In 1981, a capsule containing a balloon filled with drug was reported, which could be actuated in the gastrointestinal tract when required, by the application of a radio signal89. This technique has been used to study absorption at various sites in the gastrointestinal tract. To locate the capsule, it is swallowed with a small dose of barium sulphate to aid its localisation within the gut and is triggered when required. The absorption of frusemide was compared in 5 subjects using the device90. The drug was released in the ileo-caecal area in 3 subjects and in the ascending colon in the other two. Maximum plasma concentrations were lower after the release of the drug in the colon, and there was a forty-fold difference between absorption from the stomach and colon with bioavailabilities of 20% and 3% respectively. The capsule was also used to study the absorption of theophylline from the stomach, ileum and colon91. The mean relative bioavailability of theophylline was 86% after releasing the drug in the colon. Thus there was no evidence for a so called absorption window for theophylline which has previously been reported in the literature, suggesting that such a "window" might have been related to the pharmaceutical formulations used. The device provides a means of investigating drug absorption under normal physiological conditions but has limitations of single occasion use and the need for repeated x-rays92. Although in many studies there have been good correlations between the gamma scintigraphic data and the plasma concentration profile, there have been examples in the literature where the results have been completely inexplicable. For example, in a study of the transit and disintegration of acetylsalicylic acid from 51Cr-labelled enteric-coated tablets, in four volunteers out of twelve, the absorption of acetylsalicylic acid was delayed more than 10 hours in spite of the fact that complete disintegration and gastric emptying of the tablet seemed to have occurred93. In three volunteers, this occurred in the in postprandial state and in one subject it occurred under fasting conditions. In the remaining eight of the twelve subjects, the time of onset of absorption correlated well with the time of disintegration.
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