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Genetic Toxicology: Photomutagenicity and Phototoxicity

PHOTOMUTAGENICITY AND PHOTOTOXICITY

Phototoxicity is a toxic response from a substance applied to the body which is either elicited or increased after subsequent exposure to light, or that is induced by skin irradiation after systemic administration of a substance.

The photosafety assessment of a pharmaceutical is an integrated process that can involve an evaluation of photochemical characteristics, data from nonclinical studies and human safety information. This information is used to determine  adequate risk minimization measures to prevent adverse events in humans.

International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH), has issued  draft  Guidelines ICH S10 on  Photosafety Evaluation of Pharmaceuticals. According to this guidelines, four different effects have been discussed in connection with photosafety testing: phototoxicity, photoallergy, photogenotoxicity and photocarcinogenicity.

For a chemical to demonstrate phototoxicity and/or photoallergy, the following characteristics are critical:

  • absorbs light within the range of natural sunlight (290-700 nm);
  • generates a reactive species following absorption of UV/visible light;
  • distributes sufficiently to light-exposed tissues (e.g., skin, eye). If one or more of these conditions is not met, a compound will not present a photosafety concern.

The in vitro 3T3 NRU phototoxicity test was shown to be predictive of acute phototoxicity effects in animals and humans in vivo and, typically, a negative result in this test does not require additional photosafety studies. In the case of a positive result photogenotoxicity testing may be recommended.

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