In vivo characterization of the opioid antagonist nalmefene in mice

Abstract

Aims: The current study assessed the in vivo antagonist properties of nalmefene using procedures previously used to characterize the opioid antagonists naloxone, naltrexone, 6beta-naltrexol and nalbuphine.

Main methods: ICR mice were used to generate antagonist dose-response curves with intraperitoneal (i.p.) nalmefene against fixed A(90) doses of morphine in models of morphine-stimulated hyperlocomotion and antinociception. Additional dose-response curves for antagonist precipitated opioid withdrawal were run in mice treated acutely (100mg/kg, s.c., -4h) or chronically (75mg pellet, s.c., -72h) with morphine. Comparisons were made between antagonist potency and degree of precipitated withdrawal.

Key findings: Nalmefene produced dose- and time-related antagonism of morphine-induced increases in locomotor activity with a calculated ID(50) (and 95% confidence interval) of 0.014 (0.007-0.027)mg/kg. Nalmefene produced rapid reversal of morphine-induced locomotor activity (5.1min for 50% reduction in morphine effect). A 0.32mg/kg dose of nalmefene produced blockade of morphine-induced antinociception in the 55 degrees C tail-flick test that lasted approximately 2h. Nalmefene was able to potently precipitate withdrawal in mice treated acutely or chronically with morphine.

Significance: These results demonstrate that nalmefene is similar to naloxone and naltrexone with respect to its in vivo pharmacology in mice. Specifically, nalmefene produces potent antagonism of morphine agonist effects while precipitating severe withdrawal. The compound has a slower onset and longer duration of action compared to naloxone and naltrexone. The data allows for a more complete preclinical comparison of nalmefene against other opioid antagonists including the putative opioid neutral antagonist 6beta-naltrexol.

Figure 1

Chemical structures of naltrexone, naloxone and nalmefene.

Figure 2

Antagonist effects of nalmefene. Nalmefene produced dose-related blockade of morphine-induced locomotor activity as indexed by distance traveled (panel A) and an area under the curve analysis (panel B). A Tukey’s post-hoc analysis indicated significant differences between the morphine control and higher doses of nalmefene in combination with the morphine dose, *** p < 0.001. A significant difference was also seen between the vehicle and morphine control groups, ^††† p < 0.001.

Figure 3

Summary graphs of antagonist effects in the locomotor assay. Nalmefene was approximately equipotent to naloxone in blocking the locomotor stimulating effects of a fixed dose of morphine (panel A, 95% confidence intervals overlap). When given at supramaximal doses (1 mg/kg, i.p.), neither antagonist produced a significant effect on locomotor activity compared to vehicle treated mice (panel B, p > 0.05).

Figure 4

Nalmefene antagonism of morphine effects. Panel A depicts the reversal of established morphine-induced hyperlocomotion (32 mg/kg, s.c.) by the administration of nalmefene (0.32 mg/kg, i.p.). Panel B represents a time-course of nalmefene-pretreatment blockade (0.32 mg/kg, i.p.) of morphine-induced antinociception (32 mg/kg, s.c.) in the 55°C tail-flick assay. Naloxone is included as a comparator (1 mg/kg, i.p.).

Figure 5

Precipitation of opioid withdrawal in an acute dependence assay. ANOVA analysis is presented in the results section. Tukey’s post-hoc analyses indicated significant differences between the saline control group and antagonist treatment groups, * p < 0.05, ** p < 0.01.

Figure 6

Precipitation of opioid withdrawal in a chronic dependence assay. ANOVA analysis is presented in the results section. Tukey’s post-hoc analyses indicated significant differences between the saline control group and antagonist treatment groups, * p < 0.05, ** p < 0.01, *** p < 0.001.