We have developed a technique for specific drug targeting to digestive tract regions using electrostatic polysaccharide characteristics to cause gelation. An in situ double cross-linking process, mediated by Ca2+ and SO42-, of chitosan and alginate administered to the mouse gastrointestinal tract by double gavage, was used for gel formation. The technique not only allows researchers to target drug to different regions of the gastrointestinal (GI) tract by modulating polymer concentration but also significantly reduces degradation of the drug as well as side effects. Using this new technique, lower doses of drug can be efficiently loaded and delivered at specific target areas of the GI tract.
Techniques to deliver drugs into the gastrointestinal (GI) tract can include the provision of drugs in solution. However, such drugs will be directly affected by the pH of the stomach and are likely to be degraded under acidic pH conditions. To circumvent degradation by acidic pH or digestive enzymes, high drug doses or frequent administration are commonly used and side effects may be problematic. Recently, complicated techniques such as sprays of micro- or mini- emulsions with high drug doses have been used; these approaches lack targeting to specific areas of the gastrointestinal tract. Enemas are often used to target drugs to the colon but the procedure is cumbersome and is associated with high risk of local complications including bleeding or perforation in small animals. Thus there is an unmet need for targeted drug delivery to specific areas in the GI tract, particularly the colon.
Biomaterials must show biocompatibility, biodegradability, and bioactivity. The latter is the main criterion for a biomaterial used as a drug delivery system. Such systems are varied, ranging from metallic implants to polymers. The latter are widely used because polymers can be associated with co-polymers, and can be grafted, degraded, and acquire hybrid characteristics when associated with cells. Polymers such as poly(lactic-co-glycolic) acid (PLGA), poly(lactic) acid (PLA), and poly(ethylene glycol) (PEG) are usually modified and used to form films (1), or for nano- or microparticle-mediated (2) drug delivery. The range of possible polysaccharide uses (Table 1) is as wide as polysaccharides are diverse. Polysaccharides are biodegradable and biocompatible natural polymers (and are approved by the US Food and Drug Administration [FDA]), so such materials are widely used in applications such as the cosmetic industry (3,4) tissue engineering, or other biological projects, with attention to polysaccharide charge and mass characteristics. Thus, nanoparticles (5-7) can be used as drug delivery systems (8, 9) and, more specifically, as electrically charged molecules in electronic applications such as those requiring electroactive nanocomposites (10). When polysaccharides are used as biomaterials, size and charge regulate the kinetics of drug delivery (11, 12). Our technique is based on formation of a hydrogel by ions (Ca2+ and SO42-) that mediate cross-linking between alginate and chitosan. Other polysaccharides can be used in this technique because polysaccharides generally share similar electronic proprieties (13).
The principle point of the double gavage method for drug delivery is that it allows a "macro hydrogel" to form in the stomach (Figure 1). The first gavage contains the polysaccharide material that contains the drug. As the polysaccharides biomaterial is still liquid at the time of gavage, this technique overcomes the limitations of the size of the animal mouth and allows an easier way to administer in addition to the ability to administer higher concentration of drug. Then a second gavage will be performed with an ionic mixed solution of calcium and sulfate. As soon as the ions and the polysaccharides solution are mixed, a hydrogel is formed. The final volume of the biomaterial formed will be 150 μL.
An option of technique is to perform it twice. The method can be called double "double gavage" method. The purpose of this is to prevent early drug degradation during digestion process. The first double gavage is made with a concentrated drug solution. The second double gavage is done with a drug free polysaccharide solution to recover the biomaterial (Figure 2). This is an "onion-like" structure (8) and has two advantages (Figure 2). First, this original structure can prevent a quick release or "burst effect" of the drug from the hydrogel because the external layer, containing no drug, is the first to be degraded. Secondly, the kinetics of release (GI tract pH gradient from acid (pH=2) to neutral pH (pH=7), digestive enzymes . . . ) is surface dependant. This structure has the minimal surface contact between the hydrogel and the external medium and allows the drug to be completely separated from the degradation interface. After gavaging the polysaccharide solution into the stomach of a small animal, the ions in the solution form a hydrogel of the maximum possible size. Use of "macro-size" biomaterial reduces the contact surface between the drug and the aggressive digestive medium (Figure 3). This technique allows all types of encapsulation, including that of nanoparticles, liposomes, or drug molecules alone.
Using the unique "double gavage" method, we have shown that a combination of alginate and chitosan at a weight ratio of 7:3 is appropriate for delivery of an encapsulated product to the colon. As shown in Figure 4, most gel-loaded nanoparticles labeled with dextran-FITC were released in the colon after complete collapse of the hydrogel. The alginate and chitosan can be made with different concentrations, ratios, or types of polysaccharides to make the hydrogel collapse in a specific region of the digestive tract.