How Peptides Can Cross Buccal and Sublingual Mucosa

Peptides can cross buccal and sublingual mucosa through several potential pathways, including movement through epithelial cells, movement between cells, and transport assisted by formulation components that modify peptide-tissue interactions or epithelial permeability. Which pathway dominates depends on peptide size, charge, hydrophilicity, conformation, formulation, epithelial structure, contact time, and the specific oral site being studied. Because peptides can also bind to tissue or undergo enzymatic degradation, detecting material within mucosa does not automatically establish transport of intact peptide across the complete barrier.

Transport pathways provide the mechanistic foundation of Buccal and Sublingual Peptide Delivery Research. A peptide applied to oral mucosa must first leave its dosage form, move through saliva or the hydrated formulation layer, interact with the epithelial surface, cross the tissue barrier, and remain sufficiently intact to reach the opposite side.

Research-use notice: This article examines how peptides can cross buccal and sublingual mucosa, including transcellular, paracellular, passive-diffusion, tissue-retention, and formulation-assisted transport pathways. InStrips products are supplied solely for research and analytical use and are not intended to diagnose, treat, cure, or prevent peptide absorption disorders, oral mucosal disease, systemic disease, digestive conditions, or any other medical condition.

Researchers therefore need to distinguish several events that can occur after a peptide contacts oral tissue: surface binding, epithelial uptake, penetration into superficial layers, complete transmucosal permeation, and degradation within the tissue are not equivalent outcomes.

Transport Begins Before the Peptide Reaches the Epithelium

A peptide incorporated into a film or other oromucosal dosage form must first become available at the tissue surface.

This can require:

• film hydration
• peptide dissolution
• diffusion through the polymer matrix
• movement through the local salivary layer

A peptide that remains trapped within a formulation cannot cross the mucosa regardless of its intrinsic permeability.

Release and Permeation Are Separate Measurements

A formulation can release nearly all of its peptide into surrounding fluid while very little crosses oral epithelium.

Conversely, a formulation designed to maintain concentrated tissue contact can produce measurable permeation even when release into bulk fluid appears slower.

Researchers therefore should not use dissolution or release data as a substitute for mucosal transport data.

Buccal and Sublingual Tissue Are Related but Not Identical Barriers

Both sites are lined by stratified squamous epithelium, but they differ in anatomical and physiological properties.

Variables include:

• epithelial thickness
• degree of keratinization
• vascularization
• salivary exposure
• available contact area
• movement during speech and swallowing

Transport measured at one site should therefore not automatically be assigned to the other.

Sublingual Mucosa Is Generally Thinner

The tissue beneath the tongue is commonly considered relatively permeable compared with thicker buccal epithelium.

This can make sublingual delivery attractive for compounds capable of crossing epithelial barriers rapidly.

Buccal Mucosa Can Offer Longer Contact Opportunities

The inner cheek provides a larger and relatively accessible surface that can support:

• mucoadhesive films
• patches
• tablets
• other localized dosage forms

Longer contact can partly compensate for lower intrinsic permeability when formulation design is appropriate.

Peptide Size Creates an Important Transport Constraint

Peptides are generally larger than conventional small-molecule drugs.

Increasing molecular size can reduce passive epithelial transport because larger molecules experience:

• greater steric restriction
• slower diffusion
• greater difficulty passing narrow intercellular pathways

Hydrophilicity Can Limit Membrane Partitioning

Cell membranes contain lipid-rich domains.

Highly hydrophilic peptides generally partition poorly into these environments, making direct passage through epithelial cells difficult.

Increasing Lipophilicity Does Not Guarantee Better Peptide Transport

Oral mucosal permeation also depends on:

• molecular size
• charge
• conformation
• tissue binding
• aqueous solubility

A peptide cannot be predicted from lipophilicity alone.

The Transcellular Route Passes Through Epithelial Cells

Transcellular transport requires a molecule to cross cellular membrane barriers.

A simplified path is:

• enter the apical membrane
• move through or across the cell
• exit through the basolateral side

For large hydrophilic peptides, this route can be highly restrictive without special molecular or formulation features.

Transcellular Does Not Necessarily Mean Simple Lipid Diffusion

Material can enter cells through several processes.

Depending on the formulation, researchers may investigate:

• membrane partitioning
• endocytosis
• cell-penetrating-peptide-associated uptake
• vesicular transport

Each requires different experimental evidence.

Cellular Uptake Alone Does Not Demonstrate Complete Transcellular Transport

A peptide can enter an epithelial cell and remain:

• inside endosomes
• associated with membranes
• degraded intracellularly

without reaching the opposite side of the tissue.

The Paracellular Route Passes Between Cells

Paracellular transport occurs through spaces between neighboring epithelial cells.

This route is influenced by:

• cell-cell junctions
• intercellular lipids
• molecular dimensions
• charge interactions

Oral Epithelium Is Multilayered

Buccal and sublingual mucosa differ from simple intestinal monolayer cell models because oral epithelium contains several layers of cells.

A peptide traveling between cells must therefore negotiate a three-dimensional intercellular path rather than one junctional barrier.

Intercellular Lipids Contribute to the Buccal Barrier

Studies of oral mucosal permeability have identified organized lipid material in superficial epithelial layers as an important barrier component.

These lipids can restrict movement of hydrophilic macromolecules even when the epithelium is nonkeratinized.

Tight-Junction Proteins Are Present in Buccal Epithelium

Proteins including:

• ZO-1
• claudins

have been identified across multiple layers of buccal mucosa.

Their organization differs from a simple monolayer epithelial system.

The Barrier Is Therefore More Complex Than One Junction

A peptide may encounter:

• mucus
• surface epithelium
• intercellular lipid regions
• junctional proteins
• deeper epithelial layers

before reaching underlying tissue.

Passive Diffusion Is Commonly Used to Describe Baseline Peptide Transport

Classic oral-mucosal literature indicates that passive diffusion is a major transport mechanism for peptides and proteins.

This means movement occurs down a concentration gradient rather than through a dedicated peptide transporter.

Carrier-Mediated Transport Should Not Be Assumed

For many experimental peptides, there is no evidence that oral mucosa contains a transporter designed specifically to carry the intact peptide across the epithelium.

A measured flux should therefore not automatically be described as active transport.

Concentration Gradients Drive Passive Flux

Increasing donor concentration can increase the amount crossing per unit time if:

• the barrier remains unchanged
• the peptide remains soluble
• transport does not become saturated through another mechanism

Linear Flux Can Support Passive-Diffusion Interpretation

If steady-state flux rises proportionally with donor concentration, this can support passive transport under the tested conditions.

It does not by itself identify whether the dominant path is transcellular or paracellular.

Charge Can Change Interaction With Both Cells and Intercellular Pathways

A peptide's net charge depends on:

• amino-acid composition
• terminal groups
• environmental pH

Charged peptides may interact differently with:

• cell membranes
• mucins
• junctional regions
• formulation polymers

Conformation Adds Another Variable

Two peptides with similar molecular weight can occupy different effective shapes.

One may be:

• extended
• compact
• cyclic
• partly folded

which can influence interaction with barriers.

Enzymatic Stability Can Change Apparent Permeation

Oral mucosa contains enzymes capable of degrading selected peptides.

Therefore, researchers may detect:

• intact peptide
• degradation fragments
• both

during a permeation experiment.

A Classic Endomorphin Study Demonstrates This Problem

Endomorphin-1 was shown to degrade substantially in the presence of porcine buccal epithelium during an extended incubation.

The same study also measured intact peptide permeation across the tissue.

Degradation and Permeability Were Not the Same Limiting Step

In that experiment, inhibiting a major degrading enzyme improved peptide stability but did not increase transmucosal permeation substantially.

This illustrates that:

• stability
• epithelial transport

can be independent formulation barriers.

Peptidase Inhibitors Address Stability Rather Than Barrier Permeability

If a peptide is degraded at the epithelial surface, an enzyme inhibitor may preserve more intact material.

That does not necessarily make the intact peptide cross epithelial layers more readily.

Permeation Enhancers Address a Different Limitation

Enhancers are intended to modify epithelial permeability or peptide-tissue interaction.

They can act through mechanisms involving:

• cell membranes
• intercellular pathways
• junction-associated structures

Different Enhancers Can Favor Different Pathways

A bile salt may affect barrier structure differently from:

• a cell-penetrating peptide
• a surfactant
• a thiolated polymer

Mechanism should therefore be tested rather than inferred from increased flux alone.

Cell-Penetrating Peptides Can Increase Cellular Association

Cell-penetrating peptides have been investigated as tools for increasing buccal uptake of larger therapeutic peptides and proteins.

Their presence can increase:

• cellular internalization
• tissue penetration
• measured transmucosal flux

depending on the system studied.

Insulin Research Provides a Mechanistic Example

Studies using cell-penetrating peptide-insulin constructs have reported increased uptake and transport across multilayer buccal models.

Mechanistic experiments with endocytosis inhibitors can help determine whether uptake is:

• energy dependent
• energy independent
• electrostatically associated

Nanocarriers Can Introduce More Than One Transport Route

A peptide loaded into a lipid or particulate carrier may move differently from free peptide.

Possible observations include:

• carrier penetration
• release of peptide within tissue
• transcellular uptake
• paracellular passage

Carrier Transport Does Not Automatically Equal Intact Peptide Transport

If fluorescence appears on the receiver side, researchers still need to determine what crossed:

• free peptide
• degradation fragments
• carrier-associated peptide
• intact carrier

Fluorescence Can Be Useful but Requires Chemical Confirmation

Fluorescent labeling can show:

• where material accumulates
• how deeply it penetrates
• whether cells internalize it

but the label may remain detectable even if the parent peptide changes chemically.

Radiolabeling Has Similar Interpretive Limits

A radioactive signal can quantify material movement very sensitively.

It does not automatically prove that the signal still belongs to intact peptide.

Chromatography Can Help Confirm Molecular Identity

Methods such as HPLC can separate:

• intact peptide
• degradation products

and therefore strengthen interpretation of permeation experiments.

Mass Spectrometry Can Add Greater Structural Specificity

Where sensitivity permits, mass-spectrometric analysis can help establish that a transported molecular species retains the expected peptide mass.

Ex Vivo Diffusion Cells Measure Complete Tissue Transport

Researchers may mount excised oral tissue between:

• a donor chamber
• a receiver chamber

and quantify how much material reaches the receiver over time.

Franz and Ussing-Type Systems Are Common

Different diffusion apparatuses allow investigators to control:

• temperature
• surface area
• donor concentration
• receiver conditions

while studying tissue permeability.

Tissue Integrity Must Be Verified

Damaged epithelium can produce artificially high permeability.

Researchers may use:

• electrical resistance
• histology
• marker permeation

to identify compromised tissue.

Apparent Permeability Provides a Normalized Transport Metric

Researchers commonly calculate an apparent permeability coefficient from:

• steady-state flux
• donor concentration
• membrane area

This helps compare formulations or molecules under matched experimental conditions.

Papp Is Model-Specific

A Papp value obtained using:

• porcine buccal mucosa

cannot be assumed to equal permeability across:

• human buccal mucosa
• sublingual mucosa
• a cell monolayer

Research Note: Buccal Peptide Transport Combines Permeability and Stability Questions

A primary porcine-buccal study examined both stability and in vitro permeation of the peptide endomorphin-1. The researchers found substantial tissue-associated degradation while also measuring low but detectable intact-peptide permeability. Enzyme inhibition improved stability without producing a corresponding increase in permeability.

This experiment illustrates why the phrase peptide crossed the mucosa requires more detail. Researchers need to know whether the peptide remained intact, how much crossed, which barrier process limited transport, and whether the tissue itself altered the peptide during the experiment.

Transcellular Transport Requires Its Own Evidence

When researchers propose that peptide crosses through epithelial cells, they need measurements capable of distinguishing cellular uptake from simple intercellular movement.

Those approaches are examined in Transcellular Peptide Transport Across Oral Epithelium: What Researchers Measure.

What Oral-Mucosal Transport Studies May Establish

A well-designed experiment may establish that under its conditions:

• intact peptide crosses oral mucosa
• transport rate differs among formulations
• peptide accumulates within tissue
• one enhancer increases apparent permeability
• degradation occurs during mucosal contact
• buccal and sublingual models behave differently

What They Do Not Establish

These findings do not independently establish:

• human systemic bioavailability
• clinical effectiveness
• the exact transport route unless directly investigated
• equivalent transport across buccal and sublingual mucosa
• that every detected signal represents intact peptide
• equivalent results with a different formulation
• performance of a finished commercial product

Transport Is a Sequence of Barriers, Not One Permeability Number

A peptide must be released from its formulation, remain stable, reach the epithelial surface, interact with the barrier, cross one or more epithelial layers, and emerge intact on the other side before complete transmucosal transport has occurred.

Buccal and sublingual research therefore needs to separate transcellular movement, paracellular movement, tissue uptake, tissue binding, degradation, and receiver-side recovery rather than treating every form of mucosal association as equivalent absorption.