Evaluation of oxprenolol and metoprolol Oros systems in the dog: comparison of in vivo and in vitro drug release, and of drug absorption from duodenal and colonic infusion sites.

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Abstract
The performance of oxprenolol and metoprolol Oros systems has been evaluated in the dog. One study compared in vivo and in vitro release from both systems over 2‐14 h. The other compared the systemic availabilities of both drugs after 3 h infusion at a constant rate into the cephalic and hepatic portal veins, and into the lumen of the duodenum and colon. In the in vivo release studies, Oros systems were recovered throughout the gut from the stomach to the colon. The amounts of drug remaining in the systems corresponded closely to those measured in a parallel in vitro release experiment. In vitro testing is thus a reliable indicator of in vivo system performance. In the absorption studies, both metoprolol and oxprenolol were shown to be subject to substantial first‐pass metabolism. Additionally, for metoprolol the data indicated a significant loss during transport from the gut lumen into the portal circulation. For both drugs the availability from the colon was equal to that from the duodenum. These results provide some justification for the development of oral dosage forms with extended durations of release even for drugs which undergo significant first‐ pass metabolism.