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Phototoxicity (photoirritation) is defined as a toxic response that is elicited after the initial exposure of skin to certain chemicals and subsequent exposure to light, or that is induced by skin irradiation after systemic administration (oral, intravenous) of a chemical substance.

If a chemical absorbs UV or visible light, it needs to be determined if it is likely to cause adverse phototoxic effects when intended for human use.

The outcome of an international EU/ECVAM/COLIPA validation exercise (Spielmann et al., 1998) documented that the phototoxic potential of chemicals can be predicted by applying the in vitro 3T3NRU phototoxicity test (3T3-NRU-PT).

This assay consists of the immortalized mouse fibroblast cell line, Balb/c 3T3 and is based on a comparison of the cytotoxicity of a chemical when tested in the presence and in the absence of exposure to a non-cytotoxic dose of simulated solar light.

Cytotoxicity in this test is expressed as a concentration-dependent reduction of the uptake of the vital dye Neutral Red when measured 24 hours after treatment with the test chemical and irradiation (Borenfreund, 1985).

The test chemical together with the irradiation may alter the cell surface and in effect may result in a decreased uptake and binding of the Neutral Red Dye.

Differences in this uptake can be measured with a spectrophotometer, which allows in essence the distinction and quantification between viable, damaged or dead cells.

The 3T3-NRU-PT-assay gained regulatory acceptance in all EU Member States in 2000 and in the OECD Member States in 2004 as Test Guideline no 432. It is now widely used in the chemical and cosmetics industries.

EURL ECVAM validated test methods

Test methods currently under validation by EURL ECVAM

Development/optimisation/improvement of alternative methods