Elevated concentrations of morphine 6-beta-D-glucuronide in brain extracellular fluid despite low blood-brain barrier permeability

Improved Brain Uptake and Pharmacological Activity Profile of Morphine-6-Glucuronide Using a Peptide Vector-Mediated Strategy

Morphine-6-glucuronide (M6G), an active metabolite of morphine, has been shown to have significantly attenuated brain penetration relative to that of morphine. Recently, we have demonstrated that conjugation of various drugs to peptide vectors significantly enhances their brain uptake. In this study, we have conjugated morphine-6-glucuronide to a peptide vector SynB3 to enhance its brain uptake and its analgesic potency after systemic administration. We show by in situ brain perfusion that vectorization of M6G (Syn1001) markedly enhances the brain uptake of M6G. This enhancement results in a significant improvement in the pharmacological activity of M6G in several models of nociception. Syn1001 was about 4 times more potent than free M6G (ED(50) of 1.87 versus 8.74 micromol/kg). Syn1001 showed also a prolonged duration of action compared with free M6G (300 and 120 min, respectively). Furthermore, the conjugation of M6G results in a lowered respiratory depression, as measured in a rat model. Taken together, these data strongly support the utility of peptide-mediated strategies for improving the efficacy of drugs such as M6G for the treatment of pain.

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.

The potential of nasal application for delivery to the central brain-a microdialysis study of fluorescein in rats

Previous animal studies have shown that various types of nasally administered drugs and model substances can access the central nervous system (CNS) via direct transport across the olfactory epithelium, and thereby circumventing the blood-brain barrier (BBB). These compounds, however, have mainly been identified in the cerebrospinal fluid and the olfactory bulbs which are usually not pharmacologically relevant targets. The aim of the present study was to evaluate the potential of targeting the central brain by olfactory absorption by use of sodium fluorescein as a hydrophilic model substance with limited permeability across the blood-brain barrier. Microdialysis probes were implanted in blood and in right and left side of the brain (striatum) in rats. The pharmacokinetics of sodium fluorescein was studied from 0 to 180min following intravenous and unilateral nasal administration without occlusion of the oesophagus. Pharmacokinetic modelling showed a significantly higher absorption rate and lower T(max) in the ipsilateral striatum (0.097min(-1) and 41min) compared with the contralateral side (0.056min(-1) and 54min). The rate of elimination in brain was significantly lower after nasal administration (0.004min(-1)) compared with intravenous administration (0.012min(-1)). However, the brain to plasma area under the curve ratios of model substance were low (2-3%) and not significantly different between right and left side of the brain, regardless of the route of administration. The results obtained by microdialysis were supported by findings in whole brain homogenates where concentrations of fluorescein were approximately 40% higher in the right striatum compared with the left side initially after nasal administration to the right nostril of rats. Despite some indications of olfactory transport to the central rat brain it was concluded that the drug targeting potential of sodium fluorescein and most likely other hydrophilic compounds is limited.

Blood-brain barrier transport of morphine in patients with severe brain trauma

AIMS

In experimental studies, morphine pharmacokinetics is different in the brain compared with other tissues due to the properties of the blood-brain barrier, including action of efflux pumps. It was hypothesized in this clinical study that active efflux of morphine occurs also in human brain, and that brain injury would alter cerebral morphine pharmacokinetics.

METHODS

Patients with traumatic brain injury, equipped with one to three microdialysis catheters in the brain and one in abdominal subcutaneous fat for metabolic monitoring, were studied. The cerebral catheter locations were classified as 'better' and 'worse' brain tissue, referring to the degree of injury. Morphine (10 mg) was infused intravenously over a 10-min period in seven patients in the intensive care setting. Tissue and plasma morphine concentrations were obtained during the subsequent 3-h period with microdialysis and regular blood sampling.

RESULTS

The area under the concentration-time curve (AUC) ratio of unbound morphine in brain tissue to plasma was 0.64 (95% confidence interval 0.40, 0.87) in 'better' brain tissue (P < 0.05 vs. the subcutaneous fat/plasma ratio), 0.78 (0.49, 1.07) in 'worse' brain tissue and 1.00 (0.86, 1.13) in subcutaneous fat. The terminal half-life and T(max) were longer in the brain vs. plasma and fat, respectively. The relative recovery for morphine was higher in 'better' than in 'worse' brain tissue. The T(max) value tended to be shorter in 'worse' brain tissue.

CONCLUSIONS

The unbound AUC ratio below unity in the 'better' human brain tissue demonstrates an active efflux of morphine across the blood-brain barrier. The 'worse' brain tissue shows a decrease in relative recovery for morphine and in some cases also an increase in permeability for morphine over the blood-brain barrier.

Altered brain exposure of morphine in experimental meningitis studied with microdialysis

BACKGROUND

During pathologic conditions such as meningitis and traumatic brain injury the function of the blood-brain barrier (BBB) is disturbed. In the present study we examined the cerebral pharmacokinetic pattern of morphine in the intact brain and during experimentally induced meningitis using a pig model. Secondly, the use of intracerebral microdialysis as a potential tool for monitoring damage in the BBB by studying the pharmacokinetics of morphine is addressed.

METHODS

Six pigs were studied under general anaesthesia. One occipital and two frontal microdialysis probes and one pressure transducer were inserted into the brain tissue. Another probe was placed into the jugularis interna. Morphine 1 mg kg(-1) was administered as a 10-min infusion, and morphine concentrations were then measured for 3 h. Meningitis was subsequently induced by injecting lipopoly-saccharide into the cisterna magna. When meningitis was established, the morphine experiment was repeated.

RESULTS

The unbound area under the concentration-time curve (AUCu) ratio of morphine in brain to blood was 0.47 (0.19) during the control period, and 0.95 (0.20) (P < 0.001) during meningitis. The increase in the brain/blood AUCu ratio during meningitis implies decreased active efflux and increased passive diffusion of morphine over the BBB. The half-life of morphine in brain was longer than in blood during both periods, and was unaffected by meningitis.

CONCLUSION

This study demonstrates that the morphine exposure to the brain is significantly increased during meningitis as compared with the control situation.

Different distribution of morphine and morphine-6 beta-glucuronide after intracerebroventricular injection in rats

(1) We investigated the distribution of morphine and morphine-6beta-glucuronide (M6G) in the brain and spinal cord after intracerebroventricular (i.c.v.) injection of each drug in rats. (2) The cerebrospinal fluid (CSF) concentration of M6G was 5-37 times greater than that of morphine 10, 60 and 120 min after the i.c.v. injection. The apparent elimination clearance of M6G from the CSF was 10 times lower than that of morphine. (3) The intrathecal CSF concentration of M6G measured by the microdialysis method was 29-79 times greater than that of morphine, and M6G was rapidly distributed into the intrathecal space after the i.c.v. injection. (4) M6G was detected in the cerebrum, brainstem, cerebellum and spinal cord at concentrations 2-21 times higher than morphine after the i.c.v. injection of each drug. The distribution volume of M6G in rat brain slices was three times lower than that of morphine, and close to the extracellular fluid space in the brain regions corresponding to the vicinity of the opioid receptors. (5) These brain distribution characteristics of M6G, namely, low clearance from the central nervous system, localization in the extracellular fluid and rapid distribution into the intrathecal space, may contribute to the potent analgesic effect of M6G after i.c.v. injection.

Relationships among morphine metabolism, pain and side effects during long-term treatment: an update

The two metabolites of morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), have been studied intensively in animals and humans during the past 30 years in order to elucidate their precise action and possible contribution to the desired effects and side effects seen after morphine administration. M3G and M6G are formed by morphine glucuronidation, mainly in the liver, and are excreted by the kidneys. The metabolites are found in the cerebrospinal fluid after single as well as multiple doses of morphine. M6G binds to opioid receptors, and animal studies have demonstrated that M6G may be a more potent analgesic than morphine. Results from human studies regarding the analgesic effect of M6G are not unanimous. The potency ratio between systemic M6G and morphine in humans has not been settled, but is probably lower than previously assumed. Hitherto, only a few studies have found evidence for a contributory effect of M6G to the overall effects observed after morphine administration. Several studies have demonstrated that administration of M6G is accompanied by fewer and a milder degree of opioid-like side effects than observed after morphine administration, but most of the studies have used lower doses of M6G than of morphine. M3G displays very low affinity for opioid receptors and has no analgesic activity. Animal studies have shown that M3G may antagonize the analgesic effect of morphine and M6G, but no human studies have demonstrated this. M3G has also been connected to certain neurotoxic symptoms, such as hyperalgesia, allodynia and myoclonus, which have been observed after administration of M3G or high doses of morphine in animals. The symptoms have been reported sporadically in humans treated primarily with high doses of morphine, but the role of M3G in eliciting the symptoms is not fully elucidated.

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.

Blood-brain barrier transport and brain distribution of morphine-6-glucuronide in relation to the antinociceptive effect in rats--pharmacokinetic/pharmacodynamic modelling

1. The objective of this study was to investigate the contribution of the blood-brain barrier (BBB) transport to the delay in antinociceptive effect of morphine-6-glucuronide (M6G), and to study the equilibration of M6G in vivo across the BBB with microdialysis measuring unbound concentrations. 2. On two consecutive days, rats received an exponential infusion of M6G for 4 h aiming at a target concentration of 3000 ng ml(-1) (6.5 microM) in blood. Concentrations of unbound M6G were determined in brain extracellular fluid (ECF) and venous blood using microdialysis and in arterial blood by regular sampling. MD probes were calibrated in vivo using retrodialysis by drug prior to drug administration. 3. The half-life of M6G was 23+/-5 min in arterial blood, 26+/-10 min in venous blood and 58+/-17 min in brain ECF (P<0.05; brain vs blood). The BBB equilibration, expressed as the unbound steady-state concentration ratio, was 0.22+/-0.09, indicating active efflux in the BBB transport of M6G. A two-compartment model best described the brain distribution of M6G. The unbound volume of distribution was 0.20+/-0.02 ml g brain(-1). The concentration-antinociceptive effect relationships exhibited a clear hysteresis, resulting in an effect delay half-life of 103 min in relation to blood concentrations and a remaining effect delay half-life of 53 min in relation to brain ECF concentrations. 4. Half the effect delay of M6G can be explained by transport across the BBB, suggesting that the remaining effect delay of 53 min is a result of drug distribution within the brain tissue or rate-limiting mechanisms at the receptor level.

Hydrophobic forms of morphine-6-glucosides

NMR spectroscopy of 6-acetylmorphine (6-AM), a chloroform-soluble model compound for the hydrophilic, highly potent analgesic drug morphine-6-glucoronide (M6G), in a hydrophobic solvent indicates one hydrogen bonded water molecule per molecule of 6-AM. By analysis of nuclear Overhauser enhancements (NOEs) we find a 6-AM dimer in which the monomers are linked by two water molecules. Molecular modeling studies underscore the stability of such dimeric structures involving water molecules for 6-AM and point out their more lipophilic character allowing penetration of the blood-brain barrier.

The use of microdialysis in CNS drug delivery studies. Pharmacokinetic perspectives and results with analgesics and antiepileptics

Microdialysis can give simultaneous information on unbound drug concentration-time profiles in brain extracellular fluid (ECF) and blood, separating the information on blood-brain barrier (BBB) processes from confounding factors such as binding to brain tissue or proteins in blood. This makes microdialysis suitable for studies on CNS drug delivery. It is possible to quantify influx and efflux processes at the BBB in vivo, and to relate brain ECF concentrations to central drug action. The half-life in brain ECF vs. the half-life in blood gives information on rate-limiting steps in drug delivery and elimination from the CNS. Examples are given on microdialysis studies of analgesic and antiepileptic drugs.