Application of cryopreserved precision-cut liver slices in pharmacotoxicology--principles, literature data and own investigations with special reference to CYP1A1-mRNA induction

The Use of Long-term Hepatocyte Cultures for Detecting Induction of Drug Metabolising Enzymes: The Current Status

In this report, metabolically competent in vitro systems have been reviewed, in the context of drug metabolising enzyme induction. Based on the experience of the scientists involved, a thorough survey of the literature on metabolically competent long-term culture models was performed. Following this, a prevalidation proposal for the use of the collagen gel sandwich hepatocyte culture system for drug metabolising enzyme induction was designed, focusing on the induction of the cytochrome P450 enzymes as the principal enzymes of interest. The ultimate goal of this prevalidation proposal is to provide industry and academia with a metabolically competent in vitro alternative for long-term studies. In an initial phase, the prevalidation study will be limited to the investigation of induction. However, proposals for other long-term applications of these systems should be forwarded to the European Centre for the Validation of Alternative Methods for consideration. The prevalidation proposal deals with several issues, including: a) species; b) practical prevalidation methodology; c) enzyme inducers; and d) advantages of working with independent expert laboratories. Since it is preferable to include other alternative tests for drug metabolising enzyme induction, when such tests arise, it is recommended that they meet the same level of development as for the collagen gel sandwich long-term hepatocyte system. Those tests which do so should begin the prevalidation and validation process.

Towards personalized medicine with a three-dimensional micro-scale perfusion-based two-chamber tissue model system

A three-dimensional micro-scale perfusion-based two-chamber (3D-μPTC) tissue model system was developed to test the cytotoxicity of anticancer drugs in conjunction with liver metabolism. Liver cells with different cytochrome P450 (CYP) subtypes and glioblastoma multiforme (GBM) brain cancer cells were cultured in two separate chambers connected in tandem. Both chambers contained a 3D tissue engineering scaffold fabricated with biodegradable poly(lactic acid) (PLA) using a solvent-free approach. We used this model system to test the cytotoxicity of anticancer drugs, including temozolomide (TMZ) and ifosfamide (IFO). With the liver cells, TMZ showed a much lower toxicity to GBM cells under both 2D and 3D cell culture conditions. Comparing 2D, GBM cells cultured in 3D had much high viability under TMZ treatment. IFO was used to test the CYP-related metabolic effects. Cells with different expression levels of CYP3A4 differed dramatically in their ability to activate IFO, which led to strong metabolism-dependent cytotoxicity to GBM cells. These results demonstrate that our 3D-μPTC system could provide a more physiologically realistic in vitro environment than the current 2D monolayers for testing metabolism-dependent toxicity of anticancer drugs. It could therefore be used as an important platform for better prediction of drug dosing and schedule towards personalized medicine.

Cryopreservation of rat precision-cut liver and kidney slices by rapid freezing and vitrification

Precision-cut tissue slices of both hepatic and extra-hepatic origin are extensively used as an in vitro model to predict in vivo drug metabolism and toxicity. Cryopreservation would greatly facilitate their use. In the present study, we aimed to improve (1) rapid freezing and warming (200 degrees C/min) using 18% Me(2)SO as cryoprotectant and (2) vitrification with high molarity mixtures of cryoprotectants, VM3 and VS4, as methods to cryopreserve precision-cut rat liver and kidney slices. Viability after cryopreservation and subsequent 3-4h of incubation at 37 degrees C was determined by measuring ATP content and by microscopical evaluation of histological integrity. Confirming earlier studies, viability of rat liver slices was maintained at high levels by rapid freezing and thawing with 18% Me(2)SO. However, vitrification of liver slices with VS4 resulted in cryopreservation damage despite the fact that cryoprotectant toxicity was low, no ice was formed during cooling and devitrification was prevented. Viability of liver slices was not improved by using VM3 for vitrification. Kidney slices were found not to survive cryopreservation by rapid freezing. In contrast, viability of renal medullary slices was almost completely maintained after vitrification with VS4, however vitrification of renal cortex slices with VS4 was not successful, partly due to cryoprotectant toxicity. Both kidney cortex and medullary slices were vitrified successfully with VM3 (maintaining viability at 50-80% of fresh slice levels), using an optimised pre-incubation protocol and cooling and warming rates that prevented both visible ice-formation and cracking of the formed glass. In conclusion, vitrification is a promising approach to cryopreserve precision-cut (kidney) slices.

Phase I and phase II metabolic activities are retained in liver slices from mouse, rat, dog, monkey and human after cryopreservation

Precision-cut liver slices are described as a valuable tool for in vitro metabolism studies of potential drug candidates. Recently, some papers reported successful cryopreservation conditions for liver slices, facilitating a broader and more efficient use of the tissue (particularly of human origin). The aim of this study is to evaluate the effect of cryopreservation on both phase I and phase II metabolism in liver slices prepared from mouse, rat, dog, monkey and human, using rapid freezing in the presence of 18% DMSO. Glucuronidation and sulfation activities (phase II) in both freshly prepared and cryopreserved liver slices were determined by rapid LC-MS/MS analyses using 7-hydroxycoumarin as a marker substrate. Testosterone was used as a marker substrate for cytochrome P450 mediated drug metabolism (phase I). Although the metabolic patterns and rates varied among the different species, the phase I and phase II metabolic capacities of the liver slices were well maintained after cryopreservation. Despite the good biotransformation capacity of cryopreserved slices a decrease in viability, expressed as ATP content and LDH leakage, was observed. MTT reduction was well maintained after cryopreservation. The possibility to cryopreserve liver slices will allow a more efficient utilisation of tissue, in particular from human, but also from dog and monkey. Finally, cryopreserved liver slices from mouse, rat, dog, monkey and human with good phase I and II metabolism activities are a useful in vitro tool to compare metabolite profiles of new chemical entities between species.

New hepatocyte in vitro systems for drug metabolism: metabolic capacity and recommendations for application in basic research and drug development, standard operation procedures

Primary hepatocytes represent a well-accepted in vitro cell culture system for studies of drug metabolism, enzyme induction, transplantation, viral hepatitis, and hepatocyte regeneration. Recently, a multicentric research program has been initiated to optimize and standardize new in vitro systems with hepatocytes. In this article, we discuss five of these in vitro systems: hepatocytes in suspension, perifusion culture systems, liver slices, co-culture systems of hepatocytes with intestinal bacteria, and 96-well plate bioreactors. From a technical point of view, freshly isolated or cryopreserved hepatocytes in suspension represent a readily available and easy-to-handle in vitro system that can be used to characterize the metabolism of test substances. Hepatocytes in suspension correctly predict interspecies differences in drug metabolism, which is demonstrated with pantoprazole and propafenone. A limitation of the hepatocyte suspensions is the length of the incubation period, which should not exceed 4hr. This incubation period is sufficiently long to determine the metabolic stability and to allow identification of the main metabolites of a test substance, but may be too short to allow generation of some minor, particularly phase II metabolites, that contribute less than 3% to total metabolism. To achieve longer incubation periods, hepatocyte culture systems or bioreactors are used. In this research program, two bioreactor systems have been optimized: the perifusion culture system and 96-well plate bioreactors. The perifusion culture system consists of collagen-coated slides allowing the continuous superfusion of a hepatocyte monolayer with culture medium as well as establishment of a constant atmosphere of 13% oxygen, 82% nitrogen, and 5% CO2. This system is stable for at least 2 weeks and guarantees a remarkable sensitivity to enzyme induction, even if weak inducers are tested. A particular advantage of this systemis that the same bioreactor can be perfused with different concentrations of a test substance in a sequential manner. The 96-well plate bioreactor runs 96 modules in parallel for pharmacokinetic testing under aerobic culture conditions. This system combines the advantages of a three-dimensional culture system in collagen gel, controlled oxygen supply, and constant culture medium conditions, with the possibility of high throughput and automatization. A newly developed co-culture system of hepatocytes with intestinal bacteria offers the possibility to study the metabolic interaction between liver and intestinal microflora. It consists of two chambers separated by a permeable polycarbonate membrane, where hepatocytes are cultured under aerobic and intestinal bacteria in anaerobic conditions. Test substances are added to the aerobic side to allow their initial metabolism by the hepatocytes, followed by the metabolism by intestinal bacteria at the anaerobic side. Precision-cut slices represent an alternative to isolated hepatocytes and have been used fo the investigation of hepatic metabolism, hepatotoxicity, and enzyme induction. A specific advantage of liver slices is the possibility to study toxic effects on hepatocytes that are mediated or modified by nonparenchymal cells (e.g., by cytokine release from Kupffer cells) because the physiological liver microarchitecture is maintained in cultured slices. For all these in vitro systems, a prevalidation has been performed using standard assays for phase I and II enzymes. Representative results with test substances and recommendations for application of these in vitro systems, as well as standard operation procedures are given.

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.

Cryopreserved precision-cut rat liver slices: morphology and cytochrome P450 isoforms expression after prolonged incubation

Precision-cut liver slices are an accepted in vitro system for toxicological investigations. However, cryopreservation of slices would make a more efficient utilisation, particularly of human liver tissue possible. In the present study sections of cryopreserved male rat liver slices were examined immunohistochemically for cytochrome P450 (CYP) isoforms expression after prolonged incubation and after exposure to typical inducers. Morphologically, with just thawed slices no major alterations were seen, but remarkable cell damage was observed even after 2 h of incubation mainly in the middle of the slices and in the periportal and intermediate regions of the lobules. After 24 h of incubation, viable cells were only observed at the edges of the slices or around bigger vessels. In the viable cells of the cryopreserved liver slices after 2 h of incubation CYP expression pattern was similar to that in normal liver specimens: a low CYP1A1, but a strong CYP2B1 and 3A2 expression predominantly in the central and intermediate lobular zones. After 24 h, the immunostaining for CYP2B1 and 3A2 in the viable cells was reduced, but that for CYP1A1 was increased. Incubation with beta-naphthoflavone further elevated CYP1A1 and 2B1 expression. Phenobarbital caused an enhanced CYP2B1 and 3A2 and dexamethasone and pregnenolone 16 alpha-carbonitrile an increased CYP3A2 immunostaining. These results show that also in cryopreserved liver slices and after a prolonged incubation, a distinct expression pattern and an in vitro induction of phase I enzymes can be demonstrated immunohistochemically.

Characterization of testosterone metabolism and 7-hydroxycoumarin conjugation by rat and human liver slices after storage in liquid nitrogen for 1 h up to 6 months

1. Slices of human and rat liver were cryopreserved in 18% dimethyl sulphoxide (DMSO) and subsequently stored in liquid nitrogen for periods up to as long as 6 months. After thawing, the metabolism of testosterone to hydroxylated products and conjugation of 7-hydroxycoumarin were investigated. 2. Rat liver slices stored in liquid nitrogen for 6 months exhibited rates of formation of 7alpha-, 6beta- 16alpha- and 2alpha-hydroxytestosterone, and of androstenedione that did not differ significantly from those observed with fresh slices. 3. No formation of 2alpha-hydroxytestosterone was detected with slices of human liver. However, in contrast with the rat, human slices produced 2beta-hydroxytestosterone. The rates of formation of 7alpha-, 6beta-, 16alpha- and 2beta-hydroxytestosterone and of androstenedione by human liver slices after 6 months of storage in liquid nitrogen were 82, 71, 236, 66 and 92%, respectively, of the corresponding rates by fresh slices. 4. The rates of sulphation and glucuronidation of 7-hydroxycoumarin by slices from rat liver were 97 and 119%, respectively, of the corresponding fresh values after 6 months of storage in liquid nitrogen. 5. 7-Hydroxycoumarin glucuronidation by human liver slices was 53% of the corresponding fresh values after 6 months of storage. However, human slices showed little or no capacity to conjugate 7-hydroxycoumarin with sulphate. 6. It was demonstrated that slices of both human and rat liver can be cryopreserved and stored in liquid nitrogen for at least 6 months without major changes in their rates of metabolism of testosterone to its hydroxylated products and of 7-hydroxycoumarin conjugation. These findings further emphasize that cryopreservation of liver slices can be an effective tool in the use of biological material of limited availability.

Incubation at 37 degrees C prior to cryopreservation decreases viability of liver slices after cryopreservation by rapid freezing

Precision-cut liver slices are to some extent resistant to ice formation induced by rapid freezing. Susceptibility to rapid freezing damage has been shown to be (partly) dependent on intrinsic properties of cells. In the present study an attempt was made to decrease the susceptibility of rat liver slices for rapid freezing damage: the slices were pre-incubated at 37 degrees C under oxygen, prior to cryopreservation to recover from low ATP levels, impaired ion regulation and cell swelling induced by their preparation. It was shown that, unexpectedly, recovery of cellular homeostasis prior to the cryopreservation procedure by the 37 degrees C pre-incubation markedly decreased viability of rapidly frozen slices (in which ice was formed), but not of vitrified slices (in which no ice was formed), in a time- and temperature-dependent manner. UW was found to protect slices from this 'warm pre-incubation phenomenon.' Apparently, pre-incubation prior to freezing causes certain cellular alterations that render slices more susceptible to rapid freezing damage.

In vitro induction of cytochrome P450 2B1- and 3A1-mRNA and enzyme immunostaining in cryopreserved precision-cut rat liver slices

With the exception of cytochrome P450 (CYP) 1A1 and its mRNA, in vitro induction of other CYP forms has not been demonstrated in cryopreserved liver slices until now. Therefore precision-cut rat liver slices were cultured after cryopreservation and thawing in William's medium E for up to 24 h in the presence of inducers to demonstrate CYP2B1- and CYP3A1-mRNA induction. CYP-mRNA expression was determined by competitive RT-PCR. Exposure to 100 microM phenobarbital caused a more than 20-fold increase in CYP2B1-mRNA expression within 24 h, reaching concentrations comparable with those of PB-exposed fresh rat liver slices. Exposure to 1 microM pregnenolone 16 alpha-carbonitrile enhanced CYP3A1-mRNA expression by more than 30-fold within 24 h. This is in the same range, although with higher variability, as detected with fresh liver slices. In spite of considerable variability among the thawed slices, the induction factors are high enough for a sensitive detection of an induction at mRNA level. Additionally, immunostaining of respective CYP-forms was performed in sections of few samples, indicating CYP increase in viable cells of cryopreserved slices.

Water crystallization within rat precision-cut liver slices in relation to their viability

This study examined whether tissue vitrification, promoted by partitioning within the tissue, could be the mechanism explaining the high viability of rat liver slices, rapidly frozen after preincubation with 18% Me2SO or VS4 (a 7.5 M mixture of Me2SO, 1,2-propanediol, and formamide with weight ratio 21.5:15:2.4). To achieve this, we first determined the extent to which crystallization or vitrification occurred in cryoprotectant solutions (Me2SO and VS4) and within liver slices impregnated with these solutions. Second, we determined how these events were related to survival of slices after thawing. Water crystallization was evaluated by differential scanning calorimetry and viability was determined by histomorphological examination of the slices after culturing at 37 degrees C for 4 h. VS4-preincubated liver slices indeed behaved differently from bulk VS4 solution, because, when vitrified, they had a lower tendency to devitrify. Vitrified VS4-preincubated slices that were warmed sufficiently rapid to prevent devitrification had a high viability. When VS4 was diluted (to 75%) or if warming was not fast enough to prevent ice formation, slices had a low viability. With 45% Me2SO, low viability of cryopreserved slices was caused by cryoprotectant toxicity. Surprisingly, liver slices preincubated with 18% Me2SO or 50% VS4 had a high viability despite the formation of ice within the slice. In conclusion, tissue vitrification provides a mechanism that explains the high viability of VS4-preincubated slices after ultrarapid freezing and thawing (>800 degrees C/min). Slices that are preincubated with moderately concentrated cryoprotectant solutions (18% Me2SO, 50% VS4) and cooled rapidly (100 degrees C/min) survive cryopreservation despite the formation of ice crystals within the slice.

Monooxygenation, conjugation and other functions in cryopreserved rat liver slices until 24 h after thawing

For the extensive use of precision-cut liver slices (particularly of human origin) for toxicological investigations successful cryopreservation is necessary. But so far, survival of thawed slices was limited to few hours. This was now overcome by modification of previous procedures. The concentration of DMSO as a cryoprotectant was enhanced to 30%, and washing steps after rapid thawing were omitted. The slices were frozen in liquid nitrogen, thawed at 38 degrees C and incubated immediately in Williams medium E. Protein and potassium contents were stable until 24 h. Glutathione content, amounting to nearly 50% of fresh slices, increased during incubation. High initial lactate dehydrogenase leakage dropped after medium change to less than half during 2-24 h. Testosterone hydroxylation and 7-ethoxycoumarin O-deethylation rates were similar to fresh slices, the latter reaction was inducible by beta-naphthoflavone within 24 h. Methylumbelliferone glucuronidation and p-nitrophenol glucuronidation and sulfation were well measurable and either maintained or decreased by about 50% until 24 h.Altogether, the results are encouraging for further experiments to standardise cryopreservation conditions and to investigate the suitability of this cryopreservation protocol with human liver slices.

Cryopreservation of precision-cut rat liver slices using a computer-controlled freezer

Precision-cut liver slices are frequently used to study hepatic toxicity and metabolism of xenobiotics in vitro. Successful cryopreservation techniques will enhance an efficient and economic use of scarcely available (human) liver tissue. For primary hepatocytes, slow freezing has been accepted as the best approach towards successful cryopreservation. For slices, however, no agreement exists on the optimal way of cryopreservation and both slow and fast freezing techniques have been reported. The aim of the present study was to determine the applicability of a computer-controlled slow freezing technique for the cryopreservation of (rat) liver slices. Thus far, this technique has not been described in detail. Our studies confirmed that slow freezing was most successful in the cryopreservation of primary rat hepatocytes. Based on this observation, the slow freezing technique was applied to the cryopreservation of rat liver slices. Directly after thawing, slice viability was between 60 and 100% of fresh values, depending on the parameter determined. However, after additional culturing, slice viability was reduced. This decrease in slice viability was more pronounced in comparison to primary hepatocytes. In conclusion, the slow freezing technique was confirmed to be a successful approach for the cryopreservation of primary rat hepatocytes, and was found to be of limited use for the cryopreservation of rat liver slices.

High conservation of both phase I and II drug-metabolizing activities in cryopreserved rat liver slices

1. Xenobiotic-metabolizing enzymes, including both cytochrome P450 and phase II-conjugating systems, have been characterized in rat liver slices cryopreserved in 12 or 18% dimethylsulphoxide (DMSO). 2. Several cytochrome P450 isoforms in rat liver slices metabolized testosterone to a variety of hydroxylated products. The rates of formation of these same products were well maintained during cryopreservation of the slices in both 12 or 18% DMSO. 3. After cryopreservation of rat liver slices in 18% DMSO, the rates of metabolism of ropivacaine to 3-hydroxyropivacaine, 4-hydroxyropivacaine and PPX (all catalysed by different cytochrome P450 isoforms) were approximately 94, 79 and 82% respectively of the corresponding rates observed with fresh slices. 4. The rates of conjugation of 7-hydroxycoumarin and 1-naphthol by rat liver slices were significantly decreased after cryopreservation in 12% DMSO, but they were maintained when the concentration of this cryopreservant was increased to 18% 5. After cryopreservation in 12% DMSO, the mitochondrial reduction of the tetrazolium salt MTT by rat liver slices was significantly lowered. In contrast, slices cryopreserved in 18% DMSO demonstrated no significant decrease in their capacity to reduce MTT. 6. Thus, in agreement with previous studies, it was found that cytochrome P450-dependent activities are retained after cryopreservation of liver slices. Although phase II-conjugating enzyme activities are more sensitive to cryopreservation, it was shown that increasing the concentration of DMSO present during cryopreservation could circumvent the problem. This modification improves the usefulness of cryopreserved rat liver slices as a tool in drug metabolism studies.

Cryopreserved rat liver slices: a critical evaluation of cell viability, histological integrity, and drug-metabolizing enzymes

The effects of a cryopreservation procedure on the biochemical, morphological and functional integrity of rat liver slices just after thawing and after 24 h culture were evaluated. Freshly prepared slices were incubated in modified University of Wisconsin solution containing 50% fetal calf serum and 10% dimethyl sulfoxide for 20 min on ice prior to a rapid cooling in liquid nitrogen. After 10-40 days, slices were thawed rapidly at 42 degrees C. Total protein content and (3-[4,5-dimethylthiazol-2-yl]-2, 5-diphenyltetrazolium bromide) (MTT) reduction were well preserved at thawing, whereas ATP content was markedly decreased relative to freshly prepared slices (-83%). The major microscopic findings in sections of just-thawed liver slices consisted of hepatocellular dissociation and minimal apoptosis. The qualitative profile of antipyrine (AP) metabolism was well preserved in cryopreserved slices, but the amounts of phase I and phase II AP metabolites produced over a 3-h incubation period were markedly reduced relative to fresh slices (-58 to -71%). When cryopreserved slices were cultured for 24 h after thawing, the viability was markedly reduced, as reflected by the almost complete absence of MTT reduction and the loss of ATP content. Histological examinations showed extensive cellular necrosis. The amount of AP metabolites produced by cryopreserved slices was similar after a 3- or a 24-h culture period, indicating that AP metabolism capacities were lost at 24 h culture. In conclusion, our results suggest that cryopreserved rat liver slices may be a useful model for short-term in vitro determination of drug metabolism pathways. Further work is required to extend their use for toxicological studies.

Assessment of some critical factors in the freezing technique for the cryopreservation of precision-cut rat liver slices

A number of studies on the cryopreservation of precision-cut liver slices using various techniques have been reported. However, the identification of important factors that determine cell viability following cryopreservation is difficult because of large differences between the various methods published. The aim of this study was to evaluate some important factors in the freezing process in an effort to find an optimized approach to the cryopreservation of precision-cut liver slices. A comparative study of a slow and a fast freezing technique was carried out to establish any differences in tissue viability for a number of endpoints. Both freezing techniques aim at the prevention of intracellular ice formation, which is thought to be the main cause of cell death after cryopreservation. Subsequently, critical variables in the freezing process were studied more closely in order to explain the differences in viability found in the two methods in the first study. For this purpose, a full factorial experimental design was used with 16 experimental groups, allowing a number of variables to be studied at different levels in one single experiment. It is demonstrated that ATP and K(+) content and histomorphology are sensitive parameters for evaluating slice viability after cryopreservation. Subsequently, it is shown that freezing rate and the cryopreservation medium largely determine the residual viability of liver slices after cryopreservation and subsequent culturing. It is concluded that a cryopreservation protocol with a fast freezing step and using William's Medium E as cryopreservation medium was the most promising approach to successful freezing of rat liver slices of those tested in this study.

Induction of cytochrome P450 2B1-mRNA and pentoxyresorufin O-depentylation after exposure of precision-cut rat liver slices to phenobarbital

Precision-cut rat liver slices were prepared from male Wistar rats with a Krumdieck slicer and cultured in William's medium E for up to 24 h. In untreated control slices, CYP2B1-mRNA concentration, which was quantified by competitive RT-PCR, did not decrease during this time. After exposure of the slices to 100 microM phenobarbital, CYP2B1-mRNA increased by about 10- or 60-fold after 6 or 24 h, respectively. The extent of this in vitro induction was similar to that after in vivo administration of 60 mg/kg phenobarbital. Pentoxyresorufin O-depentylation (PROD) was also inducible in vitro after 24 h, but to a lesser extent than the corresponding CYP-mRNA. Precision-cut liver slices proved to be a simple and reliable in vitro system for the sensitive detection of an induction by phenobarbital.

Monooxygenation, cytochrome P450-mRNA expression and other functions in precision-cut rat liver slices

Precision-cut rat liver slices (KRUMDIECK slicer, slice thickness 200-250 microm) were incubated in rollers containing modified William's medium E at 37 degrees C for 2, 24 and 48 hrs. Protein, DNA, potassium and glutathione concentrations did not decrease during 48 hrs. Lactate dehydrogenase (LDH) leakage into the medium was relatively marked during the first 2 hrs of incubation, from the 2nd to the 48th hr LDH leakage was very low. The same is true of the release of thiobarbituric acid-reactive substances. Albumin synthesis and transport into the medium decreased to about 70% after 48 hrs. Cytochrome P450 (CYP)-dependent 7-ethoxycoumarin O-deethylation rate was relatively stable up to 48 hrs, whereas testosterone hydroxylation decreased significantly without alterations of the proportions of the 7 quantified hydroxylated metabolites. After exposure of the slices to beta-naphthoflavone for 6 hrs CYP1A1-mRNA expression, measured by competitive RT-PCR, was increased by a factor of at least 1000. Precision-cut liver slices are a useful tool for the study of various hepatic functions, drug metabolism and its induction in vitro.