Antinociceptive/anti-edema effects of liposomal morphine during acute inflammation of the rat paw.

We evaluated the anti-edema/antinociceptive effects of subcutaneous free and liposomal morphine in rats with carrageenan-induced inflammation of the paw. We assessed antinociception by the paw pressure test and edema by plethysmography. Unilamellar liposomes (150-200 nm) with 0.3% morphine hydrochloride were used; encapsulation significantly reduced the rate for release of morphine in vitro. During inflammation, the antinociceptive potency of free, but not liposomal morphine increased 2.5 times; moreover, duration of the effects was prolonged by encapsulation (p < 0.001). The anti-edema effects of liposomal morphine were more pronounced (p < 0. 001) and of longer duration (p < 0.05). All the effects were reversed by naloxone. The results show that morphine encapsulation enhances the anti-edema effects and prolongs antinociception.