Background. A long-acting analgesic may be particularly desirable in patients suffering from long-lasting pain. The aim of the study was to evaluate the antinociceptive effect of a novel nalbuphine preparation and to determine its duration of action. Methods. The antinociceptive effects of i.m. nalbuphine HCl in saline and nalbuphine base in sesame oil were evaluated in rats. The in vitro drug-releasing profiles of nalbuphine HCl and base in different preparations were also evaluated. Results. We found that i.m. nalbuphine HCl 25, 50 and 100 μmol kg-1 produced dose-related antinociceptive effects with a duration of action of 1.5, 2 and 3 h, respectively. I.M. nalbuphine base 100, 200 and 400 μmol kg-1 also produced dose-related antinociceptive effects but with longer durations of action: 27, 49 and 55 h, respectively. In vitro studies demonstrated that nalbuphine base in sesame oil had the slowest drug-releasing profile of the different preparations. Conclusions. I.M. injection of an oil formulation of nalbuphine base produced a long-lasting antinociceptive effect.
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