Opioid antagonists: a review of their role in palliative care,focusing on use in opioid-related constipation

Opioid antagonists have well-established indications in the reversal of life-threatening opioid toxicity, but also hold considerable promise for other applications in palliative care practice, particularly management of opioid-related constipation. We briefly review current understanding of opioid receptors, focusing on their complex role in gastrointestinal physiology. We summarize the pharmacology, conventional indications, and clinical usage of three major groups of opioid antagonists, including a promising new peripherally acting agent, methylnaltrexone, which is not commercially available. We suggest an approach to administering opioid antagonists for reduction of life-threatening opioid toxicity in patients with pain. The literature on opioid-induced constipation and its treatment with opioid-antagonists is reviewed in detail. Finally, other potential uses of opioid antagonists in palliative care are described, especially strategies for reducing such opioid side effects as nausea and pruritus and for improving analgesia or reducing tolerance by concomitantly administrating both an opioid agonist and low dosages of an antagonist.     

Transdermal fentanyl.

OBJECTIVE: To review the use of transdermal fentanyl for the treatment of moderate to severe chronic pain. The article provides background on the pharmacology and pharmacokinetics of the drug, as well as the properties of the transdermal system. In addition, clinical trials, adverse effects, and therapeutic considerations and recommendations are presented. DATA SOURCES: Clinical trials, review articles, and reference texts. STUDY SELECTION: Comparative clinical trials involving the use of transdermal fentanyl on postoperative and chronic pain patients. DATA EXTRACTION: Data from clinical human trials published in the English language were reviewed. Trials were assessed by sample size, opioid dosage regimen, and therapeutic outcome. DATA SYNTHESIS: Transdermal fentanyl was found to be effective in the control of chronic and postoperative pain. In one trial the overall patient satisfaction with pain control was 79 percent for the transdermal fentanyl group and 44 percent for the placebo group. In another trial, the amount of additional parenteral morphine was significantly lower for the group receiving transdermal fentanyl than for the placebo group (49.9 +/- 4.9 vs. 77.0 +/- 6.3 mg, respectively, p < 0.01). The most common adverse effects recorded were nausea (45-85 percent), pruritus (14-60 percent), and sedation (40-59 percent). The cost of analgesic therapy with this delivery system is higher than that of parenteral opioid analgesia, but less than patient-controlled analgesia. CONCLUSIONS: The transdermal fentanyl formulation offers some minor advantages over other forms of conventional pain management. Results of early clinical trials are promising, but more studies are needed to evaluate its long-term effectiveness and adverse effects. Specifically, comparisons with standard parenteral and patient-controlled opioid analgesia in chronic malignant and nonmalignant pain are necessary for adequate evaluation of transdermal fentanyl.     

Methylnaltrexone antagonizes opioid-mediated enhancement of HIV infection of human blood mononuclear phagocytes

Opioid abuse has been postulated as a cofactor in the immunopathogenesis of human immunodeficiency virus (HIV) infection and AIDS. We and others have recently demonstrated that opioid enhances HIV infection of human macrophages through modulation of beta-chemokines and the CCR5 receptor and that this effect is reversed by naltrexone, a tertiary opioid antagonist. Tertiary opioid antagonists cannot be used in opioid-dependent patients because they precipitate withdrawal or reversal of analgesia. We determined whether the quaternary opioid antagonist methylnaltrexone (MNTX), now in phase III clinical trials for opioid-induced constipation, reverses the opioid-mediated enhancement of HIV infection of macrophages at clinically relevant doses. MNTX completely abrogated morphine-induced HIV Bal strain infection of macrophages. MNTX also inhibited the R5 strain (ADA) envelope-pseudotyped HIV replication induced by morphine. Furthermore, MNTX abolished morphine-mediated up-regulation of CCR5 receptor expression. The ability of MNTX to block opioid-induced CCR5 expression and HIV replication at clinically relevant doses may have additional benefit for opioid abusers with HIV infection, or patients with AIDS pain receiving opioids.     

Aspiration pneumonitis caused by delayed respiratory depression following intrathecal morphine administration

Opioid analgesia is the primary pharmacologic intervention for managing pain. However, opioids can cause various adverse effects including pruritus, nausea, constipation, and sedation. Respiratory depression is the most fatal side effect. Therefore, cautious monitoring of respiratory status must be done after opioid administration. Here, we report a patient who suffered from respiratory depression with deep sedation and aspiration pneumonitis after intrathecal morphine administration.     

Antagonistes opioïdes et constipation

Constipation is a common adverse effect associated with opioid therapy; it is mediated primarily by stimulation of opioid receptor in the gastrointestinal tract. In patients with pain, uncontrolled constipation can add to their discomfort and may serve as a barrier to effective pain management, limiting therapy, or prompting discontinuation. Prolonged-release of naloxone and methylnaltrexone are effective, respectively, for preventing and treating the symptoms of constipation in patients treated with opioids. Early studies confirm that they are effective without entering the central nervous system and affecting analgesia. With a better understanding of the prevalence and pathophysiology of opioid-induced constipation, a new approach to reducing and treating constipation is also possible and will likely improve the quality of life of patients with pain.     

Methylnaltrexon

Chronic pain patients using opioids frequently suffer from constipation which compromises well-being. Such an opioid-induced gastro-intestinal complication can occur regularly in patients in palliative care as well as in analgesic sedated intensive care patients or during prolonged perioperative pain therapy. Discomfort and distress in the affected patients can be so severely pronounced that they would rather suffer from the pain than from the side effect of constipation. Conventional therapy can be insufficient in providing satisfactory relief of constipation, mostly because this opioid-induced bowel hypomotility can be laxative-resistant. Moreover, constipation does not decrease during the course of therapy as do other side effects. It is well known that opioid-induced constipation is mediated via activation of µ-opioid receptors in the gastrointestinal tract. Selective peripheral µ-receptor antagonists (such as methylnaltrexone, Relistor®) can effectively treat opioid-induced constipation. An interference with central analgesia does not occur as the molecules cannot pass the blood-brain barrier due to their charged states. A reduction of opioid therapy or the development of withdrawal symptoms can be avoided. Studies have shown that methylnaltrexone is not only safe and efficient for chronically constipated palliative care patients but offers promising therapeutic options for further patient collectives.     

Management of opioid-induced bowel dysfunction in cancer patients

The gastrointestinal (GI) effects of morphine and other opioids may result in opioid-induced bowel dysfunction (OBD) and the need for treatment. Although OBD is very common in morphine-treated patients, it is usually under-diagnosed. Opioids deliver their GI effect through central and peripheral mechanisms. Laxatives are the pharmaceuticals prescribed most in this area. Prokinetics as well as cholinergic agonists have been used satisfactorily. One-third of patients with OBD have to be treated rectally. The use of opioid antagonists has been favored, but the bioavailability of oral forms is poor. Opioid antagonists with a quaternary structure have a high affinity for peripheral opioid receptors and therefore do not interfere with the analgesia, nor do they generate alkaloid withdrawal syndrome. Opioid rotation is another strategy for maintaining or improving analgesic quality directed toward decreasing the effects of previous opiates on the GI tract.     

Management of common opioid-induced adverse effects

Opioid analgesics are useful agents for treating pain of various etiologies; however, adverse effects are potential limitations to their use. Strategies to minimize adverse effects of opioids include dose reduction, symptomatic management, opioid rotation, and changing the route of administration. Nausea occurs in approximately 25 percent of patients; prophylactic measures may not be required. Patients who do develop nausea will require antiemetic treatment with an antipsychotic, prokinetic agent, or serotonin antagonist. Understanding the mechanism for opioid-induced nausea will aid in the selection of appropriate agents. Constipation is considered an expected side effect with chronic opioid use. Physicians should minimize the development of constipation using prophylactic measures. Monotherapy with stool softeners often is not effective; a stool softener combined with a stimulant laxative is preferred. Sedation and cognitive changes occur with initiation of therapy or dose escalation. Underlying disease states or other centrally acting medications often will compound the opioid's adverse effects. Minimizing unnecessary medications and judicious use of stimulants and antipsychotics are used to manage the central nervous system side effects. Pruritus may develop, but it is generally not considered an allergic reaction. Antihistamines are the preferred management option should pharmacotherapy treatment be required.     

Adjuvant ketamine analgesia for the management of cancer pain

OBJECTIVE: To review the clinical literature evaluating the utilization of intravenous ketamine for the management of cancer-related pain, to summarize the data that suggest ketamine is an appropriate adjuvant method of providing analgesia and to report a case of successful pain management using ketamine in a patient with recurrent testicular cancer at our institution. DATA SOURCES: Primary literature was identified through a MEDLINE search (1966-March 2002), and additional information was obtained through secondary and tertiary sources. DATA SYNTHESIS: The available data suggest that supplementation of morphine with ketamine improves analgesia in patients with cancer, and also provides insight to the controversy regarding the efficacy and adverse effects of various ketamine doses. At subanesthetic doses, ketamine may be beneficial at reducing opioid requirements and related adverse effects. CASE SUMMARY: A 34-year-old white man with recurrent testicular cancer was admitted with radiating neuropathic pain of the legs and lower back. The patient was suspected to also be experiencing opioid adverse effects; therefore, alternative analgesic options were warranted. Ketamine was successful in reducing patient-reported pain and was also well tolerated. CONCLUSIONS: Ketamine is an adjuvant analgesic for the treatment of cancer-related pain when other agents either fail or are intolerable. Accordingly, there are several factors that may prevent adequate pain control with opioid use; therefore, alternative analgesic options should be considered. Promise exists for ketamine as a contemporary analgesic in the appropriate patient.     

The involvement of the mu-opioid receptor in gastrointestinal pathophysiology: therapeutic opportunities for antagonism at this receptor

The localization of opioid receptors and their endogenous peptide ligands within the gastrointestinal (GI) tract and their role in the coordination of propulsion and secretion underscores the importance of opioid receptors in the maintenance of GI homeostasis. The peripherally acting micro-opioid receptor antagonists alvimopan and methylnaltrexone (MNTX) are currently under investigation as therapeutic agents to treat the deleterious GI side effects associated with opioid administration. These compounds have demonstrated efficacy in numerous animal models of GI function, and clinical studies have revealed their efficacy in the treatment of postoperative ileus (POI) and opioid-induced bowel dysfunction. Preservation of opioid-mediated analgesia has been demonstrated for these compounds in both the preclinical and clinical settings. Future studies exploring the benefits of selective antagonism of the peripheral mu-opioid receptor in the treatment of other GI conditions may open new therapeutic opportunities for alvimopan and MNTX.     

Management of opioid side effects in cancer-related and chronic noncancer pain: a systematic review.

Side effects can limit opioid dosage and reduce quality of life. The purpose of this systematic review was to assess the management of opioid side effects in the context of cancer pain management or, in the event that no evidence was available for cancer pain, for chronic noncancer pain. The side effects studied were constipation, pruritus, nausea and vomiting, myoclonus, sedation, respiratory depression, and delirium. Opioid rotation to manage side effects was also studied. For each side effect, we searched MEDLINE and the Cochrane Controlled Trials Register and identified 657 possible titles for inclusion. Of these, 67 studies met inclusion criteria for analysis. The lack of well-designed, randomized controlled trials and the heterogeneity of populations and study designs made the drawing of firm conclusions difficult and precluded performance of meta-analysis. The type, strength, and consistency of evidence for available interventions to manage opioid side effects vary from strong (eg, on the use of naloxone to reverse respiratory depression or constipation) to weak (eg, changing from the oral to epidural route of morphine administration to manage sedation). Well-designed trials in the specified populations are required to furnish clinicians with secure evidence on managing opioid side effects successfully.     

Opioid-induced abnormal pain sensitivity

Opioid analgesics are effective for treating many pain conditions. Opioid analgesic tolerance is a pharmacologic phenomenon that could affect the clinical use of opioid analgesics. Recent studies have shown that neural plasticity associated with the development of opioid tolerance may activate a pronociceptive mechanism that could counteract the analgesic effects of opioids. Thus, exposure to opioids could lead to two seemingly unrelated cellular processes (ie, the development of opioid tolerance and opioid-induced pain sensitivity). Their converging effects may be part of the mechanisms leading to the reduced opioid analgesic efficacy in chronic opioid therapy.     

American Society for Pain Management Nursing Guidelines on Monitoring for Opioid-Induced Sedation and Respiratory Depression

As the complexity of analgesic therapies increases, priorities of care must be established to balance aggressive pain management with measures to prevent or minimize adverse events and to ensure high quality and safe care. Opioid analgesia remains the primary pharmacologic intervention for managing pain in hospitalized patients. Unintended advancing sedation and respiratory depression are two of the most serious opioid-related adverse events. Multiple factors, including opioid dosage, route of administration, duration of therapy, patient-specific factors, and desired goals of therapy, can influence the occurrence of these adverse events. Furthermore, there is an urgent need to educate all members of the health care team about the dangers and potential attributes of administration of sedating medications concomitant with opioid analgesia and the importance of initiating rational multimodal analgesic plans to help avoid adverse events. Nurses play an important role in: 1) identifying patients at risk for unintended advancing sedation and respiratory depression from opioid therapy; 2) implementing plans of care to assess and monitor patients; and 3) intervening to prevent the worsening of adverse events. Despite the frequency of opioid-induced sedation, there are no universally accepted guidelines to direct effective and safe assessment and monitoring practices for patients receiving opioid analgesia. Moreover, there is a paucity of information and no consensus about the benefits of technology-supported monitoring, such as pulse oximetry (measuring oxygen saturation) and capnography (measuring end-tidal carbon dioxide), in hospitalized patients receiving opioids for pain therapy. To date, there have not been any randomized clinical trials to establish the value of technologic monitoring in preventing adverse respiratory events. Additionally, the use of technology-supported monitoring is costly, with far-reaching implications for hospital and nursing practices. As a result, there are considerable variations in screening for risk and monitoring practices. All of these factors prompted the American Society for Pain Management Nursing to approve the formation of an expert consensus panel to examine the scientific basis and state of practice for assessment and monitoring practices for adult hospitalized patients receiving opioid analgesics for pain control and to propose recommendations for patient care, education, and systems-level changes that promote quality care and patient safety.     

Development of peripheral opioid antagonists' new insights into opioid effects

The recent approval by the US Food and Drug Administration of 2 medications--methylnaltrexone and alvimopan--introduces a new class of therapeutic entities to clinicians. These peripherally acting mu-opioid receptor antagonists selectively reverse opioid actions mediated by receptors outside the central nervous system, while preserving centrally mediated analgesia. Methylnaltrexone, administered subcutaneously, has been approved in the United States, Europe, and Canada. In the United States, it is indicated for the treatment of opioid-induced constipation in patients with advanced illness (eg, cancer, AIDS) who are receiving palliative care, when response to laxative therapy has not been sufficient. Alvimopan, an orally administered medication, has been approved in the United States to facilitate recovery of gastrointestinal function after bowel resection and primary anastomosis. Clinical and laboratory studies performed during the development of these drugs have indicated that peripheral receptors mediate other opioid effects, including decreased gastric emptying, nausea and vomiting, pruritus, and urinary retention. Laboratory investigations with these compounds suggest that opioids affect fundamental cellular processes through mechanisms that were previously unknown. These mechanisms include modifications of human immunodeficiency virus penetration, tumor angiogenesis, vascular permeability, and bacterial virulence.     

Opioid-induced bowel dysfunction

Opioid-induced bowel dysfunction is a distressing condition that may persist indefinitely in the clinical setting. As we understand more about normal gastrointestinal (GI) physiology, we are also beginning to understand more fully how opioids cause bowel dysfunction. Current therapeutic interventions for opioid-induced bowel dysfunction can be burdensome and sometimes lack efficacy. Systemic opioid antagonists administered orally can induce laxation, but can unpredictably induce systemic or local GI tract withdrawal symptoms. Two new investigational agents, alvimopan and methylnaltrexone, are peripherally acting opioid antagonists that do not cross the blood-brain barrier. Studies to date show promise that these agents may relieve opioid-induced bowel dysfunction in a well-tolerated manner without reversing central analgesia.     

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.     

Cannabinoid analgesia as a potential new therapeutic option in the treatment of chronic pain

OBJECTIVE: To review the literature concerning the physiology of the endocannabinoid system, current drug development of cannabinoid agonists, and current clinical research on the use of cannabinoid agonists for analgesia. DATA SOURCES: Articles were identified through a search of MEDLINE (1966-August 2005) using the key words cannabis, cannabinoid, cannabi*, cannabidiol, nabilone, THC, pain, and analgesia. No search limits were included. Additional references were located through review of the bibliographies of the articles identified. STUDY SELECTION AND DATA EXTRACTION: Studies of cannabinoid agonists for treatment of pain were selected and were not limited by pain type or etiology. Studies or reviews using animal models of pain were also included. Articles that related to the physiology and pharmacology of the endocannabinoid system were evaluated. DATA SYNTHESIS: The discovery of cannabinoid receptors and endogenous ligands for these receptors has led to increased drug development of cannabinoid agonists. New cannabimimetic agents have been associated with fewer systemic adverse effects than delta-9-tetrahydrocannabinol, including recent development of cannabis medicinal extracts for sublingual use (approved in Canada), and have had promising results for analgesia in initial human trials. Several synthetic cannabinoids have also been studied in humans, including 2 cannabinoid agonists available on the international market. CONCLUSIONS: Cannabinoids provide a potential approach to pain management with a novel therapeutic target and mechanism. Chronic pain often requires a polypharmaceutical approach to management, and cannabinoids are a potential addition to the arsenal of treatment options.     

Novel opioid antagonists for opioid-induced bowel dysfunction and postoperative ileus

Methylnaltrexone and alvimopan are two new and potentially useful agents in the management of opioid-induced bowel dysfunction and prevention of postoperative ileus. Both agents have promising prokinetic properties and appear to be capable of reversing the effects of opioids on delayed gastrointestinal transit. This article reviews currently available published literature to provide an overview of the clinical trials and to provide insight for the potential use of these agents for patients requiring opioid based analgesia. These compounds represent a new class of compounds that may impact the therapeutics for opioid induced bowel dysfunction as well as postoperative ileus.     

Improving opioid effectiveness: from ideas to evidence.

Opioid effectiveness can be improved by individualizing dosing, route of administration and the drug. Particularly in the treatment of chronic non-cancer pain, careful patient selection is essential. The current review concentrates on new ideas about improving opioid effectiveness by increasing efficacy or reducing adverse effects by combining other drugs that modulate opioid receptor mediated effects. These pharmacological "oipioid adjuvants" include e.g. alpha(2)-adrenergic agonists, non-steroidal anti-flammatory analgesics, NMDA-receptor antagonists, CCK-antagonists, gabapentinoids and NK-1 receptor antagonists. The theoretical background and the clinical evidence of these combinations will be discussed.