Intranasal Peptide Delivery: 5 Key Bioavailability Factors Explained

Introduction

Intranasal peptide delivery has gained significant attention as a non-invasive alternative to traditional injection methods. Therapeutic peptides typically have poor oral availability due to enzymatic degradation and low gastrointestinal absorption, so parenteral administration (especially subcutaneous injection) has been the mainstay for systemic delivery.

In recent decades, intranasal delivery has emerged as a route that bypasses first-pass metabolism and offers rapid absorption via the richly vascularized nasal mucosa. Several small peptide drugs (e.g. desmopressin and calcitonin) have been approved for intranasal use, but for larger peptide therapeutics, injections remain standard.

This report provides a comparative analysis of peptide bioavailability and pharmacokinetics when delivered intranasally versus subcutaneously, based on data from human trials (supplemented by animal studies where human data are limited). Transdermal routes are excluded due to a lack of viable data in the literature—the skin barrier has proven extremely difficult to overcome for peptides without invasive techniques.

We focus on five key areas:

1. Absolute bioavailability (%F) via intranasal and subcutaneous routes
2. The influence of molecular weight on peptide absorption
3. Differences in pharmacokinetic profiles (C_max, T_max, AUC) between the two routes
4. Formulation strategies (e.g. absorption enhancers and carriers) to improve intranasal uptake
5. Safety and tolerability considerations (nasal mucosal effects and injection site reactions)

The goal is to elucidate for which peptides intranasal delivery is a feasible alternative to injections, and what challenges must be addressed for larger molecules.

Absolute Bioavailability by Route of Administration

Absolute bioavailability (%F) is defined as the fraction of an administered dose that reaches systemic circulation intact, typically measured by comparing plasma AUC to an intravenous reference. Subcutaneous (SC) injection is generally very efficient for peptides, with high absolute bioavailability in the range of ~75–100% for most therapeutic peptides in both animals and humans.

Intranasal (IN) delivery, by contrast, shows highly variable and often much lower bioavailability, depending on the peptide’s properties and formulation. Table 1 summarizes representative data from comparative studies of intranasal vs subcutaneous administration for several peptides of varying sizes.

Table 1: Absolute Bioavailability of Selected Peptides via Intranasal vs Subcutaneous Routes

Sources: Representative data extracted from comparative pharmacokinetic studies (human and animal). Intranasal values are absolute bioavailability relative to IV unless otherwise noted. Subcutaneous values are absolute bioavailability relative to IV (or range across subjects).

Key Patterns in Bioavailability Data

Several clear patterns emerge from these data. Subcutaneous injection consistently provides high systemic availability across the board—for example, parathyroid hormone (PTH) 1–34 (teriparatide) given subcutaneously had ~77% absolute bioavailability in a sheep model, in line with ~95% observed in humans.

In contrast, intranasal bioavailability ranges from as high as ~70–80% for very small peptides (e.g. 800 Da octapeptide) down to essentially 0–2% for peptides of a few kilodaltons without special formulation. Molecules under ~1 kDa can achieve intranasal %F on the order of 50–70%, approaching the efficiency of injections. For example, an octapeptide (~800 Da) reached 73% via nasal dosing in a rat study.

Desmopressin (1.1 kDa, 9 amino acids), an active peptide hormone, is effectively delivered by the nasal route in humans—while its absolute F is not often reported, clinical dosing shows that only a few micrograms intranasally achieve therapeutic plasma levels, indicating a reasonable fraction absorbed. One study demonstrated that optimizing the delivery device (fine spray vs drops) for desmopressin can more than double the plasma exposure.

However, for peptides above roughly 3–4 kDa, intranasal absorption drops dramatically. Insulin (~5.8 kDa) is a classic example—early trials found only ~3% absorption intranasally without adjuvants, improved to about 8% with a potent phospholipid enhancer. Likewise, the 34-amino-acid PTH 1–34 fragment (4.1 kDa) achieved ≤1% nasal bioavailability in human trials despite formulation with an absorption enhancer.

Large proteins in the tens of kilodaltons (e.g. a 34 kDa test protein) exhibit essentially negligible uptake (0.5–1%) via the nasal route. These findings indicate that without special strategies, the nasal route is not viable for most larger therapeutic peptides, which typically fall in the 3–6 kDa range or higher (e.g. glucagon, growth hormone fragments, etc.).

In summary, subcutaneous injection delivers the vast majority of a peptide dose systemically (generally >75%), whereas intranasal delivery can be effective but is highly peptide-dependent. Small, potent peptides (sub-1 kDa, or up to ~10 amino acids) may achieve high nasal bioavailability, but as molecular size increases, %F plummets in the absence of absorption-enhancing measures.

Molecular Weight Thresholds and Peptide Absorption

Molecular weight (and molecular size/length, which correlates with MW for peptides) is the single strongest determinant of intranasal absorption efficiency. Across the studies reviewed, there is a striking inverse relationship between peptide size and intranasal bioavailability.

Peptides with molecular weight under ~1000 Da (roughly 5–10 amino acids) achieved on average ~70% bioavailability intranasally (in the absence of special enhancers). In one analysis, small peptides (~800 Da) were absorbed nasally almost as well as through IV or SC routes. But above a ~1 kDa threshold, bioavailability declines precipitously.

Even medium-sized peptides of 3–5 kDa typically have intranasal %F in the single digits or low teens at best. For instance, insulin (5.8 kDa) only reached ~8% with a permeation enhancer, and PTH 1–34 (4.1 kDa) was around 1% in human subjects. An experimental 34 kDa protein was practically unabsorbed at 0.6%.

Why Size Matters for Intranasal Peptide Delivery

Why does size matter so much? The primary transport mechanism for peptides across the nasal epithelium is believed to be paracellular diffusion (i.e. passage between cells through tight junctions) for hydrophilic compounds. Small molecules and tiny peptides can wiggle through the tight junctions relatively quickly.

Larger peptides, however, diffuse much more slowly—often too slowly to outpace the mucociliary clearance and enzymatic degradation that are constantly working to eliminate substances from the nasal cavity. In essence, there is a race between absorption and clearance: small peptides can permeate the epithelium and enter the bloodstream before being cleared, while large peptides are swept away or broken down before they manage to cross the mucosal barrier.

This dynamic explains the large discrepancy between the optimistic result of McMartin et al. (73% for an 800 Da peptide) and the disappointing result of Pearson et al. (≤1% for a 4 kDa peptide)—both are accurate, but applicable to different size regimes.

Expanding the Effective Size Limit

Notably, the practical upper size limit for significant intranasal absorption can be shifted slightly with powerful absorption enhancers. Some studies show that with certain permeation enhancers, peptides in the 4–6 kDa range can achieve a modest increase in uptake (several-fold higher than without enhancers).

For example, surface-active agents enabled nasal absorption of molecules up to ~6000 Da that would otherwise be largely impermeable. In one case, a cell-penetrating peptide allowed a 20 kDa interferon to reach ~11% nasal bioavailability, which is remarkably high for such a large protein.

These results suggest the effective molecular weight cutoff can be extended to some extent—perhaps into the low tens of kDa—but only with advanced formulation strategies, and even then the absorption remains far below that of injection. In summary, without special interventions, intranasal delivery is best suited for peptides roughly 3 kDa or smaller, while larger peptides (4–10 kDa and above) generally cannot achieve therapeutic systemic levels intranasally.

Pharmacokinetic Profiles: Intranasal vs Subcutaneous

In addition to total bioavailability, the pharmacokinetic (PK) profiles of intranasal versus subcutaneous administration can differ substantially. Key PK parameters—peak plasma concentration (C_max), time to peak (T_max), and exposure (AUC)—are influenced by the absorption kinetics of each route.

Onset and T_max

Intranasal delivery generally produces a faster absorption and earlier T_max compared to subcutaneous injection. The nasal mucosa is highly vascular and the diffusion distance to the bloodstream is small, so absorbed drug can appear in circulation rapidly—often within minutes.

For example, in a clinical study of PTH 1–34, the intranasal formulation reached peak levels in about 9 minutes, whereas the same peptide given subcutaneously peaked around 21 minutes post-injection. Insulin shows a similar trend: intranasal insulin (with enhancer) peaked at ~23 minutes on average, significantly faster than the subcutaneous route.

The rapid rise of intranasal absorption can be advantageous for achieving a quick onset of action—for instance, in emergency or acute settings (e.g. intranasal naloxone is used for rapid opioid reversal, intranasal glucagon for severe hypoglycemia, etc.).

Indeed, one study noted that intranasal insulin not only acted faster but also showed reduced inter-subject variability in absorption timing compared to SC (p < 0.001), likely because a metered nasal spray may have more consistent delivery than an injection which can be affected by tissue differences or technique.

C_max and Plasma Concentrations

A major drawback of intranasal delivery is the much lower C_max and plasma concentrations achieved for a given dose, relative to subcutaneous. Because only a fraction of the intranasal dose is absorbed, the peak concentration is correspondingly lower even if the nasal dose is equal or higher.

This was dramatically illustrated in the PTH 1–34 human trial: a 90 µg intranasal dose achieved a mean C_max of only 5 pg/mL, whereas a 20 µg subcutaneous injection (4.5-fold smaller dose) produced 253 pg/mL peak—over 50 times higher concentration.

In other words, the intranasal route required a much larger dose and still resulted in a far lower peak level. This is a general pattern: intranasal dosing leads to lower plasma peaks unless extremely high doses or potent enhancers are used.

AUC and Overall Exposure

The area under the curve (AUC) is directly related to bioavailability for the same dose. Thus, the AUC of intranasal administration is typically only a few percent of that of subcutaneous administration (when normalized for dose), in line with the %F values.

For example, if a peptide has 5% nasal bioavailability, the AUC from an intranasal dose will be ~5% of the AUC from the same dose given IV. In the PTH 1–34 study, the relative bioavailability of the nasal spray was reported as ≤1% in humans.

In practice, to achieve the same total exposure (AUC) intranasally as via injection, one would have to administer a much larger nasal dose—which may be limited by volume (nasal cavity can only hold ~100–200 µL per nostril comfortably) and by local side effects at high concentrations.

Distribution and Elimination

Once absorbed, the distribution and elimination phases of a peptide are generally route-independent—the molecule is the same and will have the same volume of distribution, clearance, and half-life regardless of how it entered the bloodstream.

For instance, an exenatide study found that the elimination rate constant (and half-life) was similar across IV, SC, and nasal routes (median k_e ~0.017 min⁻¹, corresponding to t_1/2 ~40 min). This indicates that the differences are primarily in the absorption phase.

In summary, intranasal delivery yields a faster T_max but much lower C_max and AUC than subcutaneous delivery for most peptides. The rapid absorption of nasal dosing can be beneficial for quick action, but achieving adequate exposure is challenging. Subcutaneous injections provide higher peaks and exposures, but with a moderately delayed onset due to absorption from the injection site.

Formulation Strategies to Improve Intranasal Bioavailability

Given the formidable barriers for intranasal peptide delivery (mucosal tight junctions, enzymatic degradation, rapid clearance by cilia), a variety of formulation and delivery strategies have been explored to boost nasal absorption. These include use of permeation enhancers, carrier molecules, mucoadhesive formulations, optimized devices, and chemical modifications.

Table 2: Formulation Strategies for Enhancing Intranasal Peptide Absorption

Each of these strategies addresses a different barrier in intranasal delivery:

Permeation Enhancers (Surfactants and Detergents)

These compounds (like Solutol HS15, bile salts, laureates, etc.) transiently increase the permeability of the nasal epithelium by interacting with cell membranes or tight junction proteins. Solutol HS15, for example, is a surfactant that was incorporated into both liquid and powder nasal formulations of PTH 1–34. Its mechanism involves altering cell membrane structure and the actin cytoskeleton to facilitate paracellular transport.

In preclinical studies (rats), a Solutol-formulated PTH spray achieved a very high relative bioavailability (~78% of SC). However, in the human trial, despite using the same excipient, the absolute F was still only ~0.5–1%, illustrating that success in animals did not fully translate to humans.

Other surface-active enhancers like chitosan or certain polymeric mucoadhesives have shown the ability to boost nasal absorption for peptides up to 5–6 kDa by opening tight junctions wider. The trade-off is that many surfactant-type enhancers can cause irritation.

Cell-Penetrating Peptides (CPPs)

CPPs are short peptides (often 10–30 amino acids, typically rich in arginine/lysine) that can ferry cargo across cell membranes. Penetratin is a well-studied CPP derived from an Antennapedia homeodomain. When co-administered intranasally with peptide drugs, penetratin can form non-covalent complexes or simply be present to help transiently destabilize membranes and promote transcellular uptake.

In a notable study, Khafagy et al. tested penetratin with GLP-1, exendin-4, and IFN-β via both nasal and intestinal routes. Nasally, penetratin was remarkably effective: GLP-1 absorption jumped to ~15.9% (versus near-zero without it), and even the large 20 kDa interferon saw ~11% absorption.

For comparison, the intestinal (jejunal) route with penetratin yielded only 0.17–5% for those peptides, highlighting that the nasal epithelium is inherently more permeable than the GI tract. CPPs have the advantage of being less damaging to membranes; penetratin at 0.5 mM did not cause obvious histological damage or ciliary loss in these studies.

Medium-Chain Fatty Acids and Phospholipids

These are classical absorption enhancers used in many transmucosal formulations. They insert into cell membranes and loosen tight junctions, increasing paracellular transport. Drejer et al. demonstrated that adding a medium-chain phosphatidylcholine (MCPC) to intranasal insulin enabled ~8% of the dose to be absorbed in healthy humans.

Without the phospholipid, prior attempts had only ~1–3% absorption. Importantly, the tolerability was good: nasal irritation was reported as absent or only slight despite the presence of this enhancer. Overall, MC fatty acids can produce a 2–5 fold increase in peptide bioavailability.

Mucoadhesive Formulations and Device Design

A significant cause of nasal delivery inefficiency is the rapid clearance of dosage forms from the nasal cavity (mucociliary clearance can remove substances in 15–30 minutes). Mucoadhesive polymers (e.g. chitosan, carbomers, gellan gum) are added to nasal formulations to make them stick to the mucosal surface longer, giving more time for absorption.

Delivery devices also play a crucial role: the nasal spray vs drops example with desmopressin is telling. A metered spray can deposit the formulation in the anterior nose (where it can adhere to the mucosa and slowly clear), whereas instilling drops often sends the liquid to the back of the nasal cavity or throat where it is swallowed.

By using an optimized spray device, researchers achieved significantly higher peptide absorption than with an unoptimized delivery. Modern nasal devices can create small droplets (~50 µm) that cover the nasal epithelium and avoid lung inhalation or immediate runoff.

Cyclodextrins

Cyclodextrins are cyclic oligosaccharides often used to improve drug solubility and stability. They can also enhance absorption by transiently extracting cholesterol from cell membranes, which can open tight junctions slightly. Kharuk et al. reported that adding dimethyl-β-cyclodextrin to an insulin nasal formulation raised its bioavailability from ~3% to ~5%, and using a dry-powder insulin with cyclodextrin achieved up to 13%.

PEGylation and Chemical Modifications

Another interesting approach is chemically modifying the peptide to improve its properties for nasal uptake. PEGylation (attaching polyethylene glycol chains) can mask peptide from proteases and increase its hydrophilicity and half-life on the mucosal surface. Other modifications include making more lipophilic prodrugs of peptides or using nanocarriers (liposomes, nanoparticles) to ferry peptides across.

In conclusion, formulation innovations can partially overcome the nasal barrier. The most effective single strategy in our dataset was the use of cell-penetrating peptides, achieving intranasal absorption levels (10–15%) that start to approach clinical relevance for moderate-sized peptides. Combining strategies (e.g. a CPP + mucoadhesive + optimal device) might further improve outcomes.

Safety and Tolerability Considerations

A crucial aspect of comparing intranasal and subcutaneous routes is their safety and tolerability profiles. Any delivery method must not only be effective but also safe for patients, especially if chronic administration is needed.

Local Nasal Tolerability

The nasal route, when using simple formulations, is generally well tolerated. Human studies of intranasal PTH 1–34 reported no serious adverse events, with subjects experiencing no sneezing, no significant irritation or discomfort from the nasal spray. This is notable because PTH nasal formulation did include a surfactant enhancer (Solutol HS15), yet at the dose used in humans it did not cause acute irritation.

Another example: intranasal insulin with a phospholipid enhancer resulted in no or minimal nasal irritation in healthy volunteers. Subjects sometimes reported a mild transient burning or sniffle, but overall tolerability was good and no lasting mucosal damage was noted on examination.

However, the picture is not universally rosy. Many penetration enhancers are known to irritate or damage the nasal mucosa if used in high concentration or repeatedly. A clinical review by Harris (1993) concluded that although nasal delivery of peptides is attractive, the aggressive absorption enhancers required for larger peptides tend to be poorly tolerated, causing irritation or even ciliostatic effects that could harm mucociliary clearance.

For instance, bile salt surfactants and SDS (sodium dodecyl sulfate) can cause epithelial shedding and inflammation at effective concentrations—clearly not acceptable for chronic use. In short, there is a safety trade-off with many enhancers: the more they enhance absorption, often the more they disrupt the nasal lining.

Mucosal Health and Long-Term Use

One major gap in the current evidence is data on chronic intranasal administration of peptides. Most studies to date have been single-dose or short-term. Thus, we do not know the long-term effects of daily intranasal peptide sprays on the nasal mucosa.

Potential concerns include: chronic irritation, development of nasal ulcers or lesions, mucosal remodeling (e.g. squamous metaplasia or loss of cilia) from repeated exposure to enhancers, and alterations in the microbiome or local immune response in the nasal cavity.

The reviewed studies explicitly note that long-term safety was not evaluated, representing a significant knowledge gap. Therefore, even if a formulation shows good acute tolerability, chronic dosing studies would be needed before widespread use.

Systemic Safety

Systemic side effects of the peptide itself should be comparable regardless of route, if equivalent plasma exposures are achieved. In practice, intranasal delivery often gives lower systemic exposure, so one might expect fewer systemic side effects for a given dose. No studies reported any unexpected systemic toxicity from intranasal delivery.

Injection Site Reactions

Subcutaneous injections, while generally safe, are not entirely without local issues. Repeated injections can cause bruising, pain, or induration at the injection sites. However, the data indicate that SC injections were well tolerated with minimal local reactions. In the PTH 1–34 trial, patients using a teriparatide injection pen had no significant injection site reactions.

The key point is that SC administration’s invasiveness is its main drawback—patients may dislike needles or have poor adherence—but the local tolerability is usually acceptable, and serious injection-site complications are rare.

Comparative Safety Summary

In comparing the two routes: Intranasal delivery avoids needles and thus eliminates needle-stick injuries and injection pain, which is a big advantage for patient comfort and compliance. No route showed any severe adverse effect in the reviewed studies—nasal irritation was at worst “slight” and injection site reactions were negligible.

Intranasal peptide delivery appears to have a favorable safety profile for acute use, especially for smaller peptides or when using mild formulation strategies. The main safety concerns arise with more potent absorption enhancers and the unknown effects of chronic daily nasal administration.

Discussion and Clinical Implications

Intranasal versus subcutaneous delivery of peptides represents a trade-off between patient convenience and pharmacokinetic performance. The evidence synthesized here indicates that intranasal delivery is a viable alternative for relatively small peptides (on the order of a few amino acids to a couple dozen amino acids), where absorption can be high enough to achieve therapeutic levels.

For these molecules—such as desmopressin, oxytocin (9 aa), or maybe certain peptide analogs—a nasal spray can offer nearly the same systemic exposure as an injection, with the advantages of pain-free administration and faster onset. Indeed, desmopressin has been available as a nasal formulation for decades for diabetes insipidus and bedwetting, validating that concept.

However, for larger, modern therapeutic peptides (e.g. glucagon-like peptide-1 agonists, PTH analogs, high-molecular-weight hormones), intranasal delivery in its current state cannot match the consistency and extent of absorption provided by subcutaneous injection.

Teriparatide is a case in point: despite substantial research, an intranasal teriparatide could not achieve more than 1–2% bioavailability in humans. To get the same clinical effect as a 20 µg SC teriparatide injection, one would need perhaps 20-fold higher dose intranasally, which might be impractical or unsafe.

Thus, subcutaneous administration remains essential for delivering adequate systemic doses of peptides above ~3–4 kDa, especially for chronic therapies where variability and partial efficacy would be unacceptable.

Molecular Weight Threshold Implications

The molecular weight threshold identified (roughly 1 kDa for efficient nasal uptake without help, up to ~5–6 kDa with strong enhancers) has important implications in drug development. When designing novel peptide therapeutics, if a non-invasive route is desired, keeping the molecule as small as possible (while retaining potency) could make intranasal delivery feasible.

Alternatively, high-potency peptides (effective at low plasma concentrations) might succeed intranasally despite low %F. For instance, calcitonin (3.4 kDa) has only ~3% nasal bioavailability, but it’s so potent in regulating calcium that a 200 IU intranasal dose works for osteoporosis treatment.

Alternative Routes: Pulmonary Delivery

One interesting alternative route to mention is pulmonary (inhalation) delivery of peptides. Though outside our main scope, one of the reviewed studies (Gedulin et al.) tested intratracheal administration of exenatide in animals and found up to 13.6% bioavailability—significantly higher than nasal.

The large surface area and permeability of the lung can potentially absorb peptides better than the nose, albeit with its own challenges. In fact, inhaled insulin (Exubera, Afrezza) has been commercialized, indicating that pulmonary route can deliver even a ~6 kDa peptide in meaningful amounts. The nasal route is more accessible and user-friendly, so it remains an attractive middle ground if its efficacy can be improved for larger peptides.

Future Formulation Improvements

Future formulation improvements will likely focus on combinations of strategies: for example, coupling a mild permeation enhancer with a mucoadhesive gel and an optimized spray device, possibly along with peptide engineering. Nanoparticle carriers (lipid or polymer nanoparticles) are also being explored to chaperone peptides across the epithelium.

From a safety perspective, any intranasal peptide intended for chronic use will require rigorous evaluation of nasal mucosa over time—including endoscopic exams and biopsies in long-term trials—to ensure no cumulative damage.

Conclusion

In conclusion, intranasal and subcutaneous routes each have distinct roles in peptide drug delivery. Intranasal delivery offers rapid absorption and convenient, non-invasive administration, but it is inherently constrained by peptide size and formulation factors, leading to low bioavailability for larger molecules.

Subcutaneous injection, while less convenient, provides consistently high bioavailability and thus remains necessary for ensuring adequate systemic exposure for most therapeutic peptides above a certain size. Current evidence supports intranasal use mainly for small peptides or niche applications where its advantages outweigh the lower efficiency.

Continued research into formulation technologies and absorption enhancers may expand the range of peptides that can be effectively delivered through the nose—bridging some of the gap between nasal sprays and the gold-standard injection. Until then, clinicians and drug developers must weigh the pros and cons of each route, considering the therapeutic context.

References

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