Mu receptor binding of some commonly used opioids and their metabolites

Sex differences in the antinociceptive effect of codeine and its peripheral but not central metabolism in adult mice

Codeine is a natural opiate extracted from opium poppy (Papaver somniferum) and used to alleviate mild to moderate pain. The analgesic effect of this molecule results from its metabolism into morphine which is an agonist of the mu opioid receptor. Morphine's major metabolite morphine-3-glucuronide induces both thermal and mechanical hypersensitivies while codeine-6-glucuronide has been proposed to be antinociceptive. However, sex differences in codeine antinociceptive effect and pharmacokinetics were barely studied. To this purpose, we injected male and female mice with codeine (2.5, 5, 10, 20 and 40 mg/kg) and thermal hypersensitivity was assessed 30 min after injection using the Tail Immersion Test. Moreover, both peripheral and central metabolism of codeine were evaluated respectively in the blood or pain-related brain structures in the central nervous system. The amounts of codeine and its metabolites were quantified using the isotopic dilution method by liquid chromatography coupled to a mass spectrometer. Our results show that codeine induces a greater antinociceptive effect in males than females mice independently of the estrous cycle. Moreover, major sex differences were found in the peripheral metabolism of this molecule, with higher amounts of pronociceptive morphine-3-glucuronide and less antinociceptive codeine-6-glucuronide in females than in males. Concerning the central metabolism of codeine, we did not find significant sex differences in pain-related brain structures. Collectively, these findings support a greater codeine antinociceptive effect in males than females in mice. These sex differences could be influenced by a higher peripheral metabolism of this molecule in female mice rather than central metabolism.

Respiratory Depression Associated with Opioids: A Narrative Review

OPINION

All opioids have a risk of causing respiratory depression and reduced cerebral circulation. Fentanyl has the greatest risk of causing both. This is particularly a concern when combined with illicit opioids such as diamorphine (also known as heroin). Fentanyl should not be used as a frontline potent opioid due its significant risks. Buprenorphine, a schedule III opioid, morphine, or hydromorphone is preferred, followed by oxycodone, which has a significant risk of abuse relative to buprenorphine and morphine. Although all opioids were equally effective in producing analgesia, the relative safety of each opioid is no longer a secondary concern when prescribing. In the face of an international opioid epidemic, clinicians need to choose opioid analgesics safely, wisely, and carefully.

Non-linear blood-brain barrier transport and dosing strategies influence receptor occupancy ratios of morphine and its metabolites in pain matrix

BACKGROUND AND PURPOSE

Morphine is important for treatment of acute and chronic pain. However, there is high interpatient variability and often inadequate pain relief and adverse effects. To better understand variability in the dose-effect relationships of morphine, we investigated the effects of its non-linear blood-brain barrier (BBB) transport on μ-receptor occupancy in different CNS locations, in conjunction with its main metabolites that bind to the same receptor.

EXPERIMENTAL APPROACH

CNS exposure profiles for morphine, M3G and M6G for clinically relevant dosing regimens based on intravenous, oral immediate- and extended-release formulations were generated using a physiology-based pharmacokinetic model of the CNS, with non-linear BBB transport of morphine. The simulated CNS exposure profiles were then used to derive corresponding μ-receptor occupancies at multiple CNS pain matrix locations.

KEY RESULTS

Simulated CNS exposure profiles for morphine, M3G and M6G, associated with non-linear BBB transport of morphine resulted in varying μ-receptor occupancies between different dose regimens, formulations and CNS locations. At lower doses, the μ-receptor occupancy of morphine was relatively higher than at higher doses of morphine, due to the relative contribution of M3G and M6G. At such higher doses, M6G showed higher occupancy than morphine, whereas M3G occupancy was low throughout the dose ranges.

CONCLUSION AND IMPLICATIONS

Non-linear BBB transport of morphine affects the μ-receptor occupancy ratios of morphine with its metabolites, depending on dose and route of administration, and CNS location. These predictions need validation in animal or clinical experiments, to understand the clinical implications.

Simultaneous quantification and confirmation of oxycodone and its metabolites in equine urine using ultra-high performance liquid chromatography-tandem mass spectrometry

Oxycodone, an opioid commonly used to treat pain in humans, has the potential to be abused in racehorses to enhance their performance. To understand the pharmacokinetics of oxycodone and its metabolites in horses, as well as to detect the illegal use of oxycodone in racehorses, a method for quantification and confirmation of oxycodone and its metabolites is needed. In this study, we developed and validated an ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method that can simultaneously quantify and confirm oxycodone and eight metabolites in equine urine. Samples were subjected to enzymatic hydrolysis and then liquid-liquid extraction using ethyl acetate. The analyte separation was achieved on a Hypersil Gold C18 sub-2 µm column and analytes were detected on a triple quadrupole mass spectrometer. The limit of detection (LOD) and lower limit of quantification (LLOQ) were 25-50 pg/mL and 100 pg/mL, respectively. Excellent linearity of the calibration curves was observed over a range of 100-10000 pg/mL for all nine analytes. Retention time, signal-to-noise ratio, and product ion ratios were utilized as confirmation criteria, with the limits of confirmation (LOC) ranging from 100 to 250 pg/mL. The data from a pilot pharmacokinetic (PK) study suggested that oxycodone metabolites have longer detection periods in equine urine compared to oxycodone itself; thus, the detection of metabolites in equine urine extends the ability to detect oxycodone exposure in racehorses.

Exploring the impact of CYP2D6 and UGT2B7 gene-drug interactions, and CYP-mediated DDI on oxycodone and oxymorphone pharmacokinetics using physiologically-based pharmacokinetic modeling and simulation

Oxycodone is one of the most commonly used opioids to treat moderate to severe pain. It is metabolized mainly by CYP3A4 and CYP2D6, while only a small fraction of the dose is excreted unchanged into the urine. Oxymorphone, the metabolite primarily formed by CYP2D6, has a 40- to 60-fold higher mu-opioid receptor affinity than the parent compound. While CYP2D6-mediated gene-drug-interactions (GDIs) and drug-drug interactions (DDIs) are well-studied, they only account for a portion of the variability in oxycodone and oxymorphone exposure. The combined impact of CYP2D6-mediated GDIs and DDIs, CYP3A4-mediated DDIs, and UGT2B7 GDIs is not fully understood yet and hard to study in head-to-head clinical trials given the relatively large number of scenarios. Instead, we propose the use of a physiologically-based pharmacokinetic model that integrates available information on oxycodone's metabolism to characterize and predict the impact of DDIs and GDIs on the exposure of oxycodone and its major, pharmacologically-active metabolite oxymorphone. To this end, we first developed and verified a PBPK model for oxycodone and its metabolites using published clinical data. The verified model was then applied to determine the dose-exposure relationship of oxycodone and oxymorphone stratified by CYP2D6 and UGT2B7 phenotypes respectively, and administered perpetrators of CYP-based drug interactions. Our simulations demonstrate that the combination of CYP2D6 UM and a UGT2B7Y (268) mutation may lead to a 2.3-fold increase in oxymorphone exposure compared to individuals who are phenotyped as CYP2D6 NM / UGT2B7 NM. The extent of oxymorphone exposure increases up to 3.2-fold in individuals concurrently taking CYP3A4 inhibitors, such as ketoconazole. Inhibition of the CYP3A4 pathway results in a relative increase in the partial metabolic clearance of oxycodone to oxymorphone. Oxymorphone is impacted to a higher extent by GDIs and DDIs than oxycodone. We predict oxymorphone exposure to be highest in CYP2D6 UMs/UGT2B7 PMs in the presence of ketoconazole (strong CYP3A4 index inhibitor) and lowest in CYP2D6 PMs/UGT2B7 NMs in the presence of rifampicin (strong CYP3A4 index inducer) covering a 55-fold exposure range.

Oxycodone, an opioid like the others?

The over-prescription of opioid analgesics is a growing problem in the field of addiction, which has reached epidemic-like proportions in North America. Over the past decade, oxycodone has gained attention as the leading opioid responsible for the North America opioid crisis. Oxycodone is the most incriminated drug in the early years of the epidemic of opioid use disorder in USA (roughly 1999-2016). The number of preclinical articles on oxycodone is rapidly increasing. Several publications have already compared oxycodone with other opioids, focusing mainly on their analgesic properties. The aim of this review is to focus on the genomic and epigenetic regulatory features of oxycodone compared with other opioid agonists. Our aim is to initiate a discussion of perceptible differences in the pharmacological response observed with these various opioids, particularly after repeated administration in preclinical models commonly used to study drug dependence potential.

Effects and Limitations of Naldemedine for Opioid-Induced Urinary Retention: A Case Report

A 78-year-old man with postoperative recurrence of esophageal cancer was admitted to the hospital due to chest pain and dyspnea. Oral short-acting opioids provided some relief, but chest pain persisted and worsened, leading to the initiation of a transdermal fentanyl patch. However, the patient developed opioid-induced urinary retention, which was treated with a naldemedine, a medication used for opioid-induced constipation and urinary retention. Opioid switching led to recurrent urinary retention, requiring placement of a urinary catheter. The patient ultimately required continuous deep sedation for refractory symptoms and died several days later.

Hydrocodone, Oxycodone, and Morphine Metabolism and Drug-Drug Interactions

Awareness of drug interactions involving opioids is critical for patient treatment as they are common therapeutics used in numerous care settings, including both chronic and disease-related pain. Not only do opioids have narrow therapeutic indexes and are extensively used, but they have the potential to cause severe toxicity. Opioids are the classical pain treatment for patients who suffer from moderate to severe pain. More importantly, opioids are often prescribed in combination with multiple other drugs, especially in patient populations who typically are prescribed a large drug regimen. This review focuses on the current knowledge of common opioid drug-drug interactions (DDIs), focusing specifically on hydrocodone, oxycodone, and morphine DDIs. The DDIs covered in this review include pharmacokinetic DDI arising from enzyme inhibition or induction, primarily due to inhibition of cytochrome p450 enzymes (CYPs). However, opioids such as morphine are metabolized by uridine-5'-diphosphoglucuronosyltransferases (UGTs), principally UGT2B7, and glucuronidation is another important pathway for opioid-drug interactions. This review also covers several pharmacodynamic DDI studies as well as the basics of CYP and UGT metabolism, including detailed opioid metabolism and the potential involvement of metabolizing enzyme gene variation in DDI. Based upon the current literature, further studies are needed to fully investigate and describe the DDI potential with opioids in pain and related disease settings to improve clinical outcomes for patients. SIGNIFICANCE STATEMENT: A review of the literature focusing on drug-drug interactions involving opioids is important because they can be toxic and potentially lethal, occurring through pharmacodynamic interactions as well as pharmacokinetic interactions occurring through inhibition or induction of drug metabolism.

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

Oxycodone, a semisynthetic derivative of naturally occurring thebaine, an opioid alkaloid, has been available for more than 100 years. Although thebaine cannot be used therapeutically due to the occurrence of convulsions at higher doses, it has been converted to a number of other widely used compounds that include naloxone, naltrexone, buprenorphine, and oxycodone. Despite the early identification of oxycodone, it was not until the 1990s that clinical studies began to explore its analgesic efficacy. These studies were followed by the pursuit of several preclinical studies to examine the analgesic effects and abuse liability of oxycodone in laboratory animals and the subjective effects in human volunteers. For a number of years oxycodone was at the forefront of the opioid crisis, playing a significant role in contributing to opioid misuse and abuse, with suggestions that it led to transitioning to other opioids. Several concerns were expressed as early as the 1940s that oxycodone had significant abuse potential similar to heroin and morphine. Both animal and human abuse liability studies have confirmed, and in some cases amplified, these early warnings. Despite sharing a similar structure with morphine and pharmacological actions also mediated by the μ-opioid receptor, there are several differences in the pharmacology and neurobiology of oxycodone. The data that have emerged from the many efforts to analyze the pharmacological and molecular mechanism of oxycodone have generated considerable insight into its many actions, reviewed here, which, in turn, have provided new information on opioid receptor pharmacology. SIGNIFICANCE STATEMENT: Oxycodone, a μ-opioid receptor agonist, was synthesized in 1916 and introduced into clinical use in Germany in 1917. It has been studied extensively as a therapeutic analgesic for acute and chronic neuropathic pain as an alternative to morphine. Oxycodone emerged as a drug with widespread abuse. This article brings together an integrated, detailed review of the pharmacology of oxycodone, preclinical and clinical studies of pain and abuse, and recent advances to identify potential opioid analgesics without abuse liability.

Classics in Chemical Neuroscience: Dextromethorphan (DXM)

Dextromethorphan (DXM) was introduced in 1958 as the first non-opioid cough suppressant and is indicated for multiple psychiatric disorders. It has been the most used over-the-counter cough suppressant since its emergence. However, individuals quickly noticed an intoxicating and psychedelic effect if they ingested large doses. DXM's antagonism at N-methyl-d-aspartate receptors (NMDAr) is thought to underly its efficacy in treating acute cough, but supratherapeutic doses mimic the activity of dissociative hallucinogens, such as phencyclidine and ketamine. In this Review we will discuss DXM's synthesis, manufacturing information, drug metabolism, pharmacology, adverse effects, recreational use, abuse potential, and its history and importance in therapy to present DXM as a true classic in chemical neuroscience.

The role of opioid transmission in music-induced pleasure

Studies conducted in rodents indicate a crucial role of the opioid circuit in mediating objective hedonic reactions to primary rewards. However, it remains unclear whether opioid transmission is also essential to experience pleasure with more abstract rewards, such as music. We addressed this question using a double-blind within-subject pharmacological design in which opioid levels were up- and downregulated by administering an opioid agonist (oxycodone) and antagonist (naltrexone), respectively, before healthy participants (n = 21) listened to music. Participants also performed a monetary incentive delay (MID) task to control for the effectiveness of the treatment and the specificity of the effects. Our results revealed that the pharmacological intervention did not modulate subjective reports of pleasure, nor the occurrence of chills. On the contrary, psychophysiological (objective) measures of emotional arousal, such as skin conductance responses (SCRs), were bidirectionally modulated in both the music and MID tasks. This modulation specifically occurred during reward consumption, with greater pleasure-related SCR following oxycodone than naltrexone. These findings indicate that opioid transmission does not modulate subjective evaluations but rather affects objective reward-related psychophysiological responses. These findings raise new caveats about the role of the opioidergic system in the modulation of pleasure for more abstract or cognitive forms of rewarding experiences, such as music.

Differential dosing of oxycodone in combination with propofol in diagnostic painless gastroscopy in elderly patients: A prospective randomized controlled trial

OBJECTIVE

The aim of this study is to investigate the safety and efficacy of different doses of oxycodone in combination with propofol during painless gastroscopy.

METHODS

120 patients underwent painless gastroscopy under general anesthesia. According to the different doses of oxycodone, patients were divided into 4 groups, group A (oxycodone 0.025 mg/kg + propofol), group B (oxycodone 0.05 mg/kg + propofol) and group C (oxycodone 0.1 mg/kg + propofol), control group (propofol alone), with 30 cases in each group. The general characteristics of all patients were then evaluated. Mean arterial pressure (MAP), heart rate (HR) and peripheral capillary oxygen saturation (SpO2) were recorded at different time points, including the time before anesthesia (T0), failure of the lash reflex (T1), successful placement of the mirror (T2), removal of the mirror (T3) and waking up (T4). The intraoperative propofol dosage and the operative time of gastroscopy were recorded. The occurrence of adverse effects in the 4 groups was also compared.

RESULTS

General characteristics, gastroscopy operative time and SpO2 did not differ significantly between the 4 groups (P > .05). However, group C had the lowest amount of propofol during gastroscopy (P < .05). At T1, groups A, B, and C had a faster HR than the control group (P < .05). At T2, groups A, B, and C had a lower MAP than the control group (P < .05). Groups B and C had fewer adverse effects than groups A and the control group (P < .05). Importantly, groups B and C had a shorter recovery time than groups A and the control group (P < .05), but no statistically significant differences were found between groups B and C.

CONCLUSION

0.05 mg/kg oxycodone in combination with propofol can be used safely and effectively for painless gastroscopy, with the advantages of a low propofol dose, maintenance of hemodynamic stability and few adverse effects.

Anterior cingulate cortex and its projections to the ventral tegmental area regulate opioid withdrawal, the formation of opioid context associations and context-induced drug seeking

Clinical evidence suggests that there are correlations between activity within the anterior cingulate cortex (ACC) following re-exposure to drug-associated contexts and drug craving. However, there are limited data contributing to our understanding of ACC function at the cellular level during re-exposure to drug-context associations as well as whether the ACC is directly related to context-induced drug seeking. Here, we addressed this issue by employing our novel behavioral procedure capable of measuring the formation of drug-context associations as well as context-induced drug-seeking behavior in male mice (8-12 weeks of age) that orally self-administered oxycodone. We found that mice escalated oxycodone intake during the long-access training sessions and that conditioning with oxycodone was sufficient to evoke conditioned place preference (CPP) and drug-seeking behaviors. Additionally, we found that thick-tufted, but not thin-tufted pyramidal neurons (PyNs) in the ACC as well as ventral tegmental area (VTA)-projecting ACC neurons had increased intrinsic membrane excitability in mice that self-administered oxycodone compared to controls. Moreover, we found that global inhibition of the ACC or inhibition of VTA-projecting ACC neurons was sufficient to significantly reduce oxycodone-induced CPP, drug seeking, and spontaneous opioid withdrawal. These results demonstrate a direct role of ACC activity in mediating context-induced opioid seeking among other behaviors, including withdrawal, that are associated with the DSM-V criteria of opioid use disorder.

Reports of Adverse Events Associated with Use of Novel Psychoactive Substances, 2017-2020: A Review

An important role of modern forensic and clinical toxicologists is to monitor the adverse events of novel psychoactive substances (NPS). Following a prior review from 2013 to 2016, this critical literature review analyzes and evaluates published case reports for NPS from January 2017 through December 2020. The primary objective of this study is to assist in the assessment and interpretation of these cases as well as provide references for confirmation methods. Chemistry, pharmacology, adverse events and user profiles (e.g., polypharmacy) for NPS are provided including case history, clinical symptoms, autopsy findings and analytical results. Literature reviews were performed in PubMed and Google Scholar for publications using search terms such as NPS specific names, general terms (e.g., 'designer drugs' and 'novel psychoactive substances'), drug classes (e.g., 'designer stimulants') and outcome-based terms (e.g., 'overdose' and 'death'). Government and website drug surveillance databases and abstracts published by professional forensic science organizations were also searched. Toxicological data and detailed case information were extracted, tabulated, analyzed and organized by drug category. Case reports included overdose fatalities (378 cases), clinical treatment and hospitalization (771 cases) and driving under the influence of drugs (170 cases) for a total of 1,319 cases providing details of adverse events associated with NPS. Confirmed adverse events with associated toxidromes of more than 60 NPS were reported including synthetic cannabinoid, NPS stimulant, NPS hallucinogen, NPS benzodiazepine and NPS opioid cases. Fifty of these NPS were reported for the first time in January 2017 through December 2020 as compared to the previous 4 years surveyed. This study provides insight and context of case findings described in the literature and in digital government surveillance databases and websites during a recent 4-year period. This review will increase the awareness of adverse events associated with NPS use to better characterize international emerging drug threats.

Epidural Oxycodone for Acute Pain

Epidural analgesia is commonly used in labour analgesia and in postoperative pain after major surgery. It is highly effective in severe acute pain, has minimal effects on foetus and newborn, may reduce postoperative complications, and enhance patient satisfaction. In epidural analgesia, low concentrations of local anaesthetics are combined with opioids. Two opioids, morphine and sufentanil, have been approved for epidural use, but there is an interest in evaluating other opioids as well. Oxycodone is one of the most commonly used opioids in acute pain management. However, data on its use in epidural analgesia are sparse. In this narrative review, we describe the preclinical and clinical data on epidural oxycodone. Early data from the 1990s suggested that the epidural administration of oxycodone may not offer any meaningful benefits over intravenous administration, but more recent clinical data show that oxycodone has advantageous pharmacokinetics after epidural administration and that epidural administration is more efficacious than intravenous administration. Further studies are needed on the safety and efficacy of continuous epidural oxycodone administration and its use in epidural admixture.

Morphine-3-Glucuronide, Physiology and Behavior

Morphine remains the gold standard painkiller available to date to relieve severe pain. Morphine metabolism leads to the production of two predominant metabolites, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). This metabolism involves uridine 5'-diphospho-glucuronosyltransferases (UGTs), which catalyze the addition of a glucuronide moiety onto the C3 or C6 position of morphine. Interestingly, M3G and M6G have been shown to be biologically active. On the one hand, M6G produces potent analgesia in rodents and humans. On the other hand, M3G provokes a state of strong excitation in rodents, characterized by thermal hyperalgesia and tactile allodynia. Its coadministration with morphine or M6G also reduces the resulting analgesia. Although these behavioral effects show quite consistency in rodents, M3G effects are much more debated in humans and the identity of the receptor(s) on which M3G acts remains unclear. Indeed, M3G has little affinity for mu opioid receptor (MOR) (on which morphine binds) and its effects are retained in the presence of naloxone or naltrexone, two non-selective MOR antagonists. Paradoxically, MOR seems to be essential to M3G effects. In contrast, several studies proposed that TLR4 could mediate M3G effects since this receptor also appears to be essential to M3G-induced hyperalgesia. This review summarizes M3G's behavioral effects and potential targets in the central nervous system, as well as the mechanisms by which it might oppose analgesia.

Contribution of CYP2D6 Functional Activity to Oxycodone Efficacy in Pain Management: Genetic Polymorphisms, Phenoconversion, and Tissue-Selective Metabolism

Oxycodone is a widely used opioid for the management of chronic pain. Analgesic effects observed following the administration of oxycodone are mediated mostly by agonistic effects on the μ-opioid receptor. Wide inter-subject variability observed in oxycodone efficacy could be explained by polymorphisms in the gene coding for the μ-opioid receptor (OPRM1). In humans, oxycodone is converted into several metabolites, particularly into oxymorphone, an active metabolite with potent μ-opioid receptor agonist activity. The CYP2D6 enzyme is principally responsible for the conversion of oxycodone to oxymorphone. The CYP2D6 gene is highly polymorphic with encoded protein activities, ranging from non-functioning to high-functioning enzymes. Several pharmacogenetic studies have shown the importance of CYP2D6-mediated conversion of oxycodone to oxymorphone for analgesic efficacy. Pharmacogenetic testing could optimize oxycodone therapy and help achieve adequate pain control, avoiding harmful side effects. However, the most recent Clinical Pharmacogenetics Implementation Consortium guidelines fell short of recommending pharmacogenomic testing for oxycodone treatment. In this review, we (1) analyze pharmacogenomic and drug-interaction studies to delineate the association between CYP2D6 activity and oxycodone efficacy, (2) review evidence from CYP3A4 drug-interaction studies to untangle the nature of oxycodone metabolism and its efficacy, (3) report on the current knowledge linking the efficacy of oxycodone to OPRM1 variants, and (4) discuss the potential role of CYP2D6 brain expression on the local formation of oxymorphone. In conclusion, we opine that pharmacogenetic testing, especially for CYP2D6 with considerations of phenoconversion due to concomitant drug administration, should be appraised to improve oxycodone efficacy.

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.

Fentanyl-related substance scheduling as an effective drug control strategy

Fentanyl is now the primary driver of the current opioid crisis. Fentanyl and its analogues are subject to the Controlled Substances Act of 1970, the Controlled Substances Analogue Enforcement Act of 1986 (Federal Analogue Act), state laws, international treaties, and the laws of foreign countries. The appearance of novel psychoactive substances led to further legislative developments in scheduling. New fentanyl analogues proliferated in a manner previously unseen since about 2016. Overdose deaths of these fentanyl analogues prompted the Drug Enforcement Administration to reactively emergency schedule each new fentanyl analogue as it appeared. The international community also acted. Finally, on February 6, 2018, a proactive temporary (emergency) class-wide scheduling of fentanyl-related substances was implemented based upon the fentanyl core structure to save lives. This action spurred a similar action in China. Fentanyl analogues fell dramatically in the marketplace, despite further increases in fentanyl itself. Congress temporarily extended this scheduling, but it will soon expire. Opposition to permanent class-wide was lodged due to concerns over law enforcement overreach, inadequate Health and Human Services input, and hindrance of research. This paper reaffirms the importance of a class-based scheduling strategy while also arguing for increased research of schedule I controlled substances.

Pharmacogenomics of oxycodone: a narrative literature review

Oxycodone is a semisynthetic μ- and κ-opioid receptor with agonist with a broad scope of use including postoperative analgesia as well as control of neuropathic and cancer pain. Advantages over other opioids include prolonged duration of action, greater potency than morphine and lack of histamine release or ceiling effect. Individual responses to oxycodone can vary due to genetic differences. This review article aims to summarize the oxycodone literature and provide context on its pharmacogenomics and pharmacokinetics. The evidence for clinical effect of genetic polymorphisms on oxycodone is conflicting. There is stronger evidence linking polymorphic genetic enzymes CYP2D6 and CYP3A with therapeutic outcomes. Further, research is needed to discern all of oxycodone's metabolites and their contribution to the overall analgesic effect.

The molecular neurobiology and neuropathology of opioid use disorder

The number of people diagnosed with opioid use disorder has skyrocketed as a consequence of the opioid epidemic and the increased prescribing of opioid drugs for chronic pain relief. Opioid use disorder is characterized by loss of control of drug taking, continued drug use in the presence of adverse consequences, and repeated relapses to drug taking even after long periods of abstinence. Patients who suffer from opioid use disorder often present with cognitive deficits that are potentially secondary to structural brain abnormalities that vary according to the chemical composition of the abused opioid. This review details the neurobiological effects of oxycodone, morphine, heroin, methadone, and fentanyl on brain neurocircuitries by presenting the acute and chronic effects of these drugs on the human brain. In addition, we review results of neuroimaging in opioid use disorder patients and/or histological studies from brains of patients who had expired after acute intoxication following long-term use of these drugs. Moreover, we include relevant discussions of the neurobiological mechanisms involved in promoting abnormalities in the brains of opioid-exposed patients. Finally, we discuss how novel strategies could be used to provide pharmacological treatment against opioid use disorder.

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Brain abnormalities caused by opioid intoxication.

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Intoxication of opioids leads to defects in brain neurocircuitries.

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Insight into the molecular mechanisms associated with craving in heroin addicts.

Unnarmicin D, an Anti-inflammatory Cyanobacterial Metabolite with δ and μ Opioid Binding Activity Discovered via a Pipeline Approach Designed to Target Neurotherapeutics

To combat the bottlenecks in drug discovery and development, a pipeline to identify neuropharmacological candidates using in silico, in vitro, and receptor specific assays was devised. The focus of this pipeline was to identify metabolites with the ability to reduce neuroinflammation, due to the implications that chronic neuroinflammation has in chronic pain and neurodegenerative diseases. A library of pure compounds isolated from the cyanobacterium Trichodesmium thiebautii was evaluated using this method. In silico analysis of drug likelihood and in vitro permeability analysis using the parallel artificial membrane permeability assay (PAMPA) highlighted multiple metabolites of interest from the library. Murine BV-2 microglia were used in conjunction with the Griess assay to determine if metabolites could reduce lipopolysaccharide induced neuroinflammation followed by analysis of pro-inflammatory cytokine concentrations in the supernatant of the treated cell cultures. The nontoxic metabolite unnarmicin D was further evaluated due to its moderate permeability in the PAMPA assay, promising ADME data, modulation of all cytokines tested, and prediction as an opioid receptor ligand. Molecular modeling of unnarmicin D to the μ and δ opioid receptors showed strong theoretical binding potential to the μ opioid receptor. In vitro binding assays validated this pipeline showing low micromolar binding affinity for the μ opioid receptor launching the potential for further analysis of unnarmicin D derivatives for the treatment of pain and neuroinflammation related diseases.

Morphine-element interactions - The influence of selected chemical elements on neural pathways associated with addiction

Addiction is a pressing social problem worldwide and opioid dependence can be considered the strongest and most difficult addiction to treat. Mesolimbic and mesocortical dopaminergic pathways play an important role in modulation of cognitive processes and decision making and, therefore, changes in dopamine metabolism are considered the central basis for the development of dependence. Disturbances caused by excesses or deficiency of certain elements have a significant impact on the functioning of the central nervous system (CNS) both in physiological conditions and in pathology and can affect the cerebral reward system and therefore, may modulate processes associated with the development of addiction. In this paper we review the mechanisms of interactions between morphine and zinc, manganese, chromium, cadmium, lead, fluoride, their impact on neural pathways associated with addiction, and on antinociception and morphine tolerance and dependence.

Use of a Noninvasive Brain-Penetrating Peptide-Drug Conjugate Strategy to Improve the Delivery of Opioid Pain Relief Medications to the Brain

The analgesic potency of morphine-6-glucuronide (M6G) has been shown to be 50-fold higher than morphine after intracerebral injection. However, the brain penetration of M6G is significantly lower than morphine, thus limiting its usefulness in pain management. Here, we created new entities by the conjugation of the angiopep-2 peptide (An2) that crosses the blood-brain barrier (BBB) by low-density lipoprotein receptor-related protein 1 receptor-mediated transcytosis with either morphine or M6G. We demonstrated improvement of BBB permeability of these new entities compared with that of unconjugated M6G and morphine. Intravenous or subcutaneous administration of the An2-M6G conjugate exerted greater and more sustained analgesic activity than equivalent doses of either morphine or M6G. Likewise, subcutaneous An2-morphine induced a delayed but prolonged antinociceptive effect. The effects of these conjugates on the gastrointestinal tract motility were also evaluated. An2-morphine significantly reduced the intestinal transit time, whereas An2-M6G exhibited a reduced constipation profile, as compared with an equimolar dose of morphine. In summary, we have developed new brain-penetrant opioid conjugates exhibiting improved analgesia to side effect ratios. These results thus support the use of An2-carrier peptides as an innovative BBB-targeting technology to deliver effective drugs, such as M6G, for pain management. SIGNIFICANCE STATEMENT: The metabolite morphine-6-glucuronide (M6G) does not efficiently cross the blood-brain barrier. The low-density lipoprotein receptor-related protein 1 peptide ligand angiopep-2 may serve as an effective drug delivery system to the brain. Here, we demonstrated that the coupling of M6G to angiopep-2 peptide (An2) improves its brain penetration and significantly increases its analgesic potency. The An2-M6G conjugate has a favorable side effect profile that includes reduction of developing constipation. An2-M6G exhibits a unique pharmacodynamic profile with a better therapeutic window than morphine.

Inhibition of Fast Nerve Conduction Produced by Analgesics and Analgesic Adjuvants-Possible Involvement in Pain Alleviation

Nociceptive information is transmitted from the periphery to the cerebral cortex mainly by action potential (AP) conduction in nerve fibers and chemical transmission at synapses. Although this nociceptive transmission is largely inhibited at synapses by analgesics and their adjuvants, it is possible that the antinociceptive drugs inhibit nerve AP conduction, contributing to their antinociceptive effects. Many of the drugs are reported to inhibit the nerve conduction of AP and voltage-gated Na⁺ and K⁺ channels involved in its production. Compound action potential (CAP) is a useful measure to know whether drugs act on nerve AP conduction. Clinically-used analgesics and analgesic adjuvants (opioids, non-steroidal anti-inflammatory drugs, ₂-adrenoceptor agonists, antiepileptics, antidepressants and local anesthetics) were found to inhibit fast-conducting CAPs recorded from the frog sciatic nerve by using the air-gap method. Similar actions were produced by antinociceptive plant-derived chemicals. Their inhibitory actions depended on the concentrations and chemical structures of the drugs. This review article will mention the inhibitory actions of the antinociceptive compounds on CAPs in frog and mammalian peripheral (particularly, sciatic) nerves and on voltage-gated Na⁺ and K⁺ channels involved in AP production. Nerve AP conduction inhibition produced by analgesics and analgesic adjuvants is suggested to contribute to at least a part of their antinociceptive effects.

Comparisons of In Vivo and In Vitro Opioid Effects of Newly Synthesized 14-Methoxycodeine-6-O-sulfate and Codeine-6-O-sulfate

The present work represents the in vitro (potency, affinity, efficacy) and in vivo (antinociception, constipation) opioid pharmacology of the novel compound 14-methoxycodeine-6-O-sulfate (14-OMeC6SU), compared to the reference compounds codeine-6-O-sulfate (C6SU), codeine and morphine. Based on in vitro tests (mouse and rat vas deferens, receptor binding and [³⁵S]GTPγS activation assays), 14-OMeC6SU has µ-opioid receptor-mediated activity, displaying higher affinity, potency and efficacy than the parent compounds. In rats, 14-OMeC6SU showed stronger antinociceptive effect in the tail-flick assay than codeine and was equipotent to morphine, whereas C6SU was less efficacious after subcutaneous (s.c.) administration. Following intracerebroventricular injection, 14-OMeC6SU was more potent than morphine. In the Complete Freund’s Adjuvant-induced inflammatory hyperalgesia, 14-OMeC6SU and C6SU in s.c. doses up to 6.1 and 13.2 µmol/kg, respectively, showed peripheral antihyperalgesic effect, because co-administered naloxone methiodide, a peripherally acting opioid receptor antagonist antagonized the measured antihyperalgesia. In addition, s.c. C6SU showed less pronounced inhibitory effect on the gastrointestinal transit than 14-OMeC6SU, codeine and morphine. This study provides first evidence that 14-OMeC6SU is more effective than codeine or C6SU in vitro and in vivo. Furthermore, despite C6SU peripheral antihyperalgesic effects with less gastrointestinal side effects the superiority of 14-OMeC6SU was obvious throughout the present study.

Pharmacogenomics of Pain Management: The Impact of Specific Biological Polymorphisms on Drugs and Metabolism

PURPOSE OF REVIEW

Pain is multifactorial and complex, often with a genetic component. Pharmacogenomics is a relative new field, which allows for the development of a truly unique and personalized therapeutic approach in the treatment of pain.

RECENT FINDINGS

Until recently, drug mechanisms in humans were determined by testing that drug in a population and calculating response averages. However, some patients will inevitably fall outside of those averages, and it is nearly impossible to predict who those outliers might be. Pharmacogenetics considers a patient's unique genetic information and allows for anticipation of that individual's response to medication. Pharmacogenomic testing is steadily making progress in the management of pain by being able to identify individual differences in the perception of pain and susceptibility and sensitivity to drugs based on genetic markers. This has a huge potential to increase efficacy and reduce the incidence of iatrogenic drug dependence and addiction. The streamlining of relevant polymorphisms of genes encoding receptors, transporters, and drug-metabolizing enzymes influencing the pain phenotype can be an important guide to develop safe new strategies and approaches to personalized pain management. Additionally, some challenges still prevail and preclude adoption of pharmacogenomic testing universally. These include lack of knowledge about pharmacogenomic testing, inadequate standardization of the process of data handling, questionable benefits about the clinical and financial aspects of pharmacogenomic testing-guided therapy, discrepancies in clinical evidence supporting these tests, and doubtful reimbursement of the tests by health insurance agencies.

MoS2-enabled dual-mode optoelectronic biosensor using a water soluble variant of μ-opioid receptor for opioid peptide detection

Owing to their unique electrical and optical properties, two-dimensional transition metal dichalcogenides have been extensively studied for their potential applications in biosensing. However, simultaneous utilization of both optical and electrical properties has been overlooked, yet it can offer enhanced accuracy and detection versitility. Here, we demonstrate a dual-mode optoelectronic biosensor based on monolayer molybdenum disulfide (MoS₂) capable of producing simultaneous electrical and optical readouts of biomolecular signals. On a single platform, the biosensor exhibits a tunable photonic Fano-type optical resonance while also functioning as a field-effect transistor (FET) based on a optically transparent gate electrode. Furthermore, chemical vapor deposition grown MoS₂ provides a clean surface for direct immobilization of a water-soluble variant of the μ-opioid receptor (wsMOR), via a nickel ion-mediated linker chemistry. We utilize a synthetic opioid peptide to show the operation of the electronic and optical sensing modes. The responses of both modes exhibit a similar trend with dynamic ranges of four orders of magnitude and detection limits of <1 nM. Our work explores the potential of a versatile multimodal sensing platform enabled by monolayer MoS₂, since the integration of electrical and optical sensors on the same chip can offer flexibility in read-out and improve the accuracy in detection of low concentration targets.

Pharmacokinetics and Bioequivalence Evaluation of a New Oxycodone Tamper-Resistant Tablet Administered with an Opioid Antagonist in Patients with Chronic Pain

BACKGROUND AND OBJECTIVES

Oxycodone tamper resistant (OTR) is a new extended-release abuse-deterrent formulation providing improvements in the tamper resistant characteristics. This study aimed to investigate the pharmacokinetic properties of the new OTR tablets and evaluate the bioequivalence of oxycodone from OTR and the original extended release (ER) formulation tablets administered with an opioid antagonist in patients with chronic pain.

METHODS

In this open-label, randomized, cross-over study, the enrolled patients were randomised to receive a single dose of 40 mg OTR or 40 mg OXYCONTIN^® (OXY) tablet administered with naltrexone blockade under fasting conditions. Serial blood samples for pharmacokinetic analysis were collected. Plasma oxycodone was quantified by a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method. Tolerability was evaluated by monitoring adverse events, physical examinations, 12-lead ECG and laboratory tests.

RESULTS

A total of 38 patients were enrolled and 33 subjects completed the study. After a single dose of 40 mg tablets, pharmacokinetic results of the new OTR tablet were found to be similar to those of original extended-release oxycodone tablet. OTR 40 mg was bioequivalent to OXY 40 mg and was well tolerated in patients with chronic pain.

CONCLUSIONS

The new OTR formulation could provide a new choice in the treatment of chronic pain and reduce the potential for oxycodone abuse. Chictr.org identifier: ChiCTR1800017253.

Update review of pain control methods of tonsil surgery

Pain after tonsil surgery is troublesome because it causes discomfort. In addition, handling patients with postoperative pain is challenging to otolaryngologists. Many laboratory studies have assessed the use of analgesics and surgical techniques to discover methods for effective control of postoperative pain associated with tonsil surgery. In this review article, we summarize and provide a comprehensive overview of current methods for the control of pain after tonsil surgery based on findings of recent studies. Although powered intracapsular tonsillotomy is not popular yet, it seems to be an effective option among various surgical techniques. More discussion about powered intracapsular tonsillotomy should be done in the future. On the other hand, surgery with a harmonic scalpel, fibrin glue, or cryoanalgesia seems ineffective. When reviewing medical treatment methods, the use of nonsteroidal anti-inflammatory drugs, steroids, and/or gabapentin/pregabalin seems to be effective. However, the use of opioid (especially codeine) for children should be avoided because of possible respiratory insufficiency. Ketorolac is dangerous because of the risk of hemorrhage. We should continue to focus on the development of novel postoperative pain control techniques with no or low complications.

Dextromethorphan Analgesia in a Human Experimental Model of Hyperalgesia

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

Central pain sensitization is often refractory to drug treatment. Dextromethorphan, an N-methyl-d-aspartate receptor antagonist, is antihyperalgesic in preclinical pain models. The hypothesis is that dextromethorphan is also antihyperalgesic in humans.

Methods

This randomized, double-blind, placebo-controlled, crossover study explores the antihyperalgesic effect of single and repeated 30-mg dose of oral dextromethorphan in 20 volunteers, using the freeze-injury pain model. This model leads to development of primary and secondary hyperalgesia, which develops away from the site of injury and is associated with central sensitization and activation of N-methyl-d-aspartate receptor in the spinal cord. The primary outcome was antihyperalgesia calculated with the area under the curve of the percentage change in mechanical pain threshold (electronic von Frey) on the area of secondary hyperalgesia. The secondary outcomes were mechanical pain threshold on the area of primary hyperalgesia and cognitive (reaction time) effect.

Results

Single 30-mg results are reported. Antihyperalgesia (% · min) is significantly higher on the area of secondary hyperalgesia with dextromethorphan than placebo (median [interquartile range]: 3,029 [746; 6,195] vs. 710 [–3,248; 4,439], P = 0.009, Hedge’s g = 0.8, 95% CI [0.1; 1.4]). On primary hyperalgesia area, mechanical pain threshold 2 h after drug intake is significantly higher with dextromethorphan (P = 0.011, Hedge’s g = 0.63, 95% CI [0.01; 1.25]). No difference in antinociception is observed after thermal painful stimuli on healthy skin between groups. Reaction time (ms) is shorter with placebo than with dextromethorphan (median [interquartile range]: 21.6 [–37.4; 0.1] vs. –1.2 [–24.3; 15.4], P = 0.015, Hedge’s g = 0.75, 95% CI [0.12; 1.39]). Nonserious adverse events occurrence (15%, 3 of 20 volunteers) was similar in both groups.

Conclusions

This study shows that low-dose (30-mg) dextromethorphan is antihyperalgesic in humans on the areas of primary and secondary hyperalgesia and reverses peripheral and central neuronal sensitization. Because dextromethorphan had no intrinsic antinociceptive effect in acute pain on healthy skin, N-methyl-d-aspartate receptor may need to be sensitized by pain for dextromethorphan to be effective.

An update on the safety of prescribing opioids in pediatrics

INTRODUCTION

The opioid abuse epidemic and its toll on the adolescent population have heightened awareness for safer opioid prescribing practices in pediatric pain management. Opioids remain the mainstay of therapy for severe pain, although there is an emphasis on multimodal therapy. Areas covered: In this update, the authors present information on parenteral/oral opioids commonly used in pediatrics. Recommendations for opioid use in special circumstances including neonates and developmental pharmacokinetic concerns are discussed. Due to noticeable interindividual variability, pharmacogenomics may be important for tailoring pain regimens. In particular, the role of CYP2D6 phenotypes on opioid selection/dosing and clinical implications are discussed. A summary of adverse effects and opioid safety data, and the role of regulations, risk assessment, Centers for Disease Control and Prevention guidelines, follow-up, and monitoring for compliance in opioid prescribing, are detailed. Expert opinion: 'One size does not fit all' describes the need for public policies focused on pediatric pain and opioid use, as children are not 'little adults.' Clinical trials to evaluate pharmacokinetics-pharmacodynamics of opioids are currently lacking. Development of novel biased opioid agonists, clinical integration of genetics in informed decision-making, and emphasis on top-down approaches to pain management will be key to decrease opioid reliance.

DARK Classics in Chemical Neuroscience: Morphine

As the major psychoactive agent in opium and direct precursor for heroin, morphine is a historically critical molecule in chemical neuroscience. A structurally complex phenanthrene alkaloid produced by Papaver somniferum, morphine has fascinated chemists seeking to disentangle pharmacologically beneficial analgesic effects from addiction, tolerance, and dependence liabilities. In this review, we will detail the history of morphine, from the first extraction and isolation by Sertürner in 1804 to the illicit use of morphine and proliferation of opioid use and abuse disorders currently ravaging the United States. Morphine is a molecule of great cultural relevance, as the agent that single-handedly transformed our understanding of pharmacognosy, receptor dynamics, and substance abuse and dependence disorders.

Linking Aromatic Hydroxy Metabolic Functionalization of Drug Molecules to Structure and Pharmacologic Activity

Drug functionalization through the formation of hydrophilic groups is the norm in the phase I metabolism of drugs for the modification of drug action. The reactions involved are mainly oxidative, catalyzed mostly by cytochrome P450 (CYP) isoenzymes. The benzene ring, whether phenyl or fused with other rings, is the most common hydrophobic pharmacophoric moiety in drug molecules. On the other hand, the alkoxy group (mainly methoxy) bonded to the benzene ring assumes an important and sometimes essential pharmacophoric status in some drug classes. Upon metabolic oxidation, both moieties, i.e., the benzene ring and the alkoxy group, produce hydroxy groups; the products are arenolic in nature. Through a pharmacokinetic effect, the hydroxy group enhances the water solubility and elimination of the metabolite with the consequent termination of drug action. However, through hydrogen bonding, the hydroxy group may modify the pharmacodynamics of the interaction of the metabolite with the site of parent drug action (i.e., the receptor). Accordingly, the expected pharmacologic outcome will be enhancement, retention, attenuation, or loss of activity of the metabolite relative to the parent drug. All the above issues are presented and discussed in this review using selected members of different classes of drugs with inferences regarding mechanisms, drug design, and drug development.

Bilateral carotid sinus nerve transection exacerbates morphine-induced respiratory depression

Opioid-induced respiratory depression (OIRD) involves decreased sensitivity of ventilatory control systems to decreased blood levels of oxygen (hypoxia) and elevated levels of carbon dioxide (hypercapnia). Understanding the sites and mechanisms by which opioids elicit respiratory depression is pivotal for finding novel therapeutics to prevent and/or reverse OIRD. To examine the contribution of carotid body chemoreceptors OIRD, we used whole-body plethysmography to evaluate hypoxic (HVR) and hypercapnic (HCVR) ventilatory responses including changes in frequency of breathing, tidal volume, minute ventilation and inspiratory drive, after intravenous injection of morphine (10 mg/kg) in sham-operated (SHAM) and in bilateral carotid sinus nerve transected (CSNX) Sprague-Dawley rats. In SHAM rats, morphine produced sustained respiratory depression (e.g., decreases in tidal volume, minute ventilation and inspiratory drive) and reduced the HVR and HCVR responses. Unexpectedly, morphine-induced suppression of HVR and HCVR were substantially greater in CSNX rats than in SHAM rats. This suggests that morphine did not compromise the function of the carotid body-chemoafferent complex and indeed, that the carotid body acts to defend against morphine-induced respiratory depression. These data are the first in vivo evidence that carotid body chemoreceptor afferents defend against rather than participate in OIRD in conscious rats. As such, drugs that stimulate ventilation by targeting primary glomus cells and/or chemoafferent terminals in the carotid bodies may help to alleviate OIRD.

Update of the Scientific Opinion on opium alkaloids in poppy seeds

Poppy seeds are obtained from the opium poppy (Papaver somniferum L.). They are used as food and to produce edible oil. The opium poppy plant contains narcotic alkaloids such as morphine and codeine. Poppy seeds do not contain the opium alkaloids, but can become contaminated with alkaloids as a result of pest damage and during harvesting. The European Commission asked EFSA to provide an update of the Scientific Opinion on opium alkaloids in poppy seeds. The assessment is based on data on morphine, codeine, thebaine, oripavine, noscapine and papaverine in poppy seed samples. The CONTAM Panel confirms the acute reference dose (ARfD) of 10 μg morphine/kg body weight (bw) and concluded that the concentration of codeine in the poppy seed samples should be taken into account by converting codeine to morphine equivalents, using a factor of 0.2. The ARfD is therefore a group ARfD for morphine and codeine, expressed in morphine equivalents. Mean and high levels of dietary exposure to morphine equivalents from poppy seeds considered to have high levels of opium alkaloids (i.e. poppy seeds from varieties primarily grown for pharmaceutical use) exceed the ARfD in most age groups. For poppy seeds considered to have relatively low concentrations of opium alkaloids (i.e. primarily varieties for food use), some exceedance of the ARfD is also seen at high levels of dietary exposure in most surveys. For noscapine and papaverine, the available data do not allow making a hazard characterisation. However, comparison of the dietary exposure to the recommended therapeutical doses does not suggest a health concern for these alkaloids. For thebaine and oripavine, no risk characterisation was done due to insufficient data. However, for thebaine, limited evidence indicates a higher acute lethality than for morphine and the estimated exposure could present a health risk.

Natural product modulators of human sensations and mood: molecular mechanisms and therapeutic potential

Humans perceive physical information about the surrounding environment through their senses. This physical information is registered by a collection of highly evolved and finely tuned molecular sensory receptors. A multitude of bioactive, structurally diverse ligands have evolved in nature that bind these molecular receptors. The complex, dynamic interactions between the ligands and the receptors lead to changes in our sensory perception or mood. Here, we review our current knowledge of natural products and their derived analogues that interact specifically with human G protein-coupled receptors, ion channels, and nuclear hormone receptors to modulate the sensations of taste, smell, temperature, pain, and itch, as well as mood and its associated behaviour. We discuss the molecular and structural mechanisms underlying such interactions and highlight cases where subtle differences in natural product chemistry produce drastic changes in functional outcome. We also discuss cases where a single compound triggers complex sensory or behavioural changes in humans through multiple mechanistic targets. Finally, we comment on the therapeutic potential of the reviewed area of research and draw attention to recent technological developments in genomics, metabolomics, and metabolic engineering that allow us to tap the medicinal properties of natural product chemistry without taxing nature.

Characterizing the subjective, observer-rated, and physiological effects of hydromorphone relative to heroin in a human laboratory study

BACKGROUND

This study compared the effects of the several doses of the opioid agonists heroin and hydromorphone across two routes of administration in humans. The goal was to guide development of human laboratory studies of opioid effects and inform subsequent injection pharmacotherapy trials of hydromorphone-assisted treatment.

METHODS

A within-subject (N = 16), double-blind, double-dummy, placebo-controlled, evaluation of acute doses of heroin and hydromorphone was completed at four dose levels (placebo, low, medium, high) across two routes of administration (intravenous, subcutaneous) in non-physically dependent, opioid-experienced individuals. Subject and observer ratings, as well as physiological outcomes, were assessed.

RESULTS

Within each route of administration, heroin and hydromorphone produced effects that were qualitatively similar on most variables across the doses examined. All effects were dose-dependent. The drugs produced different effects on VAS ratings of "Feels Like Heroin," a Heroin Identification Test, observer agonist ratings, and oxygen saturation levels. Drug-dependent differences emerged at the highest doses in all cases. Few significant main effects of Route were identified and their pattern was not uniform. Relative potency calculations across all subject, observer, and physiological outcomes that met analysis criteria revealed similar profiles and resulted in mean heroin:hydromorphone potencies of 3.35:1 and 2.88:1 for the intravenous and subcutaneous routes, respectively, and intravenous:subcutaneous potencies of 0.47:1 and 0.49:1 for heroin and hydromorphone, respectively.

CONCLUSIONS

Hydromorphone produced similar subjective and physiological effects as heroin, but was more potent than heroin. The current findings support the use of hydromorphone as a model for heroin in human laboratory and clinical treatment studies, and help identify appropriate hydromorphone dose conversion ratios to produce effects qualitatively similar to heroin.

Codeine and opioid metabolism: implications and alternatives for pediatric pain management

PURPOSE OF REVIEW

Use of perioperative opioids for surgical pain management of children presents clinical challenges because of concerns of serious adverse effects including life-threatening respiratory depression. This is especially true for children with history of obstructive sleep apnea. This review will explore current knowledge of clinically relevant factors and genetic polymorphisms that affect opioid metabolism and postoperative outcomes in children.

RECENT FINDINGS

Within the past several years, an increasing number of case reports have illustrated clinically important respiratory depression, anoxic brain injuries and even death among children receiving appropriate weight-based dosages of codeine and other opioids for analgesia at home setting particularly following tonsillectomy. Several national and international organizations have issued advisories on use of codeine in pediatrics, based on cytochrome P450 family 2 subfamily D type 6 (CYP2D6) pharmacogenetics. We have discussed the pros and cons of alternatives to codeine for pain management.

SUMMARY

Although routine preoperative genotyping to identify children at risk and personalized opioid use for pediatric perioperative pain management is still a distant reality, current known implications of CYP2D6 pharmacogenetics on codeine use shows that pharmacogenetics has the potential to guide anesthesia providers on perioperative opioid selection and dosing to maximize efficacy and safety.

Relationship between receptor occupancy and the antinociceptive effect of mu opioid receptor agonists in male rats

The analgesic mechanisms of mu opioid receptor (MOR) agonists, including receptor occupancy at the site of action, are not completely understood. The aims of the present study were to evaluate: (i) receptor occupancy in the rat brain after administration of MOR agonists; (ii) the relationship between occupancy and the antinociceptive effect. Morphine (2 or 4 mg/kg) or oxycodone (1 or 3 mg/kg) was subcutaneously administered to rats. The antinociceptive effect of these drugs was measured by the hot-plate test. MOR occupancy in the thalamus was assessed by conducting an ex vivo receptor binding assay using [³H] [D-Ala², N-MePhe⁴, Gly-ol]-enkephalin, followed by autoradiographic analysis. Both drugs produced antinociception in a dose-dependent manner, and these effects disappeared after the time point at which the maximal effect was elicited. Thalamic MOR occupancy was observed in a dose-dependent manner at the time point at which maximal antinociception was elicited, and relatively low occupancy was observed when the antinociceptive effect was decreasing. Good correlation between thalamic MOR occupancy and the antinociceptive effect was observed. These findings provide direct evidence for the receptor occupancy of MOR agonists at the site of action and its relationship with the analgesic effect.

Characterization of oxycodone in vitro metabolism by human cytochromes P450 and UDP-glucuronosyltransferases

The hepatic metabolism of oxycodone by cytochromes P450 (CYP) and the UDP-glucuronosyltransferases (UGT), the main metabolic enzymes of phase I and phase II, respectively, was assessed in vitro. The N-demethylation by CYP3A4/5 and the O-demethylation by CYP2D6 in human liver microsomes (HLM) followed Michaelis-Menten kinetics, with intrinsic clearances of 1.46μL/min/mg and 0.35μL/min/mg, respectively. Although noroxycodone and oxymorphone mainly contribute to the elimination of oxycodone, the simulated total in vivo clearance using in vitro phase I metabolism was underestimated. For the first time, metabolism of oxycodone by UGT was deeply investigated using HLM, recombinant enzymes and selective inhibitors. Oxycodone-glucuronide was mainly produced by UGT2B7 (K_m=762±153μM, V_max=344±20 peak area/min/mg) and to a lesser extent by UGT2B4 (K_m=2454±497μM, V_max=201±19 peak area/min/mg). Finally, the kinetics of the drug-drug interactions were assessed using two CYP and UGT cocktail approaches. Incubations of HLM with phase I and phase II drug probes showed that oxycodone mainly decreased the in vitro activities of CYP2D6, CYP3A4/5, UGT1A3, UGT1A6 and UGT2B subfamily with an important impact on UGT2B7.

The role of cytochrome P450 pharmacogenomics in chronic non-cancer pain patients

INTRODUCTION

Pharmacogenomics is the field that studies an individualized treatment approach for patients' medication regimen that can impact drug safety, productivity, and personalized health care. Pharmacogenomics characterizes the genetic differences in metabolic pathways which can affect a patient's individual responses to drug treatments. Areas covered: The various responses to pharmacological agents are mainly determined by the different types of genetic variants of the CYP450. CYP2D6 polymorphism is well known for its variation in the metabolism of drugs from many therapeutic arenas, including some analgesic drugs such as codeine, hydromorphone, oxycodone and tramadol. Allele combinations determine the phenotypic expression, characterized as either: extensive metabolizer, intermediate metabolizer, ultra-rapid metabolizer and poor metabolizer. Expert opinion: The Human Genome Project (HGP) revolutionized the future of medicine and the way health care providers approach individualized patient treatment, and chronic pain management is one of those areas. The key findings in the literature appear to be related to the CYP2D6 expression and its high polymorphism influencing the metabolism of opioid medications, and the impact of that on the patient's therapeutic outcome thus exemplifying the importance of genetic testing for CYP2D6 in the process of physician therapeutic decision making.

Pharmacokinetic considerations and recommendations in palliative care, with focus on morphine, midazolam and haloperidol

INTRODUCTION

A variety of medications are used for symptom control in palliative care, such as morphine, midazolam and haloperidol. The pharmacokinetics of these drugs may be altered in these patients as a result of physiological changes that occur at the end stage of life.

AREAS COVERED

This review gives an overview of how the pharmacokinetics in terminally ill patients may differ from the average population and discusses the effect of terminal illness on each of the four pharmacokinetic processes absorption, distribution, metabolism, and elimination. Specific considerations are also given for three commonly prescribed drugs in palliative care: morphine, midazolam and haloperidol).

EXPERT OPINION

The pharmacokinetics of drugs in terminally ill patients can be complex and limited evidence exists on guided drug use in this population. To improve the quality of life of these patients, more knowledge and more pharmacokinetic/pharmacodynamics studies in terminally ill patients are needed to develop individualised dosing guidelines. Until then knowledge of pharmacokinetics and the physiological changes that occur in the final days of life can provide a base for dosing adjustments that will improve the quality of life of terminally ill patients. As the interaction of drugs with the physiology of dying is complex, pharmacological treatment is probably best assessed in a multi-disciplinary setting and the advice of a pharmacist, or clinical pharmacologist, is highly recommended.

Genetic determinants of fetal opiate exposure and risk of neonatal abstinence syndrome: Knowledge deficits and prospects for future research

Opiate-dependent pregnant women receive opiate maintenance medications to prevent illicit use and withdrawal. Fetal opiate exposure causes central nervous system (CNS) alterations which manifest as postnatal physical withdrawal. The extensive variability in the Neonatal Abstinence Syndrome phenotype remains unexplained and may be related to variability in fetal exposure and response. Improved understanding of functionally significant genetic variants in pathways influencing placental opiate transfer and fetal response can lead to personalized maternal therapy and optimized neonatal outcomes.

Mechanistic basis of altered morphine disposition in nonalcoholic steatohepatitis

Morphine is metabolized in humans to morphine-3-glucuronide (M3G) and the pharmacologically active morphine-6-glucuronide (M6G). The hepatobiliary disposition of both metabolites relies upon multidrug resistance-associated proteins Mrp3 and Mrp2, located on the sinusoidal and canalicular membrane, respectively. Nonalcoholic steatohepatitis (NASH), the severe stage of nonalcoholic fatty liver disease, alters xenobiotic metabolizing enzyme and transporter function. The purpose of this study was to determine whether NASH contributes to the large interindividual variability and postoperative adverse events associated with morphine therapy. Male Sprague-Dawley rats were fed a control diet or a methionine- and choline-deficient diet to induce NASH. Radiolabeled morphine (2.5 mg/kg, 30 µCi/kg) was administered intravenously, and plasma and bile (0-150 or 0-240 minutes), liver and kidney, and cumulative urine were analyzed for morphine and M3G. The antinociceptive response to M6G (5 mg/kg) was assessed (0-12 hours) after direct intraperitoneal administration since rats do not produce M6G. NASH caused a net decrease in morphine concentrations in the bile and plasma and a net increase in the M3G/morphine plasma area under the concentration-time curve ratio, consistent with upregulation of UDP-glucuronosyltransferase Ugt2b1. Despite increased systemic exposure to M3G, NASH resulted in decreased biliary excretion and hepatic accumulation of M3G. This shift toward systemic retention is consistent with the mislocalization of canalicular Mrp2 and increased expression of sinusoidal Mrp3 in NASH and may correlate to increased antinociception by M6G. Increased metabolism and altered transporter regulation in NASH provide a mechanistic basis for interindividual variability in morphine disposition that may lead to opioid-related toxicity.