Bioavailability
| Category | Glossary |
|---|---|
| Also known as | Systemic Bioavailability, Oral Bioavailability, F% |
| Last updated | 2026-04-13 |
| Reading time | 4 min read |
| Tags | pharmacologypharmacokineticsabsorptionglossary |
Overview
Bioavailability (often abbreviated as F or expressed as F%) is a pharmacokinetic parameter that describes the fraction of an administered dose of a substance that reaches the systemic circulation in an unchanged, active form. It is one of the most critical factors determining whether a compound will produce its intended biological effects at the target tissue.
By definition, a substance administered intravenously has 100% bioavailability, since it is delivered directly into the bloodstream. All other routes of administration — subcutaneous, intramuscular, oral, intranasal, transdermal — yield bioavailability values below 100% due to various barriers and degradation processes the compound must survive before reaching circulation.
Detailed Explanation
How Bioavailability Is Measured
Bioavailability is typically determined by comparing the area under the plasma concentration-time curve (AUC) for a given route of administration against the AUC for intravenous administration of the same dose:
F = (AUC_route / AUC_IV) x 100%
A compound with an oral bioavailability of 10% means that only one-tenth of the ingested dose ultimately reaches the bloodstream in active form. The remaining 90% is lost to degradation, poor absorption, or first-pass metabolism.
Factors Affecting Bioavailability
Several physiological and chemical factors determine a compound's bioavailability:
- First-pass metabolism: Orally administered compounds must pass through the gastrointestinal tract and liver before reaching systemic circulation. Hepatic enzymes can metabolize a significant portion of the compound during this first pass, dramatically reducing bioavailability.
- Enzymatic degradation: Proteases and peptidases in the stomach, intestinal lumen, and intestinal wall rapidly break down most peptides before they can be absorbed.
- Molecular size and polarity: Larger, more hydrophilic molecules have difficulty crossing biological membranes. Most peptides, being relatively large and polar, have poor membrane permeability.
- Stability in gastric acid: The low pH environment of the stomach denatures many peptide structures.
- Efflux transporters: P-glycoprotein and other membrane transporters can actively pump absorbed compounds back into the intestinal lumen.
Bioavailability by Route of Administration
| Route | Typical Bioavailability | Notes |
|---|---|---|
| Intravenous (IV) | 100% (by definition) | Reference standard |
| Intramuscular (IM) | 75–100% | Depends on blood flow at injection site |
| Subcutaneous (SC) | 60–95% | Depot effect; slower but sustained absorption |
| Intranasal | 10–50% | Bypasses first-pass; variable for peptides |
| Oral | < 1–5% (most peptides) | Extensive degradation; rare exceptions exist |
| Transdermal | < 1–10% | Limited by molecular weight and skin barrier |
Relevance to Peptide Research
Bioavailability is a central challenge in peptide science. Most peptides are composed of amino acids linked by peptide bonds that are readily cleaved by digestive enzymes, making oral administration largely ineffective. This is why the vast majority of peptides in research and clinical settings are administered via injection — typically subcutaneous injection — to bypass the gastrointestinal barrier entirely.
Several strategies are under active investigation to improve peptide bioavailability:
- Cyclization: Forming ring structures protects against proteolysis by exopeptidases and can improve membrane permeability.
- D-amino acid substitution: Replacing susceptible L-amino acids with D-enantiomers renders the peptide resistant to most natural proteases.
- PEGylation: Attaching polyethylene glycol chains shields the peptide from enzymatic degradation and reduces renal clearance.
- Nanoparticle encapsulation: Liposomal or polymeric carriers protect the peptide during transit through hostile environments.
- Permeation enhancers: Co-administration with absorption-enhancing agents can transiently increase intestinal permeability.
Examples
BPC-157 is a notable exception among peptides, as some research suggests it retains biological activity when administered orally, possibly due to inherent stability in gastric acid. However, its exact oral bioavailability has not been definitively quantified in human studies.
Semaglutide (a GLP-1 receptor agonist) represents a landmark achievement in oral peptide delivery, using a permeation enhancer (SNAC) to achieve clinically meaningful oral bioavailability — though its oral formulation still requires specific dosing conditions (fasting, minimal water) to maintain consistent absorption.
Related Terms
Understanding bioavailability requires familiarity with pharmacokinetics (the broader study of how compounds move through the body), half-life (how long a compound persists once in circulation), and molecular weight (which influences absorption characteristics). The process of lyophilization and reconstitution with bacteriostatic water are practical steps taken before administration that can influence the effective bioavailability of peptide preparations.
Related entries
- Bacteriostatic Water— Sterile water containing 0.9% benzyl alcohol as a preservative, used as the standard solvent for reconstituting lyophilized peptides and allowing multi-dose use from a single vial.
- Half-Life— The concept of biological half-life as it applies to peptide pharmacokinetics — how long a compound remains active in the body and its implications for dosing frequency.
- Pharmacokinetics— The study of how the body processes a drug or peptide over time — encompassing absorption, distribution, metabolism, and excretion (ADME) — which determines dosing schedules and effective concentrations.
- Subcutaneous Injection— A comprehensive overview of subcutaneous injection technique, the most common delivery method for research peptides, including site selection, proper technique, and safety considerations.