Peptide Bioavailability by Route: What the Research Literature Says About Subcutaneous, Intranasal, Oral and Topical Delivery

Bioavailability is the fraction of an administered dose that reaches systemic circulation intact, and for peptides it varies enormously by route. Oral peptide delivery is reported at well under 1% bioavailability because of gastric acid, digestive enzymes and a tight intestinal barrier, while subcutaneous injection is the route pharmacokinetic studies most often use as the higher-absorption reference for smaller peptides. This article summarizes what the published literature reports about oral, subcutaneous, intranasal and topical peptide delivery, and where dissolving-strip and buccal formats fit conceptually, for researchers designing study protocols.

Why do most peptides have near-zero oral bioavailability?

Peptides are chains of amino acids held together by bonds that digestive enzymes are built to break. A review of oral peptide and protein drug delivery describes three compounding barriers: an acidic gastric environment (pH roughly 1.5-2.5) that denatures peptide structure, proteolytic enzymes including pepsin, trypsin and chymotrypsin that cleave peptide bonds at specific residues, and an intestinal epithelium whose tight junctions restrict paracellular transport of anything beyond very small molecules (Barriers and Strategies for Oral Peptide and Protein Therapeutics Delivery, PMC). Larger, hydrophilic peptides cannot passively diffuse across the lipid-rich epithelial membrane the way small, lipophilic drugs can. The combined effect is that an orally dosed peptide is degraded, trapped in mucus, or too large to cross into the bloodstream, which is why oral peptide bioavailability is described in the literature as a fraction of a percent for most unmodified sequences.

Is there a published exception to the oral bioavailability problem?

Yes, and it is a heavily studied one: oral semaglutide, formulated with the absorption enhancer SNAC (sodium salcaprozate). Buckley et al. (2018, Science Translational Medicine) reported that in preclinical and clinical studies, absorption of the GLP-1 receptor agonist occurred specifically in the stomach, was confined to the area immediately adjacent to the dissolving tablet, and required co-formulation with SNAC. The mechanism described was transcellular (through cells) rather than paracellular (between cells via tight junctions), with SNAC transiently raising local gastric pH and facilitating membrane transit without evidence of an effect on tight junction integrity. Even with this enhancer, the FDA label for the approved product states that population pharmacokinetic modeling estimated the absolute oral bioavailability of semaglutide at approximately 0.4% to 1%. Semaglutide is an FDA-approved prescription medicine under its own labeling; the research-grade semaglutide lot page referenced on this site is not that medicine and is not for human or animal use. The SNAC case is notable precisely because reaching even a fraction of a percent of oral bioavailability required a specific formulation strategy and a molecule engineered for it, not a route that works generally for peptides.

What does the literature report about intranasal peptide delivery?

The nasal cavity is lined with a vascularized, thin mucosa that researchers have used as an alternative absorption surface to the gut. Two barriers dominate the intranasal literature: mucociliary clearance, which physically moves material out of the nasal cavity within minutes, and local peptidase activity that can degrade peptides before absorption occurs. A review of nose-to-brain delivery describes the nasal mucosa's architecture and clearance kinetics, noting that formulation strategy strongly affects how much material is cleared before absorption (Tailoring Formulations for Intranasal Nose-to-Brain Delivery, PMC). Despite these barriers, intranasal delivery is widely used in rodent studies of small peptides because it at least partly bypasses first-pass hepatic metabolism and the gastrointestinal barriers described above.

The nose-to-brain hypothesis for small peptides

Small heptapeptides such as Semax and Selank, both studied primarily as intranasal preparations, are frequently cited in the nose-to-brain literature. A rat study using radiolabeled Semax reported that roughly 0.093% of the total administered radioactivity per gram of tissue was detected in brain tissue within two minutes of intranasal dosing, with most of that signal corresponding to intact peptide rather than metabolites (Shevchenko et al., Russian Journal of Bioorganic Chemistry, 2006). Selank has been studied by the same route; a separate intranasal study in rodents reported measurable changes in central nervous system markers, consistent with the hypothesis that olfactory and trigeminal pathways allow direct transport from nasal mucosa toward the brain, alongside conventional systemic absorption. Compound-specific detail is covered on this site's Semax nasal spray research page and Selank nasal spray research page.

What are the limits of transdermal and topical peptide delivery?

The skin's outermost layer, the stratum corneum, is a densely packed lipid barrier built specifically to keep hydrophilic molecules out, and most peptides are hydrophilic. This is the central problem topical peptide formulations face in the published literature. GHK-Cu, the copper-binding tripeptide, is one of the more extensively studied peptides in a topical research context because of its long history in skin biology research. An ex vivo skin permeation study reported a permeability coefficient on the order of 2.4 x 10^-4 cm/h for GHK-Cu through dermatomed skin, with roughly 136 micrograms of copper per square centimeter permeating the tissue over 48 hours, while a larger amount (approximately 82 micrograms per square centimeter) stayed retained in the skin as a depot rather than passing through (Are We Ready to Measure Skin Permeation of Modern Antiaging GHK-Cu Tripeptide Encapsulated in Liposomes?, PMC). That retained fraction recurs across the topical peptide literature: unmodified peptides tend to accumulate in upper skin layers rather than achieve deep-dermal or systemic penetration, which is why enhanced-delivery research frequently tests encapsulation strategies such as liposomes or physical enhancers such as microneedles. Background on GHK-Cu's structure is covered on this site's GHK-Cu research page, and lot-specific material is listed on the GHK-Cu product page.

How does subcutaneous absorption differ from the other routes?

Subcutaneous administration is the route most pharmacokinetic peptide literature treats as the practical reference standard for reaching systemic circulation without the gastric and hepatic first-pass barriers that limit oral delivery. After deposition into the subcutaneous space, a molecule must transit the interstitial extracellular matrix before reaching a blood capillary or a lymphatic vessel. Population pharmacokinetic modeling describes this as a competition between two absorption pathways, with molecular size as a key variable: smaller peptides are absorbed predominantly through direct capillary uptake, while larger molecules rely more heavily on the slower lymphatic pathway (Population Physiologically-Based Pharmacokinetic Model Incorporating Lymphatic Uptake for a Subcutaneously Administered Pegylated Peptide, PMC). Local factors described in this literature, including blood flow at the injection site, injection depth, and local proteolytic activity, also affect how much of the administered material is absorbed intact rather than degraded at the site.

Where do dissolving strips and buccal delivery fit conceptually?

Buccal and sublingual mucosa sit between the extremes of the gut and the skin: thinner and more vascularized than gut epithelium, but still a barrier that restricts larger, hydrophilic molecules, and subject to dilution and washout from saliva. A dissolving-strip format is a delivery vehicle, not a route in itself; the strip dissolves and the peptide it carries still has to cross the oral mucosa the way any buccally or sublingually administered molecule would. This differs from oral tablet delivery discussed above, because the strip is designed to release its contents in the mouth rather than survive transit to the stomach. This site's format overview, Peptide Delivery Formats in Research, and the NAD+ dissolving strip research page, describe how these formats are used in a research setting without claiming a bioavailability figure not established in the literature reviewed here.

Frequently asked questions

Why can't peptides just be taken orally like small-molecule drugs?

Peptides are larger, more hydrophilic and structurally more fragile than typical small-molecule drugs. Gastric acid and digestive proteases degrade the peptide bond structure, and the intestinal epithelium's tight junctions block passive diffusion of molecules above a certain size and polarity, which is why the oral peptide literature reports bioavailability figures well under 1% for most unmodified sequences.

Does the SNAC-based approach used for oral semaglutide work for any peptide?

No. The mechanism described by Buckley et al. was specific to the formulation tested and required a compound engineered for stomach-localized, transcellular absorption alongside the SNAC enhancer. Even with that engineering, the FDA label for the approved product describes absolute oral bioavailability of approximately 0.4% to 1%, which the literature treats as a formulation achievement rather than evidence that oral delivery works broadly for peptides.

Is there good evidence that intranasal peptides reach the brain directly?

Rodent studies using radiolabeled Semax and gene-expression studies following intranasal Selank both report central nervous system detection or effects after intranasal dosing, which researchers cite as support for a nose-to-brain transport pathway alongside conventional systemic absorption. The sources reviewed for this article are preclinical, rodent-based pharmacokinetic and pharmacodynamic studies, not human bioavailability data.

Why does GHK-Cu mostly stay in the skin rather than passing through it?

Ex vivo skin permeation studies report that GHK-Cu is retained in skin tissue as a depot at a higher amount than the fraction that permeates fully through the tissue over 48 hours. This is attributed to the tripeptide's hydrophilicity meeting the lipid-based barrier of the stratum corneum, and it is a key reason researchers studying enhanced topical delivery test encapsulation and physical permeation-enhancement methods.

Why is subcutaneous injection used as the reference route in peptide pharmacokinetic studies?

Subcutaneous administration avoids the gastric and hepatic first-pass barriers that limit oral bioavailability, and pharmacokinetic modeling literature describes it as reaching systemic circulation via a combination of direct capillary and lymphatic uptake, with the balance between those two pathways depending largely on molecular size.

Are dissolving strips a separate route from oral tablets?

Conceptually yes. A dissolving strip is designed to release its payload in the mouth for buccal or sublingual absorption across oral mucosa, distinct from an oral tablet, which is designed to survive into the stomach or intestine. The two formats face different barriers (mucosal permeability and salivary washout versus gastric acid and intestinal proteolysis) even though both are taken by mouth.

References

1. Buckley ST, et al. "Transcellular stomach absorption of a derivatized glucagon-like peptide-1 receptor agonist." Science Translational Medicine, 2018.
2. FDA. RYBELSUS (semaglutide) tablets, prescribing information, Clinical Pharmacology section.
3. "Barriers and Strategies for Oral Peptide and Protein Therapeutics Delivery: Update on Clinical Advances." PMC.
4. "Tailoring Formulations for Intranasal Nose-to-Brain Delivery: A Review on Architecture, Physico-Chemical Characteristics and Mucociliary Clearance of the Nasal Olfactory Mucosa." PMC.
5. Shevchenko KV, et al. "Kinetics of Semax penetration into the brain and blood of rats after its intranasal administration." Russian Journal of Bioorganic Chemistry, 2006.
6. Inozemtseva LS, Karpenko EA, Dolotov OV, et al.. Intranasal administration of the peptide Selank regulates BDNF expression in the rat hippocampus in vivo. Dokl Biol Sci. 2008. PubMed 18841804.
7. "Are We Ready to Measure Skin Permeation of Modern Antiaging GHK-Cu Tripeptide Encapsulated in Liposomes?" PMC.
8. "Population Physiologically-Based Pharmacokinetic Model Incorporating Lymphatic Uptake for a Subcutaneously Administered Pegylated Peptide." PMC.

Research use only. Homegrown Peptides products are for laboratory research and are not for human or animal use. Nothing in this article is medical advice.