Bioavailability (symbol F) is a pharmacokinetic parameter that quantifies the proportion of an administered dose of a compound that reaches the systemic circulation in an unchanged, active form. It is measured by comparing the area under the concentration-time curve (AUC) of a non-intravenous route against an intravenous reference dose, which is defined as F = 1.
Peptides administered orally typically show very low bioavailability because they are hydrolysed by gastric and intestinal proteases and have limited transport across the intestinal epithelium. Parenteral routes (subcutaneous, intramuscular) can achieve substantially higher F values; pegylation, lipidation and cyclisation are structural strategies used to raise it further.
Bioavailability defines the effective dose that reaches its target and is a central variable in any preclinical pharmacokinetic study of a research peptide.