Oral modified-release formulations in motion: the relationship between gastrointestinal transit and drug absorption.

Oral modified-release dosage forms can be designed with the aim of achieving specific pharmacokinetic profiles, delivering to specific gut localities or reducing the number of drug administrations. Multiple-unit systems, such as pellets, beads or granules, often claim superiority to single-unit modified-release formulations in terms of predictability and reproducibility of behaviour in the gastrointestinal tract. This is an oversimplification and in this review we discuss the effect of the highly variable gastrointestinal transit on the bioperformance of multiple-unit dosage forms, relative to their single-unit counterparts. We examine the sometimes contradictory literature in this area and highlight specific case studies which demonstrate the effect of intestinal transit on dosage form performance and drug absorption

Mucoadhesion and the Gastrointestinal Tract

The concept of mucoadhesion is one that has the potential to improve the highly variable residence times experienced by drugs and dosage forms at various sites in the gastrointestinal tract, and consequently, to reduce variability and improve efficacy. Intimate contact with the mucosa should enhance absorption or improve topical therapy. A variety of approaches have been investigated for mucoadhesion in the gastrointestinal tract, particularly for the stomach and small intestine. Despite interesting results in these sites, mucoadhesive approaches have not yet shown success in humans. The potential of the lower gut for these applications has been largely neglected, although the large intestine in particular may benefit, and the colon has several factors that suggest mucoadhesion could be successful there, including lower motility and the possibility of a lower mucus turnover and thicker mucus layer. In vitro studies on colonic mucoadhesion show promise, and rectal administration has shown some positive results in vivo. This review considers the background to mucoadhesion with respect to the physiological conditions of the gastrointestinal tract as well as the principles that underlie this concept. Mucoadhesive approaches to gastrointestinal drug delivery will be examined, with particular attention given to the lower gut

Marker-ion analysis for quantification of mucoadhesivity of microparticles in particle-retention assays

The objective of the present work was to develop an improved method to quantify particle retention on mucosal tissue under dynamic flow conditions with simultaneous determination of drug dissolution. The principle was to dissolve the collected inert carrier material and quantify specific marker ions by reliable analytical methods. The mucoadhesive model particles consisted of drug-loaded porous calcium carbonate microcarriers coated with chitosan, and quantification of calcium ions by capillary electrophoresis enabled to determine particle-retention kinetics on colonic mucosal tissue. The method was validated by image analysis, and the particle-retention assay was successfully applied to granulate material (125-250 mm) and small particles (<90 μm) with mucoadhesive properties. Particle retention on colonic mucosa was improved by increasing the chitosan content, demonstrating the sensitivity and usefulness of marker-ion analysis for quantification of detached particles. Furthermore, we showed that drug dissolution from mucoadhesive microparticles followed comparable kinetics in the particle-retention assay and the standard USP IV method. Our findings are helpful for the development of micro-sized colonic drug delivery systems, in particular for optimization of mucoadhesive properties and sustained drug release kinetics of porous drug carriers

Mucoadhesive microparticles for local treatment of gastrointestinal diseases

Mucoadhesive microparticles formulated in a capsule and delivered to the gastrointestinal tract might be useful for local drug delivery. However, swelling and agglomeration of hydrophilic polymers in the gastrointestinal milieu can have a negative influence on particle retention of mucoadhesive microparticles. In this work, we investigated the impact of dry-coating with nano-sized hydrophilic fumed silica on dispersibility and particle retention of mucoadhesive microparticles. As a model for local treatment of gastrointestinal diseases, antibiotic therapy of Clostridium difficile infections with metronidazole was selected. For particle preparation, we used a two-step fluidized-bed method based on drug loading of porous microcarriers and subsequent outer coating with the mucoadhesive polymer chitosan. The prepared microparticles were analysed for drug content, and further characterized by thermal analysis, X-ray diffraction, and scanning electron microscopy. The optimal molecular weight and content of chitosan were selected by measuring particle retention on porcine colonic mucosa under dynamic flow conditions. Mucoadhesive microparticles coated with 5% (weight of chitosan coating/total weight of particles) of low molecular weight chitosan showed good in vitro particle retention, and were used for the investigation of dispersibility enhancement. By increasing the amount of silica, the dissolution rate measured in the USPIV apparatus was increased, which was an indirect indication for improved dispersibility due to increased surface area. Importantly, mucoadhesion was not impaired up to a silica concentration of 5% (w/w). In summary, mucoadhesive microparticles with sustained-release characteristics over several hours were manufactured at pilot scale, and dry-coating with silica nanoparticles has shown to improve the dispersibility, which is essential for better particle distribution along the intestinal mucosa in humans. Therefore, this advanced drug delivery concept bears great potential, in particular for local treatment of gastrointestinal diseases