Bioavailability
The proportion of an administered drug that reaches the systemic circulation and is available to produce its effect, expressed as a percentage. Intravenous administration has 100% bioavailability by definition. Subcutaneous peptide injection typically achieves 60-90%, while oral peptide bioavailability is usually below 1-2%.
Technical Context
Absolute bioavailability (F) is calculated as AUC(non-IV route)/AUC(IV) × 100%. Factors affecting peptide bioavailability: Oral: destruction by gastric acid and GI proteases, poor epithelial permeability, and first-pass metabolism yield F typically <1-2%. Oral semaglutide with SNAC achieves F ~0.4-1% — enough for efficacy at the 14mg daily dose because the GLP-1R requires very low concentrations. Subcutaneous: F is 60-90%, limited by incomplete absorption from the injection site and local degradation. Intramuscular: F is similar to SC but absorption may be faster. Nasal: F is 1-10% for peptides, limited by the nasal epithelial barrier and mucociliary clearance. Bioavailability directly determines the relationship between administered dose and drug exposure.