The road is long and the road is coming——On the oral administration of protein and polypeptide drugs

Protein and polypeptide drugs have been widely used in the clinical treatment of various diseases due to their high specificity and high safety, but their main route of administration is injection, which reduces patient compliance
.
Oral administration has gradually become the focus of research as a route of administration with high safety and high compliance.
However, the structural organization and physiological functions of the gastrointestinal tract make protein and polypeptide drugs low in bioavailability and short half-life after oral administration
.
The absorption of protein and peptide drugs after oral administration has become a bottleneck in the development of oral administration of such drugs.
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With the rapid progress of biotechnology, more and more proteins and peptides have been developed as drugs for the treatment of various diseases
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Protein and peptide drugs have become one of the alternatives to small molecule drugs because of their high selectivity and efficacy with low toxicity
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However, the physicochemical properties of proteins and peptides are quite different from those of chemical drugs
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Most proteins and polypeptides are highly hydrophilic, but some cyclic peptides, such as cyclosporine, are hydrophobic
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Proteins and polypeptides have isoelectric points due to the ionization of amino and carboxyl groups, resulting in different charges at different pH values
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The biggest difference between protein and peptide drugs and chemical drugs is that conformation has an absolute influence on the pharmacological activity of proteins and peptides
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Therefore, unlike traditional small-molecule drugs, it is impossible to develop clinically applicable products without some kind of cutting-edge pharmaceutical technology
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An appropriate route of administration can not only ensure the efficacy of the drug, but also ensure patient compliance
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However, due to their poor oral bioavailability, the route of administration of proteins and polypeptides is usually by injection
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Long-term continuous injection can pose significant challenges to medication compliance, including pain, aversion to injection, and local irritation
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Therefore, many scientific teams try to develop their alternative routes, of which the oral route is the most attractive alternative because of its higher safety and compliance
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According to a joint market research report, the global oral protein and peptide market is expected to grow from $643 million in 2016 to $8.
23 billion in 2028
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In addition, some new technologies are currently being developed around the world to improve oral absorption, such as the use of transient penetration enhancers
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However, current commercial products for oral proteins and peptides are very limited to a few specific peptides, such as Neoral® for cyclosporine A and Rybelsus® for semaglutide
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The main obstacles to the development of oral delivery systems for proteins and peptides include easy enzymatic inactivation in the gastrointestinal tract, large molecular weight, high hydrophilicity, and poor transmembrane permeability
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Although it is indeed difficult to develop an oral drug delivery system for proteins and peptides, since the discovery of insulin, various attempts by countries around the world have never stopped
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Sandimmune®, approved by the FDA in 1990, was the first oral formulation of cyclosporine A, a cyclic peptide with a molecular weight of 1202 that, although generally classified as a poorly soluble drug, is also recognized as the first A peptide oral dosage form
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Five years later, Novartis developed and approved Neoral®, a modification of cyclosporin
A.
Since then, nanoemulsion self-emulsifying drug delivery systems (SNEDDS) have been recognized as an important strategy to improve oral absorption of drug molecules
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However, SNEDDS cannot greatly improve the oral bioavailability of hydrophilic proteins and peptides
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In 2019, Rybelsus®, an oral formulation of semaglutide developed by Novo Nordisk, was approved by the FDA for the treatment of type 2 diabetes
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Subsequently, octreotide extended-release capsules (Mycapssa®) developed through transient permeability enhancer (TPE®) technology were also approved by the FDA in 2020
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These two successful oral peptide products will bring revolutionary changes to the development of protein and peptide oral drug delivery systems
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1 Physiological disorder After oral administration, the drug first passes through gastric juice and then enters the small intestine, where the drug is absorbed in the stomach or small intestine
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However, the physiological environment of the stomach and intestines such as pH, enzymes, mucus and even the permeability of the epithelium, all of which can affect the stability and absorption of proteins and peptides
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Differences in pH gradients in the human body: The pH of the gastrointestinal tract is completely different in various parts of the gastrointestinal tract and is affected by various factors, including food, pathological conditions, age, and gender
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The complex pH environment of the gastrointestinal tract can lead to conformational changes or enzymatic degradation of proteins and peptides, resulting in poor therapeutic efficacy
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In general, proteins are stable in a narrow pH range not far from the isoelectric point
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Therefore, some proteins may be inactivated in gastric juice due to pH-induced unfolding
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In addition, the activity of the enzyme depends on pH.
For example, pepsin has the strongest degradation ability at pH 2~3, and is completely inactivated at pH>5
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Most proteins and peptides are rapidly degraded in the stomach of healthy adults
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Enzymatic degradation: Proteins and peptides are highly sensitive to various proteolytic enzymes, including those in the gastrointestinal tract and pancreatic secretions
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Therefore, ensuring the stability of gastrointestinal proteins and peptides in vivo is one of the important prerequisites for the successful development of oral protein and peptide drugs
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Mucus Layer: Mucus is the viscous and viscoelastic gel layer that covers the entire gastrointestinal tract
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Mucus traps foreign bodies and protects the epithelium from foreign pathogens
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The mucus throughout the gastrointestinal tract consists of two layers, with a loose and firm layer of mucus from the lumen to the epithelium
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There were significant differences in the thickness of the mucus layer in different parts of the gastrointestinal tract
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In addition, a pH gradient exists throughout the mucus layer, especially the gastric mucosa
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The pH of gastric mucus on the luminal surface is approximately 1–2, similar to that of the stomach, but increases to a neutral pH on the epithelial surface
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Mucus creates multiple barriers to the transport of drugs into the submucosal tissue
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High viscosity reduces the ability of proteins and peptides to diffuse through mucus
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Transcellular barriers: The epithelium beneath the mucus is also another major limiting factor for the absorption of oral protein drugs
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The absorption of protein and peptide drugs in the gastrointestinal tract mainly depends on the transcellular pathway, and the paracellular pathway is the main pathway for some hydrophilic small molecules
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Due to the extremely low transcellular permeability of proteins and peptides, it is difficult for proteins and peptides to be absorbed by the portal vein through transcellular pathways, even if gastrointestinal penetration enhancers are added to the formulation, such as transient permeability enhancer SNAC, protein and Oral bioavailability of peptides remains extremely low
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Individual differences: Huge inter-individual variability is also a barrier to the development of oral proteins and peptides
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For oral administration, interindividual variability in the gastrointestinal tract significantly affects the bioavailability of orally administered proteins and polypeptides, such as mucus status, enzyme secretion, and gastrointestinal motility rates
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2 Formulation factors In addition to physiological barriers, formulation formulation is also a major challenge in the development and commercialization of oral protein peptides
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The chemical and physical stability of proteins and peptides is the most important consideration in formulation development, which aims to keep proteins and peptides stable during production, transportation, storage, and administration
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In addition, in order to enhance the permeability of drugs, some penetration enhancers, such as SNAC, bile salts and nonionic surfactants, can be added to the oral formulation
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Although there are various strategies to increase the oral absorption of proteins and peptides, they are currently mainly based on three aspects, including enhanced stability, mucus penetration or adhesion, and the use of penetration enhancers
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1 Enhanced stability pH regulation: Gastrointestinal enzymes are major factors in degrading orally administered proteins and peptides, but they require an optimum pH to function
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For example, pepsin can easily cleave many proteins or polypeptides in an acidic environment, but when pH exceeds 5, pepsin begins to lose its effect
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Therefore, if the pH of the microenvironment can be adjusted to 5, the degradation of proteins and peptides in the stomach can be protected
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Enteric coatings are often used to overcome the degradation of proteins and peptides in the stomach, but unfortunately, proteolytic enzymes in the small intestine can also degrade proteins and peptides
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Enzyme Inhibitors: In addition to adjusting pH, the most important way to inhibit enzymes is to use enzyme inhibitors
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Enzyme inhibitors inactivate target enzymes by reversibly or irreversibly binding to specific sites on the enzyme
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However, caution should be exercised when using enzyme inhibitors
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2 Mucous Penetration and Mucoadhesion System Mucus Penetration System: For the mucus penetration system, a mucolytic agent is used to disrupt the mucus barrier, causing it to temporarily lower the mucus barrier of healthy mucosa, thereby facilitating absorption
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Mucoadhesive system: that is, by using adhesives to electrostatically adhere to proteins to improve drug absorption
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Due to the adhesion of proteins and polypeptides, hydrogels can make proteins and polypeptides stay in specific intestinal areas for a long time, and at the same time resist enzymatic degradation, so complexed hydrogels are better for oral administration of proteins and polypeptides.
select
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3 Enhanced absorption Absorption enhancers: The biggest obstacle to oral administration of proteins and peptides is poor gastrointestinal epithelial permeability
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Absorption enhancers are recognized drugs that improve the permeability of the gastrointestinal tract by transiently changing the epithelial structure, so they are widely used in the oral preparation of proteins and peptides
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The mechanisms that may be involved in current absorption enhancers are shown in the figure below
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Sodium caprylate/caprate and their derivatives are the most promising absorption enhancers and have been used for oral proteins and peptides, such as sodium caprylate in oral octreotide and SNAC in oral Semagluide
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The SNAC developed by Emisphere in the United States was approved for the first time for the oral administration of vitamin B12
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It is a derivative of sodium caprylate, and its structure and mechanism of action are shown in the figure below
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Chitosan and its derivatives are also the most commonly used polymers to improve the oral administration of proteins and polypeptides, mainly depending on their positive charge density and bioadhesion ability.

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Oral proteins and peptides are becoming more and more attractive in drug research and development, but there is still a long way to go in enhancing the stability and permeability of proteins and peptides in oral formulations.
The bioavailability of the preparation is still very low, and it is expected that the technical barriers will be broken one day in the future
.
References: [1] Zhu Q , Chen Z , Paul PK , et al.
Oral delivery of proteins and peptides: Challenges, status quo and future perspectives[J].
Acta Pharmaceutica Sinica B, 2021.