Imaging of oily formulations in the gastrointestinal tract

Feasibility of capsule endoscopy for direct imaging of drug delivery systems in the fasted upper-gastrointestinal tract

PURPOSE

To develop a minimally-invasive method for direct visualization of drug delivery systems in the human stomach and to compare the obtained results with an established in vitro model. The method should provide the capsule rupture, dispersion characteristics, and knowledge regarding the surrounding physiological environment in the stomach.

METHODS

A capsule endoscopic method was developed. The disintegration time, dispersion characteristics and the impact of the physiological environment on different lipid based delivery systems in different gelatin capsules in the fasted stomach of nine healthy volunteers were visualized. Biorelevant dissolution studies using a USP II apparatus and a droplet size analysis of the released SNEDDS were performed.

RESULTS

Visualization of the behavior of both hard and soft gelatin capsules formulations was possible. The disintegration and dispersion of EP oil in a soft capsule and SNEDDS in a hard shell capsule were visualized. The in vitro release rates were different from the in vivo release rates of the soft capsule due to volume, fluid composition and motility differences but not for the hard capsule containing SNEDDS.

CONCLUSIONS

A minimally-invasive capsule endoscopic method was developed for direct visualizing of drug delivery systems in the human stomach and maybe later, in the duodenum.

The transit of dosage forms through the colon

Colonic transit is a subject of great relevance when considering in vivo/in vitro relationships for oral controlled release dosage forms. Our knowledge of colonic motility has first come from the clinic, where measurement of the whole gut transit of different excreted markers was used as a method of discriminating pathologies. X-ray contrast, although widely available, was used sparing due to the accumulating dosimetry associated with each exposure. Although such methods were used for swallowing studies, gamma scintigraphy allowed physicians to measure colon function with a more moderate radiation burden. The ability to label meal and dosage form separately and to measure dispersion with more certainty, prompted the use in pharmaceutical sciences; finally, the relationship between blood concentrations and transit of different sized dosage began to be understood. This mini-review considers the development of colon transit measurements and how different designs of clinical assessment assist in elucidating size and shape influence on colon transit in man.

Preparation and evaluation of self-microemulsifying drug delivery systems (SMEDDS) containing atorvastatin

Atorvastatin is insoluble in aqueous solution and the bioavailability after oral administration is low. Self-microemulsifying drug delivery systems (SMEDDS) containing atorvastatin have been successfully prepared to improve its bioavailability. SMEDDS is a mixture of lipid, surfactant, and cosurfactant, which are emulsified in aqueous medium under gentle digestive motility in the gastrointestinal tract. Pseudo-ternary phase diagrams composed of various excipients were plotted. Droplet size, zeta-potential and long-term physical stability of the formulations were investigated. The release of atorvastatin from SMEDDS capsules was studied using the dialysis bag method in 0.1 m HCl and phosphate buffer (pH 7.4), compared with the release of atorvastatin from a conventional tablet. A pharmacokinetic study was performed in 6 beagle dogs after oral administration of 6 mg kg−1 atorvastatin. The bioavailability of atorvastatin SMEDDS capsules was significantly increased compared with that of the conventional tablet. SMEDDS capsules consisting of Labrafil, propylene glycol and Cremophor RH40 provided the greatest bioavailability. Our studies indicate that the use of SMEDDS for the delivery of atorvastatin can improve its bioavailability.

Magnetic images of the disintegration process of tablets in the human stomach by ac biosusceptometry

Oral administration of solid dosage forms is usually preferred in drug therapy. Conventional imaging methods are essential tools to investigate the in vivo performance of these formulations. The non-invasive technique of ac biosusceptometry has been introduced as an alternative in studies focusing on gastrointestinal motility and, more recently, to evaluate the behaviour of magnetic tablets in vivo. The aim of this work was to employ a multisensor ac biosusceptometer system to obtain magnetic images of disintegration of tablets in vitro and in the human stomach. The results showed that the transition between the magnetic marker and the magnetic tracer characterized the onset of disintegration (t(50)) and occurred in a short time interval (1.1 +/- 0.4 min). The multisensor ac biosusceptometer was reliable to monitor and analyse the in vivo performance of magnetic tablets showing accuracy to quantify disintegration through the magnetic images and to characterize the profile of this process.

Hydrophilic and lipophilic radiopharmaceuticals as tracers in pharmaceutical development: in vitro--in vivo studies

BACKGROUND

Scintigraphic studies have been performed to assess the release, both in vitro and in vivo, of radiotracers from tablet formulations. Four different tracers with differing physicochemical characteristics have been evaluated to assess their suitability as models for drug delivery.

METHODS

In-vitro disintegration and dissolution studies have been performed at pH 1, 4 and 7. In-vivo studies have been performed by scintigraphic imaging in healthy volunteers. Two hydrophilic tracers, (99mTc-DTPA) and (99mTc-MDP), and two lipophilic tracers, (99mTc-ECD) and (99mTc-MIBI), were used as drug models.

RESULTS

Dissolution and disintegration profiles, differed depending on the drug model chosen. In vitro dissolution velocity constants indicated a probable retention of the radiotracer in the formulation. In vivo disintegration velocity constants showed important variability for each radiopharmaceutical. Pearson statistical test showed no correlation between in vitro drug release, and in vivo behaviour, for 99mTc-DTPA, 99mTc-ECD and 99mTc-MIBI. High correlation coefficients were found for 99mTc-MDP not only for in vitro dissolution and disintegration studies but also for in vivo scintigraphic studies.

CONCLUSION

Scintigraphic studies have made a significant contribution to the development of drug delivery systems. It is essential, however, to choose the appropriate radiotracers as models of drug behaviour. This study has demonstrated significant differences in release patterns, depending on the model chosen. It is likely that each formulation would require the development of a specific model, rather than being able to use a generic drug model on the basis of its physicochemical characteristics.

Usage of radiopharmaceuticals in the development of pharmaceutical drug delivery systems: validation of [99mTc]DTPA and [99mTc]ECD

Tablets containing drugs of different lipophilicity, ranitidine and cinarizine, and placebo were prepared and their in vitro behaviour was studied by dissolution and disintegration tests. [(99m)Tc]Diethylenetriamine-pentaacetic acid ([(99m)Tc]DTPA) and [(99m)Tc]ethyl cysteinate dimer ([(99m)Tc]ECD) were used as tracers of the process. Both of them were added to tablets during wet granulation. Dissolution and disintegration profiles were assessed at different pH values (1, 4 and 7). Radioactivity was evaluated in filtered samples and scintigraphic studies were carried out in gamma camera. Stability in dissolution media was confirmed for both tracers under these conditions. Dissolution and disintegration velocity constants were calculated. [(99m)Tc]DTPA proved to be an appropriate tracer for polar drugs such as ranitidine. Nevertheless, it was not a suitable tracer for lipophilic active drugs such as cinarizine. On the other hand, the most lipophilic tracer, [(99m)Tc]ECD, exhibited the opposite behaviour. Scintigraphic studies of the disintegration process did not show significant differences between placebos and tablets containing active drugs. As disintegration is a physical process it does not discriminate between chemical differences in tablet formulations. Both methods complement each other because the dissolution process can be followed when a suitable radiotracer is chosen according to the physicochemical characteristics of the active drug.

Parenteral water/oil emulsions containing hydrophilic compounds with enhanced in vivo retention: formulation, rheological characterisation and study of in vivo fate using whole body gamma-scintigraphy

The preparation and characterization of parenteral water-in-oil (w/o) emulsions with a potential for sustained release of hydrophilic drugs was described with emphasis on rheological behaviour and spreading phenomenon after intramuscular (i.m.) injection in rabbit thigh muscle. Both steady state and dynamic rheological parameters were investigated showing Newtonian behaviour at low fraction of disperse phase ratio as opposed to viscoelastic and pseudoplastic behaviour at high fraction of disperse phase. Disappearance and spreading behaviour of hydrophilic radioactive markers, aprotinin (6512 g/mol) and pertechnetate (193 g/mol) entrapped in w/o emulsions from an i.m. injection site was studied by whole body gamma-scintigraphy. The retention of entrapped aprotinin 24 h postinjection was 83 +/- 5% for a low spreading emulsion and 76 +/- 6% for a high spreading emulsion. The corresponding values for pertechnetate were 50 +/- 11 and 23 +/- 2%, respectively. The relatively long retention times were suggested to be related to the good physical stability properties of the present emulsions. It was concluded that the presented w/o emulsions are promising vehicles for sustained release of hydrophilic drugs from an i.m. injection site.

Imaging techniques for assessing drug delivery in man

In vivo imaging technologies have a vital role to play in the pharmaceutical development process. Gamma scintigraphy, comprising two-dimensional 'planar' imaging, is used widely to visualize and to quantify drug delivery, particularly by the oral and pulmonary routes. However, three-dimensional imaging modalities - single photon emission computed tomography (SPECT), positron emission tomography (PET) and magnetic resonance imaging (MRI) - may also have applications within this area. Single photon emission computed tomography and PET offer potential advantages over gamma scintigraphy in the assessment of regional lung deposition from aerosol inhalers, but these advantages are greatly outweighed by the practical problems associated with conducting SPECT and PET studies. It is concluded that, for the foreseeable future, gamma scintigraphy is the imaging modality of choice in assessing the delivery of new oral and pulmonary drug products.

In-vivo monitoring of dosage forms

The use of imaging techniques including gamma scintigraphy to follow the behaviour of drug formulations has revolutionized our knowledge of absorption and distribution in drug delivery. The development of gamma camera techniques as physiological tools to explore organ function became routine by the mid-seventies. Several research groups started to explore the applications of technique in drug delivery. Within 5 years, the utility of the technique became obvious and scintigraphy is now widely accepted as an important investigation tool in formulation research. Gamma scintigraphy is especially useful in exploring sources of inter-subject variation, especially in examining food effects in pharmacokinetic estimations and establishing windows of absorption for oral delivery. As a tool to examine drug delivery to the lung and to the eye, scintigraphy is the method of choice. Magnetic Resonance Imaging (MRI) became more generally employed in medicine two decades after the gamma camera. The superior soft-tissue contrast and resolution compared to computed X-ray tomography rapidly established MRI in clinical investigation. Recent applications in oral drug research has allowed the pharmaceutical scientist to explore new facets of delivery and ultimately combine MRI and scintigraphy in human clinical trials.