Anatomical, biochemical and physiological considerations of the colon in design and development of novel drug delivery systems.

The colon is composed of four distinct layers such as serosa, muscularis externa, sub mucosa and mucosa. There exists a difference in the anatomy, neural and blood supply and absorption characteristics as the length of the colon is traversed. At birth the mucosal surface of the colon is similar to that of the small intestine but rapid changes occur with the loss of the villi leaving flat mucosa with deep crypts. The existence of receptors like muscarinic M3, cholecystokinin1, Eph, Erb B, estrogen (α, β), gastrin releasing peptide, killer Ig like receptor, lymphocyte-endothelial receptor, notch, pregnane X, substance P and peroxisome proliferator-activated γ receptor can be utilized as a promising approach for targeting. The inner compact firm mucus is impervious to bacteria, making it a defensive barrier for the colossal bacterial load. The mucus thus provides innate immunity to maintain the homeostasis in colon. The physiological properties of the colon such as pH, transit time, luminal pressure of the colon, and the presence of microbial flora localized in the colon are utilized in the drug design. The drug delivery systems exploit enteric coating and biodegradable polymers to reach colon in an intact form by surpassing the barriers in the stomach and small intestine. The presence of azo-reductase, glucuronidase, dextranase, pectinase, glycosidase, polysaccharidase made it feasible to design prodrug and enzyme based drug delivery. Drug designing methodologies in colon specific drug delivery include pH- based systems, enzymedepended systems, timed- release systems and pressure/osmotically release systems.