Pharmacokinetics and Bioavailability Overview

Key Concepts in Biopharmaceutics

• Biopharmaceutics: The field that studies how the chemical and physical properties of a drug, its formulation, and administration route influence the rate and extent of drug absorption into the bloodstream.

• ADME: Acronym representing the processes of Absorption, Distribution, Metabolism, and Excretion.

  • Absorption: Uptake of drug into the bloodstream.

  • Distribution: Movement of drugs from the absorption site to the target site, and sites of metabolism or excretion.

  • Metabolism: Chemical modification of a drug, primarily occurring in the liver.

  • Excretion: Elimination of the drug from the body.

  • Liberation: The passage of drugs out of the dosage form.

Bioavailability

• Definition: Refers to the rate and extent to which an active drug is absorbed and made available at the site of action.

  • Intravenous (IV): 100% bioavailability (total amount reaching systemic circulation).

  • Oral (PO): Typically less than 100% bioavailability.

• Absolute Bioavailability (F):

  • Calculated using the formula:
    $ext{Abs F} = rac{AUC<em>{test}}{AUC</em>{iv}}$

  • If expressed as a percentage:
    $F = rac{AUC<em>{test}/D</em>{test}}{AUC<em>{iv}/D</em>{iv}} imes 100$

  • Where $AUC$ signifies the area under the concentration-time curve and $D$ is the drug dose.

Factors Influencing Drug Absorption

• Co-administration: Impacts from food or other drugs which may affect absorption.

• Patient Population: Variations in disease state, age, and other demographics can affect drug absorption.

• Drug Properties: Chemical and physical properties, along with formulation, play a key role in how a drug is absorbed.

• Dosage Form: Different forms (tablet, liquid, sustained-release) can influence pharmacokinetics.

Therapeutic Windows

• Definition: The range between the minimum effective concentration and the maximum safe concentration of a drug.

  • Narrow Therapeutic Window: Small difference between effective and toxic concentrations; requires careful monitoring.

  • Wide Therapeutic Window: Larger safety margin allowing for variable absorption without risk of toxicity.

Plasma Concentration-Time Curves

• Key Terms:

  • Cmax: The maximum concentration of drug in plasma after administration.

  • AUC: Area under the curve, representing total drug exposure over time.

Relative Bioavailability

• Assessment: If a drug cannot be administered via the IV route, bioavailability can be compared to a standard formulation:

  • $F<em>{rel} = rac{AUC</em>{test}}{AUC_{standard}}$

  • Used to assess bioequivalence, indicating if two products have similar efficacy and safety profiles.

Dosage Form Considerations

• Steps in Drug Administration:

  1. Drug Release: Release from dosage form.

  2. Drug Dissolution: Dissolves in body fluids.

  3. Drug Stability: Maintains efficacy during storage and use.

  4. Membrane Permeation: Ability to pass through biological membranes.

  5. First Pass Metabolism: Initial metabolism of the drug before it enters systemic circulation, primarily affecting oral medications.

Learning Outcomes

• Understand the significance of biopharmaceutics in drug administration and effects.

• Differentiate how delivery routes impact drug exposure levels.

• Define key bioavailability terms and perform calculations to determine bioavailability efficiency based on data.