Photo irritation factor

As previously described (Epigraph 3.2.1.4), the NR penetrates cell membranes and accu-mnlates intracellularly in lysosomes. Alterations of the cell surface or sensitive lysosomal membrane lead to a decreased uptake of the NR. To discriminate between photo-irritant and non-photo-irritant chemicals, the photo-irritation factor (PIF) was defined as the ratio of the IC50 values, determined in the absence of UVAand the presence of UVA (PlF=lC5o (-UV) IC50 (+UV)) (Spielmann et al, 1994, 1996). In a more sophisticated data analysis procedure, the mean photo-effect (MPE) is determined, which uses a complete comparison of the area under the curve AUC of the concentration response curves obtained with a chemical in the presence and absence of UV light (Peters and Holzhutter, 2002). 

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