Protein and Peptide Drug Delivery

Abstract

Protein and peptide-based drugs have great potential applications as therapeutic agents since they have higher efficacy and lower toxicity than chemical drugs. However, difficulty with their delivery has limited their use. In particular, their oral bioavailability is very low, and the transdermal delivery faces absorption limitations. Therefore, most of the protein and peptide-based drugs are administered by the parenteral route. However, this route also has some problems, such as patient discomfort, especially for pediatric use. Extensive research has been performed over the past few decades to develop protein and peptide delivery systems that circumvent the problems mentioned above. Various strategies that have been employed during this time include nanoparticle carriers, absorption enhancers, enzyme inhibitors, mucoadhesive polymers, and chemical modification of protein or peptide structures. However, most of these strategies are focused on the delivery of proteins or peptides via the oral route since it is the most preferred route considering its high level of patient acceptance, long-term compliance, and simplicity. However, other routes of administration such as transdermal, nasal, pulmonary can also be attractive alternatives for protein and peptide delivery. This chapter will discuss the most effective approaches used to develop protein and peptide drug delivery systems.

1. Introduction

Proteins and peptides play vital roles in many biological processes, including catalysis, transportation, regulation of gene expression, immunity-related functions, etc. They are also involved in many pathological conditions such as diabetes, hypertension, cancer, etc. [1]. Because of their wide range of functions and their involvement in diseases, proteins and peptides are attractive therapeutic agents for combatting many diseases. Currently, there are more than 100 approved peptide-based therapeutics on the market [2]. The market for peptide and protein drugs is growing much faster than for small molecule drugs. One reason for this is that peptides and proteins can be highly selective as they have multiple points of interaction with the target. This increased selectivity will also lead to decreased side effects and toxicity.

However, the physicochemical properties of proteins and peptides make their use as drugs difficult. Firstly, proteins and peptides are not suitable for administration via the oral route because of their instability in the gastrointestinal tract (GIT). Secondly, the size and hydrophilicity of proteins and peptides lead to their poor bioavailability [3, 4]. Other routes of delivery also have some drawbacks. For example, administration via intravenous injection may not be suitable for achieving optimal therapeutic effects since many proteins and peptides have low circulation half-life [5]. This may also lead to pain or discomfort [6], severe reaction at the injection site [7, 8], scarring [9], local allergic reactions [10], cutaneous infections [11], etc. Transdermal delivery leads to absorption limitations due to the skin barrier, which prevents the passage of drug molecules with molecular weight greater than 500 Da, especially hydrophilic molecules [12, 13].

Extensive research has been performed over the past few decades to develop protein and peptide delivery systems that circumvent the drawbacks mentioned above. Different strategies that have been employed for this purpose include nanoparticle carriers, absorption enhancers, enzyme inhibitors, mucoadhesive polymers, and chemical modification of protein or peptide structures [14, 15]. Most of these approaches aim to deliver proteins and peptides via the oral route since it is the most convenient route of drug administration. However, other routes of administration such as transdermal [16], nasal [17], buccal [18], pulmonary [19] also have some attractive features such as avoidance of harsh environment of the GIT and non-invasiveness of the nature of administration. Thus, these routes can also be excellent alternatives for protein and peptide delivery [20]. This chapter will discuss the most effective approaches to developing protein and peptide drug delivery systems.

Proteins and peptides consist of amino acids connected via peptide bonds (Figure 1). Protein and peptide structures can be primary structure, secondary structure, tertiary structure, and quaternary structure. The primary structure provides information about the number and types of amino acids in a protein or peptide. Secondary structure gives us information about the presence of α-helix, β-sheets, loops, and turns in the protein or peptide. For example, hemoglobin is a predominantly helical protein (Figure 2). The tertiary structure indicates the overall three-dimensional structure of the protein. For example, the tertiary structure of hemoglobin consists of a globin fold (Figure 2). The quaternary structure indicates the oligomeric state of the protein. For example, hemoglobin has a tetrameric (or dimer of dimer) quaternary structure (Figure 2).

Figure 1.

General structure of a peptide fragment.

Figure 2.

Structure of hemoglobin.

Various routes for delivery of protein and peptide drug has been mentioned above. Their advantages and disadvantages have been discussed briefly. The strategies adopted for enhancing the delivery of protein and peptides via various routes is discussed below.

2. Oral route

2.1 Chemical modification

Chemical modification of proteins and peptides is one of the strategies adopted to make these more clinically applicable [21]. This strategy eliminates some of the undesirable properties of proteins and peptides, such as susceptibility to enzyme hydrolysis, improper solubility, poor membrane permeability, etc. There are several ways of performing the chemical modification, including replacement of a specific amino acid or modification of the structure of amino acids. For example, replacement of an L-amino acid with its D-counterpart may lead to resistance to enzymatic hydrolysis, enhanced cell-membrane permeability, etc. In desmopressin (an analog of vasopressin), the C-terminal L-Arg has been replaced by D-Arg (Figure 3). Also, the N-terminal amino group (of cysteine) has been deaminated (Figure 3). These chemical modifications resulted in substantially higher oral bioavailability of desmopressin than that of vasopressin [22].

Figure 3.

Structure of vasopressin (left) and desmopressin (right).

Another method of chemical modification is to incorporate a lipophilic moiety (e.g., fatty acid) in the protein or peptide. The lipophilic moiety will increase the overall hydrophobicity of the molecule, which may lead to an increase in its intestinal absorption. Increased hydrophobicity may also lead to increased stability of the protein or peptide [23]. For example, three amino acids (Gly, Phe, and Lys) in insulin were attached to 1, 3-dipalmitoyl glycerol moiety. This modification resulted in an increased hydrophobicity of insulin as well as increased intestinal absorption. The enzymatic hydrolysis of insulin was also reduced, which resulted in increased bioavailability [24].

2.1.1 PEGylation

PEGylation involves the covalent attachment of one or more polyethylene glycol (PEG) molecule(s) to a protein or peptide. PEG has some special features that make it suitable for protein and peptide delivery systems [25, 26, 27, 28]. One important feature is that it is soluble in both organic and aqueous solvents. Another feature is that PEG can be obtained in a wide range of molecular weights. It can be obtained both as a linear or branched chain. PEG is more hydrophilic than other polymers of comparable sizes. Also, PEG is highly flexible since there is no bulky substituent around the backbone to hinder the rotation. Finally, PEG is non-toxic and non-immunogenic.

PEG improves the pharmacokinetic properties of proteins and peptides by protecting these from enzymatic hydrolysis (Figure 4) and by increasing solubility. PEG acts as a shield that prevents the hydrolysis of the protein or peptide by blocking its access to the proteolytic enzyme (Figure 5) [29]. The attachment of PEG with a protein or peptide increases its size, which increases the circulation half-life of the protein or peptide by decreasing the clearance by the renal filtration process. The attachment of PEG to a protein or peptide also reduces immunogenicity by reducing the recognition by the β-cell or antibodies. Finally, the attachment of PEG to proteins or peptides prevents their possible aggregation [30].

Figure 4.

Enzymatic hydrolysis of the peptide bond.

Figure 5.

PEGylaiton blocks enzymatic hydrolysis of peptides.

PEGylation process can interfere with the molecular recognition between the protein or peptide and the receptor due to the modified structure of the PEGylated protein or peptide. For example, PEGylation of a protein or peptide may cause steric hindrance, which may prevent proper binding of the protein or peptide to the target. However, this drawback is balanced by the improvements in pharmacokinetic properties, such as enhanced absorption and circulation half-life [31]. The development of Nobex Corporation’s HIM2 was based on the PEGylation approach. HIM2 is hexyl insulin monoconjugate 2 that is attached to a PEG. The attachment of PEG improves the stability of the molecule and enhances bioavailability that allows oral administration [32].

2.1.2 Peptidomimetics

Peptidomimetics mimic the structure of a protein or peptide and show acceptable pharmacokinetic properties while retaining biological activity. Different modifications (N-alkylation, isosteric replacement of the amide bond, etc.) can be performed on a peptide to improve its pharmacokinetic properties [33]. Structural modifications are mainly performed to make the peptide more stable in the GIT by making it less susceptible to enzymatic degradation. Solubility, lipophilicity, and the flexibility of the peptide can also be altered to enhance stability as well as absorption. Modification of a peptide may occur at the backbone of the peptide or the side chains of amino acids, or both. The alkylation of the backbone N-atom has been shown to improve the bioavailability of peptide molecules [34].

N-alkylation, in general, leads to an increase in lipophilicity of the peptide as well as a steric hindrance. N-alkylation also leads to a reduction in H-bonding since the H-atom of the backbone amide has been replaced by an alkyl group. This decrease in H-bonding may lead to destabilization of α-helix and β-sheets, which may alter the conformation of the peptide. N-alkylation method has been used to make cyclosporine (Figure 6) where the N-atoms of the backbone have been alkylated by methyl groups (green spheres) [35].

Figure 6.

Backbone N-alkylation in cyclosporine.

Isosteric replacement of amide bond is another strategy for peptidomimetics design. This process can lead to alteration in H-bonding as well as peptide-folding. Thus, this process may affect the conformation of the peptide. For example, the replacement of a carbon-atom by an N-atom gives azapeptide class. Azapeptides can be of two types – azatides and peptoids. In azatides, all the α-carbon in the peptide backbone has been replaced by nitrogen atom. In peptoids, the α-carbon is replaced by N-atom, and the N-atom of the backbone is replaced by C-atoms. Ritonavir (Figure 7) is an example of a drug developed by the peptidomimetics approach [36, 37].

Figure 7.

Ritonavir.

2.2 Enzyme inhibition

Inhibition of enzymes like proteases is a good way of improving the stability of proteins and peptides in the GIT since these enzymes hydrolyze peptide bonds in proteins and peptides administered via the oral route. Proteases can be classified into different categories depending on the catalytic amino acid residue (Ser, Thr, Asp, Glu, etc.) in the active site of the enzyme [38]. For example, serine proteases contain a serine residue in the active that acts as a nucleophile during the hydrolysis of the peptide bond. Another class of proteases is metalloproteases, where the water molecule used for hydrolysis is ligated to a metal ion (Zn²⁺, Co²⁺, Mn²⁺, etc.) in the active site of the enzyme. Thus, one way of preventing hydrolysis of the protein or peptide drug is to co-administer protease inhibitors [39]. Insulin, for example, undergoes degradation in the GIT by different enzymes like trypsin, chymotrypsin, etc. Therefore insulin is co-administered with various synthetic (e.g., camostat mesylate) and naturally occurring inhibitors (soybean trypsin inhibitor) of trypsin and chymotrypsin. This co-administration leads to enhanced bioavailability of insulin [40]. However, the administered protein other than the therapeutic protein will not be degraded, leading to toxic side effects. Besides, the non-degraded proteins may also cause metabolic changes in the GIT [41].

Another way of protease inhibition is to alter the pH of the medium in which these enzymes work. This pH change may lead to the inactivation of the proteases. It has been reported that lowering the pH of the intestine to 4.5 or below leads to the inhibition of trypsin and chymotrypsin [23].

2.3 Absorption enhancers

Absorption enhancers are substances co-administered with protein or peptide drugs to enhance their absorption. In general, these absorption enhancers reversibly damage the physical barrier in the cell wall, which prevents a protein or peptide from crossing the intestinal wall. Thus, absorption enhancers provide a temporal path for the proteins or peptides to cross the intestinal wall and to be absorbed [42]. There are two main ways by which absorption enhancers cause the temporal opening – transcellular pathway and paracellular pathway. The transcellular mechanism involves the structural change in the cell membrane of the epithelial cells. This structural alteration leads to enhanced passive diffusion of proteins and peptides through the cell. In the paracellular pathway, the absorption enhancers facilitate the opening of tight junctions between the epithelial cells, which allows the protein and peptide to diffuse through the intercellular space present between the epithelial cells. Another absorption enhancing mechanism involves the reduction of viscosity of the mucus in the intestinal wall, which enhances the diffusion of proteins and peptides [25].

There are different types of absorption enhancers based on their molecular structure and mechanism of action. For example, ethylene diamine tetraacetic acid (EDTA) works by chelating Ca-ions, which are important in maintaining the tight junction between the cells. Therefore, when Ca-ions are complexed by EDTA, the tight junction between the cells will be opened, allowing the proteins and peptides to cross the intercellular space. However, surfactants (sodium lauryl sulfate, Tween 40, etc.) work by disrupting the intestinal membrane allowing the protein and peptide to cross the cells via the transcellular pathway [43].

It is worth mentioning that the absorption enhancers are potentially toxic since some of them disrupt the integrity of the intestinal membrane. Damage of the intestinal membrane can cause proteins or peptides other than the protein or peptide of interest to be absorbed, leading to toxicity. Besides, pathogens (virus, bacteria, etc.) may also get absorbed, which may lead to various pathological conditions. Severe damage of the intestinal membrane may also lead to inflammatory conditions and ulceration of the epithelium. Therefore, the toxicology of the absorption enhancers needs to be fully understood before their long-term applications [44].

2.4 Site specific delivery in GIT

Different regions of the GIT show differences in the absorption of proteins and peptides. These differences are due to different pH values and different distribution of proteolytic enzymes at different regions of the GIT. The pH affects both the solubility and the stability of the protein or peptide, while the proteolytic enzymes are responsible for the degradation of the protein or peptide. There is also variability in the distribution of active transporters involved in peptide transport and the efflux pumps that can lower the absorption of proteins or peptides along the GIT [45, 46].

Extensive research has been performed to locate the optimum absorption site in the GIT for proteins or peptides [47]. These results indicate that the colon region of the GIT is one of the optimum sites for protein or peptide absorption mainly due to the lower protease activity in the colon area compared to other areas in the GIT, such as the stomach and small intestine. Therefore, several strategies have been employed to deliver the therapeutic protein or peptide intact to the colon. One approach is to design a prodrug (Figure 8) with adequate stability in the other regions of the GIT [48]. However, it should be converted to the parent therapeutic protein or peptide in the colon. Since the microflora in the colon produces reductive enzymes, the prodrug should be designed by linking the therapeutic protein or peptide via a bond (e.g., azo bond) that can be cleaved by reductive enzymes in the colon [49]. These reducing enzymes can also be utilized to attach a polymeric carrier to the protein or peptide (Figure 9). This polymer will protect the therapeutic peptide molecule along the GIT and will release the protein or peptide at the colon site, enhancing its absorption.

Figure 8.

Prodrugs with an azo bond.

Figure 9.

A prodrug with a polymeric carrier.

2.5 Membrane transporters

Epithelial cells express various amino acid and oligopeptide transporter proteins that transport various amino acids and oligopeptide found in nutrients to facilitate their absorption. Therefore, designing a therapeutic drug with structural similarity with the natural substrate of the transporter proteins will help in transporting the molecule across the epithelium to the systemic circulation. This can also be achieved by attaching a natural substrate of the transporter proteins to the bioactive molecule [50]. This linkage will allow the recognition of the attached peptide by the transporter and its binding to the transporter. However, for this process to occur, the attached peptide should be enzymatically stable in the GIT. Otherwise, hydrolysis of the peptide before reaching the transporter will prevent the recognition of the molecule by the transporter, and thus, transportation will not happen. PepT1 is a transporter protein that plays a vital role in the oral delivery of biomolecules. PepT1 can transport dipeptides and tripeptides with varying degrees of substrate specificity. PepT1 is involved in the transport of peptidomimetic drugs such as acyclovir which resembles a dipeptide [51, 52].

Generally, only small molecules can be transported by membrane transport proteins. However, molecules with relatively larger sizes are usually transported by receptor-mediated endocytosis. Endocytosis involves the binding of the large molecule to a membrane receptor, and the resulting complex gets inside the cell. In some cases, the internalized molecule can be subjected to degradation by the lysosome. In other cases, the components of the complex are kept intact following internalization and exit the cell by exocytosis. Thus, these components are transported into the systemic circulation. This kind of receptor-mediated endocytosis is called transcytosis. Epidermal growth factor, vitamin B12, and immunoglobulins are some of the substances absorbed by the transcytosis [53]. It has been reported that by attaching a therapeutic protein or peptide (e.g., erythropoietin, α-interferon, etc.) to vitamin B12, the bioavailability of the protein or peptide can be enhanced [54].

2.6 Mucoadhesive systems

Mucoadhesive systems utilize the bioadhesive phenomenon where certain components of the system (usually a polymer) form adhesion bonds with the mucosal membrane at the absorption site. This adhesion may lead to a high concentration of the therapeutic molecule, which increases absorption of the therapeutic molecule. Adhesion of the polymer in the delivery system also leads to an increase the residence time of the therapeutic molecule at the site of absorption and thus, further enhances the absorption and the bioavailability of the therapeutic agent [55].

The properties of the mucoadhesive system depends on the nature of the polymer used. The polymer should be hydrophilic enough to properly interact with the high amount of water present in the mucus layer. The polymer should be large enough (high molecular weight) to increase the possibility of interactions. The polymer should also have proper surface tension to allow the spreading of the polymer on the mucus layer. The polymer should contain functional groups (e.g., COOH, OH, etc.) to form strong H-bonds. Finally, the polymer should be non-toxic and non-immunogenic [41]. Although the main function of polymer in mucoadhesive drug delivery system is to form adhesive bonds with the mucus membrane to enhance the bioavailability of the protein or peptide, some polymers may have additional functions. For example, some polymers can act as absorption enhancers by modifying tight junctions between epithelial cells and by inhibiting proteases that hydrolyze proteins or peptides [56].

Cellulose derivatives (e.g., methylcellulose, hydroxyethyl cellulose, carboxymethyl cellulose) are used in mucoadhesive drug delivery systems. Other commonly used polymers are polyacrylic acid derivatives such as carbapol and polyacrylate. Thiol-containing polymers are also used in mucoadhesive systems. These polymers show strong adhesive binding with the mucus layer due to the formation of covalent bonds in addition to the non-covalent interactions. The thiol group in these polymers forms disulfide bonds with the cysteine residues present in the glycoproteins of the mucus. It has been reported that the increase in the thiol group in the polymer increases the strength of adhesive binding [23].

2.7 Liposomes

Liposomes are spherical microscopic vesicles containing one or more phospholipid bilayer (Figure 10). The inner core of the liposomes consists of hydrophilic parts of the phospholipid, while the lipophilic parts tend to remain in the lipid portion of the phospholipid bilayer. Extensive research has been carried out on the liposome-based delivery system for their application in the delivery of proteins or peptides, mainly via the oral delivery route. The advantage of liposome in oral delivery is mainly protecting the protein or peptide (e.g., insulin) from enzymatic hydrolysis in the GIT [57]. This protection is due to the encapsulation of the protein or peptide in the interior of the liposome, and thus, the protein or peptide is inaccessible to the proteolytic enzymes in the GIT. Liposomes also enhance the absorption of proteins or peptides (e.g., insulin) in the small intestine. The activity of the insulin-containing liposome depends on various factors such as the lipid components of the liposome, the charge on the surface of the liposome, etc. [58].

Figure 10.

General structure of liposomes.

It is worth mentioning that the bile salts in the GIT can solvate the liposome resulting in their rupture and consequent release of the encapsulated proteins or peptides in the GIT. This solvation of liposomes is a problem with the application of liposomes as the oral delivery system. However, the problem with the in vivostability of liposomes can be improved by adopting strategies like coating the liposome with a polymer or using dehydrated forms of liposomes [31].

2.8 Microspheres

Microspheres (Figure 11) can be prepared from polymers using various methods such as double emulsification, spray drying, complexation of macromolecules with opposite charges. Microspheres have sizes ranging from 1 μm to 1 mm. Much research has been performed on the use of microspheres for the oral delivery of protein or peptide drugs [59]. Microsphere-based delivery systems can protect the protein or peptide of interest from the harsh environment of the GIT such as enzymatic hydrolysis, acidic pH, etc. Microspheres can also enhance the absorption of the protein or peptide of interest, mainly through the paracellular pathway. The microsphere-based delivery system also allows the controlled release of the protein or peptide of interest at a specific area in the GIT by using pH-sensitive polymers. Insulin was loaded into the microsphere formed by the polymer poly (methacrylic-g-ethylene-glycol) [60]. This microsphere prevents the enzymatic degradation of insulin in the acidic environment of the stomach. However, swelling of the microsphere and consequent release of the insulin occurred in the basic environment of the intestine.

Figure 11.

General structure of microspheres.

2.9 Nanoparticle-based delivery systems

Nanoparticles can be used as a delivery system to administer protein or peptide therapeutics via different routes such as oral, intravenous, subcutaneous, and transdermal. Delivery of proteins or peptides via nanoparticles can be achieved by employing different approaches such as encapsulation of the drug molecule by the nanoparticle, adsorption of the drug molecule on the surface of the nanoparticle, etc. [61]. Nanoparticle-based delivery system protects the protein or peptide of interest from enzymatic degradation in the GIT [62]. Nanoparticles can also deliver the protein or peptide to the desired location, such as tumor cell, inflammation site, etc. This site-specificity results in reduced side effects of the therapeutic agent [63, 64].

The uptake of nanoparticles by the cells usually occurs via endocytosis. This process involves phagocytosis, receptor-mediated phagocytosis, and pinocytosis. Endocytosis starts with the association of the nanoparticle with the cell membrane to form an endosome, followed by the internalization of the endosome. Subsequent degradation of the nanoparticle by the lysosome leads to the release of the protein or peptide inside the cell [65]. Other ways of the release of drug molecules from the nanoparticle depend on factors such as the solubility of the therapeutic agent at a specific pH, polymer swelling, composition of the nanoparticle, etc. [41].

2.9.1 Solid lipid nanoparticle

Solid lipid nanoparticles (SLN, Figure 12) are composed of lipids that are solid at room temperature as well as body temperature and dispersed in water or an aqueous surfactant solution [66]. The lipids that can form SLN can be complex acylglycerol mixtures, highly purified triacylglycerol, or waxes. Extensive research has been performed on the solid lipid nanoparticles for their application in delivering proteins or peptides mainly due to their biocompatibility, biodegradation, and good tolerability [67]. It has been reported that SLN can enhance the bioavailability of the protein or peptide therapeutics and prolong their residence time in blood [68]. SLN can also enhance the oral absorption of many drugs [69, 70]. When SLNs are administered orally, they can be absorbed either through the M-cells (membranous epithelial cells) of the Peyer’s patches in the gut-associated lymphoid tissue (GALT) or transcellularly [71].

Figure 12.

General structure of solid lipid nanoparticles.

Surface modification of nanoparticles with chitosan is an excellent way of enhancing the penetration of encapsulated proteins or peptides (e.g., insulin) through mucosal surfaces. This chitosan modified SLN has antimicrobial, mucoadhesive, absorption-enhancing properties and low toxicity in addition to good biocompatibility and biodegradation [72]. Mucoadhesive properties of chitosan may enhance drug uptake due to the longer contact period with the intestinal epithelium. This prolonged contact of the nanoparticle with the intestinal membrane leads to enhanced penetration of the protein or peptide. Also, chitosan is an effective permeability enhancer as it reversibly changes tight junctions [73, 74]. Fonte et al. showed the ability of the chitosan-coated SLN to enhance the intestinal uptake of insulin by comparing with the uncoated SLN [75]. Significant improvement of hypoglycemic effect was observed for the chitosan-coated SLN compared to the uncoated SLN. Improvement of hypoglycemic effect could be due to mucoadhesive property of chitosan, which not only overcome the degradation of insulin in the GIT, but also promotes intestinal insulin uptake. However, one major limitation of the oral delivery of insulin-loaded nanoparticles is their elimination by the mononuclear phagocyte system [76, 77]. Macrophages present in various tissues such as the liver, spleen and bone marrow are also responsible for eliminating nanoparticles. The use of PEG [78] and other hydrophilic polysaccharides [79] coating to avoid phagocytosis of nanoparticles by macrophages has been reported. SLNs coated with chitosan were not internalized by the murine macrophage cell line, while the uncoated SLN were taken up by these cells [80]. Thus, chitosan was able to provide stealth properties to the SLN.

2.9.2 Chitosan based nanoparticles

Chitosan is a natural polysaccharide of glucosamine and N-acetyl glucosamine. Chitosan has some attractive features for being used in protein or peptide delivery [81]. Firstly, chitosan has high biocompatibility and low immunogenicity. Secondly, chitosan is a biodegradable polymer with high abundance. In the acidic environment, chitosan is protonated (Figure 13). This protonated form enhances the absorption of chitosan due to the interaction of the positively charged amino group with the cellular membrane [82, 83, 84]. This interaction leads to structural changes that opens the tight junction and allows the entry of proteins or peptides across the membrane. It is worth mentioning that chitosan exhibits absorption enhancing ability only in the acidic environment since it requires the protonated form of chitosan. Therefore, chitosan cannot act as an absorption enhancer in the neutral or basic environment [85, 86]. However, N-trimethyl chitosan (Figure 13) possess the absorption enhancing property over a wide range of pH, including the physiological pH. N-trimethyl chitosan-based nanoparticles loaded with insulin showed enhanced bioavailability of insulin [87]. N-trimethyl chitosan enhances the transport of proteins or peptides via both transcellular and paracellular pathways.

Figure 13.

Protonated (middle) and methylated (right) forms of chitosan (left).

2.9.3 Inorganic nanoparticles

Inorganic nanoparticles are receiving more attention in the development of protein or peptide carriers due to some special properties. The protein or peptide of interest can be encapsulated inside the nanoparticle, which can provide protection against denaturation of the protein or peptide of interest. Thus, nanoparticles preserve both the structure and the biological function of the protein or peptide. Nanoparticles also prevent possible enzymatic hydrolysis and other degradation in the harsh environment of the GIT. Finally, nanoparticles improve the shelf life of the incorporated protein or peptide [88]. CaP nanoparticle has been used in delivering protein (e.g., insulin) via the oral route. In this formulation, PEG-insulin and casein are encapsulated in the CaP nanoparticle (Figure 14). This formulation was shown to increase the half-life of insulin [89].

Figure 14.

Structure of a CaP-based insulin carrier.

2.9.4 Aquasomes

Aquasomes have recently emerged as solid nanoparticle drug carriers with a three-layered structure – core, coating, and drug (Figure 15) [90, 91]. It consists of a ceramic core coated with poly hydroxyl oligomer on which the protein or peptide of interest can be adsorbed. This polyhydroxy oligomer film protects proteins and peptides from changing shape and being damaged when they are surface-bound [92]. The layers that form aquasomes are assembled through the non-covalent bond, ionic bond, and van der Waals interactions [93].

Figure 15.

General structure of aquasomes.

Ceramics are mainly used as core material. These materials provide structural regularity and a high degree of order due to their crystallinity. This structure leads to efficient carbohydrate-binding on its surface resulting in the stable structure of aquasomes. Common materials used as the ceramic core in aquasomes are tin oxide, calcium phosphate, diamond nanoparticles, and hydroxyapatite. Among these, calcium phosphate and hydroxyapatite have excellent biocompatibility, biodegradability, and stability.

Carbohydrate coating provides a molecular layer capable of adsorbing therapeutic proteins or peptides without modification. Carbohydrates provide an environment that resembles water to the protein or peptide but keeps it in a dry solid-state, protecting the three-dimensional structure of the protein or peptide of interest [94, 95]. Carbohydrates commonly used for coating are trehalose, cellobiose, lactose and sucrose. The coating is achieved by the adsorption of the carbohydrate onto the core. The protein or peptide of interest interacts with the coating film by non-covalent or ionic interactions. Trehalose was previously reported to induce stress tolerance in bacteria, yeast, fungi, and some plants. Trehalose protects protein within the plant cell during the dehydration process and thus preserves cell structure [96]. It was observed that trehalose increased the transition temperature of proteins, resulting in increased stability [97]. Also, the hydroxyl group of carbohydrates interacts with the polar and charged group of proteins. Upon drying, the large number of hydroxyl groups of the carbohydrate replaces the water around polar groups in protein, thus maintaining their integrity [98].

A nanosized ceramic core-based drug delivery system has been developed for the oral administration of serratiopeptidase [99]. In this method, the calcium phosphate core was coated with chitosan, and the enzyme was adsorbed by the coating. The enzyme was further stabilized by encapsulating the enzyme-loaded core into alginate gel. The results indicated the ability of aquasomes to protect the structural integrity of the enzyme, resulting in a more potent therapeutic effect.

2.10 Hydrogels

Hydrogels are three-dimensional networks made of polymeric chains that are hydrophilic in nature. The polymer can be natural or synthetic and should contain a large amount of water. Although natural polymers show good biocompatibility, they are not suitable for protein or peptide delivery due to their improper mechanical strength. Natural polymers can also lead to an autoimmune response. However, hydrogels with the synthetic polymer can be designed to avoid these problems. Hydrogels can control the release of the protein or peptide of interest in response to pH [100, 101]. Hydrogels can also enhance transportation of the protein or peptide of interest. The difference of pH at different parts of the GIT has been utilized to control the release of insulin encapsulated in the hydrogels at the intestine (Figure 16). Hydrogels also showed rapid release of the encapsulated insulin once in the intestine. Finally, the encapsulation process of the protein (e.g., insulin) inside the hydrogel is highly efficient. These properties of the hydrogels make them promising systems for protein or peptide delivery [102].

Figure 16.

Protein or peptide therapeutic (black spheres) encapsulated in a hydrogel.

2.11 Injectable nanocomposite cryogels

Although hydrogels are promising systems for protein or peptide delivery, they have some issues regarding the rapid release of the encapsulated protein or peptide of interest. This rapid release leads to a short duration of action which may not be optimal for some therapies. Another issue associated with the hydrogels is the possible denaturation of the encapsulated protein or peptide. Injectable nanocomposites have been developed to overcome some of the issues related to hydrogels. Kinetics of the release of the therapeutic protein or peptide can be controlled by using suitable components of the system [103]. This delivery system can also be used for the sustained release of various therapeutic proteins and peptides.

2.12 Cell-penetrating peptides

Cell-penetrating peptides (CPP) are capable of transporting the attached molecule across the cell membrane. Therefore, one way of enhancing the permeability of therapeutic proteins or peptides across the cell membrane is to attach them to a CPP (Table 1) [104]. One way CPP can penetrate the cell membrane is via endocytosis. Another way could be the perturbation of the lipid bilayer of the cell membrane by the CPP. Although minor disturbances in the membrane structure were found, toxic effects of CPP on the cell membrane have not been reported. Insulin attached to TAT (Figure 17) showed better permeability across the cell membrane [105].

CPP	Protein or peptide
TAT	β-galactosidase, Rnase A, Horseradish peroxidase, Domain-III of pseudomonas exotoxin A, peptides derived from VHL tumor suppressor, insulin
11-poly arginine peptide	p53
Antennapedia	p16-derived synthetic peptide
hCT (9–32)	Green fluorescence protein

Table 1.

Examples of protein and peptides delivered by CPPs.

Figure 17.

Insulin attached to a CPP.

2.13 Protein crystallization

Crystallization of therapeutic proteins offers many advantages over the traditionally used protein solution or amorphous form of the protein. The protein in the crystalline form shows significantly higher stability than the amorphous form. This higher stability is advantageous for therapeutic proteins since the high stability of the proteins maintains their biological function in different environments [106]. Crystalline form of lipase enzyme is used as a therapeutic enzyme in pathological conditions related to abnormalities in the digestion of lipid. However, a major limitation of crystallization approach is that not all proteins can be crystallized. Also, in some cases, protein crystallization is not efficient. Nevertheless, protein crystallization remains a promising approach for developing protein or peptide delivery systems [107, 108].

3. Transdermal delivery

3.1 Microneedle

Microneedle technology uses small needles, which creates small pores in the skin, allowing the protein or peptide to cross the outermost physical barrier [109]. Since one goal of transdermal delivery is to increase efficacy while still retaining an easy, non-invasive technique, microneedles are designed to breach only the stratum corneum [110]. Since the microneedles do not reach the viable dermis, both the capillaries and the nerve ends are avoided. Thus, this approach leads to painless feelings during the drug delivery. These needles have been made using various materials, including silicon, different metals, and biodegradable material such as polymers or sugars [111].

Different types of microneedles, as well as drug introduction routes, have been tested for efficient delivery of protein or peptide therapeutics [112]. In one method, needles are used to puncture the skin to create pores, followed by the drug administration. Another method involves coating the microneedle with the protein or peptide of interest. Thus, the creation of pores by the microneedles will allow the drug to enter the body. Another method involves encapsulating the drug in biodegradable microneedles. In this method, the drug will be released slowly as the microneedles degrade. Another method utilizes hollow microneedles, through which the therapeutic protein or peptide can be infused following the puncture of the skin.

3.2 Thermal ablation

Like microneedle technology, thermal ablation makes the stratum corneum more permeable [113]. Both these methods avoid the breach of deeper capillary and nerve-containing tissues. However, the difference is that short pulses of high heat (~100°C) are used (instead of needles) to create small, reversible channels in the micron range [114]. Thus, following the application of short bursts of heat, the drug can be applied to the area for entering the circulation. Multiple systems have been designed to deliver drugs via thermal ablation successfully. While these systems successfully deliver smaller drug molecules, delivery of protein or peptide via this method is a work in progress.

3.3 Iontophoresis

Iontophoresis does not require physical disruption of the skin’s outer barrier. This method uses principles of electrorepulsion (for charged particles) and electroosmosis (for uncharged particles) to act on drug molecule rather than the skin [112]. During this process, a device capable of generating electric current is placed on the skin. When delivering a negatively charged peptide or protein, for example, the battery will build up a strong negative charge on the anode, which is placed in the same area of the skin as the drug molecule. Thus, the anode will drive the negatively charged protein or peptide into the skin due to charge–charge repulsion [115, 116]. It is important to note that the rate of drug release can be controlled easily by this method since the entry of the protein or peptide into the body is proportional to the current being applied to the skin [114]. Although the system successfully delivers small molecules (e.g., lidocaine), delivery of proteins or peptides via this method is still a work in progress.

3.4 Electroporation

Electroporation uses very short pulses of high voltage (10–100 V) to perforate the skin. Like microneedles and iontophoresis, this is also a non-invasive method of drug delivery since this method breaches only the stratum corneum [117]. Application of electric current disrupts the structure of the lipid-bilayer. This disruption allows the protein or peptide of interest to penetrate the skin. One advantage of this method is that the delivery of drugs can be easily changed by changing the voltage, number and duration of pulses, etc. [112].

3.5 Sonophoresis

Sonophoresis uses sound waves to increase the permeability of the skin. Similar to electroporation, sonophoresis also targets the lipid bilayer underneath the stratum corneum [112]. Sound waves (20–100 kHz) increases the pore sizes on the skin due to the increased fluidity of the lipid bilayers. Thus, sound waves allow the transcellular entry of drug molecules through the stratum corneum [114].

3.6 Biochemical enhancers

Biochemical enhancement utilizes biomolecules to enhance the permeability of the skin towards the peptide or protein of interest. This method aims to allow the entry of the drug molecule into circulation while being non-toxic, non-irritating, and non-allergenic [118]. Magainin, a 23-amino acid-containing peptide, has been reported to form pores in the bacterial cell membrane [119, 120]. Another small peptide TD1 increases the transdermal penetration capability of hEGF when fused together [121]. This system may play a vital role in delivering hydrophilic peptides transdermally.

3.7 Nanocarriers

Nanocarriers have been developed for administering protein or peptide therapeutics via the transdermal route. Nanocarriers have been found to be more effective penetration enhancers than biochemical enhancers. Nanocarriers commonly employed for delivering protein or peptide therapeutics are described below.

3.7.1 Transfersomes

Transfersomes are elastic or deformable liposomes (discussed in Section 2.7) [122]. These were developed to overcome the localization of liposomes on the skin. The membrane of transfersomes is formed by phospholipid and a single chain surfactant molecule (sodium cholate, sodium deoxycholate, Tween 20, Tween 60, dipotassium glycyrrhizinate, etc.). Transfersomes, by virtue of its tendency to avoid dry surroundings, enter into deeper layers of the skin that have higher moisture content than the surface layer. The elasticity of the membrane helps to breach the narrow gap on the surface of the skin. The enhanced drug transport by the transfersomes can be explained by its distribution on the skin surface after the adsorption on the skin [123]. The ability of transfersomes to transport protein or peptide therapeutics (e.g., insulin) has been reported [124, 125]. Encouraging results were obtained in the case of low molecular weight heparin [126].

3.7.2 Microemulsions

Water in oil microemulsions are stable dispersions consist of small water droplets dispersed within a continuous oil layer stabilized by incorporating a high concentration of surfactant/emulsifying molecules. Lipophilic surroundings of the external phase of the microemulsion resemble the environment of the upper layer of skin. This resemblance makes microemulsions ideal for application on the skin surface [127]. Also, the ease of administering microemulsions on the skin makes these ideal for passive delivery of proteins or peptides across the skin [128].

The efficacy of topically applied protein formulated with a microemulsion was investigated [129]. Rapid penetration of the molecule into the skin immediately below the application site was observed. Another study performed with desmopressin not only validated the capability of microemulsion as a transdermal carrier system but also indicated its superiority over creams and gels in delivering drugs across the skin [130]. The superior transdermal delivery of microemulsion has been attributed to the high loading capacity of the microemulsion and penetration enhancing effect of the constituents. Microemulsions have the capacity of encompassing a large amount of drugs without an increase in vehicle affinity. This capacity leads to a higher concentration gradient and higher transdermal flux from microemulsions. Also, the surfactant (e.g., isopropyl palmitate) can augment permeation by disrupting the intracellular lipid structure of stratum corneum [131, 132].

3.8 Prodrug

A prodrug is a reversible chemical modification of the drug to enhance its solubility, bioavailability, and stability without altering its pharmacological properties [133]. There are many examples where prodrug has been synthesized for proteins or peptides. For example, the thyrotropin-releasing hormone (TRH) has been successfully transported through human skin [134]. This was done by using the lipophilic prodrug of TRH, N-octyloxycarbonyl-TRH (Figure 18). The good skin penetration behavior of this prodrug was attributed to its high water solubility and lipophilicity. Conjugation of proteins or peptides with carriers that selectively transport them across the biological membranes can be used to deliver protein or peptide therapeutics across the skin [135, 136]. For example, the toxic protein ricin is transported across the cell membrane via binding to the ricin B chain found on the surface, followed by internalization. The active component is liberated upon entering the cell, where it exerts its toxicological effects. Thus, ricin behaves essentially as a prodrug of the ricin A chain.

Figure 18.

TRH and its prodrug.

4. Nasal route

Delivery of protein or peptide via nasal route has been employed in delivering desmopressin, calcitonin, and seasonal influenza vaccine [137, 138]. Advantages of this method include patient convenience and comfort, elimination of needle-related injuries and infections, and decreased syringe-related medical waste. Some disadvantages of this method include nasal irritation, limitation on the amount of drug that can be delivered, peptidases, and large interpatient variability in absorption. However, highly effective and non-irritating absorption enhancers have been developed to overcome some of the limitations [139]. This route could be important for drugs used in crisis treatment (e.g., pain, sleep induction, panic attack, nausea, heart attack, etc.) due to the rapid absorption of drugs from the nasal cavity to the systemic circulation. In some of these cases, the putative pathway from nose to brain might provide a faster and more specific therapeutic effect [140]. This route is also very important for delivering drugs against respiratory infections since this route provides not only systemic immune response but also local mucosal immune response. The latter should provide a much higher immune response against these diseases. Considering the potential benefits of this route of delivery, we can expect novel nasal products in the near future. Some nasal delivery systems are described below.

4.1 Chitosan

Chitosan has attracted much attention as a nasal delivery system recently. Chitosan is produced by the deacetylation of chitin found in crustacean shells. The resulting free amino groups allow chitosan to exist in the protonated form (see Section 2.9.2) in the acidic environment. Chitosan glutamate is a pharmaceutically acceptable chitosan salt for nasal drug delivery. It has an average molecular weight of ~250 kDa and a degree of deacetylation > 80% [140]. A nasal morphine product containing chitosan as an absorption enhancer is being investigated. Morphine is a polar drug and is not readily absorbed via the nasal route using simple formulations [141]. However, the addition of chitosan to the nasal formulation leads to a remarkable increase in nasal morphine absorption. As discussed in Section 2.9.2, chitosan improves the transport of polar drugs across the epithelial membrane via the transient opening of the tight junctions in the cell membrane [142, 143, 144]. Another mechanism of improving the transport of polar drug across the epithelial membrane by chitosan is via bioadhesion. Finally, chitosan is non-toxic and non-irritant to the nasal membrane [145].

Other cationic polymers, such as poly-L-Arg and aminated gelatin has also been investigated for their application as absorption enhancers. These polymers work in a way similar to chitosan. These polymers improve the absorption of fluorescein-isothiocyanate (FITC)-dextran and insulin with negligible nasal toxicity [146, 147, 148].

4.2 Cyclodextrins

Cyclodextrins have been used as absorption enhancers in animal models [149, 150]. But, their usefulness has not been confirmed in humans. However, cyclodextrin systems are in use in nasal formulations for drug stabilization.

4.3 Lipids and phospholipids

Phospholipid material didecanoyl-L-α-phosphatidyl choline was used as an absorption enhancer to develop the nasal delivery system for insulin. However, the development was halted due to low bioavailability in diabetic patients [151]. A nasal insulin formulation with a bioavailability of 35% has been reported. Irritation problems were also reported.

5. Buccal route

Delivery of proteins or peptides via buccal route involves administration of therapeutics through the mucosal membrane lining the cheeks [152]. In this process, the drug is placed in the mouth between the gum and cheek [153]. Buccal delivery has many advantages such as ease of use, bypassing the GIT, large contact surface area, etc. This method can also be employed to deliver hydrophilic macromolecules [154]. However, this method has some disadvantages such as irritation of the mucosa, low permeability of the peptide, and bitter taste of many buccal drugs. Absorption enhancers and adhesive polymers are being used to overcome some of these problems. Several drugs such as insulin, oxytocin have been successfully delivered via this route. Mucoadhesion (discussed in Section 2.6) is very important during the development of buccal drug delivery systems. Some methods employed for increased drug delivery via this route are described below.

5.1 Absorptions enhancers

Absorption enhancers are essential for delivering protein or peptide therapeutics, which generally show low buccal absorption rates. Some absorption enhancers are aprotinin, benzalkonium chloride, cyclodextrin, polyoxyethylene, sodium EDTA, etc.

5.2 pH

Permeability of acyclovir was investigated in the presence of sodium glycocholate (absorption enhancer) at a pH range of 3.3 to 8.8. The permeability of acyclovir was found to be pH dependent.

5.3 Patch design

Much research have been performed to understand the relationship between the type and amount of support materials and the drug release profile. Results indicated that these two factors are interrelated. It was also shown that single- and multi-layer patches have different drug release profiles.

6. Pulmonary route

The pulmonary route has been used to successfully deliver peptide therapeutics such as desmopressin, calcitonin, human growth hormone, parathyroid hormone, etc. [155]. There are several barriers to this route of peptide or protein delivery, such as respiratory mucus, mucociliary clearance, pulmonary enzymes, and macrophages that secrete peroxidases and proteases [156]. However, the large surface area, good vascularization, high capacity for solute exchange, and ultra-thinness are some of the attractive features of the alveolar epithelium that can facilitate systemic delivery of proteins or peptides via this route. The passage of large hydrophilic molecules through alveolar epithelium and capillary endothelium is limited. Absorption enhancers and enzyme inhibitors have been used to increase the absorption of peptide or protein therapeutics. However, these can be damaging to lung tissues. Some pulmonary delivery systems are described below.

6.1 Absorption enhancers

Cyclodextrins has been used as pulmonary absorption enhancers [157]. The relative efficiency of cyclodextrins in enhancing pulmonary insulin absorption follows the following order – dimethyl-β-cyclodextrin > α-cyclodextrins > β-cyclodextrin > γ-cyclodextrin > hydroxypropyl-β-cyclodextrin (Figure 19). Lanthanide ions are also effective in enhancing pulmonary insulin absorption [158].

Figure 19.

Structures of cyclodextrins (top) and the cyclodextrin-drug complex (bottom).

6.2 Enzyme inhibition

Enzyme inhibitors (e.g., bacitracin, bestatin, nafamostat mesylate, soybean trypsin inhibitor, potato carboxypeptidase inhibitor, phosphoramidon) has been shown to enhance pulmonary absorption of proteins and peptides [159, 160, 161]. In addition to insulin, the absorption of calcitonin has been shown to increase [161, 162, 163, 164].

6.3 Microparticles

The human lung is equipped with mechanisms to remove deposited particles by mucociliary clearance and phagocytosis. These clearance mechanisms should be considered while designing protein or peptide therapeutics and nanoparticles as a vehicle. The aim should be to achieve more efficient absorption and sustained therapeutic effect. It has been reported that inhalation of large porous insulin particles resulted in an elevated systemic level of insulin [165]. On the other hand, small non-porous insulin particles did not sustain therapeutic effect (4 hours vs. 96 hours for large porous particles). It has also been reported that the pulmonary delivery of insulin with nebulized DL-lactic/glycolide copolymer nanoparticle resulted in more sustained release (48 hours vs. 6 hours) compared to the nebulized aqueous solution [166].

6.4 Liposomes

Enhanced pulmonary absorption of protein or peptide therapeutics (e.g., insulin) has been shown using the Liposome (discussed in Section 2.7) as a carrier. Intratracheal administration of insulin liposomes led to increased pulmonary uptake of insulin [167]. The ability of liposome to promote pulmonary insulin absorption depends on the concentration, charge, and acyl chain length of the phospholipid [168].

6.5 PEGylation

Pulmonary absorption of protein or peptide therapeutics has been shown to increase using PEG. The pulmonary absorption of PEGylated r-huG-CSF generated a more intense response as compared to r-huG-CSF alone [169].

7. Rectal route

Rectal delivery of drugs is sometimes necessary if other routes of delivery are not applicable. This method offers some advantages such as rapid absorption of many low molecular weight drugs, partial avoidance of the first-pass metabolism, potential absorption into the lymphatic system, retention of large volumes, etc. This method is also beneficial considering the low amount of proteases compared to other parts of the GIT. This route allows both local and systemic delivery of drugs. Local delivery of drugs can be used to treat constipation, hemorrhoids, inflammation, hyperkalemia, etc. Systemic delivery of drugs can be used to treat pain, fever, nausea and vomiting, migraines, and sedation. Controlled absorption enhancement can also be achieved via this route due to the constant condition of the rectal environment. However, this method also has limitations, including limited absorption surface area, dissolution problem due to low fluid content, drug metabolism, etc. [170]. Several drugs such as insulin and pentagastrin have been successfully delivered via this route. Some rectal delivery systems are described below.

7.1 Absorption enhancers

Since the bioavailability of the peptide is low via the rectal route, various absorption enhances such as enamines, salicylate and its derivatives, surfactants, micelles, etc., have been investigated. It has been shown that enamine derivatives enhanced rectal absorption of CMZ (a hydrophilic antibiotic) [171]. Salicylate and its derivatives enhance rectal absorption of insulin, heparin, gastrin, and pentagastrin [172].

7.2 Protease inhibitors

Protease inhibitors reduce the degradation of various proteins and peptides due to the inhibition of proteases at the absorption site. Therefore, the use of protease inhibitors is one of the promising approaches for delivering protein or peptide therapeutics. Protease inhibitors (e.g., bacitracin, aprotinin, bestatin, trypsin inhibitor, puromycin, etc.) has been shown to enhance rectal absorption of protein and peptide therapeutics [173].

7.3 Chemical modification

Chemical modification of protein or peptide therapeutics is a potentially useful approach to deliver the protein or peptide of interest via the rectal route. For example, the acyl derivative of insulin improves its membrane permeability by making it more lipophilic [173]. Chemical modification may also protect the protein or peptide of interest from enzymatic degradation.

7.4 Cyclodextrins

Cyclodextrins are cyclo oligosaccharides consisting of several glucopyranose units (see Section 6.1). These act as host molecules that form inclusion complexes. Parent cyclodextrins can be modified to extend physicochemical properties and inclusion capacity. Cyclodextrins were shown to help rectal drug delivery with respect to stabilization, release, bioavailability, and alleviation of local irritation. Enhanced rectal absorption of lipophilic drugs by cyclodextrins is due to improved release from the vehicle and dissolution rate in the rectal fluid. However, the enhanced absorption of protein or peptide therapeutics is due to the action of cyclodextrins on the rectal epithelial cells [174].

8. Conclusion

Proteins and peptides are essential in various biochemical processes. These are also involved in various pathophysiological conditions. Therefore, the application of proteins and peptides to combat diseases, including cancer and diabetes, will be beneficial. However, unfavorable physicochemical properties of protein and peptides such as large size, hydrophilicity, and stability limit their use. Various approaches (discussed in this chapter) have been developed to overcome these problems. However, there is no general strategy for the delivery of protein or peptide therapeutics. One reason for the absence of a general strategy is the complex nature and variety of peptides and proteins. Thus, many strategies discussed in this chapter were focused on the delivery of the protein or peptide of interest. The long-term safety and efficacy of all these strategies should be considered. So, there are challenges to overcome in delivering the protein or peptide therapeutics. However, the future of protein and peptide delivery is bright considering the growing number of materials and combinatorial approaches. Also, emphasis is placed on developing cost-effective, tunable, biodegradable, and biocompatible materials for protein and peptide delivery. In the near future, it will be excellent to have a system that can be used for the delivery and systemic stability of different proteins and peptides.