Professor Dr P. Boesiger, Institute of Biomedical Engineering, ETH Zurich, Gloriastrasse 35, CH-8092 Zurich, Switzerland. E-mail: firstname.lastname@example.org
Controlled delivery of drugs to the small intestine in relation to emptying of an ingested meal is important in various pathophysiological conditions. We investigated the effects of different food consistencies and the amount of co-ingested liquid on the intragastric distribution of a contrast marker.
Five healthy subjects received four meals (each 650 kcal: A, mashed potato with 100 mL water; B, rice with 100 mL water; C, hamburger meal with 100 mL water; D, hamburger meal with 300 mL water). A capsule filled with gadolinium tetra-azacyclododecane tetra-acetic acid solution (as contrast marker) was ingested following meal termination, and its intragastric distribution was assessed by magnetic resonance imaging.
Initially, marker distribution was confined to the fundus, and subsequently extended along the inner curvature of the stomach. The maximum distribution volume of the marker was lower in meal A than in meal B (P < 0.05). No differences in marker distribution were observed when the hamburger meal was given with 100 or 300 mL water.
The intragastric distribution kinetics of the marker gadolinium tetra-azacyclododecane tetra-acetic acid appeared to depend on meal consistency, but not on the amount of water co-ingested. Three-dimensional magnetic resonance imaging allows detailed analysis of the intragastric distribution of a drug model in relation to meal emptying and intragastric meal distribution.
Knowledge of the fate of an orally administered drug in the gastrointestinal tract may be of crucial importance for the development of drugs. For example, orally administered enzyme preparations aiming to improve fat digestion, and consequently to control steatorrhoea in chronic exocrine pancreatic insufficiency, are often not distributed homogeneously within the gastric contents. Therefore, they do not empty simultaneously with the food into the small intestine, diminishing their efficacy to improve maldigestion.1–3
Among the various techniques used to study the release and distribution of orally administered drugs in vivo, scintigraphy has been most successful in visualizing the fate of pharmaceutical dosage forms in the gastrointestinal tract.4,5 However, when a detailed knowledge of the location of drug–food interactions or of the properties of complex release systems is sought, the two-dimensional nature of scintigraphy imposes limitations that can be overcome by the three-dimensional capabilities of magnetic resonance imaging (MRI). We recently used MRI to follow the intragastric release of a colloidal drug model containing gadolinium tetra-azacyclododecane tetra-acetic acid (Gd-DOTA) from a gelatine capsule in the food-filled human stomach.6 We observed that the distribution of the drug model in the stomach (i.e. within the meal) was dependent on the composition and consistency of the meal. The drug model appeared to distribute predominantly in the accessible liquid compartment of the meal. Our data also suggested that the degree of homogeneity of the drug model distribution in the stomach may influence the time course of drug release into the small intestine (i.e. gastric emptying).
The first aim of this study was to investigate the effects of test meals of different consistencies and containing different amounts of liquid ingested with the meal on the intragastric distribution of a contrast marker. This was achieved by administering a gelatine capsule containing Gd-DOTA solution, an MRI contrast agent, and visualizing its release from the capsule after ingestion of meals of different composition. The second aim was to demonstrate the three-dimensional capabilities of MRI in clarifying the distribution processes in the stomach. We hypothesized that the intragastric distribution of a marker will be related to the amount of accessible liquid contained in the meal, and that the consistency of the meal will affect the spatial distribution of the contrast marker in the stomach, resulting in large differences in the timing of its delivery to the small intestine.
Five healthy male subjects, aged between 22 and 30 years, participated in the study. The subjects were of normal weight for height (body mass index: 22.7 ± 0.9), had not taken any medication prior to or during the study and had no history of gastrointestinal disease. The study was carried out with the approval of the Ethics Committee at the University Hospital Zurich. All subjects gave their informed written consent prior to participation.
The intragastric distribution of the MRI contrast agent Gd-DOTA (Dotarem, Laboratoire Guerbet, Aulnay-sous-Bois, France), administered in a hard gelatine capsule (see below for details), was examined after the ingestion of four test meals differing with regard to their consistency and the amount of water co-ingested, but equal in energy and fat content (see Table 1 for the composition of test meals). The study was divided into two parts. In the first part, we used a homogeneous meal (A) and a particulate meal (B) to assess the influence of different meal consistencies on the intragastric distribution of the contrast marker. In the second part, we examined the effect of different amounts of water ingested with a hamburger meal on the distribution of the contrast marker (meals C and D). This meal was chosen because it more closely resembles a ‘normal’ mixed meal.
Table 1. Composition of the test meals
Magnetic resonance imaging
The MRI investigations were performed with a commercial 1.5 T MRI system (Gyroscan ACS-NT, Philips Medical Systems, Best, The Netherlands). After the subjects had been positioned in the MRI system, sets of 20 parallel images of the gastric region were acquired at regular time intervals (turbo spin echo technique, TR/TE=500/9 ms, 2562 matrix). The spatial resolution was 1.7 mm × 1.7 mm × 8 mm, and each image set was acquired in 20 s. In each image, the outline of the gastric content was determined in a semiautomatic procedure based on the anatomical structures visible in the images. This yielded all volume elements (voxels) of gastric content, which were then analysed on the basis of their signal intensity. The contrast agent Gd-DOTA led to a local increase in signal intensity by affecting the protons in its vicinity. To differentiate between gastric regions containing Gd-DOTA and those not containing the contrast agent (background), a threshold signal intensity was chosen based on the intensity distribution of the voxels in the gastric content in a scan taken prior to capsule administration (reference scan). Each voxel with an intensity above the highest intensity in the reference scan was then assumed to be a volume element containing Gd-DOTA. The image intensity was standardized between images and over the course of the study by referencing all images to the signal intensity of an external control.
Preparation of Gd-DOTA-containing capsules
An aqueous solution of the MRI contrast agent Gd-DOTA served as a contrast marker. Gd-DOTA solution (500 μL) (0.5 mM/mL) was administered to the subjects in a double capsule (to increase the time before release into the stomach), consisting of a transparent, closed and sealed hard gelatine capsule (size 1; nominal volume, 500 μL) inside a hydroxypropylmethylcellulose (HPMC) capsule (size 0; nominal volume, 680 μL). After sealing of the injection hole, the capsule was immediately given to the subjects positioned in the MRI system. Although the encapsulated Gd-DOTA solution does not truly resemble a drug, it allows, at least in principle, the investigation of the intragastric distribution of an aqueous preparation relative to the meal with which it was ingested.
The subjects were allowed a light breakfast before 08.00 hours, but no food or drink except water thereafter, and arrived at the MRI centre in the early afternoon (after 13.00 hours). Each subject received, on four different days and in randomized order, one of the four test meals described in detail in Table 1. The butter used in meals A and B was thoroughly mixed into the mashed potato or rice prior to ingestion and was added to match the fat content of the hamburger meals C and D. The water was consumed with the meal. Immediately after meal ingestion, subjects were positioned in the MRI system in a supine, 30° right tilted position, and a reference scan was performed to map the signal intensity of the gastric content. Then, the Gd-DOTA-filled capsule was administered to the subjects. Opening of the capsule and, subsequently, the dynamic distribution of the contrast marker were followed over 60 min. MRI scans were performed continuously until release of the marker from the capsule was first observed (time t=0 min), and thereafter every 5 min until t=20 min, and then at t=30 min, 45 min and 60 min.
The construction of the MRI system necessitates investigations with the subjects in a supine position. As this is not ideal for gastric emptying studies, as it potentially forces the fundus (proximal stomach) in a more dependent position relative to the antrum (distal stomach), we adjusted the position of the subjects by turning them into a (supported) 30° right tilted position. In this way, the redistribution of food from the proximal to the distal stomach was possible, as is the case during ‘normal’ gastric emptying.
Gastric meal emptying.
The gastric content was outlined in each image in all 20 slices to obtain the gastric volume at each time interval.7 Gastric emptying was expressed as the percentage of gastric content remaining at t=60 min.
Distribution of the contrast marker.
The contrast marker was differentiated from the background intensity in the stomach by using the highest intensity found in the reference scans as a threshold value for subsequent analysis. Marker distribution was calculated for the whole stomach and for fundic and antral subregions, and was expressed as absolute (mL), maximum and relative distribution volumes. The maximum distribution volume of the marker described the greatest volume occupied by the marker during the study relative to the total volume, expressed as a percentage. The relative distribution volume was defined as the number of volume elements above the threshold value (as defined above) divided by the volume of the respective gastric region, expressed as a percentage. For the calculation of the distribution volumes of the whole stomach, all voxels within the gastric content were analysed for their signal intensities. Gastric subregions were identified from the coronal projection of the stomach. The gastric incisure as an anatomical landmark was used to define the partition of the stomach into the fundus and antrum. For analysis, two anatomical axes were defined that were aligned with the long axis of the fundus or antrum, respectively. All subsequent calculations were performed based on these two axes. To analyse the distribution volumes of the two subregions, the fundus and antrum, the volume elements (voxels) in the gastric content were accumulated in partitions orthogonal to the defined axes and analysed for their signal intensities. Joining these two axes resulted in a residual volume at the non-continuous transition (Figure 1A). Due to our primary interest in the distribution processes in the proximal and distal stomach regions, the absolute, maximum and relative distribution volumes were analysed for the fundus and antrum only.
Visualization of distribution.
The stomach surface, meal volumes and distribution of the contrast marker were visualized using a software package for the development of interactive three-dimensional graphics applications (OpenGL Inventor, SGI, Mountain View, CA, USA).
Data are expressed as medians (interquartile ranges). The Wilcoxon rank test was used to test for statistical significance. For multiple testing, Friedman analysis of variance was used. P < 0.05 was regarded as significant.
Effect of different meal consistencies on contrast marker distribution
We used a homogeneous meal (meal A) and a particulate meal (meal B) to study the effect of different meal consistencies on the intragastric distribution of a contrast marker. The magnetic resonance images in Figure 1(B) show the intragastric distribution of the marker volume at 20 min after release from the capsule in meal B. The maximum distribution volume of the marker in the entire stomach was lower in meal A than in meal B (meal A, 23% (22–37%); meal B, 49% (45–65%); P=0.043). In the fundus, no differences were found between the two meals (meal A, 11% (3–15%); meal B, 34% (13–49%); P=0.138, N.S.). In the antrum, the maximum distribution volume of the marker was lower in meal A than in meal B (meal A, 51% (42–55%); meal B, 72% (72–83%); P=0.043). The distribution (relative volumes) of the marker over the course of the study is shown in Figure 2 for meals A and B. The characteristics of the spatial distribution of meals A and B are illustrated in the three-dimensional images at t=45 min after capsule opening (Figure 3) and in the distribution volume at two time points for meal B (Figure 4). This example illustrates that the distribution from the capsule commenced in the fundus and extended along the inner curvature of the stomach in both meals. However, in meal A, the marker did not significantly distribute in the antrum. This is further illustrated by Table 2, which gives an overview of the absolute distribution volumes of the marker and meal volumes in the total stomach and in the fundic and antral subregions. The data show that the meal volume in the fundus amounts to approximately twice the volume in the antrum, while the marker is distributed evenly between the antrum and the fundus.
Table 2. Absolute maximum distribution volume of marker (mL) and absolute meal volume (mL) for the entire gastric lumen, fundus and antrum
Effect of ingested liquid
The effect of the amount of water ingested with the meal on the intragastric marker distribution was analysed in the two hamburger meals with 100 or 300 mL water (meals C and D, respectively). No differences in the maximum distribution volume of the marker were observed between these two meals for any of the gastric regions (total stomach: meal C, 28% (25–31%); meal D, 34% (25–40%); P=0.500, N.S.; fundus: meal C, 20% (8–47%); meal D, 13% (4–28%); P=0.345, N.S.; antrum: meal C, 30% (22–66%); meal D, 59% (50–64%); P=0.686, N.S.). The relative distribution volumes of the marker between the two meals did not differ over the course of the study (Figure 5). The assessment of the absolute distribution of the marker and the meals in the different gastric regions (Table 2) indicated that a greater proportion of the meal (at the beginning of the study) was present in the fundus and only a small proportion (approximately a quarter of the meal) was in the antrum. On the other hand, after release from the capsule, the contrast marker distributed into a greater meal volume in the antrum than in the fundus.
Gastric emptying during the study was not significantly different between any of the meals (meal A, 63% (60–67%); meal B, 74% (73–91%); meal C, 75% (68–84%); meal D, 71% (66–81%); P=0.178, N.S.).
In this study, we have shown that meal consistency can strongly affect the intragastric distribution of a contrast marker after the ingestion of meals with similar gastric emptying rates. On the other hand, the amount of liquid ingested with the meal has only a small effect on intragastric marker distribution.
We observed significant differences in contrast marker distribution between meals A (homogeneous) and B (particulate). Generally, in the antrum, meal and contrast marker were mixed more homogeneously, while a large part of the meal in the fundus was not accessible to the marker. This may indicate that, under certain circumstances, a drug may be emptied from the stomach before the meal, even when the capsule is ingested after the food, or when given in a liquid form. The differences in intragastric marker distribution between the rice (particulate) and mashed potato (homogeneous) meals may perhaps be explained by the different meal consistencies. The particulate nature of the rice meal made the meal more easily accessible to the liquid, while the mashed potato meal, with its denser consistency, could only be infiltrated by liquid, and hence the marker, to a limited extent. This interpretation is also in line with our hypothesis that the intragastric distribution of a contrast marker is related to the accessible liquid volume.6 We tested this hypothesis further by administering to our subjects two meals that varied only in the amounts of water ingested. Accordingly, the contrast marker should have distributed in a larger volume in meal D, containing 200 mL more water than meal C. However, the distribution volumes did not differ between the two meals. One explanation may be the rapid gastric emptying rate of non-nutrient liquid,8 preventing a homogeneous distribution of the marker in the liquid phase over the course of the study. A nutrient-containing drink, as used in our previous study,6 may have provided different results, as it is emptied from the stomach more slowly and may therefore allow more time for the distribution process to occur.
For all meals, the contrast marker showed a preferential distribution from the fundus along the inner curvature of the stomach wall into the antrum. Consequently, a large proportion of the fundic content did not come into contact with the marker. According to our results, it is likely that the marker bypassed a large part of the meal, in particular in the fundus, and was emptied from the stomach before any significant mixing with the meal had taken place.
Our data show that MRI is a valuable technique to study the behaviour of oral dosage forms, as three-dimensional gastric images can be reconstructed and important parameters, such as gastric emptying and motility, can be monitored.6,9,10 It is important to note that our studies have focused on the assessment of the intragastric distribution of the marker rather than on small intestinal exposure to the marker. As our calculations of marker concentrations are based on measured intensities, very small concentrations, such as those emptied into the small intestine, may, at this stage, fall below the detection limit. Also, MRI allows the performance of repeated studies in healthy subjects, as it does not involve exposure to ionizing radiation. A variety of techniques have been used previously to study the fate of pharmaceutical dosage forms. The most commonly used method for in vivo imaging to date has been γ-scintigraphy.4 Dual-isotope imaging techniques allow the labelling of the drug and meal, and thus the assessment of drug distribution relative to the meal.3,11 This is an advantage over MRI, where direct labelling of the drugs with an MRI marker has so far not been approved for clinical studies. Significant limitations of γ-scintigraphy include the restriction to two dimensions and the exposure to ionizing radiation. As alternative methods, gastrointestinal magnetomarkergraphy and a superconducting quantum interference device (SQUID) have been proposed to study transit times through the gastrointestinal tract.12,13 However, studies utilizing these methods only focus on the location of solid, non-disintegrating capsules and do not provide information on the release of a dosage form.
Our data indicate that meals with a similar energy content empty from the stomach at similar rates regardless of their composition and consistency. Meals with a more particulate consistency (rice, hamburger meal) probably mixed reasonably well with gastric secretions and ingested liquid, were ground and then emptied. The homogeneous meal (mashed potato) did not require trituration, but emptied from the stomach at a similar rate. This may be due to the denser consistency of this meal, resulting in a longer period of time for gastric secretions to penetrate and liquefy this meal. The results are in agreement with earlier studies showing that meal consistency only has a modest influence on gastric emptying.14,15
In summary, our data show that meal consistency has a significant impact on the intragastric distribution of a contrast marker, even at similar gastric emptying rates of the meals. Furthermore, our data indicate that non-nutrient fluid ingested with the meal does not appear to increase the intragastric distribution of the marker. We also demonstrated that MRI can simultaneously visualize the distribution kinetics of the marker in relation to gastric anatomy in three dimensions and at high resolution. This technique will therefore be of great value for optimizing the behaviour of new dosage forms and for the analysis and clarification of basic distribution mechanisms in the stomach.
This study was supported by the Swiss National Science Foundation (SNF grants: 32-54056.98 and 31-55932.98) and the Kamillo-Eisner-Foundation, Hergiswil, Switzerland.