Science and Technology of Bioadhesive-Based Targeted Oral Delivery Systems

The use of bioadhesive polymers as a means of delivering therapeutic agents to the gastrointestinal tract (GIT) has been a focus of attention during the past two decades (1, 2). Bioadhesive oral drug delivery systems exploit the interaction between the mucus and bioadhesive polymers and thus offer significant advantages (3, 4) (see sidebar). Oral delivery systems formulated with bioadhesive polymers may increase GIT residence time, thereby improving oral bioavailability as the formulation achieves greater opportunity to interact closely with the absorption site (5, 6). Bioadhesive polymeric systems may also be useful for coating damaged esophageal and intestinal wall tissues, thus providing defense against various irritants (7). The ability to maintain an oral delivery system at the target location for an extended period of time has great appeal for the treatment of both local conditions and sustained systemic absorption (8–10).

AUTHOR

Despite significant research on bioadhesive polymers, the phenomenon of bioadhesion is not yet fully understood. Adhesion of bioadhesive polymers to mucus is a complex interaction and is regulated to a great extent by the intrinsic properties of the polymer, the biological substrate, and the surrounding environment (11). The term bioadhesion is defined as adhesion to a biological surface (i.e., mucus and/or mucosal surface). Instances in which the polymeric system interacts with the mucus coating only are referred to as mucoadhesion (12). To develop an ideal oral bioadhesive system, one must have a thorough understanding of mucosa, bioadhesive polymers, and mucous-polymer interactions in the physiological environment.

GI mucosa is composed of high molecular weight glycoproteins that are hydrated with a continuous adherent blanket of mucin. Mucin glycoproteins are rich with fucose and sialic acid groups at the terminal ends that provide a net negative charge in the acidic environment. The thickness of the mucin gel layer varies in various regions of the GIT, with thickness ranging between 50 and 500 μm in the stomach to 15–150 μm in the colon. The thickness of the mucus gel that covers the GI epithelium is attributed to the steady state between the mucus secretions and its erosion via enzymatic and mechanical degradation. Cohesion of the mucin gel depends on the glycoprotein concentration.

Major attributes of oral bioadhesive based systems (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

The mucus layer is created biologically to protect the underlying tissues from diffusing or corrosive elements such as enzymes, acids, and other toxic molecules. As a viscoelastic gel, it helps in the transport of food over the epithelium, thereby minimizing potential erosive damage. The mucus layer, in addition to providing protection, imparts a barrier to drug absorption. The following mucin–polymer interactions have been proposed:

• Wetting and swelling of the polymer to permit intimate contact with the biological tissue
• Interpenetration of bioadhesive polymer chains and entanglement of polymer and mucin chains
• Formation of weak chemical bonds
• Sufficient mobility to allow spreading
• Water transport followed by mucosal dehydration.

As the bioadhesive delivery system comes into contact with the mucus layer, various nonspecific (van der Waals, hydrogen bonding, and hydrophobic interactions) or specific interactions occur between the complimentary structures (see Figure 1). However, these interactions are of limited duration because of the turnover process of mucin. Although this phenomenon is favorable from the toxicological perspective of keeping unwanted materials from gaining access to the body, it is not optimal for maintaining the bioadhesive delivery system at the mucosal surface. Hence, for a bioadhesive delivery system to be successful, it should release drug contents during the limited adhesion time (13).

Figure 1: Schematic representation of interactions between bioadhesive and mucus polymer chains. (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

Prerequisites for successful bioadhesive oral dosage form

The promise of bioadhesive-based oral delivery systems has fostered numerous investigations with limited success. Different types of transmucosal oral systems have been explored using various bioadhesive polymers. A majority of these systems are based on hydrophilic hydrogel polymers and are designed primarily for buccal or sublingual applications. When these hydrophilic polymers are used for oral bioadhesive systems for drug delivery in the GIT, they typically hydrate prematurely upon contact with the stomach contents before developing interactions with the mucosal surface. In the event that some weak interactions do occur, these systems cannot withstand the high turbulence of the stomach environment, and the result is premature emptying. Therefore, although the range of hydrophilic bioadhesive polymers and their application in various low-turbulence conditions is quite broad, their usefulness in oral dosage forms, especially in designing of systems for systemic delivery, is generally limited.

An ideal bioadhesive oral dosage form must meet several prerequisites to be successful. The first prerequisite to target a gastrointestinal site is that the behavior of the dosage form must be reproducible. Although many bioadhesive polymers have exhibited promising results in vitro and in vivo in animals, few benefits have been shown in human trials. The results of human clinical trials with bioadhesive oral dosage forms are summarized in Table I. Recently, Säkkinen evaluated the passage and retention of chitosan granules in the small intestine by gamma-scintigraphy in fasted human volunteers (14). Although chitosan showed marked bioadhesive capabilities in vitro, retention of the chitosan formulation in the upper small intestine was not sufficiently reproducible, and the duration of retention was similar to lactose granules used as a control. In developing a site-specific bioadhesive system for furosemide, a model drug with a narrow absorption window in the upper GIT, administration of furosemide in chitosan granules resulted in bioavailability lower than that from a conventional immediate-release formulation, thereby indicating that the bioadhesive formulation could not be retained long enough in the upper GIT in humans (15).

Table I: Bioadhesive oral drug delivery systems used in various human trials. (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

The second prerequisite for a bioadhesive system is that it should rapidly attach to the mucosal surface and maintain a strong interaction to prevent displacement. Spontaneous adhesion of the system at the target site is critical and can be achieved through bioadhesion promoters that use tethered polymers (16). Contact time should also be sufficiently long at the target site, normally longer than that needed for complete drug release. As hydrophilic bioadhesive polymers tend to lose adhesiveness upon hydration, restricted hydration and formation of a rigid gel network would be desirable for prolonged adhesion (17). A short retention time, in relation to the drug release rate, will compromise bioavailability.

The third prerequisite for a successful and effective bioadhesive system is that the bioadhesion performance should not be impacted by surrounding environmental pH. Studies have shown that the bioadhesiveness of polymers with ionizable groups are affected by surrounding pH. For example, polyacrylic acid is more bioadhesive when the majority of the carboxylic acid groups are in the ionized state. Polyanhydride-based hydrophobic bioadhesive polymers (e.g., Spheromers, Spherics, Mansfield, MA) undergo erosion that is mainly affected by the aqueous environment and not by pH of the surrounding medium. Studies have shown that as anhydride-based polymers degrade at the mucus surface, carboxylic acid groups are formed at the transected polymer chain ends, which generate a new polymer surface rich in carboxylic acid end groups (18). These hydrophilic functional groups then form hydrogen bonds with surrounding mucin strands that in turn penetrate the newly created surfaces. The result is the formation of both chemical and mechanical bonds. As the degradation process proceeds, a more porous surface rich with carboxyl groups is created, allowing for even greater adhesion that is essential to the success of an oral bioadhesive system. In earlier studies, one family of rapidly degradable polyanhydrides [poly (FA:SA)] produced bioadhesive interactions with rat small intestine tissue that were substantially stronger than all other polymers in this class (18). The fact that these bioadhesive polymers are stable in the acidic environment of the stomach and eventually degrade at pH ≥7.4, make them ideal for targeted delivery to the stomach and small intestine (19).

Another prerequisite for an ideal bioadhesive delivery system is that the bioadhesive and drug-release functions are independent of each other. Often, the bioadhesive polymer used in the dosage form is also used to regulate the release of drug. Generally, these formulations are made by mixing bioadhesive polymer and drug or by coating drug-loaded beads or tablets with the bioadhesive polymer. These approaches of using bioadhesive polymers to achieve both bioadhesion and drug-release functions have compromised results.

An effective bioadhesive formulation must not cause local tissue irritation or long-term tissue toxicity as a result of the bioadhesive polymer or other absorption enhancers used to promote drug absorption. Also, if encapsulated bioadhesive nanoparticles or multiparticulates beads are used as the delivery system, the particles may have a tendency to form agglomerates because of the charge or hydration within the capsule. Accordingly, measures should be taken to keep these structures monodisperse to allow maximum interaction with the mucosal surface upon release from the capsule. Other desirable characteristics of a bioadhesive dosage include high drug loading, complete drug release, and convenient administration.

Although the prerequisites described above apply to bioadhesive dosage forms, the potential impact of formulation excipients on the adhesive behavior of bioadhesive drug delivery systems and mucosal surfaces also should be carefully taken into account. For example, excipients containing hydroxyl groups could form hydrogen bonds with the hydrophilic functional group of bioadhesive polymers and, as a result, prevent their interaction with the mucosal surface (20). In addition, hydrophobic lubricants (e.g., magnesium stearate and talc) tend to hinder the formation of strong bioadhesive bonds and thus reduce the bioadhesive strength significantly (21). Structural breakdown of mucin has been observed by the addition of surfactants. A number of agents (e.g., tetracycline and progesterone) may alter the viscosity of mucus by altering its molecular composition. Integrity of mucin layers is also disrupted in some disease states (e.g., inflammation and ulceration). Therefore, in developing a bioadhesive dosage form, drug and excipient characteristics as well as the presence of disease states need to be taken into account.

Oral bioadhesive dosage forms targeting the GIT

Bioadhesive systems have been targeted to many sites within the GIT to increase the residence time available for absorption and thereby increase the overall bioavailability. Although bioadhesive polymers have been successfully used for oromucosal (buccal and sublingual) drug delivery (22), delivery to other GI sites has been a challenging task. The following sections review the drug delivery to various GI sites using bioadhesive formulations and discuss the degree of success.

Targeting to the esophagus. The esophagus is lined with stratified squamous epithelium and is continuously lubricated and coated by the swallowed saliva containing mucin. Bioadhesive materials have been used in various liquid dosage forms to specifically target the esophageal mucosa (23, 24). However, because the esophagus is associated with poor blood supply, drug delivery for systemic absorption through this site is not feasible.

Because of a relatively short esophageal transit time of dosage forms in supine subjects (10–15 s), liquid bioadhesive formulations have been explored as a vehicle for localized delivery to the esophageal mucosa or to provide a protective bandage for the underlying esophageal lining from gastric reflux (25). One study investigated the incorporation of antifungal agents into an oral formulation that coated the esophagus and provided drugs at the target site for localized delivery to treat esophageal candidiasis (26). Drug delivery to the esophagus was achieved in rabbits using magnetic particles in conjunction with hydroxypropyl cellulose and carboxyvinyl polymer as bioadhesive excipients. However, retention of the formulation (using bleomycin in hydroxypropyl cellulose:Carbopol) was insufficient for effective therapy. It was concluded that a stronger bioadhesive may help retain particles at the desired site of action (27).

Sodium alginate in a range of water-miscible vehicles was evaluated as a bioadhesive liquid for targeting the esophageal mucosa for the treatment of gastro-esophageal reflux disease (GERD). The study showed that vehicle composition had a considerable impact on the initial contact and retention of suspended alginate; however, the rate of detachment of the adhered layer was similar for each vehicle. Thus, the ability to modulate the mucosal retention of an alginate suspension as a function of hydration offers a novel strategy for the future development of formulations with tailored bioadhesive properties (28).

Localized delivery of hexylaminolevulinate to the Barrett's esophagus was achieved in human volunteers using poloxamer, chitosan, and sodium carboxymethylcellulose polymers as bioadhesive excipients. This study measured the esophageal transit time by endoscopic examination and suggested that a topical formulation delivered orally to treat adenocarcinoma of esophagus is feasible (29).

Strategies for successful drug delivery to the esophagus should involve drugs that are readily soluble in bioadhesive polymers, rapidly smear the esophageal epithelium, and are not washed away by the flow of saliva and other liquids. The risk of bioadhesive dosage forms getting dislodged in the esophagus when taken with little or no water could also pose problems and should be considered as part of the formulation design (30). These problems, attributed to esophageal blockage or drug-induced injuries resulting from a high concentration of drag at the epithelium layer corrosive to the local tissue, may have serious clinical implications.

Targeting to the stomach and small intestine. One of the feasible approaches for achieving a prolonged and predictable release profile in the GIT is to maintain the dosage form in the stomach over an extended duration. This is of great importance for drugs in which the absorption site is restricted to the stomach or the proximal small intestine. Specifically, it is important for controlled-release dosage forms designed to deliver drugs to narrow absorption windows (e.g., levodopa, metformin, furosemide, gabapentin, and amoxicillin). A bioadhesive dosage form for gastric delivery should achieve a rapid interaction with the mucosal surface and be sufficiently strong to resist the propulsion forces of the stomach wall. The effectiveness of the system also should be maintained irrespective of continuous production of mucus by the gastric mucosa to replenish the mucus that is lost through peristaltic contractions, as well as dilution of the stomach contents.

Few studies have been conducted to specifically investigate the transit time of oral bioadhesive tablet formulations containing hydrophilic polymers in animals and humans. An increase in transit time and improved absorption of griseofulvin, by three- to four-fold was observed from a bioadhesive oral dosage form containing a cross-linked acrylic acid polymer as the bioadhesive material (31). In a radioscintigraphic study (32), two different capsule formulations, based on the hydrophilic bioadhesive polymers polycarbophil and Carbopol, were evaluated for gastric emptying and small intestine transit time. Results indicated that both bioadhesive formulations were not dramatically different from the nonadhesive controlled formulation in terms of stomach emptying, small intestinal transit time, and time to reach the colon. Similar results were obtained with an oral bioadhesive controlled-release formulation of furosemide (33). A bioadhesive formulation containing Carbopol 934 and furosemide, a model drug with erratic absorption and a narrow absorption window, was evaluated in a radioscintigraphy transit time study in humans. The extent of absorption from the bioadhesive formulation was less than the control non-bioadhesive formulation. The reasons suggested for this poor performance were that in vivo adhesive properties were insufficient to overcome the powerful gastric contractions during Phase III of the housekeeping motor migrating complex waves in the fasted state. Clearly, the applications of hydrophilic polymers in designing oral dosage forms, in particular, controlled-release dosage forms, has been limited.

Figure 2: X-ray images of barium sulfate spheres (bioadhesive and control non-bioadhesive) taken at different time intervals in fasted beagles (34). At 1.5 h: bioadhesive spheres retained in the stomach while control spheres are entering the intestine; at 4.5 h: bioadhesive spheres in the stomach while control spheres entering ascending colon; at 8.5 h: bioadhesive spheres scattered evenly in stomach, small and large intestine, while control spheres are in distal descending colon; at 24 h: bioadhesive spheres still present in the intestine and no control spheres in the intestine. (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

Based on the poor performance of hydrophilic polymer-based bioadhesive systems in humans, interest has been focused in using hydrophobic polymers. It is perceived that nonswellable polymers, with a hydrophobic polymeric backbone and various hydrophilic functional groups, may improve bioadhesiveness to mucosa. Extensive progress has been made at Spherics, Inc. into the research and development of proprietary hydrophobic polymers (Spheromers), in which oral bioadhesive dosage form approaches have been explored for sustained and targeted delivery, including the following:

• The mechanisms of Spheromer interactions with mucosal surfaces as well as the design of novel targeting strategies

• The development of novel bioadhesive controlled release formulations of challenging drugs with improved and reproducible performance in humans.

Figure 3: Mean itraconazole plasma concentration versus time following a single dose of itraconazole bioadhesive extended release (XR) tablet, 100 mg, or Sporanox capsule, 100 mg, in healthy volunteers, n = 8. (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

Animal studies have been performed with bioadhesive oral delivery systems based on bioerodible Spheromers of the polyanhydride type (34). Among the various thermoplastic polymers studied, poly(fumaric-co-sebacic) (p[FA:SA] in a 20:80 ratio) had the most bioadhesive characteristics. In vivo radiographic transit studies in fasted beagle dogs on bioadhesive-coated, barium sulfate–loaded microspheres showed that the microspheres remained in the stomach and intestine longer than microspheres made of non-bioadhesive polymers (Eudragit RS 100), clearly demonstrating that strong adhesive interactions delayed the passage of microspheres through the GIT (see Figure 2).

Figure 4: Percent coefficient of variation (%CV) of Cmax and AUCo-t for itraconazole bioadhesive extended release tablet, 100 mg, compared with Sporanox capsule, 100 mg, in healthy volunteers, n = 8. (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

Bioadhesive extended-release formulation of drugs with narrow absorption windows and erratic absorption behavior have been developed using Spheromers to increase the residence time in the stomach and to allow drug release downstream in the small intestine in a controlled manner. Using itraconazole as a model drug, bioadhesive extended-release formulations were assessed for their pharmacokinetic performance in healthy volunteers (see Figure 3). Bioadhesive extended-release formulations provided an improved bioavailability compared with the immediate-release Sporanox capsule, while maintaining effective itraconazole plasma levels for more than 24 h and reducing intersubject variability (see Figure 4). A reduction in C_max is critical to product performance because side effects are associated with peak itraconazole plasma levels (68). These results may help prevent the risk of developing a fungal infection, especially in immunosuppressed patients.

Figure 5: Plasma levodopa profiles following a single dose administration of levodopa-carbidopa bioadhesive extended-release (XR) tablets and Sinemet CR tablet in fed beagles (n = 6). Both tablets contain 200 mg levodopa and 50 mg carbidopa (35). (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

Levodopa-carbidopa was also explored in a bioadhesive oral drug delivery system as an oral dopaminergic treatment strategy to maintain the system at the target site over an extended duration and provide stable blood levels of drug in advanced Parkinson's Disease. Figure 5 shows the pharmacokinetic performance of levodopa–carbidopa administered as a bioadhesive, multilayer tablet to beagle dogs. In comparison with Sinemet controlled-release tablets, this novel bioadhesive extended-release dosage form resulted in an extended plasma levodopa profile for as long as 12 h and improved bioavailability both in fasting and fed animals as a result of prolonged residence time (see Tables II and III) (35). By providing a continuous delivery of levodopa at a constant rate, the system should not only reduce the motor complications related to abnormal intermittent pulsatile dopaminergic stimulation but also improve patient compliance and quality of life. Figure 6 shows the levodopa profile of bioadhesive extended release tablets and Sinemet controlled-release tablets in young healthy volunteers. Because the gastroretentive systems are highly dependent on the calorie content of the meal, both formulations were administered following a light breakfast (approximately 380 calories). The bioadhesive system provides a rapid and reliable levodopa level that is needed to achieve the "morning kick," followed by a smooth delivery of levodopa over an extended duration. Reduced intersubject variability data showed superior reproducibility in the in vivo performance compared with the marketed product, Sinemet controlled-release tablets (36).

Table II: Pharmacokinetic data for levodopa-carbidopa bioadhesive extended release (XR) tablets and Sinemet CR tablets in fasted beagles (n = 6). (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

Figure 7 shows the pharmacokinetic profile of a delayed-release bioadhesive capsule formulation containing 0.375 mg pramipexole (a dopamine receptor agonist) administered once daily compared with immediate-release Mirapex tablets, 0.125mg, given three times daily in healthy human volunteers. By allowing the drug to be released in the small intestine, the novel bioadhesive dosage form extended drug release to 22 h with a similar extent of exposure compared with Mirapax tablets given three times daily. Also, the bioadhesive formulation showed reduced fluctuations in pramipexole levels and eliminated nausea and vomiting (37).

Table III: Pharmacokinetic data for levodopa-carbidopa bioadhesive extended Release (XR) tablets and Sinemet CR tablets in fed beagles (n = 6). (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

These examples cited demonstrate that systems based on hydrophobic bioadhesive polymers are likely to show great promise in terms of extended residence time and bring new possibilities for controlled delivery of drugs with narrow absorption windows.

Figure 6: Plasma levodopa profiles following a single dose administration of levodopa-carbidopa bioadhesive extended release (XR) tablets and Sinemet CR tablets in healthy volunteers. Both tablets contain 200 mg levodopa and 50 mg carbidopa, n = 12 (36). (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

Targeting to the colon. The need to target drugs to the colon is well recognized and considered to be an effective approach in the treatment of local disorders such as irritable bowl syndrome and inflammatory bowl diseases such as Crohn's disease and ulcerative colitis. It is also a preferred absorption site for oral administration of protein and peptide drugs because of relatively low proteolytic enzyme activity (38).

Figure 7: Plasma pramipexole profile following administration of pramipexole bioadhesive extended release (XR) capsules, 0.375 mg, given once daily versus Mirapex tablets, 0.125 mg given three times daily to health volunteers, n = 12 (37). (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

Because of its lower mucin turnover and lower sensitivity to mucus secretory stimulus, the colon is a suitable location for bioadhesion in the GIT (13). Slow transit of dosage forms through the colon will prolong contact time between the formulation and the absorptive surface, thereby enhancing drug exposure. This has led to the development of oral dosage forms for selective drug delivery for various colonic conditions. Typically, systems have been designed to release drugs rapidly in the upper portion of the colon. However, this may result in unnecessary exposure of drug to noninflamed tissues, thereby reducing the efficiency at the desired site of inflammation and increasing adverse effects. Therefore, a colon-specific bioadhesive multiparticulate delivery system that remains at the inflamed target site for a prolonged period of time will provide drug exposure in the local mucosa and reduce the potential for side effects (39). Achieving desired levels of drug exposure at the inflammatory target site requires careful considerations in terms of the intersubject variabilities in intestinal transit time, intraluminal pH profile, disease pattern, and drug disposition (40).

At Spherics, an effort of exploiting Spheromer bioadhesive polymers to target delivery to distal parts of the GIT has been conducted. Proof-of-concept studies have been completed with promising results. In fluoroscopic studies in beagle dogs, bioadhesive beads coated with Spheromer bioadhesive polymers uniformly lined the ascending large bowel and remained in close apposition to colonic mucosa without mixing with digested food contents (see Figure 8). In contrast, beads coated with non-bioadhesive cellulose acetate polymer were evenly distributed in the lower small bowel, mixed with food, moved freely with the peristaltic movements, and did not attach to the intestinal mucosa.

Figure 8: Spheromer-coated multiparticulate beads lining the large colon 8 h postdosing (top) and control non-bioadhesive multiparticulate beads 8 h postdosing (bottom). (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

A colonic-specific bioadhesive system should prevent drug release in the stomach and small intestine and provide rapid adhesion and drug release upon reaching the colon. The selection of triggering mechanisms (e.g., prodrug, pH, or microflora activated) that respond to the physiological conditions in the colon should be carefully selected such that the bioadhesive performance of the system remains unaffected.

Targeting to the rectum. Although peroral administration is the most common route for targeting drugs, oral administration may not be feasible. Use of a bioadhesive liquid suppository, based on in situ gelling phenomenon, has been explored as an option for the local treatment of diseases of the anorectal area as well as for systemic drug delivery (41–44). Choi examined a bioadhesive liquid suppository that combined a bioadhesive polymer with a thermal gelling polymer. With acetaminophen as a model compound, a bioadhesive system that gelled strongly at physiological temperature gave the most prolonged plasma level of acetaminophen in vivo (45).

Bioadhesive delivery system options

Several investigators have explored unique design concepts in an effort to enhance retention of bioadhesive dosage forms in the GIT. These novel bioadhesive dosage forms include solutions, suspensions, gels, powders, microparticles or nanoparticles, pellets, patches, and tablets (including minitablets and multilayer tablets). During the development of a bioadhesive delivery system, the focus should be not only to achieve the desired therapeutic outcome but also to overcome the unfavorable environmental condition and challenges found in various regions of the GI tract.

Solution, suspension, and gel-forming liquids. Viscous bioadhesive liquids have been investigated primarily to coat the esophagus to act as a protectant or a vehicle for drug delivery for the treatment of local disorders, including motility dysfunction, fungal infections, and esophageal cancer. For the treatment of GERD and other esophageal disorders, a delivery system retained within the lower esophagus would be highly desirable (46). Using sodium alginate suspension as a novel bioadhesive liquid, researchers showed that the esophageal surface can be coated to protect against refluxate and can deliver therapeutic agents to the damaged mucosa (28, 47). The bioadhesive gel of δ-5-aminolevulinic acid for local action within the esophagus has been investigated (48). The retention behavior of various bioadhesive formulations was evaluated on the esophageal surface under conditions mimicking the saliva flow. Both polycarbophil and xanthum gum demonstrated excellent bioadhesive potential, and carmellose sodium and theromosensitive poloxamer (Lutrol 407) demonstrated poor retention. Recent work by Potts has examined the esophageal retention of liquid formulations of Smart Hydrogel (GelMed, Lexington, MA), a thermosensitive hydrogel of poloxamer covalently linked to polyacrylic acid and carbopol. This "esophageal bandage," upon oral administration, demonstrated significant retention within the esophagus (49).

Multiparticulates, microparticles, and nanoparticles. Oral delivery systems based on multiparticulates, microparticles, and nanoparticles often exhibit improved performance in comparison with monolithic matrix tablets. By diffusing into the mucous gel layer by virtue of their relatively small size, these small immobilized carriers show a prolonged gastrointestinal residence time (50, 51). Figure 9 shows Spheromer-coated beads adhering to the mucosal layer of everted rat jejunum. Studies have shown that these beads make a rapid interaction with the mucosal surface and form a strong and long-lasting adhesive interaction (52). Rapid degradation of these polymers on the surface generates new carboxylic acid groups that further aid in bioadhesion. Recent work has shown that, in addition to size and chemistry, shape is also a critical feature of bioadhesive drug delivery particles and can dictate particle velocity, diffusion and adhesion to the mucus surface in a complex manner (53).

Figure 9: An everted sac of rat jejunum incubated with Spheromer-coated microspheres, adhering to the mucosal surface, after washing three times (52). (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

Fine particles of ion-exchange resins display bioadhesive properties as a result of interactions between the highly charged surface of polymers and the mucus (54). The residence time of cholestyramine, an anionic-exchange resin, was explored in fasting and fed human subjects by gamma-scintigraphy. This study showed that materials with adherent properties can resist the housekeeper sequences in the fasted subjects and will not be dislodged by food in the fed state. Contrary to conventional wisdom, this study revealed that charge-based interactions associated with ion-exchange resins play a minor role and only a small percentage of resin particles (approximately 20%) that made contact with the stomach lining were retained over an extended duration (55).

Figure 10: AUC and Cmax values following oral gavage administration of micronized (stock) paclitaxel, non-bioadhesive paclitaxel formulation, and bioadhesive paclitaxel formulation in rats (57). (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

Recent work has shown that hydrophobic bioadhesive polymers can prolong the residence time of orally administered insulin microspheres by enhancing the uptake of particles. Suppression of blood glucose levels was demonstrated in Type 1 diabetic rats and dogs (56). To evaluate the possibility of increasing the potential for greater oral uptake of bioadhesive nanoparticles of paclitaxel (BCS Class IV), formulations were prepared and tested in rats via oral gavage. Figure 10 shows the effect of Spheromer particles on the oral uptake of paclitaxel, in comparison with micronized (stock) paclitaxel and non-bioadhesive paclitaxel. In contrast with micronized paclitaxel with no oral bioavailability, the uptake of bioadhesive nanoparticles resulted in a significant increase (approximately 12 fold) in AUC compared to non-bioadhesive polylactide-co-glycolide coated nanoparticles (57).

Figure 11: A schematic comparing a bioadhesive microsphere and a microfabricated bioadhesive flat chip. The flat chip allows maximum contact area with the mucosa. It incorporates two drug reservoirs for unidirectional release (59). (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

In a recent in vivo study, the efficacy of gliadin bioadhesive nanoparticles containing amoxicillin was evaluated to eradicate Helicobacter pylori in the GIT. When compared with an amoxicillin suspension, the bioadhesive system resulted in the complete clearance of infection as a result of prolonged residence time attributed to bioadhesion (58).

Although spherical particles have been commonly used over the years, bioadhesive microdevices are being explored as a viable platform for improved residence time (59). These microdevices or microreservoirs, in the shape of thin, flat, disk-like structures are modified on one surface with a bioadhesive agent (see Figure 11). This type of asymmetric coating orients the delivery system toward the target intestinal lining and thus provides maximum contact time because of a large surface area with minimum resistance to passing fluids. The reservoir, which may be filled with drugs or biomolecules, provides unidirectional drug release. These structures can be further complemented with various lectins that can make site-specific interactions (60). Although this technology is at an early stage for drug delivery, the application of these nano-platforms has tremendous potential in developing new therapeutic modalities. They can be used to deliver drugs, and various absorption enhancers–enzyme inhibitors can be coreleased from the dosage form in a regulated manner, which can further increase the effectiveness of incorporated drugs.

Figure 12: Schematic showing the adherence of trilayer bioadhesive tablet to the mucosal surface of stomach. The trilayer tablet consists of a middle slow eroding core laminated with outer bioadhesive layers. (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

Bioadhesive tablets. Bioadhesive polymer-based monolithic matrix tablets (61, 62), multilayered tablets (36), or BIOROD (BIOadhesive Rate-controlled Oral Dosage) systems (Spherics, Mansfield, MA) with various geometrical configurations (63) offer several advantages for controlled drug release. Bioadhesive polymers are easily mixed with drugs and compressed into matrix tablets. In the multilayer tablet design, the layers can be configured such that the bioadhesive layer is in contact with the mucosa and independent of drug release that occurs from the peripheral ends (see Figure 12). By modifying the surface texture and hydration rate of the compressed bioadhesive layers, the system can be kept at the target site over an extended duration.

Figure 13: Cross-section view of BIOROD system for two-pulse delivery. (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

The BIOROD system (see Figure 13) comprises a longitudinally compressed capsule shape and inner drug core encased in an impermeable bioadhesive cylinder. One of the significant advantages of this system is its flexibility in constructing various designs to allow release kinetics to meet therapeutic goals. Figure 14 shows two pulse-release behaviors of gabapentin from a BIOROD system. The drug released from such a bioadhesive system can be easily adjusted by:

• Variation of the architecture of the inner core
• Variation of rate controlling polymers, including type and concentration in the core
• Inclusion of placebo and barrier layers within the core
• Variation of bioadhesive coating shell.

Figure 14: Two pulse release profile from the Gabapentin BIOROD system. (ALL FIGURES AND TABLES ARE COURTESY OF THE AUTHOR.)

Conclusions

The results and overview presented in this article leave no doubt that bioadhesive-based targeted oral delivery systems have the potential to enable many drugs to be delivered at the desired absorption site in a prolonged and tailored manner. The successful fabrication of the bioadhesive system should take into consideration a multitude of aspects of polymer-mucus interactions, preferred site of absorption, drug loading, GI physiology, dosage form size and shape, and contact duration. Though each of these factors plays a critical role, optimization must be performed to give the best performance in all categories. Fabrication of oral bioadhesive formulations for existing drugs potentially provides access to new and expanded markets. Many drugs with a narrow absorption window that must be administered multiple times per day could potentially be developed into longer-lasting effective therapies that offer reduced daily administrations, greater patient compliance, reduced maximum plasma–related adverse effects, and improved intersubject variability. These novel dosage forms may create added value from the existing therapeutic franchise through the classic marketing strategies of product proliferation. And, finally, at a time when many drug companies are facing expiration of key patents, new oral bioadhesive formulations with improved performance can provide patent life extension and facilitate product life cycle management.

Although further research is required, orally administered pharmaceuticals of the future are likely to include bioadhesive microparticles and nanoparticles that will provide site-specific uptake of drugs and allow tailored drug release to meet biological needs. With the recent development in nanotechnology, bioadhesive nanodevices offer yet another tool to further expand the benefits. With the growing realization of the importance of nonspherical-shape particles, the functional behavior of these bioadhesive carriers can be tuned to optimize their performance in humans (64).

Acknowledgment

The author would like to thank Dr. George Grandolfi and Aliceann Hagopian for their valuable input.

Avinash Nangia, PhD, is senior vice-president of research and development at Spherics, Inc., 375 Forbes Blvd., Mansfield, MA 02048, tel. 508.452.7000, fax 508.452.7070, anangia@spherics.com.

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