Development of Nanoparticle-Based Mucosal Drug Delivery Systems for Controlling Pharmacokinetic Behaviors

Abstract

The mucosal layer in various mucosal tissues acts as a barrier that protects the epithelial membrane from foreign substances. However, in the process of mucosal absorption of drugs, the mucus layer, a smart biological sieve to particles and molecules, can be an obstacle to effective drug delivery. Recently, functional nanoparticles (NPs) have attracted considerable interest in the field of biopharmaceutical science owing to their delivery potential and effectiveness. Among various pharmaceutical technologies, mucopenetrating NPs (MPP) and mucoadhesive NPs (MAP) are viable dosage options for controlling pharmacokinetic behavior by modifying drug absorption from the mucosal site. MPP and MAP can rapidly deliver encapsulated drugs to the absorption site by passing through the mucus layer and/or retaining NPs near the absorption membrane, possibly resulting in better drug delivery than that of conventional approaches. Modifying the surface properties of NPs is critical for determining their potential diffusiveness within the mucus layer owing to various types of interactions between the mucosal components and the surface of NPs. Additionally, the physiological characteristics of the mucus layer (thickness, viscosity, and turnover time) differ depending on the mucosal site. Thus, a deeper understanding of the design of NPs and the functional properties of the administration site is essential for developing mucosal drug delivery systems (mDDS) to maximize the potential of target drugs. This review summarizes the basic information and functions of the mucosal layer, highlights the recent progress in designing functional NPs for mDDS, and discusses the advantages and disadvantages of mucosal administration at major mucosal sites.

1. INTRODUCTION

In recent advancements of medical sciences, effective drug delivery has been a major challenge in achieving desirable pharmacokinetics. Although various therapeutic modalities, not only small molecules but also medium and large molecules, have been strategically developed to overcome incurable diseases that still lack effective treatment strategies, optimized drug delivery systems (DDS) are necessary in most cases.¹⁾ Nanoparticulate delivery systems have attracted considerable attention over the past few decades and constitute a wide area of interest in pharmaceutical research. Nanoparticles (NPs) have unique physicochemical properties derived from their particle size, surface properties, solubility, and other modifications, including (i) improvement of dissolution behavior by increasing the active surface area, (ii) stabilization of inner compounds by encapsulation, (iii) controlled release of encapsulated drugs, (iv) enhancement of intestinal cellular uptake, and (v) addition of various functionalities by modifying surface properties. Because of the potential of NPs to provide effective and safe drug delivery, NP-based technologies have been investigated by researchers worldwide and applied to drug delivery, immunotherapy, and diagnosis.^2–5)

To maximize the therapeutic potential of encapsulated drugs, various parenteral routes, including intravenous, intramuscular, and subcutaneous routes, have been used; however, significant attention has been paid to the development of alternative administration routes that do not involve any pain owing to invasiveness through skin penetration. In this regard, mucosal absorption routes through the oral, nasal, pulmonary, ocular, and vaginal mucosa could be the best alternative sites for administration being easy, non-invasive, safe, cost-effective, and ability of self-administration by patients.^6,7) The mucosal surface of the body is considerably large and represents an extensive area for therapeutic delivery. Despite various advantages of mucosal absorption, several factors make it challenging to control and estimate the absorption of target drugs.

Mucus layer is a major obstacle to effective drug delivery via the mucosal route.⁸⁾ Particle-based delivery systems tend to become trapped in the mucosal layer because they are larger than molecules and are eliminated from the absorption site by mucus turnover. Thus, understanding the pathological characteristics of mucus and its influence on the drug absorption process is important for realizing desirable drug delivery through mucosal absorption. Mucus layer is a ubiquitous system composed of viscous substances secreted from intraepithelial cells in various parts of the body, including the buccal region, stomach, intestine, trachea, and eyes.⁹⁾ These organs are always exposed to the external environment; thus, the mucus layer can act as a smart physiological barrier against pathogens and other undesired xenobiotics.¹⁰⁾ The mucosal layer provides protection against chemical, physical, and immunological risks. Similarly, the mucus layer can disturb the effective delivery of particulates by trapping them, possibly leading to a reduction in the bioavailability of target drugs. Thus, these sites offer versatile physiological and anatomical milieus, contributing to the ease of administration, as well as challenges in selecting suitable formulation systems and administration sites.

Mucosal drug delivery system (mDDS) has recently been investigated for the mucosal administration of pharmaceutical agents.¹¹⁾ mDDS can control the diffusiveness of particulate systems by modifying the interactions between the particle surface and mucus components present in the mucus layer, providing mucopenetrating and mucoadhesive properties to the particles. The strategic application of mDDS could enhance pharmacokinetics and enable targeted delivery of various drugs. Therefore, the mucosal barrier has attracted considerable attention because of its functional role in DDS studies. This review briefly summarizes the general physiology of the mucosal sites and approaches to make them attractive for controlling the mucosal absorption of drugs. This review also focuses on the advantages and disadvantages of delivering NPs through each administration route with a mucus layer to emphasize the applicability and limitations of these systems.

2. GENERAL PHYSIOLOGY OF THE MUCUS LAYER AND PHARMACEUTICAL STRATEGIES FOR MDDS

The mucus layer is a viscoelastic hydrogel layer with shear-thinning behavior. Mucus is composed of water (90–98%), glycoproteins (1–5%), electrolytes, lipids (1–2%), and proteins.¹²⁾ Mucin, a main component of the mucosal layer, is a unique glycoprotein with high molecular weight; membrane-bound mucins and secreted mucins (gel-forming mucins) are the two types of mucins. Mucus layers are composed of gel-forming mucins secreted from goblet cells.¹³⁾ The structure of mucin includes sulfate groups on N-acetyl glucosamine and galactose, and carboxylic groups on sialic acid sugars, providing an overall negative charge to mucins under most physiological pH conditions. The surface of the epithelium at mucosal sites is covered with mucus, which consists of mucin polymers connected via disulfide bonds. Mucins are continuously secreted from functional cells such as goblet and glandular cells, and the thickness of the mucus layer differs depending on the balance between its production and turnover.⁹⁾ There are some differences of physiological characteristics between each mucosal site (Table 1). The mucus layer is the thinnest in the pulmonary tract (0.1–0.2 µm) and relatively thick at the buccal site (500–800 µm) and rectal site (1000–2000 µm). Because of the presence of mucin, a large complex glycosylated protein, mucus can form mesh-like matrix structures with viscous gel layers on various mucosal tissues such as the gastrointestinal tract, respiratory tract, nose, and eyes.¹⁴⁾ This mesh-like structure can act as a biological sieve to protect the epithelial surface against pathogens and pernicious materials and inhibit cellular uptake by trapping and eliminating particles before the membrane permeation process.

Table 1. The Brief Specifications of Main Mucosal Tissues, Including Physiological Characteristics and Advantages/Disadvantages of Drug Delivery

Site	Mucosal thickness	Surface area	Biological secretions	Advantages	Disadvantages
Oral cavity	500–800 µm (Buccal)　 100–200 µm (Sublingual)	200 cm2	0.5–2.0 L/d	•Easy administration　 •Avoids first pass metabolism	•High clearance by saliva　 •Limited absorption through epithelium
Gastrointestinal tracts	100–500 µm	200–300 m2	10 L/d	•Easy administration　 •High surface area	•Severe environment (pH and metabolic enzymes)　 •Limited absorption through epithelium
Nasal	700–1000 µm*	160 cm2	1–1.5 L/d	•Easy administration　 •Potential for brain drug delivery　 •Highly vascularized	•Mucociliary clearance　 •Difficulty of penetrating mucus　 •Enzymatic degradation
Pulmonary	0.1–0.2 µm	70–80 m2	5–40 mL/d	•Rapid absorption　 •Highly vascularized　 •Large surface area　 •Limited enzymatic activity	•Difficulty of particle delivery to deeper lung　 •Mucociliary clearance　 •Phagocytosis by alveolar macrophage

* Estimation of normal septum mucosa.

Two mechanisms of the mucosal barrier may contribute to the elimination of drug particles: (i) size-dependent theory by mucin mesh-like matrix structures and (ii) molecular interactions of mucin with drug molecules and/or particulate systems, including entanglement via hydrophobic and electrostatic interactions.¹⁵⁾ The mucus barrier can be formed by a high-density fiber network of mucin with an average pore size of 20–200 nm. Essential substances such as water, gas, and nutrients can easily penetrate the mucin mesh. In contrast, molecules/particles (>30000 Da¹⁶⁾ and >500 nm¹⁷⁾) larger than the pore size likely disrupt the penetration of the mucus layer due to steric hindrance and are trapped in the mesh structure. In theory, the smaller the molecules and particles, the easier is their passage through the mucus layer. However, the interactions between mucin and drugs can influence the permeation process through the mucus later, even if the size of the drug is much smaller than the pore size of the mucin mesh, owing to increased solute-solvent resistance.¹⁸⁾ Considering these mechanisms, not only the size but also the surface properties significantly contribute to determining the diffusiveness through the mucus layer. Appropriate drug delivery systems based on the clearance mechanism of the mucus system should be considered to achieve effective and safe drug delivery. Therefore, several strategies have been developed to control the diffusion of drug nanoparticles through the mucus layer, including mucopenetration and mucoadhesion of NPs (mucus-penetrating particles (MPP) and mucoadhesive particles (MAP))¹¹⁾ (Fig. 1). Understanding the appropriate interactions and mechanisms of penetration and/or adhesion of NCs in the mucus layer has enabled researchers to identify, select, and develop materials for designing functional NPs.

Fig. 1. Schematic Illustrations of the Mechanisms for Mucopenetrating and Mucoadhesive Properties of NPs after Mucosal Administration¹¹⁾

2.1. Mucopenetration Strategies

To achieve sufficient drug absorption for systemic delivery of target drug, avoiding the protective system and/or exiting the barrier mechanisms of mucus layer should be considered. MPP are a promising delivery system to improve the oral absorption and bioavailability of encapsulated drugs as they can deliver the particulate system close to the epithelium at the absorption site.¹⁹⁾ As described in the previous section, various interactions, including physical entanglement, hydrophobic interactions, and electrostatic interactions, can capture foreign substances, making it challenging for NPs to penetrate the mucus layer. To minimize the interactions between mucin and the particle surface, formulation design of a bioinert surface with enhanced penetrability is critical. In addition, the reduction in the net charge and charge density of NPs can contribute to making the particle surface more bioinert because electrostatic interactions such as hydrogen bonding and ionic interactions can be suppressed.

2.1.1. Polyethylene Glycol (PEG)-Coated Surface

PEG is widely known as a hydrophilic polymer with high biocompatibility.²⁰⁾ Surface modifications of PEG chains are commonly applied to provide bioinert properties against various physiological barriers, not only at the mucosal site but also in the blood, for the development of various DDS approaches.²¹⁾ PEG surface can protect the inner compounds from enzymatic degradation and interactions with mucin, owing to a dense hydrated layer on the surface of NPs.²¹⁾ In addition, the surface PEG chains being neutral in charge can reduce the net charge of NPs. The bio-inert neutral surface of PEG-coated NPs can easily penetrate the mucus layer. The chemical properties of PEG, such as its length and structure, significantly affect the penetration potential of NPs.²²⁾ To design an MPP, an appropriate length of PEG chains is required, and 2000–5000 Da PEG is frequently selected as the PEG chain for surface modification in many studies.²³⁾ Poly(lactic-co-glycolic acid) (PLGA) coated with 5000 Da PEG NPs exhibited better mucus penetration properties than those coated with 1000 Da PLGA NPs.²⁴⁾ Moreover, Inchaurraga et al. reported a higher mucopenetration of 2000 and 5000 Da PEG-coated NPs, consisting of a copolymer of methyl vinyl ether and maleic anhydride, than those coated with 10000 Da PEG.²⁵⁾ Considering these findings, an optimum length of PEG chains is required for MPP because too long PEG chains may entangle with the mucus, resulting in impaired mucopenetration and rather shows mucoadhesive properties. In addition, a high-density PEG coating on the particle surface is more effective for obtaining MPP. Although surface PEG enables NPs to pass through the mucus layer, high bioinertness of PEG surface also weakens the interactions with the cellular surface, possibly limiting drug absorption if the drug is effectively released from NPs. Therefore, a balance between the bioinertness of mucin and its interactions with the cellular surface should be considered to achieve efficient systemic drug delivery.

2.1.2. Zwitterionic (Virus-Mimicking) Surface

Some viruses such as Hepatitis B, human papilloma virus, and Norwalk virus can move within the viscous mucus layer as quickly as in aqueous or saline solution.²⁶⁾ Thus, imitating the surface characteristics of such viruses for NPs is a desirable approach to ease their penetration through the protective mucus layer. Viruses have unique surface properties, with a high density of net charge composed of equal amounts of cationic and anionic substances; the total net charge can be neutral,²⁷⁾ resulting in high mucopenetration. The highly charged or highly polar surface of viruses can inhibit nonpolar interactions with the hydrophobic domains of mucus, enabling the surface of viruses to be bioinert. Additionally, the zwitterionic neutral surface can form a stable aqueous layer that protects it from interactions with biological substances owing to its highly polar properties, forming ion-dipole interactions and hydrogen bonding with the surrounding water molecules. This hydration shell may contribute to the enhanced mucopenetrating properties of NPs. Amphipathic materials such as phospholipids, polycarboxybetaine, polyphosphorylcholine, polysulfobetaine, and polydopamine can be used as coating materials to develop NPs with zwitterionic surfaces.^28–30) Combining cationic and anionic components is another option for preparing NPs with completely neutral and highly charged surfaces. Anionic polymers, such as polyacrylic acid, alginate, chondroitin sulfate, hyaluronic acid, carrageenan, and pectin, and cationic polymers, such as chitosan, protamine, and polymethacrylates with amino or ammonium substructures, have been reported for the design of NCs with highly charged neutral surfaces for MPP formulations. MPP composed of chitosan and chondroitin sulfate exhibited better mucopenetrating properties than control PLGA NPs.³¹⁾

2.1.3. Mucolytic Strategies

Mucolytic strategies focus on weakening the mucin mesh barrier system by degrading the mucus layer in the mucosal tissues.³²⁾ These are known as active mucopenetrating systems and mainly include two strategies based on the use of (i) mucolytic drugs and (ii) mucolytic enzymes. Although mucolytic strategies would enhance the mucopenetration efficiency of particulate systems by cleaving the mucus barrier, simultaneous risk of invading pathogens and foreign substances increases. Thus, an optimum system should be designed to achieve localized mucolysis around the target absorption site to minimize such risks.

N-Acetyl cysteine (NAC) is a well-known mucolytic agent used clinically as an expectorant drug and has been applied to MPP in some studies. NAC has a free sulfur group and can form disulfide bonds with cysteine groups in the mucus layer, leading to disruption of the mucin mesh network. Thus, NAC can cleave disulfide bonds in the mucus layer and reduce cross-linking of mucus gels, possibly leading to enhanced mucopenetration of NCs. Dithiothreitol, thiobutyl-amidine dodecylamide, and thioglycolic acid-octylamine have also been reported as mucolytic compounds that can degrade the mucus layer.³³⁾ These mucolytic agents are generally co-encapsulated with the target drug in NCs to avoid unexpected disruption of a wide area of the mucosal site.

The conjugation of mucolytic enzymes on the surface of NPs is another strategy for developing a mucolytic delivery system, and papain, bromelain, and trypsin have been applied as mucolytic agents for MPP system. Papain-conjugated poly(acrylic acid) (PAA) NPs showed a 2.5-fold higher mucopenetrating potential than that of control NCs without surface conjugation.³²⁾ Papain-, trypsin-, and bromelain-conjugated PLGA NPs exhibited 2–3-fold higher mucosal permeation in porcine mucin than that of reference NPs.³⁴⁾ Although these strategies can enhance the penetration potential of NPs through the mucus layer, susceptibility of these enzymes to physiological conditions at the target site should be considered to prevent their deactivation, indicating the necessity for stabilization and protection from these factors using other DDS strategies.

2.2. Mucoadhesive Strategies

MAP has also gained much attention for controlling the pharmacokinetic behavior and prolonging the residence time of NPs at the absorption sites in the mucosa, possibly leading to enhanced drug absorption and prolonged drug exposure of the disease site. Mucoadhesion is a complex process related to six different theories that explain the possible mechanisms, including wetting, adsorption, electronic, diffusion, dehydration, and fracture theories.³⁵⁾ In the contact process, the first step of mucoadhesion, the material must be in close contact with the mucus layer surface. If the attractive forces (van der Waals forces and electrostatic attraction) between the materials and the mucus layer are not strong enough to overcome the repulsive forces (e.g., osmotic pressure and electrostatic repulsion), the adhered particles can be easily removed by clearance mechanisms at the mucosal site.

Wetting theory can be applied in case of low viscosity or liquid adhesion and depends on the spreadability of the liquid sample on the target surface.³⁶⁾ According to this theory, the smaller the contact angle, the higher the adhesion potential. The absorption theory can be explained by secondary bonds (molecular interactions), including hydrogen bonds and van der Waals forces.³⁵⁾ Based on the electronic theory, net charges of the sample and mucus are important for enhancing mucoadhesion. Negatively charged mucus surface can form an electrically charged double layer with a positively charged mucoadhesive system. This leads to the generation of an attractive force. Diffusion theory describes physical entanglement between mucus components and mucoadhesive materials.³⁷⁾ The adhesion force increases depending on the degree of polymer penetration within the mucus chains. The flexibility of polymer chains, mobility, contact time, and diffusion coefficient are critical factors that determine the adhesion potential. The dehydration theory considers dehydration from the mucus layer by the gelling process of mucoadhesive materials upon contact.³⁷⁾ Different osmotic pressures can be a driving force that causes dehydration of the mucosal layer and gel-forming materials. Molecules remove water from the mucus until an osmotic balance is achieved. The dehydration process enhances the mixing of the material and mucus, resulting in increased contact time with the mucus membrane. The fracture theory helps determine the mechanical strength of mucoadhesion by measuring the force required to separate two surfaces. The type of adhesion bonds can affect the detachment forces on the surfaces.

2.2.1. Cationic and Anionic Charged Surfaces

Cationic and anionic surfaces have mucoadhesive potential because of their ability to form ionic interactions and/or hydrogen bonds with mucin components. Chitosan, alginate, PAA, and cellulose derivatives have been used as mucoadhesive materials in several previous studies.

Cationic materials can interact with negatively charged moieties within the chemical structure of mucin, which is primarily composed of sialic acid, resulting in a negatively charged mucus layer. Thus, enhanced bioadhesion at the mucosal site can prolong the residence time at the absorption site, possibly leading to improved and long-lasting absorption of encapsulated drugs. Chitosan is a well-known semisynthetic polysaccharide produced by the deacetylation of chitin and is used in various types of DDS carriers, such as mucoadhesive NCs, owing to its unique characteristics and high biocompatibility.^38–40) Some researchers have attributed the enhanced membrane permeability of chitosan to the interactions between cationic chitosan and the membrane surface, resulting in the opening of epithelial tight junctions. In previous reports, chitosan-based NPs have been applied not only to small molecules but also to macromolecules such as peptides and insulin; this may be possible because of their mucoadhesive and membrane-permeable potentials,⁴¹⁾ which are significant barriers for the absorption of macromolecules. Combination with other biocompatible polymers, such as PEG, can also improve the mucoadhesiveness of chitosan-based NCs.^42,43)

The carboxyl group is one of the main contributors to creating the anionic surface of NPs and has the potential to form hydrogen bonds, hydrophobic interactions, and van der Waals bonds with mucosal components, such as sialic acid groups and sulfate residues, within the oligosaccharide chains of mucin proteins, depending on the pH and ionic composition. Among the many reported anionic mucoadhesive materials, alginate and PAA have attracted considerable attention in MAP because of their high biocompatibility.⁴⁴⁾ Synthetic derivatives of PAA were first synthesized and patented in 1957 and have a negative net charge derived from their carboxyl groups. Several derivatives with various molecular weights and polymer structures are available as DDS materials. Their pH-sensitive characteristic enables NPs to achieve localized delivery and long-term absorption from adhesion site depending on the environmental pH at the absorption sites.⁴⁵⁾ Alginate is a polysaccharide extracted from seaweeds and consists of 1–4 linked α-L-guluronic acid and β-D-mannuronic acid residues.⁴⁶⁾ The carboxyl groups in its structure can form hydrogen bonds with the sialic acid and sulfate residues in mucin, contributing to its relatively strong mucoadhesive properties. There are some advantages to using alginate as a carrier material; for example, it shows stronger mucoadhesion than that of non-ionic and polycationic polymers and has biodegradable properties. Hyaluronic acid and chondroitin sulfate with anionic net charges have also been investigated for the development of MAP systems.⁴⁷⁾

2.2.2. Formation of Disulfide Bonds

The formation of disulfide bonds between the MAP surface and mucin molecules containing a thiol group-rich domain can help NPs to achieve stronger adhesive characteristics than noncovalent bonds owing to the difference in the strength of bonds.⁴⁸⁾ Different types of thiolated polymers, including thiolated chitosan, PAA, and alginate have been developed to improve mucoadhesion.^49,50) Similar to thiolated chitosan, chitosan-cysteine, chitosan-thioglycolic acid, chitosan-thioethylamidine, and chitosan-4-thiobutyl-amidine have been investigated for the development of MAP. Thiolated chitosan can form two strong molecular interactions with mucin: electrostatic interactions between cationic amino moieties of chitosan and negatively charged sialic acid of mucin (described above), and disulfide bond with cysteine-rich domain in the mucin structure.

To design an MAP system with thiolated surfaces, a suitable reactivity should be considered.⁵¹⁾ Excessively high reaction leads to rapid formation of disulfide bonds, resulting in adhesion of the particulate system only to the surface of the mucus layer. Such poor interpenetration may result in rapid clearance of the system from the mucosal surface during mucus turnover. Thus, mucoadhesion within the deeper area of the absorption site would be ideal for the effective prolongation of drug exposure and absorption. Thiolated NPs can be more reactive in deeper areas of the mucus layer because the pH conditions close to the absorption membrane (pH 7.2) are more suitable for the formation of disulfide bonds by thiol-disulfide exchange reactions than the pH conditions on the surface of the mucus layer.

3. ADMINISTRATION SITES FOR MUCOSAL DRUG DELIVERY SYSTEMS

3.1. Oral Administration

Oral administration route includes the largest continuous mucosal membrane in the body, from the oral cavity to the rectum, suggesting a desirable administration site for efficient absorption from the mucosal site. Owing to the ease and safety of the administration route by the patients themselves, there are many reports on the development of oral mDDS not only for systemic delivery but also for site-specific delivery. However, many obstacles, including severe pH gradient conditions from the stomach to the colon, enzymatic degradation, permeation of the cellular membrane, and clearance system of the mucus layer, must be overcome to achieve efficient oral delivery. For oral administration, two major parts of the mucosal site are used to design oral mDDS.

3.1.1. Oral Cavity

The oral cavity plays a significant role in drug delivery, particularly for systemic and local treatments.⁵²⁾ From a physiological perspective, its structure and environment provide unique opportunities for drug absorption. The oral cavity is lined with a mucosal membrane composed of stratified squamous epithelium, which varies in permeability depending on the region. For example, the sublingual and buccal areas are highly vascularized and thin, allowing efficient absorption of certain drugs directly into the systemic circulation. Sublingual drug delivery, commonly used for nitroglycerin in angina treatment, takes advantage of rapid onset due to the high permeability of the sublingual mucosa. Buccal delivery, on the other hand, is used for sustained-release formulations, providing prolonged therapeutic effects. There are many advantages of drug delivery through the oral cavity, including ease of administration for elderly or very young patients, less digestive activity, limited influence of food intake compared to the intestinal site, and avoidance of first-pass metabolism through the buccal route. One of the major disadvantages is the continuous secretion and movement of saliva, which is related to the clearance system in the oral cavity, resulting in high clearance and short residence time of particulate systems in the oral cavity.⁵³⁾ To prevent elimination from the administration site in the oral cavity, MAP can be a promising approach for enhancing the retention of NPs and active compounds.

3.1.2. Intestinal Site

The intestine is an important region for mucosal drug delivery and absorption and plays a central role in systemic pharmacotherapy. The intestine, particularly the small intestine, provides an extensive surface area for drug absorption owing to the presence of villi and microvilli, which collectively form the brush border. This large surface area, combined with rich blood supply, facilitates efficient drug uptake by the intestinal tissues. Similar to oral administration, which is the most convenient route for most patients and allows self-administration without any pain, this route is also safe for administration owing to the lack of need of special tools such as syringes. In terms of regulatory issues in the manufacturing process of formulations, oral formulations are favorable owing to the non-aseptic process of production and preparation. In addition to the portal venous system, the lymphatic absorption is an important pathway for lipophilic drugs, relatively large molecules, and colloids. The capillaries of the endothelium in the lymph have greater permeability than that of blood vessels. The lymphatic pathway can avoid the first-pass metabolism and improve the bioavailability of administered drugs. Although the advantages of intestinal absorption are significant, complex physiological conditions, such as pH variations, enzymes, and mucosal layers, should be considered when designing an appropriate oral DDS. The pH gradients through the gastrointestinal sites influence the ionization state of drug molecules and surface properties of NPs, resulting in different absorption processes depending on the site. Drugs susceptible to metabolic enzymes, such as esterases, proteases, and lipases, should be protected from enzymatic degradation for sufficient absorption. Since the thickness and protective function of the mucus layer change depending on the mucosal site, it might influence the absorption of drugs.¹⁰⁾ In recent studies, strategic applications of NP systems have been reported because of their controlled release potential, protective effects of encapsulated drugs, and functionalization via surface modifications.⁵⁴⁾ Our group developed cyclosporine A (CsA)-loaded MPP and MAP using PEG-polystyrene (PS) and PAA-PS to control the intestinal absorption process using the mDDS approach⁵⁵⁾ (Fig. 2). Oral administration of PEG-CsA and PAA-CsA resulted in significant improvements in oral absorption, as evidenced by 50- and 25-fold higher oral bioavailability, respectively, than that of unprocessed CsA. In terms of absorption rate, PAA-CsA exhibited a more sustained and slower oral absorption of CsA than that of PEG-CsA, possibly because of the different diffusion behaviors within the mucus layer.

Fig. 2. Controlling Oral Absorption Behavior of Cyclosporine A Using mDDS Approach⁵⁵⁾

(A) Plasma concentration profiles of cyclosporine A after oral administration of each CsA sample, including PEG-CsA (●), PAA-CsA (■), and amorphous CsA (▼). (B) Schematic illustrations for possible mechanisms to control the drug absorption and biodistribution of fluorescence probe-loaded PEG-CsA and PAA/CsA at intestinal tissues after the intestinal perfusion of NP suspensions in rats (Red, nanoparticles; and blue, nuclei). BA, bioavailability; and T_max, time to maximum concentration.

Although these results indicate the potential of mDDS to control intestinal absorption and pharmacokinetic behavior, there are still some issues to overcome. For the particulate system, permeation of the epithelial membrane seems to be one of the major hurdles owing to the tight connections between cellular membranes to protect against invading harmful substances. Endocytosis and phagocytosis contribute to the absorption of relatively large substances; however, the absorbable amounts are limited. Thus, the combination of absorption enhancers, which can alter the permeability of the cellular membrane, improves the mucosal absorption of relatively large substances.

3.2. Nasal Administration

Nasal administration is a non-invasive drug delivery route that leverages the unique anatomy and physiology of the nasal cavity. The nasal cavity is highly vascularized and lined with a thin mucosal membrane, making it an effective site for both local and systemic drug delivery. Anti-inflammatory steroids, antihistamines, vasoconstrictors, and various other drugs can be used to treat topical symptoms, including sinusitis, rhinitis, coryza, nasal bleeding, and nasal polyps. This route offers rapid absorption and onset of action as it bypasses the gastrointestinal tract and first-pass metabolism in the liver. The nasal cavity provides two primary absorption pathways: the respiratory epithelium and olfactory region. The nasal respiratory epithelium facilitates systemic drug delivery through dense capillary network. By contrast, the olfactory region provides a direct route to the central nervous system via the olfactory bulb. Thus, intranasal administration has recently attracted much interest for the delivery of drugs that target the brain as a potential alternative route to pass through the blood-brain barrier.⁵⁶⁾ However, there are some limitations that hinder effective drug absorption from the nasal mucus. Rapid mucociliary clearance can eliminate the drug and particles adhered to the surface of the mucus, limiting absorption. In addition, drug solubility (dissolution rate) under a limited liquid volume leads to less drug absorption because the drug particles can be cleared from the nasal cavity by the mucociliary clearance mechanism. Formulation strategies such as the use of mucoadhesive agents or nanoparticles are often employed to enhance drug retention and absorption. There have been some reports of improved residence time and mucosal absorption using MAP. Furthermore, a combination of absorption enhancers such as cell-penetrating peptides can achieve efficient brain delivery of macromolecules such as oligonucleotide therapeutics.⁵⁷⁾

3.3. Pulmonary Administration

Pulmonary administration is a drug delivery method that uses the respiratory system, particularly the lungs, for local and systemic drug delivery. Drugs administered via the pulmonary route can reach the lungs in the form of aerosols, powders, or solutions typically via inhalers or nebulizers. Pulmonary administration is commonly employed for the treatment of respiratory diseases such as asthma and chronic obstructive pulmonary disease (COPD) because of its potential for efficient topical drug exposure of the disease site with limited systemic exposure. Although most of clinically available medicines used for pulmonary administration have topical action in the respiratory tract,⁵⁸⁾ this route has attracted much attention as a systemic delivery route owing to the large surface area of alveoli, extensive capillary network, relatively low enzymatic activity, and thin alveolar-capillary barrier, which facilitate rapid drug absorption and onset of action.⁵⁹⁾ The delivery efficiency of respirable formulations depends on the size of inhaled aerosols. Generally, particles in the range of 1–5 µm are suitable for delivery to the deep of the respiratory tract or alveoli, the main absorption site for systemic delivery. Larger particles are more likely to be deposited in the oral cavity and/or upper airway, whereas smaller particles are exhaled. Pulmonary administration bypasses the first-pass hepatic metabolism and improves the bioavailability of systemic drugs. Variability in the respiratory functions and inhalation techniques of patients influence aerosolization efficiency of the formulation, resulting in sufficient drug delivery. In addition, mucociliary clearance and potential irritation or inflammation of the respiratory tract alter the biodistribution of inhaled drugs, suggesting the need to develop an appropriate DDS approach. Advanced formulations, such as liposomes, nanoparticles, and dry powder inhalers, aim to enhance drug delivery efficiency and therapeutic outcomes. Previously, our group developed and applied MAP and MPP systems for pulmonary delivery to clarify the delivery potential of these functional NPs⁶⁰⁾ (Fig. 3). In this study, visualization of biodistribution by fluorescence imaging indicated a longer residence potential of the MPP system than that of MAP because the MPP can rapidly reach the vicinity of the respiratory epithelium. In contrast, MAP is likely eliminated by the mucociliary clearance system, possibly because of adhesion, mainly around the surface of the mucus layer. Thus, MPP may be preferable in terms of retention in the respiratory system for long-lasting pharmacological action of drugs.

Fig. 3. Biodistributions of Mucoadhesive and Mucopenetrating Nanoparticles after Pulmonary Administration in Rats⁶⁰⁾

(A) Mucoadhesive NPs showed the aggregations (arrows) on the surface of pulmonary mucus layer. The NPs adhered on the surface of mucus could be rapidly removed from the lung area. (B) Mucopenetrating NPs showed uniform distribution within the mucus layer, and longer retention than mucoadhesive NPs.

4. CONCLUSION

Recent studies on NPs with controlled diffusion properties in the mucus layer have highlighted that surface-modified NPs can substantially enhance both mucopenetration and mucoadhesion of various DDS. Depending on the physiological characteristics of the target mucosal site, an appropriate application of mDDS approach can be used to control the pharmacokinetic behavior of the target drugs. Nevertheless, challenges such as potential safety concerns, side effects, and scalability of most surface modification techniques are major hurdles to their clinical applications. To develop ideal NPs for specific target diseases, close communication and collaboration with formulators, physiologists, and toxicologists are imperative, all while keeping in mind the relevant safety regulations. If these challenges can be addressed through further non-clinical studies, clinical trials, and optimization of manufacturing processes, NPs based on mDDS hold promise for treating numerous diseases.

Acknowledgments

I would like to acknowledge Dr. Satomi Onoue and Dr. Kohei Yamada at Graduate School of Pharmaceutical Sciences, University of Shizuoka, Dr. Yoshiki Seto at Gifu University of Medical Science, and Dr. Shingen Misaka at Fukushima Medical University. I wish to thank Dr. Robert K. Prud’homme at Princeton University and Dr. Hak-Kim Chan at the University of Sydney. I also thank all laboratory members at the Laboratory of Biopharmacy, School of Pharmaceutical Sciences, University of Shizuoka, and all collaborators. This work was supported in part by the JSPS KAKENHI [Grant-in-Aid for Scientific Research (C) (No. 23K006083: H.S.)].

Conflict of Interest

The author declares no conflict of interest.

Notes

This review of the author’s work was written by the author upon receiving the 2024 Pharmaceutical Society of Japan Award for Young Scientists.

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