Glossary
Bioavailability
Bioavailability is the share of a given amount that enters the circulation unchanged. Why oral peptide values hover near 1% and injected ones reach about 80%.
Bioavailability (F) describes what proportion of an administered amount arrives in the systemic circulation in chemically intact form. Intravenous delivery equals 100% by definition; every other route is expressed against it, calculated from the ratio of the areas under the respective concentration-time curves. The value has to be measured for a given molecule, route and species — it cannot be deduced from the structure.
Why peptides make it difficult
Peptides are a particularly tough class. Their size, charge and hydrophilicity make membrane passage inefficient, and the intestinal lumen is packed with proteases that break them down before they can be absorbed. Oral bioavailability of an average peptide without help is usually reported at under 1%; that is why oral semaglutide depends on the absorption enhancer SNAC and still ends up in the low single digits. In animal models the subcutaneous route typically recovers most of the amount given, while intranasal and sublingual delivery sit in between and depend heavily on the formulation.
In study design the figure is important because formats can only be compared fairly once the route has been factored in. An amount listed for an oral capsule product is not the same input as the same amount in an injectable vial. The oral peptides guide covers the trade-offs between formats.
Related terms
half-life · lipophilic