Opioid receptor agonist potencies of morphine and morphine-6-glucuronide in the guinea-pig ileum

Morphine-6-glucuronide induces contraction of the ileal circular muscle more potently than morphine in mice

It has been reported that morphine-6-glucuronide inhibits small intestinal transit in mice more potently than morphine. In this study, we investigated the effects of morphine, morphine-6-glucuronide and morphine-3-glucuronide on the contractile response of the circular muscle isolated from mouse ileum. Morphine and morphine-6-glucuronide induced tonic contraction dose-dependently, and the contractile force of morphine-6-glucuronide was greater than that of morphine. Morphine-3-glucuronide induced slight contraction at high dose. These results suggest that the strong contraction induced by morphine-6-glucuronide contributed to the inhibition of small intestinal transit in mice.

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.

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.

Morphine-6-glucuronide: an analgesic of the future?

Morphine-6-beta-glucuronide (M6G) is an opioid agonist that plays a role in the clinical effects of morphine. Although M6G probably crosses the blood-brain barrier with difficulty, during long term morphine administration it may reach sufficiently high CNS concentrations to exert clinically relevant opioid effects. As a consequence of its almost exclusive renal elimination, M6G may accumulate in the body of patients with impaired renal function and cause severe opioid adverse effects with insidious onset and long persistence. Its profile of receptor affinities, however, gives reason to speculate that M6G may exhibit analgesic effects while causing fewer adverse effects than morphine. This is supported by reports of the good tolerability of intrathecal and intravenous injections of M6G in humans with intact renal function. M6G may thus be contemplated as an analgesic for short term postoperative analgesia, especially for intrathecal analgesic therapy. In addition, its possibly higher potency than morphine makes M6G a candidate opioid for local or peripheral analgesic therapy. However, current knowledge is too incomplete to finally judge the clinical usefulness of M6G. The next topics for clinical research on M6G should include: (i) a comparison of the potencies of M6G and morphine to cause wanted and unwanted clinical effects; (ii) development of a predictive population pharmacokinetic-pharmacodynamic model of M6G with calculation of the transfer half-life between plasma and effect site; and (iii) identification of cofactors influencing the action of M6G that can serve as predictors for the clinical outcome of morphine/M6G therapy in an individual including the pharmacogenetics of M6G.

Morphine-6 beta-glucuronide has a higher efficacy than morphine as a mu-opioid receptor agonist in the rat locus coeruleus

1. The pharmacological properties of the active morphine metabolite, morphine-6 beta-D-glucuronide (M6G), and the parent compound were compared in rat locus coeruleus neurons by electrophysiological recording in brain slices. 2. M6G and morphine activated potassium currents in voltage clamped neurons, which were blocked by the opioid receptor antagonist naloxone. 3. Both M6G and morphine behaved as partial agonists that produced maximal responses smaller than the system maximum, which was measured using [Met(5)]-enkephalin. M6G produced a larger maximal response (78%) than morphine (62%), which we estimated was due to a 2 - 4 fold difference in the relative efficacy of the agonists. 4. 3-O-methoxynaltrexone, which has been reported to behave as a selective antagonist of a M6G preferring receptor, was equally effective at blocking currents produced by M6G and the selective mu-opioid receptor agonist DAMGO. 5. M6G currents were occluded by a prior application of morphine, and were reduced when mu-opioid receptors were desensitized by using [Met(5)]-enkephalin. 6. Morphine-3 beta-D-glucuronide did not affect action potential firing or membrane currents in locus coeruleus neurons and had no effect on currents produced by M6G. 7. These results show that the relative efficacy of M6G is higher than morphine in locus coeruleus neurons, contrary to what has been shown using mu-opioid receptors expressed in cell clones.

The pharmacological basis of contemporary pain management

Pain management has become an increasingly well researched area in medicine over recent years, and there have been advances in a number of areas. While opioids remain an integral part of pain-management strategies, there is now an emphasis on the use of adjuvant drugs, such as paracetamol and anti-inflammatory agents, which through physiological or pharmacological synergism, both enhance pain control and reduce opioid use. The management of neuropathic pain continues to be a challenge. Anti-epileptics and antidepressants, together with clonidine and ketamine, provide the foundations for treatment. Another area of interest has been the widespread use of patient-controlled analgesia and the administration of some drugs, especially opioids, by means other than traditional oral and parenteral routes. The number of new drugs that have reached the stage of clinical trials has been small, yet they offer exciting possibilities. The epibatidine analogue ABT-594 and zinconitide both offer novel approaches to the management of neuropathic pain states, while selective cyclo-oxygenase-2 inhibitors and nitroaspirins may see advances in the management of nociceptive pain states.

Pharmacokinetics of morphine-6-glucuronide and its formation from morphine after intravenous administration

BACKGROUND

Morphine-6-beta-glucuronide is a primary morphine metabolite with potent opioid action. However, its low and slow brain permeability eventually prevents its central opioid effects after short-term intravenous administration. Research is needed to establish whether morphine-6-beta-glucuronide qualifies as an analgesic; this study provides the pharmacokinetic bases for such studies.

METHODS

Plasma concentration-time data of morphine-6-beta-glucuronide and morphine obtained from 20 healthy volunteers after short-term intravenous administration of either morphine-6-beta-glucuronide or morphine were described by a biexponential disposition curve. Disposition parameters of morphine-6-beta-glucuronide and morphine were estimated by nonlinear regression, and basic pharmacokinetic parameters (clearance, volume of distribution at steady state, and mean disposition residence time) were derived. A new model of metabolite kinetics was applied, and the disposition parameters of morphine and morphine-6-beta-glucuronide were then used to fit the plasma concentration-time profile of morphine-6-beta-glucuronide formed from morphine. Thereby the fraction of morphine metabolized to morphine-6-beta-glucuronide and the mean transit time of morphine across the site of metabolism were estimated.

RESULTS

The extent and time course of morphine-6-beta-glucuronide formation from morphine could be well described by a parametric model, with a fraction of morphine metabolized to morphine-6-beta-glucuronide of 7.55% +/- 1.24% and a mean metabolic transit time for morphine to morphine-6-beta-glucuronide of 0.28 +/- 0.21 hour. The underlying disposition of morphine and morphine-6-beta-glucuronide was characterized by clearance (morphine clearance, 32.7 +/- 6 ml.min-1.kg-1, morphine-6-beta-glucuronide clearance, 2.2 +/- 0.4 ml.min-1.kg-1), volume of distribution at steady state (morphine, 1.8 +/- 0.3 L.hr-1; morphine-6-beta-glucuronide, 0.12 +/- 0.02 L.hr-1), and mean disposition residence time (morphine, 1.8 +/- 0.4 hours; morphine-6-beta-glucuronide, 1.7 +/- 0.4 hours).

CONCLUSIONS

The time course of morphine-6-beta-glucuronide formation kinetics was analyzed with use of the information on the disposition kinetics of both morphine and preformed morphine-6-beta-glucuronide, which was obtained by separate data fits. The transformation of morphine to morphine-6-beta-glucuronide could be described by two parameters characterizing the extent and delay of metabolite formation. The results of this study will serve as pharmacokinetic bases of future investigations of morphine-6-beta-glucuronide in human beings.

Morphine metabolites

Morphine is a potent opioid analgesic widely used for the treatment of acute pain and for long-term treatment of severe pain. Morphine is a member of the morphinan-framed alkaloids, which are present in the poppy plant. The drug is soluble in water, but its solubility in lipids is poor. In man, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) are the major metabolites of morphine. The metabolism of morphine occurs not only in the liver, but may also take place in the brain and the kidneys. The glucuronides are mainly eliminated via bile and urine. Glucuronides as a rule are considered as highly polar metabolites unable to cross the blood-brain barrier. Although morphine glucuronidation has been demonstrated in human brain tissue, the capacity is very low compared to that of the liver, indicating that the M3G and M6G concentrations observed in the cerebrospinal fluid (CSF) after systemic administration reflect hepatic metabolism of morphine and that the morphine glucuronides, despite their high polarity, can penetrate into the brain. Like morphine, M6G has been shown to be relatively more selective for mu-receptors than for delta- and kappa-receptors while M3G does not appear to compete for opioid receptor binding. The analgesic properties of M6G were recognised in the early 1970s and more recent work suggests that M6G might significantly contribute to the opioid analgesia after administration of morphine. The analgesic potency of M6G after intracerebroventricular (ICV) or intrathecal (IT) administration in rats is from 45-800 timer greater than that of morphine, depending on the animal species and the experimental antinociceptive test used. Furthermore, the development of a sensitive high-performance liquid chromatography (HPLC) assay for the quantitative determination of morphine, M6G and M3G has revealed that M6G and M3G were present in abundance after chronic oral morphine administration and that the area under the plasma concentration-time curve exceeded that of morphine. M3G has been found to antagonise morphine and M6G induced analgesia and ventilatory depression in the rat, which has led to the hypothesis that M3G may influence the development of morphine tolerance. M3G exhibits no analgesic effect after ICV or IT administration. Some studies do, however, indicate that M3G may cause non-opioid mediated hyperalgesia/allodynia and convulsions after IT administration in rats. These observations led to the hypothesis that M3G might be responsible for side-effects, hyperalgesia/allodynia and myoclonus seen after high-dose morphine treatment.

Pharmacokinetics of morphine and its glucuronides after intravenous infusion of morphine and morphine-6-glucuronide in healthy volunteers

Steady-state pharmacokinetics of morphine and morphine-6-glucuronide (M-6-G) after intravenous administration of either morphine or M-6-G were determined in healthy volunteers. With a dosing regimen calculated on the basis of data obtained in a first series of experiments in four subjects (morphine: intravenous loading dose of 0.24 mg/kg for 5 minutes and an intravenous infusion of 0.069 mg.kg-1.hr-1 for 4 hours; M-6-G: loading dose of 0.011 mg/kg for 5 minutes and an infusion of 0.006 mg.kg-1.hr-1 for 4 hours), it was possible to yield plasma concentrations of morphine and M-6-G in another four subjects close to predefined targeted levels (35 and 45.5 ng/ml morphine and M-6-G, respectively). This dosing regimen may be used in further pharmacodynamic studies to compare the analgesic effects of morphine and M-6-G. In addition, metabolite kinetics of M-6-G were calculated as a function of time with use of a linear systems approach to the estimation of rate and fraction of morphine glucuronidation to M-6-G.