High-performance liquid chromatographic method for the direct determination of 4-methylumbelliferone and its glucuronide and sulfate conjugates. Application to studies in the single-pass in situ perfused rat intestine-liver preparation

Pharmacokinetic analysis of factors determining elimination pathways for sulfate and glucuronide metabolites of xenobiotics. iii: mechanisms for sinusoidal efflux of 4-methylumbelliferone sulfate

1. To elucidate the mechanisms involved in the sinusoidal efflux of sulfate and glucuronide metabolites of 4-methylumbelliferone (4MU), isolated rat liver perfusion studies were performed under several conditions. 2. The effect of sodium azide on the hepatic handling of both conjugates was examined. The net sinusoidal efflux clearance (CL(eff)) based on the unbound concentration in the liver did not change for 4MU glucuronide (4MUG) or significantly increase for 4MU sulfate (4MUS), suggesting that the sinusoidal efflux of both conjugates is not mediated by the transport systems dependent on adenosine triphosphate. 3. Under Cl(-)-depleted conditions, the CL(eff) of 4MUG significantly decreased, but the saturation of its sinusoidal efflux rather than the transport system dependent on Cl(-) might be involved because the hepatic concentration of 4MUG was extensively higher than that of the control study due to the extremely attenuated biliary excretion. The CL(eff) of 4MUS also significantly decreased, but its hepatic concentration was not different from that in the control study, suggesting that the transport system using Cl(-) is involved in the sinusoidal efflux of 4MUS. 4. The effect of glutathione was examined. CL(eff) of 4MUG was not affected by the additional glutathione, but CL(eff) of 4MUS decreased significantly, suggesting that some transport system sensitive to glutathione is involved in the sinusoidal efflux of 4MUS, but not of 4MUG. 5. Transporters such as Oatp1, Oatp2 and/or Npt1 might be involved in the sinusoidal efflux of 4MUS, but 4MUG is secreted from the sinusoidal membrane via the systems that are totally different from those for 4MUS.

A hyaluronan synthase suppressor, 4-methylumbelliferone, inhibits liver metastasis of melanoma cells

4-Methylumbelliferone (MU) inhibits the cell surface hyaluronan (HA) formation, and that such inhibition results in suppression of adhesion and locomotion of cultured melanoma cells. Here, we examine the effect of MU on melanoma cell metastasis in vivo. MU-treated melanoma cells showed both decreased cell surface HA formation and suppression of liver metastasis after injection into the mice. Oral administration of MU to mice decreased tissue HA content. These HA knock-down mice displayed suppressed liver metastasis. Thus, both cell surface HA of melanoma cells and recipient liver HA can promote liver metastasis, indicating that MU has potential as an anti-metastatic agent.

Role of Breast Cancer Resistance Protein (Bcrp1/Abcg2) in the Extrusion of Glucuronide and Sulfate Conjugates from Enterocytes to Intestinal Lumen

The purpose of this study is to examine the significance of efflux transporters in the small intestine to extrude glucuronide (G) and sulfate (S) conjugates into the intestinal lumen. From this standpoint, we performed in situ intestinal perfusion experiments by using Eisai hyperbilirubinemic rats (EHBRs) in which the multidrug resistance protein 2 (Mrp2/Abcc2) is hereditarily defective and breast cancer resistance protein (Bcrp1/Abcg2) knockout mice. The intestinal lumen of EHBRs and Bcrp1 (-/-) mice was perfused with medium containing 4-methylumbelliferone (4MU) and E3040 [6-hydroxy-5,7-dimethyl-2-methylamino-4-(3-pyridilmethyl) benzothiazole] to determine the efflux of metabolites into the outflow. The efflux of E3040-glucuronide (G) in EHBRs was significantly lower compared with that in normal rats. However, no significant difference was observed for the efflux of 4MU-G, 4MU-sulfate (S), and E3040-S between EHBRs and normal rats. In contrast, the efflux of intracellularly formed 4MU-G, 4MU-S, and E3040-G in Bcrp1 (-/-) mice was significantly lower than that in normal mice. Therefore, Bcrp1 has an important role in extruding glucuronide and sulfate conjugates formed in enterocytes into the intestinal lumen, whereas Mrp2 is responsible for the efflux of some glucuronide conjugates.

Pharmacokinetic analysis of factors determining elimination pathways for sulfate and glucuronide metabolites of xenobiotics II: Studies with isolated perfused rat liver

1. To elucidate the determining factors for elimination pathways of sulfate and glucuronide metabolites of xenobiotics, a single-pass perfusion of 4-methylumbelliferone (4MU) or p-nitrophenol (pNP) was performed with an isolated rat liver preparation. 2. Without bovine serum albumin in the perfusion system, clearance calculated based on the unbound concentration in the liver clearly showed that the net efflux clearances (CLeff) of sulfates from the sinusoidal membrane were much higher than those of glucuronides and that the biliary excretion clearances (CLb) of glucuronides were approximately two times larger than those of sulfates. 3. The ratios of CLeff to CLb were much higher for sulfates than those for glucuronides. The bile-oriented elimination of glucuronides or sinusoidal efflux-oriented elimination of sulfates was observed even using the perfusate including 3% bovine serum albumin, but the sinusoidal efflux of sulfates was extensively enhanced by bovine serum albumin in the perfusate. The mechanisms behind this stimulatory effect remain to be elucidated. 4. For both compounds, CLb of glucuronide was comparable with CLb of sulfate, meaning that CLb is not responsible for the biliary excretion of glucuronides at extensively higher rate than sulfates. 5. Higher concentration of glucuronides in the liver, partly caused by much lower CLeff of glucuronides than that of sulfates, is likely responsible for the bile-oriented excretion of glucuronides. The extensive sinusoidal efflux of sulfates, leading to the urine-oriented excretion, is attributed to the substantially higher CLeff than CLb. 6. In conclusion, the sinusoidal efflux is an important factor for determining elimination pathways of both sulfates and glucuronides, although further studies are needed to clarify the mechanisms of the sinusoidal efflux.

A novel mechanism for the inhibition of hyaluronan biosynthesis by 4-methylumbelliferone

Specific inhibitors of hyaluronan (HA) biosynthesis can be valuable therapeutic agents to prevent cancer invasion and metastasis. We have found previously that 4-methylumbelliferone (MU) inhibits HA synthesis in human skin fibroblasts and in group C Streptococcus. In this paper, the inhibition mechanism in mammalian cells was investigated using rat 3Y1 fibroblasts stably expressing HA synthase (HAS) 2. Exposure of the transfectants to the inhibitor resulted in significant reduction of HA biosynthesis and matrix formation. The evaluation of HAS transcripts and analysis of cell-free HA synthesis demonstrated the post-transcriptional suppression of HAS activity by MU. Most interesting, the post-transcriptional suppression of HAS activity was also observed using p-nitrophenol, a well known substrate for UDP-glucuronyltransferases (UGT). We investigated whether the inhibition was exerted by the glucuronidation of MU using both high pressure liquid chromatography and TLC analyses. The production of MU-glucuronic acid (GlcUA) was consistent with the inhibition of HA synthesis in HAS transfectants. MU-GlcUA was also detected at a similar level in control cells, suggesting that the glucuronidation was mediated by an endogenous UGT. Elevated levels of UGT significantly enhanced the inhibitory effects of MU. In contrast, the inhibition by MU was diminished to the control level when an excess of UDP-GlcUA was added to the cell-free HA synthesis system. We propose a novel mechanism for the MU-mediated inhibition of HA synthesis involving the glucuronidation of MU by endogenous UGT resulting in a depletion of UDP-GlcUA.

Influence of t-butylhydroquinone and beta-naphthoflavone on formation and transport of 4-methylumbelliferone glucuronide in Caco-2/TC-7 cell monolayers

Human Caco-2 cells have been established as a model system for intestinal biotransformation and permeability. When grown on Transwell polycarbonate filters they develop morphologic and biochemical characteristics of enterocytes with well separated apical and basolateral surfaces. In addition, Caco-2/TC-7 cells have proven to be useful to study regulation of human UDP-glucuronosyltransferases (UGTs) by Ah receptor agonists and antioxidant-type inducers such as beta-naphthoflavone (BNF) and t-butylhydroquinone (TBHQ). In the present investigation, formation and transport of 4-methylumbelliferone glucuronide was studied in intact Caco-2 cell monolayers. The following results were obtained: when loaded with 50-200 microM MUF either apically or basolaterally, MUF-GA was the major metabolite which was mostly released (80%) at the basolateral surface, probably via the multidrug resistance protein isoform MRP3; MUF sulfate formation was low (5 +/-2%). Pretreatment of cells with 80 microM TBHQ or 50 microM BNF for 72 hr before addition of 100 microM MUF enhanced basolateral secretion of MUF-GA 1.4- and 1.7-fold, respectively. However, at >200 microM MUF, MUF-GA secretion and induction was smaller, probably due to inhibition of intracellular UGT activity. MRP3 protein was localized to the basolateral surface of Caco-2 cells but was not induced by TBHQ or BNF. The results suggest that MUF-GA is mostly secreted basolaterally in Caco-2 cell monolayers. Treatment with TBHQ or BNF significantly enhanced MUF-GA formation in the intact cell.

Determination of UDP-glucuronosyltransferase UGT1A6 activity in human and rat liver microsomes by HPLC with UV detection

A simple and sensitive method for the determination of UDP-glucuronosyltransferase UGT1A6 activity using 4-methylumbelliferone (4-MU) and 4-nitrophenol (4-NP) as substrates in human and rat liver microsomes by high-performance liquid chromatography (HPLC) with uv detection is reported. The method was validated for the determination of 4-methylumbelliferyl beta-D-glucuronide (4-MUG) and 4-nitrophenyl beta-D-glucuronide (4-NPG) with respect to specificity, linearity, detection limit, recovery, stability, precision and accuracy. There was no interference from matrix and non-enzymatic reactions. Calibration curves for 4-MUG and 4-NPG are linear from 0.5 to 500 microM. Average recoveries ranged from 98 to 100% in spiked liver microsomes samples. 4-MUG and 4-NPG were stable at 4 degrees C for at least 72 h in spiked liver microsomes samples. The method was found to be more sensitive than previous methods using a spectrophotometer, a spectrofluorometer and HPLC. The detection limit for 4-MUG and 4-NPG (signal-to-noise ratio of 3) was 14 and 23 nM, respectively. The intra- and inter-day precision (relative S.D. (RSD)) and accuracy (relative mean error (RME)) was <5 and 9%, respectively. The intra- and inter-day reproducibility (RSD) of UGT1A6 enzyme assay in liver microsomes was <6%. With this improved sensitivity, the kinetics of UGT activities toward 4-MU and 4-NP in human and rat liver microsomes could be determined more precisely. In addition, the method could determine the non-inducible, and 3-methylcholanthrene- and phenobarbital-inducible activities of UGT1A6 in rat liver microsomes under the same assay conditions. Therefore, this method is applicable to in vivo and in vitro studies on the interaction of xenobiotic chemicals with UGT1A6 isoform in mammals using small amounts of biological samples.

Determination of the kinetics of rat UDP-glucuronosyltransferases (UGTs) in liver and intestine using HPLC

Uridinediphosphoglucuronosyl transferases (UGTs) are a group of membrane bound proteins which catalyze the transfer of glucuronic acid from UDP-glucuronic acid to a wide variety of xenobiotics and drug molecules enabling them to be eliminated. The major UGT isoforms found in the rat are 1A1, 1A6, 2B1 and 2B12. Conventional methods for the assay of glucuronides (GLs) include TLC, extraction and colorimetry or quantification of the aglycone, liberated after hydrolyzing the GL with beta-glucuronidase. However these techniques cannot distinguish between isomeric GLs or GLs of multiple acceptor site substrates. Therefore the purpose of this study was to develop simple and sensitive HPLC methods for the direct and simultaneous analysis of the GL(s) and their aglycones without the drawbacks of the conventional methods. The three classical substrates we chose were 4-methylumbelliferone (4MU), testosterone (TES) and 8-hydroxyquinoline (8HOQ) representing UGT isoforms 1A6, 2B1 and 2B12 of the rat family, respectively. Here we report the validated HPLC conditions, for the detection and separation of 4-methylumbelliferone glucuronide (4MUG), testosterone glucuronide (TESG) and 8-hydroxyquinoline glucuronide (8HOQG) and their aglycones in incubation media containing male Sprague-Dawley rat liver and intestinal microsomal preparations. The separations were achieved on a Zorbax SB-CN column (150 x 4.6 mm, 5 micron). The analysis time for the separation of TES, 8HOQ and 4MU and their glucuronides were 17, 12 and 30 min, respectively. The methods showed excellent linearity (r2 > 0.99) over the concentration ranges tested (0.25-5.0 nmoles of TESG; 0.125-18.75 nmoles of 8HOQG and 0.125-12.5 nmoles of 4MUG), good precision and accuracy (RSD<2.5%). Inter-day variability studies (n = 3) showed no significant difference between the regression lines obtained on the three days. Recoveries were good ( > 90%) at all three points (low, mid-point, high) of the standard curve. The limits of detection were 0.125, 0.1 and 0.1 nmole for TESG, 8HOQG and 4MUG. respectively. The above methods were used to estimate kinetic parameters such as Vmax and Km for the GLs of the three substrates in both liver and intestinal tissue preparations and the values were comparable with previously reported results. UGT2B1 was found primarily in the liver while UGTs 1A6 and 2B12 were present in comparable amounts in both tissues.

Carrier-mediated entry of 4-methylumbelliferyl sulfate: characterization by the multiple-indicator dilution technique in perfused rat liver

The hepatocellular entry of 4-methylumbelliferyl sulfate (4MUS) a highly ionized and highly bound anion capable of futile cycling, was examined in the single-pass albumin-free perfused rat liver preparation. Desulfation of 4MUS to 4-methylumbelliferone (4MU) was verified in vitro to be a low-affinity, high-capacity process (Km = 731 micromol/L; Vmax = 414 nmol min(-1) g(-1) liver). With 4MUS given to the perfused rat liver, sulfation of 4MU, the formed metabolite, was attenuated in the presence of 2,6-dichloro-4-nitrophenol (DCNP), a sulfation inhibitor, and when sulfate ion was substituted by chloride ion. 4MU sulfation, being a high-affinity system, was reduced most effectively at the lowest 4MUS concentration (15 micromol/L) used, evidenced by the increased (24%) net hepatic extraction ratio of 4MUS and reduced utilization (72%) of infused tracer 35SO4(2-) by 4MU for 4MU35S formation. Single-pass multiple indicator dilution (MID) studies were thus conducted under identical conditions (DCNP and absence of inorganic sulfate), with injection of [3H]4MUS and a set of noneliminated vascular and cellular reference indicators into the portal vein (prograde) or hepatic vein (retrograde), against varying background bulk concentrations of 4MUS (5 to 900 micromol/L). The steady-state removal rate of 4MUS and formation rates of 4MU and its glucuronide conjugate (4MUG) were not altered with perfusion flow direction, suggesting the presence of even or parallel distributions of 4MUS desulfation and 4MU glucuronidation activities. When the outflow dilution profile of [3H]4MUS was evaluated with the barrier-limited model of Goresky, a slight red cell carriage effect was found for 4MUS. The permeability surface area product for cellular entry for prograde showed a dramatic concentration-dependent decrease (from 0.13 to 0.01 mL sec(-1) g(-1), or 7.4 to 0.56 times the blood perfusate flow rate) and was resolved as saturable and nonsaturable components, while data for retrograde were more scattered, varying from 2.8 to 1 times the blood perfusate flow rate. Efflux (coefficient = 0.0096 +/- 0.0024 and 0.0088 +/- 0.0062 mL sec(-1) g(-1), respectively) was relatively insensitive to concentration and flow direction. The same was observed for the removal capacity for metabolism and excretion (sequestration coefficient: for prograde, 0.0056 +/- 0.0017 mL sec(-1) g(-1); for retrograde, 0.0056 +/- 0.003 mL sec(-1) g(-1)). The decrease in the apparent partition coefficient (ratio of 4MUS concentration estimated in tissue to unbound plasma concentration) and the increase in relative throughput component with concentration further substantiate the claim on the presence of concentrative processes at the sinusoidal membrane.

First-pass effect: significance of the intestine for absorption and metabolism

The occurrence of low systemic availability due to significant metabolism or poor absorption of orally administered drugs has been well recognized. Three rate controlling factors affecting the oral absorption: unstirred water layer, membrane limitation, or flow limitation, have been identified. These are much affected by the physicochemical properties of the drug: pKA, water/lipid solubility, structural mimicry to endogenous substrates for transport proteins, and the physiology of the GI tract. Drug metabolizing enzymes are found to be present in the intestine, albeit the content is lower than that found in liver. The presence of pre-absorptive versus post-absorptive intestinal metabolism is presently discussed in experimental sets of data with luminal and systemic administration of the drugs in the vascularly perfused rat small intestine preparation. The effect of the anterior anatomical placement of the intestine and its contribution to metabolism, in relation to that for the liver, has been examined in our laboratory by the perfused intestine-liver preparation. The effect of concentration and flow have been studied and general principles governing drug absorption and metabolism in the intestine and the subsequent effects on the liver have been discussed.

Extracorporeal application of a gel-entrapment, bioartificial liver: demonstration of drug metabolism and other biochemical functions

Metabolic activity of a gel-entrapment, hollow fiber, bioartificial liver was evaluated in vitro and during extracorporeal hemoperfusion in an anhepatic rabbit model. The bioartificial liver contained either 100 million rat hepatocytes (n = 12), fibroblasts (n = 3), or no cells (n = 7) during hemoperfusion of anhepatic rabbits. Eight other anhepatic rabbits were studied without hemoperfusion as anhepatic controls, and three sham rabbits served as normal controls. Albumin production rates (mean +/- SEM) were similar during in vitro (17.0 +/- 2.8 micrograms/h) and extracorporeal (18.0 +/- 4.0 micrograms/h) application of the hepatocyte bioartificial liver. Exogenous glucose requirements were reduced (p < 0.01) and euglycemia was prolonged (p < 0.001) in anhepatic rabbits treated with the hepatocyte bioartificial liver. The maximum rate of glucose production by the hepatocyte bioartificial liver ranged from 50-80 micrograms/h. Plasma concentrations of aromatic amino acids, proline, alanine, and ammonia were normalized in anhepatic rabbits during hepatocyte hemoperfusion. Gel-entrapped hepatocytes in the bioartifical liver performed sulfation and glucuronidation of 4-methylumbelliferone. P450 activity was demonstrated during both in vitro and extracorporeal application of the BAL device by the formation of 3-hydroxy-lidocaine, the major metabolite of lidocaine biotransformation by gel-entrapped rat hepatocytes. In summary, a gel-entrapment, bioartificial liver performed multiple hepatocyte-specific functions without adverse side effects during extracorporeal application in an anhepatic, small animal model. With its potential for short term support of acute liver failure, scale-up of the current bioartificial liver device is indicated for further investigations in large animal, preclinical trials.

Pharmacokinetics and bioavailabilities of hymecromone in human volunteers

Specific and ultrasensitive reverse-phase HPLC assays of the choleretic and biliary antispasmodic hymecromone (down to 0.05 ng ml-1) and its glucuronide, using fluorimetric detection, and sulfate metabolites using UV detection, were developed. Sodium salt solutions of 400 mg (over 3 min) and 800 mg (over 5 min) were infused i.v. into 6-8 normal human volunteers. The half-life of the major rate constant averaged 28 +/- 2 min (SE). Subsequently, less than 0.8 per cent of the dose was eliminated with terminal half-lives of 70-359 min. The apparent volume of distribution of hymecromone, referenced to the total plasma concentration, averaged 20.8 +/- 1.41 (Vc, central compartment volume) and 36.4 +/- 2.11 (Vss steady state volume). Hymecromone's total body clearance averaged 1413 +/- 89 ml min-1. The pharmacokinetics of hymecromone were dose-independent. Only 0.3 +/- 0.3 per cent unchanged hymecromone was renally excreted. Mostly dose-independent glucuronidated drug (93 +/- 4 per cent of the dose) was excreted in the urine; a smaller amount was renally excreted as the sulfate (1.4 +/- 0.3 per cent of the dose). The oral bioavailability estimated from the relative areas under the hymecromone plasma concentration-time curves following oral and i.v. administration of hymecromone to six volunteer subjects showed no dose-dependence and was 1.8 +/- 0.6 per cent. However, an anomalous c. 200 per cent of the glucuronide produced by i.v. hymecromone was produced from orally administered hymecromone as determined from the ratio of the AUC values of glucuronide obtained after peroral and i.v. administration of the same dose of hymecromone to demonstrate a high first-pass effect and implicate renal glucuronidation.