The disposition of morphine and its metabolites in the in-situ rat isolated perfused liver

Central metabolism as a potential origin of sex differences in morphine antinociception but not induction of antinociceptive tolerance in mice

BACKGROUND AND PURPOSE

In rodents, morphine antinociception is influenced by sex. However, conflicting results have been reported regarding the interaction between sex and morphine antinociceptive tolerance. Morphine is metabolised in the liver and brain into morphine-3-glucuronide (M3G). Sex differences in morphine metabolism and differential metabolic adaptations during tolerance development might contribute to behavioural discrepancies. This article investigates the differences in peripheral and central morphine metabolism after acute and chronic morphine treatment in male and female mice.

EXPERIMENTAL APPROACH

Sex differences in morphine antinociception and tolerance were assessed using the tail-immersion test. After acute and chronic morphine treatment, morphine and M3G metabolic kinetics in the blood were evaluated using LC-MS/MS. They were also quantified in several CNS regions. Finally, the blood-brain barrier (BBB) permeability of M3G was assessed in male and female mice.

KEY RESULTS

This study demonstrated that female mice showed weaker morphine antinociception and faster induction of tolerance than males. Additionally, female mice showed higher levels of M3G in the blood and in several pain-related CNS regions than male mice, whereas lower levels of morphine were observed in these regions. M3G brain/blood ratios after injection of M3G indicated no sex differences in M3G BBB permeability, and these ratios were lower than those obtained after injection of morphine.

CONCLUSION

These differences are attributable mainly to morphine central metabolism, which differed between males and females in pain-related CNS regions, consistent with weaker morphine antinociceptive effects in females. However, the role of morphine metabolism in antinociceptive tolerance seemed limited.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Prediction of Drug Tissue to Plasma Concentration Ratios Using a Measured Volume of Distribution in Combination With Lipophilicity

One of the drug specific parameters needed in physiologically based pharmacokinetic (PBPK) models is the tissue to plasma drug concentration ratios (K(p) values). The aim of this study was to develop an empirical method for predicting K(p) values using a preclinically determined in vivo volume of distribution, in combination with descriptors for drug lipophilicity. Pharmacokinetic data in laboratory animals for a wide range of drug compounds were collected. Obtained correlations between K(p) values for muscle and other tissues, in a training set of 49 compounds, were used to predict K(p) values for a test set of 22 compounds, based on their volume of distribution and lipophilicity. Predicted K(p) values agreed well with experimentally determined values (n = 118), especially for noneliminating tissues (r(2) = 0.81) with 72% and 87% being within a factor +/-2 and +/-3, respectively. In conclusion, we present an empirical method based on a measured volume of distribution and a drug lipophilicity descriptor, which can be used to predict tissue K(p) values with reasonable accuracy.

The effect of intrahippocampal injection of group II and III metobotropic glutamate receptor agonists on anxiety; the role of neuropeptide Y

Earlier studies conducted by our group and by other authors indicated that metabotropic glutamatergic receptor (mGluR) ligands might have anxiolytic activity and that amygdalar neuropeptide Y (NPY) neurons were engaged in that effect. Apart from the amygdala, the hippocampus, another limbic structure, also seems to be engaged in regulation of anxiety. It is rich in mGluRs and contains numerous NPY interneurons. In the present study, we investigated the anxiolytic activity of group II and III mGluR agonists after injection into the hippocampus, and attempted to establish whether hippocampal NPY neurons and receptors were engaged in the observed effects. Male Wistar rats were bilaterally microinjected with the group II mGluR agonist (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I), group III mGluR agonist O-Phospho-L-serine (L-SOP), NPY, the Y1 receptor antagonist BIBO 3304, and the Y2 receptor antagonist BIIE 0246 into the CA1 or dentate area (DG). The effect of those compounds on anxiety was tested in the elevated plus-maze. Moreover, the effects of L-CCG-I and L-SOP on the expression of NPYmRNA in the hippocampus were studied using in situ hybridization method. It was found that a significant anxiolytic effect was induced by L-SOP injection into the CA1 region or by L-CCG-I injection into the DG. The former effect was inhibited by BIBO 3304, the latter by BIIE 0246. NPY itself showed antianxiety action after injection into both structures. In the CA1 area, the effect of NPY was prevented by BIBO 3304, whereas in the DG by BIIE 0246. Both the mGluR agonists L-CCG-I and L-SOP induced a potent increase in NPYmRNA expression in the DG region of the hippocampal formation. The obtained results indicate that group II and III mGluR agonists, L-CCG-I and L-SOP, as well as NPY display anxiolytic activity in the hippocampus, but act differently in the CA1 and DG. It was observed that group III mGluRs and Y1 receptors were engaged in the response in the CA1 area, whereas group II mGluRs and Y2 receptors played a pivotal role in the DG region.

Transport is not rate-limiting in morphine glucuronidation in the single-pass perfused rat liver preparation

Binding, transport, and metabolism are factors that influence morphine (M) removal in the rat liver. For M and the morphine 3beta-glucuronide metabolite (M3G), modest binding existed with 4% bovine serum albumin (unbound fractions of 0.89 +/- 0.07 and 0.98 +/- 0.09, respectively), and there was partitioning of M into red blood cells. Transport studies of M (<750 microM) showed similar, concentration-independent uptake clearances (CLs) of 1.5 ml min(-1) g(-1) among zonal and homogeneous, isolated rat hepatocytes. Transport of M3G, ascertained in multiple indicator dilution studies at various steady-state M3G concentrations (10-262 microM), uncovered a low and concentration-independent influx clearance (<10% of flow rate). The outflow dilution curve of [(3)H]M3G was superimposable onto that of [(14)C]sucrose, the extracellular reference, displaying similarity in transit times (23.5 and 22.2 s), negligible biliary excretion, and almost complete dose recovery from perfusate. In contrast, M3G occurred abundantly in both perfusate and bile in single-pass perfusion studies of the precursor, M, and revealed a biliary clearance of formed M3G that was 12.3-fold that of preformed M3G, suggesting a sinusoidal, diffusional barrier for M3G. With increasing concentrations of M (9-474 microM), clearance decreased, and metabolism and biliary excretion displayed concentration-dependent kinetics. Fitting of the data to a physiologically based liver model revealed that M removal mechanisms were saturable, with a K(m,met) of 52.2 microM and V(max,met) of 58.8 nmol min(-1) g(-1) for metabolism, and a K(m,ex) of 41.2 microM and V(max,ex) of 8.1 nmol min(-1) g(-1) for excretion. Sinusoidal transport was not rate-limiting for M removal.

Uptake of l-Carnitine and Its Short-Chain Ester Propionyl-l-carnitine in the Isolated Perfused Rat Liver

Hepatic uptake of propionyl-L-carnitine (PLC) and L-carnitine (LC) was assessed with the impulse-response technique in the single-pass perfused rat liver. The experiments involved a rapid injection (impulse) of a mixture of the radiolabeled test compound (PLC or LC) and a reference compound (sucrose) into portal vein inflow and collection and radiochemical analysis (response) of the venous outflowing perfusate samples. The impulse injection was made in the presence of increasing unlabeled background concentrations of PLC (0-50 microM) or LC (50-500 microM) perfusing the liver. The hepatic uptake was minimal or negligible for LC, whereas the hepatic influx clearance was found to be low (0.095 ml/s equivalent to 5.7 ml/min) for PLC relative to the perfusate flow rate (30 ml/min). When background concentrations of PLC were increased (from 1-50 microM), the influx clearance was reduced in a concentration-dependent behavior, indicating partial saturation of the entry of compound into hepatocytes. PLC was taken up into hepatocytes via a unidirectional transport process with negligible efflux. The hepatic uptake of PLC was significantly reduced in the presence of unlabeled LC (500 microM), indicating an inhibition of the sinusoidal membrane transport of PLC by LC. The study showed the sinusoidal membrane is a permeability barrier to the entry of PLC and LC into hepatocytes, and it is the site of a common carrier-mediated transporter for both compounds.

The effects of the phytoestrogenic isoflavone genistein on the hepatic disposition of preformed and hepatically generated gemfibrozil 1-O-acyl glucuronide in the isolated perfused rat liver

Foods and complementary medicines contain phytoestrogenic isoflavones such as genistein, which undergo hepatic glucuronidation and excretion into bile and can potentially interfere with the hepatic elimination of other compounds. To investigate this potential, livers from Sprague-Dawley rats were perfused in single-pass mode with preformed gemfibrozil 1-O-acyl glucuronide (GG) (1 microM, n = 12) for 60 min followed by a 30-min washout phase, or with gemfibrozil (1 microM, n = 10) for 120 min. Half of each group of livers were co-perfused with genistein (10 microM) throughout the experiment. Perfusate and bile were analyzed for GG and gemfibrozil by HPLC. Co-perfusion with genistein significantly (P< 0.05) decreased the biliary extraction ratio of preformed GG from a mean of 0.82 to 0.65 and the first-order rate constant for transport of GG into bile from 0.054 +/- 0.010 to 0.032 +/- 0.008 min(-1), but increased the first-order rate constant for sinusoidal efflux of GG from 0.128 +/- 0.023 to 0.227 +/- 0.078 min(-1). Co-perfusion with genistein also significantly decreased the biliary extraction ratio of hepatically generated GG from 0.95 +/- 0.01 to 0.83 +/- 0.05. The findings confirm that genistein increases the potential for hepatic and systemic exposure to hepatically generated glucuronides, which may be important for patients on conventional drugs who consume isoflavones.

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.

The study of codeine-gluthetimide pharmacokinetic interaction in rats

A high-performance liquid chromatographic (HPLC) assay with native fluorescence detection was developed for the simultaneous quantification of codeine and its two metabolites, morphine and morphine-3-glucuronide (M-3-G), in rat plasma. Solid-phase extraction was used to separate codeine and its metabolites from plasma constituents. Extraction efficiencies of codeine, morphine and M-3-G from rat plasma samples were 97, 92 and 93%, respectively. The chromatographic separation was performed using a reversed-phase C18 column and an elution gradient at ambient temperature. Using native fluorescence detection (excitation at 245 nm and emission at 345 nm), the detection limits of 50 ng/ml for morphine, 25 ng/ml for codeine and 20 ng/ml for M-3-G were obtained. The method had good precision, accuracy and linearity, and was applied to the study of glutethimide's influence on codeine metabolism in rat, following single doses of codeine-glutethimide association. The results confirmed the fact that glutethimide was responsible for a significant increase of morphine plasma levels and for their maintenance in time, concomitant with a significant decrease of M-3-G plasma levels, explained by the inhibition of morphine glucuronidation. In conclusion, glutethimide potentiates and prolongs the analgesic effect of codeine by a pharmacokinetic mechanism.

Cerebral and lung kinetics of morphine in conscious sheep after short intravenous infusions

BACKGROUND

The analgesic effects of morphine are delayed relative to its concentration in blood. The rate of equilibration of morphine between blood and brain may contribute to this delay, but the kinetics of this process have not been modelled. This was determined in conscious instrumented sheep. The lung kinetics of morphine were also determined given their importance in defining systemic kinetics after i.v. bolus administration.

METHODS

Sheep were given short i.v. infusions of morphine (30 mg over 4 min). Cerebral kinetics were inferred from arterio-sagittal sinus concentration gradients and cerebral blood flow, and lung kinetics from the pulmonary artery-aortic gradient and cardiac output. These data were fitted to flow- and membrane-limited models of the kinetics in each organ.

RESULTS

Morphine had minimal cardiovascular effects, did not alter cerebral blood flow and caused insignificant respiratory depression. Lung kinetics were best described by a single distribution volume (2036 ml) with a first-order loss (1370 ml min(-1)), which was attributed to deep distribution. The cerebral kinetics of morphine were characterized by a significant permeability barrier. Permeability across the barrier (7.44 ml min(-1)) was estimated with good precision, and was approximately one-fifth of the nominal cerebral blood flow. The distribution volume of morphine in the brain was estimated with less precision, but was described by a brain:blood partition coefficient of approximately 1.4. The time required for 50% equilibration between brain and blood concentrations was approximately 10.3 min.

CONCLUSION

The cerebral equilibration of morphine was relatively slow, and was characterized by significant membrane limitation.

The effects of phytoestrogenic isoflavones on the formation and disposition of paracetamol sulfate in the isolated perfused rat liver

This study examines the potential for the phytoestrogenic isoflavones, a type of complementary medicine, to be involved in pharmacokinetic interactions in the liver. Rat livers were isolated and perfused to steady state, in single-pass mode, with either 5 microM paracetamol (n = 6), or 5 microM paracetamol with a 50:50 molar mixture of genistein and biochanin A or daidzein and formononetin, at a total isoflavone concentration of 1 and 10 microM (n = 6 for each mixture at each concentration). At 1 microM, neither isoflavone mixture had any effect, while at 10 microM both mixtures decreased the clearance of paracetamol and the formation clearance to paracetamol sulfate. Genistein and biochanin A (10 microM) also increased the biliary extraction of hepatically-generated paracetamol sulfate. Additional livers were perfused with an infusion of 5 microM (14)C-paracetamol in the absence (n = 4), or presence, of a 10 microM genistein and biochanin A mixture (n = 4). Analysis of washout perfusate and bile samples (up to 30 min after stopping the infusion) revealed that the isoflavones reduced the first-order rate constant for paracetamol sulfate transport into perfusate, but not for transport into bile. The results indicate that isoflavones can reduce the formation of paracetamol sulfate and that its enhanced excretion into bile arises from the inhibition of sinusoidal efflux transport.

Hepatic disposition of fexofenadine: influence of the transport inhibitors erythromycin and dibromosulphothalein

PURPOSE

To examine the disposition of fexofenadine in the isolated perfused rat liver and the influence of erythromycin and dibromosulphthalein (DBSP) on the hepatic uptake and biliary excretion of fexofenadine.

METHODS

Livers from four groups of rats were perfused in a recirculatory manner with fexofenadine HCl added as a bolus (125, 250, 500, or 1000 microg) to perfusate. Livers from another three groups of rats were perfused with 250 microg of fexofenadine HCl. With one group as control, erythromycin (4.0 microg/ml) or DBSP (136 microg/ml) was added to the perfusate of the other groups. In all experiments, perfusate and bile were collected for 60 min; in addition, livers from the second experiment were retained for assay. Fexofenadine was determined in perfusate, bile, and homogenized liver by HPLC.

RESULTS

The area under the curve (AUC) of fexofenadine was linearly related to concentration. It was unchanged from control (12,800 +/- 200 ng x h/ml) by erythromycin (14,400 +/- 2000 ng x h/ml), but was increased 95% by DBSP (25,000 +/- 2600 ng x h/ml, P <0.001). The ratios of the concentrations of fexofenadine in liver/perfusate were decreased significantly by DBSP; those for bile/liver were increased by erythromycin.

CONCLUSIONS

Erythromycin reduced the canalicular transport of fexofenadine into bile, whereas DBSP reduced uptake across the sinusoidal membrane.

Effect of uranyl nitrate-induced renal failure on morphine disposition and antinociceptive response in rats

1. The aims of the present study were to administer morphine (14.0 mumol/kg, s.c.) to male Hooded Wistar rats and to determine the effect of uranyl nitrate-induced renal failure on: (i) the antinociceptive effect of morphine; (ii) the pharmacokinetics of morphine and morphine-3-glucuronide (M3G); and (iii) the relationship between antinociceptive effect and the pharmacokinetics of morphine in plasma and brain. 2. Renal failure was induced by a single s.c. injection of uranyl nitrate and kinetic/dynamic studies were performed 10 days after its administration, when creatinine clearance was 17% of the control group. Antinociceptive effect was measured by the tail-flick method at various times up to 2 h post-drug administration. Concentrations of morphine and M3G in plasma and brain and concentrations of creatinine in urine and serum were determined by specific HPLC methods. 3. After morphine administration, the area under the antinociceptive effect-time curve was decreased by 44% in renal failure rats. There were no differences between control and renal failure rats in: (i) plasma morphine concentration-time curves; (ii) brain morphine concentration-time curves; and (iii) plasma M3G concentration-time curves. Morphine-6-glucuronide was not detected in any plasma or brain sample from rats administered morphine and no M3G was detected in brain. 4. For both control and renal failure rats, the relationships between antinociceptive effect and plasma morphine concentration were characterized by counterclockwise hysteresis loops, probably reflecting a delay for the relatively polar morphine to cross the blood-brain barrier. The relationship between antinociceptive effect and brain morphine concentration in control rats revealed no evidence of acute tolerance and was described by a sigmoidal function. In contrast, the relationship in renal failure rats was characterized by clockwise hysteresis, which is consistent with acute tolerance development.

Application of a loading wash-out method for investigating the hepatocellular efflux of a hepatically-generated metabolite, morphine-3-glucuronide

Previous studies using the rat isolated perfused liver demonstrated that the hepatic disposition of morphine-3-glucuronide is membrane permeability-rate limited, and that the movement of the metabolite across hepatic sinusoidal and canalicular membranes is partly via carrier-mediated transport systems. As a consequence of the membrane permeability-limitation, the biliary excretion of hepatically-generated morphine-3-glucuronide is much more efficient than that of morphine-3-glucuronide reaching the liver via the vasculature. We have quantitated the cellular efflux kinetics (cell-to-blood and cell-to-bile) of morphine-3-glucuronide in the rat isolated perfused liver using a loading wash-out design. In the 'loading' phase, morphine was infused into the liver (2.7 microM) and the biliary excretion and sinusoidal efflux of morphine-3-glucuronide was assessed under steady-state conditions. Subsequently, the infusion was stopped and the concentration vs time profile of morphine-3-glucuronide in outflow perfusate (the wash-out phase) was determined. A physiologically-based pharmacokinetic model was used to determine the rate-constants for the movement of hepatically-generated morphine-3-glucuronide into the sinusoidal and canalicular spaces of the liver, and the associated membrane permeability terms. The mean (+/-s.d.) rate constants for the biliary excretion and sinusoidal efflux of morphine-3-glucuronide were determined to be 0.160 +/- 0.043 and 0.169 +/- 0.068 min(-1), respectively, and the corresponding membrane permeability parameters were 1.12 and 1.18 mL min(-1), respectively. The sinusoidal membrane permeability term was significantly less than hepatic blood flow in the rat. The volume of distribution of hepatically-generated morphine-3-glucuronide (207.5 +/- 74.8 mL) was found to be approximately 50-times the intracellular space of the rat liver, suggesting that hepatically-generated morphine-3-glucuronide accumulates within hepatocytes. The results indicate that hepatically-generated morphine-3-glucuronide undergoes intracellular accumulation, probably as a consequence of poor membrane permeability.

Neuropeptide Y suppresses epileptiform activity in rat hippocampus in vitro

Neuropeptide Y (NPY) potently inhibits glutamate-mediated synaptic transmission in areas CA1 and CA3 of the rat hippocampus without affecting other synaptic inputs onto principal cells of the hippocampal formation, suggesting that its biological role may include the regulation of excitability within the hippocampus. Here we examine NPY's actions in three in vitro models of epilepsy [0 Mg2+-, picrotoxin-, and stimulus-train-induced bursting (STIB)] with the use of extracellular and whole cell patch-clamp recordings from rat hippocampal-entorhinal cortex slices. Perfusion of the slice with saline that had Mg2+ omitted (0 Mg2+) or that had picrotoxin (100 microM) added resulted in brief spontaneous bursts (SBs) resembling interictal discharges. SB frequency is significantly reduced in both models by 1 microM NPY and by the Y2-preferring agonists peptide (P)YY(3-36) (1 microM) and 1-4-(6-aminohexanoic acid)-25-36 ([ahx(5-24)] NPY; 3 microM). The Y1-preferring agonist Leu31-Pro34NPY (1 microM) is considerably less potent, but also reduces burst frequency, even in the presence of the selective Y1 receptor antagonist GR231118, suggesting the involvement of a different receptor. In STIB, high-frequency stimulus trains to stratum radiatum of area CA2/CA3 result in clonic or tonic-clonic ictaform primary afterdischarges (primary ADs) as well as longer, spontaneous secondary ictaform discharges and SBs similar to those in the other models. Primary AD duration is greatly reduced or abolished by Y2- but not Y1-preferring agonists. SBs, although variable, were inhibited by both Y1 and Y2 agonists. In single and dual whole cell recordings from CA3 pyramidal cells, we frequently observed spontaneous, rhythmic synchronous events (SRSEs) arising after several STIB stimuli. Once established, SRSEs persist in the absence of further stimuli and are insensitive to the application of NPY. SRSEs in pyramidal cells typically occur at 2-4 Hz, are outward currents when cells are clamped near rest (>100 pA at a holding potential of -55 mV), reverse between -60 and -70 mV, and are inhibited by 100 microM picrotoxin, indicating involvement of gamma-aminobutyric acid-A receptors. They are inhibited by blockers of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) but not N-methyl-D-aspartate receptors. Whole cell patch-clamp recordings from interneurons in CA3 after STIB reveal NPY-insensitive, rhythmic, inward AMPA-receptor-mediated currents that are similar in frequency to SRSEs seen in pyramidal cells. We conclude that NPY, acting predominantly via Y2 receptors, can dramatically inhibit epileptiform activity in three fundamentally different in vitro models of epilepsy without affecting endogenous inhibitory activity. The results also provide support for the hypothesis that endogenous NPY may normally control excitability in the hippocampus and suggest the potential for NPY receptors as targets for anticonvulsant therapy.

Comparison of the disposition of hepatically-generated morphine-3-glucuronide and morphine-6-glucuronide in isolated perfused liver from the guinea pig

PURPOSE

Humans and guinea pigs metabolise morphine extensively, forming the isomers morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) in relatively similar ratios. Both metabolites are formed in the liver, and their greater polarity relative to the parent aglycone may limit their permeability across hepatic membranes. This study compared the disposition of hepatically-generated M3G and M6G in perfused livers isolated from guinea pigs.

METHODS

Livers were perfused at 30 ml/min in a non-recirculating manner with Krebs bicarbonate buffer containing morphine (6 to 7 microM). Perfusing medium, venous perfusate and bile were collected at regular intervals and concentrations of morphine, M3G and M6G determined by reversed-phase HPLC.

RESULTS

Concentrations of morphine, M3G and M6G in perfusate and the rates of biliary excretion of M3G and M6G were consistent between 20 and 50 min of perfusion. The mean (+/-s.d.) ratio for the rate of formation of M3G relative to M6G was 3.7 +/- 1.5. A mean 33 +/- 3% of morphine extracted by the liver was recovered as summed M3G and M6G. Of the M3G and M6G formed during a single passage, 19 +/- 11% and 9 +/- 9%, respectively, was excreted into bile; the values were significantly different (P = 0.002).

CONCLUSIONS

A greater fraction of hepatically-generated M3G excreted into bile compared to that for M6G reflects differences in their relative transport across sinusoidal and canalicular membranes of hepatocytes, possibly via carrier-mediated systems.

In vivo and in vitro formation of morphinone from morphine in rat

1. Morphinone, a toxic metabolite, and its glutathione adduct (MO-GSH) were identified in the bile of rat after subcutaneous injection of morphine (25 mg/kg) by hplc procedures. The amounts of morphinone and MO-GSH excreted in the 12-h bile were 0.8 +/- 0.3 and 8.4 +/- 4.3% respectively. 2. The 9000 g supernatants of rat, guinea pig, rabbit, mouse, hamster and bovine livers produced morphinone from morphine in the presence of either NAD+ or NADP+, NAD+ was a more efficient cofactor than NADP+ except in the guinea pig which equally utilized both cofactors. With NAD+ as cofactor, the amounts of morphinone formed in rat and guinea pig were 5.70 and 5.82 mumol/g liver/30 min respectively and were three-to-four times those in other species. 3. The enzyme activity responsible for formation of morphinone from morphine in the rat was almost exclusively distributed in the microsomal fraction, whereas guinea pig, hamster and bovine expressed the enzyme activity mainly in the cytosolic fraction. Rabbit and mouse gave higher activity in the cytosolic and microsomal fractions respectively, but other fractions of both species contained considerable activity. 4. The enzyme activities in male and female rat microsomes were characterized with respect to developmental pattern, kinetic parameters, pH dependency and susceptibility to inhibitors. 5. In conclusion the metabolism of morphine to morphinone in rat was confirmed by in vivo and in vitro experiments. It is also suggested that this pathway is a common route in morphine metabolism in several mammalian species.

Effects of albumin on the disposition of morphine and morphine-3-glucuronide in the rat isolated perfused liver

1. The effect of albumin on the disposition of morphine and hepatically generated morphine-3-glucuronide (M3G) was investigated in the single-pass rat isolated perfused liver. 2. Using a balanced cross-over design, each of 10 livers was perfused at 30 mL/min with medium containing 2.7 mumol/L morphine in the presence and absence of 10 g/L bovine serum albumin (BSA). 3. Both bile flow rate and hepatic oxygen consumption were significantly higher (P < 0.005) when BSA was present in the perfusion medium, suggestive of a change in the functional performance of the perfused liver. 4. The binding of morphine and M3G was negligible in both BSA-free and -containing perfusate. 5. Outflow perfusate contained both morphine and M3G, while the metabolite but not morphine was found in bile. The recovery of the administered morphine was approximately 100% and was not altered (P > 0.05) by the presence or absence of BSA. 6. The fraction of morphine escaping heptic extraction in the absence of BSA (mean +/- SD; 0.41 +/- 0.14) was not altered significantly (P > 0.05) by the presence of the protein in perfusate (0.35 +/- 0.13), indicating no change in the intrinsic clearance or morphine despite the difference in oxygen consumption. 7. The fraction of hepatically generated M3G excreted in bile was significantly higher (P < 0.005) when BSA was present in the perfusate than when it was not (0.44 +/- 0.14 vs 0.38 +/- 0.16, respectively). 8. The results are consistent with the concept that BSA modifies the ability of solutes, including M3G, to move through the paracellular pathway from the canalicular to the vascular space. 9. It is concluded that because albumin may modify not only the unbound fraction of a ligand in perfusate, but also the functional performance of the liver, care is needed in the interpretation of studies examining the influence of the protein on the hepatic disposition of drugs and their metabolites.

Biochemical synthesis, purification and preliminary pharmacological evaluation of normorphine-3-glucuronide

Normorphine was synthesised from morphine by thermal decomposition of an N-alpha-chloroethylchloroformate adduct, and purified (> 98% purity) using semi-preparative HPLC with ultraviolet detection. Normorphine-3-glucuronide (NM3G) was biochemically synthesised using the substrate normorphine, uridine diphosphoglucuronic acid and Sprague-Dawley rat liver microsomes in a 75% yield (relative to normorphine base). The synthesised NM3G was purified by precipitation and washing with acetonitrile. Determinations of purity using HPLC with electrochemical and ultraviolet detection confirmed that the NM3G produced was of high (> 99%) purity. Mass spectrometry, fourier transform infrared spectrophotometry and nuclear magnetic resonance spectrometry confirmed the structure, especially placement of the glucuronide moiety at the 3-phenolic position and not at the 17-nitrogen. Administration of NM3G by the intracerebroventricular (icv) route to rats in doses of 2.5 and 7.5 microg resulted in the development of central nervous system (CNS) excitatory behavioural effects including myoclonus, chewing, wet-dog shakes, ataxia and explosive motor behaviour. At an icv dose of 7.5 microg, NM3G also induced short periods of tonic-clonic convulsive activity. Thus, NM3G elicits CNS excitation following supraspinal administration in a manner analogous to morphine-3-glucuronide (M3G), the major metabolite of morphine (1). Further studies are required to determine whether NM3G attenuates morphine-induced antinociception in a similar manner to M3G.

The disposition of morphine and morphine-3-glucuronide in the isolated perfused rat liver: effects of altered perfusate flow rate

The rat single-pass isolated perfused liver preparation was used to study the effects of altered perfusate flow rate on the hepatic disposition of morphine and its polar metabolite morphine-3-glucuronide (M3G). Using a balanced, cross-over design, livers of female Sprague-Dawley rats (n = 6) were perfused at 15 and 30 mL min-1 with erythrocyte- and protein-free perfusion medium containing a constant concentration of morphine (2.7 microM). After reaching steady-state, inflow and outflow perfusate and bile samples were collected and morphine and M3G were measured by HPLC. Doubling of perfusate flow rate was associated with a significant increase (P < 0.05) in the availability of morphine (mean +/- s.d. of 0.19 +/- 0.06 at 15 mL min-1 and 0.29 +/- 0.08 at 30 mL min-1). The magnitude of the change in morphine availability was consistent with the predictions of the well-stirred model of hepatic elimination. The fate of hepatically generated M3G was assessed by the biliary extraction ratio of M3G; alterations in perfusate flow rate had no significant effect on this ratio (mean +/- s.d. of 0.49 +/- 0.14 at a perfusate flow rate of 15 mL min-1 and 0.47 +/- 0.22 at 30 mL min-1). A physiologically-based mathematical model, in which the vascular and intracellular spaces of the liver were represented by two well-mixed compartments, was utilized to derive an equation for the biliary extraction ratio of M3G. According to the model, the value of this extraction ratio will become insensitive to changes in perfusate flow rate when the permeability for M3G of the membrane separating the intracellular and vascular compartments is low compared with perfusate flow rate. Hence, the experimental results are consistent with the concept that the hepatic sinusoidal membrane represents a diffusional barrier to M3G.