Combined in vitro assay for photohaemolysis and haemoglobin oxidation as part of a phototoxicity test system assessed with various phototoxic substances

In Vitro Alternatives and Phototoxicity Testing. II. Effects of Reactive Oxygen Species in In Vitro Phototoxicity Assays

The effects of reactive oxygen species (including singlet oxygen) in two in vitro phototoxicity assays — the 3T3 cell neutral red uptake phototoxicity (3T3 NRU PT) assay and the photohaemolysis assay — were assessed by using scavengers. Fifteen test substances, which had previously been shown to be phototoxic in vitro, were assessed. Eleven of these produced singlet oxygen. The major factor in the photodynamic reaction of bithionol was thought to be a Type I reaction, because bithionol did not produce singlet oxygen and did not react to histidine. Acridine was regarded as a Type II substance, because of the evident effect of histidine as a scavenger. 8-Methoxypsoralen and 5-methoxypsoralen produced singlet oxygen, but their actions were not affected by the scavengers. In this study, we confirmed that reactive oxygen species have great effects in in vitro phototoxicity, and that the 3T3 NRU PT assay can be used to detect effects which are thought to be the direct reaction of an excited photosensitiser to biological substrates (Type III reaction), for example, 8-methoxypsoralen. Therefore, we suggest that photohaemolysis and phototoxicity could be used to evaluate the photodynamic mechanisms of photosensitising chemicals.

In Vitro Alternatives and Phototoxicity Testing. I. Evaluation of In Vitro Phototoxicity Assays

Three in vitro phototoxicity assays — the photohaemolysis assay, the haemoglobin (Hb) photo-oxidation assay, and the 3T3 cell neutral red uptake phototoxicity (3T3 NRU PT) assay — were evaluated for use as screening methods in predicting the phototoxicity of test substances. Twenty-seven test substances, including cosmetic ingredients and drugs, were assessed in this study. The phototoxicity predicted in each assay was compared with that in guinea-pigs. In total, nine phototoxic substances used in this study could be detected by some of the in vitro phototoxicity assays. Eleven test substances regarded as non-phototoxic in vivo were predicted as non-phototoxic in the in vitro assays. Of the eight false positives revealed in some of the in vitro assays, five are believed to be photo-allergens. This suggests that in vitro phototoxicity assays might not be able to discriminate clearly between in vivo phototoxicity and in vivo photo-allergy, because the mechanisms in both processes are based on photodynamic reactions. The predictability of the three in vitro assays was comparatively good, but positive predictive values were low. The equivalence values of the photohaemolysis assay, the Hb photo-oxidation assay and the 3T3 NRU PT assay were 81%, 70% and 70%, respectively. From these results, we suggest that the three in vitro phototoxicity assays could be used as screens in predicting phototoxicity.

An In Vitro Phototoxicity Assay Battery (Photohaemolysis and 3T3 NRU PT test) to Assess Phototoxic Potential of Fragrances

The purpose of this study was to compare the in vivo and in vitro phototoxicity potentials of 13 fragrances. We used the 3T3 neutral red uptake phototoxicity (3T3 NRU PT) test and the photohaemolysis test as in vitro phototoxicity assays. In the 3T3 NRU PT test, all of the fragrances were non-phototoxic. Six fragrances were phototoxic in the photohaemolysis test. Three of the six photohaemolytic fragrances were phototoxic in the guinea-pig photoirritation test. These phototoxic fragrances did not cause cellular phototoxicity, but showed a photohaemolytic reaction. The photohaemolysis test was more sensitive than the 3T3 NRU PT test for screening for the phototoxicity of fragrances. The accuracy of this in vitro phototoxicity test battery was 82%. It is thought that the major phototoxic mechanism of fragrances is cell membrane damage. We suggest that a battery composed of the 3T3 NRU PT test and the photohaemolysis test is a simple and effective model for the in vitro phototoxicity assay of fragrances.

Dark and Photoinduced Cytotoxic Activity of the New Chlorophyll-a Derivatives with Oligoethylene Glycol Substituents on the Periphery of Their Macrocycles

In the present work, we investigated the dark and photoinduced cytotoxic activity of the new chlorophyll-a derivatives which contain the substituents of oligoethylene glycol on the periphery of their macrocycles. These compounds were tested using human cell lines to estimate their potential as photosensitizers for photodynamic therapy of cancer. It was shown that all the tested compounds have expressed photoinduced cytotoxic activity in vitro. Detailed study of the biological activity of one of the most perspective compound in this series—pyropheophorbide-a 17-diethylene glycol ester (Compound 21) was performed. This new compound is characterized by lower dark cytotoxicity and higher photoinduced cytotoxicity than previously described in a similar compound (DH-I-180-3) and clinically used Photolon^TM. Using fluorescent microscopy, it was shown that Compound 21 quickly penetrates the cells. Analysis of caspase-3 activity indicated an apoptosis induction 40 min after exposure to red light (λ = 660 nm). The induction of DNA damages and apoptosis was shown using Comet assay. The results of expression analysis of the stress-response genes indicate an activation of the genes which control the cell cycle and detoxification of the free radicals after an exposure of HeLa cells to Compound 21 and to red light. High photodynamic activity of this compound and the ability to oxidize biomolecules was demonstrated on nuclear-free mice erythrocytes. In addition, it was shown that Compound 21 is effectively activated with low energy 700 nm light, which can penetrate deep into the tissue. Thus, Compound 21 is a prospective substance for development of the new drugs for photodynamic therapy of cancer.

The red blood cell phototoxicity test (photohaemolysis and haemoglobin oxidation): EU/COLIPA validation programme on phototoxicity (phase II)

In the EU/COLIPA validation programme on "Photoirritation in vitro", two core tests and a number of mechanistically based tests were carried out to examine their suitability as regulatory tests for phototoxicity testing. In the meantime, one core test, the 3T3 neutral red uptake phototoxicity test (NRU PT) has been validated and has been accepted by ECVAM and the European Commission. The second core test, the red blood cell phototoxicity test (Photo-RBC test), has passed through a prevalidation process during this programme. This test protocol combines two endpoints, photohaemolysis and met-haemoglobin (met-Hb) formation. These endpoints are determined by measuring changes in the optical density of the haemoglobin spectrum at 525 nm and 630 nm, respectively. In addition, a prediction model was inserted into the Standard Operating Procedure (SOP) with two cut-off values: a photohaemolysis factor (PHF) > or = 3.0 for photohaemolysis, and a deltaOD(max) > or = 0.05 for met-Hb formation. Three laboratories agreed to implement the SOP and to perform the study by testing 30 selected test chemicals (25 phototoxicants and 5 non- phototoxic chemicals). The outcome of the study presents a good overall fit, including acceptable accuracy, sensitivity, and positive predictivity. The specificity and the negative predictivity are comparably low, due to the low number of non-phototoxic substances among the test chemicals. Further analysis of the data showed that the transfer of the SOP from between laboratories could have been more efficient. The results, especially of the lead laboratory, clearly indicate that an experienced laboratory can handle the SOP with high predictivity for phototoxicants and non-phototoxic substances. Finally, it was concluded that the combined Photo-RBC test can be considered as a second in vitro test, which can be used advantageously to obtain some mechanistic information, in particular on photodynamic effects on cellular proteins and biomembranes.

Validation of in vitro methods to single out photoirritants using mechanistically based tests

In the recent OECD draft proposal for a new guideline on acute dermal photoirritation testing, in vitro screening tests have been included as part of the sequential test strategy. These screening tests were placed directly prior to animal tests proposed. In Europe some in vitro techniques - cell culture and mechanistic tests - are under validation in a joint project of the European Center for the Validation of Alternative Methods (ECVAM) and the European Cosmetic, Toiletry, and Perfumery Association (COLIPA). Two promising cellular in vitro tests are presented and discussed as tool for the screening of photoirritancy. The first one as a general core test performed in each participating laboratory is the 3T3 mouse fibroblast Neutral Red Uptake Phototoxicity test determining the cell viability by uptake of Neutral Red as end point, whereas the second performed only by three participating laboratories was the the Red Blood Cell phototoxicity test comprising a combination of two end points, the photohaemolysis and the oxyhaemoglobin oxidation. Besides this, other mechanistic tests can be used as additional support. Identification of photoirritation is generally considered to be one area for the successful research and validation of in vitro techniques.