Considerations in excipient selection for colon-targeted dosage forms

The colon is a potential organ for drug delivery systems because it has several advantages, such as long transit time and neutral pH conditions. Moreover, colon targeting can avoid enzymatic reactions in the stomach and small intestine which increase the bioavailability of the drug. A colon-targeted drug delivery system (CDDS) requires excipients that have distinctive characteristics to prevent drug release in the upper gastrointestinal tract and enhance drug release in the colon. Polysaccharides are suitable in CDDS due to the degradation mechanism by colonic microflora enzymes. Furthermore, polysaccharides can swell when hydrated so that drugs can be released from the matrix system by diffusion and/or relaxation. Polysaccharides can swell when hydrated, which will form a gel-like matrix that enables drug release through diffusion, where the drug diffuses through the swollen matrix. Additionally, drug release can occur through a relaxation mechanism, where the polymer chains gradually unwind and erode, further facilitating the drug release. This degradation and swelling mechanism can lead to the release of the drug at the colon site. Some polysaccharides that can be utilized for CDDS are alginate, amylose, arabinoxylan, dextran, guar gum, inulin, carrageenan, chitosan, chondroitin sulfate, lactulose, locust bean gum, pectin, and cyclodextrin. Moreover, polymers can also be employed as excipients in CDDS due to conformation changes in different pH of the gastrointestinal tract. These pH-dependent properties can ensure that the drug is protected during transit through the stomach and small intestine and is only released when the dosage form reaches the colon. Some polymers that are potential for CDDS are cellulose derivatives, poly (acrylic acid) (PAA), poly (metha-acrylic acid) (PMAA), Eudragit^®, poly (lactic-co-glycolic acid) (PLGA), Kollicoat^®, and Shellac^®. The combination and/or modification of these excipients can increase the effectiveness of CDDS and prevent the early release of drugs in the upper gastrointestinal tract which can lead to the drug release being more precisely controlled in the colonic site.