Correlation of acute lethal potency with in vitro cytotoxicity

Hepatocytes in Primary Culture: An Alternative to LD50 Testing? Validation of a Predictive Model by Multivariate Analysis

The cytotoxicity of 30 chemicals was assessed in rat hepatocyte primary cultures using four methods: lactate dehydrogenase release, neutral red uptake, the MTT assay, and measurement of total protein content.

Comparison of the data obtained in vitro (IC50 values) and in vivo (LD50 values) resulted in a significant correlation (p<0.001) between IC50 values and intravenous LD50 values. The validity, as well as the predictability of the model, were determined by multivariate analysis (principal component analysis and correspondence analysis). The predictability area, expressed in IC50 values, was in the range of 0–l,500μg/ml and reached 95%, with a 75–100% confidence interval (p = 0.05).

Assessment of the cytotoxicity of 54 additional chemicals would provide a more accurate predictability limit around l,500μg/ml and the estimated predictability confidence interval could be reduced to 90–100%.

The Predictive Potential of a Battery of Ecotoxicological Tests for Human Acute Toxicity, as Evaluated with the First 50 MEIC Chemicals

The acute toxicities of the first 50 chemicals (dextropropoxyphene hydrochloride excluded) of the multicentre evaluation of in vitro cytotoxicity (MEIC) programme were determined on four aquatic invertebrates and a bacterial strain (Photobacterium phosphoreum for the Microtox™ test) commonly used in ecotoxicology testing. Three of the aquatic invertebrate tests consisted of cyst-based toxicity tests (Artoxkit M with Artemia salina, Streptoxkit F with Streptocephalus proboscideus, and Rotoxkit F with Brachionus calyciflorus), and the Daphnia magna test. Results of simple linear regression analyses indicated that the rodent tests (rat and/or mouse) were better than the ecotoxicological tests for predicting acute oral lethal doses in man. However, it appears that the batteries of ecotoxicological tests resulting from the partial least squares method appear to be better than the rodent tests for predicting human oral lethal doses.

Comparison of Xenobiotic-mediated Cytotoxicity in Rat Cultured Hepatocytes and the V79 Chinese Hamster Lung Fibroblast Cell Line: Can Metabolically-activated Hepatotoxins be Identified by Selective Cytotoxicity to Hepatocytes?

The effects of 17 xenobiotics on rat hepatocytes and V79 cells were evaluated under identical exposure conditions (confluent monolayer for 24 hours) and endpoint measurement (MTT reduction). The data indicated that the majority of metabolically-activated rat hepatotoxins could be identified by greater cytotoxicity to rat hepatocytes relative to V79 cells, but that direct-acting hepatotoxins (galactosamine and ethionine) and a group of eight compounds likely to act through interference of basal functions in non-dividing and dividing cells produced similar toxicity in each cell type. It is possible that the greater water-solubility of the direct-acting hepatotoxins relative to the indirect-acting hepatotoxins may contribute to the lack of effect seen with the former group under the conditions of the assay (top concentration of 1mM).

Two Procedures for the Prediction of Acute Toxicity (LD50) from Cytotoxicity Data

Single linear regression analysis was used to characterise the relationship between cytotoxicity in a variety of mammalian cell culture systems and acute oral toxicity (LD50) in experimental animals. The following results were obtained.

Firstly, in a cytotoxicity assay using the calf aortic endothelial cell line BKEz-7, IC50 values determined for 44 chemicals in culture showed significant correlation with the oral LD50 values for rat and mouse (computed correlation coefficient r=0.546). After eliminating three chemicals that were characterised by extreme lethality indices (LI = IC50/LD50), the correlation coefficient of the remaining 41 chemicals increased to a value of r=0.728. By using the linear regression model for these 41 chemicals, the oral LD50 for rat and mouse can be predicted correctly from the IC50 values for 83% of substances from a variety of chemical substance classes within a range of approximately one order of magnitude of dosage unit of LD50 for rat and mouse.

Secondly, the mean IC50 values (IC50x¯) determined as the geometrical mean of two or more IC50 values per substance, which were generated in a wide spectrum of mammalian cell lines and collected in a “Registry of Cytotoxicity” (RC), gave similar results (r=0.644). Likewise, with the aid of this method, the oral LD50 for rat and mouse can be predicted for 74% of non-selected chemicals from structurally-different classes in the same dosage range, e.g., 1–25 millimoles per kg body weight.

The prediction of LD50 values from in vitro cytotoxicity data may permit the calculation of a more precise dose range-finding and offers a new way for reducing the number of animals in acute toxicity testing.

Validation of In Vitro Cytotoxicity Tests — Past and Present Strategies

In recent years, conventional toxicity testing in animals has been reinforced by in vitro methods. As a result, toxicity testing in some sectors has become more effective and at the same time more ethical. This trend is probably only at its beginning, as many of the newly-developed methods have not yet won general acceptance as a basis for the large-scale replacement and reduction of animal experimentation. What limits the wider use of these methods is validation, i.e. the evaluation of their reliability and relevance. The present paper is a short review of the validation efforts made hitherto, including projects being planned and under discussion. Our own MEIC approach is compared with other strategies. Finally, our opinion on the effectiveness of one large consensus project relative to several different smaller validation programmes is expressed — we advocate the latter strategy, because it will save time and reduce costs.

Application of fish cell lines for evaluating the chromium induced cytotoxicity, genotoxicity and oxidative stress

In the present study, we hypothesize that cytotoxicity, genotoxicity and oxidative stress play a key role in chromium induced toxicity in SISS, SISK, IEE, IEK, IEG, SICH and ICG cell lines after 24 h exposure. Three fish species namely Lates calcarifer, Etroplus suratensis and Catla catla were exposed to the concentrations of 0, 10, 20, 30, 40 and 50 mg/L of chromium for 96 h under static conditions for conducting acute toxicity tests. LC₅₀ was then calculated. The percentage cell survival was assessed by multiple endpoints such as MTT, NR, AB and CB assays in the seven fish cell lines exposed to different concentrations of chromium and EC₅₀ values of all the four endpoints were calculated. High significances were noted in the correlations between each in vitro cytotoxicity assays and in vivo mortality data. Cell shrinkage, cell detachment, vacuolations and cell swelling at the highest concentration of chromium (50 mg/L) were seen on microscopic examination of cell morphology. Comet assay and Hoechst staining were carried out to assess DNA damage and nuclear fragmentation in the seven fish lines exposed to chromium. The results of antioxidant parameters obtained indicate a significant reduction in the level of catalase, superoxide dismutase, glutathione S-transferase and Glutathione peroxidase, and increased level of lipid peroxidation in all the cell lines exposed to chromium. These results confirm that fish cell lines could be used as an alternative to whole fish for cytotoxicity, genotoxicity and oxidative stress assessment in chromium toxicity studies.

Development and characterization of cell line from the gill tissue of Catla catla (Hamilton, 1822) for toxicological studies

Catla gill cell line (ICG) was established from gill tissue of Indian major carp (Catla catla), a freshwater fish cultivated in India. The cell line was maintained in Leibovitz's L-15 supplemented with 10% fetal bovine serum. These cells have been sub-cultured more than 55 passages over a period of 2 years. The ICG cell line consists predominantly of epithelial-like cells. The cells were able to grow at a wide range of temperatures from 24°C to 32°C with an optimum temperature of 28°C. The growth rate of gill cells increased as the fetal bovine serum (FBS) proportion increased from 2% to 20% at 28°C with optimum growth at the concentrations of 10% or 15% FBS. Amplification of mitochondrial gene 12s rRNA using primers specific to C. catla confirmed the origin of this cell line from C. catla. The cells were successfully cryopreserved and revived at passage numbers 25, 35, 45 and 55. The cytotoxicity of three metal salts (ZnCl(2), CuSO(4) and CdCl(2)) was assessed in ICG cell line using multiple endpoints such as MTT, Neutral Red assay, Alamar Blue assay and Coomassie Blue protein assay. Acute toxicity assay on fish were conducted by exposing C. catla for 96 h to three metal salts under static conditions. Statistical analysis revealed good correlation with r(2)=0.908-0.985 for all combinations between endpoints employed. Linear correlations between each in vitro EC(50) and the in vivo LC(50) data were highly significant.

IN VIVO HEPATOTOXICITY STUDY OF RATS IN COMPARISON WITH IN VITRO HEPATOTOXICITY SCREENING SYSTEM

For the establishment of a high throughput screening system using primary cell cultures, investigation of elucidated toxicities to assess the correlation between in vitro and in vivo hepatotoxicity is necessary in the safety evaluation of the compound. In the previous study, we reported the usability of rat primary cultured hepatocytes for establishment of high throughput screening system. To confirm the reliability of rat primary hepatocytes culture screening system, we conducted a single-dose in vivo study with relatively high dose of hepatotoxicant in rats using 4 reference compounds (acetaminophen, amiodarone, tetracycline, carbon tetrachloride), and investigated histopathological changes and expression of oxidative stress-related proteins by immunohistochemistry. We also carried out a proteomics analysis for estimating the reliable and sensitive biomarkers. Histopathologically, compound-specific hepatotoxicity was detected at 24 hr after administration in all compounds except amiodarone, which is known to induce phospholipidosis. Immunohistochemically, oxidative stress-related proteins were increased within 6 hr after administration in all treated groups. Proteomics analysis revealed several protein biomarkers related to oxidative stress and mitochondrial metabolism-regulation, which had been previously detected by proteomics analysis in in vitro screening system. Oxidative stress-related proteins were considered as useful biomarkers of hepatotoxicity; since they were detected by immunohistochemistry and proteomics analysis prior to appearance of compound-specific histopathological changes detected by light microscopy. Considering the relevance of in vitro system to in vivo system from the aspect of new biomarkers related to the toxicogenomics/toxicoproteomics, in vitro primary cell culture system would be sufficient to detect hepatotoxicity in the early stage of drug discovery.

SIRC-CVS CYTOTOXICITY TEST: AN ALTERNATIVE FOR PREDICTING RODENT ACUTE SYSTEMIC TOXICITY

An in vitro crystal violet staining method using the rabbit cornea-derived cell line (SIRC-CVS) has been developed as an alternative to predict acute systemic toxicity in rodents. Seventy-nine chemicals, the in vitro cytotoxicity of which was already reported by the Multicenter Evaluation of In vitro Toxicity (MEIC) and ICCVAM/ECVAM, were selected as test compounds. The cells were incubated with the chemicals for 72 hrs and the IC(50) and IC(35) values (microg/mL) were obtained. The results were compared to the in vivo (rat or mouse) "most toxic" oral, intraperitoneal, subcutaneous and intravenous LD(50) values (mg/kg) taken from the RTECS database for each of the chemicals by using Pearson's correlation statistics. The following parameters were calculated: accuracy, sensitivity, specificity, prevalence, positive predictability, and negative predictability. Good linear correlations (Pearson's coefficient; r>0.6) were observed between either the IC(50) or the IC(35) values and all the LD(50) values. Among them, a statistically significant high correlation (r=0.8102, p<0.001) required for acute systemic toxicity prediction was obtained between the IC(50) values and the oral LD(50) values. By using the cut-off concentrations of 2,000 mg/kg (LD(50)) and 4,225 microg/mL (IC(50)), no false negatives were observed, and the accuracy was 84.8%. From this, it is concluded that this method could be used to predict the acute systemic toxicity potential of chemicals in rodents.

Experimental survey of non-clonogenic viability assays for adherent cells in vitro

Results of rapid cell viability assays were experimentally compared in order to reveal the most suitable test for in vitro investigations of the combination of photodynamic therapy (PDT) with chemotherapeutic drugs. meso-Tetra(3-hydroxyphenyl)-chlorin (m-THPC) accumulating in cell membranes and meso-tetra(4-sulfonatophenyl)-porphin (TPPS4) accumulating in lysosomes were used as photosensitisers. Doxorubicin that localises, mainly, to nucleus and vincristine that binds to microtubules were used as cytostatic drugs. Two adherent rodent cell lines, baby hamster kidney (BHK-21) and murine hepatoma (MH-22A), were used to examine the contribution of a cell. We tested cytotoxicity assays of the main groups of fast (non-clonogenic) methods of cell viability measuring. Plasma membrane integrity was estimated by trypan blue exclusion and LDH leakage, metabolic activity was tested by [3H]-thymidine incorporation and MTT assay, loss of monolayer adherence was measured by staining with crystal violet and CyQUANT. The most sensitive test in each case was the assay related to the site of the direct damage, and measurement of the loss of monolayer adherence proved to be as sensitive assay as the damage-specific one. All the assays applied, except for the LDH release, revealed a higher effect of combination of m-THPC-mediated phototreatment and doxorubicin compared to either of the single treatments.

IDENTIFICATION OF OXIDATIVE STRESS-RELATED PROTEINS FOR PREDICTIVE SCREENING OF HEPATOTOXICITY USING A PROTEOMIC APPROACH

We investigated the effects of three hepatotoxicants, acetaminophen (APAP), amiodarone (AD) and tetracycline (TC), on protein expression in primary cultured rat hepatocytes with toxicoproteomic approach, which is two-dimensional gel electrophoresis (2DE) and mass spectrometry. The objectives of this study were to search for alternative toxicity biomarkers which could be detected with high sensitivity prior to the appearance of morphological changes or alterations of analytical conventional biomarkers. The related proteins in the process of cell degeneration/necrosis such as cell death, lipid metabolism and lipid/carbohydrate metabolism were mainly affected under exposure to APAP, AD and TC, respectively. Among the differentially expressed proteins, several oxidative stress-related proteins were clearly identified after 24-hr exposure, even though they were not affected for 6-hr exposure. They were glutathione peroxidase (GPX) as a down-regulated protein as well as peroxiredoxin 1 (PRX1) and peroxiredoxin 2 (PRX2) as up-regulated proteins, which are known to serve as antioxidative enzymes in cells. These findings suggested that the focused proteins, GPX and PRXs, could be utilized as biomarkers of hepatotoxicity, and they were useful for setting high throughput screening methods to assess hepatotoxicity in the early stage of drug discovery.

INVESTIGATION OF A HEPATOTOXICITY SCREENING SYSTEM IN PRIMARY CELL CULTURES-"WHAT BIOMARKERS WOULD NEED TO BE ADDRESSED TO ESTIMATE TOXICITY IN CONVENTIONAL AND NEW APPROACHES?"-

High throughput toxicological estimation is required for safety evaluation in the early stage of drug discovery. In this context, establishment of an in vitro screening system reflecting in vivo toxicity is demanded for earlier safety assessment. We investigated LDH release and mitochondrial respiration (WST-1 reduction assay; WST-1) to detect cytotoxicity, morphological evaluation, and proteomics for estimating the reliable and sensitive biomarkers by using rat primary hepatocytes exposed to the compounds (acetaminophen, amiodarone, tetracycline and carbon tetrachloride) that are known to induce hepatotoxicity. In LDH release, no significant difference was detected between the control and compound exposed cells after exposure for 3 or 6 hr, but a dose-dependent increase was observed after exposure for 24 hr. Regarding the WST-1 assay, a dose-dependent reduction was detected after exposure for 6 and 24 hr to all of the compounds evaluated. In the proteomics analysis, 31 candidate proteins were identified from among the 103 demonstrating altered expression spots after exposure to acetaminophen. It was concluded that the cytotoxicity was detected earlier by measuring WST-1 than by measuring LDH release because the reduction of mitochondrial respiration is an expressions of earlier toxicity for cellular function, while the measured increase in the LDH release occurs after the failure of the cell membrane. Mitochondrial respiration ability was a useful parameter for cytotoxicity in in vitro hepato-toxicity screening, as cytotoxicity can be detected during the early stage of exposure. In addition to the conventional biomarkers, several protein biomarkers which relate to oxidative stress and metabolism-regulation were detected. Further comprehensive analysis of defined proteins would be necessary to estimate the more sensitive toxicology biomarker.

Advantages of in vitro cytotoxicity testing by using primary rat hepatocytes in comparison with established cell lines

We investigated and compared the cytotoxicity of 16 reference compounds in four in vitro systems: primary cultured rat hepatocytes, hepatoma HepG2 cell line, non-hepatic HeLa and Balb/c 3T3 cell lines. After 24 hr of exposure to the test compounds, the water-soluble tetrazolium salts WST-1 assay was used as an endpoint to evaluate cytotoxicity. Acetaminophen, diclofenac sodium cyclophosphamide and disulfiram displayed from 2 to more than 10 times higher IC50 values in three cell lines than in rat primary cultured hepatocytes. The cytotoxic effects of aspirin, amiodarone, clorfibiric acid, chlorpromazine, erythomycin, lithocholic acid, cisplatin and quinidine in rat hepatocytes were similar or 2 times stronger than those observed in cell lines. Ketoconazole resulted in the lowest IC50 value in the HeLa cell line. The data suggested that the compounds which are known to be metabolism-mediated liver toxicants have a differential hepatotoxicity in vitro and that primary cultured rat hepatocytes could represent a valuable tool for both screening and study of the effects of bio-transformation on the cytotoxicity of new chemical entities and xenobiotics in vitro.

Development of a high throughput in vitro toxicity screen predictive of high acute in vivo toxic potential

At an early stage of drug discovery high throughput screens are an invaluable tool to de-select compounds with undesirable properties. A high throughout in vitro toxicity screen has been developed and validated to identify compounds that have a high potential to be acutely toxic in vivo. This screen is based on treating Chinese hamster ovary (CHO) cells with test compounds for 24 h and then determining the degree of cytotoxicity by the reduction of Resazurin. Twenty-six structurally unrelated compounds were chosen that spanned a range of acute LD(50) values and mechanisms of toxicity. The acute LD(50) values (intraperitoneal and intravenous routes) from rat and mouse were taken from the RTECS database. Experimentally derived in vitro IC(35) results were compared to the 'most toxic' (lowest) LD(50) values for each compound. The resulting correlation was statistically significant (r=0.8475). However, due to the scatter of the data points, it was considered not appropriate to rank compounds according to their degree of in vivo toxicity on the basis of the in vitro result. However, by defining cut-off concentrations for both the in vivo (LD(50)) and the in vitro (IC(35)) values it was possible, using the in vitro result (IC(35) <10 microM), to identify compounds that had a high potential to be acutely toxic in vivo ('most toxic' LD(50) <25 micromol/kg). Further development led to a high throughput screen capable of giving a 'Yes', 'No' or 'Borderline' classification as to whether a compound has a high acute in vivo toxic potential. This screen is highly specific (no false positive classifications) and has a sensitivity of approximately 80%. This is deemed acceptable for a first tier toxicity screen at an early stage in the drug discovery process. Transfer of this screen from GlaxoSmithKline UK to sites in Italy, Spain and the USA resulted in very similar findings indicating the inter-laboratory robustness of this screen and therefore the ability to compare results across the GlaxoSmithKline sites.

Assessing organic toxic pollutants in fish-canning factory effluents using cultured fish cells

The fibroblastic RTG-2 fish cell line has been used to assess the cytotoxicity of organic compounds present in the effluents discharged by a canning factory. Organic compounds of the effluents were extracted and concentrated, and their cytotoxicity determined by measuring three end points: the cellular mass, the cell viability and the intracellular ATP content of the cells. Three out of the four organic concentrates were found to be highly cytotoxic, and these three toxic concentrates were fractionated, into 210 organic fractions, using an HPLC system. Up to six toxic fractions, present in all three organic concentrates, could be detected. Thus, when assessing the environmental effect of fish-canning factory effluents, the presence of specific toxic chemicals must be considered, in addition to toxicological problems due to fish-offal sewage.

The cytotoxicity of 50 chemicals from the MEIC study determined by growth inhibition of ascites sarcoma BP8 cells: a comparison with acute toxicity data in man and rodents

In this study, 50 chemicals selected on the basis of existence of particularly reliable human toxicity data were screened in a cytotoxicity test involving inhibition of the growth of Ascites Sarcoma BP8 cells. These test results are part of an international validation program, the Multicenter Evaluation of In Vitro Cytotoxicity (MEIC), the aim of which is to recommend batteries of in vitro tests to be used for prediction of human toxicity. The cytotoxicities (expressed as the concentrations causing 50% inhibition of cell growth) were compared to acute toxicity data in humans (LDL0) and rodents (LD50), showing the best correlation to rodent data. The results are discussed in relationship to what is usually referred to as basal cytotoxic mechanisms as a cause of in vivo toxicity. It could be concluded that the predicted results on the basis of mechanistic reasoning were not always obtained.

A Multicentre Study of Acute In Vitro Cytotoxicity in Rat Hepatocytes: Tentative Correlation Between In Vitro Toxicities and In Vivo Data

A multicentre validation study of the acute in vitro cytotoxicities of 31 liquid or solid chemicals was carried out by six laboratories, using primary rat hepatocyte cultures as a model system. We report here a comparison of neutral red uptake IC50 and LD50 values. Oral, i.p. and i.v. LD50 values were available for 27, 24 and 18 chemicals, respectively, and an IC50 value was obtained for 15, 14 and 11 of these compounds, respectively. A significant correlation was found only between IC50 and i.v. LD50 values.