Morphine-3-glucuronide Formation Research Articles

Inhibition of Morphine Metabolism by Major Cannabinoids and their Metabolites

Opioids including morphine are mu‐opioid receptor (MOR) agonists that suppress excitatory neurotransmitter release, hyperpolarize neuronal cells, and decrease overall excitability, resulting in the desired analgesic effect. As a result of their over‐prescription, the still‐growing opioid epidemic has devastated many communities, with many individuals often using them in combination with other drugs including cannabis. Morphine is primarily metabolized by the phase II enzyme UDP‐glucuronosyltransferase (UGT) 2B7 to an inactive metabolite, morphine‐3‐glucuronide (M3G), and an active metabolite, morphine‐6‐glucuronide (M6G), both of which are extensively excreted into the urine. Previous studies in our lab have shown that major cannabis constituents including Δ9‐tetrahydrocannabinol (THC), cannabidiol (CBD), and cannabinol (CBN) inhibit several major UGT enzymes. To examine whether cannabinoids or their major metabolites inhibit morphine glucuronidation by UGT2B7, screening assays were performed in pooled human liver microsomes (HLM) with the major cannabinoids and THC metabolites, 11‐hydroxy‐Δ9‐tetrahydrocannabinol (11‐OH‐THC), 11‐nor‐9‐carboxy‐Δ9‐tetrahydrocannabinol (11‐COOH‐THC), and 11‐nor‐Δ9‐tetrahydrocannabinol‐carboxylic acid glucuronide (THC‐COO‐Gluc). IC50 values were then determined for those cannabinoids that exhibited at least 50% inhibition at 100 µM final concentration. THC, 11‐OH‐THC, THC‐COO‐Gluc, and CBD all exhibited greater than 50% inhibition of both M3G and M6G formation at either 10 or 100 µM. The IC50 values for THC, 11‐OH‐THC, THC‐COO‐Gluc, and CBD against M3G formation were determined to be 27 ± 9.97 µM, 5.97 ± 2.31 µM, 15.58 ± 12.28 µM, and 15.66 ± 19.98 µM, respectively. The IC50 values for THC, 11‐OH‐THC, THC‐COO‐Gluc, and CBD against M6G formation were 14.49 ± 4.03 µM, 6.26 ± 0.76 µM, 13.22 ± 5.28 µM, and 8.12 ± 9.64 µM, respectively. To confirm results from these inhibition screenings, M3G and M6G formation were further examined using microsomal fractions from HEK293 cells overexpressing recombinant UGT2B7. THC, 11‐OH‐THC, THC‐COO‐Gluc, and CBD all exhibited >50% inhibition of both M3G and M6G formation at either 10 or 100 µM in the recombinant enzyme system. These data suggest that several cannabinoids and their metabolites inhibit morphine metabolism, and that the co‐use of these agents may lead to adverse drug events in humans.

<i>In Vivo</i> Interaction of Morphine and Diclofenac

The number of studies on possible pharmacokinetic interactions between opioid analgesics and nonsteroidal anti-inflammatory drugs (NSAIDs), which are commonly used in combination for the treatment of chronic pain, is limited. In rats, the major metabolic pathway of morphine is glucuronidation to morphine-3-glucuronide (M3G) by UDP-glucuronosyltransferase. In this study, we investigated the influence of diclofenac (NSAID) on the formation of M3G in vitro using rat liver tissue homogenates. Competitive inhibition of M3G formation by diclofenac was observed with an average Ki of 19.9 μM. Because these in vitro findings suggested that a pharmacokinetic interaction occurs in vitro , we investigated whether diclofenac inhibits the glucuronidation of morphine in rats. A single dose of diclofenac increased serum concentrations of both morphine and M3G and showed a higher analgesic efficacy in the Von Frey test. Furthermore, diclofenac caused a net decrease in morphine urine concentrations, but the excretion of M3G through biliary and urinary routes was unchanged. These results demonstrated that in contrast to in vitro data a single dose of diclofenac did not alter the glucuronidation of morphine in vitro .

Pivaloylcodeine, a new codeine derivative, for the inhibition of morphine glucuronidation. An in vitro study in the rat

We have previously found that phenanthrenic opioids, including codeine, modulate morphine glucuronidation in the rat. Here codeine and five of its derivatives were compared in their effects on the synthesis of morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) from morphine by rat liver microsomal preparations, and by primary cultures of rat hepatocytes previously incubated for 72h with either codeine or its derivatives. Acetylcodeine and pivaloylcodeine shared the capability of the parent compound of inhibiting the synthesis of M3G by liver microsomes through a noncompetitive mechanism of action. Their IC50 were 3.25, 2.27, and 4.32μM, respectively. Dihydrocodeine, acetyldihydrocodeine, and lauroylcodeine were ineffective. In all the experimental circumstances M6G was undetectable in the incubation medium. In primary hepatocyte cultures codeine only inhibited M3G formation, but with a lower efficacy than that observed with microsomes (IC50 20.91 vs 4.32μM). Preliminary results show that at micromolar concentrations codeine derivatives exhibit a low rate of affinity for μ opiate receptors. In conclusion, acetyl and pivaloyl derivatives of codeine noncompetitively inhibit liver glucuronidation of morphine interacting with microsomes. This study further strengths the notion that phenanthrenic opioids can modulate morphine glucuronidation independently from their effects on μ opiate receptors.

Repeated exposure to codeine alters morphine glucuronidation by affecting UGT gene expression in the rat

We have previously found that phenantrenic opioids, such as heroin or naltrexone, modulate morphine glucuronidation in the rat. Here we further investigated the effects of phenantrenic opioids on morphine glucuronidation comparing the effects of codeine and heroin. In particular, we measured the synthesis of morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) from morphine: in the liver microsomal preparations obtained from rats repeatedly treated with two different doses of codeine (ex vivo study); in primary cultures of rat hepatocytes previously incubated for 72h with codeine, or heroin (in vitro study); in the latter conditions, the levels of expression of genes coding for uridine-5′-diphosphate-glucuronosyltransferases (UGTs) A1, A6, A7 and 2B1 were also determined; finally, the levels of glucuronic acid in rat hepatocytes previously incubated for 72h with codeine or heroin were assessed. The ex vivo study shows that codeine exposure in vivo stimulated liver microsomal M3G formation and de novo synthesis of M6G. Differently, in primary hepatocyte cultures both codeine and heroin inhibited M3G formation, whereas heroin only stimulated de novo synthesis of M6G; moreover, codeine significantly reduced UGT2B1 expression at 6h and caused a trend toward inhibition of UGT1A1 expression at 72h; heroin enhanced UGT2B1 expression and inhibited that of UGT1A1 at 72h; finally, both codeine and heroin depleted UDPGA content of hepatocytes. In conclusion, codeine affects liver glucuronidation of morphine enlightening the possible contribution of changes in the spectrum of UGT gene expression and co-factor synthesis in this phenomenon.

Inhibition of Morphine Metabolism by Ketamine

Clinical observation of a synergistic effect of ketamine on morphine analgesia remains controversial. Although a pharmacodynamic basis for an interaction has been explored in animal and clinical studies, the possibility of a pharmacokinetic mechanism has not been investigated. Whereas both morphine and morphine-6-glucuronide are effective analgesics, morphine-3-glucuronide (M3G) lacks activity. Thus, changes in the metabolism and disposition of morphine may result in an altered response. First, we investigated the interaction between morphine and ketamine in the isolated perfused rat liver preparation. The clearance of morphine was decreased from 16.8 +/- 4.6 ml/min in the control period to 7.7 +/- 2.8 ml/min in the ketamine-treatment period, with the formation clearance of M3G decreasing from 8.0 +/- 4.1 ml/min to 2.1 +/- 1.1 ml/min. Fractional conversion of morphine to M3G was significantly decreased from 0.46 +/- 0.17 in the control period to 0.28 +/- 0.14 upon the addition of ketamine. The possible mechanism of the interaction was further investigated in vitro with rat liver microsomes as the enzyme source. The formation of M3G followed single-enzyme Michaelis-Menten kinetics, with a mean apparent K(m) of 2.18 +/- 0.45 mM and V(max) of 8.67 +/- 0.59 nmol/min/mg. Ketamine inhibited morphine 3-glucuronidation noncompetitively, with a mean K(i) value of 33.3 +/- 7.9 microM. The results demonstrate that ketamine inhibits the glucuronidation of morphine in a rat model.

Lack of interaction between metoclopramide and morphine in vitro and in mice

1. This study examined interactions via common metabolism or via common pharmacodynamic pathways between frequently co-prescribed metoclopramide (a prokinetic) and morphine (an opioid analgesic).2. In human liver microsomes, morphine 3-glucuronide and morphine 6-glucuronide formation had Vmax estimates of 6.2 ± 0.07 and 0.75 ± 0.01 (nmole min−1 mg−1 protein) and Km estimates of 1080 ± 37 and 665 ± 55 (µM), respectively. The in vitro Ki for morphine 3-glucuronide formation in the presence of metoclopramide in human liver microsomes or recombinant uridine diphosphoglucuronosyltransferase 2B7 predicted a lack of in vivo interaction.3. Morphine (2 mg kg−1 subcutaneously) delayed gastrointestinal meal transit in mice, metoclopramide (10 mg kg−1 subcutaneously) had no effect on meal transit, and metoclopramide did not alter this effect of morphine.4. Morphine (2 or 5 mg kg−1 subcutaneously) was antinociceptive in mice (hot plate test) and metoclopramide (10 mg kg−1 subcutaneously) did not alter the antinociceptive effects of morphine.5. Together, the data suggest a lack of interaction between morphine and metoclopramide.

Single nucleotide polymorphism discovery and functional assessment of variation in the UDP-glucuronosyltransferase 2B7 gene

UDP-glucuronosyltransferase 2B7 (UGT2B7) plays a central role in the liver-mediated biotransformation of endogenous and exogenous compounds. The genetic basis of interindividual variability in UGT2B7 function is unknown. This study aimed to discover novel gene variants of functional significance. Caucasian human livers (n=54) were used. UGT2B7 was resequenced in 12 samples [(six highest and six lowest for the formation of morphine-3-glucuronide (M3G)]. Haplotype-tagging single nucleotide polymorphisms were genotyped in the entire sample set. Samples were phenotyped for mRNA expression. 10 haplotype-tagging single nucleotide polymorphisms were identified and their haplotypes were inferred. Haplotype 4 (-45597G; -6682_-6683A; 372A; IVS1+9_IVS1+10A; IVS1+829T; IVS1+985G; IVS1+999C; IVS1+1250G; 801T; IVS4+185C) (frequency of 0.12) was associated with an increase in enzyme activity and gene expression. The 1/4 and 4/6 diplotypes had higher M3G formation compared with 1/1 (P<0.05) and 2/3 (P<0.01) diplotypes. Diplotypes containing haplotype 4 resulted in a significant 45% average increase in the formation of M3G compared with diplotypes without haplotype 4 (P=0.002). There was also an association between haplotype 4 and increased mRNA expression. IVS1+985A>G, 735A>G, and 1062C>T are the putative functional variants of haplotype 4. We also identified two mRNA splicing variants (UGT2B7_v2 and UGT2B7_v3) splicing out exon 1, 4, 5, and 6 but sharing exons 2 and 3 with the involvement of additional 5' exons. UGT2B7_v2 was detected in all livers tested, but UGT2B7_v3 was present at much lower levels compared with UGT2B7_v2. The UGT2B7 reference sequence mRNA is now named UGT2B7_v1. UGT2B7 haplotype 4 is functional and its effects on the biotransformation of UGT2B7 substrates should be tested in controlled clinical trials. Biochemical studies should investigate the functional role of the newly discovered mRNA splicing variants.

Non-opioid induction of morphine-6-glucuronide synthesis is elicited by prolonged exposure of rat hepatocytes to heroin

Background Liver metabolism of morphine leads to the formation of morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), the latter possessing strong opioid activity that however differs from that of the parent compound. In previous studies conducted in rats we have shown that repeated in vivo exposure to phenanthrene class of mu opioid receptor (MOR) agonists or antagonists (heroin, morphine, and naltrexone), but not to non-phenanthrene class of MOR agonist methadone, affects morphine glucuronidation by liver microsomes. Methods In the present study, we measured the in vitro formation of M3G and M6G by rat hepatocytes incubated for 120 min with morphine (0.1–1.0 mM) after 72 h pre-incubation with one of the following MOR agonists: heroin (3.3 or 6.6 μM), morphine (7.8 μM), or methadone (12 μM). The MOR antagonist naltrexone (10 or 25 μM) was also tested, alone or in combination with heroin. The amount of M3G and M6G synthesized was then measured by HPLC method. Results Heroin inhibited M3G synthesis and induced the formation of M6G, which under basal conditions is not synthesized in rats. Heroin effects were not blocked by naltrexone. Morphine, but not methadone, produced effects similar to those of heroin but more modest in intensity. Pre-incubation with naltrexone alone slightly increased M3G synthesis, but had no effect on M6G formation. Conclusions These results are in agreement with those of previous ex vivo studies and indicate that exposure to heroin or, to a lesser extent, morphine, can affect morphine glucuronidation via direct non-opioid actions on the hepatocytes.

In vivo chronic exposure to heroin or naltrexone selectively inhibits liver microsome formation of estradiol-3-glucuronide in the rat

We have previously found that repeated exposure to heroin reduces liver synthesis of morphine-3-glucuronide (M3G) and increases the production of morphine-6-glucuronide (M6G), which normally is not formed in the rat. By contrast repeated exposure to naltrexone does not activate M6G synthesis but increases the V max of M3G formation. M3G synthesis depends on the activity of two isoforms of the UDP-glucuronosyltransferase (UGT), UGT1A1 and UGT2B1. These isozymes also activate the formation of estradiol-3-glucuronide (E3G) and estradiol-17-glucuronide (E17G), respectively. The goal of the present study was to investigate the role of UGT1A1 and UGT2B1 in the effects of heroin and naltrexone by determining their influence on the synthesis of E3G and E17G. Estradiol glucuronidation was performed using microsomes of rats treated daily, for 10 days, with saline, heroin (10 mg/kg, i.p.), or naltrexone (40 mg/kg, i.p.). Moreover, liver expression of both UGT1A1 and UGT2B1 was studied in the same experimental conditions by polymerase chain reaction analysis. Kinetic analysis showed that the V max for E3G formation was significantly reduced by both heroin (168.82 ± 9.73 nmol/mg/min) and naltrexone (194.60 ± 16.6) relative to saline (624.60 ± 17.6). Moreover, homotropic kinetic of E3G formation (Hill coefficient: 1.8) was transformed in Michaelis–Menten kinetic by both heroin (0.88) and naltrexone (1.15). The synthesis of E17G was not affected by either opioid. The expression of liver UGT1A1 and UGT2B1 did not differ across groups. The present results suggest that heroin and naltrexone can reduce estradiol glucuronidation via a specific interaction with UGT1A1 isoform.

Endogenous Morphine in SH-SY5Y Cells and the Mouse Cerebellum

BackgroundMorphine, the principal active agent in opium, is not restricted to plants, but is also present in different animal tissues and cell types, including the mammalian brain. In fact, its biosynthetic pathway has been elucidated in a human neural cell line. These data suggest a role for morphine in brain physiology (e.g., neurotransmission), but this hypothesis remains a matter of debate. Recently, using the adrenal neuroendocrine chromaffin cell model, we have shown the presence of morphine-6-glucuronide (M6G) in secretory granules and their secretion products, leading us to propose that these endogenous alkaloids might represent new neuroendocrine factors. Here, we investigate the potential function of endogenous alkaloids in the central nervous system.Methodology and Principal FindingsMicroscopy, molecular biology, electrophysiology, and proteomic tools were applied to human neuroblastoma SH-SY5Y cells (i) to characterize morphine and M6G, and (ii) to demonstrate the presence of the UDP-glucuronyltransferase 2B7 enzyme, which is responsible for the formation of M6G from morphine. We show that morphine is secreted in response to nicotine stimulation via a Ca2+-dependent mechanism involving specific storage and release mechanisms. We also show that morphine and M6G at concentrations as low as 10−10 M are able to evoke specific naloxone-reversible membrane currents, indicating possible autocrine/paracrine regulation in SH-SY5Y cells. Microscopy and proteomic approaches were employed to detect and quantify endogenous morphine in the mouse brain. Morphine is present in the hippocampus, cortex, olfactory bulb, and cerebellum at concentration ranging from 1.45 to 7.5 pmol/g. In the cerebellum, morphine immunoreactivity is localized to GABA basket cells and their termini, which form close contacts on Purkinje cell bodies.Conclusions/SignificanceThe presence of morphine in the brain and its localization in particular areas lead us to conclude that it has a specific function in neuromodulation and/or neurotransmission. Furthermore, its presence in cerebellar basket cell termini suggests that morphine has signaling functions in Purkinje cells that remain to be discovered.

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.

Pharmacokinetic modeling to predict morphine and morphine-6-glucuronide plasma concentrations in healthy young volunteers.

This investigation focused on the development of a predictive model of morphine, including morphine-6-glucuronide (M6G) for healthy young volunteers after morphine administration. Population compartmental pharmacokinetic modeling with NONMEM was applied to the plasma concentration-time data of morphine and M6G obtained from 8 healthy volunteers (4 men and 4 women; age range, 23 to 30 years) after intravenous bolus injection of 5.64 mg morphine base (7.5 mg morphine sulfate) and of 1 mg deuterium-labeled M6G. Two models were identified that described the plasma concentration versus time courses of morphine and M6G after administration of morphine. The model consisted of a standard 3-compartment model for morphine and a standard 2-compartment model for M6G, with input into and output from the central compartments. The formation of M6G from morphine was modeled as a fraction of morphine clearance of about 14%, which accounted for the formation of M6G, and a delay of the appearance of M6G in plasma modeled as a first-order process, with a mean metabolic transit time of 17.2 minutes. An alternative model assigned the formation of M6G among the first peripheral compartment of morphine and the central compartment of M6G. Therefore the alternative 3-compartment model of morphine had the input into the central compartment and renal excretory elimination from the central compartment, but the metabolic clearance of morphine started from the first peripheral compartment. M6G was again modeled with a standard 2-compartment model. Both models predicted morphine and M6G plasma concentrations available from an independent study with acceptable accuracy and without bias. Two models are provided that can predict plasma concentrations of morphine and M6G with acceptable accuracy in healthy young volunteers.

Sex-specific differences in levels of morphine, morphine-3-glucuronide, and morphine antinociception in rats

A number of studies reported striking differences in antinociceptive responses to morphine as a function of sex. Although sex differences in the sensitivity to morphine are widely characterized in rodents, the underlying causes are not identified. Gonadal steroids are believed to contribute to sex differences in response to opioid-induced antinociception. In rats, morphine is metabolized by glucuronidation to morphine-3-glucuronide (M3G). M3G was found to be a functional antagonist of the actions of morphine. Knowledge about the role morphine glucuronides play in sex-specific responses to the antinociceptive effect of morphine may be useful in evaluating therapeutic outcomes of morphine treatment. The purpose of this project was to investigate the effects of sex on the systemic formation of M3G in rats and to correlate glucuronidation variability with differences in antinociceptive responses to morphine. Female rats showed significantly lower morphine-induced antinociception as compared to male rats; 4.6±0.5 s vs. 11.7±2.2 s, respectively. Female rats also demonstrated about three-fold higher maximum plasma levels of M3G compared with male rats; 6.2±2.2 μg/ml vs. 1.9±0.7 μg/ml, respectively. The M3G:morphine AUC ratio was 6.6:1 in female rats and 0.7:1 in male rats. Gonadectomy only partially eliminated sex differences in morphine antinociception and plasma levels of M3G. The results of this study demonstrate that sex and sex differences in the M3G:morphine plasma ratio may play a role in male–female differences observed in morphine antinociception.

Tissue distribution and interindividual variation in human UDP-glucuronosyltransferase activity: relationship between UGT1A1 promoter genotype and variability in a liver bank

The variability in a liver bank and tissue distribution of three probe UDP-glucuronosyltransferase (UGT) activities were determined as a means to predict interindividual differences in expression and the contribution of extrahepatic metabolism to presystemic and systemic clearance. Formation rates of acetaminophen-O-glucuronide (APAPG), morphine-3-glucuronide (M3G), and oestradiol-3-glucuronide (E3G) as probes for UGT1A6, 2B7, and 1A1, respectively, were determined in human kidney, liver, and lung microsomes, and in microsomes from intestinal mucosa corresponding to duodenum, jejunum and ileum. While formation of E3G and APAPG were detectable in human kidney microsomes, M3G formation rates from kidney microsomes approached the levels seen in liver, indicating significant expression of UGT2B7. Interestingly, rates of E3G formation in human intestine exceeded the hepatic rates by several fold, while APAPG and M3G formation rates were low. The intestinal apparent Km value for E3G formation was essentially identical to that seen in liver, consistent with intestinal UGT1A1 expression. No UGT activities were observed in lung. Variability in APAPG and M3G activity across a bank of 20 human livers was modest (< or = 7-fold), compared to E3G formation, which varied approximately 30-fold. The E3G formation rates were found to segregate by UGT1A1 promoter genotype, with wild-type (TA)6 rates significantly greater than homozygous mutant (TA)7 individuals. Kinetic analyses were performed to demonstrate that the promoter mutation altered apparent Vmax without significantly affecting apparent Km. These results suggest that glucuronidation, and specifically UGT1A1 activity, can profoundly contribute to intestinal first pass metabolism and interindividual variability due to the expression of common allelic variants.

Morphine Metabolism in the Pregnant Guinea Pig and Her Pups

The study of chronic in utero exposure to heroin and morphine in the human is limited by polysubstance abuse. The guinea pig was used as a model for the human to determine the in vivo and in vitro effect of chronic morphine exposure on morphine metabolism in the pregnant dam and her offspring. In vivo pharmacokinetics of morphine were examined in pregnant guinea pigs following pretreatment with either saline or morphine. In vitro hepatic enzyme kinetics were also examined in a similar group of pregnant dams and their fetuses. Additional pregnant dams were allowed to give birth and their pups’ enzyme kinetics were studied at 1, 3, and 7 days. Apparent V_MAX for the formation of both morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) formation was significantly increased in the morphine-treated pregnant guinea pig. However, no effect of morphine treatment was detectable on the in vivo pharmacokinetics of morphine in the pregnant dam. The apparent morphine K_M for the formation of M3G was significantly different than the apparent K_M for the formation of M6G. Significant age effects on the enzyme kinetics were found. The apparent V_MAX for the formation of both glucuronides increased through the neonatal period. Through literature comparisons, the guinea pig was shown to have in vivo pharmacokinetics similar to the pregnant human, and the guinea pig pups were found to have enzyme development consistent with in vivo pharmacokinetic development seen in human neonates, infants and children.

Effect of prior morphine-3-glucuronide exposure on morphine disposition and antinociception

Morphine-3-glucuronide (M3G), the primary metabolite of morphine in humans and rats, has been reported to antagonize morphine-induced pharmacologic effects. The present experiment was conducted to evaluate the effect of prior systemic M3G exposure on morphine disposition and antinociceptive response in male Sprague-Dawley rats. Saline (N = 6), low dose M3G (0.15 mg/hr, N = 7), or high dose M3G (0.30 mg/hr, N =6) was infused for 720 min prior to the administration of morphine by i.v. bolus (2 mg/kg). Tail-flick latencies in response to hot water (50°) were assessed prior to and for 180 min after the morphine test dose. M3G exposure had no significant effect on morphine pharmacokinetics, although a disproportionate increase in M3G concentrations was observed following the morphine i.v. bolus dose in rats infused with high dose M3G. Morphine-induced antinociception, expressed as the percent of maximum response (%MPR), was maximum 15 min after morphine administration and returned to baseline by 180 min. A pharmacokinetic-pharmacodynamic model was constructed to relate tail-flick latencies to morphine serum concentrations. In saline-exposed rats, the antinociceptive response to morphine was characterized by a sigmoidal E max model, with an ec 50 of 328 ng/mL, a Hill coefficient (γ) of 4.5, and a half-life for the offset of pharmacologic effect of 11 min. No statistically significant differences in the intensity or duration of morphine-induced response were detected between salineand M3G-exposed animals. These results suggest that systemic formation of M3G is unlikely to contribute significantly to the development of tolerance to morphine antinociception.

CSF and Plasma Concentrations of Morphine and Morphine Glucuronides in Cancer Patients Receiving Epidural Morphine

Abstract Thirty-five cancer patients, treated with chronic epidural morphine, were assayed for plasma and cerebrospinal fluid (CSF) minimum steady-state concentrations (Css min) of morphine (M), morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) by high performance liquid chromatography (HPLC). A linear dose-concentration relationship was found for the 3 substances in plasma and for morphine and M3G in CSF. The mean ± S.E.M. CSF/plasma morphine ratio was 158 ± 43. In CSF, the concentrations of morphine exceeded those of the metabolites substantially and, normalized to morphine, the mean CSF M M3G M6G ratio was 1:0.05:0.02. In plasma, the metabolite concentrations were higher than the parent drug and the plasma M M3G M6G ratio was 1:12:3. The mean M3G and M6G concentrations in CSF were 40–60% of those found in plasma. Indication of cerebral formation of M3G was found in 1 patient. Pain relief, evaluated by a visual analogue scale (VAS), did not correlate with the CSF M3G concentrations or with the M3G M ratio. CSF M6G concentrations were low and did not contribute to any detectable analgesia. We conclude that after epidural administration of morphine, the M3G and M6G metabolites in CSF are low compared to unchanged morphine and seem to have little influence on analgesia. Howewer, the fact that a significant passage of the glucuronide metabolites occurs to the CSF may indicate a role in morphine analgesia after other routes of administration.

CSF and plasma concentrations of morphine and morphine glucuronides in cancer patients receiving epidural morphine

Thirty-five cancer patients, treated with chronic epidural morphine, were assayed for plasma and cerebrospinal fluid (CSF) minimum steady-state concentrations (Css min) of morphine (M), morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) by high performance liquid chromatography (HPLC). A linear dose-concentration relationship was found for the 3 substances in plasma and for morphine and M3G in CSF. The mean ± S. E. M. CSF/plasma morphine ratio was 158 ± 43. In CSF, the concentrations of morphine exceeded those of the metabolites substantially and, normalized to morphine, the mean CSF M M3G M6G ratio was 1:0.05:0.02. In plasma, the metabolite concentrations were higher than the parent drug and the plasma M M3G M6G ratio was 1:12:3. The mean M3G and M6G concentrations in CSF were 40–60% of those found in plasma. Indication of cerebral formation of M3G was found in 1 patient. Pain relief, evaluated by a visual analogue scale (VAS), did not correlate with the CSF M3G concentrations or with the M3G M ratio. CSF M6G concentrations were low and did not contribute to any detectable analgesia. We conclude that after epidural administration of morphine, the M3G and M6G metabolites in CSF are low compared to unchanged morphine and seem to have little influence on analgesia. Howewer, the fact that a significant passage of the glucuronide metabolites occurs to the CSF may indicate a role in morphine analgesia after other routes of administration.