Which is a commonly used phototoxicity assay?

• A. Ames test
• B. Draize test
• C. HET-CAM test
• D. 3T3 NRU phototoxicity test

Answer: (D)

Phototoxicity arises when a chemical becomes toxic only after exposure to light, usually UVA. The 3T3 NRU phototoxicity test is a standard in vitro assay designed to detect this risk. In this test, cultured 3T3 mouse fibroblasts are treated with the chemical both with and without UVA irradiation. Cell viability is measured by the neutral red uptake; viable cells incorporate this dye into lysosomes, so a decrease in dye uptake after irradiation indicates phototoxic potential. This approach is widely used because it is rapid, cost-effective, and has regulatory acceptance (OECD guidelines), all while avoiding animal testing. The other options do not assess phototoxicity: the Ames test screens for mutagenicity in bacteria, the Draize test evaluates dermal irritation in animals, and the HET-CAM test is an ocular irritation model.