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

ABCC3 genetic variants are associated with postoperative morphine-induced respiratory depression and morphine pharmacokinetics in children

Respiratory depression (RD) is a serious side effect of morphine and detrimental to effective analgesia. We reported that variants of the ATP binding cassette gene ABCC3 (facilitates hepatic morphine metabolite efflux) affect morphine metabolite clearance. In this study of 316 children undergoing tonsillectomy, we found significant association between ABCC3 variants and RD leading to prolonged postoperative care unit stay (prolonged RD). Allele A at rs4148412 and allele G at rs729923 caused a 2.36 (95% CI=1.28-4.37, P=0.0061) and 3.7 (95% CI 1.47-9.09, P=0.0050) times increase in odds of prolonged RD, respectively. These clinical associations were supported by increased formation clearance of morphine glucuronides in children with rs4148412 AA and rs4973665 CC genotypes in this cohort, as well as an independent spine surgical cohort of 67 adolescents. This is the first study to report association of ABCC3 variants with opioid-related RD, and morphine metabolite formation (in two independent surgical cohorts).

Development and application of a simplified sample preparation method for determination of TEGDMA and related metabolites

Analysis of biological samples obtained from in vivo experiments can be often challenging. In general it is not possible to apply the commonly used matrices that are necessary for the experiments to the desired analysis systems without further conditioning or sample purification steps. Besides possible adverse effects for instruments, interference between analytes and matrices can affect the correct measurement of analytes. Different methods of sample preparation can be used to convert biological samples into samples suitable for analysis; SPE and HS-SPME are two well established methods. Research of in vivo metabolism of triethyleneglycoledimethacrylate (TEGDMA), one of the most frequently contained comonomer in dental restorative materials, demands sample preparation methods that offer separation of TEGDMA and its related metabolites from biological matrices. In the presented study two methods for sample preparation were developed in order to analyze TEGDMA as well as its metabolites triethyleneglycole (TEG), 2,3-epoxymethacrylicacid methylester (2,3-EMME), and methacrylacid methylester (MAME) in Krebs-Henseleit buffer samples to facilitate a subsequent analysis via GC-MS. An easy and time-saving separation protocol was developed. Recovery rates of TEGDMA and TEG after SPE were 21 +/- 3% and 105 +/- 12%, respectively, recovery rate after headspace extraction of 2,3-EMME and MAME was higher at 48 degrees C compared with 20 degrees C extraction temperature. The tested range for 2,3-EMME and MAME concentration after HS-SPME extraction was 0.1-100 mg/L and both analytes showed a good linearity.

Multidrug Resistance Proteins 2 and 3 Provide Alternative Routes for Hepatic Excretion of Morphine-Glucuronides

Glucuronidation is a major hepatic detoxification pathway for endogenous and exogenous compounds, resulting in the intracellular formation of polar metabolites that require specialized transporters for elimination. Multidrug resistance proteins (MRPs) are expressed in the liver and can transport glucuronosyl-conjugates. Using morphine as a model aglycone, we demonstrate that morphine-3-glucuronide (M3G), the predominant metabolite, is transported in vitro by human MRP2 (ABCC2), a protein present in the apical membrane of hepatocytes. Loss of biliary M3G secretion in Mrp2(-/-) mice results in its increased sinusoidal transport that can be attributed to Mrp3. Combined loss of Mrp2 and Mrp3 leads to a substantial accumulation of M3G in the liver, from which it is transported across the sinusoidal membrane at a low rate, resulting in the prolonged presence of M3G in plasma. Our results show that murine Mrp2 and Mrp3 provide alternative routes for the excretion of a glucuronidated substrate from the liver in vivo.

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.

Mice lacking multidrug resistance protein 3 show altered morphine pharmacokinetics and morphine-6-glucuronide antinociception

Glucuronidation is a major detoxification pathway for endogenous and exogenous compounds in mammals that results in the intracellular formation of polar metabolites, requiring specialized transporters to cross biological membranes. By using morphine as a model aglycone, we demonstrate that multidrug resistance protein 3 (MRP3/ABCC3), a protein present in the basolateral membrane of polarized cells, transports morphine-3-glucuronide (M3G) and morphine-6-glucuronide in vitro. Mrp3(-/-) mice are unable to excrete M3G from the liver into the bloodstream, the major hepatic elimination route for this drug. This results in increased levels of M3G in liver and bile, a 50-fold reduction in the plasma levels of M3G, and in a major shift in the main disposition route for morphine and M3G, predominantly via the urine in WT mice but via the feces in Mrp3(-/-) mice. The pharamacokinetics of injected morphine-glucuronides are altered as well in the absence of Mrp3, and this results in a decreased antinociceptive potency of injected morphine-6-glucuronide.

Isolated perfused liver model: the rat and guinea pig compared

OBJECTIVE

Although the rat is the most commonly used species for the study of hepatic metabolism, the physiology of the guinea pig is closer to human physiology. We compared the model of isolated perfused guinea pig liver with the classic model of isolated perfused rat liver, especially with respect to amino acid metabolism.

METHODS

After validation of an anesthetic mixture of ketamine, diazepam, and xylazine for the guinea pig, isolated perfused livers were harvested for both species. Three groups of animals were compared for the study of liver metabolic fluxes: 6-wk-old male Sprague-Dawley rats (R; 230 +/- 10 g, n = 5), young male Hartley guinea pigs (YG; 223 +/- 8 g, n = 6) matched to rats by liver weight, and adult male Hartley guinea pigs (AG; 389 +/- 5 g, n = 6) matched to rats by age. Results (mean +/- standard error of the mean) were compared by analysis of variance and Newman-Keuls tests.

RESULTS

Both models displayed a satisfactory hepatic viability, but differences were noted, with higher portal flows (R: 3.1 +/- 0.3 versus YG: 4.5 +/- 0.3 and AG: 4.2 +/- 0.3 mL. min(-1). g(-1); P < 0.05, YG and AG versus R) and bile flows (R: 0.34 +/- 0.01 versus YG: 2.38 +/- 0.22 versus AG: 3.17 +/- 0.28 microL. min(-1). g(-1); P < 0.05, YG and AG versus R, and YG versus AG) and higher amino acid fluxes (P < 0.05) leading to greater nitrogen uptake (P < 0.05) in guinea pigs. We performed a second set of experiments to evaluate the influence of anesthesia and portal flow on this last parameter. In these experiments, rats were anesthetized with ketamine, diazepam, and xylazine and guinea pig livers were perfused at rat blood flow. Apart from a 50% anesthesia-related mortality for rats, bile flow and metabolic parameters were only slightly modified. However, some amino acid fluxes were statistically different (aspartate, serine, and histidine; P < 0.05), as confirmed by a higher transfer constant.

CONCLUSION

Our results indicate that the isolated perfused guinea pig liver is a suitable model for the study of hepatic metabolism.

Effect of apoE/ATP-containing liposomes on hepatic energy state

BACKGROUND/AIMS

ATP-containing liposomes partially prevent ATP depletion in the cold-stored liver. As hepatocytes can specifically bind apoE, we investigated whether the addition of apoE to large (200 nm) ATP-containing liposomes increases their uptake by the liver and further improves hepatic energy stores.

METHODS

Livers from fasted male Hartley guinea-pigs (231 +/- 3 g) were perfused for 90 min under our standard conditions (Control, n = 6) or after a single bolus addition of plain liposomes (Lip, n = 6), ATP (5 micromol)-containing liposomes (ATP-Lip, n = 6) or apoE/ATP-containing liposomes (0.8 or 8mg apoE/g phospholipids; apoE1-Lip and apoE10-Lip, respectively, n = 6 in each group). Liposome uptake and its impact on energy and nitrogen metabolism were studied.

RESULTS

At its highest concentration, apoE significantly increased liposome uptake (apoE10-Lip: - 9.17 +/- 0.69 vs apoE1-Lip: - 6.18 +/- 0.44 vs ATP-Lip: - 6.40 +/- 0.88 nmol min(-1) g(-1) P < 0.05). This was associated with a significant increase in intrahepatic ATP (apoE10-Lip: 1033 +/- 137 vs apoE1-Lip: 811 +/- 98 and ATP-Lip: 648 +/- 36 nmol g(-1); P < 0.05), which was restored to its level in non-perfused livers. Hepatic viability and nitrogen metabolism were not affected.

CONCLUSIONS

Hepatic ATP content being a key factor in the maintenance of liver graft function, apoE/ATP-containing liposomes should be useful in liver preservation for transplantation.

Enterohepatic circulation: physiological, pharmacokinetic and clinical implications

Enterohepatic recycling occurs by biliary excretion and intestinal reabsorption of a solute, sometimes with hepatic conjugation and intestinal deconjugation. Cycling is often associated with multiple peaks and a longer apparent half-life in a plasma concentration-time profile. Factors affecting biliary excretion include drug characteristics (chemical structure, polarity and molecular size), transport across sinusoidal plasma membrane and canniculae membranes, biotransformation and possible reabsorption from intrahepatic bile ductules. Intestinal reabsorption to complete the enterohepatic cycle may depend on hydrolysis of a drug conjugate by gut bacteria. Bioavailability is also affected by the extent of intestinal absorption, gut-wall P-glycoprotein efflux and gut-wall metabolism. Recently, there has been a considerable increase in our understanding of the role of transporters, of gene expression of intestinal and hepatic enzymes, and of hepatic zonation. Drugs, disease and genetics may result in induced or inhibited activity of transporters and metabolising enzymes. Reduced expression of one transporter, for example hepatic canalicular multidrug resistance-associated protein (MRP) 2, is often associated with enhanced expression of others, for example the usually quiescent basolateral efflux MRP3, to limit hepatic toxicity. In addition, physiologically relevant pharmacokinetic models, which describe enterohepatic recirculation in terms of its determinants (such as sporadic gall bladder emptying), have been developed. In general, enterohepatic recirculation may prolong the pharmacological effect of certain drugs and drug metabolites. Of particular importance is the potential amplifying effect of enterohepatic variability in defining differences in the bioavailability, apparent volume of distribution and clearance of a given compound. Genetic abnormalities, disease states, orally administered adsorbents and certain coadministered drugs all affect enterohepatic recycling.