The antinociceptive effect of a long-acting nalbuphine preparation in rabbits.

Background: An analgesic with a long-lasting effect is particularly desirable to patients suffering from long-standing pain. The aim of this study was to evaluate the antinociceptive effect and duration of action of an oily dosage form of nalbuphine base and to see whether its effect could last longer compared with nalbuphine HCl.

Methods: Male New Zealand White rabbits (n = 6 in each group) were used. Two studies were carried out. In study 1, we evaluated the antinociceptive effect of intramuscular nalbuphine HCl (in saline) with dosages of 25, 50, and 100 mumol/kg. In study 2, we evaluated the antinociceptive effects of intramuscular nalbuphine base (in sesame oil) with dosages of 100, 200, and 400 mumol/kg. In these studies, the vehicles (saline or sesame oil) were used as controls. A paw pressure test was used to detect the antinociceptive effect of the testing drugs.

Results: We found that nalbuphine HCl 25, 50 and 100 mumol/kg produced a dose-related antinociceptive effect with durations of action of 1, 1, and 1.3 h, respectively. Nalbuphine base 100, 200 and 400 mumol/kg also produced a dose-related antinociceptive effect but with longer durations of action of 8, 24, and 48 h, respectively. On an equimolar basis (100 mumol/kg), nalbuphine base produced a 6-fold increase in the duration of action than did nalbuphine HCl.

Conclusions: We conclude that intramuscular nalbuphine base produced a relatively longer duration of action than did intramuscular nalbuphine HCl.