Bioavailability

Definition

Bioavailability is a key pharmacokinetic parameter that quantifies how much of an administered dose actually reaches the systemic circulation in an unchanged (or therapeutically active) form. For intravenous administration, bioavailability is 100% by definition, as the entire dose is directly introduced into the bloodstream. For other routes of administration (oral, subcutaneous, intramuscular, intranasal), bioavailability is typically lower because the compound must be absorbed from the site of administration and may be subject to metabolism or degradation. Bioavailability is usually expressed as a percentage; for example, a peptide might have 20% bioavailability when given orally, meaning only 20% of the dose reaches systemic circulation intact. Bioavailability is measured through comparison of plasma concentration versus time curves following different routes of administration. Peptides, which are proteins, typically have low oral bioavailability because they are digested by proteolytic enzymes in the gastrointestinal tract, which is why most peptide medications are administered by injection.

Factors affecting bioavailability include the chemical stability of the compound in body fluids, its rate of absorption from the site of administration, and its susceptibility to first-pass metabolism (degradation by the liver before reaching systemic circulation). Injectable peptides often have higher bioavailability than oral formulations because they bypass the gastrointestinal tract and hepatic metabolism. However, subcutaneous bioavailability may be lower than intravenous because the peptide must be absorbed through tissue; different peptides have different absorption rates. Intramuscular injection of peptides may provide intermediate bioavailability and longer duration of action compared to subcutaneous injection, depending on the compound and injection site.

Bioavailability has practical importance for dosing and efficacy. If a compound has low bioavailability, a larger dose may be required to achieve therapeutic concentrations. Conversely, a compound with high bioavailability may produce strong effects at lower doses. For research applications, understanding bioavailability is essential for interpreting pharmacokinetic data and for comparing results across studies using different routes of administration. Researchers must ensure they account for bioavailability differences when comparing efficacy or safety across different dosing routes.