Human hepatocyte cultures: a model of pharmaco-toxicological studies

A study of the metabolism of methamphetamine and 4-bromo-2,5-dimethoxyphenethylamine (2C-B) in isolated rat hepatocytes

The metabolism of methamphetamine (MA) and 4-bromo-2,5-dimethoxyphenethylamine (2C-B) in freshly isolated rat hepatocytes was investigated, and compared with in vivo results. A suspended hepatocyte culture, established from male Wistar rats using a collagenase perfusion technique, was incubated in the presence of MA or 2C-B. After enzymatic hydrolysis of the conjugated forms, the metabolites were extracted by liquid-liquid partition and analyzed by gas chromatography/mass spectrometry (GC/MS). Amphetamine, p-hydroxymethamphetamine and p-hydroxyamphetamine were detected in the culture fluids of the rat hepatocytes inoculated with MA. The alcohol derivative, carboxylic acid derivative, 2-O-desmethyl-2C-B, 2-O-desmethyl-N-acetyl-2C-B and 5-O-desmethyl-N-acetyl-2C-B were detected in the case of 2C-B. The major metabolite of MA in rat hepatocytes was p-hydroxymethamphetamine. This is in good agreement with the urinary excretion profile for rats that were fed MA. 2-O-Desmethyl-2C-B and the carboxylic acid derivative were the major recovered metabolites of 2C-B in the rat hepatocyte culture, a slight deviation from the in vivo findings, in which 5-O-desmethyl-N-acetyl-2C-B was found to be the main component. Metabolites with a hydroxy group were largely present in their conjugated forms in the culture fluids, except for 2-O-desmethyl-2C-B. Taking these results into consideration, a primary hepatocyte culture system has the potential to provide a quick and handy method for estimating the in vivo metabolic fate of abused drugs.

A commentary on the use of hepatocytes in drug metabolism studies during drug discovery and development

Isolated hepatocytes and liver slices, in short-term suspension or longer-term culture, offer the prospect of providing qualitative metabolic information and quantitative pharmacokinetic parameters from key animal species and man at early stages of the drug discovery-development continuum. The propensity for changes in the fidelity of drug metabolism after removal of hepatocytes from the organ has long been recognized. The many and varied approaches which have been undertaken in an attempt to compensate for physiological shortcomings of in vitro hepatocyte systems are reviewed. In this respect, short-term suspension culture may provide a baseline against which to measure the success of extended culture methods, but it should be remembered that even freshly isolated hepatocyte preparations have deficiencies and liabilities that may affect the nature of information gathered. This article discusses the current advances and shortcomings of hepatocyte suspensions and cultures, along with liver slice technology, at both quantitative and qualitative levels.

Long-term culture of hepatocytes from human adults

A long-term primary human hepatocyte culture retraining liver-specific functions is important and essential for basic research and for the future development of hepatocyte-based applications. We established a normal hepatocyte culture system from excess normal tissues obtained from adult liver cancer patients who received partial liver resection. Hepatocytes were isolated after perfusion and enzymatic disaggregation, and were first maintained in hormonally defined media on a Matrigel matrix, and then transferred to collagen sandwich gel. The hepatocytes formed clusters on the Matrigel matrix and increased in size and numbers with time of culture and eventually grew into spheroids of variable sizes. After being transferred to collagen gel, the cells migrated outward from spheroids to form a monolayer with cuboidal or polygonal cell shapes with granular cytoplasm and continued to proliferate. Cellular functions specific for hepatocytes were analyzed using immunoblot assay for proteins specifically secreted by the liver cells on different days of culture. The cells secreted albumin, transferrin and alpha-fetoprotein consistently for more than 100 days, to a maximum of 150 days. Thus, we have established a long-term culture of hepatocytes from human adults, which will be useful for basic studies of liver physiology such as metabolism and morphogenesis, as well as for other applications in the study of infectious hepatitis, pharmacology, pharmacokinetics, and toxicology.

Comparative study of the biotransformation of bepridil analogs in isolated liver cells from one rat. Relationships between structure and in vitro liver toxicity

The biotransformation of several analogs of the anti-calcium agent bepridil was studied comparatively in liver cells isolated from one rat. Three types of metabolites were identified by mass spectrometry, resulting from three phase I reactions: hydroxylation, N-debenzylation and pyrrolidine ring opening. The amount of each bepridil analog untransformed after 18 h of incubation depended on its liver toxicity rather than on its concentration in the culture medium. The proportion of phase I metabolites identified remained constant regardless of toxicity. The difference delta c (in %) between the initial concentration of the analog tested and the sum of the concentrations of untransformed material and of identified metabolites decreased with the increasing hepatocyte toxicity. The analogs tested were responsible for the liver toxicity. The presence of substituents in different positions on the N-phenyl moiety increased liver toxicity; ortho-substituted analogs were more toxic than para- or meta-substituted ones.

The effect of amino acid composition of serum-free medium on DNA synthesis in primary hepatocyte cultures in the presence of epidermal growth factor

The presence of optimal nutritional elements in cell culture medium is very important in studies of cultured cells. For this reason, several researchers have experimented with adding or increasing the concentration of one or more amino acids to the medium they were using to determine "essential" amino acids and optimal concentrations. We studied how leaving out one amino acid at a time from Dulbecco's modified Eagle's medium would affect epidermal growth factor-induced DNA synthesis in primary hepatocytes of the rat. Our "modified" DMEM contained only eight amino acids: arginine, cysteine, isoleucine, leucine, lysine, phenylalanine, tryptophan, and valine. Proline was found to be an essential amino acid in normal DMEM but not in the modified DMEM, and some other amino acids reduced DNA synthesis in this medium. This study showed that perhaps no single amino acid such as proline can be called "essential," but rather an optimal balance of amino acids is required for each major function of each cell type cultured.

Calcium antagonists can be classified using in vitro toxicity and potency indices

The toxicity of eleven calcium antagonists from different chemical families was determined in rat hepatocyte primary cultures. The calcium antagonist potency of the same compounds was also determined in isolated rabbit aortic rings contracted with high K+. The hepatocytotoxicity of the calcium antagonists was not directly linked to blockade of voltage-operated calcium channels, since there was no correlation between the rank order of hepatotoxicity and that for calcium antagonist potency. The toxicity and calcium antagonist potency of each calcium antagonist examined were used to calculate an in vitro therapeutic index value for each compound. It was observed that therapeutic indices fell into three distinct groups and we therefore propose that the in vitro therapeutic index can be used to subclassify the calcium antagonist group of drugs. The proposed classification corresponds very closely with one already suggested by Spedding on pharmacological grounds. In conclusion, the in vitro therapeutic index may provide a useful tool in the characterization and subclassification of novel calcium antagonist compounds.

Thawed human hepatocytes in primary culture

In drug metabolism studies, isolated and cultured human hepatocytes provide a useful model for overcoming the difficulty of extrapolating from animal data. In vitro studies with human hepatocytes are scarce because of the lack of livers and suitable methods of storage. After developing a new method for cryopreservation of human hepatocytes, we evaluated the effects of deep freezing storage on their viability, morphology, and functional and toxicological capabilities in classical culture conditions. Freshly isolated human hepatocytes were cryopreserved in medium containing 10% Me2SO and 20% fetal calf serum, using a Nicool ST20 programmable freezer (-1.9 degrees C/min for 18 min and -30 degrees C/min for 4 min). Cells were stored in liquid nitrogen. Viability of thawed human hepatocytes was 50-65% as assessed by erythrosin exclusion test prior to purification on a Percoll density gradient. Morphological criteria showed that thawed human hepatocytes require an adaptation period to the medium after seeding. Functional assessments showed that human hepatocytes which survive freezing and thawing preserve their protein synthesis capabilities and are able to secrete a specific protein, anionic peptidic fraction, which is involved in the hepatic uptake of bile-destined cholesterol. We then studied Midazolam biotransformation to test metabolic functions, and erythromycin toxicity by Neutral Red test (cell viability) and 3-(4,5-dimethylthiazol-2-yl)-diphenyl tetrazolium bromide test (cell metabolism). All of these experiments indicated that thawed human hepatocytes should be used 38 h after seeding for optimum recovery of their functions: membrane integrity, protein synthesis, and stabilization of drug metabolism enzymes.

Investigations on the in vitro metabolism of five synthetic 19-norprogestins using hepatocyte suspensions isolated from five laboratory animal species

Five synthetic progestins of the 19-nortestosterone type (norethisterone, NET; levonorgestrel, LN; gestodene, GEST; NET-3-oxime, NETO; norgestimate, NGM) were investigated in the in vitro hepatocyte model. Radiolabelled progestins were added to hepatocyte suspensions (3 x 10(6) cells/ml) freshly prepared from female rat, guinea pig, rabbit, dog (beagle) and cynomolgus monkey. Drug level decreases (NET, LN, GEST) and prodrug conversions (NETO, NGM) were followed by radiochromatography (HPLC) for 60 min. In the case of NET and NETO the conversion into ethinyl estradiol (EE2) was quantified by RIA after HPLC separation. Half-lives of drug level decreases (t1/2), areas under the curves (AUC) and metabolic clearance rates (MCR) were estimated for all progestins. For NETO and NGM the percentages of conversion into NET and LN were calculated, respectively, and levels of EE2 determined in the case of NET and NETO. Rat hepatocytes showed an extremely high metabolic activity towards NET, LN and GEST resulting in t1/2 values of below 2 min. Respective values for rabbit hepatocytes ranged from 5-8 min, whereas half-lives calculated for liver cells from guinea pig, dog and monkey were generally above 30 min. A drastic increase in t1/2 was found for NETO (as compared to NET) in hepatocytes from rat, rabbit and monkey but not from guinea pig. Dog hepatocytes degraded NETO about 3 times more rapidly than NET. NGM was degraded much faster than LN in hepatocytes from all species except the rat. Liver cells from guinea pig and dog seem to be able to metabolize the 3-oxime group much more rapidly than hepatocytes from the other animal species. The lowest degree of prodrug conversion of 4% was observed for NGM and dog hepatocytes. Elevated EE2 levels were found in all experiments with NET and NETO. Results of NET, LN and GEST were compared with published in vivo experiments. No correlations were found for t1/2, MCR, and AUC.

Isolation and culture of hepatocytes from the cynomolgus monkey (Macaca fascicularis)

Isolation and culture techniques for hepatocytes from whole livers of the cynomolgus monkey, Macaca fascicularis, are described. Hepatocytes were isolated by two-step perfusion of livers, using collagenase with hyaluronidase; fructose and trypsin inhibitor were included to reduce cell loss. Yields from a single liver average 4 X 10(9) cells with viabilities of 90.8 +/- 5.7%. Cells, plated on collagen substrates, were assessed for changes in morphology and various marker enzyme activities over a period of 7 d in culture. Cells exhibited a morphology similar to that observed for this species in vivo; little change in attached and spread cells was observed over the length of time monitored. Enzyme activities for catalase, succinate dehydrogenase, and tyrosine aminotransferase were observed to decrease significantly (though considerable activity remained), whereas acid phosphatase and 5'-nucleotide phosphodiesterase remained unchanged. Activity of cytochrome P-450 reductase was observed to increase slightly for the first 2 d, then decrease to about 60% of initial levels. Activity of alpha-mannosidase was stable for 4 d but was observed to be increased at Day 7. Cells were observed to retain metabolic responsiveness, demonstrated by glucose production by both gluconeogenesis and glycogenolysis in response to glucagon stimulation. The monkey hepatocytes obtained by methods described here thus retain hepatocellular morphology and activity through at least 1 wk in culture without medium or culture modification.

Studies on the biotransformation of lonazolac, bromerguride, lisuride and terguride in laboratory animals and their hepatocytes

1. Metabolic patterns and the extents of metabolism of four drugs, namely [14C]lonazolac (LON), [14C]bromerguride)BRO), [14C]lisuride (LIS) and [3H]terguride (TER) have been studied in three experimental models, namely hepatocyte suspensions of rat, guinea pig, beagle dog and cynomolgus monkey, isolated perfused liver of rat and guinea pig and intact animals (rat, guinea pig, dog and monkey). 2. Selection of compounds was based on differences in phase I metabolic pathways. LON is exclusively hydroxylated in the N-substituting aromatic ring, BRO is mainly N-deethylated in the urea moiety, and LIS and TER are both degraded into numerous metabolites. 3. The decrease in unchanged drug levels in hepatocyte suspensions was characterized by half-lives, with LON as the most stable and LIS as the least stable compound. Marked interspecies differences were found. De-ethylation and aromatic hydroxylation were much slower in rat hepatocytes than in the liver cells of other species; BRO was slowly biodegraded in dog hepatocytes while LIS was broken down extremely quickly. 4. Liver perfusion experiments and studies in vivo were evaluated for the extents of metabolism of each drug. 5. Metabolism studies in hepatocytes did not show any quantitative correlation to those of metabolism in vivo. The suitability of evaluating parameters for in vitro studies is discussed.

Studies of chemical toxicity to fresh and cryopreserved rat hepatocytes

Isolated hepatocytes are useful for studying the metabolism and mechanisms of hepatic toxicity of foreign chemicals. A problem with using human hepatocytes is the limited and irregular availability of normal human liver. Cryopreservation could provide a useful way of storing hepatocytes until they are needed. As a preliminary step to using human hepatocytes we have compared the toxic response to chemical toxicants of primary cultures of fresh rat hepatocytes and rat hepatocytes cryopreserved as previously described (G. Powis, K. S. Santone, D. C. Melder, L. Thomas, D. J. Moore, and T. J. Wilke, 1987. Drug Metab. Dispos. 15, 826). After 24 hr in culture the cryopreserved hepatocytes had a plating efficiency 75% that of noncryopreserved hepatocytes. The cultured cryopreserved hepatocytes showed a small increase in spontaneous lactate dehydrogenase release compared to that of cultured noncryopreserved hepatocytes. A similar toxic chemical-induced increase in lactate dehydrogenase release occurred in the cultured cryopreserved as in the noncryopreserved hepatocytes. The 50% effective concentrations (EC50) for lactate dehydrogenase release (+/- SE, n = 3 preparations) from cultured cryopreserved and noncryopreserved hepatocytes for chlorpromazine were 235 +/- 20 and 215 +/- 30 microM, for cadmium chloride 200 +/- 5 and 272 +/- 23 microM, and for menadione (2-methyl-1,4-naphthoquinone) 24 +/- 7 and 44 +/- 8 microM, respectively. The EC50 values for intracellular glutathione depletion in cultured cryopreserved and noncryopreserved hepatocytes were for chlorpromazine 200 +/- 8 and 235 +/- 8 microM, for cadmium chloride 242 +/- 19 and 213 +/- 7 microM, and for menadione 22 +/- 2 and 21 +/- 3 microM, respectively. The results show that cryopreservation offers a practical way of storing rat hepatocytes for studies of chemical toxicity.

In vitro screens from CNS, liver and kidney for systemic toxicity

The development and evaluation of in vitro systems from target organs for preliminary assessments of the potential for systemic toxic effects has been receiving increased attention. This review presents a synopsis of progress made in developing toxicity screens for three common target organs and identifies further work needed for more complete validation.

Genotoxicity of carcinogens in human hepatocytes: application in hazard assessment

Evaluation of chemical genotoxicity has been used in assessing human cancer hazard, based on the observation that most human carcinogens are known to be DNA-reactive. The availability of data on the DNA-reactivity of compounds in metabolically competent human cells would assist hazard assessment by providing direct information of human genotoxicity. To evaluate the reliability of human hepatocytes for this purpose, the induction of DNA repair by DNA-reactive carcinogens of several structural classes and related noncarcinogens was studied. All the carcinogens elicited DNA repair synthesis, whereas the noncarcinogens did not. These studies provide additional support for the use of human hepatocytes in a DNA repair test in the investigation of genotoxicity. The demonstration of genotoxicity in human cells is suggested to provide important information for hazard assessment.

Hepatotoxicity and molecular aspects of hepatocyte function in primary culture

1. The application of primary cultures of hepatocytes in testing for hepatotoxicity of drugs is reviewed. 2. Hepatotoxicity results principally from the biotransformation of toxic agents. This process is very complex and specific and involves a powerful system of multigenic isozyme families for both phase I and phase II drug metabolizing reactions. Many of the isozymes are specifically expressed in the liver in relation to the maturation or differentiation state, and are specifically induced, possibly through a complex temporally programmed gene regulation. 3. This highly specific, coordinated, molecular regulation is difficult to maintain in vitro. Isolation of hepatocytes induces a prompt differential decline of liver-specific gene transcription, which leads to preferential loss of the most specific functions, including those of the drug metabolizing isozymes, whereas repair of cell damage remains active. 4. The use of serum-free, hormonally defined media stabilizes specific hepatic functions, but not transcriptional activity, for 4-5 days. Defined media retain active DNA replication but do not permit clonal growth of hepatocytes. Co-culturing hepatocytes with primitive biliary cells prolongs cell survival and their functional capacities for several weeks, including some of the transcriptional activity.

Enhancement of aldehyde dehydrogenase activity in human and rat hepatocyte cultures by 3-methylcholanthrene

Aldehyde dehydrogenase was measured in primary cultures of hepatocytes obtained with a two-step collagenase perfusion either from human hepatic tissue or from livers of Fisher rats. Basal enzyme activity declines gradually as a function of time in culture, but remains at all times higher when measured with propionaldehyde and NAD (P/NAD) than with benzaldehyde and NADP (B/NADP). Treatment of the cultures with 2 microM of 3-methylcholanthrene for four days significantly increased the B-NADP activity of human and rat hepatocytes (tenfold and eightfold respectively). In human hepatocytes 3-methylcholanthrene increases also the P/NAD activity, but to a lesser extent (twofold), compared to the B/NADP activity. Due to the significant enhancement of B/NADP activity in cultures of human and rat hepatocytes after application of 3-methylcholanthrene, the initial difference in the basal activity levels between the P/NAD and B/NADP forms diminishes or, in the case of human hepatocytes, is even inverted. These results show for the first time that aldehyde dehydrogenase activity is increased in cultured human hepatocytes. This biochemical property is preserved in human and rat hepatocyte cultures, despite the rather quick loss of the basal aldehyde dehydrogenase activity.