Opponents have long criticized animal-based skin tests as too variable because the responses observed can differ among individual animals. Results also often vary among labs, partly because the measures of the reactions are qualitative.
Others are concerned that adverse reactions identified in animals do not always reflect how humans may respond to the chemical exposure. "The rabbit is not a particularly good model for human irritation or corrosion, largely because the barrier properties in rabbit [skin] are far less robust that in the human," says John Sheasgreen, president of MatTek Corp., which manufactures one of the approved models. He explains that when his company compared results from chemical testing on their in vitro skin with available human and rabbit data, the "in vitro model correlates much better with the human experience than the rabbit experience."
The artificial skin models come from normal human skin cells, which are cultured in specialized media to form a three-dimensional reconstruction of the real thing. Sheasgreen explains that the in vitro skin closely resembles intact human skin both structurally and biochemically—it consists of multiple layers of cells and has a stratum corneum, the dead layer of cells on the surface that provides a protective barrier. These properties make it amenable for use in toxicity testing.
Proponents of the in vitro alternatives claim that they yield more reproducible and more quantitative results, because the response to irritancy is based on measures of cell viability. Furthermore, because the new tests are based on human cells, the data they provide may better represent how people would react.
In formally testing the alternatives, the European Center for the Validation of Alternative Methods (ECVAM) concluded that the models can reliably and accurately predict whether a chemical is an irritant.
According to the new OECD guideline, the in vitro skin can serve as a "stand-alone replacement test for in vivo skin irritation testing," depending on a country's hazard classification requirements. The regulatory system in E.U. countries, for example, classifies substances as corrosive, irritant or neither, which in vitro tests can adequately identify. By contrast, some regulatory authorities in some countries, such as the U.S. Environmental Protection Agency, require identification and labeling of an additional category of so-called "mild" irritants, which the in vitro tests cannot discern. Additional testing, such as animal tests or ethical trials on human volunteers, may be necessary to identify such low-level irritants.
The approval of the human skin models would not eliminate animal testing, explains Laurence Musset, principal administrator of the OECD Environment Directorate's Environment, Health and Safety Division. "The in vitro tests have a limited scope and don't work for all chemicals," Musset says, adding that each country's regulatory agencies will decide whether the in vitro or the animal test should be used for skin irritation studies.
OECD guidelines recommend using the human skin tests as part of a sequential testing strategy, in which in vitro methods are used before animals in combination with a weight-of-the-evidence (WoE) analysis of existing data on a substance or related chemical. In this testing strategy if a corrosive or irritant is identified in vitro, follow-up testing in animals is not necessary; if a substance tests negative in vitro, however, a WoE analysis may be used to back up that finding. Otherwise, in vivo testing may be necessary to rule out potential false negatives.
William Stokes, the executive director of the U.S. Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM), says his organization is currently evaluating the applicability of the in vitro irritation tests for chemical testing in this country. Stokes points out that the in vitro corrosion tests miss nearly 20 percent of known corrosives tested, which is worrisome because there is no clear strategy to follow when a substance tests negative.