Phototoxicity: Its Mechanism and Animal Alternative Test Methods

Abstract

The skin exposure to solar irradiation and photoreactive xenobiotics may produce abnormal skin reaction, phototoxicity. Phototoxicity is an acute light-induced response, which occurs when photoreacive chemicals are activated by solar lights and transformed into products cytotoxic against the skin cells. Multifarious symptoms of phototoxicity are identified, skin irritation, erythema, pruritis, and edema that are similar to those of the exaggerated sunburn. Diverse organic chemicals, especially drugs, are known to induce phototoxicity, which is probably from the common possession of UV-absorbing benzene or heterocyclic rings in their molecular structures. Both UVB (290~320 nm) and UVA (320~400 nm) are responsible for the manifestation of phototoxicity. Absorption of photons and absorbed energy (hv) by photoactive chemicals results in molecular changes or generates reactive oxygen species and depending on the way how endogenous molecules are affected by phototoxicants, mechanisms of phototoxcity is categorized into two modes of action: Direct when unstable species from excited state directly react with the endogenous molecules, and indirect when endogeneous molecules react with secondary photoproducts. In order to identify phototoxic potential of a chemical, various test methods have been introduced. Focus is given to animal alternative test methods, i.e., in vitro, and in chemico assays as well as in vivo. 3T3 neutral red uptake assay, erythrocyte photohemolysis test, and phototoxicity test using human 3-dimensional (3D) epidermis model are examples of in vitro assays. In chemico methods evaluate the generation of reactive oxygen species or DNA strand break activity employing plasmid for chemicals, or drugs with phototoxic potential.

Keywords: Animal alternative test method; In vitro toxicology; Phototoxicity; Sunlight; UV.

Fig. 1.. Phototoxicity is initiated when photoactive or photoreactive chemicals are activated by UV lights. Photoreactivity is exemplified by photo-addition, production of reactive oxygen species (ROS). In final consequence, the reactions which derived by solar UV irradiation cause cytotoxicity against skin cells and, ultimately, skin irritation occurs.

Fig. 2.. Solar spectrum and irradiance on a clear day. (from Newport website, http://www.newport.com).

Fig. 3.. Two types of modes of phototoxicity: Direct and Indirect modes. Direct mode is when unstable phototoxic compound reacts with endogenous molecules directly. On the other hand, indirect mode is initiated when endogenous molecules bind to photoproducts of the phototoxic compound. Both modes result in the alteration of biological substrate, in which induces biological responses.

Fig. 4.. Photochemical reaction of representative phototoxicants: (A) Chlorpromazine, (adopted from Irene K.E. Kochevar (15)), (B) Avobenzone (From Sayre et al. (19)).

Fig. 5.. Commercial solar simulators: Newport, Suntest CPS+ or CPS (Atlas), SXL-2500V2 (Seric).

Fig. 6.. Phototoxicity and its evaluations: Spectral power distribution of a filtered solar simulator (adopted from OECD TG432 , %RCEE, Relative Cumulative Erythemal Effectiveness (27)).

Fig. 7.. Prediction model of Photocytotoxicity by PIF (Photo-irritation factor).

Fig. 8.. Photo effect calculation: The photo effect (PEC) at an arbitrary concentration C is defined as the product of the response effect (REC) and the dose effect (DEC), i.e. PEC = REC × DEC. The definition is illustrated as adopted from . Calculation of the photo effect at the concentration 0.4 follows the equations given in the text gives: response effect RE0.4 = (66% − 11%)/100% = 0.55, dose effect DE0.4 = (0.4/0.16 − 1)/(0.4/0.16 + 1) = 0.43, and photo effect PE0.4 = 0.24. The mean photo effect is obtained by averaging over the values for the photo effect at various concentrations .