Metabolite Morphine-6-glucuronide Research Articles

Bridging the gap between in silico and in vivo by modeling opioid disposition in a kidney proximal tubule microphysiological system

Opioid overdose, dependence, and addiction are a major public health crisis. Patients with chronic kidney disease (CKD) are at high risk of opioid overdose, therefore novel methods that provide accurate prediction of renal clearance (CLr) and systemic disposition of opioids in CKD patients can facilitate the optimization of therapeutic regimens. The present study aimed to predict renal clearance and systemic disposition of morphine and its active metabolite morphine-6-glucuronide (M6G) in CKD patients using a vascularized human proximal tubule microphysiological system (VPT-MPS) coupled with a parent-metabolite full body physiologically-based pharmacokinetic (PBPK) model. The VPT-MPS, populated with a human umbilical vein endothelial cell (HUVEC) channel and an adjacent human primary proximal tubular epithelial cells (PTEC) channel, successfully demonstrated secretory transport of morphine and M6G from the HUVEC channel into the PTEC channel. The in vitro data generated by VPT-MPS were incorporated into a mechanistic kidney model and parent-metabolite full body PBPK model to predict CLr and systemic disposition of morphine and M6G, resulting in successful prediction of CLr and the plasma concentration–time profiles in both healthy subjects and CKD patients. A microphysiological system together with mathematical modeling successfully predicted renal clearance and systemic disposition of opioids in CKD patients and healthy subjects.

Physiologically based pharmacokinetic/pharmacodynamic model for the prediction of morphine brain disposition and analgesia in adults and children.

Morphine is a widely used opioid analgesic, which shows large differences in clinical response in children, even when aiming for equivalent plasma drug concentrations. Age-dependent brain disposition of morphine could contribute to this variability, as developmental increase in blood-brain barrier (BBB) P-glycoprotein (Pgp) expression has been reported. In addition, age-related pharmacodynamics might also explain the variability in effect. To assess the influence of these processes on morphine effectiveness, a multi-compartment brain physiologically based pharmacokinetic/pharmacodynamic (PB-PK/PD) model was developed in R (Version 3.6.2). Active Pgp-mediated morphine transport was measured in MDCKII-Pgp cells grown on transwell filters and translated by an in vitro-in vivo extrapolation approach, which included developmental Pgp expression. Passive BBB permeability of morphine and its active metabolite morphine-6-glucuronide (M6G) and their pharmacodynamic parameters were derived from experiments reported in literature. Model simulations after single dose morphine were compared with measured and published concentrations of morphine and M6G in plasma, brain extracellular fluid (ECF) and cerebrospinal fluid (CSF), as well as published drug responses in children (1 day- 16 years) and adults. Visual predictive checks indicated acceptable overlays between simulated and measured morphine and M6G concentration-time profiles and prediction errors were between 1 and -1. Incorporation of active Pgp-mediated BBB transport into the PB-PK/PD model resulted in a 1.3-fold reduced brain exposure in adults, indicating only a modest contribution on brain disposition. Analgesic effect-time profiles could be described reasonably well for older children and adults, but were largely underpredicted for neonates. In summary, an age-appropriate morphine PB-PK/PD model was developed for the prediction of brain pharmacokinetics and analgesic effects. In the neonatal population, pharmacodynamic characteristics, but not brain drug disposition, appear to be altered compared to adults and older children, which may explain the reported differences in analgesic effect.

Deep proteomics and phosphoproteomics reveal novel biological pathways perturbed by morphine, morphine-3-glucuronide and morphine-6-glucuronide in human astrocytes.

Tolerance and hyperalgesia associated with chronic exposure to morphine are major limitations in the clinical management of chronic pain. At a cellular level, neuronal signaling can in part account for these undesired side effects, but unknown mechanisms mediated by central nervous system glial cells are likely also involved. Here we applied data-independent acquisition mass spectrometry to perform a deep proteome and phosphoproteome analysis of how human astrocytes responds to opioid stimulation. We unveil time- and dose-dependent effects induced by morphine and its major active metabolites morphine-3-glucuronide (M3G) and morphine-6-glucuronide that converging on activation of mitogen-activated protein kinase and mammalian target of rapamycin signaling pathways. We also find that especially longer exposure to M3G leads to significant dysregulation of biological pathways linked to extracellular matrix organization, antigen presentation, cell adhesion, and glutamate homeostasis, which are crucial for neuron- and leukocyte-astrocyte interactions.

CaMKII is activated in opioid induced conditioned place preference, but αCaMKII Thr286 autophosphorylation is not necessary for its establishment

Activation of calcium/calmodulin-dependent protein kinase II (CaMKII), particularly its α isoform, is known to be important for neuronal processes central for learning and memory and has also been implicated in the maladaptive learning involved in drug addiction.Thr286 autophosphorylation of αCaMKII has been shown to be indispensable for establishment of cocaine-induced CPP (Easton et al., 2014). To study the contribution of CaMKII in opioid induced conditioned learning, we examined how establishment of conditioned place preference (CPP) induced by 10 or 30 μmol/kg morphine or its active metabolite morphine-6-glucuronide (M6G) affects the levels and Thr286 autophosphorylation of the α- and β-isoforms of CaMKII, as well as β-actin levels, in dorsal and ventral striatum and in hippocampus of mice. An acute and a sub-chronic treatment were used as controls. Whereas an acute single administration of morphine or M6G caused increases in CaMKII levels and phosphorylation at Thr286 and β-actin in striatal areas, CPP induced by these opioids was accompanied primarily by an increase in the protein levels of both CaMKII isoforms and β-actin in dorsal striatum and hippocampus. Decreases in CaMKII Thr286 phosphorylation were observed in dorsal striatum after the sub-chronic pharmacological treatment. Despite the changes observed in αCaMKII activity in wild type mice, morphine-induced CPP was not affected in αCaMKIIT286A autophosphorylation-deficient mice. These results indicate that opioid-induced CPP is accompanied by activation of α- and βCaMKII in striatum and hippocampus, but, in opposition to what has been observed with cocaine, αCaMKII autophosphorylation is not essential for establishment of opioid-induced CPP.

A Randomized, Crossover Study on the Effect of Food on the Pharmacokinetic Characteristics of Morphine ARER (MorphaBond™ ER), an Abuse-Deterrent Formulation of Extended-Release Morphine.

IntroductionFood can alter the pharmacokinetics of certain abuse-deterrent formulations. Morphine ARER is an oral abuse-deterrent formulation of ER morphine sulfate tablets formulated with physical and chemical properties that contribute to the abuse-deterrent aspects of the drug. This study compared the relative bioavailability of Morphine ARER in the presence and absence of food.MethodsThis was a randomized, single-dose, two-treatment, crossover study in which healthy adults received Morphine ARER 100 mg under fasting and fed conditions. Subjects were given naltrexone 50 mg to limit opioid effects. Plasma concentrations of morphine and its active metabolite morphine-6-glucuronide (M6G) were obtained up to 48 h post-dose; area under the plasma concentration-time curve (AUC) from time 0 extrapolated to infinity (AUC0–∞), maximum observed plasma concentration (Cmax) and time to Cmax (Tmax) were calculated. Safety was evaluated by observation or report of adverse events, which were monitored during the treatment periods.ResultsOf 28 enrolled subjects, 27 completed all treatments; 1 subject in the fasted group withdrew voluntarily. Under fed conditions, the Cmax for morphine was 33% higher (44.78 vs. 33.30 ng/ml for fed and fasted conditions, respectively) and the median Tmax was 30 min longer than under fasted conditions. The overall morphine exposure (AUC0–∞) was similar for fed (440.6 ng · h/ml) vs. fasted conditions (395.1 ng · h/ml). For M6G, the Cmax and AUC0–∞ were similar under both conditions, and the median Tmax for M6G was 60 min longer under fed conditions. Common adverse events were somnolence and nausea.ConclusionMorphine ARER can be administered without regard to food.Plain Language SummaryPlain language summary available for this article.FundingInspirion Delivery Sciences, LLC.

Advances in the Physicochemical Profiling of Opioid Compounds of Therapeutic Interest.

This review focuses on recent developments in the physicochemical profiling of morphine and other opioids. The acid‐base properties and lipophilicity of these compounds is discussed at the microscopic, species‐specific level. Examples are provided where this type of information can reveal the mechanism of pharmacokinetic processes at the submolecular level. The role of lipophilicity in quantitative structure–activity relationship (QSAR) studies of opioids is reviewed. The physicochemical properties and pharmacology of the main metabolites of morphine are also discussed. Recent studies indicate that the active metabolite morphine‐6‐glucuronide (M6G) can contribute to the analgesic activity of systemically administered morphine. The unexpectedly high lipophilicity of M6G partly accounts for its analgesic activity. When administered parenterally, another suspected minor metabolite, morphine‐6‐sulfate (M6S) has superior antinociceptive effects to those of morphine. However, because sulfate esters of morphine derivatives cannot cross the blood‐brain barrier these esters may be good candidates to develop peripheral analgesic drugs.

Morphine Use in Renal Failure: A Case Report of Single-Dose Morphine Toxicity in a Patient Requiring Peritoneal Dialysis

Objective: We describe a case of morphine toxicity presumably caused by accumulation of its active metabolite morphine-6-glucuronide (M6G) secondary to reduced clearance by peritoneal dialysis. Methods: We present the relevant history and laboratory data and review pertinent literature regarding the use of opioids in renal failure. Results: A 76-year-old African-American female received one dose of morphine sulfate and developed signs and symptoms of morphine toxicity for 2 days, even after multiple peritoneal dialysis sessions. Conclusion: Because of reduced renal clearance of morphine and its metabolites in patients requiring peritoneal dialysis, morphine should be avoided as an analgesic option in this population due to increased risk of morphine toxicity.

Relative Oral Bioavailability of an Abuse-deterrent, Extended-release Formulation of Morphine Versus Extended-release Morphine: A 2-period, Single-dose, Randomized Crossover Study in Healthy Subjects

PurposeMorphine ARER is a novel oral, abuse-deterrent, extended-release (ER) formulation of morphine sulfate with physical and chemical properties that deter misuse and abuse by nonoral routes of administration. Here we evaluate the relative bioavailability of morphine ARER and extended-release morphine. MethodsThis single-dose, 2-treatment, 2-period, 2-sequence, randomized crossover study in healthy adult subjects compared the relative bioavailability of morphine ARER 100 mg to that of ER morphine 100 mg in the fasted condition. At 12 and 1.5 hours before dosing and 12 hours after dosing, all subjects received a 50-mg oral naltrexone tablet to minimize opioid-related side effects. Pharmacokinetic parameters including the AUC0–t, AUC0–∞, and Cmax of morphine and its metabolite morphine-6-glucuronide (M6G) were determined at various times up to 48 hours postdose. The bioequivalence of morphine ARER and ER morphine was determined using an ANOVA of the least-squares mean values of morphine and M6G bioavailability. FindingsForty-nine subjects completed the study. Both morphine ARER and ER morphine exhibited peak plasma morphine and M6G concentrations of ∼30 ng/mL and ∼200 ng/mL, respectively, at 3 hours postdose. The 90% CIs of the ln-transformed values of morphine AUC0–t, AUC0–∞, and Cmax were within the 80% to 125% range for bioequivalence. M6G values also indicated bioequivalence of morphine ARER and ER morphine. The most common adverse events were nausea and somnolence. ImplicationsThese data show that, in these subjects, morphine ARER was bioequivalent to ER morphine, a treatment for pain with well-established efficacy and safety profiles.

Activation of μ-opioid receptor and Toll-like receptor 4 by plasma from morphine-treated mice

In this study, we quantified the ability of opioids present in biological samples to activate the μ-opioid receptor and TLR4 using cell-based assays. Each assay was standardised, in the presence of plasma, using morphine, its μ receptor-active metabolite morphine-6 glucuronide (M6G) and its μ receptor-inactive, but TLR4-active metabolite morphine-3 glucuronide (M3G). Specificity was verified using antagonists. Morphine- and M6G-spiked plasma samples exhibited μ receptor activation, which M3G-spiked plasma lacked. In contrast, M3G showed moderate but consistent activation of TLR-4. Plasma samples were collected at a number of time points from mice administered morphine (1 or 10mg/kg every 12h for 3days) or saline. Morphine administration led to intermittent μ receptor activation, reversed by μ receptor antagonists, and to TRL4 activation at time points where M3G is measured in plasma. Interestingly, this protocol of morphine administration also led to TLR4-independent NF-κB activation, at time points where M3G was not detected, presumably via elevation of circulating cytokines including, but not limited to, TNFα. Circulating TNFα was increased after three days of morphine administration, and TNFα mRNA elevated in the spleen of morphine-treated mice.

Post-mortem levels and tissue distribution of codeine, codeine-6-glucuronide, norcodeine, morphine and morphine glucuronides in a series of codeine-related deaths

This article presents levels and tissue distribution of codeine, codeine-6-glucuronide (C6G), norcodeine, morphine and the morphine metabolites morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) in post-mortem blood (peripheral and heart blood), vitreous fluid, muscle, fat and brain tissue in a series of 23 codeine-related fatalities. CYP2D6 genotype is also determined and taken into account. Quantification of codeine, C6G, norcodeine, morphine, M3G and M6G was performed with a validated solid phase extraction LC–MS method. The series comprise 19 deaths (83%) attributed to mixed drug intoxication, 4 deaths (17%) attributed to other causes of death, and no cases of unambiguous monointoxication with codeine. The typical peripheral blood concentration pattern in individual cases was C6G≫codeine≫norcodeine>morphine, and M3G>M6G>morphine. In matrices other than blood, the concentration pattern was similar, although in a less systematic fashion. Measured concentrations were generally lower in matrices other than blood, especially in brain and fat, and in particular for the glucuronides (C6G, M3G and M6G) and, to some extent, morphine. In brain tissue, the presumed active moieties morphine and M6G were both below the LLOQ (0.0080mg/L and 0.058mg/L, respectively) in a majority of cases. In general, there was a large variability in both measured concentrations and calculated blood/tissue concentration ratios. There was also a large variability in calculated ratios of morphine to codeine, C6G to codeine and norcodeine to codeine in all matrices, and CYP2D6 genotype was not a reliable predictor of these ratios. The different blood/tissue concentration ratios showed no systematic relationship with the post-mortem interval. No coherent degradation or formation patterns for codeine, morphine, M3G and M6G were observed upon reanalysis in peripheral blood after storage.

Quantification of morphine, morphine 6-glucuronide, buprenorphine, and the enantiomers of methadone by enantioselective mass spectrometric chromatography in whole blood.

Deaths among drug addicts are frequently caused by intoxication with methadone and/or morphine. These drugs are often used in combination with other drugs, such as buprenorphine. In addition, methadone is generally used as a mixture of R- and S-enantiomers. To date, a method for separation and quantitation of these specific drugs has not been developed. The aim of this study was to develop a sensitive enantioselective method for quantitation of morphine, its active metabolite morphine 6-glucuronide, buprenorphine, and R- and S-methadone, in a single analytical run. Whole blood samples were diluted with 0.5 mol/L ammonium carbonate buffer and extracted on a Bond Elut C18 solid-phase extraction column with an automatic solid-phase extraction system. Chromatographic separation was performed on a chiral alpha-1-acid glycoprotein column with an acetonitrile/ammonium acetate buffer (10 mmol/L, pH 7.0, 22:78 v/v) mobile phase. The whole blood concentrations of the drugs were quantified by mass spectrometry using their stable isotope-labeled compounds as internal standards. The method was validated with respect to specificity, linearity, precision, limits of detection, and quantification and matrix effects. The precision (coefficient of variation) was below 15%, and the accuracy was between 90 and 115%. This method will be useful for routine analyses in forensic laboratories where blood samples are frequently analyzed for drugs of abuse. In some cases, sudden death from methadone overdose is caused by the enantiomeric form of the methadone, which makes the enantiomer separation capability of this method important.

A validated method for simultaneous determination of codeine, codeine-6-glucuronide, norcodeine, morphine, morphine-3-glucuronide and morphine-6-glucuronide in post-mortem blood, vitreous fluid, muscle, fat and brain tissue by LC-MS.

The toxicodynamics and, to a lesser degree, toxicokinetics of the widely used opiate codeine remain a matter of controversy. To address this issue, analytical methods capable of providing reliable quantification of codeine metabolites alongside codeine concentrations are required. This article presents a validated method for simultaneous determination of codeine, codeine metabolites codeine-6-glucuronide (C6G), norcodeine and morphine, and morphine metabolites morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) in post-mortem whole blood, vitreous fluid, muscle, fat and brain tissue by high-performance liquid chromatography mass spectrometry. Samples were prepared by solid-phase extraction. The validated ranges were 1.5-300 ng/mL for codeine, norcodeine and morphine, and 23-4,600 ng/mL for C6G, M3G and M6G, with exceptions for norcodeine in muscle (3-300 ng/mL), morphine in muscle, fat and brain (3-300 ng/mL) and M6G in fat (46-4,600 ng/mL). Within-run and between-run accuracy (88.1-114.1%) and precision (CV 0.6-12.7%), matrix effects (CV 0.3-13.5%) and recovery (57.8-94.1%) were validated at two concentration levels; 3 and 150 ng/mL for codeine, norcodeine and morphine, and 46 and 2,300 ng/mL for C6G, M3G and M6G. Freeze-thaw and long-term stability (6 months at -80°C) was assessed, showing no significant changes in analyte concentrations (-12 to +8%). The method was applied in two authentic forensic autopsy cases implicating codeine in both therapeutic and presumably lethal concentration levels.

Quantification of morphine and its major metabolites M3G and M6G in antemortem and postmortem samples.

Morphine is one of the most effective agents for the control of significant pain, primarily metabolized to morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). While M6G is a potent opioid agonist, M3G has no opioid action and seems to have a role in side-effects caused by morphine. In this study, a reversed-phase high-performance liquid chromatographic method with diode-array and electrochemical detection was developed for the simultaneous determination of morphine, M3G and M6G in antemortem and postmortem samples (plasma, whole blood, urine, liver, kidney and brain). Morphine, glucuronides and internal standard were extracted by double solid-phase extraction and the separation was carried out with a Waters Spherisorb® ODS2 reversed-phase column and potassium phosphate buffer (pH = 2.2)–acetonitrile containing sodium dodecyl sulfate as the mobile phase. The method proved to be specific with good linearity for all analytes in a calibration range from 1 to 600 ng/mL and proved to be accurate and have adequate precision and recovery. Limits of detection in the studied matrices were 0.4–4.5 ng/mL for morphine, 2.7–6.1 ng/mL for M3G and 0.8–4.4 ng/mL for M6G. The proposed method can be successfully applied to quantify morphine and its metabolites in several biological samples, covering the major routes of distribution, metabolism and elimination of morphine.

In vitro morphine metabolism by rat microglia

Morphine is mainly transformed to morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) in the liver. Glucuronidation is also performed by rat brain homogenates and UDP-glucuronosyltransferases (UGTs) are present in the brain. Here we investigated the possibility that microglia transforms morphine into its metabolites M3G and M6G. Primary cultures of neonatal rat microglia were incubated for different intervals of time in basal conditions or with different concentrations of morphine. The following measures were performed on these cultures and/or in the medium: (i) morphine as well as M3G and M6G concentrations; (ii) levels of mRNA coding for UGT1A1, UGT1A6, UGT1A7, and UGT2B1 as well as their protein levels; (iii) released prostaglandin (PG)E2 and nitrite concentrations. Results show that in basal conditions morphine and M3G are produced by microglia; accordingly, these cells expressed UGT1A1, UGT1A6 and UGT1A7, but not UGT2B1. When cultures were exposed to different concentrations of exogenous morphine, M6G was also synthesized. This shift in the glucuronidation was associated with variations in the expression of UGT isozymes. In particular, UGT1A7 expression was rapidly upregulated and this event was translated into enhanced protein levels of UGT1A7; lesser effects were exerted on UGT1A1 and UGT1A6. Upon prolonged exposure to morphine, microglial cell UGT expression returned to baseline conditions or even to reduced levels of expression. Morphine exposure did not affect the synthesis of both PGE2 and nitrites, ruling out a generalized priming of microglia by morphine. In conclusion, this study suggests that morphine glucuronides found in the cerebrospinal liquor upon peripheral morphine administration may at least in part be brain-born, reconciling the conceptual gap between the high hydrophilic features of morphine glucuronides and their presence beyond the blood–brain barrier.

Inhibition of Morphine Glucuronidation in the Liver Microsomes of Rats and Humans by Monoterpenoid Alcohols

Morphine is an important drug used to alleviate moderate to severe pain. This opiate is mainly metabolized by glucuronidation to a non-analgesic metabolite, morphine-3-glucuronide (M-3-G) and an active metabolite morphine-6-glucuronide (M-6-G). To understand the modulation of morphine glucuronidation activity by environmental factors, the effect of endogenous and food-derived compounds on morphine uridine 5'-diphosphate (UDP)-glucuronosyltransferase (UGT) in rat and human microsomes was evaluated examining the 50% inhibitory concentration (IC(50)). The liver microsomes from Sprague-Dawley rats (RLM) and humans (HLM, 150 donors, pooled microsomes) were used as enzyme sources. Of 27 compounds tested, monoterpenoid alcohols, such as borneol and iso-borneol, exhibited a strong inhibitory effect on morphine glucuronidation in rat liver microsomes (RLM), whereas we failed to detect any inhibitory effect of endogenous substances including amino acids and sugars. The substances which have the ability to inhibit the activity in RLM are also inhibitory toward morphine glucuronidation in HLM and UGT2B7 baculosomes. However, the difference was that while the strongest inhibitory effect was observed for iso-menthol in HLM, borneol was the strongest inhibitor of the activity mediated by RLM. Although zidovudine is a typical substrate of UGT2B7, the inhibition of morphine glucuronidation by zidovudine was far weaker than that of monoterpenoid alcohols. These results demonstrate that dietary and supplementary monoterpenoid alcohols modify the pharmacokinetics and pharmacodynamics of morphine through inhibition of UGT2B7.

Correlation between myoclonus and the 3-glucuronide metabolites in patients treated with morphine or hydromorphone: A pilot study

The 3-glucuronide metabolites of morphine and hydromorphone have been implicated as a causative factor for patients exhibiting myoclonus. The primary goal of this study was to determine plasma levels of morphine-3-glucuronide (M3G) or hydromorphone-3-glucuronide (H3G) in patients demonstrating myoclonus and identify any trends or associations between the two. Patients were recruited from San Diego Hospice and the Institute for Palliative Medicine's inpatient unit. A prospective convenience sample comprised of 17 subjects, 12 with myoclonus and 5 without myoclonus. Analysis included demographic, metabolic and clinical variables. Plasma was assayed via high performance liquid chromatography for morphine, M3G, and morphine-6-glucuronide or hydromorphone and hydromorphone-3-glucuronide. No trends or associations were identified between plasma levels of M3G or H3G and myoclonus. Ratio levels of 3-glucuronide metabolite to their corresponding parent opioid were dramatically lower than anticipated. In this small pilot study, it appears that the serum levels of metabolites M3G and H3G do not correlate with myoclonus.

Influence of the CYP2D6 polymorphism and hemodialysis on codeine disposition in patients with end-stage renal disease

We studied the influence of three factors on drug disposition: genetic polymorphism, impaired renal excretion of drug metabolites, and the possible elimination by hemodialysis (HD), using codeine as a model drug. Based on the genotyping of three CYP2D6 polymorphisms in 228 HD patients, nine extensive metabolizers (EMs) and two poor metabolizers (PMs) were given a single oral dose of 50 mg codeine phosphate. Plasma concentrations of its metabolites codeine-6-glucuronide (C6G), morphine-3-glucuronide (M3G), and morphine-6-glucuronide (M6G) were determined after 2, 4, 6, 8 and 24 h (beginning of the HD session) and again after 4 h of HD (28 h). Codeine metabolites in plasma were quantitated by liquid chromatography-mass spectrometry (LC-MS). The concentrations of C6G in plasma were high and similar in EMs and PMs. Two hours after the codeine intake, the mean concentration of M3G was 210 nM in EMs vs. 3.5 nM in PMs. The M6G metabolite concentrations could be quantitated in EMs but were below the limit of quantification in PMs (<1 nM). All three codeine metabolites/glucuronides remained unchanged or even increased until the start of HD, and thereafter, the concentrations decreased dramatically during the HD procedure. Formation of the codeine metabolites M3G and M6G was dependent on the CYP2D6 genotype, as previously shown in healthy individuals. Elimination of glucuronides in these patients was absent until HD was performed. These factors need to be taken into consideration when drugs metabolized by CYPs are prescribed in HD patients.

Comparative Pharmacological Profiles of Morphine and Oxycodone under a Neuropathic Pain-Like State in Mice: Evidence for Less Sensitivity to Morphine

The present study was undertaken to investigate pharmacological actions induced by morphine and oxycodone under a neuropathic pain-like state. In the mu-opioid receptor (MOR) binding study and G-protein activation, we confirmed that both morphine and oxycodone showed MOR agonistic activities. Mice with sciatic nerve ligation exhibited the marked neuropathic pain-like behavior. Under these conditions, antinociception induced by subcutaneously (s.c.) injected morphine was significantly decreased by sciatic nerve ligation, whereas s.c. injection of oxycodone produced a profound antinociception in sciatic nerve-ligated mice. There were no significant differences in spinal or supraspinal antinociception of morphine and oxycodone between sham operation and nerve ligation. Moreover, either morphine- or oxycodone-induced increase in guanosine-5'-o-(3-thio) triphosphate ([(35)S]GTPgammaS) binding in the spinal cord, periaqueductal gray matter and thalamus in sciatic nerve-ligated mice was similar to that in sham-operated mice. Antinociception induced by s.c., intrathecal, or intracerebroventricular injection of the morphine metabolite morphine-6-glucuronide (M-6-G) was significantly decreased by sciatic nerve ligation. Furthermore, the increase in the G-protein activation induced by M-6-G was eliminated in sciatic nerve ligation. In addition, either morphine- or oxycodone-induced rewarding effect was dramatically suppressed under a neuropathic pain-like state. The increased [(35)S]GTPgammaS binding by morphine or oxycodone was significantly lower in the lower midbrain of mice with sciatic nerve ligation compared with that in control mice. These findings provide further evidence that oxycodone shows a profound antinociceptive effect under a neuropathic pain-like state with less of a rewarding effect. Furthermore, the reduction in G-protein activation induced by M-6-G may, at least in part, contribute to the suppression of the antinociceptive effect produced by morphine under a neuropathic pain-like state.

Morphine‐3‐glucuronide inhibits morphine induced, but enhances morphine‐6‐glucuronide induced locomotor activity in mice

The main metabolite of morphine, morphine‐3‐glucuronide (M3G) has no opioid effects. Some studies have rather indicated that it antagonizes the antinociceptive and respiratory depressive effects of both morphine and the active metabolite morphine‐6‐glucuronide (M6G). We studied the possible influence of M3G on the psychostimulant properties of morphine and M6G measured by locomotor activity. Mice were given two injections, one with either 80, 240 or 500 μmol/kg M3G or saline followed by an injection of 20 or 30 μmol/kg morphine or M6G. M3G influenced the locomotor activity induced by both morphine and M6G, but in opposite directions. M3G reduced the morphine induced locomotor activity during the first hour following morphine injection in a concentration dependent manner. M3G pretreatment did not significantly influence brain concentrations of morphine indicating that the interaction was of a pharmacodynamic type. In contrast M3G pretreatment increased the M6G induced locomotor activity. M3G pretreatment increased serum and brain M6G concentrations to an extent indicating that this interaction was mainly of a pharmacokinetic type. In conclusion our results disclose complicated interactions between morphine and its two metabolites with respect to induction of locomotor activity and possibly also with respect to mechanisms related to drug reward.

Conditioned place preference induced by morphine and morphine-6-glucuronide in mice

Morphine-6-glucuronide (M6G), an active metabolite of morphine has been shown to produce analgesia and fewer side effects than morphine, and the introduction of M6G as a new drug for treatment of postoperative pain is planned in 2007. Following morphine intake in humans, the metabolites morphine-3-glucuronide (M3G) and M6G are present in substantial concentrations and for longer periods than the parent drug. The possible reward effects of the morphine glucuronides have previously not been well studied. In the present study, conditioned place preference (CPP) was recorded after conditioning with subcutaneous injections of 5, 10, 20, 30 or 50 μmol/kg morphine or M6G, or 240 or 500 μmol/kg M3G in C57BL/6J-Bom mice, using a biased two compartment (“closed” and “open”) counterbalanced paradigm. CPP was induced after treatment with both morphine and M6G with dose dependent increase up to 30 μmol/kg after treatment in the “closed” compartment. No dose response was observed in the “open” compartment, with maximal CPP after 10 μmol/kg morphine or M6G. M3G caused a tendency of condition place aversion (CPA), although not statistically significant. In the present study morphine and M6G demonstrated comparable reward effects, at doses that differed depending on which compartment the mice were conditioned in. M3G showed a tendency to exhibit aversive properties.