The emergence of sophisticated technologies in molecular and cellular biology has spurred a relatively rapid evolution in the life sciences, including toxicology. In effect, the focus of toxicology is shifting from whole animals and populations of whole animals to the cells and molecules of individual animals and humans. Since the mid-1980s, toxicologists have begun to employ these new methodologies in assessing the effects of chemicals on living systems. As a logical progression, such methods are being adapted for the purposes of toxicity testing. These scientific advances have worked together with social and economic factors to effect change in the evaluation of product safety and potential risk.

Economic factors are specifically related to the volume of materials that must be tested. A plethora of new cosmetics, pharmaceuticals, pesticides, chemicals and household products is introduced into the market every year. All of these products must be evaluated for their potential toxicity. In addition, there is a backlog of chemicals already in use that have not been adequately tested. The enormous task of obtaining detailed safety information on all of these chemicals using traditional whole animal testing methods would be costly in terms of both money and time, if it could even be accomplished.

There are also societal issues that relate to public health and safety, as well as increasing public concern about the use of animals for product safety testing. With regard to human safety, public interest and environmental advocacy groups have placed significant pressure on government agencies to apply more stringent regulations on chemicals. A recent example of this has been a movement by some environmental groups to ban chlorine and chlorine-containing compounds in the United States. One of the motivations for such an extreme action lies in the fact that most of these compounds have never been adequately tested. From a toxicological perspective, the concept of banning a whole class of diverse chemicals based simply on the presence of chlorine is both scientifically unsound and irresponsible. Yet, it is understandable that from the public’s perspective, there must be some assurance that chemicals released into the environment do not pose a significant health risk. Such a situation underscores the need for more efficient and rapid methods to assess toxicity.

The other societal concern that has impacted the area of toxicity testing is animal welfare. The growing number of animal protection groups throughout the world have voiced considerable opposition to the use of whole animals for product safety testing. Active campaigns have been waged against manufacturers of cosmetics, household and personal care products and pharmaceuticals in attempts to stop animal testing. Such efforts in Europe have resulted in the passage of the Sixth Amendment to Directive 76/768/EEC (the Cosmetics Directive). The consequence of this Directive is that cosmetic products or cosmetic ingredients that have been tested in animals after January 1, 1998 cannot be marketed in the European Union, unless alternative methods are insufficiently validated. While this Directive has no jurisdiction over the sale of such products in the United States or other countries, it will significantly affect those companies that have international markets that include Europe.

The concept of alternatives, which forms the basis for the development of tests other than those on whole animals, is defined by the three Rs: reduction in the numbers of animals used; refinement of protocols so that animals experience less stress or discomfort; and replacement of current animal tests with in vitro tests (i.e., tests done outside of the living animal), computer models or test on lower vertebrate or invertebrate species. The three Rs were introduced in a book published in 1959 by two British scientists, W.M.S. Russell and Rex Burch, The Principles of Humane Experimental Technique. Russell and Burch maintained that the only way in which valid scientific results could be obtained is through the humane treatment of animals, and believed that methods should be developed to reduce animal use and ultimately replace it. Interestingly, the principles outlined by Russell and Burch received little attention until the resurgence of the animal welfare movement in the mid-1970s. Today the concept of the three Rs is very much in the forefront with regard to research, testing and education.

In summary, the development of in vitro test methodologies has been influenced by a variety of factors that have converged over the last ten to 20 years. It is difficult to ascertain if any of these factors alone would have had such a profound effect on toxicity testing strategies.

Concept of In Vitro Toxicity Tests

This section will focus solely on in vitro methods for evaluating toxicity, as one of the alternatives to whole-animal testing. Additional non-animal alternatives such as computer modelling and quantitative structure-activity relationships are discussed in other articles of this chapter.

In vitro studies are generally conducted in animal or human cells or tissues outside of the body. In vitro literally means “in glass”, and refers to procedures carried out on living material or components of living material cultured in petri dishes or in test tubes under defined conditions. These may be contrasted with in vivo studies, or those carried out “in the living animal”. While it is difficult, if not impossible, to project the effects of a chemical on a complex organism when the observations are confined to a single type of cells in a dish, in vitro studies do provide a significant amount of information about intrinsic toxicity as well as cellular and molecular mechanisms of toxicity. In addition, they offer many advantages over in vivo studies in that they are generally less expensive and they may be conducted under more controlled conditions. Furthermore, despite the fact that small numbers of animals are still needed to obtain cells for in vitro cultures, these methods may be considered reduction alternatives (since many fewer animals are used compared to in vivo studies) and refinement alternatives (because they eliminate the need to subject the animals to the adverse toxic consequences imposed by in vivo experiments).

In order to interpret the results of in vitro toxicity tests, determine their potential usefulness in assessing toxicity and relate them to the overall toxicological process in vivo, it is necessary to understand which part of the toxicological process is being examined. The entire toxicological process consists of events that begin with the organism’s exposure to a physical or chemical agent, progress through cellular and molecular interactions and ultimately manifest themselves in the response of the whole organism. In vitro tests are generally limited to the part of the toxicological process that takes place at the cellular and molecular level. The types of information that may be obtained from in vitro studies include pathways of metabolism, interaction of active metabolites with cellular and molecular targets and potentially measurable toxic endpoints that can serve as molecular biomarkers for exposure. In an ideal situation, the mechanism of toxicity of each chemical from exposure to organismal manifestation would be known, such that the information obtained from in vitro tests could be fully interpreted and related to the response of the whole organism. However, this is virtually impossible, since relatively few complete toxicological mechanisms have been elucidated. Thus, toxicologists are faced with a situation in which the results of an in vitro test cannot be used as an entirely accurate prediction of in vivo toxicity because the mechanism is unknown. However, frequently during the process of developing an in vitro test, components of the cellular and molecular mechanism(s) of toxicity are elucidated.

One of the key unresolved issues surrounding the development and implementation of in vitro tests is related to the following consideration: should they be mechanistically based or is it sufficient for them to be descriptive? It is inarguably better from a scientific perspective to utilize only mechanistically based tests as replacements for in vivo tests. However in the absence of complete mechanistic knowledge, the prospect of developing in vitro tests to completely replace whole animal tests in the near future is almost nil. This does not, however, rule out the use of more descriptive types of assays as early screening tools, which is the case presently. These screens have resulted in a significant reduction in animal use. Therefore, until such time as more mechanistic information is generated, it may be necessary to employ to a more limited extent, tests whose results simply correlate well with those obtained in vivo.

In Vitro Tests for Cytotoxicity

In this section, several in vitro tests that have been developed to assess a chemical’s cytotoxic potential will be described. For the most part, these tests are easy to perform and analysis can be automated. One commonly used in vitro test for cytotoxicity is the neutral red assay. This assay is done on cells in culture, and for most applications, the cells can be maintained in culture dishes that contain 96 small wells, each 6.4mm in diameter. Since each well can be used for a single determination, this arrangement can accommodate multiple concentrations of the test chemical as well as positive and negative controls with a sufficient number of replicates for each. Following treatment of the cells with various concentrations of the test chemical ranging over at least two orders of magnitude (e.g., from 0.01mM to 1mM), as well as positive and negative control chemicals, the cells are rinsed and treated with neutral red, a dye that can be taken up and retained only by live cells. The dye may be added upon removal of the test chemical to determine immediate effects, or it may be added at various times after the test chemical is removed to determine cumulative or delayed effects. The intensity of the colour in each well corresponds to the number of live cells in that well. The colour intensity is measured by a spectrophotometer which may be equipped with a plate reader. The plate reader is programmed to provide individual measurements for each of the 96 wells of the culture dish. This automated methodology permits the investigator to rapidly perform a concentration-response experiment and to obtain statistically useful data.

Another relatively simple assay for cytotoxicity is the MTT test. MTT (3[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) is a tetrazolium dye that is reduced by mitochondrial enzymes to a blue colour. Only cells with viable mitochondria will retain the ability to carry out this reaction; therefore the colour intensity is directly related to the degree of mitochondrial integrity. This is a useful test to detect general cytotoxic compounds as well as those agents that specifically target mitochondria.

The measurement of lactate dehydrogenase (LDH) activity is also used as a broad-based assay for cytotoxicity. This enzyme is normally present in the cytoplasm of living cells and is released into the cell culture medium through leaky cell membranes of dead or dying cells that have been adversely affected by a toxic agent. Small amounts of culture medium may be removed at various times after chemical treatment of the cells to measure the amount of LDH released and determine a time course of toxicity. While the LDH release assay is a very general assessment of cytotoxicity, it is useful because it is easy to perform and it may be done in real time.

There are many new methods being developed to detect cellular damage. More sophisticated methods employ fluorescent probes to measure a variety of intracellular parameters, such as calcium release and changes in pH and membrane potential. In general, these probes are very sensitive and may detect more subtle cellular changes, thus reducing the need to use cell death as an endpoint. In addition, many of these fluorescent assays may be automated by the use of 96-well plates and fluorescent plate readers.

Once data have been collected on a series of chemicals using one of these tests, the relative toxicities may be determined. The relative toxicity of a chemical, as determined in an in vitro test, may be expressed as the concentration that exerts a 50% effect on the endpoint response of untreated cells. This determination is referred to as the EC₅₀ (Effective Concentration for 50% of the cells) and may be used to compare toxicities of different chemicals in vitro. (A similar term used in evaluating relative toxicity is IC₅₀, indicating the concentration of a chemical that causes a 50% inhibition of a cellular process, e.g., the ability to take up neutral red.) It is not easy to assess whether the relative in vitro toxicity of the chemicals is comparable to their relative in vivo toxicities, since there are so many confounding factors in the in vivo system, such as toxicokinetics, metabolism, repair and defence mechanisms. In addition, since most of these assays measure general cytotoxicity endpoints, they are not mechanistically based. Therefore, agreement between in vitro and in vivo relative toxicities is simply correlative. Despite the numerous complexities and difficulties in extrapolating from in vitro to in vivo, these in vitro tests are proving to be very valuable because they are simple and inexpensive to perform and may be used as screens to flag highly toxic drugs or chemicals at early stages of development.

Target Organ Toxicity

In vitro tests can also be used to assess specific target organ toxicity. There are a number of difficulties associated with designing such tests, the most notable being the inability of in vitro systems to maintain many of the features of the organ in vivo. Frequently, when cells are taken from animals and placed into culture, they tend either to degenerate quickly and/or to dedifferentiate, that is, lose their organ-like functions and become more generic. This presents a problem in that within a short period of time, usually a few days, the cultures are no longer useful for assessing organ-specific effects of a toxin.

Many of these problems are being overcome because of recent advances in molecular and cellular biology. Information that is obtained about the cellular environment in vivo may be utilized in modulating culture conditions in vitro. Since the mid-1980s, new growth factors and cytokines have been discovered, and many of these are now available commercially. Addition of these factors to cells in culture helps to preserve their integrity and may also help to retain more differentiated functions for longer periods of time. Other basic studies have increased the knowledge of the nutritional and hormonal requirements of cells in culture, so that new media may be formulated. Recent advances have also been made in identifying both naturally occurring and artificial extracellular matrices on which cells may be cultured. Culture of cells on these different matrices can have profound effects on both their structure and function. A major advantage derived from this knowledge is the ability to intricately control the environment of cells in culture and individually examine the effects of these factors on basic cell processes and on their responses to different chemical agents. In short, these systems can provide great insight into organ-specific mechanisms of toxicity.

Many target organ toxicity studies are conducted in primary cells, which by definition are freshly isolated from an organ, and usually exhibit a finite lifetime in culture. There are many advantages to having primary cultures of a single cell type from an organ for toxicity assessment. From a mechanistic perspective, such cultures are useful for studying specific cellular targets of a chemical. In some instances, two or more cell types from an organ may be cultured together, and this provides an added advantage of being able to look at cell-cell interactions in response to a toxin. Some co-culture systems for skin have been engineered so that they form a three dimensional structure resembling skin in vivo. It is also possible to co-culture cells from different organs—for example, liver and kidney. This type of culture would be useful in assessing the effects specific to kidney cells, of a chemical that must be bioactivated in the liver.

Molecular biological tools have also played an important role in the development of continuous cell lines that can be useful for target organ toxicity testing. These cell lines are generated by transfecting DNA into primary cells. In the transfection procedure, the cells and the DNA are treated such that the DNA can be taken up by the cells. The DNA is usually from a virus and contains a gene or genes that, when expressed, allow the cells to become immortalized (i.e., able to live and grow for extended periods of time in culture). The DNA can also be engineered so that the immortalizing gene is controlled by an inducible promoter. The advantage of this type of construct is that the cells will divide only when they receive the appropriate chemical stimulus to allow expression of the immortalizing gene. An example of such a construct is the large T antigen gene from Simian Virus 40 (SV40) (the immortalizing gene), preceded by the promoter region of the metallothionein gene, which is induced by the presence of a metal in the culture medium. Thus, after the gene is transfected into the cells, the cells may be treated with low concentrations of zinc to stimulate the MT promoter and turn on the expression of the T antigen gene. Under these conditions, the cells proliferate. When zinc is removed from the medium, the cells stop dividing and under ideal conditions return to a state where they express their tissue-specific functions.

The ability to generate immortalized cells combined with the advances in cell culture technology have greatly contributed to the creation of cell lines from many different organs, including brain, kidney and liver. However, before these cell lines may be used as a surrogate for the bona fide cell types, they must be carefully characterized to determine how “normal” they really are.

Other in vitro systems for studying target organ toxicity involve increasing complexity. As in vitro systems progress in complexity from single cell to whole organ culture, they become more comparable to the in vivo milieu, but at the same time they become much more difficult to control given the increased number of variables. Therefore, what may be gained in moving to a higher level of organization can be lost in the inability of the researcher to control the experimental environment. Table 1 compares some of the characteristics of various in vitro systems that have been used to study hepatotoxicity.

Table 1. Comparison of in vitro systems for hepatotoxicity studies

Precision-cut tissue slices are being used more extensively for toxicological studies. There are new instruments available that enable the researcher to cut uniform tissue slices in a sterile environment. Tissue slices offer some advantage over cell culture systems in that all of the cell types of the organ are present and they maintain their in vivo architecture and intercellular communication. Thus, in vitro studies may be conducted to determine the target cell type within an organ as well as to investigate specific target organ toxicity. A disadvantage of the slices is that they degenerate rapidly after the first 24 hours of culture, mainly due to poor diffusion of oxygen to the cells on the interior of the slices. However, recent studies have indicated that more efficient aeration may be achieved by gentle rotation. This, together with the use of a more complex medium, allows the slices to survive for up to 96 hours.

Tissue explants are similar in concept to tissue slices and may also be used to determine the toxicity of chemicals in specific target organs. Tissue explants are established by removing a small piece of tissue (for teratogenicity studies, an intact embryo) and placing it into culture for further study. Explant cultures have been useful for short-term toxicity studies including irritation and corrosivity in skin, asbestos studies in trachea and neurotoxicity studies in brain tissue.

Isolated perfused organs may also be used to assess target organ toxicity. These systems offer an advantage similar to that of tissue slices and explants in that all cell types are present, but without the stress to the tissue introduced by the manipulations involved in preparing slices. In addition, they allow for the maintenance of intra-organ interactions. A major disadvantage is their short-term viability, which limits their use for in vitro toxicity testing. In terms of serving as an alternative, these cultures may be considered a refinement since the animals do not experience the adverse consequences of in vivo treatment with toxicants. However, their use does not significantly decrease the numbers of animals required.

In summary, there are several types of in vitro systems available for assessing target organ toxicity. It is possible to acquire much information about mechanisms of toxicity using one or more of these techniques. The difficulty remains in knowing how to extrapolate from an in vitro system, which represents a relatively small part of the toxicological process, to the whole process occurring in vivo.

In Vitro Tests for Ocular Irritation

Perhaps the most contentious whole-animal toxicity test from an animal welfare perspective is the Draize test for eye irritation, which is conducted in rabbits. In this test, a small fixed dose of a chemical is placed in one of the rabbit’s eyes while the other eye is used as a control. The degree of irritation and inflammation is scored at various times after exposure. A major effort is being made to develop methodologies to replace this test, which has been criticized not only for humane reasons, but also because of the subjectivity of the observations and variability of the results. It is interesting to note that despite the harsh criticism the Draize test has received, it has proven to be remarkably successful in predicting human eye irritants, particularly slightly to moderately irritating substances, that are difficult to identify by other methods. Thus, the demands on in vitro alternatives are great.

The quest for alternatives to the Draize test is a complicated one, albeit one that is predicted to be successful. Numerous in vitro and other alternatives have been developed and in some cases they have been implemented. Refinement alternatives to the Draize test, which by definition, are less painful or distressful to the animals, include the Low Volume Eye Test, in which smaller amounts of test materials are placed in the rabbits’ eyes, not only for humane reasons, but to more closely mimic the amounts to which people may actually be accidentally exposed. Another refinement is that substances which have a pH less than 2 or greater than 11.5 are no longer tested in animals since they are known to be severely irritating to the eye.

Between 1980 and 1989, there has been an estimated 87% decline in the number of rabbits used for eye irritation testing of cosmetics. In vitro tests have been incorporated as part of a tier-testing approach to bring about this vast reduction in whole-animal tests. This approach is a multi-step process that begins with a thorough examination of the historical eye irritation data and physical and chemical analysis of the chemical to be evaluated. If these two processes do not yield enough information, then a battery of in vitro tests is performed. The additional data obtained from the in vitro tests might then be sufficient to assess the safety of the substance. If not, then the final step would be to perform limited in vivo tests. It is easy to see how this approach can eliminate or at least drastically reduce the numbers of animals needed to predict the safety of a test substance.

The battery of in vitro tests that is used as part of this tier-testing strategy depends upon the needs of the particular industry. Eye irritation testing is done by a wide variety of industries from cosmetics to pharmaceuticals to industrial chemicals. The type of information required by each industry varies and therefore it is not possible to define a single battery of in vitro tests. A test battery is generally designed to assess five parameters: cytotoxicity, changes in tissue physiology and biochemistry, quantitative structure-activity relationships, inflammation mediators, and recovery and repair. An example of a test for cytotoxicity, which is one possible cause for irritation, is the neutral red assay using cultured cells (see above). Changes in cellular physiology and biochemistry resulting from exposure to a chemical may be assayed in cultures of human corneal epithelial cells. Alternatively, investigators have also used intact or dissected bovine or chicken eyeballs obtained from slaughterhouses. Many of the endpoints measured in these whole organ cultures are the same as those measured in vivo, such as corneal opacity and corneal swelling.

Inflammation is frequently a component of chemical-induced eye injury, and there are a number of assays available to examine this parameter. Various biochemical assays detect the presence of mediators released during the inflammatory process such as arachidonic acid and cytokines. The chorioallantoic membrane (CAM) of the hen’s egg may also be used as an indicator of inflammation. In the CAM assay, a small piece of the shell of a ten-to-14-day chick embryo is removed to expose the CAM. The chemical is then applied to the CAM and signs of inflammation, such as vascular hemorrhaging, are scored at various times thereafter.

One of the most difficult in vivo processes to assess in vitro is recovery and repair of ocular injury. A newly developed instrument, the silicon microphysiometer, measures small changes in extracellular pH and can been used to monitor cultured cells in real time. This analysis has been shown to correlate fairly well with in vivo recovery and has been used as an in vitro test for this process. This has been a brief overview of the types of tests being employed as alternatives to the Draize test for ocular irritation. It is likely that within the next several years a complete series of in vitro test batteries will be defined and each will be validated for its specific purpose.

Validation

The key to regulatory acceptance and implementation of in vitro test methodologies is validation, the process by which the credibility of a candidate test is established for a specific purpose. Efforts to define and coordinate the validation process have been made both in the United States and in Europe. The European Union established the European Centre for the Validation of Alternative Methods (ECVAM) in 1993 to coordinate efforts there and to interact with American organizations such as the Johns Hopkins Centre for Alternatives to Animal Testing (CAAT), an academic centre in the United States, and the Interagency Coordinating Committee for the Validation of Alternative Methods (ICCVAM), composed of representatives from the National Institutes of Health, the US Environmental Protection Agency, the US Food and Drug Administration and the Consumer Products Safety Commission.

Validation of in vitro tests requires substantial organization and planning. There must be consensus among government regulators and industrial and academic scientists on acceptable procedures, and sufficient oversight by a scientific advisory board to ensure that the protocols meet set standards. The validation studies should be performed in a series of reference laboratories using calibrated sets of chemicals from a chemical bank and cells or tissues from a single source. Both intralaboratory repeatability and interlaboratory reproducibility of a candidate test must be demonstrated and the results subjected to appropriate statistical analysis. Once the results from the different components of the validation studies have been compiled, the scientific advisory board can make recommendations on the validity of the candidate test(s) for a specific purpose. In addition, results of the studies should be published in peer-reviewed journals and placed in a database.

The definition of the validation process is currently a work in progress. Each new validation study will provide information useful to the design of the next study. International communication and cooperation are essential for the expeditious development of a widely acceptable series of protocols, particularly given the increased urgency imposed by the passage of the EC Cosmetics Directive. This legislation may indeed provide the needed impetus for a serious validation effort to be undertaken. It is only through completion of this process that the acceptance of in vitro methods by the various regulatory communities can commence.

Conclusion

This article has provided a broad overview of the current status of in vitro toxicity testing. The science of in vitro toxicology is relatively young, but it is growing exponentially. The challenge for the years ahead is to incorporate the mechanistic knowledge generated by cellular and molecular studies into the vast inventory of in vivo data to provide a more complete description of toxicological mechanisms as well as to establish a paradigm by which in vitro data may be used to predict toxicity in vivo. It will only be through the concerted efforts of toxicologists and government representatives that the inherent value of these in vitro methods can be realized.

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