Metabolism of coumarin and 7-ethoxycoumarin by rat, mouse, guinea pig, cynomolgus monkey and human precision-cut liver slices

In vitro pharmacokinetic behavior in lung of harringtonine, an antagonist of SARS-CoV-2 associated proteins: New insights of inhalation therapy for COVID-19

BACKGROUND

Recent studies have shown that harringtonine (HT) could specifically bind with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) protein and host cell transmembrane serine protease 2 (TMPRSS2) to block membrane fusion, which is an effective antagonist for SARS-CoV-2.

PURPOSE

Our study focused on in-depth exploration of in vitro pharmacokinetic characteristics of HT in lung.

METHODS

HPLC-fluorescence detection method was used to detect changes of HT content. Incubation systems of lung microsomes for phase I metabolism and UGT incubation systems for phase II metabolism were performed to elucidate metabolites and metabolic mechanisms of HT, and then the metabolic enzyme phenotypes for HT were clarified by chemical inhibition method and recombinant enzyme method. Through metabolomics, we comprehensively evaluated the physiological dynamic changes in SD rat and human lung microsomes, and revealed the relationship between metabolomics and pharmacological activity of HT.

RESULTS

HPLC-fluorescence detection method showed strong specificity, high accuracy, and good stability for rapid quantification of HT. We confirmed that HT mainly underwent phase I metabolism, and the metabolites of HT in different species were all identified as 4'-demethyl HT, with metabolic pathway being hydrolysis reaction. CYP1A2 and CYP2E1 participated in HT metabolism, but as HT metabolism was not NADPH dependent, the esterase HCES1 in lung also played a role. The main KEGG pathways in SD rat and human lung microsomes were cortisol synthesis and secretion, steroid hormone biosynthesis and linoleic acid metabolism, respectively. The downregulated key biomarkers of 11-deoxycortisol, 21-deoxycortisol and 9(10)-EpOME suggested that HT could prevent immunosuppression and interfere with infection and replication of SARS-CoV-2.

CONCLUSION

HT was mainly metabolized into 4'-demethyl HT through phase I reactions, which was mediated by CYP1A2, CYP2E1, and HCES1. The downregulation of 11-deoxycortisol, 21-deoxycortisol and 9(10)-EpOME were key ways of HT against SARS-CoV-2. Our study was of great significance for development and clinical application of HT in the treatment of COVID-19.

Coumarin-Induced Hepatotoxicity: A Narrative Review

Coumarin is an effective treatment for primary lymphoedema, as well as lymphoedema related to breast cancer radiotherapy or surgery. However, its clinical use is limited in several countries due to the possible occurrence of hepatotoxicity, mainly in the form of mild to moderate transaminase elevation. It is worth noting that only a few cases of severe hepatotoxicity have been described in the literature, with no reported cases of liver failure. Data available on coumarin absorption, distribution, metabolism, and excretion have been reviewed, focusing on hepatotoxicity studies carried out in vitro and in vivo. Finally, safety and tolerability data from clinical trials have been thoroughly discussed. Based on these data, coumarin-induced hepatotoxicity is restricted to a small subset of patients, probably due to the activation in these individuals of alternative metabolic pathways involving specific CYP450s isoforms. The aim of this work is to stimulate research to clearly identify patients at risk of developing hepatotoxicity following coumarin treatment. Early identification of this subset of patients could open the possibility of more safely exploiting the therapeutical properties of coumarin, allowing patients suffering from lymphoedema to benefit from the anti-oedematous activity of the treatment.

Metabolomics reveals the role of isopentenyl group in coumarins metabolism

Coumarins are a group of natural compounds commonly found in the families of Rutaceae and Umbelliferae. 7-Isopentenyloxycoumarin (ISC), auraptene (AUR), and umbelliprenin (UM) belong to prenyloxycoumarins (PYCs), which link isopentenyl, geranyl, and farnesyl group at C7 position, respectively. The substituent of 7-ethoxycoumarin (ETC) is the ethyl group. In this study, UPLC-ESI-QTOF-MS (ultra-performance liquid chromatography-electrospray ionization-quadrupole time of flight-MS)-based metabolomics was used to evaluate the in vivo and in vitro metabolism of PYCs. Results showed that ETC produced 10 known metabolites, and ISC was transformed into 17 metabolites in vivo and in vitro, which were undescribed compounds. A total of 35 AUR metabolites, including 34 undescribed metabolites were identified, and 21 metabolites were reported for the first time in UM. The results indicated that hydroxylation and N-acetylcysteine conjugation were the common metabolic reactions for PYCs. The metabolic rates of ETC, ISC, AUR and UM were 26%, 36%, 81%, and 38%, respectively, in human liver microsome, while they were 24%, 40%, 80%, and 37%, respectively, in mouse liver microsomes. In addition, recombinant cytochrome P450s (CYPs) screening showed that CYP1A1, 2C19, 3A4, and 3A5 were the major metabolic enzymes involved in the formation of hydroxylation metabolites. Together, these results suggest that the isopentenyl group plays an important role in the metabolism of PYCs.

Influence of Slice Thickness and Culture Conditions on the Metabolism of 7-Ethoxycoumarin in Precision-cut Rat Liver Slices

This study investigated the effects of some experimental variables on the rate of xenobiotic metabolism in precision-cut rat liver slices. Liver slices of 123 ± 8μm (mean ± SEM of six slices), 165 ± 3μm, 238 ± 6μm and 515 ± 14μm thickness were prepared from male Sprague-Dawley rats, and incubated in RPMI 1640 medium in an atmosphere of 95% O2/5% CO2by using a dynamic organ culture system. Liver slices of all thicknesses metabolised 10μM 7-ethoxycoumarin to total (free and conjugated) 7-hydroxycoumarin in a time-dependent manner. The rate of 7-ethoxycoumarin metabolism was greatest in 165μm thick slices and slowest in 515μm thick slices, being 2.74 ± 0.19pmol/minute/mg slice protein and 0.69 ± 0.07pmol/minute/mg slice protein, respectively. No marked effects on the rate of 7-ethoxycoumarin metabolism in liver slices were observed either by changing the medium to Earle's balanced salt solution (EBSS) or by changing the gas phase to 95% air/5% CO2. Moreover, the perfusion of rat livers with EBSS at 2–4°C, prior to preparation of tissue cores, did not enhance 7-ethoxycoumarin metabolism in rat liver slices. In this study, the optimal slice thickness was 175μm, with higher rates of 7-ethoxycoumarin metabolism being observed than with 250μm thick slices, which are often used for studies of xenobiotic metabolism. Variable results were obtained with slices of around 100–120μm thickness, which may be attributable to the ratio between intact hepatocytes and cells damaged by the slicing procedure in these very thin slices.

Cancer Hazard Identification Integrating Human Variability: The Case of Coumarin

Coumarin is a naturally occurring sweet-smelling benzopyrone that may be extracted from plants or synthesized for commercial uses. Its uses include as a flavoring agent, fragrance enhancer, and odor-masking additive. We reviewed and evaluated the scientific evidence on the carcinogenicity of coumarin, integrating information from carcinogenicity studies in animals with mechanistic and other relevant data, including data from toxicogenomic, genotoxicity, and metabolism studies, and studies of human variability of a key enzyme, CYP2A6. Increases in tumors were observed in multiple studies in rats and mice in multiple tissues. Our functional pathway analysis identified several common cancer-related biological processes/pathways affected by coumarin in rat liver following in vivo exposure and in human primary hepatocytes exposed in vitro. When coumarin 7-hydroxylation by CYP2A6 is compromised, this can lead to a shift in metabolism to the 3,4-epoxidation pathway and increased generation of electrophilic metabolites. Mechanistic data align with 3 key characteristics of carcinogens, namely formation of electrophilic metabolites, genotoxicity, and induction of oxidative stress. Considerations of metabolism, human variability in CYP2A6 activity, and coumarin hepatotoxicity in susceptible individuals provide additional support for carcinogenicity concern. Our analysis illustrates the importance of integrating information on human variability in the cancer hazard identification process.

In vitro Drug Metabolism Investigation of 7-Ethoxycoumarin in Human, Monkey, Dog and Rat Hepatocytes by High Resolution LC-MS/MS

Background:

Recently, it has been an increasing concern on the bioactivation and adverse re-actions associated with consumption of herbal and nature products. 7-Ethoxycoumarin is one of coumarin family compounds, but little information is available regarding its potential reactive metabolites.

Method:

7-ethoxylcoumarin was incubated individually with human, monkey, dog and rat hepatocytes for 2 hr, metabolites were detected, identified and characterized using high resolution liquid chromagraphy – tandem mass spectrometry.

Results:

Twenty-eight metabolites (M1 - M28) were detected and identified. O-deethylation, glucuronida-tion, sulfation, oxygenation, oxidative ring-opening, hydrogenation, glutathionation, dehydrogenation, cysteination, glucosidation, methylation, and hydrolysis were observed. At least sixteen metabolites not reported previously, were newly identified. M1 (O-deethylation, mono-oxygenation and glucuronidation), M3 (O-deethylation and glucuronidation), M5 (hydrolysis and mono-oxygenation), M14 (O-deethylation), M16 (hydrolysis), M22 (oxidative ring-opening and oxygenation) and M27 (mono-oxygenation) exhibited high mass spectrometric responses in human hepatocytes. M3, M5, M8, M13 (mono-oxygenation), M14, M16, M18 (O-deethylation and sulfation), M22 and M27 exhibited high mass spectrometric responses in monkey hepatocytes. M14, M16, M18, M20 (glutathionation and dehy-drogenation) and M27 exhibited high mass spectrometric responses in dog hepatocytes. M1 (O-deethylation, mono-oxygenation and glucuronidation), M3, M5, M13, M14, M16, M17 (cysteination), M18, M20, and M22 exhibited high mass spectrometric responses in rat hepatocytes.

Conclusion:

Most of new metabolites via oxidative ring-opening and glutathionation were identified. Species differences in metabolism of 7-ethoxylcoumarin in hepatocytes were observed. The analysis of metabolites suggests that 7-ethoxylcoumarin may undergo 3,4-epoxidation responsible for formation of glutathione and its derived cysteine conjugates, carboxylic acid and its glucuronides, glucosides and sul-fate.

Bioprofiling of Cosmetics with Focus on Streamlined Coumarin Analysis

Facing the widespread use of cosmetic products in daily use and recognizing the very limited information obtained by target analysis, a method suited for comprehensive characterization of cosmetics was aimed at. The biological activity of ingredients of 20 cosmetics taken from 16 different product groups and their coumarin contents were investigated via chromatography linked to bioassays (direct bioautography) and mass spectrometry. It allows for screening a large number of cosmetic products within a short time to generate a more valid database on their coumarin content and their contribution to the overall exposure. Bioactivity profiling of cosmetics with regard to bioactive ingredients opens new avenues for a comprehensive characterization of important substances in products of daily use, helpful for the legally required safety and risk assessment of cosmetic products, especially for multiple product usage. As for coumarin, a ubiquitary fragrance compound of allergenic potential, which is under recurrent discussion due to its hepatoxic properties, it is necessary to be able to estimate the regular intake via cosmetics for a valid risk assessment. This newly developed bioprofiling method allowed a selective determination of coumarin down to 1.3 mg kg-1, even for very matrix-rich cosmetics despite minimalism in sample preparation. The declaration limits according to European Cosmetics Regulation were completely covered. Mean coumarin contents of 20 cosmetic products reached up to 2218 mg kg-1. The repeatabilities (%RSD, n = 3) were between 1.1 and 2.9%, and the mean recoveries (n = 5) were between 96 and 102% for the different cosmetic matrices.

Stability of Phase II Conjugation Systems in Cultured Precision-cut Rat Hepatic Slices

The aim of the present study was to investigate the stability of Phase II conjugation enzymes in precision-cut rat liver slices that have been cultured, using the multiwell plate system, for various periods of time up to 72 hours. The enzyme activities monitored were epoxide hydrolase, glutathione S-transferase and the associated glutathione reductase, sulfotransferase and glucuronyl transferase. The activity of all enzymes studied declined with time when expressed per slice, but the rate of loss differed among the various enzymes. However, in most cases substantial Phase II activity was still present in the liver slices following a 24-hour incubation, and in some cases significant activity was even retained in slices cultured for 72 hours. These studies indicate that precision-cut liver slices maintain Phase II activity for long periods of time, thus allowing metabolic studies involving prolonged incubations to be performed.

Comparativein vitrometabolism of the suspected pro-oestrogenic compound, methoxychlor in precision-cut liver slices from male and female rats

The in vitro metabolism of [14C]methoxychlor (MXC), a suspected pro-oestrogenic compound, by male and female Fischer rats (F344) was compared in precision-cut liver slices. The results demonstrated time-dependent metabolism of MXC with integrated phase I and II reactions, and the sex differences were detected in the metabolic profiles. In liver slices from male rats, MXC was metabolized to bis-demethylated MXC (bis-OH-MXC) by sequential O-demethylation followed by subsequent O-glucuronidation. The doubly conjugated metabolite, bis-OH-MXC 4-O-sulphate 4'-O-glucuronide was additionally produced. In the case of the female rat, the glucuronides of both mono- and bis-OH-MXC were formed as the main metabolites, and the mono-OH-MXC glucuronide appeared to be specific to the female rat. The ratios of bis-/mono-demethylated metabolite, which include the amounts of corresponding conjugates, were approximately 95/5 for the male rats and 40/60 for the female. These results imply that demethylation to the intermediate metabolite, (S)-mono-OH-MXC, is a key step for the sex-dependent metabolism of MXC in the rats. The phase I metabolites produced were extensively conjugated with D-glucuronic acid in both male and female rats.

Metabolic and structural viability of precision-cut rat lung slices in culture

1. The principal objective was to evaluate the functional and structural integrity of precision-cut rat lung slices in culture over 72 h. 2. Lung slices metabolized 7-ethoxycoumarin in a time-dependent fashion, the major metabolites being the sulphate and glucuronide of 7-hydroxycoumarin with very low levels of the free compound. Prior treatment of rats with beta-naphthoflavone elevated markedly the rate of metabolism. The optimum slice thickness, as exemplified by the metabolism of 7-ethoxycoumarin, was about 600 microm. 3. Lung slices retained metabolic viability towards 7-ethoxycoumarin for 8 h, but after this point a marked decline in metabolic activity was noted. However, very low levels of activity were still evident following a 72 h incubation. 4. Morphological examination of lung slices revealed nuclear degeneration and loss of tissue architecture following 24h incubation. When cellular integrity was assessed using lactate dehydrogenase, a time-dependent leakage was evident with maximum loss occurring within 24h; longer incubations did not result in further leakage. 5. It is concluded that precision-cut rat lung slices, of 600 microm thickness, can be maintained metabolically viable in culture for some 8 h.

In vitrometabolism of [14C]methoxychlor in rat, mouse, Japanese quail and rainbow trout in precision-cut liver slices

1. The in vitro metabolism of [14C]methoxychlor (MXC) has been studied using precision-cut liver slices from the Sprague-Dawley male rat, CD-1 male mouse, WE strain male Japanese quail and juvenile rainbow trout (Oncorhynchus mykiss). The results demonstrated integrated phase I and II metabolism of MXC and species differences in the metabolic profiles were observed. 2. In rat liver slice preparations, MXC was rapidly metabolized to bis-OH-MXC by sequential O-demethylation followed by subsequent O-glucuronidation forming bis-OH-MXC glucuronide. No mono-OH-MXC glucuronide was detected. The doubly conjugated metabolite, bis-OH-MXC 4-O-sulphate 4'-O-glucuronide, was also detected as a rat-specific metabolite. 3. Formation of mono-OH-MXC and its glucuronide was the main metabolic pathway in the mouse and Japanese quail. In contrast to the rat, only minor amounts of bis-OH-MXC glucuronide were detected. A reductively dehalogenated metabolite, dechlorinated mono-OH-MXC glucuronide, was observed only in mouse preparations. 4. In rainbow trout, comparative amounts of both mono- and bis-OH-MXC glucuronide were formed as the major metabolites. Unconjugated forms of these metabolites were detected only as minor products. 5. The different metabolic profiles of MXC observed in the four animal species are possibly due to substrate specificity of contributing CYP450 monooxgenase enzyme(s) in different animal species.

Comparative in vitro metabolism of methoxychlor in male and female rats: metabolism of demethylated methoxychlor metabolites by precision-cut rat liver slices

The in vitro metabolism of demethylated methoxychlor (MXC) metabolites, mono-OH-MXC (including (R)- and (S)-isomers) and bis-OH-MXC (mono- and bis-demethylated MXC, respectively), was conducted using precision-cut liver slices to understand the sex-dependent metabolism of MXC in rats. In the study with bis-OH-MXC, the substrate underwent extensive conjugation producing its glucuronide and glucuronide/sulphate diconjugate, and no significant sex differences were found. On the contrary, the metabolism of mono-OH-MXC appeared to exhibit the sex differences in the metabolic profiles. The bis-OH-MXC glucuronide and glucuronide/sulphate diconjugate were major metabolites in male rat, whereas the mono- and bis-OH-MXC glucuronides predominated in the female. The per cent distribution of the demethylated products (sum of bis-OH-MXC derivatives) was approximately 90% for the male (for both isomers) and 81 (R-) to 56% (S-) for the female. The metabolic profiles in (S)-mono-OH-MXC, which is the predominant enantiomer preferentially produced in MXC metabolism in rats, showed a similar pattern to that of MXC compared with the (R)-isomer. The results indicate that the sex differences in oxidative demethylation of the intermediate, (S)-mono-OH-MXC, could be one of the probable reasons for the sex-dependent metabolism of MXC in rats, and the stereo-structural preference of the contributing demethylase enzymes appear to be involved.

Lack of strain-related differences in drug metabolism and efflux transporter characteristics between CD-1 and athymic nude mice

CD-1 mice are commonly used in oncology metabolism and toxicity to support drug discovery and development and to examine drug metabolism and toxicity properties of new chemical entities. On the other hand, athymic nude mice are the preferred animals to investigate tumor growth inhibition. Therefore, a frequently asked question is: are the metabolic and pharmacokinetic characteristics of xenobiotics in these two mouse strains comparable or not? To address this issue, we characterized drug metabolism and efflux transporter properties in both strains and in different organs. The metabolic stability of a set of 20 compounds and metabolite formation of cytochrome P450 (CYP) marker substrates (testosterone, ethoxyresorufin and pentoxyresorufin) were measured in liver microsomes. Drug conjugation was studied by following the disappearance of 7-hydroxycoumarin and the formation of its glucuronide and sulfate conjugates in freshly prepared liver slices. In addition, mRNA expression levels of the main cyp genes and drug efflux transporters were investigated by real-time RT-PCR in the liver, kidney, intestine and adrenal glands. No significant differences in enzymatic activities and metabolite formation were observed between the two strains. Also mRNA expression profiles of cyp and drug transporter genes were similar between CD-1 and nude mice.

Metabolic Detoxification Determines Species Differences in Coumarin-Induced Hepatotoxicity

Hepatotoxicity of coumarin is attributed to metabolic activation to an epoxide intermediate, coumarin 3,4-epoxide (CE). However, whereas rats are most susceptible to coumarin-induced hepatotoxicity, formation of CE is greatest in mouse liver microsomes, a species showing little evidence of hepatotoxicity. Therefore, the present work was designed to test the hypothesis that detoxification of CE is a major determinant of coumarin hepatotoxicity. CE can either rearrange spontaneously to o-hydroxyphenylacetaldehyde (o-HPA) or be conjugated with gluatathione (GSH). o-HPA is hepatotoxic and is further detoxified by oxidation to o-hydroxyphenylacetic acid (o-HPAA). In vitro experiments were conducted using mouse liver microsomes to generate a constant amount of CE, and cytosols from F344 rats, B6C3F1 mice, and human liver were used to characterize CE detoxification. All metabolites were quantified by HPLC methods with UV detection. In rats and mice, GSH conjugation occurred non-enzymatically and through glutathione-S-transferases (GSTs), and the kinetics of GSH conjugation were similar in rats and mice. In rat liver cytosol, oxidation of o-HPA to o-HPAA was characterized with a high affinity K(m) of approximately 12 microM, and a V(max) of approximately 1.5 nmol/min/mg protein. In contrast, the K(m) and V(max) for o-HPA oxidation in mouse liver cytosol were approximately 1.7 microM and 5 nmol/min/mg protein, respectively, yielding a total intrinsic clearance through oxidation to o-HPAA that was 20 times higher in mouse than in rats. Human cytosols (two separate pools) detoxified CE through o-HPA oxidation with an apparent K(m) of 0.84 microM and a V(max) of 5.7 nmol/min/mg protein, for a net intrinsic clearance that was more than 50 times higher than the rat. All species also reduced o-HPA to o-hydroxyphenylethanol (o-HPE), but this was only a major reaction in rats. In the presence of a metabolic reaction replete with all necessary cofactors, GSH conjugation accounted for nearly half of all CE metabolites in rat and mouse, whereas the GSH conjugate represented only 10% of the metabolites in human cytosol. In mouse, o-HPAA represented the major ring-opened metabolite, accounting for the remaining 50% of metabolites, and in human cytosol, o-HPAA was the major metabolite, representing nearly 90% of all CE metabolites. In contrast, no o-HPAA was detected in rats, whereas o-HPE represented a major metabolite. Collectively, these in vitro data implicate o-HPA detoxification through oxidation to o-HPAA as the major determinant of species differences in coumarin-induced hepatotoxicity.

Functional integrity of precision-cut liver slices from deer and cattle

Precision-cut bovine and cervine liver slices were incubated in RPMI 1640 media for up to 72 h, and cellular integrity was assessed utilizing the leakage of lactate dehydrogenase (LDH) and the formation of the formazan metabolite of 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT). Leakage of LDH (80%) from the bovine and cervine slices was noted only following 10 h of culture, and similarly, the generation of MTT-formazan declined. Metabolic viability was determined using 7-ethoxycoumarin as the model substrate, which was metabolized by slices from both animal species in a time-dependent manner for at least 6 h to generate 7-hydroxycoumarin, which was secreted into the media primarily as glucuronide and sulphate conjugates. With both cervine and bovine slices metabolic activity decreased markedly after a 4-h preincubation as assessed following a further 2-h incubation in the presence of 7-ethoxycoumarin. Subsequently, ethoxycoumarin metabolism by bovine slices did not decrease further until 24 h and was still detectable at 72 h. In the case of cervine slices, activity declined gradually after 8 h, with no activity being detectable at 72 h. It may be concluded that cervine and bovine slices may be maintained metabolically active for 8-10 h, which should prove sufficient for xenobiotic metabolism studies to be performed.

PRECLINICAL CHARACTERIZATION OF 2-[3-[3-[(5-ETHYL-4′-FLUORO-2-HYDROXY[1,1′-BIPHENYL]-4-YL)OXY]PROPOXY]-2-PROPYLPHENOXY]BENZOIC ACID METABOLISM: IN VITRO SPECIES COMPARISON AND IN VIVO DISPOSITION IN RATS

Assessment of the pharmacokinetics of [14C]2-[3-[3-[(5-ethyl-4'-fluoro-2-hydroxy[1,1'-biphenyl]-4-yl)oxy]propoxy]-2-propylphenoxy-]benzoic acid ([14C]LY293111), an experimental anti-cancer agent, suggested long-lived circulating metabolites in rats. In vivo metabolites of LY293111 were examined in plasma, bile, urine, and feces of Fischer 344 (F344) rats after oral administration of [14C]LY293111. Metabolites were profiled by high-performance liquid chromatography-radiochromatography, and identified by liquid chromatography (LC)/mass spectrometry and LC/NMR. The major in vivo metabolites of LY293111 identified in rats were phenolic (ether), acyl, and bisglucuronides of LY293111. Measurement of radioactivity in rat plasma confirmed that a fraction of LY293111-derived material was irreversibly bound to plasma protein and that this bound fraction increased over time. This was consistent with the observed disparity in half-lives between LY293111 and total radioactivity in rats and monkeys, and is likely due to covalent modification of proteins by the acyl glucuronide. In vitro metabolism of [14C]LY293111 in liver slices from CD-1 mice, F344 rats, rhesus and cynomolgus monkeys, and humans indicates that glucuronidation was the primary metabolic pathway in all species. The acyl glucuronide was the most prevalent radioactive peak (16% of total 14C) produced by F344 rat slices, whereas the ether glucuronide was the major metabolite in all other species (26-36% of total 14C). Several minor hydroxylated metabolites were detected in F344 rat slice extracts but were not observed in other species. The data presented suggest that covalent modification of proteins by LY293111 acyl glucuronide is possible in multiple species, although the relative reactivity of this metabolite appears to be low compared with those known to cause adverse drug reactions.

Cryopreservation of precision-cut tissue slices for application in drug metabolism research

Cryopreservation of tissue slices greatly facilitates their use in drug metabolism research, leading to efficient use of human organ material and a decrease of laboratory animal use. In the present review, various mechanisms of cryopreservation such as equilibrium slow freezing, rapid freezing and vitrification, and their application to cryopreservation of tissue slices are discussed as well as the viability parameters often used to evaluate the success of cryopreservation. Equilibrium freezing prevents intracellular ice formation by inducing cellular dehydration, but (large) ice crystals are still formed in the interstitial space of the slices. Upon rapid freezing, (small) intra- and extracellular ice crystals are formed which slices from some tissues can resist. Vitrification prevents the formation of both intra- and extracellular ice crystals while an amorphous glass is formed of the slice liquid constituents. To vitrify, however, high molarity solutions of cryoprotectants are required that may be toxic to the slices. The use of mixtures of high molarity of cryoprotectants overcomes this problem. We conclude that vitrification is the approach that most likely will lead to the development of universal cryopreservation methods for tissue slices of various organs from various animal species. In the future this may lead to the formation of a tissue slice bank from which slices can be derived at any desirable time point for in vitro experimentation.

Metabolism of zaleplon by human liver: evidence for involvement of aldehyde oxidase

1. The metabolism of Zaleplon (CL-284,846; ZAL) has been studied in precision-cut human liver slices and liver cytosol preparations. 2. Human liver slices metabolized ZAL to a number of products including 5-oxo-ZAL (M2), N-desethyl-5-oxo-ZAL (M1) and N-desethyl-ZAL (DZAL), the latter metabolite being known to be formed by CYP3A forms. 3. Human liver cytosol preparations catalysed the metabolism of ZAL to M2. Kinetic analysis of three cytosol preparations revealed mean (+/- SEM) K(m) and V(max) of 93 +/- 18 mm and 317 +/- 241 pmol/min/mg protein, respectively. 4. Using 16 individual human liver cytosol preparations a 33-fold variability in the metabolism of 80 micro M ZAL to M2 was observed. Correlations were observed between M2 formation and the metabolism of the aldehyde oxidase substrates phenanthridine (r(2) = 0.774) and phthalazine (r(2) = 0.460). 5. The metabolism of 80 micro M ZAL to M2 in liver cytosol preparations was markedly inhibited by the aldehyde oxidase inhibitors chlorpromazine, promethazine, hydralazine and menadione. Additional kinetic analysis suggested that chlorpromazine and promethazine were non-competitive inhibitors of M2 formation with K(i) of 2.3 and 1.9 micro M, respectively. ZAL metabolism to M2 was also inhibited by cimetidine. 6. Incubations conducted with human liver cytosol and H(2)(18)O demonstrated that the oxygen atom incorporated into ZAL and DZAL to form M2 and M1, respectively, was derived from water and not from molecular oxygen. 7. In summary, by correlation analysis, chemical inhibition and H(2)(18)O incorporation studies, ZAL metabolism to M2 in human liver appears to be catalysed by aldehyde oxidase. With human liver slices, ZAL was metabolized to products dependent on both aldehyde oxidase and CYP3A forms.

Inhibition of zaleplon metabolism by cimetidine in the human liver: in vitro studies with subcellular fractions and precision-cut liver slices

1. The effect of cimetidine on the metabolism of zaleplon (ZAL) in human liver subcellular fractions and precision-cut liver slices was investigated. 2. ZAL was metabolized to a number of products including 5-oxo-ZAL (M2), which is known to be formed by aldehyde oxidase, N-desethyl-ZAL (DZAL), which is known to be formed by CYP3A forms, and N-desethyl-5-oxo-ZAL (M1). 3. Human liver microsomes catalysed the NADPH-dependent metabolism of ZAL to DZAL. Kinetic analysis of three microsomal preparations revealed mean (+/-SEM) S(50) and V(max) of 310 +/- 24 micro M and 920 +/- 274 pmol/min/mg protein, respectively. 4. Human liver cytosol preparations catalysed the metabolism of ZAL to M2. Kinetic analysis of three cytosol preparations revealed mean (+/-SEM), K(m) and V(max) of 124 +/- 14 micro M and 564 +/- 143 pmol/min/mg protein, respectively. 5. Cimetidine inhibited ZAL metabolism to DZAL in liver microsomes and to M2 in the liver cytosol. With a ZAL substrate concentration of 62 micro M, the calculated mean (+/-SEM, n = 3) IC50 were 596 +/- 103 and 231 +/- 23 micro M for DZAL and M2 formation, respectively. Kinetic analysis revealed that cimetidine was a competitive inhibitor of M2 formation in liver cytosol with a mean (+/-SEM, n = 3) K(i) of 155 +/- 16 micro M. 6. Freshly cut human liver slices metabolized ZAL to a number of products including 1, M2 and DZAL. 7. Cimetidine inhibited ZAL metabolism in liver slices to M1 and M2, but not to DZAL. Kinetic analysis revealed that cimetidine was a competitive inhibitor of M2 formation in liver slices with an average (n = 2 preparations) K(i) of 506 micro M. 8. The results demonstrate that cimetidine can inhibit both the CYP3A and aldehyde oxidase pathways of ZAL metabolism in the human liver. Cimetidine appears to be a more potent inhibitor of aldehyde oxidase than of CYP3A forms and hence in vivo is likely to have a more marked effect on ZAL metabolism to M2 than on DZAL formation. 9. The results also demonstrate that precision-cut liver slices may be a useful model system for in vitro drug-interaction studies.

In vitro absorption and metabolism of a citrus chemopreventive agent, auraptene, and its modifying effects on xenobiotic enzyme activities in mouse livers

We previously reported that auraptene (7-geranyloxycoumarin, AUR), widely occurring in citrus fruit, is a structurally novel type of effective cancer-preventive agent, as manifested in several rodent models. However, its bio-availability and metabolism in biological systems have yet to be investigated. In the present study, we examined the chemical stability of AUR at pH 1.57 and 37 degrees C (as a stomach digestion model) and observed its stoichiometric conversion to umbelliferone [7-hydroxycoumarin, UMB; half-life (t1/2) = 15 h; 7-ethoxycoumarin (ETC) was stable for 24 h]. Differentiated Caco-2 cells, a human colorectal adenocarcinoma cell line, were used as a small intestine model. ETC permeated the basolateral (portal vein) side of Caco-2 cells in a time-dependent manner; AUR slightly permeated the cells, but with an intracellular accumulation. Epoxyauraptene and UMB were detected when AUR was treated with the rat liver S-9 mixture. ETC was also converted to UMB, but its t1/2 of two hours was much shorter than that of AUR (> 24 h). This suggests that AUR, bearing a geranyloxyl side chain, is a relatively metabolism-resistant substrate for cytochrome P-450 enzymes and, thus, is stable in the liver compared with ETC. Oral administration of AUR by gavage at 50-200 mg/kg body wt dose dependently induced glutathione S-transferase (GST) activity in mouse livers without affecting cytochrome P-450 activity. Using 10 coumarin-related compounds, we found that only those coumarins having a 7-alkyloxyl group induced GST, but not cytochrome P-450, activity. The present study presumes that AUR accumulates in the epithelial cells of the small intestine and then gradually permeates into the portal vein. Stable localizability of AUR in the colon and liver may be associated with the induction of GST activity, which is important as the action mechanism for suppression of rodent chemical carcinogenesis.

Viability of liver slices exhibiting absorption, metabolism, and elimination of substrates in culture medium

In an attempt to improve liver slice culture for in vitro tests of newly developed drugs, we found that a relatively thick 2-mm slice was much more viable, long-lived, and metabolically functional than the conventionally used 300-microm-thick slices. Results revealed that some ionic and nonionic compounds (Na(125)I, [(14)C]Ala, and [(14)C]Xyl) dissolved in culture medium were absorbed into the slices, reaching from the cut edges deep into the inner part, in only a short period. Moreover, the slices could be observed for 24 h after incubation so that a normal histological image could be obtained. Each semi-microautoradiographic image was also clearly positive not only at the margin but also in the inner part of each slice. Furthermore, we determined the specific functioning of each donated liver for each substrate added, in uptake, metabolism, and excretion.

Simultaneous blood and biliary sampling of esculetin by microdialysis in the rat

Biliary excretion and intestinal reabsorption in enterohepatic circulation play major dispositional roles for some drugs. To circumvent multiple blood sampling and interruption of enterohepatic circulation in conventional biliary cannulation, the present study utilized the minimally invasive sampling technique of microdialysis in pharmacokinetics and biliary excretion studies. Microdialysis probes were inserted into the jugular vein and bile duct in the anesthetized rat for simultaneous and continuous sampling following intravenous administration of esculetin, a bioactive coumarin derivative. Placements of the microdialysis probes were designed to minimize obstruction to normal flows of the body fluids. Separation and quantitation of esculetin in the dialysates were achieved using high performance liquid chromatography (HPLC) coupled to UV detection. The results indicated higher drug concentrations in the bile than in the blood, suggesting active biliary excretion. The study also provided an example of successful application of in vivo microdialysis as an interesting and feasible alternative for pharmacokinetics and biliary drug excretion studies.

Coumarin metabolism, toxicity and carcinogenicity: relevance for human risk assessment

The metabolism, toxicity and results of tests for carcinogenicity have been reviewed with respect to the safety for humans of coumarin present in foodstuffs and from fragrance use in cosmetic products. Coumarin is a natural product which exhibits marked species differences in both metabolism and toxicity. The majority of tests for mutagenic and genotoxic potential suggest that coumarin is not a genotoxic agent. The target organs for toxicity and carcinogenicity in the rat and mouse are primarily the liver and lung. Moreover, the dose-response relationships for coumarin-induced toxicity and carcinogenicity are non-linear, with tumour formation only being observed at high doses which are associated with hepatic and pulmonary toxicity. Other species, including the Syrian hamster, are seemingly resistant to coumarin-induced toxicity. There are marked differences in coumarin metabolism between susceptible rodent species and other species including humans. It appears that the 7-hydroxylation pathway of coumarin metabolism, the major pathway in most human subjects but only a minor pathway in the rat and mouse, is a detoxification pathway. In contrast, the major route of coumarin metabolism in the rat and mouse is by a 3,4-epoxidation pathway resulting in the formation of toxic metabolites. The maximum daily human exposure to coumarin from dietary sources for a 60-kg consumer has been estimated to be 0.02 mg/kg/day. From fragrance use in cosmetic products, coumarin exposure has been estimated to be 0.04 mg/kg/day. The total daily human exposure from dietary sources together with fragrance use in cosmetic products is thus 0.06 mg/kg/day. No adverse effects of coumarin have been reported in susceptible species in response to doses which are more than 100 times the maximum human daily intake. The mechanism of coumarin-induced tumour formation in rodents is associated with metabolism-mediated, toxicity and it is concluded that exposure to coumarin from food and/or cosmetic products poses no health risk to humans.

Molecular modelling of the human cytochrome P450 isoform CYP2A6 and investigations of CYP2A substrate selectivity

(1) The generation of a homology model of CYP2A6, the major catalyst of human hepatic coumarin 7-hydroxylase activity, involves the use of the recently published substrate-bound CYP102 crystal structure as a template. (2) A substantial number of structurally diverse CYP2A6 substrates are found to dock satisfactorily within the putative active site of the enzyme, leading to the formulation of a structural template (or pharmacophore) for CYP2A6 specificity/selectivity. (3) The CYP2A6 model is consistent with available evidence from site-directed mutagenesis studies carried out on CYP2A subfamily isoforms, and enables some explanation of species differences in CYP2A-mediated metabolism of certain substrates. (4) Quantitative structure-activity relationship (QSAR) analysis of CYP2A5 (the mouse orthologue) mutants yields statistically significant correlations between various properties of amino acid residues and coumarin 7-hydroxylase activity.

Structural and functional integrity of precision-cut liver slices in xenobiotic metabolism: a comparison of the dynamic organ and multiwell plate culture procedures

1. Objectives were two-fold: (1) to compare the viability of precision-cut liver slices in two culture systems, namely the dynamic organ and the multiwell plate; and (2) to evaluate whether increasing the number of slices per incubation results in a proportional increase in the extent of metabolism. 2. With both culturing systems, the major products of 7-ethoxycoumarin metabolism were the sulphate and glucuronide conjugates of 7-hydroxycoumarin with very low levels of the free compound. When the multiwell plate procedure was used, metabolism increased linearly for at least 10 h, whereas it tended to plateau after 6 h in the dynamic organ culture system. At preincubations > 10 h, significantly more metabolism of 7-ethoxycoumarin was seen in the slices cultured using the multiwell system compared with the dynamic organ system. 3. Morphological evaluation employing light and electron microscopy revealed that liver slices incubated using the multiwell system were structurally better preserved compared with those incubated using the dynamic organ system. 4. Using the multiwell system, increasing the number of slices per incubation from one to two resulted in only a modest increase in the metabolism of 7-ethoxycoumarin. The rate of metabolism of this substrate was much higher with one liver slice when expressed per mg homogenate protein. 5. It is concluded that (1) the multiwell plate culture system for culturing slices is superior to the dynamic organ system in studying the metabolism of xenobiotics following long-term incubations, (2) increasing the number of slices per incubation does not result in a corresponding increase in the rate of metabolism, and (3) in both culture systems optimal viability appears to be within 24 h of incubation.

Lack of effect of coumarin on unscheduled DNA synthesis in precision-cut human liver slices

In this study the effect of coumarin on unscheduled DNA synthesis (UDS) in precision-cut human liver slices has been examined. Liver slices from tissue samples from four donors were cultured for 24 hr in medium containing [3H]thymidine and 0-5.0 mM coumarin using a dynamic organ culture system and processed for autoradiographic evaluation of UDS. As positive controls liver slices were also cultured with three known genotoxic agents, namely 0.02 and 0.05 mM 2-acetylaminofluorene (2-AAF), 0.002 and 0.02 mM aflatoxin B1 (AFB1) and 0.005 and 0.05 mM 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP). UDS was quantified as the net grain count in centrilobular hepatocytes and as the percentage of centrilobular hepatocyte nuclei with more than five net grains. Compared with control liver slice cultures, treatment with 0.05-5.0 mM coumarin had no effect on UDS. In contrast, treatment with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM AFB1 and 0.005 and 0.05 mM PhIP produced significant increases in the net grain counts of centrilobular hepatocytes. The greatest induction of UDS was observed in liver slices treated with 0.05 mM PhIP. Treatment with 2-AAF, AFB1 and PhIP also produced significant increases in the number of centrilobular hepatocyte nuclei with more than five net grains. At the concentrations examined neither coumarin. 2-AAF, AFB1 nor PhIP had any significant effect on replicative DNA synthesis in 24 hr cultured human liver slices. These results demonstrate that coumarin does not induce UDS in cultured human liver slices. However, all three positive control compounds produced marked significant increases in UDS, thus confirming the functional viability of the human liver slice preparations used in this study. The results of this study suggest that coumarin is not a genotoxic agent in human liver.

Single-dose disulfiram does not inhibit CYP2A6 activity

BACKGROUND

Disulfiram and its primary metabolite diethyldithiocarbamate are effective mechanism-based inhibitors of human liver cytochrome P450 2E1 (CYP2E1) in vitro. A single dose of disulfiram, which significantly diminishes human P450 2E1 activity in vivo, has been used to investigate the role of CYP2E1 in human drug metabolism and to prevent CYP2E1-mediated biotransformation. Nevertheless, the specificity of single-dose disulfiram toward human CYP2E1 in vivo is unknown. Because diethyldithiocarbamate also inhibits human liver CYP2A6 in vitro, this investigation explored the effect of single-dose disulfiram on human CYP2A6 activity in vivo.

METHODS

CYP2A6 activity was assessed by the 7-hydroxylation of coumarin, which is catalyzed selectively by CYP2A6. Ten healthy volunteers received 50 mg oral coumarin on two occasions in a randomized crossover design, approximately 10 hours after 500 mg oral disulfiram was administered or after no pretreatment (control group). Plasma and urine 7-hydroxycoumarin and plasma coumarin concentrations were determined by HPLC.

RESULTS

The area under the plasma 7-hydroxycoumarin versus time curve (2.69 +/- 0.90 micrograms.hr/ml) was not decreased after disulfiram pretreatment (3.33 +/- 0.93 micrograms.hr/ml). Furthermore, maximum plasma concentration (Cmax) of 7-hydroxycoumarin (1.4 +/- 0.5 versus 1.8 +/- 0.6 micrograms/ml) and time to reach Cmax (1.0 +/- 0.2 and 1.0 +/- 0.4 hour) were unchanged by disulfiram pretreatment. Urinary 7-hydroxycoumarin excretion over a 24-hour period (38.9 +/- 10.8 mg) was also undiminished by disulfiram pretreatment (45.2 +/- 6.6 mg).

CONCLUSIONS

Single-dose disulfiram does not inhibit human CYP2A6 activity in vivo. When single-dose disulfiram is used as an in vivo probe for P450, inhibition of drug metabolism suggests involvement of CYP2E1 but not CYP2A6.

Use of precision-cut rat liver slices for studies of xenobiotic metabolism and toxicity: comparison of the Krumdieck and Brendel tissue slicers

1. In this study we have compared freshly cut and cultured precision-cut rat liver slices produced by the Krumdieck and Brendel-Vitron tissue slicers. 2. No significant differences were observed in levels of protein, potassium, total glutathione (i.e. GSH and GSSG), reduced glutathione (GSH) and cytochrome P450 and activities of 7-ethoxyresorufin O-deethylase and 7-benzoxyresorufin O-debenzylase in freshly cut rat liver slices produced by the two tissue slicers. However, levels of oxidized glutathione (GSSG) were significantly greater in liver slices produced with the Brendel-Vitron tissue slicer. 3. Precision-cut rat liver slices produced with both tissue slicers were cultured for 0 (i.e. a 1-h preincubation), 24 and 72 h in a dynamic organ culture system in an atmosphere of either 95% 02/5% CO2 or 95% air/5% CO2. 4. Apart from small differences in glutathione levels in 0 and 24 h cultured liver slices, no significant differences were observed in the parameters measured between liver slices prepared with both tissue slicers and cultured in both gas phases. 5. With liver slices produced by both tissue slicers 50 microM sodium arsenite produced a greater induction of heat shock protein 70 levels in slices cultured for 24 h in a high oxygen than in an air atmosphere. 6. These results suggest that both tissue slicers can readily produce precision-cut liver slices for studies of xenobiotic metabolism and toxicity. However, the data suggest that for any given application of precision-cut tissue slices it is desirable to establish optimal culture conditions

Effect of cold and warm ischaemia on drug metabolism in isolated hepatocytes and slices from human and monkey liver

1. The influence of short-term cold storage in University of Wisconsin organ preservation solution (UW) on the ability to metabolize lidocaine, testosterone and 7-ethoxycoumarin in isolated human and cynomolgus monkey (Macaca fascicularis) hepatocytes and liver slices has been investigated. 2. The human liver tissue was obtained from two different sources, i.e. healthy liver tissue from patients undergoing partial hepatectomy because of metastases of colorectal carcinoma (PH livers) and donor tissue remaining as surgical waste after reduced size or split liver transplantation (Tx livers). Tx livers were perfused in situ with ice-cold UW avoiding warm ischaemia. This in contrast with PH livers, where the operation caused warm ischaemia for 5-90 min. 3. Liver slices and hepatocytes from cynomolgus monkey liver showed comparable metabolic rates for the substrates tested, indicating that all hepatocytes in the slice are participating in the biotransformation of the substrates. These monkey liver preparations can be stored up to 18 h with only a slight loss of their metabolic capacity. 4. Liver slices and isolated hepatocytes from the Tx livers as well as isolated cells from the PH livers could also be stored up to 18 h without losing metabolic capacity. However, for liver slices prepared from PH livers cold storage is not recommended, because metabolic function was reduced by approximately 40% after 18 h.

A rapid and simple method for cryopreservation of human liver slices

1. Precision-cut liver slices represent a suitable and convenient in vitro preparation for studying metabolism and toxicity mechanisms of drugs and toxic chemicals. Particularly in the case of human liver slices, cryopreservation would enable more efficient utilization of this scarce and irregularly available tissue. 2. Liver slices from consecutive human livers were cryopreserved using a method previously developed for rat and monkey liver slices. This procedure involves incubation in 12% dimethyl sulphoxide for 30 min on ice and direct immersion into liquid nitrogen. 3. Functional integrity of cryopreserved human liver slices, as compared with that of fresh liver slices, was maintained at 66 +/- 8% (alanine aminotransferase activity retained in the slices), 78 +/- 7% (urea synthesis), 88 +/- 14% (testosterone hydroxylation), 84 +/- 7% (N-deethylation of lidocaine) and 88 +/- 10% (total O-deethylation of 7-ethoxycoumarin). The ratios of testosterone metabolites did not change on cryopreservation. 4. These results show that the cryopreserved human liver slices retained the measured drug metabolism activities. Therefore, this cryopreservation method is suitable for storing liver slices to be used for comparing drug metabolism patterns, at least qualitatively, between species.

Influence of 48 hours of cold storage in University of Wisconsin organ preservation solution on metabolic capacity of rat hepatocytes

BACKGROUND/AIMS

Suspensions of isolated hepatocytes are a valuable tool to study liver functions. For optimal use of the isolated hepatocytes, methods are needed to preserve the hepatocytes while maintaining their viability, metabolic and transport functions. Until now, little has been known about the maintenance of the drug metabolism capacity and energy state, measured by the so-called energy charge (ATP+1/2ADP)/(ATP+ADP+AMP), in hepatocytes after storage in University of Wisconsin cold storage solution (UW). Consequently, we investigated whether UW, originally designed to preserve organs for transplantation, was suitable for preservation of isolated rat hepatocytes with respect to the maintenance of drug metabolism and levels of energy-rich substrates.

METHODS

Viability of the isolated rat hepatocytes was determined by trypan blue exclusion, ATP content and energy charge after 24 and 48 h of storage in UW at 0 degrees C. Phase I and II metabolic functions of the cells were studied by measuring the cytochrome P450 content and the metabolic rate of lidocaine and 7-ethoxycoumarin.

RESULTS

During 48 h of storage of hepatocytes in UW both phase I and phase II metabolism are preserved at control levels. After storage, the viability of the hepatocytes was not changed significantly, and the cells maintained proper cellular ATP content and overall energy charge.

CONCLUSIONS

These results imply that hepatocytes from a single isolation can be stored in UW solution and used for metabolism experiments for 3 consecutive days, allowing a reduction in the use of experimental animals.

Comparison of CYP2A6 catalytic activity on coumarin 7-hydroxylation in human and monkey liver microsomes

Comparison of 7-hydroxylation of coumarin, a CYP2A6 substrate, in human and African green and cynomolgus monkey liver microsomes was made by means of an HPLC assay with UV detection. In human liver microsomes, the Km and Vmax values for the metabolic conversion were 2.1 microM and 0.79 nmol/mg/min, respectively. While African green monkey showed Km and Vmax values of 2.7 microM and 0.52 nmol/mg/min, which were similar to human, higher Km and Vmax values were found in cynomolgus monkey. Coumarin 7-hydroxylation in human and African green monkey was selectively inhibited by methoxsalen and pilocarpine (CYP2A6 inhibitors) but not by other inhibitors, i.e. alpha-naphthoflavone (CYP1A1), orphenadrine (CYP2B6), sulfaphenazole (CYP2C9), quinidine (CYP2D6) and ketoconazole (CYP3A4). Immunoinhibition results supported CYP2A6 involvement in human and its homolog in monkey in coumarin 7-hydroxylation, as only anti-CYP2A6, but not CYP2B1, CYP2C13, CYP2D6, CYP2E1 or CYP3A antibodies, inhibited this conversion. African green monkey was found to be similar to human in catalytic activity of coumarin 7-hydroxylation and response to CYP2A6 inhibitors or antibody inhibition. However, the monkey CYP2A6 is not identical to the human in that Ki values were different, and differences were observed with some CYP2A6 inhibitors, such as nicotine and methoxsalen, suggesting that, under some circumstances, studies of nicotine kinetics and drug taking behavior in monkey may not be comparable to human.

The use of precision-cut rat lung slices for studying PGF(2α) metabolism

The suitability of a dynamic lung slice culture system as an in vitro model for studying pulmonary metabolism of PGF(2α) was assessed. [(3)H]Prostaglandin F(2α) ([(3)H]PGF(2α)), a twenty carbon fatty acid that contains a five-carbon ring and is known to be metabolized by lung in vivo, was incubated with precision-cut rat lung slices in 1.7 ml of Waymouth's buffer fortified with 10% fetal calf serum. At 0, 2, 4 and 8 h after addition of [(3)H]PGF(2α) (1.82 ng/μCi), incubation was stopped and the contents of each vial were analyzed for [(3)H]PGF(2α) and its metabolites using reversed-phase HPLC with radiochemical detection. PGF(2α) was metabolized to 15-keto PGF(2α), 13,14-dihydro-15-keto PGF(2α), and two unknown minor polar metabolites. These results indicate that PGF(2α) was metabolized in lung slices pathways similar to those seen in vivo. Slice viability was assessed by protein synthesis and light microscopic examination of lung slices through 24 h of incubation. Protein synthesis was maintained and no tissue necrosis was observed over the entire 24 h incubation, indicating that the lung slices were viable for at least 24 h. These results indicate that the dynamic lung slice culture system is an appropriate in vitro model for studying the pulmonary metabolism of PGF(2α).

Lack of effect of piperonyl butoxide on unscheduled DNA synthesis in precision-cut human liver slices

In this study the effect of piperonyl butoxide (PBO) on unscheduled DNA synthesis in precision-cut human liver slices has been examined. Liver slices prepared from tissue samples from five human donors were cultured in medium containing [3H]thymidine and 0-2.5 mM PBO using a dynamic organ culture system. After 24 h the liver slices were processed for autoradiographic examination of UDS. As positive controls, liver slices were also cultured with three known genotoxic agents, namely 2-acetylaminofluorene (2-AAF), aflatoxin B1 (AFB1) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). UDS was quantified as the net grain count in centrilobular hepatocytes and as the percentage of centrilobular hepatocyte nuclei with > 5 and > 10 net grains. Compared to control liver slice cultures PBO had no effect on UDS. In contrast, treatment with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM AFB1 and 0.005 and 0.05 mM PhIP produced significant increases in net grain counts of centrilobular hepatocytes. The greatest induction of UDS was observed in liver slices treated with 0.05 mM PhIP. Treatment with 2-AAF, AFB1 and PhIP also produced increases in the number of centrilobular hepatocyte nuclei with > 5 and > 10 net grains. At the concentrations examined neither PBO, 2-AAF nor PhIP had any significant effect on replicative DNA synthesis in 24 h cultured human liver slices. In cultured liver slices treated with 0.02, but not 0.002, mM AFB1 a significant reduction in the rate of replicative DNA synthesis was observed. These results demonstrate that PBO does not induce UDS in cultured human liver slices. However, all three positive control compounds produced marked significant increases in UDS, thus confirming the functional viability of the human liver slice preparations used in this study. In conclusion, these results provide further evidence that PBO is a non-genotoxic agent which does not damage DNA in human liver.

Analysis of the glucuronidation of 7-hydroxycoumarin by HPLC

The in-vitro metabolism of 7-hydroxycoumarin to 7-hydroxycoumarin-glucuronide was investigated in bovine liver homogenate. A metabolic reaction mixture was prepared that included a crude preparation of uridine diphosphate (UDP) glucuronyl transferase, 7-hydroxycoumarin and UDP-glucuronic acid. A HPLC method was developed to separate coumarin, 7-hydroxycoumarin, 7-hydroxycoumarin-glucuronide and an internal standard, 4-hydroxycoumarin. Samples were separated by reverse-phase HPLC, on a C18 column, with a 1 ml min-1 gradient elution with UV detection at 320 nm. The limit of quantification of the method, for 7-hydroxycoumarin-glucuronide, was 1.47 microM, and the linear range was from 0-295.7 microM. Concentrations of 7-hydroxycoumarin-glucuronide produced were calculated from a plot of 7-hydroxycoumarin-glucuronide concentration versus the mean absorbance ratio (n = 4) (7-hydroxycoumarin-glucuronide absorbance/4-hydroxycoumarin absorbance). It was possible to monitor the decrease in the 7-hydroxycoumarin content as it was metabolised as well as the increase in 7-hydroxycoumarin-glucuronide as it was produced enzymatically. The identity of the compound produced was confirmed by photodiode array spectral analysis. A plot of time versus 7-hydroxycoumarin-glucuronide produced indicates that the metabolism is linear for the first 90 min and reached a plateau at 150 min. The rate of reaction in the first 90 min was 2.96 +/- 0.06 (RSD 1.7%, n = 3) nmol of 7-hydroxycoumarin-glucuronide produced per minute per milligram of protein. After 150 min 0.34 +/- 0.005 mM (RSD 1.4%) 7-hydroxycoumarin-glucuronide was produced, from 0.77 mM 7-hydroxycoumarin introduced into the reaction mixture and 58.0% +/- 5.3% (or 0.44 +/- 0.02 mM) of the 7-hydroxycoumarin remained. These results show that it is possible to monitor the production of the phase II metabolite of coumarin with minimal sample clean-up and without the need for deconjugation of the glucuronide moiety. The method was very reliable and applicable for the direct determination of 7-hydroxycoumarin-glucuronide in an in-vitro metabolic assay.