Multiple Barriers and Breakthrough Strategies for Oral Delivery of Protein and Peptide Drugs

In the field of biopharmaceuticals, protein and peptide drugs (PPs) hold a pivotal position in treating various diseases such as diabetes and osteoporosis, owing to their well-defined mechanisms of action, high selectivity, and low side effects. However, the traditional parenteral injection route not only causes pain and inconvenience to patients but may also lead to adverse reactions like subcutaneous nodules and infections, significantly impacting medication adherence. The oral route, as the most convenient and patient-preferred non-invasive administration method, can mimic the natural secretion process of proteins to potentially reduce side effects. Nevertheless, the complex physiological environment of the gastrointestinal (GI) tract presents multiple formidable barriers, resulting in extremely low oral bioavailability for PPs, which constitutes a major challenge for the pharmaceutical industry. This article provides a systematic analysis centered on the core obstacles, breakthrough strategies, current clinical application status, existing challenges, and future directions for oral PPs delivery, offering a clear framework for research and development in this field.

1.Core Gastrointestinal Barriers Facing Oral PPs Delivery

The physiological structure and environment of the GI tract create multiple layers of obstacles to PPs absorption, primarily categorized into three major classes: biochemical barriers, mucus barriers, and intestinal epithelial barriers, each encompassing several specific challenges.

1.1 Biochemical Barriers

This represents the primary obstacle to oral PPs absorption, stemming mainly from the extreme chemical environment of the GI tract, which can be subdivided into three points:

• pH Barrier:The stomach's pH is as low as 1.2–3.0. This highly acidic environment can cause PPs to unfold, aggregate, or even hydrolyze, destroying their bioactivity. For instance, insulin rapidly inactivates in the stomach due to disulfide bond cleavage. Upon entering the intestines, the pH rises to an alkaline 6.5–8.0, which can induce deamidation and aggregation of PPs, further compromising their stability.
• Enzymatic Barrier:​ The GI tract is rich in various proteolytic enzymes. Pepsin in the stomach, and trypsin, chymotrypsin, etc., in the small intestine rapidly degrade PPs into small molecular fragments, rendering them therapeutically inactive. Studies show that free insulin cannot be detected intact in simulated gastric fluid containing pepsin after 1 hour, and is almost completely degraded within 15 minutes in simulated intestinal fluid.
• Thiol/Disulfide Exchange Reaction Barrier:​ The intestinal lumen contains abundant thiol-containing substances like glutathione, which can engage in exchange reactions with disulfide bonds in PPs. This disrupts their native structure, forming inactive conjugates, and may also accelerate PPs aggregation and clearance.

1.2 Mucus Barrier

The mucus layer coating the intestinal wall is a significant obstacle for PPs to reach epithelial cells. Primarily composed of mucins, this layer forms a viscoelastic gel-like network through cross-linking, with pore sizes ranging from 20–1800 nm, acting as a molecular sieve for PPs. Furthermore, the negatively charged mucus layer can entrap PPs via non-covalent interactions like electrostatic forces, hydrogen bonding, and hydrophobic interactions. The outer mucus layer is continuously renewed with a turnover period of 50–270 minutes, rapidly clearing trapped PPs and significantly shortening their contact time with the intestinal epithelium.

1.3 Intestinal Epithelial Barrier

Even if PPs successfully traverse the mucus layer, the physical barrier formed by intestinal epithelial cells remains the final hurdle. Tight junctions between these cells occupy less than 1% of the mucosal surface area, with gaps of only about 6 Å – far smaller than the molecular dimensions of most PPs – precluding paracellular transport. The hydrophilic nature of PPs makes it difficult for them to penetrate the epithelial cell's lipid bilayer. Even if internalized via endocytosis, they risk degradation by intracellular proteases or efflux by pumps, hindering successful transcellular transport into systemic circulation.

2.Key Strategies to Overcome Gastrointestinal Barriers

To address the barriers mentioned above, researchers have developed a series of targeted strategies spanning chemical modification, carrier encapsulation, and physical enhancement. Some strategies can simultaneously tackle multiple barriers.

2.1 Strategies Against Biochemical Barriers

• pH Regulation:​ Adjusting the local microenvironment pH using buffers (e.g., SNAC) or enteric coatings to minimize acid/base-induced damage. For example, oral semaglutide (Rybelsus®) is co-formulated with SNAC, creating a near-neutral microenvironment in the stomach to protect the drug's structure. Enteric-coated capsules allow PPs to bypass the acidic stomach and release in the intestines. Additionally, adding agents like citric acid to formulations can lower local intestinal pH, inhibiting protease activity and indirectly protecting PPs.
• Chemical Modification:​ Altering the physicochemical properties of PPs through PEGylation, lipidation, peptide cyclization, or N-/C-terminal acetylation to enhance resistance to pH extremes and enzymatic degradation. PEGylation creates a hydration shell and steric hindrance, reducing contact with protons and enzymes. Cyclization increases molecular rigidity, lowers conformational entropy, and bolsters resistance to degradation. Lipidation enhances PPs hydrophobicity, laying groundwork for subsequent epithelial crossing.
• Encapsulation Technologies:​ Encapsulating PPs within carriers like liposomes, metal-organic frameworks (MOFs), or polymer nanoparticles creates a physical barrier isolating them from acids, bases, and enzymes. For instance, acetylated inulin nanoparticles can protect insulin's secondary and tertiary structure in the stomach, with controlled release in the intestines triggered by carrier degradation via gut microbiota. A liposome-alginate hydrogel composite system effectively reduces PPs degradation and burst release in the stomach.
• Enzyme Inhibitors:​ Adding protease inhibitors (e.g., soybean trypsin inhibitor, U-Omp19) to formulations inhibits PPs degradation by binding to enzyme active sites or chelating essential metal ions (e.g., EDTA chelating Ca²⁺). However, this strategy carries potential toxicity risks, may interfere with normal digestion, and long-term use could trigger compensatory increases in protease secretion.

2.2 Strategies Against the Mucus Barrier

Strategies targeting the mucus barrier primarily fall into two categories: mucus-penetrating and mucoadhesive, aiming to enhance contact efficiency with the epithelium via "active penetration" or "prolonged residence," respectively.

① Mucus-Penetrating Strategies:

• Adding mucolytic agents (e.g., N-acetylcysteine) to cleave disulfide bonds between mucin molecules, reducing mucus gel density to create channels for PPs. This may compromise mucosal protective function and increase infection risk.
• Hydrophilic modification of carriers (e.g., PEGylation) to reduce hydrophobic interactions with mucins, enhancing mucus diffusion rate. PEG molecular weight and grafting density require precise control to optimize penetration.
• Modifying carrier surface charge to near-neutral or weakly negative to avoid strong electrostatic interactions with negatively charged mucins. Combinatorial cationic-anionic surface modifications mimicking viral surface properties can improve penetration efficiency.
• Optimizing carrier size (optimal 50–200 nm), shape (rod-shaped or short nanotubes penetrate more easily), and rigidity (moderately rigid carriers can deform to navigate the mucus mesh), reducing spatial hindrance.
• Utilizing the active propulsion of micro/nanomotors, driven by catalytic reactions, magnetic fields, etc., to overcome mucus viscoelastic resistance for deep penetration.

② Mucoadhesive Strategies:

• Hydrophobic modification of carriers to enhance interaction with hydrophobic regions of mucins, prolonging intestinal residence time of PPs.
• Cationic modification (e.g., chitosan coating) to increase adhesion via electrostatic interaction with negatively charged mucins, potentially also promoting epithelial cell uptake.
• Introducing thiol groups onto carriers to form stable disulfide bonds with cysteine residues in mucins, significantly enhancing adhesion and prolonging drug action time.

2.3 Strategies Against the Intestinal Epithelial Barrier

①Permeation Enhancers: 

Temporarily altering epithelial structure to enhance PPs permeability. Chelators (e.g., EDTA, EGTA) can open tight junctions by sequestering Ca²⁺. Surfactants (e.g., SDS, SNAC) enhance cell membrane fluidity, promoting transcellular transport. Cell-penetrating peptides (e.g., Penetratin®) mediate PPs entry into cells via electrostatic and hydrophobic interactions. However, some permeation enhancers may have toxicity, necessitating rigorous safety evaluation.

② Chemical Modification:

• Lipidation to increase PPs hydrophobicity, facilitating integration into the epithelial lipid bilayer and enhancing transmembrane transport efficiency.
• Ligand conjugation (e.g., to deoxycholic acid, vitamin B12) targets PPs to transporters on epithelial cell surfaces (e.g., ASBT, PAT1), enabling active absorption via receptor-mediated endocytosis, improving transport specificity and efficiency.

③ Nanocarrier Modification:

• Hydrophobic modification of PPs-loaded nanoparticles enhances interaction with epithelial cell membranes, promoting endocytosis.
• Cationic modification increases nanoparticle binding to negatively charged epithelial cell surfaces, boosting cellular uptake.
• Ligand functionalization (e.g., transferrin conjugation) enables targeted nanoparticle transport while protecting PPs from degradation during transit.
• Zwitterionic modification mimics viral surface properties, aiding both mucus penetration and PAT1 receptor-mediated entry into epithelial cells, achieving simultaneous breaching of mucus and epithelial barriers.

2.4 Synergistic Integration of Strategies

A single strategy is often insufficient to overcome multiple barriers. Nanocarriers have emerged as a core platform for integrating multiple strategies. To resolve the opposing surface property requirements for mucus penetration (hydrophilic/neutral) and epithelial uptake (hydrophobic/cationic), researchers have developed strategies like dissociable coatings, hydrolyzable coatings, and microneedle technology.

• Dissociable Coatings:​ Carriers are coated with a hydrophilic layer (e.g., SB12, pHPMA) that detaches after traversing the mucus layer, exposing a hydrophobic or cationic core to promote epithelial cell uptake.
• Hydrolyzable Coatings:​ Exploiting pH or enzymatic differences between the mucus layer and epithelial surface, the carrier coating undergoes hydrolysis, reversing surface charge from neutral to positive, enhancing epithelial binding.
• Microneedle Technology:​ PPs are loaded onto biodegradable microneedles delivered via oral capsules. Once in the GI tract, the microneedles physically penetrate the mucus and epithelial barriers, enabling direct drug delivery, significantly enhancing bioavailability. Some microneedle systems have achieved oral insulin bioavailability as high as 23.6%.

3.Current Status of Clinical Application for Oral PPs Delivery

3.1 Marketed Products

• Currently approved oral PPs formulations primarily employ relatively mature, simpler strategies. Examples include:
• Semaglutide tablets (Rybelsus®):​ Utilize SNAC as a permeation enhancer to improve intestinal absorption.
• Oral octreotide (Mycapssa®):​ Combines peptide cyclization, enteric coating, and permeation enhancer technology, achieving a bioavailability of 0.7%.
• Cyclosporine A (Neoral®):​ Employs a self-nanoemulsifying drug delivery system (SNEDDS), leveraging its lipophilic nature for efficient oral absorption with bioavailability of 19–40%.

3.2 Products in Clinical Development

Several investigational formulations are exploring more complex technological routes, such as:

• Diasome Pharmaceuticals' oral insulin liposomal formulation:​ Modified for hepatocyte targeting to mimic physiological insulin delivery. It has entered Phase II/III clinical trials.
• Oshadi's insulin-loaded silica nanoparticles:​ Combined with hydrophobic polysaccharide modification, currently in Phase II trials.
• Rani Therapeutics' microneedle capsule:​ Delivers PPs via mechanical piercing, having completed Phase I trials with promising safety and efficacy.

4.Existing Challenges in Oral PPs Delivery Strategies

Despite significant research progress, clinical translation of oral PPs delivery faces several bottlenecks:

• Low Bioavailability:​ Most current strategies address only partial barriers. Marketed formulations typically exhibit oral bioavailability below 1%, often insufficient to match injectable efficacy, limiting their application for serious conditions.
• Manufacturing Scalability Challenges:​ Ensuring consistent nanoparticle size and batch-to-batch reproducibility is difficult. High production costs and process instability during scale-up hinder clinical translation feasibility.
• Safety and Regulatory Issues:​ Long-term biosafety data for nanocarriers is lacking, with potential risks of accumulation and immunogenicity. Regulatory approval pathways for novel delivery systems remain unclear, adding to development hurdles.
• Inadequate Carrier Stability:​ Organic carriers (e.g., liposomes) may leak drugs, while inorganic carriers (e.g., mesoporous silica nanoparticles) may have poor biodegradability, affecting formulation efficacy and safety.

5.Future Directions

To address these challenges and advance oral PPs delivery, future research should focus on:

• Advanced Biological Model Development:​ Utilizing human stem cell-derived organoid models combined with microfluidic technology to more accurately simulate GI physiology. This will enable efficient assessment of delivery system safety and efficacy, reducing reliance on animal studies.
• Nanocarrier Engineering Optimization:​ Simplifying nanocarrier manufacturing, developing low-cost, biodegradable materials, and optimizing surface modification strategies. Goals include enhancing scalable production capability and batch consistency while maintaining barrier penetration efficiency.
• AI-Assisted R&D:​ Leveraging AI to mine vast preclinical and clinical datasets, predicting carrier biocompatibility, toxicity, and delivery efficiency. This can optimize formulation design, shorten development cycles, and reduce R&D costs.

6.Summary

The advancement of oral delivery technologies for PPs represents a significant trend in biopharmaceuticals. Its core value lies in patient-centricity, aiming to improve patient quality of life through enhanced convenience and safety, especially for those on long-term therapy. Current research indicates that single strategies are inadequate against the GI tract's multiple barriers. Multi-strategy synergistic nanocarrier systems and innovative microneedle technologies show the most promising potential. Particularly, environmentally responsive "smart" carriers that dynamically adapt to different barrier requirements offer novel approaches for boosting bioavailability.

Simultaneously, it's crucial to recognize that clinical translation of oral PPs faces substantial hurdles. Significantly enhancing bioavailability remains the core objective, requiring deeper understanding of GI physiology for precise strategy design. Safety is the prerequisite for clinical application, demanding validation of carrier biocompatibility and degradability through long-term toxicology studies and clinical observation. Furthermore, breakthroughs in production costs and scalable manufacturing are key to ensuring innovative formulations benefit a broad patient population.

Overall, interdisciplinary convergence of materials science, nanotechnology, and AI is gradually overcoming the technical bottlenecks of oral PPs delivery. With continued fundamental research and clinical validation, the future holds promise for developing oral PPs formulations with high bioavailability, excellent safety profiles, and controllable costs. This could fundamentally transform the current injection-dependent treatment paradigm, offering patients a superior therapeutic experience and driving continuous progress in the biopharmaceutical industry.