Polymers for Colon Targeted Drug Delivery: A Review

The colon targeted drug delivery has a number of important implications in the field of pharmacotherapy. Oral colon targeted drug delivery systems have recently gained importance for delivering a variety of therapeutic agents for both local and systemic administration. Targeting of drugs to the colon via oral administration protect the drug from degradation or release in the stomach and small intestine. It also ensures abrupt or controlled release of the drug in the proximal colon. Various drug delivery systems have been designed that deliver the drug quantitatively to the colon and then trigger the release of drug. This review covers different types of polymers used in formulation of colon targeted drug delivery systems. *Corresponding author, Mailing address: Neha Sharma Rayat & Bahra Institute of Pharmacy, Sahauran, District Mohali, Punjab -140104, India Email: [email protected] Article History:-----------------------Date of Submission: 12-02-2013 Date of Acceptance: 28-02-2013 Conflict of Interest: NIL Source of Support: NONE R e v ie w P a p e r Int. J. Drug Dev. & Res., January-March 2013, 5 (1): 21-31 Covered in Scopus & Embase, Elsevier 21 be improved by colon-targeted drug delivery system. Colon is used for systemic absorption of proteins and peptides also because proteolytic activity of colon mucosa is much less than that observed in small intestine. Drug targeting to specific sites of action offers several advantages over non targeted drugs such as prevention of side effects and reduction of doses. The colon as a site of drug delivery offers various therapeutic advantages because of its near neutral pH and longer transit time. To reach the colon and release the drug, a dosage form must be formulated taking into account various obstacles introduced by the gastrointestinal tract. Successful delivery of a drug to the colon requires protection of the drug from degradation or release in the stomach and then controlled release of drug in colon. [5,6] The desired properties of colon targeted drug delivery systems can be achieved by using some polymers either alone or in a combination because it is now recognized that polymers can potentially influence the rate of release and absorption of drugs and play an important role in formulating colon targeted drug delivery systems. COLON SPECIFIC POLYMERS Polymers are macromolecules having very large chains contain a variety of functional groups, can be blended with low and high molecular weight materials. Polymers are becoming increasingly important in the field of drug delivery. Advances in polymer science have led to the development of several novel drug delivery systems. A proper consideration of surface and bulk properties can aid in the designing of polymers for various drug delivery applications.[7] These newer technological development include drug modification by chemical means, career based drug delivery and drug entrapment in polymeric matrices or within pumps that are placed in desired bodily compartments. These technical developments in drug delivery approaches improve human health. Use of polymeric material in novel drug delivery approaches has attracted the scientists.[8] Pharmaceutical application of polymers: • Used to achieve taste masking • Used as a binder in tablets to viscosity and flow controlling agent in liquids, suspension and emulsions. • Used as film coating to disguise the unpleasant taste of drug and to enhance drug stability. • Used to modify drug release characteristics.[9] MECHANISM OF COLON TARGETING: A diverse range of mechanisms have been developed to achieve colon targeting of drugs. One of the widely used mechanisms is to coat the formulation using natural or synthetic polymers. In this mechanism, the drug is present in the core of the formulation which is coated with layers of polymer coatings. The first coating (next to core material) is an acid-soluble ploymer and outer coating is an enteric polymer. The core of formulation is comprised of the active material and other desirable excipients. During the transit of formulation (microsphers, tablets, capsules etc) through the GI tract, the formulation remain intact in the stomach due to enteric protection, but the enteric coating will dissolve in the small intestine , where the pH is above 6. The enteric coating starts to dissolve at the pH 5 in the small intestine. Upon entry into the colon, the polysaccharide coating will starts to dissolve. The bacteria will enzymatically degrade the polysaccharide into organic acid. This lowers the pH of the surrounding system and results in the dissolution of the surrounding system and results in dissolution of acid-soluble coating and subsequent drug release. BIODEGRADABLE POLYMERS: Biodegradation is a natural process by which organic chemicals in the environment are converted to simpler compounds, mineralized and redistributed through elemental cycles such as carbon, nitrogen R e v ie w P a p e r Sharma Neha et al: Polymers for Colon Targeted Drug Delivery: A Review Int. J. Drug Dev. & Res., January-March 2013, 5 (1): 21-31 Covered in Scopus & Embase, Elsevier 22 and sulphur cycles. Biodegradable polymers have been widely used in biomedical applications because of their known biocompatibility and biodegradability. Biodegradable polymers are intended for temporary aids, such as sutures, tissue-supporting scaffolds, and drug delivery devices.[10] Polymers within this group retain their properties for a limited period of time and then gradually degrade into soluble molecules that can be excreted from the body.[11] Biodegradable polymers are preferred for drug delivery applications, since the need for surgical removal of the depleted device is eliminated. Although the number of biodegradable polymers is large, only a limited number of polymers are suitable for drug delivery applications. Suitable candidates must not only be biodegradable but also fit the high prerequisites of biocompatibility. In addition, a polymer should ideally offer process ability, sterilizability, and storage stability if it is to be useful for biomedical applications.[12] The greatest advantage of degradable polymers is that they are broken down into biologically acceptable molecules that are metabolized and removed from the body via normal metabolic pathways. However, biodegradable materials do produce degradation by-products that must be tolerated with little or no adverse reactions within the biological environment. These degradation products—both desirable and potentially non desirable—must be tested thoroughly, since there are a number of factors that will affect the biodegradation of the original materials. • Factor affecting biodegradation of polymers:• Chemical structure. • Chemical composition. • Distribution of repeat units in multimers. • Presents of ionic groups. • Presence of unexpected units or chain defects. • Molecular weight. • Molecular-weight distribution. • Morphology (amorphous/semicrystalline, microstructures, residual stresses). • Presence of low-molecular-weight compounds. • Processing conditions.