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Toxicology Letters, 68 (1993) 91-99 Elsevier Science Publishers B.V. 91 TOXLET 02904 In vitro methodologies for enhanced toxicity testing N. J. DelRaso Toxicology Division, Occupational and Environmental Health Directorate, Armstrong Laboratory, Wright-Patterson AFB, OH (USA) Key words: In vitro; Methods; Hepatocyte; Screens SUMMARY This report will give a general overview of some of the in vitro methodologies used in toxicity testing. The use of computer-based structure-activity relationships and cell culture testing systems can provide valuable toxicological data for hazard and risk assessments. In vitro systems allow for a more rapid identification of toxic compounds and can be utilized to study mechanisms of toxicity at the cellular and subcellular level. The data derived from these types of studies can be used to improve the predictability of animal models for chemical or drug toxicity. This report focused on primary hepatocytes as an in vitro model for cytotoxicity and metabolic studies. INTRODUCTION Increased pressures from groups concerned with animal welfare has resulted in the need for alternatives to whole animal testing. However, substitution of in vitro meth- ods for in vivo techniques must not result in increased health risks to the public. With this in mind, researchers have taken a ‘3Rs’ approach to toxicity testing. The 3Rs are ‘reduction’ (the use of the least number of animals in toxicologic research that results in meaningful data), ‘refinement’ (the improvement of whole animal toxicologic re- search in such a way as to reduce or eliminate pain and discomfort), and ‘replace- ment’ (the use of alternative toxicologic testing methods that do not involve the intact animal). The status of this type of approach has been recently reviewed and animal use has been reduced by employing in vitro methods [l]. The process of hazard assessment may be envisioned as a series of steps that begins with structure-activity correlation and proceeds through physical and chemical analysis, short-term tests, screening procedures, animal studies, and finally human studies and risk assessment Correspondence to: N.J. DelRaso, Toxicology Division, Occupational and Environmental Health Director- ate, Armstrong Laboratory, Wright-Patterson AFB, OH 45433, USA.

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Toxicology Letters, 68 (1993) 91-99

Elsevier Science Publishers B.V. 91

TOXLET 02904

In vitro methodologies for enhanced toxicity testing

N. J. DelRaso

Toxicology Division, Occupational and Environmental Health Directorate, Armstrong Laboratory, Wright-Patterson AFB, OH (USA)

Key words: In vitro; Methods; Hepatocyte; Screens

SUMMARY

This report will give a general overview of some of the in vitro methodologies used in toxicity testing. The use of computer-based structure-activity relationships and cell culture testing systems can provide valuable toxicological data for hazard and risk assessments. In vitro systems allow for a more rapid identification of toxic compounds and can be utilized to study mechanisms of toxicity at the cellular and subcellular level. The data derived from these types of studies can be used to improve the predictability of animal models for chemical or drug toxicity. This report focused on primary hepatocytes as an in vitro model for cytotoxicity and metabolic studies.

INTRODUCTION

Increased pressures from groups concerned with animal welfare has resulted in the need for alternatives to whole animal testing. However, substitution of in vitro meth- ods for in vivo techniques must not result in increased health risks to the public. With this in mind, researchers have taken a ‘3Rs’ approach to toxicity testing. The 3Rs are ‘reduction’ (the use of the least number of animals in toxicologic research that results in meaningful data), ‘refinement’ (the improvement of whole animal toxicologic re- search in such a way as to reduce or eliminate pain and discomfort), and ‘replace- ment’ (the use of alternative toxicologic testing methods that do not involve the intact animal). The status of this type of approach has been recently reviewed and animal use has been reduced by employing in vitro methods [l]. The process of hazard assessment may be envisioned as a series of steps that begins with structure-activity correlation and proceeds through physical and chemical analysis, short-term tests, screening procedures, animal studies, and finally human studies and risk assessment

Correspondence to: N.J. DelRaso, Toxicology Division, Occupational and Environmental Health Director- ate, Armstrong Laboratory, Wright-Patterson AFB, OH 45433, USA.

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[2]. Based on this scheme of hazard assessment, in vitro methods can be incorporated as short-term tests and screens. Furthermore, in vitro systems can be utilized to give insight into mechanisms of chemical toxicity at the cellular level thereby allowing possible intervention with therapeutic or antidotal treatment.

The development of liver perfusion procedures to isolate viable rat hepatocytes [3,4] has led to their use in a wide range of toxicity and metabolism studies. Primary hepatocytes that maintain their in vivo characteristics make them ideal candidates for these types of studies. Therefore, it is not surprising to find that they have been the predominant cell type used in in vitro toxicology studies. Because the liver was the major target organ of many of the novel compounds screened in our laboratory, examples of in vitro/in vivo comparisons were presented using only the primary hepa- tocyte model for hazard assessment. However, it should be noted that accurate risk assessment will require information from batteries of in vitro assays performed on a number of different cell types from a number of different species.

ADVANTAGES AND DISADVANTAGES

The main objectives in toxicity testing are to identify hazardous material and to provide data estimating the quantitative exposure-response relationship for hazard- ous material in animals and ultimately in humans. The first of these objectives can be addressed by using in vitro screening systems, whereas the second can be addressed by using cell systems in mechanistic toxicity studies. There are a number of advantages to using these in vitro systems as alternatives to whole animals.

The obvious advantage in using in vitro testing methods over in vivo methods is a reduction in animal numbers and cost. Animal numbers are reduced because the ceils from a single animal can be used to conduct a complete experiment or set of experi- ments. Cost savings are realized as a result of the reduced animal requirements as well as the reduced maintenance costs in housing and feeding of animals. Another advan- tage of in vitro systems is that the cells used in experiments are of a uniform popula- tion, being derived from a single animal. This results in reduced experimental varia- bility. In vitro systems also have the added advantage of being able to assess toxicity of compounds of limited quantity because doses are typically in the range of micro- grams, or less, per milliliter instead of milligrams per kilogram body weight. Another advantage is that an in vitro system can be utilized to rapidly identify metabolites of test agents. Using whole cells, or microsomes, metabolites of chemicals can be identi- fied in a matter of hours instead of days using whole animals. Lastly, the use of in vitro systems will ultimately lead to better designed in vivo experiments. These sys- tems can be used to identify chemicals or drugs worthy of further in vivo study and to give an indication of an appropriate dose range to use in an animal study.

Although there are a number of advantages to using in vitro toxicity testing sys- tems, there are also some limitations. Because cells are usually derived from the digestion of a particular organ, these isolated cells have been removed from their normal heterogeneous cellular milieu. The loss of organ architecture and cell orienta-

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tion may result in cells responding differently to chemical or drug exposure than those observed in vivo. In addition, the digestion of a particular organ may result in altera- tion in cell membrane lipids and receptors of the isolated cell type, which can lead to erroneous conclusions in toxicity studies using these cells. Furthermore, these cells have been removed from their physiologic relationships with the intact animals circu- latory system. Therefore, factors such as chemical or drug absorption, distribution, and elimination can not be assessed in vitro.

Another limitation of in vitro systems is that cellular functions may become unsta- ble over time in culture. This is of critical importance when conducting biotransfor- mation studies with primary hepatocytes. It is well established that cytochrome P-450 (P450) content will decrease with time in culture [5]. Factors such as species and culture environment will determine the degree of P450 change [69]. However, cell function and longevity can be preserved using the appropriate culture medium and biomatrix substrate (collagen, laminin, fibronectin, etc.) for cell attachment. Al- though some of the in vitro limitations can be controlled, others cannot. One example is in the area of neurotoxicity. In vitro systems are not, as yet, capable of assessing behavioral effects such as pain, irritation, or coordination.

IN VITRO STUDIES OF METABOLIC MECHANISMS OF TOXICITY

In vitro studies of cellular metabolic mechanisms of toxicity give insight into the pathway(s) involved in chemically induced toxicity. In vitro systems are useful for dissecting cellular biochemical pathways of toxicity because the target cell can be studied directly without interference of other cell types or organ systems. Therefore, measurements of effects on specific cell functions, such as membrane integrity, mito- chondrial membrane integrity and function, cellular detoxification systems, metabol- ic capacity, and DNA damage, can be made directly. In addition, critical measure- ments of enzyme kinetics, that are often difficult or impossible to determine in vivo, can be made due to a more direct dose-response relationship. This data can be used in pharmacokinetic risk assessment models to generate more accurate predictions of risk. Furthermore, by correlating in vitro toxicity studies with in vivo toxicity studies, predictions concerning the toxicity in other species that possess similar biochemical pathways can be made.

Although microsomes can be used as an in vitro system to assess metabolism of chemicals or drugs, they may not accurately reflect the in vivo or intact cell situation [lo]. It has been shown that the metabolism profile generated from microsomes may not match that generated from whole animals or whole cells [l I]. This difference in metabolite profile results because microsome preparations lack cytosolic enzymes and cofactors that may be involved in the metabolism of certain compounds.

IN VITRO ASSAYS

Cytotoxicity assays are designed to detect alterations in cellular structure and func-

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tion that lead to irreversible cell damage. Because of the large number of biochemical pathways that exist in cells and the diversity of cell types, there are a multitude of biochemical indicators that can be chosen to study the effects of chemicals or drugs at the cellular, subcellular, and molecular level. A number of factors must be consid- ered when choosing an assay or method for use in an in vitro toxicity testing system. Species specificity, sensitivity, reproducibility, and reliability are key factors to con- sider when deciding on a particular assay or method. Furthermore, knowledge of the physical properties of the test agent and theoretical predictions of the biochemical pathways affected will also play a role in the decision of which cell type and assay system to use.

IN VITRO/IN VIVO CORRELATION EXAMPLE

ChlorotriJEuoroethylene oligomers Polychlorotrifluoroethylene (3.1 oil) is a prototype nonflammable hydraulic fluid

being evaluated for use in both military and commercial aircraft. This hydraulic fluid is composed of a mixture of chlorotrifluoroethylene (CTFE) oligomers, predominant- ly C, (trimer) and C, (tetramer) chain length.

In vivo inhalation and oral gavage studies with this 3.1 oil indicated that the liver was the principal target organ for toxicity [ 12,131. Furthermore, the oral gavage study also showed that the tetramer CTFE oligomer was more toxic than the trimer CTFE oligomer component of 3.1 oil as evidenced, in part, by significant body weight loss in orally dosed male Fischer 344 (F-344) rats over 14 days (Fig. 1). However, 3.1 oil was not found to be toxic in orally dosed Rhesus (Macaca mulutta) monkeys [14]. Differences between 3.1 oil-exposed rats and primates, with respect to liver per- oxisomal /?-oxidation rates and peroxisome number, were also found. Rats orally dosed with 3.1 oil indicated significant increases in peroxisome number and rates of /?-oxidation when compared to 3.1 oil-exposed primates (Table I).

In another study, evidence that the trimer and tetramer CTFE oligomers were metabolized to their corresponding oligomer carboxylic acids was provided [1.5]. These acids were later identified in male F-344 rats by gas chromatography/mass spectroscopy [16]. An oral gavage study using the trimer and tetramer CTFE acids has also shown the tetramer CTFE acid to be more hepatotoxic than the trimer CTFE acid [17].

In vitro results In vitro dose-response studies with the trimer and tetramer CTFE carboxylic acids,

using primary rat hepatocytes, also indicated that the tetramer CTFE acid was more toxic than the trimer CTFE acid as determined by significantly increased lactate dehydrogenase enzyme leakage when compared to control (Fig. 2). This result corre- lates very well with the previous in vivo study using the CTFE acids mentioned above. Furthermore, rat hepatocytes exposed to the CTFE acids resulted in increased rates of peroxisomal p-oxidation, whereas exposed primate hepatocytes did not (Fig. 3).

95

230

225

220

215

;; 3 210 a

r 205 3

D 200

195

190

185

180

-o- SALINE s--o-. 3.1Ca - TRIMER

dP

/O-o/

P

---o-- 3.1CaCs . ..o... TETRAMER P'

-1 0 1 2 3 4 5 6 7 8 9 1011121314 TIME (DAYS)

Fig. 1. Effect of 14 days of repeated oral dosing with CTFE oligomer-based compounds on body weight gain. (a) P < 0.05 as determined by the Bonferroni multiple comparison test when compared to saline

control. (b) P < 0.05 when compared to saline control, 3.1 oilC6, and trimer. Data taken from [13].

This data also correlates well with that found in in vivo studies mentioned above with rats and primates exposed to 3.1 oil.

Interestingly, primary rat hepatocytes exposed to noncytotoxic doses of the trimer

TABLE I

IN VIVO PEROXISOMAL j&OXIDATION IN PRIMATE AND RAT LIVER FOLLOWING EXPO- SURE TO POLYCTFE 3.1 OIL FOR 2 WEEKS

Treatment Mean peroxisome count B-Oxidation activity (pmol/min/g)

Rat saline 10.2 f 3.6 5.6 f 2.6

3.1 oil 17.9 f 7.8* 10.2 f 1.2*

Primate saline 2.5 f 0.3 3.0 + 0.8 3.1 oil 3.3 f 0.4 4.1 f 2.8

*Significant at P < 0.05 by ANOVA.

96

1200

1000

800

600

400

200

i

r

a-

O-

\ 1

0 Tetramer CTFE Acid

0 Trimer CTFE Acid

~*_.________i______________._.__.‘.._~ I I

0.0 0.5 1 .o

Concentration (mM)

15

Fig. 2, Dose-response curves for the trimer and tetramer CTFE oligomer acids in 24 h primary rat hepato- cyte cultures as determined by lactate dehydrogenase (LDH) enzyme leakage. Only control cells exhibiting ~25% of the total int~~ellular LDH were considered valid experiments. Data points represents the average

from three experiments and triplicate plates.

C6 C8

Treatment

Fig. 3. Peroxisomal &oxidation activities in primary rat and primate hepatocytes after exposure to trimer and tetramer CTFE acids. Bars represent the average from three experiments and triplicate plates. CAT; carnitine acetyltransferase. N.M. (not measurable); peroxisomal #?-oxidation of palmitoyl CoA could not be determined in either control rat or primate hepatocytes under experimental conditions. Only rat hepato- cytes exposed to the CTFE acids resulted in measurable rates of p~mitoyl CoA oxidation. *Si~ificantly

different than control at P c 0.05 using multifa~to~~ ANOVA.

97

CTFE acid indicated a significant increase in P450 activity and no effect on cell glutathione (GSH) levels (Fig. 4). In contrast, hepatocytes exposed to the tetramer CTFE acid indicated no effect on P450 activity and exhibited significantly reduced GSH levels. This data indicates that P450 activity and GSH level may play important roles in the determination of CTFE acid hepatotoxicity, and demonstrates how in vitro systems can be used to give insight into possible mechanisms of toxicity.

CONCLUSION

The example of in vitro/in vivo correlation outlined above using primary hepato- cytes exposed to CTFE oligomer acids is just one example of how primary cells can be utilized in toxicology. Recent advances in tissue culture techniques have allowed for the establishment of primary cell cultures from many different organs. Again, it should be noted that no one cell type will substitute for whole animal studies. To make predictions concerning toxicity with a high degree of confidence will require batteries of in vitro assays using multiple cell types. In addition, the advantages and disadvantages of in vitro systems must be recognized before they can be effectively

250

200

5 150

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x 100

50

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’ ORat P450’ mRat GSH @Primate GSH

C6 CTFE

Treatment

CB CTFE

Fig. 4. GSH levels and P450 activity in primary rat and primate hepatocytes after exposure to trimer and tetramer CTFE acids. Rat and primate hepatocytes were dosed with the CTFE acids at -0.1 mM for 72 h and 96 h, respectively. Bars for rat hepatocytes represent results from a typical experiment with triplicate plates repeated three times. Control values for rat hepatocyte P450 activity and GSH level ranged from 0.25 to 0.52 mnol/min/mg and 10 to 31 nmol/mg, respectively. Bar for primate hepatocytes represents results from a single experiment with triplicate plates. Control primate hepatocyte GSH level was 8.0 f 0.7 nmol/mg. Primate hepatocyte P450 activity was not determined. *Significantly different from control at P < 0.05

using multifactorial ANOVA.

98

utilized in toxicity studies. In vitro methods must be properly validated to achieve maximal utility in these type of studies. The process of validation for in vitro methods that can be used in toxicity testing has been described [ 181.

It is not realistic, at this time, to expect that in vitro methods will totally replace the whole animal. However, the use of in vitro systems to reduce animal numbers and refine in vivo experiments have been documented [ 11. There are a number of integrat- ed interrelationships between structure, function, and behavior in toxicology that will require whole animal studies. Some examples include: measurements of neurobehav- ioral effects of toxicants, strain and species differences with respect to metabolic fate of test compounds, the role of intestinal flora and enterohepatic circulation in influ- encing metabolic pathways of test compounds, and differential distribution of xenobi- otic biotransformation due to organ architecture and cell polarity. It is conceivable that advancements in in vitro technology may some day be capable of addressing the problems of integrated interrelationships in toxicology and result in the total replace- ment of animals.

ACKNOWLEDGEMENTS

Animals used to provide data in this report were handled in accordance with the principles stated in the Guide for the Care and Use of Laboratory Animals, prepared by the Committee on Care and Uses of Laboratory Animals of the Institute of Labo- ratory Animal Resources, National Research Council, DHHS, National Institute of Health Publication No. 85-23, 1985, and the Animal Welfare Act of 1966, as amended.

REFERENCES

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2 Golberg, L. (1986) Charting a course for cell culture alternatives to animal testing. Fundam. Appl. Toxicol. 6, 607.

3 Berry, M.N. and Friend, D.S. (1969) High-yield preparation of isolated rat liver parenchymal cells. J. Cell Biol. 43, 506.

4 Seglen, P.O. (1976) Preparation of isolated rat liver cells. Methods Cell Biol. 13, 29. 5 Guzelian, P.S., Bissell, D.M. and Meyer, U.A. (1977) Drug metabolism in adult rat hepatocytes in

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7 Maslansky, C.J. and Williams G.M. (1982) Primary cultures and the levels of cytochrome P-450 in hepatocytes from mouse, rat, hamster and rabbit liver. In Vitro 18,683.

8 Paine, A.J. and Hockin, L.J. (1980) Nutrient imbalance causes the loss of cytochrome P-450 in liver cell culture: formulation of culture media which maintain cytochrome P-450 in rat liver cell culture. Bio- them. Pharmacol. 31, 1175.

9 Dickins, M. and Peterson, R. (1980) Effects of a hormone-supplemented medium on cytochrome P-450

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content and mono-oxygenase activities of rat hepatocytes in primary culture. Biochem. Pharmacol. 29, 1231.

10 Benford, D.J. (1987) Biological models as alternatives to animal experimentation. ATLA 14, 318. 11 Bisgaard, H.C. and Lam, H.R. (1989) In vitro and in vivo studies on the metabolism of 1,3-diaminoben-

zene: comparison of the metabolites formed by the perfused rat liver, primary hepatocyte culture, hepatic microsomes and the whole rat. Toxicol. In Vitro 3, 167.

12 Kinkead, E.R., Kimmel, EC., Wall, H.G., Kutzman, R.S., Conolly, R.B., Whitmire, R.E. and Flem- ming, C.D. (1990) Subchronic inhalation studies of polychlorotrifluoroethylene (3.1 oil). Inhal. Toxicol. 2,431.

13 DelRaso, N.J., Godin, C.S., Jones, C.E., Wall, H.G., Mattie, D.R. and Flemming, C.D. (1991) Com- parative hepatotoxicity of two polychlorotrifluoroethylene (CTFE) oligomers in male Fischer 344 rats. Fundam. Appl. Toxicol. 17, 550.

14 Jones, C.E., Ballinger, M.B., Mattie, D.R., DelRaso, N.J., Seckel, C. and Vinegar, A. (1990) Effects of short-term oral dosing of polychlorotrifluoroethylene (PolyCTFE) on the rhesus monkey. J. Appl. Toxicol. 11, 51.

15 DelRaso, N.J., Auten, K.L., H&man, H.C. and Leahy, H.F. (1991) Evidence of hepatic conversion of C, and Cs chlorotrifluoroethylene (CTFE) oligomers to their corresponding CTFE acids. Toxicol. Lett. 59,41.

16 Brashear, W.T., Greene, R.J. and Mahle, D.A. (1992) Structural determination of the carboxylic acid metabolites of polychlorotrifluoroethylene. Xenobiotica 22, 499.

17 Kinkead, E.R., Bunger, S.K., Wolfe, R.E., Flemming, C.D., Whitmire, R.E. and Wall, H.G. (1991) Repeated-dose gavage studies on polychlorotrifluoroethylene acids. Toxicol. Indust. Health 7, 295.

18 Balls, M., Blaauboer, B., Brusick, D., Frazier, J., Lamb, D., Pemberton, M., Reinhardt, C., Roberfroid, M., Rosenkranz, H., Schmid, B., Spielmann, H., Stammati, A.-L. and Walum, E. (1990) Report and recommendations of the CAATlERGATT workshop on the validation of toxicity test procedures. ATLA 18,313.