Across the forty excerpts a consistent, quantitative hierarchy of proteolytic threat emerges: oral » nasal ≈ sub-cutaneous injection site. The difference is large enough that the same peptide can be rendered undetectable in plasma after oral dosing yet reach >80 % of IV exposure when given nasally or sub-cutaneously without any formulation aid.
1. Oral delivery – systemic protease “gauntlet”
Therapeutic Peptides and Proteins is most explicit: “the entire GI tract … digests therapeutic proteins with the same efficiency as it digests dietary proteins.” Luminal serine proteases (trypsin, chymotrypsin, elastase), brush-border aminopeptidases and cytosolic proteasomes act sequentially. Peptide Drug Discovery and Development adds that cleavage is site-specific—leuprolide and insulin survive longest in the ileum/colon, but even there only a few percent remain intact. Nanocapsules or protease inhibitors raise bioavailability, yet the highest value cited for any peptide in vivo is ~10 % for insulin in rats; most stay <2 %. Thus, unless the molecule is cyclised or N-methylated (peptidomimetic), first-pass degradation is essentially quantitative.
2. Nasal cavity – modest surface peptidase activity
Therapeutic Peptides and Proteins again supplies the key numbers: for peptides <1 kDa, “intranasal bioavailability may approach that of an IV injection.” Metkephamid (a pentapeptide) reaches rat serum levels statistically identical to IV; calcitonin and LH-RH analogues achieve 10–30 % in humans without enhancers. Degradation still occurs—monolayers of human nasal epithelium show aminopeptidase N and neprilysin activity—but the total protease load is two to three orders of magnitude lower than in the gut, and residence time is <15 min, so only a few surface cleavages are possible. Enhancers (chitosan, bile salts) mainly open tight junctions; protease inhibition is usually unnecessary.
3. Sub-cutaneous depot – slow, local cleavage
The interstitial fluid surrounding a sub-cutaneous bolus contains mast-cell proteases, cathepsins and collagenases, but their concentration is low and diffusion into the depot is slow. Peptides: Chemistry and Biology notes that “proteolytic degradation occurs on the locus of the application,” yet the same passage states that even simple depot formulations give “significantly enhanced” half-life. Measurable loss is observed—salmon calcitonin degrades via deamidation and disulphide exchange at the injection site—but the fraction destroyed before reaching the lymph/blood is typically <20 %. Consequently, commercial peptides such as GLP-1 agonists are given sub-cutaneously once daily to once weekly without protease inhibitors.
4. Practical implications that appear in more than one source
– Patient preference drives development toward nasal or oral routes, but the bioavailability gap forces dosage to scale inversely: an oral tablet may need 50–100× the sub-cutaneous dose to match exposure.
– Chemical stabilisation (N-methylation, D-amino-acid substitution) is mandatory for oral delivery; it is optional for nasal and usually unnecessary for sub-cutaneous.
– Regulatory toxicology differs: oral peptides require evaluation of both the parent and the abundant peptide fragments generated in the gut; nasal and sub-cutaneous programs focus on the parent and a few minor deamidation/oxidation products.
– Cost-of-goods follows the same rank order: oral > nasal > sub-cutaneous because the first needs both high drug load and expensive formulation technology.
5. Surprising / counter-intuitive finding
Several excerpts (Handbook of Biologically Active Peptides, Peptide Protocols) reveal that peptide half-life inside cells can now be extended for hours by nano-carriers or structural “hitch-hiking,” yet none of these technologies change the extracellular protease hierarchy. In other words, we can engineer peptides to survive after absorption, but we still cannot make the gut behave like a nose or a sub-cutaneous depot.
6. Critical gaps / disagreements
None of the books quantify how much of the nasal loss is enzymatic versus mucociliary clearance; thus the exact contribution of nasal proteases remains uncertain. Likewise, sub-cutaneous degradation rates are reported only for a handful of peptides, all biased toward those stable enough to reach the market. Finally, while Peptides: Chemistry and Biology claims that “even parenteral application is often not efficient,” other texts show >80 % bioavailability for the same route, indicating that expert opinion diverges on what constitutes “efficient.”
References
- GHK and DNA Resetting the Human Genome to Health — Loren Pickart
- Handbook of Biologically Active Peptides
- Peptide Protocols Volume One — William A Seeds MD
- Peptide drug discovery and development _ Translational — edited by Miguel Castanho and
- Peptides_ Chemistry and Biology, 2nd Edition
- Therapeutic Peptides and Proteins Formulation
- Processing — Ajay K Banga, physrev 00027 2001