

Represents the fraction of drug extracted from the renal blood flow during a single pass through the kidneys.

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Total Clearance (Cl)

The combined clearance of a drug from the body by all organs, including the liver and kidneys.

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Study Notes

Pharmacokinetics Parameters

  •  Pharmacokinetics (PK) is the study of how drugs enter, travel through, and leave the body.
  •  Key PK parameters include bioavailability, volume of distribution, clearance, and half-life.
  •  Bioavailability: Fraction of administered dose reaching systemic circulation. Affected by drug administration route, rate of absorption, and presystemic metabolism.
  •  Volume of distribution (Vd): Theoretical volume in which a drug could distribute if evenly dispersed. A larger Vd implies a greater drug distribution into tissues beyond the bloodstream.
  •  Clearance (Cl): Volume of plasma that is completely cleared of drug per unit time. High clearance often suggests rapid elimination.
  •  Half-life (t1/2): Time for drug concentration to decrease by half. Shorter half-life indicates faster elimination.

Bioavailability

  •  Bioavailability (F) is the fraction of a drug that reaches the systemic circulation after administration.
  •  Expressed in percentages (%).
  •  Measured using AUC (area under the curve) from extravascular and intravenous administration after the same dose.
  •  Determined by absorption, distribution, and metabolism.
  •  Drug absorption process is a critical factor determining bioavailability.
  •  Oral bioavailability is affected by factors such as solubility, stability, and GI transit time.

Absorption

  •  Absorption is the process where a drug moves from the site of administration into the bloodstream.
  •  The drug must cross biological membranes before entering the vascular system.
  •  Factors influencing absorption include route of administration, drug solubility, and the presence of transport mechanisms.

Bioequivalence

  •  Bioequivalence assessment is necessary for generic drugs.
  •  Determining serum concentration/time profiles for generic and original drugs under similar conditions.
  •  Bioequivalents need to have similar serum concentration/time profiles (AUC, Cmax, Tmax).
  •  Generic medications match bioavailability, concentration, and timing of the absorption of the original drug.

Volume Distribution

  •  Volume of distribution (Vd) is a theoretical value representing the apparent volume into which a drug distributes in the body.
  •  Calculated using the initial drug amount and concentration,
  •  Vd is not equal to the actual volume.
  •  Factors influencing Vd include drug properties, plasma protein binding, and tissue binding.
  •  It is essential to understand the relationship between physiological characteristics and drug distribution.

Metabolism and Elimination Clearance

  •  Clearance (Cl) is the volume of plasma that is completely cleared of a drug per unit time.
  •  Clearance describes the amount of drug cleared from the blood.
  •  The rate of elimination can vary depending on the location of metabolic processes.
  •  Factors influencing clearance include hepatic and renal blood flow and metabolic activity.

Half-Life and Elimination Rate Constant

  •  Half-life (t1/2) is the time required for the drug concentration in the body to decrease by half.
  •  Elimination rate constant (Ke): Fraction of drug removed from the body per unit time.
  •  Clearance equals Ke multiplied by Volume of distribution.

Clinical Applications of Elimination Rate Constant and Half-life

  •  Estimating steady state.
  •  Predicting non-steady-state plasma levels following initiation of an infusion.
  •  Predicting fluctuations in plasma concentration within a dosing interval.

Relationship Among Pharmacokinetic Parameter

  •  Half-life is influenced by clearance and volume of distribution.
  •  These are independent variables because one can vary without the other.

Exercise Problems

  • Several exercise problems on calculating bioavailability, clearance, volume of distribution, and half-life are given.
  •  These allow for practical application of the concepts.

Michaelis-Menten Pharmacokinetics

  •  Drug metabolism by enzymes; e.g., cytochrome P-450.
  •  Non-linear pharmacokinetics.
  •  Steady-state drug concentrations increase disproportionately with rising dosage.
  •  Enzyme and drug reactions create concentration-dependant clearance.

Zero-Order Pharmacokinetics

  •  Constant rate of metabolism regardless of concentration.
  •  Clearance decreases when concentrations rise, eventually approaching saturable conditions.

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Pharmacokinetics Parameters PDF

Description

Dive into the essential aspects of pharmacokinetics, including critical parameters such as bioavailability, volume of distribution, clearance, and half-life. This quiz will test your understanding of how these factors influence drug action and elimination in the body.

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