Colon Targeted Delivery Of Drugs Via Oral Route Biology Essay


Rakesh29 et al (2010) ., worked on Design, optimization and evaluation of pH responsive Prednisolone sustained release tablet for ileo-colonic delivery. It contains sustained drug release system of hydroxypropylmethylcellulose, ethyl cellulose, pectin, starch and polymeric coating of Eudragit polymers. DSC, FTIR and accelerated stability studies indicated that there is no possibility of interaction between Prednisolone and other ingredients. The results of the in vitro dissolution tests in pH 1.2, pH 4.5, pH 7.2 indicated absence of drug release in stomach and small intestine and controlled release in colonic medium up to 8 hrs.

Mithun3 et al (2010)., worked on formulation and evaluation of polymer-coated polysaccharide tablets of azathioprine using different ratios of avicel (MCC), inulin and triacetin. The tablets formulations containing 25 mg of azathioprine were prepared and evaluated for drug release under different pH conditions. The formulation containing Eudragit-S, Eudragit-L and cellulose acetate phthalate (ES, EL and CAP) (1:1:1) showed desired release pattern with only 9.75% drug release in first 5 (lag phase) and satisfactory release in lowered pH conditions. Drug release increased with increase in plasticizer concentration and the concentration of inulin and citric acid above 5% w/w also increases the drug release. The addition of inulin in the formulation with coating level 28% w/w shows increased drug release in presence of rat cecal content. Thus inulin containing ES, EL and CAP (1:1:1) polymer-coated formulation system showed desired release pattern for the targeted delivery of azathioprine.

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Ghassan Abdullah31 et al (2010) ., worked on colon targeted drug delivery system containing mebeverine .Matrix tablets which were prepared using ethyl cellulose (EC), Eudragit RL 100 either solely or in combination by wet granulation technique. Dissolution was carried in 0.1N hydrochloricacid for 2 followed by pH 6.8 phosphate buffer for eight hours. Uncoated forms released more than 5% drug in 0.1N hydrochloric acid therefore, Eudragit L100 was used as a coat. The results indicated slow release profile. The matrix containing 7% Eudragit RL 100 and 6% of binder was subjected to further studies to assess the effect of different coats like Eudragit L 100-55 and cellulose acetate phthalate and different binders like pectin and sodium alginate on the release profile. And finally concluded that the prepared system has the potential to deliver mebeverine HCl in vivo to the colon.

Huiming Lai32  et al (2010) ., worked to develop a novel colon targeted drug delivery system using guar gum and Eudragit as enzyme and pH-based materials. Lansoprazole, was used as model drug. Under three different conditions, the in vitro drug release behaviors were evaluated, using β-mannanase, rat cecal content, and human fecal media to simulate the pH and enzyme during intestinal transit to the colon while targeting. The released amount of drug in simulated small intestine fluid (pH 6.8) after 5 hours was less than 10% from the pH and enzyme controlled tablets compared with 80.01±0.3% in rat cecal content medium (pH 7.4).The degradation ability of human fecal slurries was independent of human age and gender. β-Mannanase did not showed a similar effect on the degradation of polysaccharide as rat cecal enzymes and human fecal enzymes .

Clive G Wilson33 et al (2010)., worked on the transit of dosage forms through the colon. Colonic transit is of great relevance when considering in vivo/in vitro relationships for oral controlled release dosage forms. X-ray contrast, was used sparing due to the accumulating dosimetry associated with each exposure. Such methods were used for swallowing studies, gamma scintigraphiation allowed physicians to measure colon function with a more moderate radiation burden, prompted the use in pharmaceutical sciences; finally, the relationship between blood concentrations and transit of different sized dosage were understood.

Hodges34 et al (2009)., worked on scintigraphic evaluation of colon targeting pectin-HPMC tablets in healthy volunteers. Pectin-hydroxypropyl methylcellulose coating was compressed into core tablets. The tablets released in the colon of all subjects; 3 in the ascending colon (AC) and 3 in the transverse colon (TC). Tablets that released in the TC had reached the AC before or just after the food (Group A). The other 3 tablets released immediately upon AC entry at least 1.5 h post-meal (Group B). Group B tablets showed a clear residence period at the ileocaecal junction (ICJ) which was not seen in Group A. Prolonged residence at the ICJ increases hydration of the hydrogel layer surrounding the core tablet. Forces applied as the tablets progressed through the ICJ may have disrupted the hydrogel layer to initiate drug release. Conversely, Group A tablets moved rapidly from AC to the TC, possibly decreasing contact times with water so there is less initiation of radiolabel release.

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Mayur35 et al (2009)., developed core tablet of mesalamine which was compression coated with hydroxyl propyl methyl cellulose (HPMC K4M) and pH-dependent polymer Eudragit L100. The simplex lattice was adopted to optimize the independent variables i.e. amount of HPMC, dextrose and polyvinyl pyrollidone (PVP) and to study their effect on the variables they depended. i.e lag time and time for 50% drug dissolution (t50). The results of the linear model, graphical representation revealed that as the amount of HPMC increases, the lag time and T50 also increases and as the amount of dextrose and PVP were increased the lag time and T50 value decreases.

Jose36 et al (2009) ., worked on different approaches of Colon targeted drug delivery .Oral colon-targeted drug delivery systems deliver a variety of therapeutic agents for both local and systemic administration. Local treatment of a variety of colonic diseases and systemic absorption of proteins. Targeting of drugs to specific sites of action (i.e colon) provides several advantages over non-targeting of drugs. The colon drug delivery, is also beneficial for the treatment of diseases sensitive to circadian rhythms. The successful targeted drug delivery to the colon via the gastrointestinal tract requires the protection of a drug from release in the stomach and small intestine and then ensures controlled release in the proximal colon. This review covered both past and present approaches for achieving colon specific drug delivery

Timucin37 et al (2007)., worked on Colonic delivery of compression coated Nisin tablets  using pectin/HPMC polymer mixture. In this study, each 100 mg core tablet of Nisin was compression coated with 100%  pectin, 90%  pectin-10% HPMC, 85% pectin-15% HPMC, 80% pectin -20% HPMC, 75% pectin -25% HPMC, 100% HPMC at a coat weight of 400 mg. Drug release studies were carried in pH 3.3 buffer solution. System erosion experiments were carried out in pH 1.2, 3.3, and 6.8 buffers using pectinolytic enzyme.

Pornsak38 et al (2007)., Investigated on swelling and erosion behaviours of hydrophilic matrix tablets using pectin and their impact on drug release, was carried out in various media. The pectin matrix tablets in contact with the aqueous medium undergoing a combination of swelling and erosion. The swelling action of pectin matrices was controlled by the rate of its hydration in the aqueous medium. Release studies showed that the swelling and erosion of matrices influenced the drug release from pectin. The extent of pectin matrix tablets swelling, erosion and diffusion of drug determined the kinetics as well as mechanism of drug release. The release data showed a good fit into the Korsmeyer-Peppas equation indicates the combined effect of diffusion and erosion mechanisms of release.

Cui Fude39 et al (2007)., worked on pH-dependent, time-based and enzyme degradable pellets for use as an oral colon drug delivery system. The pellets were prepared using extrusion-spheronizing equipment and coated with three functional polymers by an air-suspension technique. The core consisting of 5-aminosalicylic acid (5-ASA) as a model drug, CaP as an enzyme-degradable material and microcrystalline cellulose (MCC) as an additive. As far as the three coated layers were concerned, the outer most layer was coated with Eudragit L30D-55 for protection against gastrointestinal juices (stomach pH), the intermediate layer was coated with ethylcellulose to inhibit drug release during passage through the small intestine, and the inner film was coated with pectin for swelling and enzyme-degradation. Also, dissolution testing of the cores was done. The presence of calcium pectinate (CaP) markedly increased the drug release rate from the cores, as determined by scanning electron microscopy (SEM).

Raghavendra40 et al (2007) ., worked on to develop colon targeted drug delivery systems for metronidazole (MTZ).Tablets were prepared using various polysaccharides or copolymer of methacrylic acid with guar gum (GG) as a carrier. For improving the colon specificity, some selected tablet formulations were enteric coated with Eudragit-L 100 to give protection in an acidic environment. Drug release studies were performed for uncoated tablets containing xanthan gum or mixture of xanthan gum with graft copolymer showed 30-40% drug release during the initial 4-5 hr, whereas for tablets containing GG with the graft copolymer, it was 70%. After enteric coating, the release was reduced to 18-24%. Further, in vitro release was performed in the presence of rat caecal contents. Results indicated an increased release when compared to formulations studied without rat caecal contents. Release data indicated that drug release is significantly affected by the nature of the polysaccharide used and enteric coating of the tablet.

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Valentine41 et al (2006)., Investigated the in vitro dissolution characteristics of pH-responsive polymers in a variety of simulated fluids for ileo-colonic delivery. Prednisolone tablets were fabricated and coated with the following polymer systems: Eudragit S in organic solution, Eudragit S in aqueous dispersion, Eudragit FS in aqueous dispersion and Eudragit P4135 in organic solution. The results indicate that the tablets coated with the newer Eudragit FS polymer shows appropriate for drug delivery to the ileo-colonic region in comparison to the Eudragit S. However, dissolution in the physiological buffers was found to be slower for all coated tablets Therefore, their is a need to simulate the ionic composition of the intestinal fluid in the dissolution media.

Ceballos 42 et al (2005) ., worked on influence of formulation and process variables on in vitro release of theophylline from directly-compressed Eudragit matrix tablets. They prepared matrix tablets by direct compression of drug and different pH-dependent (Eudragit L100, S100 and L100-55) and pH-independent (Eudragit RLPO and RSPO) polymer combinations. The influence of varying the polymer/polymer (w/w) ratio and the drug incorporation method was also evaluated. Drug release was monitored depending on both the kind of Eudragit polymer combinations. Maintaining a constant 1:1 drug/polymers ratio, the selection of appropriate mixtures of pH-dependent and pH-independent polymers enabled achievement of a suitable control release. Finally they concluded that Eudragit RLPO showed highly reproducible drug release profiles, with an almost zero-order kinetic, and allowed 100% released drug after 360 min. As for the effect of the drug incorporation method, simple blending was better than the solid dispersion technique, because it not only did not improve the release data reproducibility, but also caused a marked slowing down in drug release rate.

Miao Xi43 et al (2005) ., Studied on the characteristics of pectin-ketoprofen (PT-KP) prodrug with the potential for colon targeted delivery has been evaluated. HPLC method was used for the determination of concentration of ketoprofen in rats. This method was also used to estimate the colon targeting property of PT-KP. KP or PT-KP which was given to rats by oral administration. Plasma and the different parts of gastrointestinal tract were taken after 2, 4, 6, 8, 10 and 12 hours of oral administration of two formulations to rats and the concentration of KP was measured by HPLC. Experiments showed KP distributes mainly in stomach, proximal small intestine and distal small intestine. However, PT-KP mainly distributes in cecum and colon. Therefore, this approach suggested that PT-KP prodrug has a good colon targeting property.

Sverre44 et al (2005)., worked on pectin-based oral drug delivery to the colon .They reviewed oral formulations intended for site-specific drug delivery to the colon with pectin as the main excipient. By using scintigraphic methods the functionality of pectin formulations were discussed. The main focus was on the various formulations reported, including matrix tablets, multiparticulate formulations as pellets and hydrogel beads, and pectin-based coatings.

Ahmed45 et al (2005)., worked on effect of acidic pH representative of gastric fluid on the release of 5-aminosalicylicacid(ASA) from beads coated with pectin/ethylcellulose as film coating to target the drug to the colon. The in vitro incubation of the beads in acidic medium was found to influence the hydration and the swelling characteristics of pectin. Moreover, the drug release profiles from the beads in simulated intestinal fluid after incubation for 2 h in simulated gastric fluid vs. no acid incubation were found to be different. The in vitro degradation of pectin by pectinolytic enzymes in simulated cecal fluid depended on whether the beads were placed in simulated gastric fluid prior to testing in simulated intestinal fluid. The percentage drug release also dependes upon the ratio of pectin to ethyl cellulose used in the coat.

Alvarez46 et al (2004) ., worked on oral drug delivery system for colon targeting of enteric-coated matrix tablets which  were prepared by direct compression of mixtures of hydroxyl ethyl cellulose (HEC), ethyl cellulose (EC) or micro-crystalline cellulose (MCC) polymers, in which theophylline was dispersed. Eudragit S100 polymer which is soluble at pH 7, was used as pH-sensitive coating polymer. The results of release studies indicated that the Eudragit S100 enteric-coated matrix tablet achieved gastric resistance and timed-release of the drug. The enteric-coating level was the critical factor in determining the duration of the lag-phase, whereas the release rate mainly depended on the matrix composition. Formulations with more HEC content showed a faster drug release rate than those with greater content in inert polymer and the MCC-HEC combinations shows good results in comparision with EC-HEC ones. The best results were given by the 27% coated 1:0.3:0.7 (w/w) drug/MCC/HEC tablets, which, after a 260 min lag time, regularly released the drug, achieving about 90% of release after 10 h.

María47 et al (2003)., aimed to developing a novel sodium diclofenac formulation for colonic release. The central core was formed by a solid dispersion of the drug into the hydrophilic polymer PEG 4000, which enabled an increase of drug dissolution properties with respect to other carriers. Eudragit® RS100 polymeric matrix for the core coating, mixed (50:50, w/w) with sodium chloride and Emdex® as channeling agents. Tablets were prepared by direct compression, and tested for dissolution properties. They concluded that by varying the sodium chloride/Eudragit w/w ratio, it was possible to suitably modulate the length of both the lag time and zero-order release phases for achieving colonic targeting.

LinShu Liu48 et al (2003)., worked on pectin-based systems for colon -specific drug delivery via oral route. Pectin-derived matrices are examined and tested for controlled drug delivery. Pectin was intact in the upper gastrointestinal tract and degraded in lower gastrointestinal tract by colonic microflora. Thus, pectin-derived drug carriers provided promising potential for colon -specific drug delivery. This paper reviewed recent developments in pectin-derived formulations. Subjects reviewed included gelation of pectin, calcium cross-linked pectinate, composites of pectin and technologies to fabricate pectin into useful drug delivery vehicle, and various methods to evaluate release kinetics of incorporated drugs. This article discussed advantages, limitations, and possible future developments in pectin-based formulations with particular emphasis on the field of colon -specific drug delivery.

Krishnaiah49 et al (2010)., worked on to develop novel tablet formulations for site-specific delivery of 5-fluorouracil to the colon by using guar gum as a carrier. Fast-disintegrating 5-fluorouracil core tablets were compression coated with 60%, 70% & 80% of guar gum, and were subjected to in vitro drug release studies. Guar gum compression-coated tablets released only 2.5-4% of the 5-fluorouracil in simulated GI fluids and in simulated colonic fluids. The compression-coated FH-60, FH-70 and FH-80 tablets released another 70, 55 & 41% of the drug. The results of the study show that compression-coated tablets containing FH-80 of guar gum are most likely to provide targeting of 5-fluorouracil for local action in the colon.

Paola50 et al (2003)., developed a combined approaches of a specifically colon-biodegradable pectin matrix with a pH-sensitive Eudragit S100 polymeric coating. This system over come the problems of pectin solubility in the upper gastrointestinal tract and low site-specificity of simple pH-dependent systems. The effects of varying the type of pectin, the pectin:Emdex ratio and the level of the pH-dependent polymeric coating on drug release behavior were investigated. After lag time, nearly zero-order profiles were obtained whose slope was depended on both the Emdex content and the pectin type.

Krishnaiah51 et al (2003)., worked on colon-targeted drug delivery systems for Ornidazole using guar gum as a carrier by direct compression. The prepared formulations were evaluated for hardness and drug content uniformity and were subjected to in vitro drug release studies. The compression-coated formulations released less than 8% of Ornidazole in the physiological environment of stomach and small intestine. The compression-coated tablets with 85%, 75%, and 65% of guar gum coat released about 21%, 38%, and 73% of drug respectively, in simulated colonic fluids shows the susceptibility of the guar gum to the rat caecal contents. The results of the study show that compression-coated ornidazole tablets with either 65% or 75% of guar gum coat are most likely to provide targeting of ornidazole for local action in the colon owing to its minimal release of the drug in the first 5 hr.

Meiling52 et al (2003)., worked on novel pH- and time-dependent delivery system for delivering drugs to the colon . The delayed time of the press-coating layer was controlled by its erosion rate, and it followed Hixson-Crowell equation. The transit profiles in two volunteers by gamma scintigraphy demonstrated that the tablets  passed through the stomach and small intestine intact and safely reached the distal end of the small intestine, where the the drug began to release from core tablet. For both of the volunteers, disintegration of the tablets occurred in the ascending colon, which shows the potential of this system for colonic drug delivery.

Sinha53 et al (2003)., worked on microbially triggered drug delivery to the colon. Colon -specific drug delivery was designed to target drug molecules specifically to this area. Development of site-specific delivery may exploit a specific property of the target site for drug release. The gastrointestinal tract is inhabited by around 400 bacterial species. Colon, the distal part of the intestine is inhabited by a large variety of gram negative microorganisms. They produce a vast number of enzymes which are being exploited for formulation of colon drug delivery systems. A number of microbially activated systems for colon-specific drug delivery are evaluated. These include prodrugs and polymer-based delivery systems. The article aimed at reviewing the various microbially activated drug delivery systems for colon -specific drug delivery with specific reference to the microflora.

Rasmane54   et al (2003) ., worked on synthesis and enzymatic degradation of epichlorohydrin Cross-Linked pectins .The water solubility of pectin was decreased by cross-linking with increasing amounts of epichlorohydrin in the reaction media. After incubating the different cross-linked pectins, (0.5% w/v) in 25 ml of 0.05 M acetate-phosphate buffer (pH 4.5), containing 50 µL of Pectinex Ultra SP-L (pectinolytic enzymes), between 60 and 80% of the pectin  bonds were broken in less than 1 h. Moreover, increasing the cross-linking resulted in a slowing of the enzymatic degradation velocity.

Krishnaiah55 et al (2002) ., worked on the development of oral colon  targeted drug delivery systems of mebendazole using guar gum as a carrier. Matrix tablets of guar gum were prepared by wet granulation technique using starch paste as binder. The results of the study showed that matrix tablets containing either 20% or 30% of guar gum were most likely to provide targeting of mebendazole for local action in the colon. The mebendazole matrix tablets containing either 20% or 30% of guar gum showed no change either in physical appearance, drug content or dissolution pattern after storage at 45°C/75% relative humidity for 3 months.

Murat56 et al (2002)., worked on in vitro evaluation of pectin -HPMC compression coated 5-aminosalicylic acid tablets for colonic delivery. Each 100 mg core tablet contained 5-aminosalicylic acid was compression coated at 20-30 kN using 100% pectin, 80% pectin -20% HPMC, or 60% pectin-40% HPMC, at two different coat weights as 400 or 500 mg. Drug dissolution/system erosion/degradation studies were carried in pH 1.2 and 6.8 buffers using a pectinolytic enzyme. It was found that pectin was not sufficient to protect the core tablets. So, HPMC addition was required to control the solubility of pectin. The optimum HPMC concentration was 20% and such system would protect up to 6 h that corresponded to 25-35% erosion and after that under the influence of pectinase the system degrades faster and delivering 5-ASA to the colon. The pectin -HPMC combination was found to be a promising drug delivery system for those drugs to be delivered to the colon.

Krishnaiah57 et al (2002)., worked on development of oral colon-targeted drug delivery systems for celecoxib using guar gum as a carrier. The tablets were evaluated for hardness, drug content and were subjected to in vitro drug release studies. In simulated colonic fluids (rat caecal content medium), the matrix tablets containing 20% of guar gum released 37% of celecoxib after degradation by the colonic bacterial action. The matrix tablets containing 30% of guar gum released about 24% of celecoxib in simulated colonic fluids. Results of the study showed that the matrix tablets containing either 20 or 30% of guar gum are most likely to target celecoxib for local action in the colon.

Krishnaiah58 et al (2001)., worked to develop colon targeted drug delivery systems for mebendazole using guar gum as a carrier. Matrix tablets containing various ratios of guar gum were prepared by wet granulation using starch as a binder. Guar gum matrix tablets released 8-15% of the mebendazole in the physiological environment of stomach and small intestine depending on the proportion of guar gum used in the formulation. It was concluded that matrix tablets containing either 20% or 30% of guar gum were most likely to provide targeting of mebendazole for local action in the colon.

Krishnaiah59 et al (2001) ., worked on the influence of metronidazole and tinidazole on the usefulness of guar gum, a colon-specific drug carrier based on the metabolic activity of colonic bacteria, using matrix tablets of albendazole (containing 20% of guar gum) as a model formulation.  The matrix tablets of albendazole were subjected to in vitro drug release studies in simulated colonic fluids (4%w/v of rat caecal contents) obtained after oral treatment of rats for 7 days either with varying doses of metronidazole/ tinidazole and 1 ml of 2%w/v of guar gum or with 1 ml of 2%w/v of guar gum alone (control study) after completing the dissolution study in 0.1 M hydrochloric acid (2 h) and pH 7.4 Sorensen's phosphate buffer (3 h).In vitro results showed that the release of the drug from guar gum formulations was found to increase with a decrease in the dose of metronidazole/tinidazole administered. They concluded that administration of either metronidazole or tinidazole with guar gum based colon-specific drug delivery systems interfere with the targeting of drugs to colon.

Sinha60 et al (2001)., worked on Polysaccharides in colon-specific drug delivery. Natural polysaccharides are extensively used for the development of solid dosage forms for delivery of drugs to colon. The rationale for the development of a polysaccharide based delivery system for colon in the presence of large amounts of polysaccharidases in the human colon they are inhabited by a large number of bacteria which secrete many enzymes e.g. D-glucosidase, D-galactosidase, amylase, pectinase, xylanase,D-xylosidase, dextranase, etc. Various approaches of polysaccharides for colon-specific delivery are fermentable coating of the drug core, embedding of the drug in biodegradable matrix, formulation of prodrugs. A large number of polysaccharides have been studied for their potential use as colon-specific drug carrier systems, such as chitosan, pectin, cyclodextrin, dextrans, guar gum, inulin, and amylose. Recent approaches exploiting these polysaccharides in colon-specific drug delivery are discussed in this review.

Tracye61 et al (2001)., worked on amebiasis. Amebiasis is caused by the protozoan parasite called Entamoeba histolytica. Invasion of the colonic mucosa in the form of trophozoite this parasite generally results in amoebic dysentery. In some patients it causes amoebic liver abscesses. Although it is estimated that 10% of the world's population is infected with E. histolytica, only 1% of these patients will manifest disease. The diagnosis is usually by microscopic identification of the microorganism in a stool specimen. The drug of choice for treatment of amebiasis is metronidazole.

Sayeh62 et al (2000)., worked on Development of pectin matrix tablets for colonic delivery of model drug ropivacaine. The aim of this study was to investigate some formulation factors that could reduce the release of the drug in the simulated gastric and intestinal fluids, increase the release in the simulated cecal fluid and improve the poor compactibility of pectins. The effect of two pectin types, the incorporation of ethylcellulose as a matrix-additive and water or ethanol as granulating liquids were investigated. Amidated pectin produced harder tablets than the calcium salts of pectin. Addition of ethyl cellulose increased the tablet strength and the dissolution rate of drug. Furthermore, directly compressed tablets were produced by addition of coarse or micronised qualities of ethyl cellulose. The also improves the crushing strength and imposing a marginal release-reducing effect. Coating this formulation with Eudragit L 100 reduced the release in the simulated upper GI conditions without interference with the subsequent enzymatic activity.

Graeme63 et al (1999)., worked on selective drug delivery to the colon  using pectin chitosan: hydroxyl propyl methylcellulose film coated tablet. Radio labelled tablets were coated with a 3:1:1, P:C:H film and administered to humans. The gastro-intestinal transit of the tablets was assessed by gamma scintigraphication study. The results showed that in all cases, the tablets were able to pass through the stomach and small intestine intact. Break up of the tablets commenced once in the colon, due to degradation of the coat by colonic bacteria. Therefore the study proved the potential of this coating system for colon drug delivery.

Zahirul64 et al (1999) ., worked on pH-dependent colon  targeted oral drug delivery system using methacrylic acid copolymers: Lactose-based placebo tablets were coated using various combinations of  methacrylic acid copolymers, Eudragit L100 and Eudragit S100, by spraying from aqueous systems. The same coating formulations were applied on tablets  containing mesalazine as a model drug and evaluated for in vitro dissolution rates. The disintegration data obtained from the placebo tablets  demonstrated that disintegration rate of the studied tablets  was dependent on: (i) the polymers combination used to coat the tablets,  (ii) pH of the disintegration media, and (iii) the coating level of the tablets. Dissolution studies performed on the mesalazine tablets and confirmed that the release profiles of the drug could be manipulated by changing the Eudragit L100 and Eudragit S100 ratios within the pH range of 5.5 to 7.0,and a coating formulation consisting of a combination of the polymers can overcome the issue of high gastrointestinal (GI) pH variability among individuals. The results also demonstrated that a combination of Eudragit L100 and Eudragit S100 could be successfully used from aqueous system to coat tablets for colon targeted delivery of drugs and adjusted to deliver drug at any other desirable site of the intestinal region of the GI tract on the basis of change in pH. For colon  targeted delivery of drugs the proposed combination system was superior to tablets coated with either Eudragit L100-55 or Eudragit S100 alone.

Fernández65 et al (1998) ., worked on pectin/chitosan mixtures as coatings for colon-specific drug delivery:  Small tablets were coated with either pectin or pectin in a 1:10 ratio with chitosan. In vitro release studies showed that pectin was able to protect the cores from premature release, when a substantially thick coat was present. Pectin/ chitosan mixtures achieved good protection at a lower coat weight. The use of pectinolytic enzymes to simulate breakdown in the colon showed that the pectin/ chitosan mixture was susceptible to enzymatic breakdown and allowed faster drug release.

Ramprasad66 et al (1998) ., worked on in vitro evaluation of guar gum as a carrier and Indomethacin as model drug for colon-specific drug delivery . Drug release studies from mouth to colon transit have shown that guar gum protects the drug from being released completely in the stomach and small intestine. The pre-treatment of rats with 1 ml of 2% w/v aqueous dispersion of guar gum for 3 days orally induces enzymes acting on guar gum thereby increasing release. A further increase in release was observed with rat caecal contents obtained after 7 days of pre-treatment. The presence of 4% w/v of caecal contents obtained after 3 days and 7 days of enzyme induction showed biphasic drug release curves. The results shows that guar gum is a potential carrier for colon-specific drug delivery.

Renaat Kinget67 et al (1998)., worked on  colonic drug targeting. Drugs targeting to the colon recognized to have several therapeutic advantages. Drugs which are destroyed by the stomach acid and/or metabolized by pancreatic enzymes were slightly affected in the colon, and sustained colonic release of drugs are useful in treatment of asthma, angina and arthritis. Treatment of colonic diseases such as ulcerative colitis, colorectal cancer and Crohn's disease is more effective with direct delivery of drugs to the affected area in colon. Likewise, colonic delivery of vermicides and colonic diagnostic agents required smaller doses. They aimed in providing insight into the design considerations and evaluation of colonic drug delivery systems. For this purpose, the anatomy and physiology of the lower gastrointestinal tract were surveyed. Furthermore, the biopharmaceutical aspects are considered for drug absorption in the colon and hence various approaches of colon-specific drug delivery are discussed.

Ashford68 et al (1994)., worked on targeting drugs to the colon. In this review they shorted approaches taken to achieve a universal system for delivery. The design of such a system required the identification and exploitation of a unique feature of the colonic environment. The use of transit times, pH and bacterial enzymes were critically assessed. In addition, they must provide protection for the drug during transit to the colon. Upper gastro-intestinal physiology and the transit of pharmaceuticals through these regions were reviewed with reference to their relevance in achieving site specificity.

Majeed69 et al (1984)., worked on Effect of long-term metronidazole (MTZ) therapy on experimental colon cancer in rats .in this study they divided eighty-one rats into four groups. Group A served as a control, groups B and C were given MTZ in their food (50 mg/kg/day). In groups C and D, a 3-cm colonic segment was isolated and brought out as a blind loop fistula. All animals received 20 weekly sc doses of 1,2-dimethylhydrazine (DMH) and were killed 25 weeks after the first injection. The mean number of colon tumours per animal (±SEM) in MTZ groups B (1.65 ± 0.29) and C (2.57 ± 0.38) were higher than A (1.44 ± 0.3) and D (1.18 ± 0.21), but the increase was only significant for group C over groups A and D (P < 0.05) and group B (P = 0.06). The mean number of tumours per animal in the isolated loop of group C (0.95 ± 0.28) was similar to group D (0.68 ± 0.16) P = 0.41, but the mean number of tumours in the functioning colon of group C (1.62 ± 0.25) was higher than group D (0.5 ± 0.12) P < 0.001. These findings suggest that (1) long-term MTZ increased the number of colon tumours per rat in the DMH model but a statistical significance (P < 0.05) was only noted in the MTZ and surgery group. (2) Surgery alone did not increase the number of tumours. (3) This effect of MTZ is dependent on the presence of the fecal stream, since there was no significant difference between the number of tumours in the empty loops of MTZ and non-MTZ groups.