Treatment of opioid-induced constipation: focus on the peripheral μ-opioid receptor antagonist methylnaltrexone

Mechanism of opioid addiction and its intervention therapy: Focusing on the reward circuitry and mu-opioid receptor

Opioid abuse and addiction have become a global pandemic, posing tremendous health and social burdens. The rewarding effects and the occurrence of withdrawal symptoms are the two mainstays of opioid addiction. Mu-opioid receptors (MORs), a member of opioid receptors, play important roles in opioid addiction, mediating both the rewarding effects of opioids and opioid withdrawal syndrome (OWS). The underlying mechanism of MOR-mediated opioid rewarding effects and withdrawal syndrome is of vital importance to understand the nature of opioid addiction and also provides theoretical basis for targeting MORs to treat drug addiction. In this review, we first briefly introduce the basic concepts of MORs, including their structure, distribution in the nervous system, endogenous ligands, and functional characteristics. We focused on the brain circuitry and molecular mechanism of MORs-mediated opioid reward and withdrawal. The neuroanatomical and functional elements of the neural circuitry of the reward system underlying opioid addiction were thoroughly discussed, and the roles of MOR within the reward circuitry were also elaborated. Furthermore, we interrogated the roles of MORs in OWS, along with the structural basis and molecular adaptions of MORs-mediated withdrawal syndrome. Finally, current treatment strategies for opioid addiction targeting MORs were also presented.

Efficacy and safety of ketamine in mechanically ventilated intensive care unit patients: a scoping review

Objectives: Mechanically ventilated patients account for about one-third of all admissions to the intensive care unit (ICU). Ketamine has been conditionally recommended to aid with analgesia in such patients, with low quality of evidence available to support this recommendation. We aimed to perform a narrative scoping review of the current knowledge of the use of ketamine, with a specific focus on mechanically ventilated ICU patients. Methods: We searched MEDLINE and EMBASE for relevant articles. Bibliographies of retrieved articles were examined for references of potential relevance. We included studies that described the use of ketamine for postoperative and emergency department management of pain and in the critically unwell, mechanically ventilated population. Results: There are few randomised controlled trials evaluating ketamine's utility in the ICU. The evidence is predominantly retrospective and observational in nature and the results are heterogeneous. Available evidence is summarised in a descriptive manner, with a division made between high dose and low dose ketamine. Ketamine's pharmacology and use as an analgesic agent outside of the ICU is briefly discussed, followed by evidence for use in the ICU setting, with particular emphasis on analgesia, sedation and intubation. Finally, data on adverse effects including delirium, coma, haemodynamic adverse effects, raised intracranial pressure, hypersalivation and laryngospasm are presented. Conclusions: Ketamine is used in mechanically ventilated ICU patients with several potentially positive clinical effects. However, it has a significant side effect profile, which may limit its use in these patients. The role of low dose ketamine infusion in mechanically ventilated ICU patients is not well studied and requires investigation in high quality, prospective randomised trials.

Opioid-Related Side Effects and Management

The following areas will be discussed in relation to opioid-related side effects and approaches to their management in the cancer patient.

The efficacy of docusate sodium and senna glycoside for the treatment of constipation after rotator cuff repair: A randomized controlled study

BACKGROUND

Although stool softeners and laxatives are commonly prescribed for postoperative constipation, it is unclear if they are effective during the postoperative period. The data gained from this study will be beneficial for advanced practitioners when examining for postoperative constipation concerns.

PURPOSE

This study aimed to investigate the efficacy of docusate sodium and senna glycoside in the prevention and treatment of constipation following rotator cuff repair (RCR).

METHODS

Patients (n = 107) were randomized to receive docusate sodium, senna glycoside, or nothing (control) in addition to a standardized postoperative protocol. Patients maintained a daily bowel-movement log for postoperative days 0-10. Constipation symptoms and quality of life were assessed preoperatively and at 2 and 6 weeks postoperatively using the Patient Assessment of Constipation Symptoms (PAC-SYM) and Patient Assessment of Constipation Quality of Life (PAC-QOL) questionnaires.

RESULTS

Sixty-seven percent of patients experienced constipation. There was no difference in the prevalence of constipation in the docusate, senna, and control groups (71.4%, 66.7%, and 64.3%, respectively; p = .88). Neither PAC-SYM nor PAC-QOL scores significantly differed between the 3 groups at any time point during 6-week follow-up (p > .05).

IMPLICATIONS FOR PRACTICE

The majority (67%) of patients experience postoperative constipation following RCR. Although docustate sodium and senna glycoside are common first-line agents for the treatment of constipation, they are ineffective during the postoperative period. Providers need to explore other treatment modalities for postoperative constipation pain.

Discovery of Axelopran (TD-1211): A Peripherally Restricted μ-Opioid Receptor Antagonist

The effects of opioids in the central nervous system (CNS) provide significant benefit in the treatment of pain but can also lead to physical dependence and addiction, which has contributed to a growing opioid epidemic in the United States. Gastrointestinal dysfunction is an additional serious consequence of opioid use, and this can be treated with a localized drug distribution of a non-CNS penetrant, peripherally restricted opioid receptor antagonist. Herein, we describe the application of Theravance's multivalent approach to drug discovery coupled with a physicochemical property design strategy by which the N-substituted-endo-3-(8-aza-bicyclo[3.2.1]oct-3-yl)-phenyl carboxamide series of μ-opioid receptor antagonists was optimized to afford the orally absorbed, non-CNS penetrant, Phase 3 ready clinical compound axelopran (TD-1211) 19i as a potential treatment for opioid-induced constipation.

Effects of Methylnaltrexone on Ticagrelor-Induced Antiplatelet Effects in Coronary Artery Disease Patients Treated With Morphine

OBJECTIVES

The aim of this study was to assess if intravenous methylnaltrexone can counteract the effects of morphine on the pharmacokinetic (PK) and pharmacodynamic (PD) profiles of ticagrelor.

BACKGROUND

Morphine delays the onset of action of oral P2Y₁₂ receptor inhibitors, including ticagrelor, by inhibiting gastric emptying and leading to delayed drug absorption. Methylnaltrexone is a peripheral opioid receptor antagonist that has the potential to prevent opioid-induced peripherally mediated side effects (e.g., gastric emptying inhibition) without affecting analgesia.

METHODS

In this prospective, randomized, double-blind, placebo-controlled, crossover study, aspirin-treated patients with stable coronary artery disease (n = 30) were randomized to receive methylnaltrexone (0.3 mg/kg intravenous) or matching placebo. After methylnaltrexone or placebo administration, all patients received morphine (5 mg intravenous). This was followed 15 min later by a 180-mg loading dose of ticagrelor. Patients crossed over to the alternative study treatment after 7 ± 2 days of washout. PK and PD assessments were performed at 12 time points (6 pre- and 6 post-crossover). PK analysis included measurement of plasma levels of ticagrelor and its major active metabolite (AR-C124910XX). PD assessments included VerifyNow P2Y₁₂, light transmittance aggregometry, and vasodilator-stimulated phosphoprotein.

RESULTS

Only marginal changes in plasma levels of ticagrelor (and its major active metabolite) were observed with ticagrelor: maximum plasma concentration and area under the plasma concentration versus time curve from time 0 to the last measurable concentration were 38% and 30% higher, respectively, in patients receiving methylnaltrexone compared with those receiving placebo, but no differences in time to maximum plasma concentration were observed. There were no differences in P2Y₁₂ reaction units by VerifyNow P2Y₁₂ between groups at each time point, including 2 h (the primary endpoint; p = 0.261). Similarly, there were no differences in PD markers assessed by light transmittance aggregometry and vasodilator-stimulated phosphoprotein.

CONCLUSIONS

In patients with coronary artery disease receiving morphine, intravenous administration of the peripheral opioid receptor antagonist methylnaltrexone leads to only marginal changes in plasma levels of ticagrelor and its major metabolite, without affecting levels of platelet reactivity. (Effect of Methylnaltrexone on the PK/PD Profiles of Ticagrelor in Patients Treated With Morphine; NCT02403830).

Methylnaltrexone-Associated Bowel Perforation in Postoperative Opioid-Induced Constipation and Ogilvie Syndrome: A Case Report

Pain management with opioids is often limited by medication side effects. One of the most common and distressing side effects is opioid-induced constipation (OIC), a syndrome that is now getting significant national attention. We report the case of an opioid-dependent 56-year-old man who underwent lumbar decompression for spinal stenosis. Postoperatively, he developed OIC and Ogilvie syndrome, then following treatment with methylnaltrexone experienced an acute bowel perforation. We briefly review the recommended management of OIC as well as indications and contraindications of methylnaltrexone and similar new medications.

Cannabinoid Receptor Type 1 and mu-Opioid Receptor Polymorphisms Are Associated With Cyclic Vomiting Syndrome

OBJECTIVES

Cyclic vomiting syndrome (CVS) is a disorder defined by recurrent, unexplained episodes of severe nausea and vomiting. Our aim was to investigate whether CVS and pathophysiological mechanisms underlying this condition are associated with selected variations in genes encoding the components of the endogenous cannabinoid and opioid systems.

METHODS

This case-control study included 65 patients with CVS-16 male and 49 female, and 1,092 healthy controls-525 male and 567 female from the 1000 Genomes Project. CVS subjects filled out study-specific questionnaires. Single-nucleotide polymorphisms (SNPs) in genes encoding cannabinoid receptors (CNR1 and CNR2), fatty acid amide hydrolase (FAAH) and mu-opioid receptor (OPRM1) were analyzed using the TaqMan SNP genotyping assay. Correlations between SNP's and clinical characteristics of CVS were ascertained.

RESULTS

Our study disclosed an increased risk of CVS among individuals with AG and GG genotypes of CNR1 rs806380 (P<0.01), whereas the CC genotype of CNR1 rs806368 and AG and GG genotypes of OPRM1 rs1799971 were associated with a decreased risk of CVS (P<0.05). In addition, AG and GG genotypes of OPRM1 rs1799971 were correlated with migraine episodes, AG and GG of OPRM1 rs1799971, and CT and CC of CNR1 rs806368 with a family history of migraines (second degree relatives), and CT and CC of CNR1 rs2023239 with a positive response to therapy.

CONCLUSIONS

Our results show for the first time that the variations in CNR1 and OPRM1 genes are associated with CVS and that different genotypes may contribute to the risk of CVS.

Opioids and GI Motility-Friend or Foe?

OPINION STATEMENT

The use of opioids for the treatment of chronic non-cancer pain is growing at an alarming rate. Opioid-induced bowel dysfunction (OBD) is a common adverse effect of long-term opioid treatment manifesting as constipation, nausea, and vomiting. These effects are primarily mediated by peripheral μ-opioid receptors with resultant altered GI motility and function. As a result, patients may present with opioid-induced constipation (OIC), opioid-induced nausea and vomiting (OINV), and/or narcotic bowel syndrome (NBS). This often leads to decreased quality of life and in many cases, discontinuation of opioid therapy. There is limited evidence to support the use of traditional anti-emetics and laxatives in the treatment of OBD. Tapering the dose of opioids, switching to transdermal application, opioid rotation, or dual-action opioids, such as tapentadol, may be helpful in the treatment of OBD. Novel agents, such as peripherally acting μ-opioid receptor antagonists which target the cause of OIC, show promise in the treatment of OBD and should be considered when conventional laxatives fail. This chapter will review the pathophysiology of OBD, including OINV and OIC, and treatment options available.

Opioid-induced constipation in chronic noncancer pain

PURPOSE OF REVIEW

Opioid-based management of noncancer pain has become much more prevalent over the last 2 decades and is responsible for a wide range of side-effects, particularly affecting the intestinal tract causing opioid-induced constipation (OIC). This review will consider results of recent clinical trials that have provided evidence of new pharmacological management options for the treatment of OIC.

RECENT FINDINGS

Supportive use of conventional agents, such as stool softeners, osmotic laxatives, and stimulating laxatives in OIC has limited efficacy. The peripheral μ-opioid receptor antagonist (PAMORA) methylnaltrexone (MNTX) was first FDA approved for OIC in patients with advanced illness and later also for OIC in noncancer pain patients; clinical trial results indicated MNTX did not reverse opioid analgesia and did not trigger central opioid withdrawal. Another PAMORA, the orally available naloxegol, has also gained recent FDA approval for the treatment of OIC in adults with chronic, noncancer pain. Lubiprostone, a bicyclical fatty acid acting via activation of intestinal chloride channel-2 (ClC-2), was also approved for OIC treatment in patients with noncancer pain.

SUMMARY

PAMORA MNTX and naloxegol and the intestinal chloride channel-2 (ClC-2) activator lubiprostone represent additional possible therapeutic options for the management of OIC in patients with chronic noncancer pain.

Management of the lower gastrointestinal system in burn: A comprehensive review

BACKGROUND

Burn produces complex gastrointestinal (GI) responses. Treatment, including large volume fluid resuscitation and opioid analgesia, may exacerbate GI dysfunction. Complications include constipation and opioid-induced bowel dysfunction (OBD), acute colonic pseudo-obstruction (ACPO), bacterial translocation and sepsis, and abdominal compartment syndrome (ACS). Contamination of perineal burns contributes to delayed healing, skin graft failure and sepsis and may impact upon morbidity and mortality. The authors carried out a literature review on management of the lower GI system in burn. This study aimed to explain: current prevention and treatment modalities; drawbacks and complications associated with available treatments, and to provide direction for development of best practice guidelines. ACS is associated with high mortality and should be treated with careful fluid resuscitation and diuresis, to minimise and remove oedema.

METHODS

A comprehensive search of English language literature was performed on PubMed, Medline and Embase. Both MeSH and keywords searches were used.

RESULTS

Evidence available on the management of lower gastrointestinal system in burn is summarised. Levels of evidence available are generally low (level III-IV).

CONCLUSION

Structured, graded interventions are required for prevention and treatment of constipation and OBD. Correction of electrolyte imbalance, adequate enteral intake and mobilisation are pre-requisites. Laxatives should be used according to World Gastroenterology Organisation recommendations. Resistant constipation may respond to changes in medication, but ACPO should be suspected and treated when present. Other complications, such as bacterial translocation and ACS are common in major burns. There is evidence that selective digestive tract decontamination reduces mortality and infectious episodes in major burns. ACS is associated with high mortality and should be treated with careful fluid resuscitation and diuresis. Surgery is reserved for non-responsive and severe cases. Perineal burns present challenges in wound and bowel management. Faecal management systems and negative pressure wound therapy (NPWT) may improve wound control and hygiene, but diversion colostomy will still be beneficial in some cases. There is a clear need for rigorous studies to guide practice more effectively in these challenging conditions.

Emerging therapies for patients with symptoms of opioid-induced bowel dysfunction

Opioid-induced bowel dysfunction (OIBD) comprises gastrointestinal (GI) symptoms, including dry mouth, nausea, vomiting, gastric stasis, bloating, abdominal pain, and opioid-induced constipation, which significantly impair patients’ quality of life and may lead to undertreatment of pain. Traditional laxatives are often prescribed for OIBD symptoms, although they display limited efficacy and exert adverse effects. Other strategies include prokinetics and change of opioids or their administration route. However, these approaches do not address underlying causes of OIBD associated with opioid effects on mostly peripheral opioid receptors located in the GI tract. Targeted management of OIBD comprises purely peripherally acting opioid receptor antagonists and a combination of opioid receptor agonist and antagonist. Methylnaltrexone induces laxation in 50%–60% of patients with advanced diseases and OIBD who do not respond to traditional oral laxatives without inducing opioid withdrawal symptoms with similar response (45%–50%) after an oral administration of naloxegol. A combination of prolonged-release oxycodone with prolonged-release naloxone (OXN) in one tablet (a ratio of 2:1) provides analgesia with limited negative effect on the bowel function, as oxycodone displays high oral bioavailability and naloxone demonstrates local antagonist effect on opioid receptors in the GI tract and is totally inactivated in the liver. OXN in daily doses of up to 80 mg/40 mg provides equally effective analgesia with improved bowel function compared to oxycodone administered alone in patients with chronic non-malignant and cancer-related pain. OIBD is a common complication of long-term opioid therapy and may lead to quality of life deterioration and undertreatment of pain. Thus, a complex assessment and management that addresses underlying causes and patomechanisms of OIBD is recommended. Newer strategies comprise methylnaltrexone or OXN administration in the management of OIBD, and OXN may be also considered as a preventive measure of OIBD development in patients who require opioid administration.

Advances in pharmacotherapy for opioid-induced constipation - a systematic review

INTRODUCTION

Opioid-induced constipation (OIC) is one of the most frequent and burdening adverse events (AE) of opioid therapy. This systematic review aimed to evaluate efficacy and safety of drugs in randomized controlled trials (RCTs) with adult OIC patients.

AREAS COVERED

Efficacy assessment focused on objective outcome measures (OOMs): bowel movement (BM) frequency, BM within 4 h and time to first BM. Twenty-one studies examining seven drugs were identified. Methylnaltrexone showed improvements in all three OOMs. RCTs in naloxone and alvimopan tended to be effective for BM frequency measures. Naloxegol (≥ 12.5 mg) improved all OOMs. Though effectiveness of lubiprostone was demonstrated for all OOMs, group differences were small to moderate. CB-5945 and prucalopride tended to increase BM frequency, especially for 0.1 mg twice daily and 4 mg daily, respectively. Besides nausea and diarrhea, abdominal pain was the most frequent AE for all drugs (risk ratio, range: 1.52 - 5.06) except for alvimopan. Treatment-related serious AEs were slightly higher for alvimopan (cardiac events) and prucalopride (severe abdominal pain, headache). Pain scores for placebo and intervention groups were similar for all drugs.

EXPERT OPINION

Finding a consensus definition and inclusion criteria for OIC plus a rational balance between efficacy and AEs of drugs remain future challenges.

Neurochemical phenotype and function of endomorphin 2-immunopositive neurons in the myenteric plexus of the rat colon

The distribution and activity of endomorphins (EMs), which are endogenous μ-opioid receptor (MOR) ligands in the gastrointestinal tract (GI), are yet to be elucidated. The current study aimed to shed light on this topic. EM2 was expressed in the enteric neurons in the myenteric plexus of the mid-colon. Of the EM2-immunoreactive (EM2-IR) neurons, 53 ± 4.6%, 26 ± 4.5%, 26 ± 2.8% and 49 ± 4.2% displayed immunopositive staining for choline acetyl transferase (ChAT), substance P (SP), vasoactive intestinal peptide (VIP) and nitric oxide synthetase (NOS), respectively. A bath application of EM2 (2 μM) enhanced spontaneous contractile amplitude and tension, which were reversed by β-FNA (an antagonist of MOR) but not NG-nitro-L-arginine methyl ether (L-NAME, a non-selective inhibitor of NOS) or VIP6-28 (an antagonist of the VIP receptor) in the colonic strips. EM2 significantly suppressed inhibitory junction potentials (IJPs) in 14 of the 17 examined circular muscle cells, and this effect was not antagonized by preincubation in L-NAME. EM2 was widely expressed in interneurons and motor neurons in the myenteric plexus and presynaptically inhibited fast IJPs, thereby enhancing spontaneous contraction and tension in the colonic smooth muscle.