Plasma and cerebrospinal fluid concentrations of morphine and morphine glucuronides in rabbits receiving single and repeated doses of morphine

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSION AND IMPLICATIONS

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

Morphine-induced hyperalgesia involves mu opioid receptors and the metabolite morphine-3-glucuronide

Opiates are potent analgesics but their clinical use is limited by side effects including analgesic tolerance and opioid-induced hyperalgesia (OIH). The Opiates produce analgesia and other adverse effects through activation of the mu opioid receptor (MOR) encoded by the Oprm1 gene. However, MOR and morphine metabolism involvement in OIH have been little explored. Hence, we examined MOR contribution to OIH by comparing morphine-induced hyperalgesia in wild type (WT) and MOR knockout (KO) mice. We found that repeated morphine administration led to analgesic tolerance and hyperalgesia in WT mice but not in MOR KO mice. The absence of OIH in MOR KO mice was found in both sexes, in two KO global mutant lines, and for mechanical, heat and cold pain modalities. In addition, the morphine metabolite morphine-3beta-D-glucuronide (M3G) elicited hyperalgesia in WT but not in MOR KO animals, as well as in both MOR flox and MOR-Nav1.8 sensory neuron conditional KO mice. M3G displayed significant binding to MOR and G-protein activation when using membranes from MOR-transfected cells or WT mice but not from MOR KO mice. Collectively our results show that MOR is involved in hyperalgesia induced by chronic morphine and its metabolite M3G.

Bioavailability of a morphine suppository is increased after intracolostomal administration in colostoma-constructed rabbits

This study was performed to assess the pharmacokinetics of morphine and its major metabolites after its rectal or colostomal administration in rectal-resected (ROP) or colostoma-constructed (SOP) rabbits, respectively. The pharmacokinetics of morphine, morphine-3-glucuronide (M3G), and M6G in normal rabbits appeared to be similar to those in human, judging from their plasma concentration-time profiles and the susceptibility of morphine to first-pass metabolism. In SOP, but not ROP, rabbits, the plasma concentrations of morphine, M3G and M6G were significantly increased compared with those in normal rabbits. The AUC of morphine and its metabolites, and the F value of the former in the SOP group were greater than those in the control group, while the elimination half-life (t(1/2)) values were comparable in the two groups. In addition, the disposition of morphine and its metabolites after intravenous (i.v.) administration to SOP rabbits was almost the same as that in normal rabbits, suggesting that an increase in the rate of absorption of morphine in SOP rabbits was not due to inflammation at the absorption site caused by operation, but probably due to its increased solubility in loose stools. Therefore, great attention should be paid when morphine suppositories are intracolostomally administered to colostoma-constructed patients.

Pharmacokinetic differences of morphine and morphine-glucuronides are reflected in locomotor activity

The main metabolites of morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), have been considered to participate in some of the effects of morphine. There is limited knowledge of the pharmacokinetics and dynamics of morphine and the main metabolites in mice, but mice are widely used to study both the analgesic effects and the psychomotor effects of morphine. The present study aimed to explore pharmacokinetic differences between morphine and morphine-glucuronides in mice after different routes of administration, and to investigate how possible differences were reflected in locomotor activity, a measure of psychostimulant properties. Mice were given morphine, M3G or M6G by different routes of administration. Serum concentrations versus time curves, pharmacokinetic parameters and locomotor activity were determined. Intraperitoneal administration of morphine reduced the bioavailability compared to intravenous and subcutaneous administration, but not so for morphine-glucuronides. The two morphine-glucuronides had similar pharmacokinetics, but morphine demonstrated higher volume of distribution and clearance than morphine-glucuronides. The present results demonstrated no locomotor effect of M3G, but a serum concentration effect relationship for morphine and M6G. When serum concentrations and effect changes were followed over time, there was some right hand shifts with respect to locomotor activity, especially during the declining phase of the concentration curve and particularly for M6G.

Uncertainty factors for chemical risk assessment: interspecies differences in glucuronidation

For the risk assessment of effects other than cancer, a safe daily intake in humans is generally derived from a surrogate threshold dose (e.g. NOAEL) in an animal species to which an uncertainty factor of 100 is usually applied. This 100-fold is to allow for possible interspecies (10-fold) and interindividual (10-fold) differences in response to a toxicant, and incorporates toxicodynamic and toxicokinetic aspects of variability. The current study determined the magnitude of the interspecies differences in the internal dose of compounds for which glucuronidation is the major pathway of metabolism in either humans or in the test species. The results showed that there are major interspecies differences in the nature of the biological processes which influence the internal dose, including the route of metabolism, the extent of presystemic metabolism and enterohepatic recirculation. The work presented does not support the refinement of the interspecies toxicokinetic default to species- and pathway-specific values, but demonstrates the necessity for risk assessments to be carried out using quantitative chemical-specific data which define the fundamental processes which will influence the internal dose of a chemical (toxicokinetics), or the interaction of toxicant with its target site (toxicodynamics).

Modelling of the blood-brain barrier transport of morphine-3-glucuronide studied using microdialysis in the rat: involvement of probenecid-sensitive transport

The objective of this study was to investigate the impact of probenecid on the blood-brain barrier (BBB) transport of morphine-3-glucuronide (M3G). Two groups of rats received an exponential infusion of M3G over 4 h to reach a target plasma concentration of 65 microM on two consecutive days. Probenecid was co-administered in the treatment group on day 2. Microdialysis was used to estimate unbound M3G concentrations in brain extracellular fluid (ECF) and blood. In vivo recovery of M3G was calculated with retrodialysis by drug, preceding the drug administration. The BBB transport was modelled using NONMEM. In the probenecid group, the ratio of the steady-state concentration of unbound M3G in brain ECF to that in blood was 0.08+/-0.02 in the absence and 0.16+/-0.05 in the presence of probenecid (P=0.001). In the control group, no significant difference was found in this ratio between the 2 days (0.11+/-0.05 and 0.10+/-0.02, respectively). The process that appears to be mainly influenced by probenecid is influx clearance into the brain (0.11 microl min(-1) g-brain(-1) vs 0.17 microl min(-1) g-brain(-1), in the absence vs presence of probenecid, P:<0.001). The efflux clearance was 1.15 microl min(-1) g-brain(-1). The half-life of M3G was 81+/-25 min in brain ECF vs 22+/-2 min in blood (P<0.0001). Blood pharmacokinetics was not influenced by probenecid. In conclusion, a probenecid-sensitive transport system is involved in the transport of M3G across the BBB.

The role of P-glycoprotein in blood-brain barrier transport of morphine: transcortical microdialysis studies in mdr1a (-/-) and mdr1a (+/+) mice

1. The aim of this study was to investigate whether blood-brain barrier transport of morphine was affected by the absence of mdr1a-encoded P-glycoprotein (Pgp), by comparing mdr1a (-/-) mice with mdr1a (+/+) mice. 2. Mdr1a (-/-) and (+/+) mice received a constant infusion of morphine for 1, 2 or 4 h (9 nmol/min/mouse). Microdialysis was used to estimate morphine unbound concentrations in brain extracellular fluid during the 4 h infusion. Two methods of estimating in vivo recovery were used: retrodialysis with nalorphine as a calibrator, and the dynamic-no-net-flux method. 3. Retrodialysis loss of morphine and nalorphine was similar in vivo. Unbound brain extracellular fluid concentration ratios of (-/-)/(+/+) were 2.7 for retrodialysis and 3.6 for the dynamic-no-net-flux at 4 h, with corresponding total brain concentration ratios of (-/-)/(+/+) being 2.3 for retrodialysis and 2.6 for the dynamic-no-net-flux. The total concentration ratios of brain/plasma were 1.1 and 0.5 for mdr1a (-/-) and (+/+) mice, respectively. 4. No significant differences in the pharmacokinetics of the metabolite morphine-3-glucoronide were observed between (-/-) and (+/+) mice. 5. In conclusion, comparison between mdr1a (-/-) and (+/+) mice indicates that Pgp participates in regulating the amount of morphine transport across the blood-brain barrier.

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

1 This study was done to find out how morphine 6-beta-D-glucuronide (M6G) induces more potent central analgesia than morphine, despite its poor blood-brain barrier (BBB) permeability. The brain uptake and disposition of these compounds were investigated in plasma and in various brain compartments: extracellular fluid (ECF), intracellular space (ICS) and cerebrospinal fluid (CSF). 2 Morphine or M6G was given to rats at 10 mg kg(-1) s.c. Transcortical microdialysis was used to assess their distributions in the brain ECF. Conventional tissue homogenization was used to determine the distribution in the cortex and whole brain. These two procedures were combined to estimate drug distribution in the brain ICS. The blood and CSF pharmacokinetics were also determined. 3 Plasma concentration data for M6G were much higher than those of morphine, with Cmax and AUC 4-5 times more higher, Tmax shorter, and VZf-1 (volume of distribution) and CL f(-1) (clearance) 4-6 times lower. The concentrations of the compounds in various brain compartments also differed: AUC values for M6G were lower than those of morphine in tissue and CSF and higher in brain ECF. AUC values in brain show that morphine levels were four times higher in ICS than in ECF, whereas M6G levels were 125 higher in ECF than in ICS. 4 Morphine entered brain cells, whereas M6G was almost exclusively extracellular. This high extracellular concentration, coupled with extremely slow diffusion into the CSF, indicates that M6G was predominantly trapped in the extracellular fluid and therefore durably available to bind at opioid receptors.

Does prolonged oral treatment with sustained-release morphine tablets influence immune function?

Opioids such as morphine are the mainstay of acute and chronic pain treatment. The purpose of this study was to investigate the immunosuppressive effects of morphine in patients with pain syndromes. We investigated 10 patients with chronic pain syndromes undergoing treatment with oral sustained-release morphine (30-240 mg/d) before and after 1,4, and 12 wk of treatment compared with healthy control subjects without morphine treatment. Immunological variables of the cellular and humoral immune axis showed that 1) total lymphocyte counts and the distribution of lymphocyte subpopulations, including helper T-cell/suppressor cytotoxic T-cell ratios (CD4/CD8 ratios), did not change compared with baseline or healthy control subjects; 2) proliferation of peripheral mononuclear cells (PMC) was not impaired by morphine treatment; 3) interleukin 2 production increased after 4 wk of treatment with morphine; and 4) immunoglobulin (Ig) production was reduced before initiation of therapy in pain patients and decreased further during morphine treatment, whereas Ig concentrations in the circulation remained at normal levels. These results indicate that treatment with oral, sustained-release morphine does not have a suppressive effect on overall PMC function. On the other hand, Ig production was impaired in patients with chronic pain and was further suppressed by morphine. Whether this suppression of humoral immune function has a clinical impact on the immune system as a whole remains to be determined.

IMPLICATIONS

Treatment of patients with chronic pain with oral, sustained-release morphine does not influence cellular immune function, but it suppresses the already attenuated production of immunoglobulins.