Synergy between mu opioid ligands: evidence for functional interactions among mu opioid receptor subtypes

Esmethadone-HCl (REL-1017): a promising rapid antidepressant

This review article presents select recent studies that form the basis for the development of esmethadone into a potential new drug. Esmethadone is a promising member of the pharmacological class of uncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonists that have shown efficacy for major depressive disorder (MDD) and other diseases and disorders, such as Alzheimer's dementia and pseudobulbar affect. The other drugs in the novel class of NMDAR antagonists with therapeutic uses that are discussed for comparative purposes in this review are esketamine, ketamine, dextromethorphan, and memantine. We present in silico, in vitro, in vivo, and clinical data for esmethadone and other uncompetitive NMDAR antagonists that may advance our understanding of the role of these receptors in neural plasticity in health and disease. The efficacy of NMDAR antagonists as rapid antidepressants may advance our understanding of the neurobiology of MDD and other neuropsychiatric diseases and disorders.

The novel uncompetitive NMDA receptor antagonist esmethadone (REL-1017) has no meaningful abuse potential in recreational drug users

Esmethadone (REL-1017) is the opioid-inactive dextro-isomer of methadone and a low-affinity, low-potency uncompetitive NMDA receptor antagonist. In a Phase 2, randomized, double-blind, placebo-controlled trial, esmethadone showed rapid, robust, and sustained antidepressant effects. Two studies were conducted to evaluate the abuse potential of esmethadone. Each study utilized a randomized, double-blind, active-, and placebo-controlled crossover design to assess esmethadone compared with oxycodone (Oxycodone Study) or ketamine (Ketamine Study) in healthy recreational drug users. Esmethadone 25 mg (proposed therapeutic daily dose), 75 mg (loading dose), and 150 mg (Maximum Tolerated Dose) were evaluated in each study. Positive controls were oral oxycodone 40 mg and intravenous ketamine 0.5 mg/kg infused over 40 min. The Ketamine study included oral dextromethorphan 300 mg as an exploratory comparator. The primary endpoint was maximum effect (Emax) for Drug Liking, assessed using a bipolar 100-point visual analog scale (VAS). A total of 47 and 51 participants completed the Oxycodone Study and the Ketamine Study, respectively (Completer Population). In both studies, esmethadone doses ranging from therapeutic (25 mg) to 6 times therapeutic (150 mg) had a meaningful and statistically significantly (p < 0.001) lower Drug Liking VAS Emax compared with the positive control. Results were consistent for all secondary endpoints in both studies. In both studies, all doses of esmethadone were statistically equivalent to placebo on Drug Liking VAS Emax (p < 0.05). In the Ketamine Study, Drug Liking VAS Emax scores for esmethadone at all tested doses were significantly lower vs. dextromethorphan (p < 0.05) (exploratory endpoint). These studies indicate no meaningful abuse potential for esmethadone at all tested doses.

REL-1017 (esmethadone; D-methadone) does not cause reinforcing effect, physical dependence and withdrawal signs in Sprague Dawley rats

REL-1017 (esmethadone, D-methadone) is the opioid-inactive d-isomer of racemic D,L-methadone. REL-1017 may exert antidepressant effects via uncompetitive N-methyl-D-aspartate receptor (NMDAR) channel block. As REL-1017 is expected to exert central nervous system activity, full characterization of its abuse potential is warranted. We evaluated lack of reinforcing effect, physical dependence, and withdrawal of REL-1017 in Sprague Dawley rats. (1) Self-administration Study Rats were trained to self-administer oxycodone intravenously (IV) and then were subjected to 3-day substitution tests where saline, oxycodone, and REL-1017 were self-delivered IV by a fixed number of lever presses; (2) Drug Discontinuation Study Rats were treated for 30 days by oral gavage with vehicle, REL-1017, ketamine or morphine and evaluated for withdrawal with functional observational batteries (FOBs). In the self-administration study, rats treated with saline, vehicle, and all REL-1017 doses showed the typical "extinction burst" pattern of response, characterized by an initial rapid increase of lever-pressing followed by a rapid decrease over 3 days. Rats treated with oxycodone maintained stable self-injection, as expected for reinforcing stimuli. In the withdrawal study, REL-1017 did not engender either morphine or ketamine withdrawal signs over 9 days following abrupt discontinuation of drug exposure. REL-1017 showed no evidence of abuse potential and did not engender withdrawal symptomatology.

Co-administration of morphine and levamisole increases death risk, produces neutropenia and modifies antinociception in mice

Levamisole is a veterinary anthelmintic drug and a common adulterant of misused drugs. This study analyses the lethal, antinociceptive and haematological effects produced by acute or repeated levamisole administration by itself or combined with morphine. Independent groups of male Swiss Webster mice were i.p. injected with 100 mg/kg morphine, 31.6 mg/kg levamisole (lethal doses at 10%, LD₁₀ ) or the same doses combined. Naloxone pretreatment (10 mg/kg, i.p.) prevented morphine-induced death, as did 2.5 mg/kg, i.p. mecamylamine with levamisole. Co-administration of levamisole and morphine (Lvm + Mor) increased lethality from 10% to 80%. This augmented effect was prevented by 30 mg/kg, i.p. naloxone and reduced with 10 mg/kg naloxone plus 2.5 mg/kg, i.p. mecamylamine. In independent groups of mice, 17.7 mg/kg, i.p. levamisole antagonized the acute morphine's antinociceptive effect evaluated in the tail-flick test. Repeated 17.7 mg/kg levamisole administration (2×/day/3 weeks) did not affect tolerance development to morphine (10 mg/kg, 3×/day/1 week). Blood samples obtained from mice repeatedly treated with levamisole showed leukopenia and neutropenia. Morphine also produced neutropenia, increased erythrocyte count and other related parameters (e.g. haemoglobin). Lvm + Mor had similar effects on leukocyte and neutrophil counts to those seen with levamisole only, but no erythrocyte-related alterations were evident. Blood chemistry analysis did not indicate liver damage but suggested some degree of electrolyte balance impairment. In conclusion, Lvm + Mor increased death risk, altered morphine-induced antinociceptive effects and produced haematologic abnormalities. The importance of studying combinations of drugs of abuse lies in the fact that drug users frequently combine drugs, which are commonly adulterated.

Subanalgesic morphine doses augment fentanyl analgesia by interacting with delta opioid receptors in male rats

Opioids are commonly used for the treatment of postoperative and post-traumatic pain; however, their therapeutic effectiveness is limited by undesirable and life-threatening side effects. Researchers have long attempted to develop opioid co-administration therapies that enhance analgesia, but the complexity of opioid analgesia and our incomplete mechanistic understanding has made this a daunting task. We discovered that subanalgesic morphine doses (100 ng/kg-10 µg/kg) augmented the acute analgesic effect of fentanyl (20 µg/kg) following subcutaneous drug co-administration to male rats. In addition, administration of equivalent drug ratios to naïve rat spinal cord membranes induced a twofold increase in G protein activation. The rate of GTP hydrolysis remained unchanged. We demonstrated that these behavioral and biochemical effects were mediated by the delta opioid receptor (DOP). Subanalgesic doses of the DOP-selective agonist SNC80 also augmented the acute analgesic effect of fentanyl. Furthermore, co-administration of the DOP antagonist naltrindole with both fentanyl-morphine and fentanyl-SNC80 combinations prevented augmentation of both analgesia and G protein activation. The mu opioid receptor (MOP) antagonist cyprodime did not block augmentation. Confocal microscopy of the substantia gelatinosa of rats treated with fentanyl, subanalgesic morphine, or this combination showed that changes in MOP internalization did not account for augmentation effects. Together, these findings suggest that augmentation of fentanyl analgesia by subanalgesic morphine is mediated by increased G protein activation resulting from a synergistic interaction between or heterodimerization of MOPs and DOPs. This finding is of great therapeutic significance because it suggests a strategy for the development of DOP-selective ligands that can enhance the therapeutic index of clinically used MOP drugs.

Positive allosteric modulation of the mu-opioid receptor produces analgesia with reduced side effects

Positive allosteric modulators (PAMs) of the mu-opioid receptor (MOR) have been hypothesized as potentially safer analgesics than traditional opioid drugs. This is based on the idea that PAMs will promote the action of endogenous opioid peptides while preserving their temporal and spatial release patterns and so have an improved therapeutic index. However, this hypothesis has never been tested. Here, we show that a mu-PAM, BMS-986122, enhances the ability of the endogenous opioid Methionine-enkephalin (Met-Enk) to stimulate G protein activity in mouse brain homogenates without activity on its own and to enhance G protein activation to a greater extent than β-arrestin recruitment in Chinese hamster ovary (CHO) cells expressing human mu-opioid receptors. Moreover, BMS-986122 increases the potency of Met-Enk to inhibit GABA release in the periaqueductal gray, an important site for antinociception. We describe in vivo experiments demonstrating that the mu-PAM produces antinociception in mouse models of acute noxious heat pain as well as inflammatory pain. These effects are blocked by MOR antagonists and are consistent with the hypothesis that in vivo mu-PAMs enhance the activity of endogenous opioid peptides. Because BMS-986122 does not bind to the orthosteric site and has no inherent agonist action at endogenously expressed levels of MOR, it produces a reduced level of morphine-like side effects of constipation, reward as measured by conditioned place preference, and respiratory depression. These data provide a rationale for the further exploration of the action and safety of mu-PAMs as an innovative approach to pain management.

How to Use Methadone in an Era of an Opioid Epidemic

OPINION STATEMENT

The opioid epidemic is one of the most important public health crises as opioid-related deaths have become a leading cause of accidental death in the USA. Various efforts have been made to understand how to safely and appropriately prescribe opioids for patients with chronic pain, including those with cancer-related pain. We find the guidelines proposed by the Expert Consensus White Paper on the use of methadone to be current, comprehensive, and practical. While methadone is a complex medication with unique pharmacokinetics and pharmacodynamics, it remains a superior choice for many patients with cancer pain given its cost and applicability in a variety of situations. Methadone should be prescribed in the context of experienced clinicians as well as an interdisciplinary team. At a critical time when preventing opioid-related deaths is a priority, we recommend implementing additional precautions for monitoring including universal screening for risk of non-medical opioid use, education on proper storage and disposal, as well as discussing a plan with patients and caregivers in the case of serious complications such as opioid overdose.

Sex Differences in Neuroplasticity- and Stress-Related Gene Expression and Protein Levels in the Rat Hippocampus Following Oxycodone Conditioned Place Preference

Prescription opioid abuse is a serious public health issue. Recently, we showed that female and male Sprague-Dawley rats acquire conditioned place preference (CPP) to the mu opioid receptor agonist oxycodone. Anatomical analysis of the hippocampus from these rats unveiled sex differences in the opioid system in a way that would support excitation and opiate associative learning processes especially in females. In this study, we examined the expression and protein densities of opioid, plasticity, stress and related kinase and signaling molecules in the hippocampus of female and male rats following oxycodone CPP. Oxycodone CPP females have: a) increases in ARC (activity regulated cytoskeletal-associated protein)-immunoreactivity (ir) in CA3 pyramidal cells; b) decreases in Npy (neuropeptide Y) gene expression in the medial hippocampus but higher numbers of NPY-containing hilar interneurons compared to males; c) increases in Crhr2 (corticotropin releasing factor receptor 2) expression in CA2/3; d) increases in Akt1 (AKT serine/threonine kinase 1) expression in medial hippocampus; and e) decreases in phosphorylated MAPK (mitogen activated protein kinase)-ir in CA1 and dentate gyrus. Oxycodone CPP males have: a) increases in Bdnf (brain derived-neurotrophic factor) expression, which is known to be produced in granule cells, relative to females; b) elevated Mapk1 expression and pMAPK-ir in the dentate hilus which harbors newly generated granule cells; and c) increases in CRHR1-ir in CA3 pyramidal cell soma. These sex-specific changes in plasticity, stress and kinase markers in hippocampal circuitry parallel previously observed sex differences in the opioid system after oxycodone CPP.

Lack of Antinociceptive Cross-Tolerance With Co-Administration of Morphine and Fentanyl Into the Periaqueductal Gray of Male Sprague-Dawley Rats

Tolerance to the antinociceptive effect of mu-opioid receptor agonists, such as morphine and fentanyl, greatly limits their effectiveness for long-term use to treat pain. Clinical studies have shown that combination therapy and opioid rotation can be used to enhance opioid-induced antinociception once tolerance has developed. The mechanism and brain regions involved in these processes are unknown. The purpose of this study was to evaluate the contribution of the ventrolateral periaqueductal gray (vlPAG) to antinociceptive tolerance and cross-tolerance between administration and co-administration of morphine and fentanyl. Tolerance was induced by pretreating rats with morphine or fentanyl or low-dose combination of morphine and fentanyl into the vlPAG followed by an assessment of the cross-tolerance to the other opioid. In addition, tolerance to the combined treatment was assessed. Cross-tolerance did not develop between repeated vlPAG microinjections of morphine and fentanyl. Likewise, there was no evidence of cross-tolerance from morphine or fentanyl to the co-administration of morphine and fentanyl. Co-administration did not cause cross-tolerance to fentanyl. Cross-tolerance was only evident to morphine or morphine and fentanyl combined in rats pretreated with co-administration of low doses of morphine and fentanyl. This finding is consistent with the functionally selective signaling that has been reported for antinociception and tolerance after morphine and fentanyl binding to the mu-opioid receptor. This research supports the notion that combination therapy and opioid rotation may be useful clinical practices to decrease opioid tolerance and other side effects. PERSPECTIVE: This preclinical study shows that there is a decrease in cross-tolerance between morphine and fentanyl within the periaqueductal gray, which is a key brain region in opioid antinociception and tolerance.

Chronic immobilization stress primes the hippocampal opioid system for oxycodone-associated learning in female but not male rats

In adult female, but not male, Sprague Dawley rats, chronic immobilization stress (CIS) increases mossy fiber (MF) Leu-Enkephalin levels and redistributes delta- and mu-opioid receptors (DORs and MORs) in hippocampal CA3 pyramidal cells and GABAergic interneurons to promote excitation and learning processes following subsequent opioid exposure. Here, we demonstrate that CIS females, but not males, acquire conditioned place preference (CPP) to oxycodone and that CIS "primes" the hippocampal opioid system in females for oxycodone-associated learning. In CA3b, oxycodone-injected (Oxy) CIS females relative to saline-injected (Sal) CIS females exhibited an increase in the cytoplasmic and total densities of DORs in pyramidal cell dendrites so that they were similar to Sal- and Oxy-CIS males. Consistent with our earlier studies, Sal- and Oxy-CIS females but not CIS males had elevated DOR densities in MF-CA3 dendritic spines, which we have previously shown are important for opioid-mediated long-term potentiation. In the dentate gyrus, Oxy-CIS females had more DOR-labeled interneurons than Sal-CIS females. Moreover, Sal- and Oxy-CIS females compared to both groups of CIS males had elevated levels of DORs and MORs in GABAergic interneuron dendrites, suggesting capacity for greater synthesis or storage of these receptors in circuits important for opioid-mediated disinhibition. However, more plasmalemmal MORs were on large parvalbumin-containing dendrites of Oxy-CIS males compared to Sal-CIS males, suggesting a limited ability for increased granule cell disinhibition. These results suggest that low levels of DORs in MF-CA3 synapses and hilar GABAergic interneurons may contribute to the attenuation of oxycodone CPP in males exposed to CIS.

Sex Differences in the Rat Hippocampal Opioid System After Oxycodone Conditioned Place Preference

Although opioid addiction has risen dramatically, the role of gender in addiction has been difficult to elucidate. We previously found sex-dependent differences in the hippocampal opioid system of Sprague-Dawley rats that may promote associative learning relevant to drug abuse. The present studies show that although female and male rats acquired conditioned place preference (CPP) to the mu-opioid receptor (MOR) agonist oxycodone (3 mg/kg, I.P.), hippocampal opioid circuits were differentially altered. In CA3, Leu-Enkephalin-containing mossy fibers had elevated levels in oxycodone CPP (Oxy) males comparable to those in females and sprouted in Oxy-females, suggesting different mechanisms for enhancing opioid sensitivity. Electron microscopy revealed that in Oxy-males delta opioid receptors (DORs) redistributed to mossy fiber-CA3 synapses in a manner resembling females that we previously showed is important for opioid-mediated long-term potentiation. Moreover, in Oxy-females DORs redistributed to CA3 pyramidal cell spines, suggesting the potential for enhanced plasticity processes. In Saline-injected (Sal) females, dentate hilar parvalbumin-containing basket interneuron dendrites had fewer MORs, however plasmalemmal and total MORs increased in Oxy-females. In dentate hilar GABAergic dendrites that contain neuropeptide Y, Sal-females compared to Sal-males had higher plasmalemmal DORs, and near-plasmalemmal DORs increased in Oxy-females. This redistribution of MORs and DORs within hilar interneurons in Oxy-females would potentially enhance disinhibition of granule cells via two different circuits. Together, these results indicate that oxycodone CPP induces sex-dependent redistributions of opioid receptors in hippocampal circuits in a manner facilitating opioid-associative learning processes and may help explain the increased susceptibility of females to opioid addiction acquisition and relapse.

Efficacy and safety of dual opioid therapy

There is some evidence for a partial opioid switching or an 'add on' approach to opioid dosing strategies. Preclinical and clinical findings suggest different activation profiles for the stimulation of the mu subtypes, raising the questions about what might occur with combinations of these substances. In the postoperative setting, it seems that the analgesic effect of the combination at equivalent doses is similar to that produced by the individual components, not adding particular advantages. However, adverse effects seem to be reduced with the combination of morphine/oxycodone, when given in doses equianalgesic to individual opioids. The reduction of opioid-induced postoperative adverse effects may have important clinical implications, given that adverse effects may prolong length of stay and hospitalization costs. Thus, in the acute postoperative setting, a reduction of adverse effects may be expected. In chronic pain, information is still in the infancy, but opioid combination therapy may have greater advantages in improving the opioid response. The possibility to clinically translate opioid combinations into practice, as demonstrated in some animal models, depends on a broad number of factors implicated in the pain process. More research is needed to better elucidate these issues in the near future.

Coadministration of intravenous remifentanil and morphine for post-thoracotomy pain: comparison with intravenous morphine alone

OBJECTIVES

In this double-blind, randomized study, the authors compared the effects of a patient-controlled remifentanil and morphine combination with morphine alone on post-thoracotomy pain, analgesic consumption, and side effects.

DESIGN

A prospective, randomized, double-blind clinical study.

SETTING

University hospital.

PARTICIPANTS

Volunteer patients at a university hospital undergoing elective thoracotomy surgery.

INTERVENTIONS

Patients were allocated randomly into 2 groups to receive patient-controlled analgesia: the morphine (M) group or the morphine plus remifentanil (MR) group. Pain, discomfort, sedation scores, cumulative patient-controlled morphine consumption, rescue analgesic (meperidine) requirement and side effects were recorded for 24 hours.

MEASUREMENTS AND MAIN RESULTS

Sixty patients were allocated randomly to receive intravenous patient-controlled analgesia with morphine alone (M) or morphine plus remifentanil (MR) in a double-blind manner. Patients were allowed to use bolus doses of morphine (0.02 mg/kg) or the same dose of a morphine plus remifentanil (0.2 µg/kg) mixture every 10 minutes without a background infusion. VAS scores were lower in the MR group than in the M group at 30 minutes (p = 0.04), 1 hour (p = 0.03), and 2 hours (p = 0.04). Mean cumulative doses of morphine were not significantly different at 27.8±15 mg for the M group and 21.9±10.5 mg for the MR group. Significantly more patients needed meperidine in the M group (p = 0.039); these also experienced more nausea (p = 0.01).

CONCLUSIONS

Coadministration of PCA remifentanil with morphine for the treatment of post-thoracotomy pain did not reduce morphine consumption but provided superior analgesia, less use of rescue analgesics, and fewer side effects compared to morphine alone.

Mu opioids and their receptors: evolution of a concept

Opiates are among the oldest medications available to manage a number of medical problems. Although pain is the current focus, early use initially focused upon the treatment of dysentery. Opium contains high concentrations of both morphine and codeine, along with thebaine, which is used in the synthesis of a number of semisynthetic opioid analgesics. Thus, it is not surprising that new agents were initially based upon the morphine scaffold. The concept of multiple opioid receptors was first suggested almost 50 years ago (Martin, 1967), opening the possibility of new classes of drugs, but the morphine-like agents have remained the mainstay in the medical management of pain. Termed mu, our understanding of these morphine-like agents and their receptors has undergone an evolution in thinking over the past 35 years. Early pharmacological studies identified three major classes of receptors, helped by the discovery of endogenous opioid peptides and receptor subtypes-primarily through the synthesis of novel agents. These chemical biologic approaches were then eclipsed by the molecular biology revolution, which now reveals a complexity of the morphine-like agents and their receptors that had not been previously appreciated.

A phase 3, randomized, double-blind comparison of analgesic efficacy and tolerability of Q8003 vs oxycodone or morphine for moderate-to-severe postoperative pain following bunionectomy surgery

Objective

Compare the efficacy and tolerability of the dual-opioid, Q8003® (morphine/oxycodone combination) 12 mg/8 mg to morphine 12 mg or oxycodone 8 mg in subjects following bunionectomy surgery.

Design

This was a randomized, double-blind study.

Setting

Hospitalized patients.

Patients

Healthy men or women aged ≥18 years with moderate or severe pain (score ≥2 on a 4-point Likert scale) and ≥4 on the 11-point numerical pain rating scale following surgery.

Interventions

Study medication was initiated after surgery and was given for 48 hours.

Outcomes

The primary efficacy variable was mean sum of the pain intensity difference (SPID) scores from the postsurgical baseline.

Results

Five hundred twenty-two subjects were randomized; 31 (5.9%) discontinued, including 19 (3.6%) for adverse events. The mean total morphine equivalent dose (MED) was 182.7 mg from Q8003 12 mg/8 mg, 92.4 mg for morphine 12 mg, and 92.1 mg for oxycodone 8 mg. SPID from baseline over 24 hours and SPID from baseline over 48 hours were significantly (P < 0.02) higher for Q8003 12 mg/8 mg vs morphine 12 mg or oxycodone 8 mg. Significantly (P < 0.015) fewer subjects in the Q8003 group required ibuprofen rescue medication, used lower doses of rescue medication, and had a longer median time to first use of rescue medication. Oxygen desaturation <90% occurred in 5.3% with Q8003, 2.8% with morphine 12 mg, and 2.3% with oxycodone 8 mg, and the cumulative median dose at first desaturation was twofold greater with Q8003.

Conclusion

Q8003 provided superior efficacy to its individual components at twice the MED with only a modest increase in the incidence of adverse events.

Methadone-induced hypoglycemia

To determine if recent observations of hypoglycemia in patients receiving high-dose methadone extended to an animal model, we explored the effects of methadone and other mu-opioids on blood glucose levels in mice. Methadone lowered blood glucose in a dose-dependent manner with 20 mg/kg yielding a nadir in average glucose levels to 55 ± 6 mg/dL from a baseline of 172 ± 7 mg/dL, an effect that was antagonized by naloxone and mu selective antagonists β-funaltrexamine and naloxonazine. The effect was stereoselective and limited to only the l-isomer, while the d-isomer was ineffective. Despite the robust decrease in blood glucose produced by methadone, a series of other mu-opioids, including morphine, fentanyl, levorphanol, oxycodone or morphine-6β-glucuronide failed to lower blood glucose levels. Similar differences among mu-opioid agonists have been observed in other systems, suggesting the possible role of selected splice variants of the mu-opioid receptor gene Oprm1. This mouse model recapitulates our clinical observations and emphasizes the need to carefully monitor glucose levels when using high methadone doses, particularly intravenously, and the need for controlled clinical trials.

Does the pharmacology of oxycodone justify its increasing use as an analgesic?

Oxycodone is a semisynthetic opioid analgesic that is increasingly used for the treatment of acute, cancer, and chronic non-malignant pain. Oxycodone was synthesized in 1917 but its pharmacological properties were not thoroughly studied until recently. Oxycodone is a fairly selective μ-opioid receptor agonist, but there is a striking discrepancy between the relatively low binding potential and G protein activation by oxycodone and its analgesic efficacy. It has been claimed that this is because of active metabolites and enhanced passage to the central nervous system by active transport. We critically review studies on the basic pharmacology of oxycodone and on its pharmacokinetics and pharmacodynamics in humans. In particular, the role of pharmacogenomics and population pharmacokinetics in understanding the properties of oxycodone is discussed in detail. We compare oxycodone with morphine, the standard opioid in clinical use.

Preclinical pharmacology and opioid combinations

Although effective alone, opioids are often used in combination with other drugs for relief of moderate to severe pain. Guidelines for acute perioperative pain recommend the use of multimodal therapy for pain management, although combinations of opioids are not specifically recommended. Mu opioid drugs include morphine, heroin, fentanyl, methadone, and morphine 6β-glucuronide (M6G). Their mechanism of action is complex, resulting in subtle pharmacological differences among them and with unpredictable differences in their potency, effectiveness, and tolerability among patients. Highly selective mu opioids do not bind to a single receptor. Rather, they interact with a large number of mu receptor subtypes with different activation profiles for the various drugs. Thus, mu-receptor-based drugs are not all the same and it may be possible to utilize these differences for enhanced pain control in a clinical setting. These differences among the drugs raise the question of whether combinations might result in better pain relief with fewer side effects. This concept has already been demonstrated between two mu opioids in preclinical studies and clinical trials on other combinations are ongoing. This article reviews the current state of knowledge about mu opioid receptor pharmacology, summarizes preclinical evidence for synergy from opioid combinations, and highlights the complex nature of the mu opioid receptor pharmacology.

Synergism between fentanyl and tramadol in tonic inflammatory pain: the orofacial formalin test

Opioids have been used for long time to management of pain, the coadministration of two opioids may induce synergism. The present study was conducted to determine the antinociceptive interaction between the dual mechanism of action of tramadol compared to the main of fentanyl antinociception in the orofacial formalin which represents a model of persistent cutaneous nociception in the region innervated by the trigeminal nerve. The i.p. administration of tramadol and fentanyl induced a dose-dependent antinociception with an ED(50) of 2.97 ± 0.32 mg/kg for phase I and 1.79 ± 0.30 mg/kg for phase II and 0.062 ± 0.0040 mg/kg in phase I and 0.041 ± 0.0039 mg/kg in phase II, respectively. The coadministration of fentanyl with tramadol induced synergism in both phases of the test with an interaction index of 0.343 and 0.163 for phase I and phase II, respectively. This finding could be explained by the more complex pharmacology of tramadol compared to fentanyl.

Acute Pain Management in Opioid-Tolerant Patients: A Growing Challenge

In Australia and New Zealand, in parallel with other developed countries, the number of patients prescribed opioids on a long-term basis has grown rapidly over the last decade. The burden of chronic pain is more widely recognised and there has been an increase in the use of opioids for both cancer and non-cancer indications. While the prevalence of illicit opioid use has remained relatively stable, the diversion and abuse of prescription opioids has escalated, as has the number of individuals receiving methadone or buprenorphine pharmacotherapy for opioid addiction. As a result, the proportion of opioid-tolerant patients requiring acute pain management has increased, often presenting clinicians with greater challenges than those faced when treating the opioid-naïve.

Treatment aims include effective relief of acute pain, prevention of drug withdrawal, assistance with any related social, psychiatric and behavioural issues, and ensuring continuity of long-term care. Pharmacological approaches incorporate the continuation of usual medications (or equivalent), short-term use of sometimes much higher than average doses of additional opioid, and prescription of non-opioid and adjuvant drugs, aiming to improve pain relief and attenuate opioid tolerance and/or opioid-induced hyperalgesia. Discharge planning should commence at an early stage and may involve the use of a ‘Reverse Pain Ladder’ aiming to limit duration of additional opioid use. Legislative requirements may restrict which drugs can be prescribed at the time of hospital discharge. At all stages, there should be appropriate and regular consultation and liaison with the patient, other treating teams and specialist services.

Role of oxycodone and oxycodone/naloxone in cancer pain management

Oxycodone is a valued opioid analgesic, which may be administered either as the first strong opioid or when other strong opioids are ineffective. In case of insufficient analgesia and/or intense adverse effects such as sedation, hallucinations and nausea/vomiting a switch from another opioid to oxycodone might be beneficial. Oxycodone is administered to opioid-naive patients with severe pain and to patients who were unsuccessfully treated with weak opioids, namely tramadol, codeine and dihydrocodeine. Oxycodone effective analgesia may be attributed to its affinity to μ and possibly κ opioid receptors, rapid penetration through the blood-brain barrier and higher concentrations in brain than in plasma. Oxycodone displays high bioavailability after oral administration and may be better than morphine in patients with renal impairment due to the decreased production of active metabolites. Recently an oral controlled-release oxycodone formulation was introduced in Poland. Another new product that was launched recently is a combination of prolonged-release oxycodone with prolonged-release naloxone (oxycodone/naloxone tablets). The aim of this review is to outline the pharmacodynamic and pharmacokinetic properties, drug interactions, dosing rules, adverse effects, equianalgesic dose ratio with other opioids and clinical studies of oxycodone in patients with cancer pain. The potential role of oxycodone/naloxone in chronic pain management and its impact on the bowel function is also discussed.

Oxycodone combinations for pain relief

No single analgesic drug provides the perfect therapeutic/adverse effect profile for every pain condition. In addition to convenience and possibly improved compliance, a combination of analgesic drugs offers the potential, requiring verification, of providing greater pain relief and/or reduced adverse effects than the constituent drugs when used individually. We review here analgesic combinations containing oxycodone. We found surprisingly little preclinical information about the analgesic or adverse effect profiles of the combinations (with acetaminophen, paracetamol, nonsteroidal anti-inflammatory drugs, morphine, gabapentin or pregabalin). Clinical experience and studies suggest that the combinations are safe and effective and may offer certain advantages. As with all combinations, the profile of adverse effects must also be determined in order to provide the clinician with the overall benefit/risk assessment.

Topical methadone and meperidine analgesic synergy in the mouse

Topical analgesics have many potential advantages over systemic administration. Prior work has shown potent analgesic activity of a number of topical opioids in the radiant heat tail-flick assay. The current study confirms the analgesic activity of morphine and extends it to two other mu opioids, methadone and meperidine. Combinations of topical morphine and lidocaine are synergistic. Similarly, the combination of methadone and lidocaine is synergistic. While there appeared to be some potentiation with the combination of meperidine and lidocaine, it did not achieve significance. Systemically, prior studies have shown that co-administration of morphine and methadone was synergistic. The combination of morphine and methadone was also synergistic when given topically. In contrast, the combination of morphine and meperidine was not synergistic systemically and it was not synergistic topically. Thus, the pharmacology of topical opioids mimics that seen with systemic administration. Their activity in the topical model supports their potential utility while the local limitation of their actions offers the possibility of a reduced side-effect profile.

Synaptic mechanism for functional synergism between delta- and mu-opioid receptors

By sustained activation of mu-opioid receptors (MORs), chronic opioids cause analgesic tolerance, physical dependence, and opioid addiction, common clinical problems for which an effective treatment is still lacking. Chronic opioids recruit delta-opioid receptors (DORs) to plasma membrane through exocytotic trafficking, but the role of this new DOR and its interaction with existing MOR in brain functions and in these clinical problems remain largely unknown. In this study, we investigated the mechanisms underlying synaptic and behavioral actions of chronic morphine-induced DORs and their interaction with MORs in nucleus raphe magnus (NRM) neurons important for opioid analgesia. We found that the emerged DOR inhibited GABAergic IPSCs through both the phospholipase A(2) (PLA(2)) and cAMP/protein kinase A (PKA) signaling pathways. MOR inhibition of IPSCs, normally mediated predominantly by the PLA(2) pathway, was additionally mediated by the cAMP/PKA pathway, with MOR potency significantly increased after chronic morphine treatment. Isobologram analysis revealed a synergistic DOR-MOR interaction in their IPSC inhibition, which was dependent on upregulated activities of both the PLA(2) and cAMP/PKA pathways. Furthermore, DOR and MOR agonists microinjected into the NRM in vivo also produced a PLA(2)-dependent synergism in their antinociceptive effects. These findings suggest that the cAMP/PKA pathway, upregulated by chronic opioids, becomes more important in the mechanisms of both MOR and DOR inhibition of GABA synaptic transmission after chronic opioid exposure, and DORs and MORs are synergic both synaptically and behaviorally in producing analgesic effects in a PLA(2)-dependent fashion, supporting the potential therapeutic use of DOR agonists in pain management under chronic opioid conditions.

The determination and application of fixed-dose analgesic combinations for treating multimodal pain

When the pathophysiology of a medical condition is multimodal, ie, related to multiple physiological causes or mediated by multiple pathways, the optimal strategy can be to use a drug or a combination of drugs that contribute multiple mechanisms to the therapeutic endpoint. In such situations, a rational multimodal approach can also result in the fewest adverse effects. We discuss the quantitative analysis of multimodal action using the treatment of pain as a practical example and give examples of its application to some widely used analgesic drugs.

PERSPECTIVE

This article reviews the medical relevance of the quantitative evaluation of drug combinations, using pain and combinations of analgesics as specific examples. Such measure can help clinicians who seek to maximize therapeutic effect while simultaneously minimizing adverse effects.

Molecular insights into mu opioid pharmacology: From the clinic to the bench

Most of the opioids used in clinical practice exert their effects through mu opioid receptors. Yet, subtle but important pharmacological differences have been observed among the mu opioids. Their potency, effectiveness, and adverse effects can vary unpredictably among patients. These clinical differences among the mu opioids strongly argue against a single receptor mediating their actions. The cloning of the mu opioid receptor has greatly enhanced our understanding of the complexity of this system and has provided possible mechanisms to explain these observations. A single mu opioid receptor gene has been identified, but we now know that it generates a multitude of different mu opioid receptor subtypes through a mechanism commonly used to enhance protein diversity, alternative splicing. Early studies identified a number of splice variants involving the tip of the C-terminus. This region of the receptor is far away from the binding pocket, explaining why these variants still exhibit the same selectivity for mu opioids. However, the differences in structure at the C-terminus influence the activation patterns of the mu opioids. In addition, a second series of variants has been isolated that involves alternative splicing at the N-terminus. Together, these sets of mu opioid receptor splice variants may help explain the clinical variability of the mu drugs among patients and provide insights into why it is so important to individualize therapy for every patient in pain.

Opioid switching and rotation in primary care: implementation and clinical utility

BACKGROUND

Opioid therapy is the standard treatment for moderate-to-severe cancer pain and is becoming a more frequent treatment for moderate-to-severe chronic noncancer pain. Response to opioids varies significantly between patients and even within the individual patient at different stages of treatment. Finding an opioid at a dose that provides adequate long-term analgesia with minimal adverse effects can be difficult. Opioid switching and opioid rotation, at different stages of therapy, represent two clinical strategies used to optimize opioid response for patients with moderate-to-severe pain.

OBJECTIVES

Review the theoretical and clinical evidence supporting the concepts of opioid switching and rotation, outline the conditions under which these practices should be considered, and briefly suggest practical steps for their implementation.

SCOPE

Clinical literature, clinical practice and guideline databases, and professional society websites were searched for articles or reports describing opioid switching or opioid rotation in chronic pain therapy; variability in patient response to opioid therapy; physiologic, pharmacologic, and genetic factors that affect clinical response to opioids; and practical approaches to maximizing analgesia and minimizing adverse effects in opioid therapy. It is outside the scope of this review to evaluate the pharmacoeconomic aspects that affect changes in opioid therapy.

FINDINGS

The variability in de novo clinical response to opioids likely represents the interaction of the varying properties of the individual opioids with the variability in individual patient biology. This interaction forms the rationale for opioid switching and explains its clinical utility. As with opioid switching, success with opioid rotation is related to the myriad of factors determining an individual patient's response to a specific opioid. However, the benefits of opioid rotation also derive from a partial reversal of tolerance at the mu-opioid receptor and the response of different micro-opioid receptor subtypes to the different opioids.

Involvement of exon 11-associated variants of the mu opioid receptor MOR-1 in heroin, but not morphine, actions

Heroin remains a major drug of abuse and is preferred by addicts over morphine. Like morphine, heroin has high affinity and selectivity for mu-receptors, but its residual analgesia in exon 1 MOR-1 knockout mice that do not respond to morphine suggests a different mechanism of action. MOR-1 splice variants lacking exon 1 have been observed in mice, humans, and rats, raising the possibility that they might be responsible for the residual heroin and morphine-6beta-glucuronide (M6G) analgesia in the exon 1 knockout mice. To test this possibility, we disrupted exon 11 of MOR-1, which eliminates all of the variants that do not contain exon 1. Morphine and methadone analgesia in the exon 11 knockout mouse was normal, but the analgesic actions of heroin, M6G, and fentanyl were markedly diminished in the radiant heat tail-flick and hot-plate assays. Similarly, the ability of M6G to inhibit gastrointestinal transit was greatly diminished in these exon 11 knockout mice, whereas the ability of morphine was unchanged. These findings identify receptors selectively involved with heroin and M6G actions and confirm the relevance of the exon 11-associated variants and raise important issues regarding the importance of atypical truncated G-protein-coupled receptors.

The role of methadone in opioid rotation-a Polish experience

BACKGROUND

To assess methadone analgesia, adverse effects, and calculation method of equianalgesic doses with oral morphine.

MATERIALS AND METHODS

Methadone was administered to 21 opioid-tolerant cancer patients because of pain (numerical rating scale [NRS] > 5) on morphine (ten patients), transdermal fentanyl (TF; four patients), morphine, ketamine, and TF (one patient), tramadol (one patient), pethidine (one patient), pain with drowsiness on morphine with ketamine (three patients), and pain with nausea on morphine (one patient). Dose ratios of equivalent daily dose of oral morphine (ddom) to daily dose of oral methadone (ddomet) were 4:1 (ddom to 100 mg), 6:1 (101-300 mg), 12:1 (301-1,000 mg), and 20:1 (over 1,000 mg). Previous opioid treatment was stopped completely (stop-start approach) in 19 patients; two received methadone and other opioids. The mean ddom before switch was 812 +/- 486 mg. Methadone was administered regularly three times daily; 20 patients received oral methadone, one patient received rectal suppositories. Breakthrough pain was treated with methadone (half of regular dose), morphine, fentanyl, metamizol, ketoprofen, or ketamine.

RESULTS

Mean time of methadone treatment was 38.3 +/- 27.1 days (range 3-95 days), mean daily doses: start 48.1 +/- 19.7 mg, maximal 148.5 +/- 104.1 mg, treatment completion 131.1 +/- 104.3 mg. Good analgesia (NRS < 4) was observed in 11 patients, partial (NRS 4-5) in nine patients, and unsatisfactory (NRS > 5) in one patient. Adverse effects such as drowsiness (six patients), constipation (six patients), nausea and vomiting (two patients), sweating (two patients), and respiratory depression (one patient) the last one resolved by methadone cessation and naloxone.

CONCLUSIONS

Results confirmed high analgesic efficacy, acceptable methadone adverse event profile, safety, and effectiveness of ddom to ddomet dose calculation method.

The successful use of parenteral methadone in a patient with a prolonged QTc interval

Recent case reports have raised concerns about the potential for methadone to prolong the QTc interval (QT corrected for heart rate) and predispose patients to torsade de pointes (TdP), a life-threatening arrhythmia. We present a case report that describes the successful use of parenteral and oral methadone in a patient with uncontrolled cancer pain and a history of QTc prolongation. We describe an approach to the use of methadone in this patient and review both case reports and recent prospective studies that have evaluated the risk of TdP and the long-term outcome with respect to the development of TdP in patients receiving methadone for chronic pain or addiction.

Interactions between drugs and occupied receptors

This review has 2 parts. Part I deals with isobolographic procedures that are traditionally applied to the joint action of agonists that individually produce overtly similar effects. Special attention is directed to newer computational procedures that apply to agonists with dissimilar concentration-effect curves. These newer procedures are consistent with the isobolographic methods introduced and used by Loewe, however, the present communications provides the needed graphical and mathematical detail. A major aim is distinguishing super and sub-addictive interactions from those that are simply additive. The detection and measurement of an interaction is an important step in exploring drug mechanism and is also important clinically. Part II discusses a new use of isoboles that is applicable to a single drug or chemical whose effect is mediated by 2 or more receptor subtypes. This application produces a metric that characterizes the interaction between the receptor subtypes. The expansion of traditional isobolographic theory to this multi-receptor situation follows from the newer approaches for 2-drug combination analysis in Part I. This topic leads naturally to a re-examination of competitive antagonism and the classic Schild plot. In particular, it is shown here that the Schild plot in the multi-receptor case is not necessarily linear with unit slope. Both parts of this review emphasize the quantitative aspects rather than the many drugs that have been analyzed with isobolographic methods. The mathematical exposition is rather elementary and is further aided by several graphs. An appendix is included for the reader interested in the mathematical details.

Morphine, Oxycodone, Methadone and Its Enantiomers in Different Models of Nociception in the Rat

We studied the effects of the commonly used mu-opioid receptor agonists morphine, oxycodone, methadone and the enantiomers of methadone in thermal and mechanical models of acute pain and in the spinal nerve ligation model of neuropathic pain in rats. Subcutaneous administration of morphine, oxycodone, and methadone produced a dose-dependent antinociceptive effect in the tail flick, hotplate, and paw pressure tests. l-methadone, racemic methadone, and oxycodone had a similar dose-dependent antinociceptive effect, whereas the dose-response curve of morphine was shallower. In the spinal nerve ligation model of neuropathic pain, subcutaneous administration of morphine, oxycodone, methadone and l-methadone had antiallodynic effects in tests of mechanical and cold allodynia. l-methadone showed the strongest antiallodynic effect of the tested drugs. d-methadone was inactive in all tests. Morphine 5.0 mg/kg, oxycodone 2.5 mg/kg, and l-methadone 1.25 mg/kg decreased spontaneous locomotion 30 min after drug administration. In conclusion, in acute nociception all mu-opioid receptor agonists produced antinociception, with morphine showing the weakest effect. In nerve injury pain, l-methadone showed the greatest antiallodynic potency in both mechanical and cold allodynia compared with the other opioids. Opioids seem to have different profiles in different pain models. l-methadone should be studied for neuropathic pain in humans.

An overview of drug combination analysis with isobolograms

Drugs given in combination may produce effects that are greater than or less than the effect predicted from their individual potencies. The historical basis for predicting the effect of a combination is based on the concept of dose equivalence; i.e., an equally effective dose (a) of one will add to the dose (b) of the other in the combination situation. For drugs with a constant relative potency, this leads to linear additive isoboles (a-b curves of constant effect), whereas a varying potency ratio produces nonlinear additive isoboles. Determination of the additive isobole is a necessary procedure for assessing both synergistic and antagonistic interactions of the combination. This review discusses both variable and constant relative potency situations and provides the mathematical formulas needed to distinguish these cases.

Controversies in pharmacotherapy of pain management

Since the establishment of the WHO three-step ladder for management of cancer pain, several controversies have arisen, which are partly due to new drug development, reformulations of older analgesics, and technological advancements. As a result, clinicians need clarification of several questions. Is morphine the opioid of choice for moderate to severe pain in cancer? Should combinations of opioids be used? When should spinal opioids be used to treat pain in cancer? What are the appropriate opioid doses for breakthrough pain? Should selective cyclo-oxygenase (COX) 2 inhibitors be used? What is the best tactic to treat neuropathic pain, and what first-line adjuvant analgesic should be used? And do bisphosphonates relieve bone pain in cancers other than breast cancer and myeloma? This review addresses these questions.