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Bioavailability

See also: pharmacokineticshalf-life

Bioavailability refers to the proportion of an administered dose that reaches systemic circulation and becomes available at the site of action. It is a key pharmacokinetic parameter that determines the effective dose of a substance.

Key Concepts

  • Absolute Bioavailability: The fraction of the administered dose that reaches systemic circulation unchanged
  • Relative Bioavailability: Comparison between two different formulations or routes of administration
  • First-Pass Metabolism: The extent to which a drug is metabolized by the liver before reaching systemic circulation

Factors Affecting Bioavailability

  • Route of Administration:

    • Intravenous (IV): ~100% bioavailability (bypasses first-pass metabolism)
    • Subcutaneous (SC): Typically 50-80% for peptides
    • Intramuscular (IM): Typically 60-90% for peptides
    • Oral: Often less than 50% due to first-pass metabolism and digestive breakdown

  • Formulation:

    • Molecular size and structure
    • Lipophilicity vs hydrophilicity
    • Carrier systems (liposomes, nanoparticles)
    • Stability in gastrointestinal tract

  • Chemical Properties:

    • Solubility
    • pKa (acid dissociation constant)
    • Protein binding
    • Molecular weight

Research Context

In peptide research, bioavailability is frequently studied because:

  • Different administration routes can produce vastly different pharmacokinetic profiles
  • Peptide stability varies by formulation and storage conditions
  • Individual variations in absorption and metabolism are significant
  • Understanding bioavailability is crucial for:
    • Dose determination
    • Comparing study results
    • Designing research protocols

Measurement

Bioavailability is typically expressed as a percentage (%) or as a fraction (0-1.0). It is calculated using area under the curve (AUC) comparisons between the test route and a reference route (usually IV).

Note: Bioavailability data in research settings may vary significantly from clinical use due to differences in formulation, purity, and administration technique. Always refer to primary literature for specific compound data.