Physiology of the colorectal barrier

Cell-surface markers for colon adenoma and adenocarcinoma

Early detection of colorectal cancer (CRC) is crucial for effective treatment. Among CRC screening techniques, optical colonoscopy is widely considered the gold standard. However, it is a costly and invasive procedure with a low rate of compliance. Our long-term goal is to develop molecular imaging agents for the non-invasive detection of CRC by molecular imaging-based colonoscopy using CT, MRI or fluorescence. To achieve this, cell surface targets must be identified and validated. Here, we report the discovery of cell-surface markers that distinguish CRC from surrounding tissues that could be used as molecular imaging targets. Profiling of mRNA expression microarray data from patient tissues including adenoma, adenocarcinoma, and normal gastrointestinal tissues was used to identify potential CRC specific cell-surface markers. Of the identified markers, six were selected for further validation (CLDN1, GPR56, GRM8, LY6G6D/F, SLCO1B3 and TLR4). Protein expression was confirmed by immunohistochemistry of patient tissues. Except for SLCO1B3, diffuse and low expression was observed for each marker in normal colon tissues. The three markers with the greatest protein overexpression were CLDN1, LY6G6D/F and TLR4, where at least one of these markers was overexpressed in 97% of the CRC samples. GPR56, LY6G6D/F and SLCO1B3 protein expression was significantly correlated with the proximal tumor location and with expression of mismatch repair genes. Marker expression was further validated in CRC cell lines. Hence, three cell-surface markers were discovered that distinguish CRC from surrounding normal tissues. These markers can be used to develop imaging or therapeutic agents targeted to the luminal surface of CRC.

Exploring Different Strategies for Efficient Delivery of Colorectal Cancer Therapy

Colorectal cancer (CRC) is the third most common cancer and the fourth leading cause of cancer death in the world. Currently available chemotherapy of CRC usually delivers the drug to both normal as well as cancerous tissues, thus leading to numerous undesirable effects. Much emphasis is being laid on the development of effective drug delivery systems for achieving selective delivery of the active moiety at the anticipated site of action with minimized unwanted side effects. Researchers have employed various techniques (dependent on pH, time, pressure and/or bacteria) for targeting drugs directly to the colonic region. On the other hand, systemic drug delivery strategies to specific molecular targets (such as FGFR, EGFR, CD44, EpCAM, CA IX, PPARγ and COX-2) overexpressed by cancerous cells have also been shown to be effective. This review aims to put forth an overview of drug delivery technologies that have been, and may be developed, for the treatment of CRC.

Oral drug delivery systems using chemical conjugates or physical complexes

Oral delivery of therapeutics is extremely challenging. The digestive system is designed in a way that naturally allows the degradation of proteins or peptides into small molecules prior to absorption. For systemic absorption, the intact drug molecules must traverse the impending harsh gastrointestinal environment. Technologies, such as enteric coating, with oral dosage formulation strategies have successfully provided the protection of non-peptide based therapeutics against the harsh, acidic condition of the stomach. However, these technologies showed limited success on the protection of therapeutic proteins and peptides. Importantly, inherent permeability coefficient of the therapeutics is still a major problem that has remained unresolved for decades. Addressing this issue in the context, we summarize the strategies that are developed in enhancing the intestinal permeability of a drug molecule either by modifying the intestinal epithelium or by modifying the drug itself. These modifications have been pursued by using a group of molecules that can be conjugated to the drug molecule to alter the cell permeability of the drug or mixed with the drug molecule to alter the epithelial barrier function, in order to achieve the effective drug permeation. This article will address the current trends and future perspectives of the oral delivery strategies.

Oral colon delivery of insulin with the aid of functional adjuvants

Oral colon delivery is currently considered of importance not only for the treatment of local pathologies, such as primarily inflammatory bowel disease (IBD), but also as a means of accomplishing systemic therapeutic goals. Although the large bowel fails to be ideally suited for absorption processes, it may indeed offer a number of advantages over the small intestine, including a long transit time, lower levels of peptidases and higher responsiveness to permeation enhancers. Accordingly, it has been under extensive investigation as a possible strategy to improve the oral bioavailability of peptide and protein drugs. Because of a strong underlying rationale, most of these studies have focused on insulin. In the present review, the impact of key anatomical and physiological characteristics of the colon on its viability as a protein release site is discussed. Moreover, the main formulation approaches to oral colon targeting are outlined along with the design features and performance of insulin-based devices.

The interaction of large bowel microflora with the colonic mucus barrier

The colonic mucus barrier is the first line of defence that the underlying mucosa has against the wide range of potentially damaging agents of microbial, endogenous, and dietary origin that occur within the colonic lumen. The functional component of mucus is the secreted, polymeric glycoprotein mucin. The mucus barrier can either act as an energy source or a support medium for growth to the intestinal microflora. The mucus barrier appears to effectively partition the vast number of microbial cells from the underlying epithelium. The normal functionality and biochemistry of this mucus barrier appears to be lost in diseases of the colorectal mucosa. Germ-free animal studies have highlighted the necessity of the presence of the colonic microflora to drive the maturation of the colonic mucosa and normal mucus production. A number of by-products of the microflora have been suggested to be key luminal drivers of colonic mucus secretion.

Colonic treatments and targets: issues and opportunities

The colon provides a plethora of therapeutic opportunities. There are multiple disease targets, drug molecules, and colon-specific delivery systems to be explored. Clinical studies highlight the potential for systemic delivery via the colon, and the emerging data on the levels of cell membrane transporters and metabolic enzymes along the gut could prove advantageous for this. Often efflux transporters and metabolic enzyme levels are lower in the colon, suggesting a potential for improved bioavailability of drug substrates at this site. The locoregional distribution of multiple metabolic enzymes (including cytochromes), efflux transporters (including P-glycoprotein and breast cancer resistance proteins), and influx transporters (including the solute carrier family) along the intestine is summarized. Local delivery to the colonic mucosa remains a valuable therapeutic option. New therapies that target inflammatory mediators could improve the treatment of inflammatory bowel disease, and old and new anticancer molecules could, when delivered topically, prove to be beneficial adjuncts to the current systemic or surgical treatments. New issues such as pharmacogenomics, chronotherapeutics, and the delivery of prebiotics and probiotics are also discussed in this review. Targeting drugs to the colon utilizes various strategies, each with their advantages and flaws. The most promising systems are considered in the light of the physiological data which influence their in vivo behavior.

Colon-specific drug delivery for mebeverine hydrochloride

Mebeverine Hydrochloride (MB-HCl), an effective spasmolytic drug, was formulated as CODES. A colon-specific drug delivery technology CODES was designed to avoid the inherent problems associated with pH- or time-dependent systems. To achieve more protection and control of drug release, MB-HCl was prepared as microspheres and compressed as core tablets of CODES (modified CODES). The core tablets contained the drug either in free form [Formula 1 (F(1))], or as microspheres with 2 different polymer:drug:lactulose ratios (1:1:0.5 [Formula 2 (F(2))] and 2:1:0.5 [Formula 3 (F(3))]. The release profiles of the coated CODES systems were compared with uncoated compressed tablets. The uncoated tablet showed a drug release of 94% after 1 h in simulated gastric condition (pH = 1.2). The release characteristics of the coated systems revealed that the enteric coating (Eudragit L(100)) prevented any drug release in simulated gastric or duodenal conditions in the first 3 h (pH 1.2-6.1), after which drug was slightly liberated in simulated intestinal fluid (pH 7.4) {Phase 1 (P1)}. After 4 h the pH was adjusted to 7 and beta-glucose-oxidase was added, which is an enzyme produced by enterobacteria present in the colon. The acid-soluble coat (Eudragit)E(100)) dissolved and the drug release suddenly increased to reach 95, 72 and 60.4% for F(1)-F(3), respectively. IR spectrum study showed a covalent bond between the drug and the polymer in the formulae F(2) and F(3) resulting in the sustained drug release from the microspheres with a significant difference (p>0.05) to F(1). The findings were confirmed by in vivo investigation using X-ray images for Guinea pigs ingested tablets containing barium sulphate (F(4)), where the tablet began to disintegrate after 10 h of tablet intake. The results of the study indicated that MB-HCl CODES colon-specific drug delivery can act as a successful trigger for drug targeting in the colon. Furthermore, a sustained release of the drug can be achieved from modified CODES containing the drug in the form of microspheres.

Absorption enhancement of poorly absorbed hydrophilic compounds from various mucosal sites by combination of mucolytic agent and non-ionic surfactant

Absorption enhancement of poorly absorbed hydrophilic compounds from various mucosal sites by co-administration of a mucolytic agent and a non-ionic surfactant was examined in rats. Fluorescein isothiocyanate-labeled dextran with average molecular weight of ca. 4.4kDa (FD-4), and salmon calcitonin (SCT) were used as model compounds. N-acetylcysteine (NAC) and p-t-octyl phenol polyoxyethylene-9.5 (Triton X-100, TX-100) were selected as a mucolytic agent and a non-ionic surfactant, respectively. Dosing solutions containing these agents were administered into various mucosal sites including the nose, the lung and the large intestine, and the bioavailabilities were determined. The combination of 5% NAC and 5% TX-100 significantly enhanced the nasal, the pulmonary and the large intestinal absorption of FD-4 compared to the control, and the enhancement ratios relative to the control were 7.2-, 2.8- and 4.5-fold, respectively. The different enhancement ratio among the administration sites explored indicates that the absorption enhancing effect of the combination of NAC and TX-100 is site-dependent. This combination also improved the nasal and the pulmonary absorption of SCT, and the enhancement ratios relative to the control were 6.1- and 8.1-fold, respectively. All these results suggest that the combination strategy of a mucolytic agent and a non-ionic surfactant may be widely applicable to various mucosal deliveries of poorly absorbed hydrophilic compounds.

Investigation of the absorption mechanisms of baicalin and baicalein in rats

To characterize and compare the absorption mechanisms of baicalin (BG) and baicalein (B), either of them was perfused in situ in rats with ligation of the bile duct as well as without it. Two RP-HPLC methods were developed to determine the drugs' concentrations in the gastric and intestinal perfusates, respectively. The result showed that BG was moderately absorbed in stomach but poorly in small intestine and colon, while B was well absorbed in stomach and small intestine but relatively less in colon. It also indicated that bile could excrete BG and significantly promote the absorption of B. When BG or B was perfused alone in the small intestine after ligation of the bile duct, there came out to be increasing B or BG in the perfusate, respectively. In addition, when B was intravenously administered to rats after ligation of the bile duct, there came out to be BG in the intestinal perfusate. In conclusion, B was more suitable to be administered orally than BG, which was absorbed as B and then restored to BG in the body. Part of the BG formed from the absorbed or intravenously administered B could be excreted back into the gut.

Gastrointestinal transit and disintegration of enteric coated magnetic tablets assessed by ac biosusceptometry

The oral administration is a common route in the drug therapy and the solid pharmaceutical forms are widely used. Although much about the performance of these formulations can be learned from in vitro studies using conventional methods, evaluation in vivo is essential in product development. The knowledge of the gastrointestinal transit and how the physiological variables can interfere with the disintegration and drug absorption is a prerequisite for development of dosage forms. The aim of this work was to employing the ac biosusceptometry (ACB) to monitoring magnetic tablets in the human gastrointestinal tract and to obtain the magnetic images of the disintegration process in the colonic region. The ac biosusceptometry showed accuracy in the quantification of the gastric residence time, the intestinal transit time and the disintegration time (DT) of the magnetic formulations in the human gastrointestinal tract. Moreover, ac biosusceptometry is a non-invasive technique, radiation-free and harmless to the volunteers, as well as an important research tool in the pharmaceutical, pharmacological and physiological investigations.

Effects of absorption promoters on insulin absorption through colon-targeted delivery

The aim of this study were to investigate the effect of sodium glycocholate (GC-Na) as an absorption promoter and the effects of the co-administration of GC-Na and various absorption promoters on orally administered insulin absorption utilizing a colon-targeted delivery system. The system containing insulin and GC-Na (CDS) was administered to dogs, and plasma glucose and insulin levels were then measured at 24h after administration. For CDS, the C(max) in plasma glucose level was significantly higher than a reference formulation without GC-Na. The pharmacological availability for CDS was not significantly higher than the reference formulation. In contrast, CDS with poly(ethylene oxide) as a gelling agent (CDSP) showed prolonged hypoglycemia effects. The pharmacological availability was 5.5% and significantly different from the reference formulation. The absolute bioavailability for CDS was 0.25%, and for CDSP it was 0.42%. Consequently, the results of this study demonstrated that colon-specific delivery of insulin with GC-Na was more effective in increasing hypoglycemic effects after oral administration, and the combination of GC-Na and poly(ethylene oxide) tended to prolong the colonic absorption of insulin and might be more effective for improvement of orally administered insulin absorption utilizing the colon-targeted delivery system.

Advances in Colonic Drug Delivery

Targeting drugs and delivery systems to the colonic region of the gastrointestinal tract has received considerable interest in recent years. Scientific endeavour in this area has been driven by the need to better treat local disorders of the colon such as inflammatory bowel disease (ulcerative colitis and Crohn's disease), irritable bowel syndrome and carcinoma. The colon is also receiving significant attention as a portal for the entry of drugs into the systemic circulation. A variety of delivery strategies and systems have been proposed for colonic targeting. These generally rely on the exploitation of one or more of the following gastrointestinal features for their functionality: pH, transit time, pressure or microflora. Coated systems that utilise the pH differential in the gastrointestinal tract and prodrugs that rely on colonic bacteria for release have been commercialised. Both approaches have their own inherent limitations. Many systems in development have progressed no further than the bench, while others are expensive or complex to manufacture, or lack the desired site-specificity. The universal polysaccharide systems appear to be the most promising because of their practicality and exploitation of the most distinctive property of the colon, abundant microflora.

Chitosan dispersed system for colon-specific drug delivery

A chitosan dispersed system (CDS), which was composed of active ingredient reservoir and the outer drug release-regulating layer dispersing chitosan powder in hydrophobic polymer, was newly developed for colon-specific drug delivery. An aminoalkyl methacrylate copolymer RS (Eudragit) RS) was selected as a hydrophobic polymer because it is hardly dissolved in acidic medium in which easily dissolves chitosan. In order to obtain the bi-functional releasing characteristics, i.e. time dependent and site specific, capsules containing the active ingredient (Drug Capsules) were coated by the chitosan dispersed hydrophobic polymer, resulting in CDS Capsules. The release rate could be controlled by changing the thickness of the layer. Furthermore, for colon-specific drug delivery, an additional outer enteric coating was necessary to prevent the drug release from CDS Capsules in the stomach, since chitosan dispersed in the layer dissolves easily under acidic conditions. Resultant enteric-coated CDS Capsules reached the large intestine within 1-3 h after oral administration and they were degraded at the colon in beagle dogs.

Studies on lactulose formulations for colon-specific drug delivery

A novel, colon-targeted delivery system (CODES), which uses lactulose, was investigated in this study. Lactulose is not absorbed in the upper GI tract, but degraded to organic acids by enterobacteria in the lower gastrointestinal tract, especially the colon. A CODES consists of three components: a core containing lactulose and the drug, an inner acid-soluble material layer, and an outer layer of an enterosoluble material. When a CODES containing a pigment was introduced into the rat cecum directly after shaking in JP 2nd fluid for 3 h, pigment release was observed 1 h after introduction. A CODES containing 5-aminosalicylic acid (5-ASA) was orally administered to fasting and fed dogs to evaluate its pharmacokinetic profiles. 5-ASA was first detected in plasma after 3 h, which is the reported colon arrival time for indigestible solids, after dosing to fasting dogs. The T(max) in fed dogs was delayed by 9 h when compared to fasting dogs. This corresponds to the gastric emptying time. However, the C(max) and AUC under fed conditions were almost as same as those under fasting conditions. The results of this study show that lactulose can act as a trigger for drug release in the colon, utilizing the action of enterobacteria.