Successful use of methadone in the treatment of chronic neuropathic pain arising from burn injuries: a case-study

Novel use of methadone intraoperatively in pediatric burn patients

Burn injury in children can cause severe and chronic physical and mental sequelae. Opioids are a mainstay in burn pain management but increasing utilization in this country has led to concern for their continued use and potential for dependence. Methadone is a long-acting analgesic that targets the N-methyl-D-aspartate (NMDA) receptor in addition to the mu opioid receptor and has benefit in adult burn patients. However, its use in the pediatric burn population has been less robustly studied. This is a retrospective cohort study at a single Level 1 Burn Center whose primary aim is to compare opioid utilization 36 hours postoperatively between pediatric burn patients who received intraoperative, intravenous methadone and those who did not. Secondary aim was to describe differences in methadone-related complications between the cohorts. There was decreased opioid utilization measured by median morphine equivalents per kilogram (ME/kg) postoperatively in the methadone cohort compared to the control cohort (0.54mg/kg v. 0.77mg/kg, p = 0.18). No adverse events were noted upon chart review. The data suggests methadone use is beneficial in pediatric burn patients, but further prospective studies are warranted on a larger population.

Methadone vs. morphine SR for treatment of neuropathic pain: A randomized controlled trial and the challenges in recruitment

Introduction: Accumulating evidence has identified a number of advantages for methadone over other opioids for the treatment of chronic pain including: agonist action at both μ and δ opioid receptors, N-methyl-d-aspartate (NMDA) antagonist activity and the ability to inhibit the reuptake of monoamines. It was hypothesized that with these three mechanisms of action methadone might be a good option for the treatment of neuropathic pain.

Methods: This was a double-blind randomized controlled trial comparing methadone to controlled-release morphine. The primary objective was to determine whether methadone is clinically inferior versus noninferior to morphine as an analgesic.

Results: We attempted recruitment at three academic pain centers over a 3-year period. In the end only 14 participants were able to be recruited; 5 withdrew and only 9 completed the trial. This study was underpowered. All participants showed a mean reduction in pain intensity according to the Numeric Rating Scale for Pain Intensity (morphine 5.86 to 4.38, methadone 6.11 to 4.5) and reported pain relief compared to pretreatment, but the sample size was too small for statistical analysis.

Discussion: Reasons for challenges in recruitment included tight inclusion and exclusion criteria and high participant burden. In addition, there was significant heterogeneity of patients between the three sites, leading to differences in reasons for exclusion. This included seemingly disparate reasons at the different sites, including few participants who were methadone naïve vs. avoidance or fear of opioids. In the end, we were unable to answer the question of the study.

On the horizon: research priorities in burns for the next decade

This review demonstrates that many advances have been made in burn care that have made dramatic differences in mortality, clinical outcomes, and quality of life in burn survivors; however, much work remains. In reality, the current standard of care is insufficient and we cannot be satisfied with the status quo. We must strive for the following goals: no deaths due to burn, no scarring, and no pain. These particular goals have only begun to be confronted.

Methadone in post-herpetic neuralgia: A pilot proof-of-concept study

OBJECTIVE

This research was designed as a pilot proof-of-concept study to evaluate the use of low-dose methadone in post-herpetic neuralgia patients who remained refractory after first and second line post-herpetic neuralgia treatments and had indications for adding an opioid agent to their current drug regimens.

METHODS

This cross-over study was double blind and placebo controlled. Ten opioid naïve post-herpetic neuralgia patients received either methadone (5 mg bid) or placebo for three weeks, followed by a 15-day washout period and a second three-week treatment with either methadone or placebo, accordingly. Clinical evaluations were performed four times (before and after each three-week treatment period). The evaluations included the visual analogue scale, verbal category scale, daily activities scale, McGill pain questionnaire, adverse events profile, and evoked pain assessment. All patients provided written informed consent before being included in the study. ClinicalTrials.gov: NCT01752699 RESULTS: Methadone, when compared to placebo, did not significantly affect the intensity of spontaneous pain, as measured by the visual analogue scale. The intensity of spontaneous pain was significantly decreased after the methadone treatment compared to placebo on the category verbal scale (50% improved after the methadone treatment, none after the placebo, p=0.031). Evoked pain was reduced under methadone compared to placebo (50% improved after the methadone treatment, none after the placebo, p=0.031). Allodynia reduction correlated with sleep improvement (r=0.67, p=0.030) during the methadone treatment. The side effects profile was similar between both treatments.

CONCLUSIONS

Methadone seems to be safe and efficacious in post-herpetic neuralgia. It should be tried as an adjunctive treatment for post-herpetic neuralgia in larger prospective studies.

Methadone for chronic non-cancer pain in adults

BACKGROUND

Methadone belongs to a class of analgesics known as opioids, that are considered the cornerstone of therapy for moderate-to-severe pain due to life-threatening illnesses; however, their use in chronic non-cancer pain (CNCP) is controversial. Methadone has many characteristics that differentiate it from other opioids, which suggests that it may have a different efficacy and safety profile.

OBJECTIVES

To assess the analgesic effectiveness and safety of methadone in the treatment of CNCP.

SEARCH METHODS

We identified both randomized controlled trials (RCTs) and non-randomized studies of methadone use in chronic pain by searching the Cochrane Central Register of Controlled Trials (CENTRAL) in The Cochrane Library 2011, issue 11, MEDLINE (1950 to November 2011), and EMBASE (1980 to November 2011), together with reference lists of retrieved papers and reviews.

SELECTION CRITERIA

We included RCTs with pain assessment as either the primary or secondary outcome. Quasi-randomized studies, cohorts and case-control trials were also considered for inclusion because we suspected that the beneficial and harmful effects of methadone in CNCP may not be adequately addressed in RCTs.

DATA COLLECTION AND ANALYSIS

Two review authors independently extracted efficacy and adverse event data and assessed risk of bias.

MAIN RESULTS

We included two RCTs and one non-randomized study, involving a total of 181 participants. Both RCTs were cross-over studies, one involving 19 participants with diverse neuropathic pain syndromes, the other involving 76 participants with postherpetic neuralgia. Study phases were 20 days and approximately eight weeks, respectively. The non-randomized study retrospectively evaluated 86 outpatients over an average of 8.8 ± 6.3 months.One RCT reported average pain intensity and pain relief, and found statistically significant improvements versus placebo for both outcomes, with 10 mg and 20 mg daily doses of methadone. The second RCT reported differences in pain reduction between methadone and morphine and found morphine to be statistically superior. The non-randomized study found that in patients initially prescribed methadone it was effective in fewer participants than in those initially prescribed other long-acting opioids (28% versus 42%, 33% and 50% for morphine, oxycodone and transdermal fentanyl, respectively).One RCT compared incidences for several individual adverse events, but found a difference between methadone and placebo for only one event, dizziness (P = 0.041).

AUTHORS' CONCLUSIONS

The three studies provide very limited evidence of the efficacy of methadone for CNCP, and there were too few data for pooled analysis of efficacy or harm, or to have confidence in the results of the individual studies. No conclusions can be made regarding differences in efficacy or safety between methadone and placebo, other opioids, or other treatments.

Methadone use in children and young adults at a cancer center: a retrospective study

OBJECTIVE

To augment the literature on methadone applications in pediatric oncology, the authors reviewed the use of methadone at a pediatric cancer center over a 5-year period.

DESIGN AND SETTING

Forty-one patients received methadone for inpatient or outpatient pain management. The authors retrospectively reviewed their demographic characteristics, diagnoses, type of pain (nociceptive, neuropathic, or mixed) and causes of pain, and the indications, dose regimens, adverse effects, and outcomes of methadone treatment.

RESULTS

There were four types of clinical uses for methadone in 41 patients (10 patients had two): nociceptive pain unresponsive to other opioids (17 patients, 33.3 percent), neuropathic pain (20 patients, 39.2 percent), facilitation of weaning from opioids (11 patients, 21.6 percent), and end-of-life pain management (3 patients, 5.9 percent). The mean age of the 24 males (58.5 percent) and 17 females (41.5 percent) at the start of treatment was 15.7 years (range, 0.6-23 years). The most common diagnoses were leukemia (n = 10, 24.4 percent), osteosarcoma (n = 7, 17.0 percent), and rhabdomyosarcoma (n = 5, 12.2 percent). The causes of pain were bone marrow transplant (n = 13, 31.7 percent), amputation (n = 6, 14.6 percent), chemotherapy (n = 5, 12.2 percent), tumor (n = 5, 12.2 percent), limb-sparing surgery (n = 4, 9.8 percent), and other (n = 8, 19.5 percent). Efficacy was assessed at the end (or after 6 months) of methadone treatment. For many patients (43.1 percent), methadone showed efficacy in achieving the purpose for which it was prescribed, including reduction of nociceptive or neuropathic pain and prevention of opioid withdrawal. Sedation was the most common side effect (24.4 percent).

CONCLUSIONS

Methadone was effective for pediatric patients with neuropathic pain or nociceptive pain unresponsive to other opioids, and it effectively prevented opioid withdrawal.

Treatment of pain in children after limb-sparing surgery: an institution's 26-year experience

A significant proportion of patients report long-term pain that is ≥5 on a 0-10 intensity scale after limb-sparing surgery for malignancies of the long bones. Patients experience several distinct types of pain after limb-sparing surgery which constitute a complex clinical entity. This retrospective study examined 26 years of experience in a pediatric institution (1981-2007) in pain management as long as 6 months after limb-sparing surgery and reviewed the historical evolution of pain interventions. One hundred fifty patients underwent 151 limb-salvage surgeries for bone cancer of the extremities in this series. Pain treatment increased progressively in complexity. Therapies included opioids, nonsteroidal antiinflammatory drugs, acetaminophen-opioid combinations, postoperative continuous epidural infusion, anticonvulsants and tricyclic antidepressants for neuropathic pain, local anesthetic wound catheters, and continuous peripheral nerve block catheters. Management of pain after limb-sparing surgery has evolved over the 26 years of this review. It currently relies on multiple "layers" of pharmacologic and nonpharmacologic strategies to address the complex mixed nociceptive and neuropathic mechanisms of pain in this patient population.

Efficacy of opioids for chronic pain: a review of the evidence

Opioid therapy for chronic pain has been popularized over the past few decades, and a concern has arisen that the analgesic efficacy of opioids is not always maintained over prolonged courses of treatment despite dose escalation and stable pain. Considering the potentially serious adverse effects of opioids, the idea that pain relief could diminish over time may have a significant impact on the decision to embark on this therapy, especially in vulnerable individuals. Possible loss of analgesic efficacy is especially concerning, considering that dependence may make it hard to withdraw opioid therapy even in the face of poor analgesia. This article first reviews the evidence on opioid efficacy when used for the treatment of chronic pain, and concludes that existing evidence suggests that analgesic efficacy, although initially good, is not always sustained during continuous and long-term opioid therapy (months to years). The theoretical basis for loss of analgesic efficacy over time is then examined. Mechanisms for loss of analgesic efficacy proposed are pharmacologic tolerance, opioid-induced hyperalgesia, subtle and intermittent withdrawal, and a number of psychologic factors including loss of the placebo component.

Successful use of gabapentin in acute pain management following burn injury: a case series

Pain after burn injury has multiple qualities, including neuropathic and hyperalgesic elements. This element of the burn patients' pain experience is frequently difficult to manage and contributes significantly to their suffering. The onset may be either immediate or delayed. Gabapentin has established efficacy in the reduction of burn-induced hyperalgesia and allodynia in animal and human experimental burn models. This article reports a case series of six patients who, following admission to hospital with burn injury, described burning dysesthesia at either the injury or graft donor site. These patients were prescribed gabapentin in addition to standard analgesia. The use of gabapentin resulted in a rapid reduction in the severity of the neuropathic element of the pain. The medication was well tolerated, with no severe adverse reactions. Conclusions. This case series introduces the use of gabapentin as a potentially important therapy in the management of neuropathic pain following burn injury. Further research is required to define the use of gabapentin in this specific setting.

Enhanced inflammatory hyperalgesia after recovery from burn injury

Severe burn induces severe pain. While chronic as well as acute pain syndromes are reported, the peripheral mechanisms of burn-induced chronic pain syndromes have not been studied. We tested the hypothesis that burn induces plastic changes in primary afferent nociceptors that predispose to chronic pain states. Mechanical nociceptive thresholds were measured using the Randall-Selitto paw-withdrawal test in male Sprague-Dawley rats, before and following a small (<1% total body surface area) partial-thickness thermal injury to the dorsal surface of one hind paw. This burn induced mechanical hyperalgesia, which lasted over 2 weeks. After recovery, local injection of prostaglandin E2 (PGE2), to mimic re-injury, induced an enhanced and markedly prolonged mechanical hyperalgesia compared to the hyperalgesic effect of PGE2 in the control contralateral paw. This prolonged PGE2-induced hyperalgesia was reversed by a selective inhibitor of protein kinase C-epsilon (PKCepsilon). Our findings suggest PKCepsilon as a peripheral mechanism for burn-induced chronic pain syndromes.

Management of pain after burn injury

PURPOSE OF REVIEW

Burn pain is often under treated. Burn patients suffer from daily background pain as well as procedural pain. Direct mechanical and chemical stimulation to peripheral nociceptors, peripheral- and central sensitization contribute to the pathophysiology of pain. The purpose of this review is to discuss the current management of burn pain and also to stimulate future studies.

RECENT FINDINGS

Background pain is best treated with mild to moderate potent analgesics administered regularly to maintain a steady plasma drug concentration. Procedural pain should be treated vigorously with intravenous opioids, local or even general anesthesia if needed. Opioids are the mainstay of treatment for severe acute pain. PCA should be used wherever applicable. Further opioids should not be substituted by high dose NSAIDs in the management of procedural pain. Hypnosis, therapeutic touch, massage therapy, distracting techniques and other behavioral cognitive techniques have demonstrated some intriguing impact on acute as well as chronic burn pain treatment.

SUMMARY

There is no clear evidence to show that the use of opioids in acute pain may increase the likelihood of developing opioid dependency. Thus, pain after burn injury should be aggressively treated using pharmacologic and non-pharmacologic approaches. Further controlled studies are yet to be conducted to define appropriate treatments for different burn patients and to establish standard treatment protocols for burn pain.

Opioids for chronic nonterminal pain

This article first reviews the evidence for and against chronic opioid therapy. Evidence supporting the opioid responsiveness of chronic pain, including neuropathic pain, includes multiple randomized trials conducted over months (up to 8 months). Observational studies are conducted for longer, and many also support opioid analgesic efficacy. Concerns have arisen about loss of efficacy with prolonged use, possibly related to tolerance or opioid-induced hyperalgesia. Mechanisms of tolerance and opioid-induced hyperalgesia are explored. Evidence on other important outcomes such as improvement in function and quality of life is mixed, and is less convincing than evidence supporting analgesic efficacy. It is clear from current evidence that many patients abandon chronic opioid therapy because of the unacceptability of side effects. There are also concerns about toxicity, especially when opioids are used in high doses for prolonged periods, related to hormonal and immune function. The issue of addiction during opioid treatment of chronic pain is also explored. Addiction issues present many complex questions that have not been satisfactorily answered. Opioid treatment of pain has been, and remains, severely hampered because of actual and legal constraints related to addiction risk. Pain advocacy has focused on placing addiction risk into context so that addiction fears do not compromise effective treatment of pain. On the other hand, denying addiction risk during opioid treatment of chronic pain has not been helpful in terms of providing physicians with the tools needed for safe chronic opioid therapy. Here, a structured goal-directed approach to chronic opioid treatment is suggested; this aims to select and monitor patients carefully, and wean therapy if treatment goals are not reached. Chronic opioid therapy for pain has not been a universal success since it was re-established during the last two decades of the twentieth century. It is now realized that the therapy is not as effective or as free from addiction risk as was once thought. Knowing this, many ethical dilemmas arise, especially in relation to patients' right to treatment competing with physicians' need to offer the treatment selectively. In the future, we must learn how to select patients for this therapy who are likely to achieve improvement in pain, function and quality of life without interference from addiction. Efforts will also be made in the laboratory to identify opioids with lower abuse potential.

Addiction to opioids in chronic pain patients: a literature review

Opioids have proven very useful for treatment of acute pain and cancer pain, and in the developed countries opioids are increasingly used for treatment of chronic non-malignant pain patients as well. This literature review aims at giving an overview of definitions, mechanisms, diagnostic criteria, incidence and prevalence of addiction in opioid treated pain patients, screening tools for assessing opioid addiction in chronic pain patients and recommendations regarding addiction problems in national and international guidelines for opioid treatment in cancer patients and chronic non-malignant pain patients. The review indicates that the prevalence of addiction varied from 0% up to 50% in chronic non-malignant pain patients, and from 0% to 7.7% in cancer patients depending of the subpopulation studied and the criteria used. The risk of addiction has to be considered when initiating long-term opioid treatment as addiction may result in poor pain control. Several screening tools were identified, but only a few were thoroughly validated with respect to validity and reliability. Most of the identified guidelines mention addiction as a potential problem. The guidelines in cancer pain management are concerned with the fact that pain may be under treated because of fear of addiction, and the guidelines in management of non-malignant pain patients include warnings of addiction. According to the literature, it seems appropriate and necessary to be aware of the problems associated with addiction during long-term opioid treatment, and specialised treatment facilities for pain management or addiction medicine should be consulted in these cases.

A Descriptive Review of Neuropathic-Like Pain After Burn Injury

This study reviews the natural history of neuropathic-like pain after burn injury. We undertook a retrospective chart review during a 24-month period of patients treated at an outpatient burn center. The medical records of patients with neuropathic-like pain complaints, including the sensation of pins and needles, burning, stabbing, shooting, or "electric" sensations, were included for analysis. Medical and demographic data were collected. We identified 72 patients for inclusion in the study. The age was 44 +/- 2 years (mean +/- SEM), and TBSA burned was 18 +/- 3%. The first complaint of neuropathic-like symptoms was at 4.3 +/- 0.5 months after injury. Documentation of improvement in the symptoms occurred at 7.0 +/- 0.8 months. Symptoms persisted for 13.1 +/- 2.2 months after the injury. Patients were followed for 14.5 +/- 2.2 months. Documented initial pain severity score was 7 +/- 1 of 10. Typical exacerbating factors included temperature change, dependent position, light touch, and weight-bearing activities. Common alleviating factors included rest, massage, compression garment use, and elevation. Treatment regimens often included gabapentin (38%) and steroid injections (21%). Hypertrophic scarring (43%), pruritus (40%), and psychiatric diagnoses (36%) were common associated problems. There is a patterned natural history for neuropathic-like pain after burn injury. This clinical entity involves significant pain complaints and persists, on average, for greater than 1 year after injury, which underscores the importance of long-term outpatient care after burn injury. Furthermore, an understanding of the natural history will assist clinicians in prognosticating and caring for burn survivors with pain after wound closure.

Response Phases in Methadone Treatment for Chronic Nonmalignant Pain

Although studies on the beliefs of persons with chronic nonmalignant pain (CNMP) are still scarce, methadone is increasingly prescribed for the treatment of CNMP. This qualitative case study uses semistructured interviews to explore the beliefs of 11 patients with CNMP and the challenges they faced coming to terms with and integrating methadone treatment into their lives. The study identifies a two-phase process of acceptance and integration. In the first phase, during acceptance of the prescribed methadone treatment, initial beliefs were mostly determined by the societal stigma that "methadone is for junkies." Different influencing factors such as knowledge about methadone for pain management, family support, and trust in physicians changed behavior in a positive way. In the second phase, patients dealt with the degree of disclosure about their treatment. Full disclosers have no problem in telling others that they were being treated with methadone, whereas partial disclosers were more selective. They were confronted with various barriers: negative encounters with family, friends, and the public; past addict experiences; safety issues; and obstacles within the health care system. As a result of these challenges, their beliefs were summarized as: "others think I'm an addict," and "methadone can harm me and/or my family."This study highlights the important role nurses have in the education of patients on the use of methadone in pain management, and in assisting patients with CNMP to gain confidence and a greater sense of control to cope with the challenging issues related to disclosing information.

Oral methadone for chronic noncancer pain: a systematic literature review of reasons for administration, prescription patterns, effectiveness, and side effects

OBJECTIVE

To assess the indications, prescription patterns, effectiveness, and side effects of oral methadone for the treatment of chronic noncancer pain.

METHODS

We conducted searches of several electronic databases, textbooks and reference lists for controlled or uncontrolled studies in humans. Effectiveness was assessed using a dichotomous classification of "meaningful" versus "nonmeaningful" outcomes.

RESULTS

Twenty-one papers (1 small randomized trial, 13 case reports, and 7 case series) involving 545 patients with multiple noncancer pain conditions were included. In half of the patients, no specific diagnosis was reported. Methadone was administered primarily when previous opioid treatment was ineffective or produced intolerable side effects. Starting dose ranged from 0.2 to 80 mg/day and maximum dose ranged from 20 to 930 mg/day. Pain outcomes were meaningful in 59% of the patients in the uncontrolled studies. The randomized trial demonstrated a statistically significant improvement in pain for methadone (20 mg/day) compared to placebo. Side effects were considered minor.

DISCUSSION

Oral methadone is used for various noncancer pain syndromes, at different settings and with no prescription pattern that could be identifiable. Starting, maintenance, and maximum doses showed great variability. The figure of 59% effectiveness of methadone should be interpreted very cautiously, as it seems overrated due to the poor quality of the uncontrolled studies and their tendency to report positive results. The utilization of oral methadone for noncancer pain is based on primarily uncontrolled literature. Well-designed controlled trials may provide more accurate information on the drug's efficiency in pain syndromes and in particular neuropathic pain.

The pharmacotherapy of chronic pain: a review

The past two decades have contributed a large body of preclinical work that has assisted in our understanding of the underlying pathophysiological mechanisms that cause chronic pain. In this context, it has been recognized that effective treatment of pain is a priority and that treatment often involves the use of one or a combination of agents with analgesic action. The current review presents an evidence-based approach to the pharmacotherapy of chronic pain. Medline searches were done for all agents used as conventional treatment in chronic pain. Published papers up to June 2005 were included. The search strategy included randomized, controlled trials, and where available, systematic reviews and meta-analyses. Further references were found in reference sections of papers located using the above search strategy. Agents for which there were no controlled trials supporting efficacy in treatment of chronic pain were not included in the present review, except in cases where preclinical science was compelling, or where initial human work has been positive and where it was thought the reader would be interested in the scientific evidence to date.

Methadone in the management of intractable neuropathic noncancer pain

OBJECTIVE

To evaluate the role of methadone in the management of intractable neuropathic noncancer pain.

METHODS

A case series of 50 consecutive noncancer pain patients who were seen at a tertiary care centre and treated with oral methadone for a variety of intractable neuropathic pain states.

RESULTS

The mean age was 52.7 years and the mean duration of follow-up was 13.9 months. Post-discectomy nerve root fibrosis, complex regional pain syndrome, peripheral neuropathy and central spinal cord pain syndromes were the most common diagnoses. Over 90% had been treated with one or more tricyclic antidepressants and anticonvulsants and a similar number had received other adjuvant analgesics. All patients had failed treatment with one or more conventional opioid analgesics (mean 2.8) at a mean maximal morphine dose of 384 mg (or equivalents) per day. Twelve patients had failed spinal cord stimulation. Nineteen patients (38%) did not tolerate initial methadone titration or thought their pain was worse on methadone. Five patients (10%) declared initial benefit but required repetitive dose escalation and eventually became non-responders. Twenty-six patients (52%) reported mild (4), moderate (15), marked (6) or complete (1) pain relief and continued on methadone at a mean maintenance dose of 159.8 mg/day for a mean duration of 21.3 months. Fourteen patients (28%) reported improved function on methadone relative to previous treatments.

CONCLUSIONS

Methadone appears to have unique properties including N-methyl-D-aspartate antagonist activity that may make it especially useful in the management of intractable neuropathic pain. This observation needs to be tested in randomized, controlled trials.

Management of chronic neuropathic pain with methadone: a review of 13 cases

The synthetic opioid methadone has generated much interest in recent years among clinicians involved in the management of intractable chronic cancer pain. Its use as an analgesic is starting to extend to the treatment of noncancer pain, particularly neuropathic pain. Unfortunately, the evidence for its use in the management of neuropathic pain is limited to a few case studies. We examined retrospectively during a 12-month study period the clinical response of all 13 patients at our pain clinic who were prescribed methadone in an attempt to control neuropathic pain resistant to conventional analgesics. A questionnaire was also administered to the 9 patients who continued to take methadone at 12 months posttreatment. A total of 4 patients (31%) discontinued it by the end of the 12-month study period. Patients discontinued methadone due to the absence of pain relief and due to various intractable, undesirable side effects. Somnolence was the most common adverse effect reported, followed by nausea, constipation, and vomiting. All patients took coanalgesics (eg, amitriptyline, gabapentin) or other analgesics (eg, morphine, nonsteroidal anti-inflammatory drugs) during methadone treatment to control pain. The 9 patients who continued to take methadone at 12 months reported experiencing on average 43% pain relief (range 0-80%), 47% improvement in quality of life (range 0-100%), and 30% improvement in quality of sleep (range 0-60%). Methadone was effective at relieving pain and ameliorating quality of life and sleep in 62% of patients. These findings suggest that methadone can offer an acceptable success rate for the treatment of neuropathic pain. Prospective randomized, placebo-controlled studies are now needed to examine more rigorously the benefits of methadone for this type of pain.

Functional inhibition by methadone of N-methyl-D-aspartate receptors expressed in Xenopus oocytes: stereospecific and subunit effects

Methadone is a strong opioid analgesic that is finding increasing use in chronic pain therapeutics. We explored its reported efficacy for inhibiting N-methyl-D-aspartate (NMDA) receptors in a functional electrophysiologic assay (Xenopus laevis oocyte expression). Racemic methadone inhibited all subtypes of rat NMDA receptors with derived 50% inhibitory concentrations in the low micromolar range. These concentrations overlap with clinically achievable concentrations reported in pharmacokinetic studies. In contrast, morphine inhibited these functional ion channels only at 8-16 times larger concentrations. The NR1/2A and NR1/2B subtype combinations were in general significantly more sensitive to inhibition by methadone and morphine compared with the NR1/2C and NR1/2D subtypes. In the presence of racemic methadone, the maximum NMDA-stimulated currents were markedly decreased, but the NMDA concentration producing 50% of maximal activation was altered only slightly, indicating that methadone blocks by a noncompetitive mechanism. Although stereoisomers of methadone showed minimal stereoselectivity in most subtypes, R(-) methadone was highly selective in its inhibition of the NR1/2A combination. These results provide further functional data describing the NMDA receptor inhibitory actions of methadone and support the hypothesis that methadone acts through both opioid and NMDA receptor mechanisms.

IMPLICATIONS

At clinically achievable concentrations, methadone inhibits functional N-methyl-D-aspartate receptors. These results indicate a unique mode of action by this opioid that may enhance its ability to treat chronic pain and to limit opioid tolerance.