Relationship between the rate of appearance of oxprenolol in the systemic circulation and the location of an oxprenolol Oros 16/260 drug delivery system within the gastrointestinal tract as determined by scintigraphy.

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Abstract
1. The position in the gastrointestinal tract of an orally administered oxprenolol Oros drug delivery system labelled with technetium‐99m DTPA was followed by gamma scintigraphy, and the corresponding plasma drug concentration‐time profiles after oral and i.v. administration were used to relate pharmacokinetic and transit data. 2. Gastric emptying time (0.8 +/‐ 0.4 h, mean +/‐ s.d.), and the time to arrival in the colon (3.8 +/‐ 0.7 h) were reasonably consistent after administration of the Oros system to fasted subjects, as were the calculated small intestine transit times (3.0 +/‐ 0.7 h). As expected there were wide individual variations in colonic transit, so that recorded values for total transit ranged from 6 to 32 h (median, 24.7 h). 3. Absorption of oxprenolol occurred throughout the GI tract including the colon. Plasma drug concentration‐time profiles and input functions (calculated by deconvolution) could be related to transit behaviour and in vitro release. Inflexions in the calculated rate of drug input when the Oros system was located in the colon corresponded with periods of stagnation at the hepatic and splenic flexures in two subjects and the ileocaecal junction in two others. The mechanism of these changes is unclear.