Prediction phototoxicity

Lelievre, D., Justine, P Christiaens, F., Bonaventure, N., Coutet, J., Marrot, L. and Cotovio, J. (2007) The EpiSkin phototoxicity assay (EPA) development of an in vitro tiered strategy using 17 reference chemicals to predict phototoxic potency. Toxicology In Vitro An International Journal Published in Association with BIBRA,... 

There are three recommended protocols for assessing topical phototoxicity potential rabbit, guinea pig, and mouse. The first described here is that for the rabbit. The traditional methodology for a predictive test for phototoxicity has been an intact rabbit test (Marzulli and Maibach, 1970). This test is conducted as follows (and illustrated diagrammatically in Figure 11.2). 

H.-G. Holzhiitter. A general measure of in vitro phototoxicity derived from pairs of dose-response curves and its use for predicting the in vivo phototoxicity of chemicals. ATLA 25 445-462 (1997). 

We do not address early toxicology profiling in this section, that is, those phenotypic assays for which molecular targets are hardly known and which attempt to be predictive of the standard later stage assays, such as those covering genotoxicity, hepatotoxicity or phototoxicity. These are all covered elsewhere in this book. 

Only a limited number of reliable prediction tools are currently available for photoinduced toxicity. This is not surprising since establishing phototoxic potential is a complex task. Phototoxicity can be the consequence of various mechanisms such as photogeneration of reactive oxygen species, production of toxic photoproducts or sensitization of DNA damage by energy transfer. In addition, so far, there are no available universal descriptors (indicators) to predict the photodynamic potency of chemicals. 

The phototoxicity test 3T3 NRU was proposed in 1994 and is so far the only in vitro method that has been validated by European regulatory authorities for predicting the photoirritant potential of substances [5,40,41]. In this test, the mouse fibroblasts cell line Balb/c 3T3 is exposed to simulated solar UV (or, more frequently, solar UVA) in the presence of the test compound after an incubation of 1 h in the dark. Evaluation of cytotoxicity is performed 24h post-exposure using the neutral red uptake (NRU) method. N RU permits to distinguish live and dead cells, since intact cells retain this dye (detailed method in INVITOX protocol 78). The validation was performed with substances selected on the basis of their in vivo photoirritant or phototoxic properties. Some of these structures are shown in Table 19.1. 

Barratt, M.D. (2004) Structure-activity relationships and prediction of the phototoxicity and phototoxic potential of new drugs. Alternatives to Laboratory Animals, 32, 511-524. 

Analytical studies on the prediction of photosensitive/phototoxic potential of pharmaceutical substances. Pharmaceutical Research, 23, 156-164. 

An important method using fungi is Daniels test for phototoxicity, which utilizes the yeast Candida albicans as the test organism. A 1988 study compared favorably the results of this test with the results of photo-patch testing in volunteers for samples from six furocoumarin-containing plants. Many test materials which produce an erythemic response in the photoirritant test are not analyzed as positive in this test. A new test method, Solatex-pi, has demonstrated capability to predict the potential for photoirritation of materials in this class as well as that of other well-known photoirritants. Solatex-pi utilizes the two compartment physicochemical model of Skintex to predict the interactive effects of specific chemicals and UV radiation. Solatex-pi is being validated by Frame and the BGA (Zebet) as an in vitro test to predict photoirritants. 

In addition to gaps in knowledge for currently existing classes of chemicals and ecotoxicological effects, other mechanisms of actions that are currently not yet studied, or other processes, may require further studies. For example, recently it has become clear that phototoxic effects may be a realistic problem for polycylic aromatic hydrocarbons (PAHs) in aquatic and benthic organisms. The amount of UV-light which is required for phototoxicity, is thus an example of a parameter which was not introduced earlier as an important environmental parameter to describe or predict toxicity [148]. Other examples are if the internal effect concept can be used for metals and organometals in risk assessment [149,150]. 

Estrada, E. and Patlewicz, G. (2004) On the usefulness of graph-theoretic descriptors in predicting theoretical parameters. Phototoxicity of polycyclic aromatic hydrocarbons (PAHs). Croat. Chem. Acta, 77, 203-211. 

The quantum yield provides information about the effectiveness of a certain photo-induced process. The quantum yield of loss of starting material or product formation will provide valuable information about structure-activity relationships. Fluorescence and phosphorescence quantum yields will indicate the fraction of molecules likely to be found in the excited singlet and triplet state. The quantum yield is a useful parameter to predict the importance of a certain reaction for example, an isolated degradation product can have a long phosphorescence lifetime and should therefore be considered as a possible sensitizer. If the quantum yield of formation of this product is very low, however, it is less likely to be formed in a biologically active concentration and may therefore play a minor role in phototoxicity reactions. 

Pharmacokinetic aspects must be taken into account in cases in which the in vitro screening indicates that the compound is likely to cause phototoxic reactions in vivo. This includes information about in vivo metabolites and degradation products (if available). Phototoxicity is generally dose dependent, i.e., dependent on the concentration of the drug sensitizer and the intensity of the incident radiation at the site of action. Prior to a phototoxic reaction, the sensitizer must be distributed to tissues that are exposed to irradiation and further absorb the light that penetrates these tissues. If no pharmacokinetic data are available, the probability of being retained in a lipophilic medium can be predicted to some extent from the pKa and/or the log P value of the compound. 

Based on the theoretical considerations stated previously, it is relatively easy to predict that a drug cannot cause ocular damage through a photoinduced event. The short screen given in Table 11.1 will dramatically reduce the number of potential substances that need to be considered for ocular phototoxicity. 

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