Neuropathic pain (NeP) is a complex and debilitating condition that impacts millions of people globally. Although various treatment options exist, their effectiveness is often limited, and they can be accompanied by significant side effects. In recent years, there has been increasing interest in targeting the N-methyl-D-aspartate receptor (NMDAR) as a potential therapeutic approach to alleviate different types of neuropathic pain. This narrative review aims to provide a comprehensive examination of NMDAR antagonists, specifically ketamine, memantine, methadone, amantadine, carbamazepine, valproic acid, phenytoin, dextromethorphan, riluzole, and levorphanol, in the management of NeP. By analyzing and summarizing current preclinical and clinical studies, this review seeks to evaluate the efficacy of these pharmacologic agents in providing adequate relief for NeP.

Introduction: Deaths due to overdose of fentanyls result primarily from depression of respiration. These potent opioids can also produce muscle rigidity in the diaphragm and the chest muscles, a phenomenon known as Wooden Chest Syndrome, which further limits ventilation. Methods: We have compared the depression of ventilation by fentanyl and morphine by directly measuring their ability to induce muscle rigidity using EMG recording from diaphragm and external and internal intercostal muscles, in the rat working heart-brainstem preparation. Results: At equipotent bradypnea-inducing concentrations fentanyl produced a greater increase in expiratory EMG amplitude than morphine in all three muscles examined. In order to understand whether this effect of fentanyl was a unique property of the phenylpiperidine chemical structure, or due to fentanyl's high agonist intrinsic efficacy or its lipophilicity, we compared a variety of agonists with different properties at concentrations that were equipotent at producing bradypnea. We compared carfentanil and alfentanil (phenylpiperidines with relatively high efficacy and high to medium lipophilicity, respectively), norbuprenorphine (orvinolmorphinan with high efficacy and lipophilicity) and levorphanol (morphinan with relatively low efficacy and high lipophilicity). Discussion: We observed that, agonists with higher intrinsic efficacy were more likely to increase expiratory EMG amplitude (i.e., produce chest rigidity) than agonists with lower efficacy. Whereas lipophilicity and chemical structure did not appear to correlate with the ability to induce chest rigidity.

Recently we proposed an isocratic enantioselective high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the separation and quantitative determination of dextro- (DXM) and levo-methorphan (LVM) and their pharmacologically relevant metabolites, dextrorphan and levorphanol, respectively, in human blood samples. This method was based on the polysaccharide-based chiral column Lux AMP, a specialty column characterized with high stability in mobile phases of pH 11.0 and above. The use of a single-source column is a limitation for any analytical method. Therefore, the major goal of the present study was to develop an enantioselective method for the differentiation of dextro- and levo-methorphan, as well as their metabolites dextrorphan and levorphanol, using Lux Cellulose-3 as alternative chiral column with methanol containing 0.1 % diethylamine mobile phase. A newly developed method uses a chiral selector part of HPLC columns available from multiple manufacturers and a fairly common mobile phase. The method was validated and applied to post-mortem blood samples. Out of 50 analyzed new samples, dextromethorphan (DXM) was detected in 17 samples. Of these 17 cases DXM was accompanied with LVM in 7 samples. The proposed analytical method is relatively simple, accurate and fast and can be adopted for routine use in forensic and clinical toxicology laboratories.

In the present work an isocratic enantioselective high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed for the separation and quantitative determination of dextro - and levo -methorphan and their pharmacologically relevant metabolites, dextrorphan and levorphanol, respectively, in human blood samples. The separation of enantiomers of methorphan and metabolites was performed on the polysaccharide-based chiral column Lux AMP in combination with acetonitrile and 5 mM aqueous ammonium bicarbonate pH 11 in the ratio 50:50 (%, v/v) as mobile phase with the flow rate 1 mL/min. The mass spectrometer was operated in scheduled multiple reaction monitoring (MRM) mode, with four transitions for each dextromethorpan, levomethorphan, dextrorphan and dextromethorphan-d3 and two transitions for each levorphanol, levorphanol-d3 and dextrorphan-d3. Application of this method to human post-mortem blood samples confirmed cases of severe overdosing with dextromethorphan, levomethorphan, and less commonly with both.

The term opiate, or more appropriately opioids, is commonly referred to as a class of compounds, such as the alkaloids morphine, codeine, and thebaine, derived from the opium poppy (Papaver somniferum), and known and utilized by humankind for millennia for both analgesia and sedation. This class of substances also includes semi-synthetic compounds such as heroin, oxycodone, hydrocodone, and hydromorphone, obtained from these natural molecules as well as fully synthetic compounds, including fentanyl, pethidine, levorphanol, methadone, tramadol, and dextropropoxyphene. Morphine was first isolated in 1806, followed by the isolation of codeine several years later. Following the development of the hypodermic needle and hollow needle in the 1850s, physicians began to use morphine for various surgical procedures as well for the treatment of chronic pain and postoperative pain.[1] With the discovery of different opioid agonists, antagonists, and partial agonist compounds such as nalorphine, various researchers postulated and later proved that there are multiple stereospecific opioid binding sites in the central nervous system (CNS) through which they exert their physiological effects. Researchers further surmised that these receptors were most likely targets of endogenous opioid compounds, which researchers then began to isolate and study.[2] In 1975, John Hughes and Hans Kosterlitz reported the first evidence of endogenous opioids in brain extracts that they noted were able to inhibit acetylcholine release from nerves in the guinea pig ileum. Further, they indicated that when treated with the opioid receptor antagonist naloxone, the inhibition was blocked. The compounds that they first isolated were termed enkephalins. Structurally, the enkephalins are pentapeptides that are distinguished into two subgroups by their carboxy-terminal amino acids, leucine, or methionine. As a consequence, the enkephalins either classify as met-encephalins and leu-encephalins, respectively: The met-enkephalins present the amino acid sequence Tyr-Gly-Gly-Phe-Met. The leu-enkephalins present the amino acid sequence Tyr-Gly-Gly-Phe-Leu. The enkephalins are one of the three peptide systems that also include beta-endorphins and dynorphins. Of note, the three classes of endogenous opioid peptides share a common N terminus sequence of Tyr-Gly-Gly-Phe and lack a C terminus amide.[3] In molecular terms, Tyr and Phe bind the receptor, and the glycine pair acts as a spacer. These peptides act as neurotransmitters and neuromodulators throughout the nervous system and various end-organ targets.[4] Additionally, research has found that met-enkephalin has an essential role in cell proliferation and tissue organization during development. When discussed in this context, met-enkephalin is often referred to as the opioid growth factor (OGF).[5]

Context: Equianalgesic tools are commonly utilized to guide dose of analgesic therapy, but there is no national consensus on equianalgesic calculations in the United States. Objectives: To propose a summary of current opioid equianalgesic data that include variations and trends among national institutions. Methods: Opioid equianalgesic tools were obtained between May and September 2021. For meperidine, tramadol, codeine, hydrocodone, morphine, oxycodone, oxymorphone, hydromorphone, levorphanol, fentanyl, and tapentadol, details of adjustment for incomplete tolerance, opioid equianalgesic ratios, and formulation types were collected and analyzed. Baseline opioid pharmaco kinetic data were obtained through manufacturer labels on FDA databases, including half-life (T1/2), volume of distribution (Vd), clearance (Cl), area under the curve (AUC), max concentration (Cmax), and time to max concentration (Tmax). Results: Thirty-two institutions' equianalgesic tools were included with each study opioid appearing on an average of 23 institutions' tools. Few tools contained guidance on levorphanol or tapentadol; or included minimum and maximum recommended doses. All tools included guidance on fentanyl, hydromorphone, oxycodone, morphine, and hydrocodone. A minority of tools included guidance on cross-tolerance considerations (n = 12, 37.5%). Oral-tramadol-to-oral-morphine and oral-hydromorphone-to-intravenous (IV)-hydromorphone had the largest variances across equianalgesic tools (6.7 ± 2.8 and 4.06 ± 1.2 mg, respectively). Conclusion: Opioid equianalgesia tools from across the United States demonstrated significant variation in their inclusion of guidance on adjustment for incomplete cross-tolerance, oral-to-IV, and oral-to-oral opioid equianalgesic ratios, and which opioids and formulations were listed. Tramadol and hydromorphone had the most variation in their equianalgesic guidance among the opioids.

Opioids and stimulants are often used in combination for both recreational and non-recreational purposes. High-efficacy mu opioid agonists generally increase the behavioral effects of stimulants, whereas opioid receptor antagonists generally attenuate the behavioral effects of stimulants; however, less is known regarding the interactions between stimulants and opioids possessing low to intermediate efficacy at the mu receptor. The purpose of this study was to examine the role of an opioid's relative efficacy at the mu receptor in altering the behavioral effects of dextro(d-)amphetamine. To this end, opioids possessing a range of relative efficacy at the mu receptor were examined alone and in combination with cumulative doses of d-amphetamine on a test of open-field, locomotor activity in male rats. Levorphanol, buprenorphine, butorphanol, nalbuphine, (-)-pentazocine, (-)-metazocine, (-)-cyclazocine, (-)-NANM, and nalorphine increased the locomotor effects of d-amphetamine in either an additive or greater-than-additive manner according to an effect-additive model. Only the selective, high-efficacy kappa agonist, spiradoline, and the non-selective opioid receptor antagonist, naloxone, failed to increase the effects of d-amphetamine under the conditions examined. These data indicate that opioids possessing a large range of relative efficacy at the mu receptor, including those possessing very low relative efficacy, significantly increase the locomotor effects of d-amphetamine.

The objective of this systematic review is to present the available evidence for the utilization of the atypical opioids tapentadol, buprenorphine, and levorphanol for the treatment of neuropathic pain.

Background: Post-stroke pain is one of the most common sequelae of stroke, which stands among the leading causes of death and adult-acquired disability worldwide. The role and clinical efficacy of opioids in post-stroke pain syndromes is still debated. Objectives: Due to the important gap in knowledge on the management of post-stroke pain, this systematic review aimed at assessing the efficacy of opioids in post-stroke pain syndromes. Methods: A literature search was conducted on databases relevant for medical scientific literature, i.e. PubMed/MEDLINE, Scopus, Web of Science and Cochrane Library databases from databases inception until August 31st, 2020 for clinical trials assessing the effects of opioids and opioid antagonists on pain reduction and pain related symptoms in patients with post-stroke pain syndromes. Studies assessing the effects of other medications (e.g., tricyclic antidepressant, pregabalin) or non - pharmacological management strategies (e.g., neurostimulation techniques) were excluded. The selected studies have been subjected to examination of the risk of bias. Results: The literature search retrieved 83,435 results. After duplicates removal, 34,285 articles were title and abstract screened. 25 full texts were assessed and 8 articles were identified to be eligible for inclusion in the qualitative summary and narrative analysis, of which three were placebo-controlled and two were dose-response. Among placebo-controlled studies, two evaluated the analgesic effect of morphine and one assessed the effects of the opioid antagonist naloxone on patients with central post-stroke pain. With regard to dose-response studies, both were on patients with central post-stroke pain, one assessing the efficacy of levorphanol, and the other on naloxone. Seven out of eight included studies showed an overall slight analgesic effect of opioids, with less consistent effects on other pain-related symptoms (e.g., mood, quality of life). The randomized controlled trials were subjected to meta-analysis and rating of the quality of evidence for the two outcomes considered according to GRADE (Grading of Recommendations, Assessment, Development and Evaluations) system. The overall results are inconclusive because of the small number of studies and of patients. Conclusions: The limited number of the included studies and their heterogeneity in terms of study design do not support the efficacy of opioids in post-stroke pain and in pain-related outcomes. Large double-blind randomized clinical trials with objective assessment of pain and related symptoms are needed to further investigate this topic.

The opioids are a large class of medications related in structure to the natural plant alkaloids found in opium that are derived from the resin of the opium poppy, Papaver somniferum. The natural alkaloids are also referred to as opiates and include morphine and codeine. Synthetic derivatives include heroin, fentanyl, hydromorphone, methadone, buprenorphine and others. The opioids are highly potent and effective analgesics, but most have a high potential for dependency and abuse. Opioids act by engagement of specific cell surface receptors; the opiate receptors, which are designated µ [mu], κ [kappa] and δ [delta]. These receptors are found predominantly in the central nervous system, brain and spinal column, but are also present on vascular, cardiac, lung, gut and even peripheral blood mononuclear cells. Engagement of the opiate receptors generates a series of intracellular signals, including inhibition of adenylate cyclase, decreased opening of calcium channels, increased potassium currents and activation of protein kinase C (PKC). The major effect of these pathways is reduction in cell excitability and neurotransmission. The natural ligands for the opiate receptors are the so-called endogenous opioid peptides such as the enkephalins, endorphins and endomorphins. The opioids have a variety of clinical effects, but are predominantly known and used for their profound pain relieving effects. Other effects that are often linked to opiate analgesia include euphoria, changes in mood, drowsiness and mental clouding. However, the distinctive feature of the analgesia induced by the opioids is the lack of loss of consciousness. The pain is often described as less intense, but still present although better tolerated. Thus, the opioids do not decrease or treat the cause of the painful stimulus, but rather decrease its perception. Other effects of opioids include respiratory depression, decreased gastrointestinal motility, sedation, nausea, vomiting, constipation and intestinal bloating. Opioids also have direct cardiovascular effects, decreasing blood pressure, causing vasodilation and decreasing cardiac work. Most opioids have similar effects and side effects, although pharmacokinetic differences, tissue distribution, and receptor type specificity probably account for the variation in effects of the various synthetic and semisynthetic derivatives of morphine. Morphine is considered the prototype opiate, against which other agents are measured for their analgesic effects as well as adverse side effects. The opioids can be categorized into subclasses on the basis of their chemical structure as opium alkaloids (opiates: codeine, morphine), semisynthetic derivatives of the natural alkaloids (hydrocodone, hydromorphone, oxycodone, buprenorphine), and various classes of synthetic opioids such as the anililopiperidines (fentanyl, alfentanil, sufentanil, remifentanil), diphenylpropylamine derivatives (propoxyphene, dextropropoxyphene, methadone, diphenoxylate, loperamide), and others (pentazocine, butorphanol, nalbuphine, levorphanol, tramadol), and, the opioid antagonists (nalmefene, naloxone and naltrexone). They can also be informally classified based upon their major use such as anesthesia (fentanyl, alfentanil, remifentanil, sufentanil), severe pain (morphine, hydromorphone, levorphanol, meperidine), moderate-to-severe acute or chronic pain (transdermal or transbuccal fentanyl, codeine, oxycodone, hydrocodone, levorphanol, methadone), diarrhea (loperamide, diphenoxylate), and cough (codeine, hydrocodone). Finally, opioids can be categorized on the basis of their action as full agonists, partial agonists or mixed agonists/antagonists, and antagonists of opiate receptors. Opioid receptor antagonists are used to reverse the effects of opioids and are invaluable in the management of opioid overdose (naloxone, naltrexone, nalmefene). Specialized opioid antagonists can be used to reverse unwanted opioid effects, such as constipation in patients with chronic pain on long-term opioids. These agents (naldemedine, naloxegol) are generally modified so as not to cross the blood brain and reverse the central nervous system effects of opiates. Opioids are rare causes of drug induced liver disease and are not mentioned in large case series of clinically apparent liver injury caused by medications. In physiological, pain relieving doses, opioids have not been implicated in causing clinically apparent liver injury, acute liver failure, chronic hepatitis or vanishing bile duct syndrome. However, overdoses of the more potent opioids have been linked to cases of acute liver injury, usually with a precipitous onset and pattern of acute toxicity with marked elevations in serum aminotransferase levels and early onset of signs of hepatic failure. This syndrome has been best characterized after buprenorphine overdose or abuse, but likely occurs with others. It is possible that the implicated opioids are not directly toxic to the liver, but cause ischemic liver injury due to respiratory failure, cardiovascular collapse, shock and anoxia that can occur with severe opioid overdose. The clinical syndrome resembles acute hepatic necrosis and liver failure, but is rapidly reversible and rarely the primary cause of death from overdose. A special form of liver injury linked to opioid use occurs with their fixed drug combinations with acetaminophen. These combinations are commonly used for moderate to moderately severe pain and can lead to abuse. If taken too frequently, acetaminophen doses may reach toxic levels, particularly with overuse for several days in the face of malnutrition, alcohol abuse or intercurrent illness. These other stresses can lower hepatic glutathione levels and predispose to acetaminophen hepatotoxicity. This constellation of events is referred to as inadvertent or unintended acetaminophen overdose or more colloquially as a “therapeutic misadventure”. Because of their potential for hepatotoxicity, opioid combinations in which the dose of acetaminophen is greater than 325 mg per tablet or capsule were discontinued. References to the safety and hepatotoxic potential of the various opiate agonists are given together at the end of this overview section. References to the opioids and the opiate antagonists used to treat substance abuse are given separately with each agent (buprenorphine, methadone, nalmefene, naloxone, naltrexone). The opioids are discussed individually or as groups of agents and links to each are given below. Full and partial agonists: • Alfentanil • Buprenorphine • Butorphanol • Codeine • Diphenoxylate • Fentanyl • Heroin • Hydrocodone • Hydromorphone • Levorphanol • Loperamide • Meperidine • Methadone • Morphine • Oliceridine • Opium • Oxycodone • Oxymorphone • Pentazocine • Remifentanil • Sufentanil • Tramadol Opiate antagonists: • Naldemedine • Nalmefene • Naloxegol • Naloxone • Naltrexone

Levorphanol is a Food and Drug Administration (FDA)-approved long-acting opioid. Most information on perioperative use of levorphanol comes from the early- and mid-1950s when this drug emerged in the field of experimental pharmacology and anesthesia. It was mainly studied during this period with some additional data being generated in the 1960s and 70s. Since this time, perioperative use has declined and research is limited. This review of literature aims to provide pharmacologic and historic description of levorphanol as a tool for perioperative pain management and as an aid to potentially decrease total postoperative opioid use during the current opioid crisis.

Abuse-deterrent formulations (ADFs) using physical/chemical barrier approaches limit abuse by providing resistance to dosage form manipulation to limit drug extraction or altered release. Standardizing in vitro testing methods to assess the resistance to manipulation presents a number of challenges, including the variation in particle sizes resulting from the use of various tools to alter the tablet matrix (e.g., grinding, chipping, crushing). A prototype, direct-compression ADF using a sintered polyethylene oxide (PEO) matrix containing dextromethorphan, an enantiomeric form of the opioid, levorphanol, was developed to evaluate testing methodologies for retention of abuse-deterrent properties following dosage form tampering. Sintered PEO tablets were manipulated by grinding, and drug content and release were evaluated for the recovered granules. Drug content analysis revealed that higher amounts of drug were contained in the smaller size granules (< 250 μm, 190% of the theoretical amount) compared with the larger particles (> 250 μm, 55-75% of theoretical amount). Release testing was performed on various size granule fractions (> 850 μm, 500-850 μm, 250-500 μm, and < 250 μm) using USP type I (basket), type II (paddle), and type IV (flow-through) apparatus. The USP type I and type II apparatus gave highly variable release results with poor discrimination among the release rates from different size granules. The observed sticking of the hydrated granules to the baskets and paddles, agglomeration of hydrated granules within the baskets/vessels, and ongoing PEO hydration with subsequent gel formation further altered the particle size and impacted the rate of drug release. The use of a flow-through apparatus (USP type IV) resulted in improved discrimination of drug release from different size granules with less variability due to better dispersion of granules (minimal sticking and aggregation). Drug release profiles from the USP type IV apparatus showed that the larger size granules (> 500 μm) offered continued resistance to drug release following tablet manipulation, but the smaller size granules (< 500 μm) provided rapid drug release that was unhindered by the hydrated granule matrix. Since < 500-μm size particles are preferred for nasal abuse, improved direct-compression ADF formulations should minimize the formation of these smaller-sized particles following tampering to maintain the product's abuse-deterrent features.