5. Naltrexone - preclinical data

Editors: Spanagel, Rainer; Mann, Karl F.

Title: Drugs for Relapse Prevention of Alcoholism, 1st Edition

Copyright 2005 Springer

> Table of Contents > Naltrexone: preclinical data

Naltrexone: preclinical data

Michael S. Cowen

Howard Florey Institute, University of Melbourne, VIC 3010, Australia

Introduction

Naltrexone (N-cylopropylmethylnoroxymorphone) is a potent opiate antagonist with only very weak agonist properties [1]. The drug is well tolerated [2] and orally available [1]. Naltrexone has been shown to precipitate withdrawal from morphine in rats [3] and monkeys [4], and to reverse morphine-induced antinociception against noxious heat and pressure stimuli in rats [5]. Naltrexone was also shown to reverse the antinociception induced by the selective -opioid receptor agonist U-69, 593 with slightly higher doses than for morphine-induced antinociception [5], reflecting the somewhat higher affinity of naltrexone for -opioid receptors compared with - and -opioid receptors [6, 7]. Prior to the publication of the first clinical studies examining the effect of naltrexone in alcohol dependence [8, 9], most pre-clinical studies examining the interaction of ethanol with opioids used the shorter-acting, pure opioid antagonist naloxone [10, 11] rather than naltrexone [12, 13, 14 and 15]. However, since that time a large body of data has been gathered examining the effect of naltrexone on ethanol consumption in various pre-clinical models.

Dosage

In general, naltrexone has been shown to cause a dose-dependent decrease in ethanol consumption and preference in a range of animal models and species, including mice, rats and monkeys. The range of effective doses appears broadly similar across species (although see below), but the effectiveness of naltrexone in decreasing ethanol consumption varies depending on the experimental model used. Thus, naltrexone has been shown to be effective at doses below 1 mg per kg body weight (0.1-0.6 mg/kg) when the period of access to ethanol is short (e.g., 1 h) and naltrexone is injected shortly prior to the access period, such as in limited access schedules [16, 17, 18, 19 and 20] or in operant self-administration protocols [21, 22, 23, 24, 25, 26, 27, 28, 29, 30 and 31]. When there is a delay between naltrexone injection and access to ethanol, the doses of naltrexone required to cause a significant reduction in ethanol consumption are much higher [22].

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As the period of access to ethanol increases, the dose of naltrexone required to cause a significant suppression of ethanol consumption also increases. When ethanol is available under a 24-h continual access regime [13, 15, 18, 32] or a 23-h operant self-administration session [33], a dose of 10 mg/kg has caused a significant reduction in ethanol consumption in some [13, 15, 18, 32, 33] but not all [33, 34] studies. Although in most of these studies dose-dependency was not established (i.e., the minimum effective dose was not determined), several other studies using doses up to 5 mg/kg [35, 36, 37 and 38] have shown no significant effect of naltrexone alone under continual access conditions. There are, however, two studies of continual access or near-continual access to ethanol consumption, in which a relatively low dose of naltrexone produced a significant reduction in ethanol consumption. Thus, in the study by H lter and Spanagel [33], whereas naltrexone up to a dose of 10 mg/kg had no significant effect on operant ethanol self-administration by rats under basal conditions (23-h sessions), following a period of enforced abstinence from ethanol (1 week), a dose of 0.1 mg/kg naltrexone caused a significant reduction in operant ethanol self-administration over the following 23 h period. The data may be explained by the differential motivation to consume ethanol in the two paradigms, deprivation-induced ethanol consumption presumably having a significant opioidergic component. Although this suggestion would appear to be borne out by the studies of Kornet et al. [39, 40], in which a low dose of 0.17 mg/kg naltrexone caused a significant reduction in ethanol consumption by rhesus monkeys in the 17 h following a period of enforced abstinence (48 h), basal ethanol consumption by these monkeys was also suppressed by a low dose of 0.5 mg/kg naltrexone. The discrepancy between the monkey studies [39, 40] and the studies in rats and mice outlined above may therefore also indicate a species difference in the effect of naltrexone on ethanol consumption; unfortunately, the effect of naltrexone on ethanol self-administration by monkeys over such long time periods does not appear to have been examined by any other group.

Williams et al. [31] have pointed out that the doses of naltrexone needed to decrease operant ethanol self-administration by rhesus monkeys are higher than those needed for reversal of morphine-induced antinociception [5], presumably a -opioid receptor mediated effect [41]. This may in part reflect the different time frames of these experiments (minutes versus hours) and the nature of the drug examined (morphine interacting directly with opioid receptors, ethanol apparently modulating [42, 43] or acting indirectly [44, 45] on opioid receptors). In the only study of selective opioid antagonists in primates [46], whereas - and -opioid receptor selective antagonists had no effect on ethanol self-administration, the -opioid receptor antagonist nor-binaltorphimine was shown to decrease ethanol self-administration. Since nor-binaltorphimine was effective only on the day of injection (whereas its antagonism of -opioid receptors was expected to continue over days), the authors [46] were led to suggest that the effect of naltrexone was not mediated by opioid receptors. However, a combination of selective antagonists was not tested, i.e., the involvement of multiple opioid receptor subtypes cannot be ruled out.

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Continuous and repeated dosing

In the studies of longer ethanol access periods, the effects of naltrexone have been observed to be most pronounced in the initial 1-4 h post-injection [33, 36, 39]. Since the effect of naltrexone appears to diminish relatively rapidly post-injection, continuous delivery systems have been tested, using either a subcutaneously planted naltrexone pellet [47, 48 and 49] or osmotic minipump [33, 37, 50]. However, particularly poor outcomes are obtained using such systems: frequently no effect [33, 37, 47] or an increase in ethanol consumption [49] is observed. Any significant decrease normally occurs only immediately (1-2 days) post-surgery [48, 50]. Cowen et al. [50] have argued that the lack of efficacy of such delivery systems may be due to the marked and rapid upregulation of opioid receptors induced by naltrexone over the experimental time period; however, the exact mechanisms remain unclear.

In a few cases using repeated daily injections of naltrexone and discrete ethanol access periods, the initial dose of naltrexone is ineffective and the effect of naltrexone on ethanol consumption appears only after repeated dosing [12, 51], or the effect of naltrexone on ethanol consumption increases over time [52], suggesting an extinction by naltrexone of ethanol-reinforced behavior [53]. In contrast, there are several reports of tolerance, or decreasing efficacy over time, with repeated daily injections of naltrexone [37, 49, 54, 55 and 56], particularly when the initial dose was effective (1-6 mg/kg in a range of experimental paradigms). Since other studies using similar [52, 57] or higher (10 mg/kg [58, 59]) doses of naltrexone report no development of tolerance over equivalent time periods, the interaction between the dose of naltrexone and the development of tolerance is complex and involves other factors. Undoubtedly, however, as for continuous delivery systems, tolerance can develop to repeated daily injections of naltrexone and this has clear clinical implications.

Acquisition and reinstatement of ethanol seeking behavior

Naltrexone has been shown to significantly retard the acquisition of ethanol consumption [49, 57, 60]; however, in these studies ethanol consumption did increase over time, probably reflecting the difficulty in maintaining opioid receptor blockade with repeated naltrexone dosing and the development of tolerance. Ethanol consumption following a period of enforced deprivation from ethanol - the so-called alcohol deprivation effect (ADE) - is also sensitive to naltrexone, as noted previously [33, 39, 54], indicating a significant opioidergic component in this motivational state. Further, the reinstatement of ethanol-seeking behavior by cues associated with the availability of ethanol has been shown to be sensitive to naltrexone [51, 61, 62, 63 and 64]. The doses used in inhibiting cue-mediated ethanol-seeking behavior are similar to those used to inhibit operant ethanol self-administration (the protocol under which cue-mediated

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ethanol-seeking behavior is learnt); however, stress-induced reinstatement of ethanol-seeking behavior is not affected by naltrexone at the same doses [62, 64], suggesting an important role for opioidergic signalling in cue- but not stress-induced reinstatement. One study has reported that the post-shock increase in ethanol consumption can be overcome by naltrexone [15]; however, in this study, unlike the reinstatement studies, the animals were able to sample ethanol; therefore, the effect of naltrexone may have been a post-ingestive effect.

Specificity of action

The original toxicology study for naltrexone indicated that chronic naltrexone, within the doses used to decrease ethanol consumption, was well tolerated in rats and monkeys with no difference in body weight relative to control animals in a timescale of weeks to months [2]. However, over a shorter timescale naltrexone has been shown to cause a reduction in food and fluid intake at doses that are germane to the effects on ethanol consumption [65, 66, 67, 68, 69, 70 and 71], although interestingly, the reductions tended to be in situations of high motivation, such as in fluid intake subsequent to deprivation [65] or injection of hypertonic saline [66], or food consumption subsequent to deprivation [69] or in response to a highly palatable diet (but not a control diet; [67]). Whereas some selectivity of naltrexone for ethanol consumption compared with water consumption has been demonstrated on occasion [25, 72]; several reports have indicated that when water or a sweetened solution are available concurrently with ethanol, naltrexone has caused a significant decrease in both ethanol and water/sweetened solution consumption [22, 30, 57, 58], although in most of these studies [30, 57, 58] water/sweetened solution consumption recovered over days. In an interesting study by Williams and Woods [27], the concentration of ethanol was varied to alter the relative preference by monkeys for ethanol versus water. When ethanol was the preferred solution, ethanol consumption was significantly reduced by naltrexone; when water was the preferred solution, water consumption was significantly reduced by naltrexone. The data clearly suggest that under these conditions, naltrexone is most effective against the most reinforcing or salient solution.

Naltrexone has been shown to decrease ethanol self-administration under operant conditions with no effect on cocaine self-administration [73]. However, in a recent set of studies, whereas naltrexone was ineffective in decreasing food and phencyclidine self-administration when these were available only during operant sessions (while having a significant impact on ethanol and saccharin self-administration) [26], naltrexone was able to decrease food and phencyclidine responding when these were available after the self-administration session [28], an open economy . In other words, the post-session availability weakened the apparent reinforcing value of these substances, thus allowing naltrexone to have greater effectiveness as an intervention [28].

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This result has some resonance with other, previous studies; thus naltrexone was shown to facilitate extinguishment of ethanol-seeking behavior (no ethanol availability following responding, therefore a weakened stimulus; [51]). In contrast, the data would appear to be difficult to reconcile with the relatively greater effect of naltrexone on food, water and ethanol consumption under high motivational states [33, 67, 69]. Clearly, several levels of need or want are involved in these consummatory behaviors, not all of which can be clearly delineated at this point in time but some of which appear to be sensitive to naltrexone. In total, the data indicate that naltrexone may diminish several motivated consummatory behaviors, not solely ethanol consumption, and the relative impairment may depend on the motivational state and the extent to which opioidergic signalling plays a role in mediating that specific behavior within the brain.

Combination pharmacotherapy

In order to enhance the effect of naltrexone on ethanol consumption, several attempts have been made to use naltrexone in combination with some other drug to facilitate a decrease in ethanol consumption. Thus naltrexone has been used in combination with the 5-HT₃ receptor antagonists ondansetron [16] and ICS 205-930 [74], the 5-HT_2A receptor antagonist amperozide [72], the calcium channel inhibitor isradipine [54, 57], the selective serotonin reuptake inhibitor fluoxetine [55], with fluoxetine and the thyrotropin-releasing hormone analogue TA-0910 [38] and acamprosate [19]. In some of these studies there was no additional benefit conferred by using naltrexone in combination with another drug, possibly because ethanol consumption was already significantly reduced by naltrexone - a floor effect [19, 55, 57]. In contrast, in the studies by Le and Sellers [16] and Rezvani et al. [38], the combination pharmacotherapy produced a significant decrease in ethanol consumption whereas individually the drugs had no significant impact. Such combinations may have an advantage in overcoming the non-specific effects of naltrexone on food and water consumption, and minimizing the development of tolerance, although the timeframes under which the combinations were examined were usually quite short.

Other interactions of naltrexone and ethanol

Naltrexone has been shown to cause a decrease in ethanol-induced place preference [37], indicating an interaction with the reinforcing properties of ethanol. Interestingly, naltrexone has also been shown to counteract ethanol-induced locomotor activity [75] and the hypnotic effects of ethanol in one strain of mouse (the BALB/c mouse strain) but not in two others, the C57BL/6 and DBA/2 mouse strains [75]. However, the interactions between the behavioral

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effects of ethanol and naltrexone on the one hand, and naltrexone's effect on ethanol consumption on the other, remain uncertain. Although naltrexone has been shown to cause an increase in aversive responses to the taste of ethanol [20, 76, 77], whether this plays a role in naltrexone's effect on ethanol consumption is still unclear, as naltrexone has been shown to significantly decrease intravenous self-administration of ethanol [12, 46]. Further, Linseman [78] demonstrated that the peripherally-acting opioid antagonist methylnaltrexone had no effect on ethanol consumption by rats in a limited access paradigm, whereas naltrexone at the same doses caused a significant decrease in ethanol consumption (although not precluding the possibility that naltrexone alters the palatability of ethanol via a central nervous system mechanism). An obvious test would be to examine the effect of methylnaltrexone on taste responsiveness to ethanol. Naltrexone has also been shown to cause a small decrease in the absorption of ethanol [79].

Conclusions

Naltrexone has been shown to decrease ethanol consumption in a range of animal models. Naltrexone appears to be particularly effective in animal models of relapse such as the alcohol deprivation effect or cue-induced reinstatement of ethanol-seeking behavior. The effects of naltrexone are centrally mediated and involve decreasing the rewarding value of ethanol (subsequent to consumption), disrupting the neural signalling involved in mediating the connection between cues and ethanol-seeking behavior and increasing the perceived aversiveness of ethanol's flavor. The pre-clinical data indicating the development of tolerance and lack of specificity for ethanol consumption (i.e., effects on food and water intake) may indicate that naltrexone is best used in a lower dose range in combination with other drugs; however, further work is needed in this regard.

Acknowledgements

I would like to acknowledge the support of the National Health & Medical Research Council (NHMRC), Australia and E. Krstew for her critical comments.

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