IN VITRO VS. IN VIVO

The ultimate goal of toxicological studies is to ensure that newly developed substances, such as therapeutic drugs, agricultural or household chemicals, food additives, and cosmetics are safe for human, animal and environmental health. Traditionally, most of the available toxicity data comes from animal toxicity testing, but these tend to have significant limitations when translating them to human risk assessment. Despite their utility, commonly employed animal-to-human extrapolation of experimental results are still challenging, since animal models do not ideally reflect the fate and effect of a chemical in humans due to interspecies differences, different dosing schedules, inadequate statistical power, selection of outcome measures or follow-up duration etc. Thus, drugs showing safety and efficacy in preclinical animal models may show very different pharmacological properties when administered to humans. For example, corticosteroids are widely teratogenic in animals but not in humans, while thalidomide is not a teratogen in many animal species but it is in humans. Recent experience in a phase 1 study of the monoclonal antibody TGN 1412 resulted in life-threatening morbidity in all six healthy volunteers, reflecting inadequate prediction even in non-human primates of the human response.

The toxicity testing paradigm was introduced in 2005 by the U.S. EPA and the U.S. National Toxicology Program (NTP), the goal of which was to develop the area using advances in toxicogenomics, in vitro biology, computational sciences, and information technology, to rely increasingly on human data as opposed to animal data, and to offer increased efficiency in design and costs. In 2007, the US National Research Council released its expert panel report appealing for moving away from whole-animal testing to one founded primarily on in vitro methods that evaluate changes in biological processes using cells, cell lines, or cellular components, preferably of human origin with the emphasize on understanding of toxicity pathways (NRC-NAS, 2007).

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ADVANTAGES AND DISADVANTAGES OF IN VITRO TEST-SYSTEMS

Current advances in in vitro methodologies facilitate rapid hazard identification using high-throughput screening approach in mammalian cells cultured in monolayer 2D or biomimetic 3D structures. In vitro assays bring a number of technical advantages to the conventional method of testing substances on animal models, such as the ability to elucidate cellular-response networks and toxicity pathways; the use of concentrations relative to human exposure etc. In vitro 3D cultures that produce biomimetic models have been widely hailed as an advanced screening method by showing how substances influence in vivo cellular interactions. Data obtained using initial in vitro toxicity testing methods help to improve subsequent study design, which may significantly reduce the use of animals. 

Overall, in vitro model systems ensure tight control of chemical and physical environment during the experiment, enable to address the toxicity pathways issue, provide human relevant and high throughput data, and are relatively cheap and simple to procure. However, the application of in vitro methods as standalone approaches in risk assessment is still a problem regarding extrapolation the intact organism. A disadvantage is the absence of the in vivo biokinetics that may lead to a misinterpretation of in vitro data. It is possible that in the in vivo systems, the cells expose to the lower concentrations, since the chemical compound cannot simply reach the target cells. On the other side, drugs may accumulate in certain organs, tissues or cells, resulting in a prolonged or enhanced exposure of the target tissue. For this reason, using an in vitro system may result in an underestimation of the biological activity of the compound under study.

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COMMONLY USED IN VITRO TEST-SYSTEMS

Some basic toxicity testing using human cells in vitro is already a conventional method and often conducted to evaluate endpoints like cytotoxicity, protein binding, enzyme induction/inhibition, membrane permeability etc. In the field of toxicology of cosmetic ingredients, regulatory agencies accept in vitro tests for dermal absorption, irritation and corrosivity, as well as ocular irritation and corrosivity. Below is a list of common validated in vitro toxicology methods.

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