Opiate antagonists and the modification of heroin self-administration behavior in man: an experimental study

Opioid receptor antagonists reduce motivated wheel-running behavior in mice

We hypothesized that opioid receptor antagonists would inhibit motivated behavior produced by a natural reward. To evaluate motivated responses to a natural reward, mice were given access to running wheels for 71.5 h in a multi-configuration testing apparatus. In addition to a running wheel activity, locomotor activity (outside of the wheel), food and water intake, and access to a food container were measured in the apparatus. Mice were also tested separately for novel-object exploration to investigate whether naloxone affects behavior unrelated to natural reward. In untreated mice wheel running increased from day 1 to day 3. The selective µ-opioid receptor antagonist β-funaltrexamine (β-FNA) (5 mg/kg) slightly decreased wheel running, but did not affect the increase in wheel running from day 1 to day 3. The non-selective opioid receptor antagonist naloxone produced a greater reduction in wheel running than β-FNA and eliminated the increase in wheel running that occurred over time in the other groups. Analysis of food access, locomotor behavior, and behavior in the novel-object test suggested that the reduction in wheel running was selective for this highly reinforcing behavior. These results indicate that opioid receptor antagonism reduces responses to the natural rewarding effects of wheel running and that these effects involve multiple opioid receptors since the non-selective opioid receptor antagonist had greater effects than the selective µ-opioid receptor antagonist. It is possible that at the doses employed, other receptor systems than opioid receptors might be involved, at least in part, in the effect of naloxone and β-FNA.

Investigation of the reinforcing potential of samidorphan and naltrexone by fixed and progressive ratio intravenous self-administration testing in heroin-maintained rats

BACKGROUND

Samidorphan is a novel μ-opioid antagonist with low intrinsic activity at κ- and δ-opioid receptors.

AIMS

Because samidorphan is central nervous system-active, we investigated whether samidorphan (13.6, 40.8, 68 μg/kg/injection) served as a positive reinforcer in rats trained to self-administer heroin on a fixed ratio-5 schedule. Samidorphan's relative reinforcing effect was evaluated by progressive ratio/break-point determination. Naltrexone (13.6, 40.8, 68 μg/kg/injection) and heroin (7.5, 15, 25 μg/kg/injection) were comparators.

RESULTS

All heroin doses maintained self-administration on fixed ratio-5 and progressive ratio/break-points at levels significantly greater than saline. Samidorphan and naltrexone had similar profiles on fixed ratio-5 with one samidorphan dose serving as a positive reinforcer and one naltrexone dose showing a strong trend ( p=0.053) for positive reinforcement. The numbers of injections of every samidorphan and naltrexone dose were significantly lower than all heroin doses. The numbers of self-administered samidorphan and naltrexone injections/session on fixed ratio-5 were not significantly different from one another. The mean inter-injection intervals for heroin were significantly shorter than for saline, whereas those of samidorphan and naltrexone were not. Progressive ratio break-points for samidorphan and naltrexone were not different from saline except for the highest dose of samidorphan. In addition, the progressive ratio break-points for samidorphan were not significantly different from those of naltrexone and were significantly lower than heroin. The samidorphan unit-doses evaluated in self-administration yielded plasma concentrations ranging between 25-109% and 10-45% of the maximum concentration values in humans.

CONCLUSIONS

Overall, the profiles of samidorphan and naltrexone, which has no abuse liability, were similar in this model.

Intranasal oxycodone self-administration in non-dependent opioid abusers

Oxycodone, an opioid with known abuse liability, is misused by the intranasal route. Our objective was to develop a model of intranasal oxycodone self-administration useful for assessing the relative reinforcing effects of opioids and potential pharmacotherapies for opioid use disorders. Healthy, sporadic intranasal opioid abusers (n = 8; 7 M, 1 F) completed this inpatient 2.5-week, randomized, double-blind, placebo-controlled, crossover study. Each intranasal oxycodone dose (0, 14 & 28 mg) was tested in a separate 3-day block of sessions. The first day of each block was a sample session in which the test dose was given. Two randomized progressive ratio sessions were conducted on the next 2 days: (1) subjects could work for the test dose over 7 trials (1/7th of total dose/trial), and (2) subjects could work for either a portion of the dose (1/7th) or money ($3) over 7 trials. Physiological and subjective measures were collected before and after drug administration for all sessions. Subjects never worked to self-administer placebo regardless of whether money was available. In both self-administration sessions, oxycodone self-administration was dose-dependent. Subjects worked less for drug (28 mg oxycodone) when money was available but only modestly so. Oxycodone dose-dependently increased VAS ratings of positive drug effects (e.g., "like") during sample sessions (p < .05). These reports were positively correlated with self-administration behavior (e.g., "like," r = .65). These data suggest that both procedures are sensitive for detecting the reinforcing properties of intranasal oxycodone and may be used to further explore the characteristics of opioid compounds and potential pharmacotherapies for treatment.

Oral naltrexone maintenance treatment for opioid dependence

BACKGROUND

Research on clinical application of oral naltrexone agrees on several things. From a pharmacological perspective, naltrexone works. From an applied perspective, the medication compliance and the retention rates are poor.

OBJECTIVES

To evaluate the effects of naltrexone maintenance treatment versus placebo or other treatments in preventing relapse in opioid addicts after detoxification.

SEARCH STRATEGY

We searched: Cochrane Central Register of Controlled Trials (CENTRAL - The Cochrane Library issue 6 2010), PubMed (1973- June 2010), CINAHL (1982- June 2010). We inspected reference lists of relevant articles and contacted pharmaceutical producers of naltrexone, authors and other Cochrane review groups.

SELECTION CRITERIA

All randomised controlled clinical trials which focus on the use of naltrexone maintenance treatment versus placebo, or other treatments to reach sustained abstinence from opiate drugs

DATA COLLECTION AND ANALYSIS

Three reviewers independently assessed studies for inclusion and extracted data. One reviewer carried out the qualitative assessments of the methodology of eligible studies using validated checklists.

MAIN RESULTS

Thirteen studies, 1158 participants, met the criteria for inclusion in this review.Comparing naltrexone versus placebo or no pharmacological treatments, no statistically significant difference were noted for all the primary outcomes considered. The only outcome statistically significant in favour of naltrexone is re incarceration, RR 0.47 (95%CI 0.26-0.84), but results come only from two studies. Considering only studies were patients were forced to adherence a statistical significant difference in favour of naltrexone was found for retention and abstinence, RR 2.93 (95%CI 1.66-5.18).Comparing naltrexone versus psychotherapy, in the two considered outcomes, no statistically significant difference was found in the single study considered.Naltrexone was not superior to benzodiazepines and to buprenorphine for retention and abstinence and side effects. Results come from single studies.

AUTHORS' CONCLUSIONS

The findings of this review suggest that oral naltrexone did not perform better than treatment with placebo or no pharmacological agent with respect to the number of participants re-incarcerated during the study period. If oral naltrexone is compared with other pharmacological treatments such as benzodiazepine and buprenorphine, no statistically significant difference was found. The percentage of people retained in treatment in the included studies is however low (28%). The conclusion of this review is that the studies conducted have not allowed an adequate evaluation of oral naltrexone treatment in the field of opioid dependence. Consequently, maintenance therapy with naltrexone cannot yet be considered a treatment which has been scientifically proved to be superior to other kinds of treatment.

Oral naltrexone maintenance treatment for opioid dependence

BACKGROUND

Research on the clinical application of oral naltrexone agrees on several things. From a pharmacological perspective, naltrexone works. From an applied perspective, however, the medication compliance and the retention rates are very poor.

OBJECTIVES

To evaluate the effects of naltrexone maintenance treatment versus placebo or other treatments in preventing relapse in opioid addicts after detoxification.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (CENTRAL - The Cochrane Library issue 6 2010), PubMed (1973- June 2010), CINAHL (1982- June 2010). We inspected reference lists of relevant articles and we contacted pharmaceutical producers of naltrexone, authors and other Cochrane review groups.

SELECTION CRITERIA

All randomised and controlled clinical trials which focus on the use of naltrexone maintenance treatment versus placebo, or other treatments to reach sustained abstinence from opiate drugs

DATA COLLECTION AND ANALYSIS

Three reviewers independently assessed studies for inclusion and extracted data. One reviewer carried out the qualitative assessments of the methodology of eligible studies using validated checklists.

MAIN RESULTS

Thirteen studies, 1158 participants, met the criteria for inclusion in this review.Comparing naltrexone versus placebo or no pharmacological treatments, no statistically significant difference were noted for all the primary outcomes considered. The only outcome statistically significant in favour of naltrexone is re incarceration, RR 0.47 (95%CI 0.26-0.84), but results come only from two studies.Comparing naltrexone versus psychotherapy, in the two considered outcomes, no statistically significant difference was found in the single study considered.Naltrexone was not superior to benzodiazepines and to buprenorphine for retention and abstinence and side effects. Results come from single studies.

AUTHORS' CONCLUSIONS

The findings of this review suggest that oral naltrexone did not perform better than treatment with placebo or no pharmacological agent with respect to the number of participants re-incarcerated during the study period. If oral naltrexone is compared with other pharmacological treatments such as benzodiazepine and buprenorphine, no statistically significant difference was found. The percentage of people retained in treatment in the included studies is however low (28%). The conclusion of this review is that the studies conducted have not allowed an adequate evaluation of oral naltrexone treatment in the field of opioid dependence. Consequently, maintenance therapy with naltrexone cannot yet be considered a treatment which has been scientifically proved to be superior to other kinds of treatment.

Human behavioral pharmacology, past, present, and future: symposium presented at the 50th annual meeting of the Behavioral Pharmacology Society

A symposium held at the 50th annual meeting of the Behavioral Pharmacology Society in May 2007 reviewed progress in the human behavioral pharmacology of drug abuse. Studies on drug self-administration in humans are reviewed that assessed reinforcing and subjective effects of drugs of abuse. The close parallels observed between studies in humans and laboratory animals using similar behavioral techniques have broadened our understanding of the complex nature of the pharmacological and behavioral factors controlling drug self-administration. The symposium also addressed the role that individual differences, such as sex, personality, and genotype play in determining the extent of self-administration of illicit drugs in human populations. Knowledge of how these factors influence human drug self-administration has helped validate similar differences observed in laboratory animals. In recognition that drug self-administration is but one of many choices available in the lives of humans, the symposium addressed the ways in which choice behavior can be studied in humans. These choice studies in human drug abusers have opened up new and exciting avenues of research in laboratory animals. Finally, the symposium reviewed behavioral pharmacology studies conducted in drug abuse treatment settings and the therapeutic benefits that have emerged from these studies.

Pharmacological options for management of opioid dependence

Methadone is currently the only opioid available for the pharmacotherapy of opioid dependence. Cross-tolerance between methadone and other opioids constitutes the pharmacological basis for substitution and attenuating the effects of illicit opioid use. However, these principles limit the utility of methadone. Potential alternative opioids include long-acting partial agonists such as buprenorphine and pure antagonists such as naltrexone. Buprenorphine is an alternative to methadone with intermediate intrinsic efficacy. It has a large margin of safety, yet displays some agonist actions similar to methadone. It has greater potential than methadone to safely and effectively block the actions of illicit opioids. Naltrexone is a safe, convenient opioid-antagonist for use following detoxification from opioid agonists. Its main use is to block the actions of other opioids, thereby attenuating or eliminating illicit use during treatment. However, it is poorly accepted by many clients, limiting its application to a sub-group who are highly motivated to detoxify. The distinct pharmacological properties of these opioids can overcome some of the drawbacks of methadone, but other limitations may emerge. Non-opioid adjuncts such as alpha2-adrenoceptor agonists can also have a role during detoxification. These drugs might be of use for specific groups of opioid users, providing therapists with the flexibility to tailor pharmacotherapy to the individual needs of clients.

Self-administration of cocaine, cannabis and heroin in the human laboratory: benefits and pitfalls

The objective of this review is to describe self-administration procedures for modeling addiction to cocaine, cannabis and heroin in the human laboratory, the benefits and pitfalls of the approach, and the methodological issues unique to each drug. In addition, the predictive validity of the model for testing treatment medications will be addressed. The results show that all three drugs of abuse are reliably and robustly self-administered by non-treatment-seeking research volunteers. In terms of pharmacotherapies, cocaine use is extraordinarily difficult to disrupt either in the laboratory or in the clinic. A range of medications has been shown to significantly decrease cocaine's subjective effects and craving without decreasing either cocaine self-administration or cocaine abuse by patients. These negative data combined with recent positive findings with modafinil suggest that self-administration procedures are an important intermediary step between pre-clinical and clinical studies. In terms of cannabis, a recent study suggests that medications that improve sleep and mood during cannabis withdrawal decrease the resumption of marijuana self-administration in abstinent volunteers. Clinical data on patients seeking treatment for their marijuana use are needed to validate these laboratory findings. Finally, in contrast to cannabis or cocaine dependence, there are three efficacious Food and Drug Administration-approved medications to treat opioid dependence, all of which decrease both heroin self-administration and subjective effects in the human laboratory. In summary, self-administration procedures provide meaningful behavioral data in a small number of individuals. These studies contribute to our understanding of the variables maintaining cocaine, marijuana and heroin intake, and are important in guiding the development of more effective drug treatment programs.

Controversies in translational research: drug self-administration

RATIONALE

Laboratory animal and human models of drug self-administration are used to evaluate potential pharmacotherapies for drug abuse, yet the utility of these models in predicting clinically useful medications is variable.

OBJECTIVE

The objective of this study was to track how antagonist, agonist, and partial agonist medication approaches influence heroin and cocaine self-administration by rodents, non-human primates, and humans and to compare these results to clinical outcomes.

RESULTS

Across species, heroin self-administration was decreased by all three medication approaches, paralleling their demonstrated clinical utility. The heroin data emphasize the importance of assessing a medication's abuse liability preclinically to predict medication abuse and compliance and of considering subject characteristics (e.g., opioid dependence) when interpreting medication effects. For cocaine, the effects of ecopipam, modafinil, and aripiprazole were consistent in the laboratory and clinic, provided that the medications were administered repeatedly before self-administration sessions. Modafinil attenuated cocaine's reinforcing effects in the human laboratory and improved treatment outcome, while ecopipam and aripiprazole increased the reinforcing effects of cocaine and do not appear promising in the clinic.

CONCLUSIONS

The self-administration model has reliably identified medications to treat opioid dependence, and the recent data with modafinil suggest that the human laboratory model also identifies medications to treat cocaine dependence. There have been numerous false positives when subjective effects are the primary outcome measure, but not when self-administration is the outcome. Factors relevant to the predictive validity of self-administration procedures include medication maintenance and the concurrent assessment of a range of behaviors to determine abuse liability and the specificity of effect.

The role of human drug self-administration procedures in the development of medications

The purpose of this review is to illustrate the utility and value of employing human self-administration procedures in medication development, including abuse liability assessments of novel medications and evaluation of potential pharmacotherapies for substance use disorders. Traditionally, human abuse liability testing has relied primarily on subjective reports describing drug action by use of questionnaires; similarly, drug interactions between putative treatment agents and the drugs of abuse have relied on these measures. Subjective reports are highly valued because they provide qualitative and quantitative information about the characteristics of central and peripheral pharmacodynamic effects as well as safety and tolerability. However, self-administration procedures directly examine the behavior of interest-that is, drug taking. The present paper (1) reviews the most commonly used human self-administration procedures, (2) discusses the concordance of subjective reports and self-administration within the context of medications development for substance use disorders, focusing primarily on illustrative examples from development efforts with opioid and cocaine dependence, and (3) explores the utility of applying self-administration procedures to assess the abuse liability of novel compounds, including "abuse-deterrent" formulations (ADFs). The review will focus on opioid and cocaine dependence because a rich database from both clinical laboratory and clinical trial research exists for these two drug classes. The data reviewed suggest that drug-induced changes in self-administration and subjective effects are not always concordant. Therefore, assessment of self-administration in combination with subjective effects provides a more comprehensive picture that may have improved predictive validity for translating to the clinical setting.

Sustained-release naltrexone: novel treatment for opioid dependence

The devastating costs of opioid abuse and dependence underscore the need for effective treatments for these disorders. At present, several different maintenance medications exist for treating opioid dependence, including methadone, buprenorphine and naltrexone. Of these, naltrexone is the only one that possesses no opioid agonist effects. Instead, naltrexone occupies opioid receptors and prevents or reverses the effects produced by opioid agonists. Despite its clear pharmacologic effectiveness, its clinical effectiveness in treating opioid dependence has been disappointing, primarily due to non-compliance with taking the medication. However, the recent availability of sustained-release formulations of naltrexone has renewed interest in this medication. The present paper describes the development of sustained-release naltrexone formulations and discusses the clinical issues associated with their use in treating opioid dependence.

Oral naltrexone maintenance treatment for opioid dependence

BACKGROUND

Research on the clinical application of oral naltrexone agrees on several things. From a pharmacological perspective, naltrexone works. From an applied perspective, however, this medication is not used since the medication compliance and the retention rates are very poor.

OBJECTIVES

To evaluate the effects of naltrexone maintenance treatment versus placebo or other treatments in preventing relapse in opioid addicts after detoxification.

SEARCH STRATEGY

We searched the Cochrane Drugs and Alcohol Group Register of Trials (January 2005), Cochrane Central Register of Controlled Trials (CENTRAL - The Cochrane Library Issue 1, 2005), MEDLINE (1973-first year of naltrexone use in humans- January 2005), EMBASE (1974- January 2005), PsycINFO (OVID-January 1985 to January 2004). We inspected reference lists of relevant articles and we contacted pharmaceutical producers of naltrexone, authors and other Cochrane review groups.

SELECTION CRITERIA

All randomised and controlled clinical trials which focus on the use of naltrexone maintenance treatment versus placebo, or other treatments to reach sustained abstinence from opiate drugs

DATA COLLECTION AND ANALYSIS

Three reviewers independently assessed studies for inclusion and extracted data. One reviewer carried out the qualitative assessments of the methodology of eligible studies using validated checklists.

MAIN RESULTS

Ten studies, 696 participants, met the criteria for inclusion in this review. Only two studies described an adequate allocation concealment. The results show that naltrexone maintenance therapy alone or associated with psychosocial therapy is more efficacious that placebo alone or associated with psychosocial therapy in limiting the use of heroin during the treatment (RR 0,72 95% confidence interval 0.58 to 0.90). If we consider only the studies comparing naltrexone with placebo, the difference do not reach the statistical significancy, RR 0.79 (95%CI 0.59 to 1.06). With respect to the number of participants re incarcerated during the study period, the naltrexone associated with psychosocial therapy is more effective than the psychosocial treatment alone; RR 0.50 (95%CI 0.27 to 0.91). No statistically significant benefit was shown in terms of retention in treatment, side effects or relapse results at follow-up for any of the considered comparisons.

AUTHORS' CONCLUSIONS

Unfortunately the studies did not provide an objective evaluation of naltrexone treatment in the field of opioid dependence. The conclusions are also limited due to the heterogeneity of the trials both in the interventions and in the assessment of outcomes.

Methadone doses of 100 mg or greater are more effective than lower doses at suppressing heroin self-administration in opioid-dependent volunteers

AIMS

Methadone maintenance has been an effective pharmacotherapy for the treatment of heroin dependence for nearly four decades. Recent clinical research suggests that methadone doses larger than those used in most clinics are more effective at suppressing illicit heroin use. This greater efficacy may result from greater cross-tolerance to the reinforcing effects of heroin.

DESIGN

The purpose of this double-blind, within-subject study was to examine the relationship between methadone maintenance dose and the reinforcing effects of heroin.

SETTING

Participants were stabilized on 50, 100 and 150 mg methadone (ascending order) during separate outpatient periods before being admitted to an inpatient research unit for testing at each maintenance dose.

PARTICIPANTS

Five opiate-dependent volunteers completed the study.

MEASUREMENTS

During each 4-week inpatient testing period, participants sampled three doses of heroin (0, 10, or 20 mg; random order; one dose per week) and were subsequently allowed seven opportunities to choose between another injection of that week's heroin dose and varying amounts of money (dollars 2-38).

FINDINGS

The number of heroin injections chosen decreased as methadone dose was increased. Larger alternative monetary reinforcers were required to suppress heroin self-administration during maintenance on 50 compared to 100 or 150 mg methadone. Larger methadone doses also completely blocked the subjective effects of heroin and produced greater withdrawal suppression during the outpatient periods.

CONCLUSIONS

These results support other clinical and laboratory-based research indicating that persistent heroin use may be reduced by providing larger methadone maintenance doses that produce more effective cross-tolerance to heroin.

A systematic review on the efficacy of naltrexone maintenance treatment in opioid dependence

AIM

To evaluate the efficacy of naltrexone maintenance treatment in preventing relapse in opioid addicts after detoxification.

DESIGN

A systematic review according to the methodology developed by the Cochrane Collaboration based on either randomized controlled trials (RCTs) or controlled clinical trials (CCTs).

PARTICIPANTS

Seven hundred and seven heroin dependent in- and out-patients, or former heroin addicts dependent on methadone and participating in a naltrexone treatment programme; 89% were male.

INTERVENTION

Maintenance treatments on opiate dependent people after detoxification, comparing naltrexone with placebo, pharmacological or behavioural treatments.

MEASUREMENTS

The outcomes considered were successfully completed treatment, opioid use under treatment (re)-incarcerations during the study period, mean duration of treatment.

FINDINGS

The outcomes tended to be slightly although not significantly in favour of the naltrexone groups. Use of naltrexone in addition to behavioural treatment significantly decreased the probability of (re-)incarceration (OR=0.30; 95% CI 0.12, 0.76). The difficulties in producing a quantitative analysis were due mainly to the heterogeneity of the included studies.

CONCLUSIONS

From the available clinical trials performed up to this time, there is insufficient evidence to justify the use of naltrexone in maintenance treatment of opioid addicts.

Depot naltrexone: long-lasting antagonism of the effects of heroin in humans

RATIONALE

Naltrexone, an opioid antagonist, is currently approved as a treatment for heroin dependence. However, naltrexone is generally not well accepted by patients, and medication non-compliance is a difficult obstacle to treatment. A sustained-release form of naltrexone may improve compliance.

OBJECTIVE

The present study was designed to evaluate the time course, safety, and effectiveness of a depot formulation of naltrexone (Depotrex).

METHODS

Twelve heroin-dependent individuals participated in an 8-week inpatient study. After a 1-week detoxification period, six participants received 192 mg naltrexone base and six participants received 384 mg naltrexone base. For safety, the low dose of depot naltrexone was tested before the high dose. The effects of heroin (0, 6.25, 12.5, 18.75, 25 mg, i.v.) were evaluated for the next 6 weeks. One dose of heroin was tested per day on Mondays through Fridays, and the entire dose range was tested each week. Active heroin doses were administered in ascending order during the week, while placebo could be administered on any day. Subjective, performance, and physiological effects were measured both before and after heroin administration. The hypotheses were that depot naltrexone would antagonize the effects of heroin, and that the high dose of depot naltrexone would produce a more effective and longer-lasting antagonism than the low dose.

RESULTS

The low and high doses of depot naltrexone antagonized heroin-induced subjective ratings for 3 and 5 weeks, respectively. Plasma levels of naltrexone remained above 1 ng/ml for approximately 3 and 4 weeks after administration of 192 mg and 384 mg naltrexone. Other than the initial discomfort associated with the injection of depot naltrexone, there were no untoward side-effects.

CONCLUSIONS

These results suggest that this depot formulation of naltrexone provides a safe, effective, long-lasting antagonism of the effects of heroin.

Naltrexone maintenance treatment for opioid dependence

BACKGROUND

Despite widespread use of naltrexone maintenance in many countries for more than a decade, the evidence of its effects has not yet been systematically evaluated.

OBJECTIVES

To evaluate the effects of naltrexone maintenance treatment in preventing relapse in opioid addicts after detoxification.

SEARCH STRATEGY

We searched MEDLINE (1973-first year of naltrexone use in humans-July 2000), EMBASE (1974-July 2000), Cochrane Controlled Trials Register (Cochrane Library issue 2001.4) and handsearched the "Bolletino per le Farmacodipendenze e l'Alcolismo" (1978 to 1997) and reference lists of relevant articles. We contacted pharmaceutical producers of naltrexone, authors and other Cochrane review groups. Date of most recent searches: December 2001.

SELECTION CRITERIA

All controlled studies of naltrexone; treatment of heroin addicts after detoxification.

DATA COLLECTION AND ANALYSIS

Reviewers evaluated data independently and analysed outcome measures taking into consideration adherence to and success of the study intervention. Data were extracted and analysed stratifying for the three categories of study quality. Where possible, meta-analysis was performed.

MAIN RESULTS

Eleven studies met the criteria for inclusion in this review, even if not all of them were randomised. The methodological quality of the included studies varied, but was generally poor. Meta-analysis could be performed to a very low degree only, because the studies and their outcome measures were very heterogeneous. A statistically significant reduction of (re-)incarcerations was found for patients treated with naltrexone and behaviour therapy in respect to those treated with behaviour therapy only. The other outcomes considered in the meta-analysis did not yield any significant results. Final conclusions on whether naltrexone treatment may be considered effective in maintenance therapy cannot be drawn from the clinical trials available so far.

REVIEWER'S CONCLUSIONS

The available trials do not allow a final evaluation of naltrexone maintenance treatment yet. A trend in favour of treatment with naltrexone was observed for certain target groups (particularly people who are highly motivated), as has been previously described in the literature.

A method of reducing the opioid withdrawal intensity using progressively increasing doses of naloxone

We assessed the withdrawal intensity in acutely morphine-dependent mice using a pretreatment with escalating doses of naloxone. All animals received a single dose of morphine (100 mg/kg) for the induction of acute opioid dependency. Group 1 (control) received three injections of normal saline and then naloxone 0.8 mg/kg. Group 2 received increasing pretreatment doses of naloxone (0.1, 0.2, and 0.4 mg/kg) and a challenge dose of 0.8 mg/kg. Group 3 received three injections of naloxone 0.1 mg/kg and a challenge dose of 0.8 mg/kg. Groups 4 and 5 were used to verify whether ED(50) found in previous studies was comparable with values obtained in the current experiments. The withdrawal intensity was determined by the number of jumps. The mice of group 1 exhibited significantly more jumps after 0.8 mg/kg of naloxone as compared with group 2. The number of jumps in response to naloxone between groups 1 and 2 and groups 2 and 3 was not significantly different. The results show that pretreatment with increasing naloxone doses significantly reduced the withdrawal intensity as compared with the control group; whereas pretreatment with repeated low antagonist did not reduce it significantly.

Effects of an alternative reinforcer on intravenous heroin self-administration by humans

Five heroin-dependent research volunteers, maintained on divided daily oral morphine doses, participated in an inpatient study designed to evaluate intravenous (i.v.) heroin self-administration when money ($10, $20 or $40) was concurrently available. Each morning participants received a single injection of heroin (placebo, 6.25, 12.5, 25, or 50 mg/70 kg, i.v.) and each afternoon, they had the opportunity to self-administer all or part of the morning dose. Participants responded under a progressive-ratio schedule (50, 100, ..., 2800) during a 10-trial self-administration task. During each trial, participants could respond for 1/10th of the sampled heroin dose or 1/10th of a single money value. The progressive-ratio value increased independently for each option. The total amount of heroin and/or money chosen during the self-administration task was administered at the end of the task. Heroin dose-dependently increased ratings of 'good drug effect' and 'high', impaired task performance and decreased pupil diameter and blood oxygen saturation. Heroin also dose-dependently increased progressive-ratio break point values, which varied as a function of the alternative money amount. Consistent with previous studies, the present results demonstrate that alternative reinforcers, depending on magnitude, are effective in reducing heroin use in opioid-dependent individuals.

Pharmacologic interventions for the treatment of opioid dependence and withdrawal

This article discusses current pharmacologic methods in the treatment of heroin dependence and withdrawal. Methadone hydrochloride, the most commonly used opiate agonist, is used for both withdrawal and maintenance therapy. However, it produces dependence and withdrawal results upon abrupt discontinuation. Other opiate agonists including L-alpha acetyl methadyl (LAAM) and propoxyphene napsylate have been used for both withdrawal and maintenance therapy. LAAM is currently available only as an investigational agent and propoxyphene is easily accessible but has been associated with hallucinations and dysphoria at high doses. Alpha 2-adrenergic agonists decrease opiate withdrawal symptoms by decreasing the central adrenergic hyperarousal that is associated with withdrawal. Clonidine effectively attenuates but does not totally eliminate withdrawal symptoms. Other alpha 2-adrenergic agonists (e.g., lofexidine hydrochloride, guanfacine hydrochloride, and guanabenz) have undergone only preliminary investigations. Although alpha 2 agonists effectively decrease most withdrawal symptoms they often cause hypotension. Buprenorphine hydrochloride is a partial opiate agonist that shows some promise in the treatment of the heroin-dependent population. It attenuates opiate craving and causes only minimal withdrawal upon abrupt discontinuation. Because it is well accepted by the heroin-dependent population, however, it may ultimately become an abused substance. Naltrexone is a potent, orally acting opiate antagonist that blocks all opiate-agonist effects and causes no euphoria. Unfortunately, it has not been well accepted by the heroin-dependent population. Scant research has been conducted concerning the use of adjunctive medications during opioid withdrawal.

Naltrexone. A review of its pharmacodynamic and pharmacokinetic properties and therapeutic efficacy in the management of opioid dependence

Naltrexone is a long acting competitive antagonist at opioid receptors which blocks the subjective and objective responses produced by intravenous opioid challenge. It is suitable for oral administration, and has been studied as an adjunct for use in opioid addiction management programmes. In non-comparative clinical trials involving detoxified patients, oral naltrexone reduced heroin craving and between 23 and 62% of patients remained in treatment after 3 to 4 weeks. However, in two studies 32 to 58% of patients who continued in treatment were opioid-free between 6 and 12 months after stopping naltrexone. As might be expected studies involving highly motivated patients have shown this type of patient group to achieve greater treatment success rates during naltrexone therapy, and remain opioid-free longer than other groups of apparently less motivated patients. In addition, when naltrexone is combined with family support, psychotherapy and counselling, patients are more likely to remain opioid-free. Naltrexone produces a low incidence of side effects, with gastrointestinal effects being the most commonly reported symptoms. Thus, despite the overall high attrition rates from trials, in selected patient groups and in combination with appropriate support mechanisms and psychotherapy, naltrexone represents a useful adjunct for the maintenance of abstinence in the detoxified opioid addict.

Heroin Maintenance: A Medical View

People who are fed up with crime in the streets think, simplistically, that heroin maintenance will solve the problem. According to the medical view presented here, heroin maintenance will not solve any problem, either for the addict or for society, and it will have predictable adverse consequences. Some problems have no complete solution. Some “solutions” can be worse than the problems they are meant to solve.

Psychopathology, craving, and mood during heroin acquisition: an experimental study

Six detoxified addict volunteers were allowed to self-administer intravenous heroin on an essentially self-determined schedule. Two periods of heroin acquisition were compared: an unmodified cycle in which patients could become intoxicated and a later cycle in which the effects of heroin were blocked with a narcotic antagonist. In the unblocked condition, patients initially experienced an increase in positive mood, but with chronic administration there was a significant rise in psychopathology and the development of a generalized dysphoric state. Similar changes did not occur when the same patients took heroin while blocked with a narcotic antagonist. Drug craving rose dramatically when "unblocked" heroin was available, but gradually fell during methadone detoxification. Following treatment with a narcotic antagonist, the presence of heroin failed to elicit any sustained rise in craving and drug taking was dramatically reduced.

Aftercare on narcotic antagonists: prospects and problems

This study examines the problems and prospects of aftercare of the multiple treatment failure opiate addict who is being treated with narcotic antagonists (primarily naltrexone). At the time of this paper, 21 patients had been admitted to the Drug Addiction Research Project at McLean Hospital, with a retention rate of 42%. "Completers," "noncompleters," and "dropouts" could not be differentiated on the basis of demographic or psychiatric variables, thus making it impossible to define the population most suited for antagonist treatment. The degree of carry-over into the first month in the community, for those who successfully completed the inpatient segment of the program, disappeared over time. The nonreinforcing (lack of euphoric effect) properties of the antagonists and the resultant high level of motivation necessary for the continuation of its use by the patient make this form of treatment suitable for only a small percentage of the "hard core" addict population.