Detailed Explanation
Bioavailability is the percentage of an administered drug dose that reaches the bloodstream in its active, functional form. For intravenous injection, bioavailability is 100% by definition. For every other route — oral, subcutaneous, intranasal, transdermal — bioavailability is lower because the peptide must survive degradation and absorption barriers.
Oral bioavailability is the great unsolved challenge of peptide therapeutics. Most peptides taken by mouth are destroyed in the stomach (acid, pepsin) and small intestine (trypsin, chymotrypsin) before they can be absorbed. Even those that survive are poorly absorbed because their size and hydrophilicity prevent them from crossing the intestinal epithelium. Typical oral bioavailability for unmodified peptides is less than 1–2%.
Semaglutide (Rybelsus) broke this barrier by co-formulating with SNAC (sodium N-[8-(2-hydroxybenzoyl)amino]caprylate), an absorption enhancer that locally raises pH and promotes transcellular uptake in the stomach. Even so, oral semaglutide's bioavailability is only about 1% — meaning 99% of the dose is wasted. This is why subcutaneous injection remains the primary delivery route for most peptide drugs. Collagen peptides, by contrast, are approximately 90% bioavailable orally because their small size and hydrophilic nature allow efficient absorption in the small intestine.
Key Facts
- IV bioavailability = 100% by definition
- Oral peptide bioavailability typically <1–2%
- Destroyed by stomach acid and digestive proteases
- Oral semaglutide (Rybelsus): ~1% bioavailability using SNAC enhancer
- Collagen peptides: ~90% oral bioavailability due to small size (~3 kDa)
- Major barrier to oral peptide drug development
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