Gastric flow and mixing studied using computer simulation

Imaging and modelling of digestion in the stomach and the duodenum

Gastroduodenal physiology is traditionally understood in terms of motor-secretory functions and their electrical, neural and hormonal controls. In contrast, the fluid-mechanical functions that retain and disperse particles, expose substrate to enzymes, or replenish the epithelial boundary with nutrients are little studied. Current ultrasound and magnetic resonance imaging allows to visualize processes critical to digestion like mixing, dilution, swelling, dispersion and elution. Methodological advances in fluid mechanics allow to numerically analyse the forces promoting digestion. Pressure and flow fields, the shear stresses dispersing particles or the effectiveness of bolus mixing can be computed using information on boundary movements and on the luminal contents. These technological advances promise many additional insights into the mechanical processes that promote digestion and absorption.

Impact of the intragastric location of extended release tablets on food interactions

Gastrointestinal motility and transport as well as concomitant food intake are factors that are known to influence pharmacokinetics derived after intake of extended release dosage forms. However, the mechanisms behind these influencing factors are mostly unknown. In this study the gastrointestinal transit and the in vivo drug release of magnetically labelled extended release tablets containing felodipine were monitored together with the drug absorption phase of pharmacokinetics under fasting and fed conditions in six healthy volunteers using Magnetic Marker Monitoring. It was found that the in vivo drug release profiles of the tablets compared well under fasting and fed conditions. However, the plasma concentration profiles were strongly influenced by concomitant food intake. This could be attributed to elongated residence of the tablets in proximal parts of the stomach, resulting in delayed drug absorption and the occurrence of late high plasma peak concentrations. The lag time until the first appearance of felodipine in plasma and the residence time of the tablets in the proximal stomach, were found to be directly correlated. The study shows that increased plasma peak drug concentrations after intake of extended release formulations together with food can be explained by poor mixing in the proximal part of the stomach and are not necessarily due to failure of the formulation to control drug release (dose dumping).

A Novel in Vitro and Numerical Analysis of Shear-Induced Drug Release from Extended-Release Tablets in the Fed Stomach

PURPOSE

To design an in vitro apparatus that could simulate the in vivo range of surface shear stresses relevant for the human stomach under fed conditions.

METHODS

Computer simulations were combined with in vitro experiments to quantify tablet erosion rate vs. surface shear stress. From two separate computer models, of tablets in the fed stomach and of tablets in vitro, we first estimated the intragastric range of surface stress and Reynolds number (Re), and then designed a dissolution apparatus and parameter space to replicate the in vivo conditions. The in vitro tablet erosion was determined by a new rotating beaker apparatus that provided predictable surface shear on tablets. Tablet mass erosion rates were measured for two different extended-release tablets at a range of in vivo relevant surface shear stresses obtained by varying viscosity of test media and rotation rate of the beaker.

RESULTS

Mass erosion rate and surface shear were found to be highly correlated. Erosion rate increased with surface shear more rapidly at "low" stresses (<35 dyne/cm2) independent of tablet material. At higher surface stress, erosion was strongly material dependent.

CONCLUSIONS

Shear force effects on drug release from matrix tablets relevant for fed state are for the first time possible to predict by in vitro dissolution testing.