Morphine-6beta-glucuronide and morphine-3-glucuronide, opioid receptor agonists with different potencies

Differential interaction patterns of opioid analgesics with µ opioid receptors correlate with ligand-specific voltage sensitivity

The µ opioid receptor (MOR) is the key target for analgesia, but the application of opioids is accompanied by several issues. There is a wide range of opioid analgesics, differing in their chemical structure and their properties of receptor activation and subsequent effects. A better understanding of ligand-receptor interactions and the resulting effects is important. Here, we calculated the respective binding poses for several opioids and analyzed interaction fingerprints between ligand and receptor. We further corroborated the interactions experimentally by cellular assays. As MOR was observed to display ligand-induced modulation of activity due to changes in membrane potential, we further analyzed the effects of voltage sensitivity on this receptor. Combining in silico and in vitro approaches, we defined discriminating interaction patterns responsible for ligand-specific voltage sensitivity and present new insights into their specific effects on activation of the MOR.

Heroin and its metabolites: relevance to heroin use disorder

Heroin is an opioid agonist commonly abused for its rewarding effects. Since its synthesis at the end of the nineteenth century, its popularity as a recreational drug has ebbed and flowed. In the last three decades, heroin use has increased again, and yet the pharmacology of heroin is still poorly understood. After entering the body, heroin is rapidly deacetylated to 6-monoacetylmorphine (6-MAM), which is then deacetylated to morphine. Thus, drug addiction literature has long settled on the notion that heroin is little more than a pro-drug. In contrast to these former views, we will argue for a more complex interplay among heroin and its active metabolites: 6-MAM, morphine, and morphine-6-glucuronide (M6G). In particular, we propose that the complex temporal pattern of heroin effects results from the sequential, only partially overlapping, actions not only of 6-MAM, morphine, and M6G, but also of heroin per se, which, therefore, should not be seen as a mere brain-delivery system for its active metabolites. We will first review the literature concerning the pharmacokinetics and pharmacodynamics of heroin and its metabolites, then examine their neural and behavioral effects, and finally discuss the possible implications of these data for a better understanding of opioid reward and heroin addiction. By so doing we hope to highlight research topics to be investigated by future clinical and pre-clinical studies.

An analog of [d-Trp]CJ-15,208 exhibits kappa opioid receptor antagonism following oral administration and prevents stress-induced reinstatement of extinguished morphine conditioned place preference

Opioid use disorder (OUD) relapse rates are discouragingly high, underscoring the need for new treatment options. The macrocyclic tetrapeptide natural product CJ-15,208 and its stereoisomer [d-Trp]CJ-15,208 demonstrate kappa opioid receptor (KOR) antagonist activity upon oral administration which prevents stress-induced reinstatement of cocaine-seeking behavior. In order to further explore the structure-activity relationships and expand the potential therapeutic applications of KOR antagonism for the treatment of OUD, we screened a series of 24 analogs of [d-Trp]CJ-15,208 with the goal of enhancing KOR antagonist activity. From this screening, analog 22 arose as a compound of interest, demonstrating dose-dependent KOR antagonism after central and oral administration lasting at least 2.5 h. In further oral testing, analog 22 lacked respiratory, locomotor, or reinforcing effects, consistent with the absence of opioid agonism. Pretreatment with analog 22 (30 mg/kg, p.o.) prevented stress-induced reinstatement of extinguished morphine conditioned place preference and reduced some signs of naloxone-precipitated withdrawal in mice physically dependent on morphine. Collectively, these data support the therapeutic potential of KOR antagonists to support abstinence in OUD and ameliorate opioid withdrawal.

Blood-Placental Barrier Transfers and Pharmacokinetics of Unbound Morphine in Pregnant Rats with Multiple Microdialysis Systems

Microdialysis coupled to an analytical system can be used to continuously monitor unbound protein analytes in any biological fluid, tissue, or organ of animals. To date, no application of microdialysis has been performed to simultaneously monitor unbound morphine and its metabolites in the placenta and fetus of pregnant rats. Our hypothesis is that morphine and its metabolite penetrate the blood-placental barrier to reach the fetus during pregnancy. To investigate this hypothesis, this study aimed to develop a microdialysis experimental animal model coupled with an analytical system to monitor morphine and morphine-3-glucuronide (M3G) in the maternal blood, placenta, fetus, and amniotic fluid of pregnant rats. To determine the analytes in dialysates, a validated ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method was developed. The pharmacokinetic results indicated that morphine fit well to a two-compartment model and exhibited nonlinear pharmacokinetic behavior within the dosage regimen. The M3G-to-morphine metabolite ratio, determined by the area under the concentration curve (AUC) ratio (AUC_M3G/AUC_morphine), was approximately 5.40 in the maternal blood. In terms of tissue distribution, the mother-to-fetus transfer ratio (AUC_fetus/AUC_blood) of morphine and M3G was about 0.34 and 0.18, respectively. In conclusion, the high metabolite ratio suggests that morphine has the characteristics of rapid biotransformation, and the mother-to-fetus transfer ratio indicates that morphine and M3G partially transfer the blood-placental barrier in pregnant rats. This newly developed multiple microdialysis coupled to UHPLC-MS/MS system can be applied to the studies of maternal pharmacokinetics and blood-placental transfer in pregnant rats.

The mixed kappa and delta opioid receptor agonist, MP1104, attenuates chemotherapy-induced neuropathic pain

Effective treatments for chronic pain without abuse liability are urgently needed. One in 5 adults suffer chronic pain and half of these patients report inefficient treatment. Mu opioid receptor agonists (MOP), including oxycodone, tramadol and morphine, are often prescribed to treat chronic pain, however, use of drugs targeting MOP can lead to drug dependency, tolerance and overdose deaths. Kappa opioid receptor (KOP) agonists have antinociceptive effects without abuse potential; however, they have not been utilised clinically due to dysphoria and sedation. We hypothesise that mixed opioid receptor agonists targeting the KOP and delta opioid receptor (DOP) would have a wider therapeutic index, with the rewarding effects of DOP negating the negative effects of KOP. MP1104, an analogue of 3-Iodobenzoyl naltrexamine, is a novel mixed opioid receptor agonist with potent antinociceptive effects mediated via KOP and DOP in mice without rewarding or aversive effects. In this study, we show MP1104 has potent, long-acting antinociceptive effects in the warm-water tail-withdrawal assay in male and female mice and rats; and is longer acting than morphine. In the paclitaxel-induced neuropathic pain model in mice, MP1104 reduced both mechanical and cold allodynia and unlike morphine, did not produce tolerance when administered daily for 23 days. Moreover, MP1104 did not induce sedative effects in the open-field locomotor activity test, respiratory depression in mice using whole-body plethysmography, or have cross-tolerance with morphine. This data supports the therapeutic development of mixed opioid receptor agonists, particularly mixed KOP/DOP agonists, as non-addictive pain medications with reduced tolerance.

Morphine plasmatic concentration in a pregnant mare and its foal after long term epidural administration

Background

Epidural administration of morphine has been shown to be an effective analgesic strategy in horses; however, the possible occurrence of side effects limits its usage. In order to decrease their frequency, it is important to target the minimal effective plasma concentration and avoid overdosing. As to date species-specific pharmacokinetics data are not available for epidural morphine, the dosing regimen is usually established on the basis of clinical reports and personal experience. In certain physiological conditions, like gestation, the outcome of an empirical dosing scheme can be unpredictable. The aim of this case report is to describe the pharmacological profile of morphine and its metabolites after prolonged epidural administration in a pregnant mare and her foal.

Case presentation

A 20 years old pregnant mare was presented to our hospital because of severe lameness, 2 months before delivery. Following an ineffective systemic pain treatment, an epidural catheter was inserted and morphine administered (initial dose 0.1 mg/kg every 8 h). Due to its efficacy in controlling pain, it was continued until end of gestation. Plasmatic concentration of morphine and its metabolites were assessed in the mare 6 weeks after starting the treatment, and in both the mare and foal during the first days after delivery. Plasmatic values similar to those previously reported in the literature following morphine short term administration through various routes and not accompanied by side effects were found in the mare, except during an excitatory period. Moreover, no evidence of dangerous drug accumulation or significant milk passage was noticed in the foal. Mild reduction of feces production with no signs of colic and two self-limiting episodes of excitement occurred during treatment in the mare. No side effects occurred during gestation and first phases of life in the foal.

Conclusion

Prolonged epidural administration of morphine in a pregnant mare allowed good pain control in absence of clinically relevant side effects, in both the mare and her foal. Sudden increase in morphine plasmatic concentration can occur and side effects appear; careful treatment to the lowest effective dose and continuous monitoring of the clinical condition of the treated horse should be performed.

Binding mode analyses of NAP derivatives as mu opioid receptor selective ligands through docking studies and molecular dynamics simulation

Mu opioid receptor selective antagonists are highly desirable because of their utility as pharmacological probes for receptor characterization and functional studies. Furthermore, the mu opioid receptors act as an important target in drug abuse and addiction treatment. Previously, we reported NAP as a novel lead compound with high selectivity and affinity towards the mu opioid receptor. Based on NAP, we have synthesized its derivatives and further characterized their binding affinities and selectivity towards the receptor. NMP and NGP were identified as the two most selective MOR ligands among NAP derivatives. In the present study, molecular modeling methods were applied to assess the dual binding modes of NAP derivatives, particularly on NMP and NGP, in three opioid receptors, in order to analyze the effects of structural modifications on the pyridyl ring of NAP on the binding affinity and selectivity. The results indicated that the steric hindrance, electrostatic, and hydrophobic effects caused by the substituents on the pyridyl ring of NAP contributed complimentarily on the binding affinity and selectivity of NAP derivatives to three opioid receptors. Analyses of these contributions provided insights on future design of more potent and selective mu opioid receptor ligands.

Comparison and analysis of the animal models used to study the effect of morphine on tumour growth and metastasis

The effect of opioids on tumour growth and metastasis has been debated for many years, with recent emphasis on the possibility that they might influence the rate of disease-free survival after tumour resection when used in the perioperative pain management of cancer surgery patients. The literature presents conflicting and inconclusive in vitro and in vivo data about the potential effect of opioids, especially morphine, on tumour growth and metastasis. To inform clinical practice, appropriate animal models are needed to test whether opioids alter the course of tumour growth and metastasis. Here, we review the literature on animal-based studies testing the effect of morphine on cancer so far, and analyse differences between the models used that may explain the discrepancies in published results. Such analysis should elucidate the role of opioids in cancer and help define ideal pre-clinical models to provide definitive answers.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Relevance of UDP-glucuronosyltransferase polymorphisms for drug dosing: A quantitative systematic review

UDP-glucuronosyltransferases (UGT) catalyze the biotransformation of many endobiotics and xenobiotics, and are coded by polymorphic genes. However, knowledge about the effects of these polymorphisms is rarely used for the individualization of drug therapy. Here, we present a quantitative systematic review of clinical studies on the impact of UGT variants on drug metabolism to clarify the potential for genotype-adjusted therapy recommendations. Data on UGT polymorphisms and dose-related pharmacokinetic parameters in man were retrieved by a systematic search in public databases. Mean estimates of pharmacokinetic parameters were extracted for each group of carriers of UGT variants to assess their effect size. Pooled estimates and relative confidence bounds were computed with a random-effects meta-analytic approach whenever multiple studies on the same variant, ethnic group, and substrate were available. Information was retrieved on 30 polymorphic metabolic pathways involving 10 UGT enzymes. For irinotecan and mycophenolic acid a wealth of data was available for assessing the impact of genetic polymorphisms on pharmacokinetics under different dosages, between ethnicities, under comedication, and under toxicity. Evidence for effects of potential clinical relevance exists for 19 drugs, but the data are not sufficient to assess effect size with the precision required to issue dose recommendations. In conclusion, compared to other drug metabolizing enzymes much less systematic research has been conducted on the polymorphisms of UGT enzymes. However, there is evidence of the existence of large monogenetic functional polymorphisms affecting pharmacokinetics and suggesting a potential use of UGT polymorphisms for the individualization of drug therapy.

Binding mode characterization of 6α- and 6β-N-heterocyclic substituted naltrexamine derivatives via docking in opioid receptor crystal structures and site-directed mutagenesis studies: application of the 'message-address' concept in development of mu opioid receptor selective antagonists

Highly selective opioid receptor antagonists are essential pharmacological probes in opioid receptor structural characterization and opioid agonist functional studies. Currently, there is no highly selective, nonpeptidyl and reversible mu opioid receptor antagonist available. Among a series of naltrexamine derivatives that have been designed and synthesized, two compounds, NAP and NAQ, were previously identified as novel leads for this purpose based on their in vitro and in vivo pharmacological profiles. Both compounds displayed high binding affinity and selectivity to the mu opioid receptor. To further study the interaction of these two ligands with the three opioid receptors, the recently released opioid receptor crystal structures were employed in docking studies to further test our original hypothesis that the ligands recognize a unique 'address' domain in the mu opioid receptor involving Trp318 that facilitates their selectivity. These modeling results were supported by site-directed mutagenesis studies on the mu opioid receptor, where the mutants Y210A and W318A confirmed the role of the latter in binding. Such work not only enriched the 'message-address' concept, also facilitated our next generation ligand design and development.

Induction of morphine-6-glucuronide synthesis by heroin self-administration in the rat

RATIONALE

Heroin is rapidly metabolized to morphine that in turn is transformed into morphine-3-glucuronide (M3G), an inactive metabolite at mu-opioid receptor (MOR), and morphine-6-glucuronide (M6G), a potent MOR agonist. We have found that rats that had received repeated intraperitoneal injections of heroin exhibit measurable levels of M6G (which is usually undetectable in this species).

OBJECTIVE

The goal of the present study was to investigate whether M6G synthesis can be induced by intravenous (i.v.) heroin self-administration (SA).

MATERIALS AND METHODS

Rats were trained to self-administer either heroin (50 μg/kg per infusion) or saline for 20 consecutive 6-h sessions and then challenged with an intraperitoneal challenge of 10 mg/kg of heroin. Plasma levels of heroin, morphine, 6-mono-acetyl morphine, M3G, and M6G were quantified 2 h after the challenge. In vitro morphine glucuronidation was studied in microsomal preparations obtained from the liver of the same rats.

RESULTS

Heroin SA induced the synthesis of M6G, as indicated by detectable plasma levels of M6G (89.7 ± 37.0 ng/ml vs. 7.35 ± 7.35 ng/ml after saline SA). Most important, the in vitro V (max) for M6G synthesis was correlated with plasma levels of M6G (r (2) = 0.78). Microsomal preparations from saline SA rats produced negligible amounts of M6G.

CONCLUSION

Both in vivo and in vitro data indicate that i.v. heroin SA induces the synthesis of M6G. These data are discussed in the light of previous studies conducted in heroin addicts indicating that in humans heroin enhances the synthesis of the active metabolite of heroin and morphine.

Morphine metabolism, transport and brain disposition

The chemical structures of morphine and its metabolites are closely related to the clinical effects of drugs (analgesia and side-effects) and to their capability to cross the Blood Brain Barrier (BBB). Morphine-6-glucuronide (M6G) and Morphine-3-glucuronide (M3G) are both highly hydrophilic, but only M6G can penetrate the BBB; accordingly, M6G is considered a more attractive analgesic than the parent drug and the M3G. Several hypotheses have been made to explain these differences. In this review we will discuss recent advances in the field, considering brain disposition of M6G, UDP-glucoronosyltransferases (UGT) involved in morphine metabolism, UGT interindividual variability and transport proteins.

A systematic review of the use of opioid medication for those with moderate to severe cancer pain and renal impairment: a European Palliative Care Research Collaborative opioid guidelines project

BACKGROUND

Opioid use in patients with renal impairment can lead to increased adverse effects. Opioids differ in their effect in renal impairment in both efficacy and tolerability. This systematic literature review forms the basis of guidelines for opioid use in renal impairment and cancer pain as part of the European Palliative Care Research Collaborative's opioid guidelines project.

OBJECTIVE

The objective of this study was to identify and assess the quality of evidence for the safe and effective use of opioids for the relief of cancer pain in patients with renal impairment and to produce guidelines.

SEARCH STRATEGY

The Cochrane Database of Systematic Reviews, Cochrane Central Register of Controlled Trials, MedLine, EMBASE and CINAHL were systematically searched in addition to hand searching of relevant journals.

SELECTION CRITERIA

Studies were included if they reported a clinical outcome relevant to the use of selected opioids in cancer-related pain and renal impairment. The selected opioids were morphine, diamorphine, codeine, dextropropoxyphene, dihydrocodeine, oxycodone, hydromorphone, buprenorphine, tramadol, alfentanil, fentanyl, sufentanil, remifentanil, pethidine and methadone. No direct comparator was required for inclusion. Studies assessing the long-term efficacy of opioids during dialysis were excluded.

DATA COLLECTION AND ANALYSIS

This is a narrative systematic review and no meta-analysis was performed. The Grading of RECOMMENDATIONS Assessment, Development and Evaluation (GRADE) approach was used to assess the quality of the studies and to formulate guidelines.

MAIN RESULTS

Fifteen original articles were identified. Eight prospective and seven retrospective clinical studies were identified but no randomized controlled trials. No results were found for diamorphine, codeine, dihydrocodeine, buprenorphine, tramadol, dextropropoxyphene, methadone or remifentanil.

CONCLUSIONS

All of the studies identified have a significant risk of bias inherent in the study methodology and there is additional significant risk of publication bias. Overall evidence is of very low quality. The direct clinical evidence in cancer-related pain and renal impairment is insufficient to allow formulation of guidelines but is suggestive of significant differences in risk between opioids.

RECOMMENDATIONS

RECOMMENDATIONS regarding opioid use in renal impairment and cancer pain are made on the basis of pharmacokinetic data, extrapolation from non-cancer pain studies and from clinical experience. The risk of opioid use in renal impairment is stratified according to the activity of opioid metabolites, potential for accumulation and reports of successful or harmful use. Fentanyl, alfentanil and methadone are identified, with caveats, as the least likely to cause harm when used appropriately. Morphine may be associated with toxicity in patients with renal impairment. Unwanted side effects with morphine may be satisfactorily dealt with by either increasing the dosing interval or reducing the 24 hour dose or by switching to an alternative opioid.

Analysis of polar metabolites by hydrophilic interaction chromatography–MS/MS

Increasing emphasis has been placed on quantitative characterization of drug metabolites during drug discovery and development. Due to the more polar nature of drug metabolites, quantitative analysis using traditional reversed-phase liquid chromatography tandem mass spectrometry (RPLC–MS/MS) can be quite challenging. As an alternative chromatographic mode, hydrophilic interaction chromatography (HILIC) offers unique advantages for analysis of polar metabolites, providing better retention/separation, higher sensitivity, higher efficiency and potential for ultra-fast analysis to improve throughput. In this article, selected case studies from the authors’ own laboratory, and examples from current literature, will be discussed to demonstrate some practical considerations for method development of HILIC–MS/MS assays. The effectiveness of using HILIC–MS/MS for mitigating analytical challenges associated with quantitation of polar metabolites, including phase I and II metabolites of drugs, as well as endogenous metabolites, will be exhibited.

Phosphorylation of GluR1, ERK, and CREB during spontaneous withdrawal from chronic heroin self-administration

Negative motivational symptoms are observed soon after withdrawal from chronic opiate administration, and are thought to mediate dependence. Examination of brain region-specific signaling changes that accompany early withdrawal may shed light on neural mechanisms underlying negative reinforcement and dependence. Thus, we measured alterations in protein phosphorylation in multiple limbic brain regions in rats undergoing 24 h spontaneous or naltrexone-precipitated withdrawal from chronic (6 h/day) i.v. heroin self-administration. Region-specific increases in cyclic AMP-dependent GluR(1) (S845) phosphorylation were found in the nucleus accumbens shell, basolateral amygdala, hippocampal CA1 and CA3 subregions, and premotor cortex from 12 to 24 h of spontaneous withdrawal, and there were no changes in prefrontal cortex, nucleus accumbens core or caudate-putamen. Increased GluR(1) (S845) phosphorylation was detected earlier (12 h withdrawal) in the central amygdala and ventral tegmental area. In contrast, prominent increases in extracellular signal-regulated kinase phosphorylation were found in both prefrontal and premotor cortex, and CA1 and CA3 between 12 and 24 h withdrawal. Phosphorylation of striatal cyclic AMP response element binding protein increased in the caudate-putamen but not in the nucleus accumbens. Naltrexone administration after 24 h withdrawal increased extracellular signal-regulated kinase phosphorylation in the central amygdala, and nucleus accumbens core and shell. Thus, spontaneous withdrawal from heroin self-administration produces region- and time-dependent changes in cyclic AMP and extracellular signal-regulated kinase activity that could contribute to the behavioral manifestation of opiate dependence.

Design, synthesis, and biological evaluation of 6alpha- and 6beta-N-heterocyclic substituted naltrexamine derivatives as mu opioid receptor selective antagonists

Opioid receptor selective antagonists are important pharmacological probes in opioid receptor structural characterization and opioid agonist functional study. Thus far, a nonpeptidyl, highly selective and reversible mu opioid receptor (MOR) antagonist is unavailable. On the basis of our modeling studies, a series of novel naltrexamine derivatives have been designed and synthesized. Among them, two compounds were identified as leads based on the results of in vitro and in vivo assays. Both of them displayed high binding affinity for the MOR (K(i) = 0.37 and 0.55 nM). Compound 6 (NAP) showed over 700-fold selectivity for the MOR over the delta receptor (DOR) and more than 150-fold selectivity over the kappa receptor (KOR). Compound 9 (NAQ) showed over 200-fold selectivity for the MOR over the DOR and approximately 50-fold selectivity over the KOR. Thus these two novel ligands will serve as leads to further develop more potent and selective antagonists for the MOR.

The effect of naloxone-3-glucuronide on colonic transit time in healthy men after acute morphine administration: a placebo-controlled double-blinded crossover preclinical volunteer study

BACKGROUND

Constipation is a significant side effect of opioid therapy. We have previously demonstrated that naloxone-3-glucuronide (NX3G) antagonizes the motility-lowering-effect of morphine in the rat colon.

AIM

To find out whether oral NX3G is able to reduce the morphine-induced delay in colonic transit time (CTT) without being absorbed and influencing the analgesic effect.

METHODS

Fifteen male volunteers were included. Pharmacokinetics: after oral administration of 0.16 mg/kg NX3G, blood samples were collected over a 6-h period. Pharmacodynamics: NX3G or placebo was then given at the start time and every 4 h thereafter. Morphine (0.05 mg/kg) or placebo was injected s.c. 2 h after starting and thereafter every 6 h for 24 h. CTT was measured over a 48-h period by scintigraphy. Pressure pain threshold tests were performed.

RESULTS

Neither NX3G nor naloxone was detected in the venous blood. The slowest transit time was observed during the morphine phase, which was significantly different from morphine with NX3G and placebo. The pain perception was not significantly influenced by NX3G.

CONCLUSIONS

Orally administered NX3G is able to reverse the morphine-induced delay of CTT in humans without being detected in peripheral blood samples. Therefore, NX3G may improve symptoms of constipation in-patients using opioid medication without affecting opioid-analgesic effects.

Role of active metabolites in the use of opioids

The opioid class of drugs, a large group, is mainly used for the treatment of acute and chronic persistent pain. All are eliminated from the body via metabolism involving principally CYP3A4 and the highly polymorphic CYP2D6, which markedly affects the drug's function, and by conjugation reactions mainly by UGT2B7. In many cases, the resultant metabolites have the same pharmacological activity as the parent opioid; however in many cases, plasma metabolite concentrations are too low to make a meaningful contribution to the overall clinical effects of the parent drug. These metabolites are invariably more water soluble and require renal clearance as an important overall elimination pathway. Such metabolites have the potential to accumulate in the elderly and in those with declining renal function with resultant accumulation to a much greater extent than the parent opioid. The best known example is the accumulation of morphine-6-glucuronide from morphine. Some opioids have active metabolites but at different target sites. These are norpethidine, a neurotoxic agent, and nordextropropoxyphene, a cardiotoxic agent. Clinicians need to be aware that many opioids have active metabolites that will become therapeutically important, for example in cases of altered pathology, drug interactions and genetic polymorphisms of drug-metabolizing enzymes. Thus, dose individualisation and the avoidance of adverse effects of opioids due to the accumulation of active metabolites or lack of formation of active metabolites are important considerations when opioids are used.

Effect of naloxone-3-glucuronide and N-methylnaloxone on the motility of the isolated rat colon after morphine

The effect of the opioid antagonists naloxone-3-glucuronide and N-methylnaloxone on rat colon motility after morphine stimulation was measured. The rat model consisted of the isolated, vascularly perfused colon. The antagonists (10(-4) M, intraluminally) and morphine (10(-4) M, intra-arterially) were administered from 20 to 30 and from 10 to 50 min, respectively. Colon motility was determined by the luminal outflow. The antagonist concentrations in the luminal and venous outflow were measured by high-performance liquid chromatography. Naloxone-3-glucuronide and N-methylnaloxone reversed the morphine-induced reduction of the luminal outflow to baseline within 10 and 20 min, respectively. These antagonists were then excreted in the luminal outflow and could not be found in the venous samples. Naloxone, produced by hydrolysis or demethylation, was not detectable. In conclusion, highly polar naloxone derivatives peripherally antagonize the motility-lowering effect of morphine in the perfused isolated rat colon, are stable, and are not able to cross the colon-mucosal blood barrier.

Effect of repeated administrations of heroin, naltrexone, methadone, and alcohol on morphine glucuronidation in the rat

RATIONALE

Heroin is rapidly metabolized to morphine that in turn is transformed in morphine-3-glucuronide (M3G), an inactive metabolite, and morphine-6-glucuronide (M6G), a potent mu-opioid receptor (MOR) agonist. We have found that heroin addicts exhibit higher M6G/M3G ratios relative to morphine-treated control subjects. We have also shown that heroin-treated rats exhibit measurable levels of M6G (which is usually undetectable in this species) and reduced levels of M3G.

OBJECTIVE

We investigated the role of MOR in these effects of heroin, by examining the effects of methadone, a MOR agonist, and of naltrexone, a MOR antagonist, on morphine glucuronidation. We also investigated the effects of alcohol, which is known to alter drug metabolism and is frequently coabused by heroin addicts.

METHODS

Morphine glucuronidation was studied in liver microsomes obtained from rats exposed daily for 10 days to saline, heroin (10 mg/kg, i.p.), naltrexone (20-40 mg/kg, i.p.), heroin + naltrexone (10 mg/kg+20-40 mg/kg, i.p.), methadone (5-20 mg/kg, i.p.), or 10% ethanol.

RESULTS

Heroin induced the synthesis of M6G and decreased the synthesis of M3G. Naltrexone exhibited intrinsic modulatory activity on morphine glucuronidation, increasing the synthesis of M3G via a low-affinity/high-capacity reaction characterized by positive cooperativity. The rate of M3G synthesis in the heroin + naltrexone groups was not different from that of the naltrexone groups. Methadone and ethanol induced a modest increase in M3G synthesis and had no effect on M6G synthesis.

CONCLUSION

The effects of heroin on morphine glucuronidation are not shared by methadone or alcohol (two drugs that figure prominently in the natural history of heroin addiction) and do not appear to depend on the activation of MOR.

Opioid metabolites

The metabolism of opioids closely relates to their chemical structure. Opioids are subject to O-dealkylation, N-dealkylation, ketoreduction, or deacetylation leading to phase-I metabolites. By glucuronidation or sulfatation, phase-II metabolites are formed. Some metabolites of opioids have an activity themselves and contribute to the effects of the parent compound. This can go as far that the main clinical activity is exerted through active metabolites while the parent compounds are only weak agonist at mu-opioid receptors, as in the case of codeine and tilidine. The clinical effects of tramadol also involve an important contribution of its active metabolite. With morphine, the active metabolite morphine-6-glucuronide exerts important clinical opioid effects when it accumulates in the plasma of patients with renal failure. However, after short-term administration of morphine, its contribution to the central nervous effects of morphine is probably poor. Morphine-6-glucuronide has recently been identified to exert important peripheral opioid effects. By this, it may play an important role in the clinical effects of morphine. Several other opioids, such as meperidine and perhaps also morphine and hydromorphone, produce metabolites with neuroexcitatory effects. In sum, the evidence suggests that the metabolites of several opioids account for an important part of the clinical effects that must be considered in clinical practice.

Morphine-6-glucuronide: Actions and mechanisms

Morphine-6-glucuronide (M6G) appears to show equivalent analgesia to morphine but to have a superior side-effect profile in terms of reduced liability to induce nausea and vomiting and respiratory depression. The purpose of this review is to examine the evidence behind this statement and to identify the possible reasons that may contribute to the profile of M6G. The vast majority of available data supports the notion that both M6G and morphine mediate their effects by activating the micro-opioid receptor. The differences for which there is a reasonable consensus in the literature can be summarized as: (1) Morphine has a slightly higher affinity for the micro-opioid receptor than M6G, (2) M6G shows a slightly higher efficacy at the micro-opioid receptor, (3) M6G has a lower affinity for the kappa-opioid receptor than morphine, and (4) M6G has a very different absorption, distribution, metabolism, and excretion (ADME) profile from morphine. However, none of these are adequate alone to explain the clinical differences between M6G and morphine. The ADME differences are perhaps most likely to explain some of the differences but seem unlikely to be the whole story. Further work is required to examine further the profile of M6G, notably whether M6G penetrates differentially to areas of the brain involved in pain and those involved in nausea, vomiting, and respiratory control or whether micro-opioid receptors in these brain areas differ in either their regulation or pharmacology.

Single- and Multiple-Dose Pharmacokinetic and Dose-Proportionality Study of??Oxymorphone Immediate-Release Tablets

INTRODUCTION

Oxymorphone hydrochloride (referred to as oxymorphone), a semisynthetic mu-opioid agonist, is known to produce a more rapid onset of action and greater analgesic potency compared with its parent compound, morphine. Until recently, oxymorphone has been available only in suppository and intravenous formulations. This study examined the pharmacokinetics and dose proportionality of a new immediate-release (IR) tablet formulation of oxymorphone and its metabolites (6-OH-oxymorphone and oxymorphone-3-glucuronide) following single- and multiple-dose administration in healthy volunteers.

STUDY DESIGN

A randomised, three-way crossover design was employed, with a target sample size of 24 healthy men and women.

METHODS

A single dose of oxymorphone IR (5, 10 and 20mg) was administered on day 1. After drug washout on day 2, study participants then received the same dose every 6 hours (22 total doses) on days 3 to 8. Treatment periods were separated by a 7-day washout. Naltrexone hydrochloride was coadministered to prevent opioid-related adverse events. Blood was collected up to 48 hours after day 1 to determine single-dose pharmacokinetics and up to 6 hours after the last dose for determination of pharmacokinetics at steady state.

RESULTS

Twenty-three of 24 enrolled subjects (12 men, 11 women) completed the study. Following a single dose of 5, 10 or 20mg, the oxymorphone IR mean area under the plasma concentration versus time curve from time zero to infinity ([AUC(infinity)] 4.5, 9.1 and 20.1 microg . h/L, respectively) and maximum plasma concentration ([C(max)] 1.1, 1.9 and 4.4 microg/L, respectively) confirmed dose proportionality. 6-OH-oxymorphone and oxymorphone-3-glucuronide also increased in an approximate 2-fold fashion. Similar results were observed for AUC and C(max) of oxymorphone and its metabolites at steady state. Steady state was achieved within 3 days of 6-hourly administration. The median t(max) (time to reach C(max)) was 0.5 hours for all single doses of oxymorphone and at steady state, and the terminal elimination half-life (t(1/2)) was approximately 7.3-9.4 hours. Adverse events were generally mild, and no clinically significant changes in laboratory or other safety variables were noted.

DISCUSSION

Because successful pain management often requires careful drug titration across a wide therapeutic dose range, it is important that opioid formulations provide predictable increases in drug concentration with increasing dose. The single-dose and steady-state pharmacokinetic profiles of oxymorphone IR tablets were linear and dose proportional across the dose range from 5 to 20mg.

Morphine metabolites as novel analgesic drugs?

PURPOSE OF REVIEW

Morphine metabolites have attracted continuing interest for their contribution to the desired and unwanted effects of morphine. Among the metabolites of morphine, morphine-6-glucuronide has been given most scientific attention. It accounts for 10% of the morphine metabolism, acts as an agonist at mu-opioid receptors and exerts antinociceptive effects. This review summarizes the recent findings on morphine-6-glucuronide and discusses its potential use as an analgesic.

RECENT FINDINGS

Morphine-6-glucuronide has a very long delay between the time course of its plasma concentrations and the time course of its central nervous effects, with 6-8 h probably the longest transfer half-life between plasma and effect site of all opioids administered in humans. This complicates the control of morphine-6-glucuronide therapy when used as an intravenous analgesic, and the long duration of action confers no advantage over other opioids because long-lasting opioid analgesia can be readily obtained with sustained release formulations of other opioids. During acute treatment, however, morphine-6-glucuronide appears to be sufficiently potent to exert peripheral analgesic effects, without exerting major central nervous opioid side effects for a short period of time. The side effects profile does not clearly separate morphine-6-glucuronide from morphine, with reports of similar side effects. There are contrasting reports, however, about similar or less respiratory depression and other side effects compared with morphine after systemic injection.

SUMMARY

Morphine-6-glucuronide might qualify as an analgesic but it has several pharmacological properties that make it far from ideal for therapeutic use. Whether it will be a useful addition to the currently established analgesics has yet to be demonstrated.

Repeated exposures to heroin and/or cadmium alter the rate of formation of morphine glucuronides in the rat

After absorption, heroin is transformed into mono-acetyl-morphine and then into morphine. Morphine, in turn, is metabolized to morphine-3-glucuronide (M3G), an inactive compound, and morphine-6-glucuronide (M6G), a potent opioid agonist. Thus, changes in the rate of formation of M6G may alter the pharmacological consequences of a treatment with heroin or morphine. In this study, we investigate the effect of repeated exposures (10 daily i.p. injections) to heroin, morphine, cadmium (which has been previously shown to inhibit M3G formation in vitro), or heroin + cadmium on morphine glucuronidation both in vivo and ex vivo (i.e., microsomal preparation obtained from rats treated in vivo). Repeated heroin (2.5, 5.0, and 10 mg/kg) increased plasma levels of M6G (which was undetectable in all other groups) and reduced those of M3G. Also, the microsomal preparations obtained from the liver of repeated heroin rats, when incubated with morphine, yielded significant amounts of M6G (which was undetectable in all other groups) and decreased levels of M3G relative to the control groups. These effects were reversible upon discontinuation of heroin administration. In contrast, repeated morphine (10, 20, and 40 mg/kg) only slightly reduced M3G formation at the dose of 40 mg/kg. Repeated cadmium (5, 15, and 45 microg/kg) reduced the rate of M3G formation without inducing M6G synthesis. The effects of the repeated coadministration of heroin (10 mg/kg) and cadmium (15 microg/kg) were virtually identical to those of repeated heroin alone. In summary, repeated exposure of rats to heroin can shift morphine glucuronidation toward the formation of the active metabolite M6G.

High levels of morphine-6-glucuronide in street heroin addicts

RATIONALE

In the body, heroin is rapidly transformed to 6-acetylmorphine (6-AM) and then to morphine, that in turn is mainly metabolized to morphine-3-glucuronide (M3G) and, at lesser extent, to morphine-6-glucuronide (M6G). Unlike M3G, M6G is a potent opioid agonist. Intravenous heroin abusers (IHU) are exposed to a wide array of drugs and contaminants that might affect glucuronidation.

OBJECTIVES

We assessed plasma and urine concentrations of M3G and M6G in four groups of subjects: the first two included long-term IHU either exposed to street heroin ( n=8) or receiving a single IV injection of morphine ( n=4), while the other two groups included non-IHU patients receiving acute IV ( n=8) or chronic oral ( n=6) administrations of morphine.

METHODS

After solid phase extraction plasma and urine concentrations of morphine metabolites were determined by HPLC analyses.

RESULTS

M3G accounted for the greater part of morphine glucuronides detected in body fluids of non-IHU patients treated with morphine. This pattern of metabolism remained stable across 15 days of oral administration of incremental doses of morphine. In contrast, the two groups of IHU (street heroin taking or morphine-treated subjects) showed a reduction of blood and urine M3G concentrations in favor of M6G. Consequently, M6G/M3G ratio was significantly higher in the two IHU groups in comparison with the non-IHU groups.

CONCLUSIONS

Chronic exposure to street heroin causes a relative increase in concentrations of the active metabolite, M6G. Since the pattern of M6G action seems closer to heroin than to morphine, the increased synthesis of M6G observed in IHU may prolong the narrow window of heroin effects.

Beta-phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids and compounds derived from them, including further products of oxidation, condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2001 to June 2002 is reviewed, with 581 references cited.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

The role of the hydrophilic Asn230 residue of the mu-opioid receptor in the potency of various opioid agonists

1. To investigate the effect of the hydrophilic Asn amino acid at position 230 of the human mu-opioid receptor (hMOR230) on the potency of various agonists, we mutated this residue to Thr and Leu (hMORN230T and hMORN230L respectively). 2. Taking advantage of the functional coupling of the opioid receptor with the heteromultimeric G-protein-coupled inwardly rectifying K(+) (GIRK1/GIRK2) channel, either the wild type hMOR or one of the mutated receptors (hMORN230L or hMORN230T) were functionally coexpressed with GIRK1/GIRK2 channels and a regulator of G-protein signalling (RGS4) in Xenopus laevis oocytes. 3. The two-microelectrode voltage clamp technique was used to measure the opioid receptor-activated GIRK1/GIRK2 channel responses. The potency of [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO), remained unaffected as measured via hMORN230T and hMORN230L, while the potency of fentanyl and morphine significantly increased via these mutated receptors. 4. Our results are indicative for the existence of hydrophobic interactions between a methyl-group of the side chain of Thr or Leu on the one hand and the piperidine-ring of fentanyl and the hexene-ring of morphine on the other. The mutations also had no influence on the potency of morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G). 5. We conclude that the hydrophilic side chain of Asn in position 230 is not involved in the formation of a H-bond with the aliphatic alcohol of morphine and that an enhancement of the potency of morphine and fentanyl can be explained by mutating this residue towards more hydrophobic amino acids.