Pharmacokinetics

Pharmacokinetics Study Guide

1. Differentiate between pharmacokinetics and pharmacodynamics.

While the source context focuses primarily on the components of pharmacokinetics, it identifies pharmacokinetics as the study of how the body handles a drug through four specific processes: Absorption, Distribution, Metabolism, and Excretion (ADME).

2. Define the four different components of pharmacokinetics.

Pharmacokinetics is defined by the acronym ADME, which represents the movement and transformation of drugs within the body:

• Absorption: The process by which drugs enter the body; specifically, the movement of a drug from its site of administration into the central compartment.

• Distribution: The delivery of a drug from the systemic circulation to the various tissues of the body.

• Metabolism (Biotransformation): The chemical alteration of the drug, primarily occurring in the liver to inactivate the substance.

• Excretion: The process by which the drug and its metabolites are eliminated from the body per unit of time.

3. Define bioavailability.

Bioavailability is the relative amount of an administered drug that successfully reaches the systemic circulation. This value is influenced by several factors:

• First-pass metabolism

• Drug formulation

• Bioequivalence

• Route of administration

• Distribution

4. Explain how a drug’s ability to permeate membranes is critical to absorption and describe what factors can influence this.

Absorption requires the drug to move from the administration site into the central compartment, a process necessitated by membrane permeation. The factors that influence this ability and the overall rate of absorption include:

• Solubility: The drug's ability to dissolve.

• Concentration: The amount of drug present at the site.

• Blood flow: The rate of circulation at the absorbing surface.

• Absorbing surface: The total area available for the drug to pass through.

• Contact time: How long the drug remains at the absorbing surface.

• pH: The acidity or alkalinity of the environment, which affects drug ionization.

5. Describe different routes of administration and explain how they influence drug absorption and bioavailability.

The route of administration determines the barriers a drug must cross, significantly affecting the speed of absorption and the total bioavailability.

6. Explain plasma protein binding and how it affects drug distribution.

Many drugs bind reversibly to plasma proteins, primarily albumin, within the vascular compartment. This binding acts as a drug reservoir, which slows the onset of the drug and prolongs its duration of action.

• High Plasma Protein Binding: Results in the drug remaining in the plasma. This leads to a low Volume of Distribution (Vd) and a prolonged half-life. Examples include warfarin and diazepam.

• Displacement and Disease: Drugs like furosemide or valproate can displace other drugs (like warfarin) from albumin. Additionally, liver disease can lower albumin concentrations, altering drug distribution.

• Volume of Distribution (Vd): This is the measure of the apparent space in the body available to contain the drug.

  • Formula: V_d = \frac{\text{Dose (IV amount)}}{\text{Co (Initial concentration)}}

  • Large Vd: Indicates the drug is concentrated in the tissues (Tissue protein binding favors a larger Vd).

  • Small Vd: Indicates the drug remains in the extracellular fluid (ECF) or plasma.

7. Describe the blood-brain barrier and explain how it affects drug distribution.

The blood-brain barrier (BBB) is a site of drug exclusion, making it difficult for many substances to enter the central nervous system. Its restrictive nature is due to:

1. Tight junctions between endothelial cells (unlike the fenestrated junctions in other tissues).

2. Glia wrappings around the capillaries.

3. Low levels of drug-binding proteins in the cerebrospinal fluid (CSF).

4. Drug-metabolizing enzymes within endothelial cells (e.g., monoamine oxidases, cytochrome P-450s).

5. Efflux transporters that pump drugs out of the brain.

8. Explain the first-pass effect and how it affects the bioavailability of oral drugs.

The first-pass effect occurs when a drug is metabolized in the liver before it reaches the systemic circulation. Oral drugs are absorbed in the gastrointestinal tract and carried to the liver via the portal vein. If the liver inactivates a significant portion of the drug during this "first pass," the bioavailability is greatly reduced. Routes such as sublingual, transdermal, and intravenous bypass this effect.

9. Describe the metabolic processes that occur in the liver and explain the role of enzymes such as cytochrome P450.

The liver is the primary site of drug metabolism (biotransformation), generally aimed at drug inactivation.

• Phase I Biotransformation: Involves oxidation, reduction, and hydrolysis. Cytochrome P450 (CYP450) enzymes are the primary catalysts for oxidation. The major CYP enzymes include CYP3A (which metabolizes the largest percentage of drugs), CYP2D6, CYP2C, CYP1A2, and CYP2E1.

• Phase II Biotransformation: Involves conjugation, where polar groups (such as sulfate or glucuronic acid) are attached to the drug via transferase enzymes (e.g., Glucuronosyltransferase, acetyltransferase).

• Phase III: Post-phase II processes involving further metabolism and excretion.

10. Explain how enzyme inducers and inhibitors affect bioavailability.

Changes to the cytochrome P-450 system are a major source of drug interactions:

• Inducers: These increase the activity of CYP450 enzymes, which increases the metabolism of drugs. This results in lower therapeutic drug levels.

• Inhibitors: These decrease the activity of CYP450 enzymes, slowing metabolism. This results in raised therapeutic drug levels, which can lead to toxicity.

11. Describe prodrugs and explain why they are useful.

A prodrug is a pharmacologically inactive compound that is converted into an active drug through metabolic processes in the body. They are useful for delivering medications that might otherwise be ineffective or poorly absorbed in their active form.

• Example: Valacyclovir is a prodrug that is metabolized into an active antiviral drug.

12. Describe various routes of excretion for drugs.

Excretion is the removal of the drug and its metabolites from the body, primarily through the kidneys and liver.

• Kidney Excretion:

  • Glomerular Filtration Rate (GFR): Depends on drug size, charge, and protein binding. Drugs that are not protein-bound and not reabsorbed are eliminated at the creatinine clearance rate (125 mL/min).

  • Tubular Secretion: Occurs in the middle segment of the proximal convoluted tubule at a rate approaching renal plasma flow (660 mL/min). It utilizes specific transporters to move drugs into the urine.

• Liver Excretion: The liver also processes drugs for excretion through metabolic products.

13. Differentiate between first-order and zero-order elimination kinetics.

Elimination kinetics describe how the concentration of a drug in the body decreases over time:

• Zero-order Kinetics: The amount of drug eliminated is constant over time, regardless of the concentration. This results in a straight line on a graph of drug amount versus time.

• First-order Kinetics: A constant fraction (percentage) of the drug is eliminated per unit of time. The rate of elimination is proportional to the drug concentration, resulting in an exponential decay curve.

14. Define the half-life of a drug.

The half-life is a pharmacokinetic parameter related to a drug's elimination and duration of action. It is influenced by the Volume of Distribution and plasma protein binding; for example, drugs with high plasma protein binding generally have a prolonged half-life because they remain in the vascular compartment and are released slowly from their protein "reservoir."