Development of assays for the detection of photomutagenicity of chemicals during exposure to UV light--I. Assay development

Insights and controversies on sunscreen safety

Although sunlight provides several benefits, ultraviolet (UV) radiation plays an important role in the development of various skin damages such as erythema, photoaging, and photocarcinogenesis. Despite cells having endogenous defense systems, damaged DNA may not be efficiently repaired at chronic exposure. In this sense, it is necessary to use artificial defense strategies such as sunscreen formulations. UV filters should scatter, reflect, or absorb solar UV radiation in order to prevent direct or indirect DNA lesions. However, the safety of UV filters is a matter of concern due to several controversies reported in literature, such as endocrine alterations, allergies, increased oxidative stress, phototoxic events, among others. Despite these controversies, the way in which sunscreens are tested is essential to ensure safety. Sunscreen regulation includes mandatory test for phototoxicity, but photogenotoxicity testing is not recommended as a part of the standard photosafety testing program. Although available photobiological tests are still the first approach to assess photosafety, they are limited. Some existing tests do not always provide reliable results, mainly due to limitations regarding the nature of the assessed phototoxic effect, cell UV sensitivity, and the irradiation protocols. These aspects bring queries regarding the safety of sunscreen wide use and suggest the demand for the development of robust and efficient in vitro screening tests to overcome the existing limitations. In this way, Saccharomyces cerevisiae has stood out as a promising model to fill the gaps in photobiology and to complete the mandatory tests enabling a more extensive and robust photosafety assessment.

Does Photosensitivity Predict Photococarcinogenicity?

Assessment of short-term and long-term effects of light (pho—totesting) is part of the safety evaluation of drugs. Results are incorporated into drug package inserts to advise patients and health care providers about the use of drug products on sun—exposed skin. We undertook an exhaustive literature search and a search of archived studies at the Center for Drug Evaluation and Research (CDER), Food and Drug Administration (FDA), in order to evaluate the potential of short—term photoassays to predict long—term effects of drugs used in sunlight (280–700 nm). The correlation between the findings from the photococarcinogenicity assays in mice that used exposure to simulated sunlight and those from photogenotoxicity and photosensitivity studies was examined. Results indicated that photosensitivity and photogenotoxicity assays did not necessarily predict effects in photococarcinogenicity studies in mice. Effects of drugs on skin that are not due to photoactivation of drug can be important factors in enhancement of UV—induced skin carcinogenesis.

The Relevance of Photomutagenicity Testing as a Predictor of Photocarcinogenicity

Today's lifestyle is associated with frequent and intense exposure to ultraviolet radiation (UVR). The tumorigenic effects of UVR are well known. Specifically, the premutagenic lesions of UVB (290-320 nm) are known to be the most important molecular events in UVR tumorigenicity. The less carcinogenic UVA (320-400 nm) mainly generates oxidative damage in the DNA via photody-namic generation of active oxygen species involving endogenous or exogenous photosen sitizers. Several pharmaceuticals are known to act as photosensitizers. Photoinstable phenothiazines, furocoumarins, and fluoroquinolones were shown to be very efficient inducers of chromosomal damage in mammalian cells in culture. Photocar cinogenicity testing in hairless mice of furocoumarins and several fluoroquinolones demonstrated a higher incidence and a shorter latent period for skin tumors compared to UVR alone. These data show a good correlation between the photomutagenic and photocarcinogenic potential of these compounds. Although mammalian cells possess effective repair mechanisms for oxidative damage, photoproducts, and dimers, these repair mechanisms can be overloaded. Eventually, unrepaired damage leads to gene mutations or chromosomal damage in exposed cells and to tumors in the skin. Therefore, in vitro photomutageni-city testing in mammalian cells may be an early and easy-to-measure predictor of the photocarcinogenic potential of a pharmaceutical.

The rise and fall of photomutagenesis

UV is the most abundant human carcinogen, and protection from extensive exposure to it is a widespread human health issue. The use of chemicals (sunscreens) for protection is intuitive and efficacious. However, these chemicals may become activated to reactive intermediates when absorbing energy from UV, thus producing damage themselves, which may manifest itself in phototoxic, photoallergenic or photocarcinogenic reactions in humans. The development of safe sunscreens for humans is of high interest. Similar issues have been observed for some therapeutically used principles such as PUVA therapy for psoriasis or porphyrins for phototherapy of human cancers. Photoactivation has also been reported as a side effect of various pharmaceuticals such as the antibacterial fluoroquinolones. In this context, the authors have been involved over more than 20 years in the development and refinement of assays to test for photomutagenicity as an unwanted side effect of UV-mediated activation of such chemicals for cosmetic or pharmaceutical use. The initial years of great hopes for simple mammalian cell-based assays for photomutagenicity to screen out substances of concern for human use were followed by many years of collaborative trials to achieve standardization. However, it is now realized that this topic, albeit of human safety relevance, is highly complex and subject to many artificial modifiers, especially in vitro in mammalian cell culture. Thus, it is not really suitable for being engineered into a general testing framework within cosmetic or pharmaceutical testing guidelines. Much knowledge has been generated over the years to arrive at the conclusion that yes, photomutagenicity does exist with the use of chemicals, but how to best test for it will require a sophisticated case-by-case approach. Moreover, in comparison to the properties and risks of exposure to UV itself, it remains a comparatively minor human safety risk to address. In considering risks and benefits, we should also acknowledge beneficial effects of UV on human health, including an essential role in the production of Vitamin D. Thus, the interrelationships between UV, chemicals and human health remain a fascinating topic of research.

An investigation of the photo-clastogenic potential of ultrafine titanium dioxide particles

Ultrafine titanium dioxide is widely used in a number of commercial products including sunscreens and cosmetics. There is extensive evidence on the safety of ultrafine titanium dioxide. However, there are some published studies indicating that some forms at least may be photogenotoxic, photocatalytic and/or carcinogenic. In order to clarify the conflicting opinions on the safety of ultrafine titanium dioxide particles, the current studies were performed to investigate the photo-clastogenic potential of eight different classes of ultrafine titanium dioxide particles. The photo-clastogenicity of titanium dioxide was measured in Chinese hamster ovary (CHO) cells in the absence and presence of UV light at a dose of 750 mJ/cm(2). The treatments were short (3 h) followed by a 17-h recovery and achieved concentrations that either induced approximately 50% cytotoxicity or reached 5000 microg/ml if non-cytotoxic. None of the titanium dioxide particles tested induced any increase in chromosomal aberration frequencies either in the absence or presence of UV. These studies show that ultrafine titanium dioxide particles do not exhibit photochemical genotoxicity in the model system used.

In vivo photochemical skin micronucleus test using a sunlight simulator: Detection of 8-methoxypsoralen and benzo[a]pyrene in hairless mice

Evaluating in vivo photochemical genotoxicity (photogenotoxicity) or photochemical carcinogenicity (photocarcinogenicity) in the skin that is actually exposed to light is important for estimating the risk of human exposure to chemicals under sunlight. With regard to the skin micronucleus test, Nishikawa et al. developed a reliable technique that is simple and in which the negative control has a stable background. In the present study, we applied 8-methoxypsoralen (8-MOP) and benzo[a]pyrene (B[a]P) to the backs of hairless mice and subjected the mice to irradiation by a sunlight simulator in order to investigate whether this test can detect photogenotoxicity of these chemicals. In the treatment with 8-MOP [0.00075% and 0.0015% (w/v)], a significant increase was observed in the frequency of micronucleated cells only under light irradiation using the sunlight simulator. At a high chemical dose, the frequency of micronucleated cells increased from 48h after the treatment, peaked at 96h, and then decreased at 168h. Furthermore, at 96h with the high dose under light irradiation, we frequently observed cells with nuclear buds. In the treatment with B[a]P [first experiment: 0.025% and 0.05% (w/v); second experiment: 0.005%, 0.01%, and 0.02% (w/v)], a significant increase was observed in the frequency of micronucleated cells at skin-irritating doses [0.01%, 0.02%, 0.025%, and 0.05% (w/v)] at 72 or 96h after the treatment only under light irradiation using the sunlight simulator. In conclusion, photogenotoxicity of 8-MOP and B[a]P was detected in the in vivo photochemical skin micronucleus study.

Octyl salicylate: a new contact sensitivity

We describe acute contact sensitivity to octyl salicylate, an ultraviolet filter, used in many cosmetics and sunscreens.

Use of the yeast Saccharomyces cerevisiae as a pre-screening approach for assessment of chemical-induced phototoxicity

Photoreactive chemicals can induce dermatological reactions when present in the skin exposed to sunlight. Thus, new chemicals absorbing above 290 nm should have their potential phototoxicity tested. In order to screen a large number of molecules with various physico-chemical properties, a microbiological method is helpful. To this end, the yeast Saccharomyces cerevisiae was evaluated for its ability to detect phototoxic compounds. Twelve products known to be phototoxic in vivo and previously used as standards for validating the regulatory test 3T3 NRU were used in this work. Eleven of them could be detected in the yeast assay and, among them, 5-methoxypsoralen (5-MOP), 8-methoxypsoralen (8-MOP), angelicin and, to a lower extend, tiaprofenic acid induced genetic alterations. Interestingly, a pre-incubation with yeast cells in the dark before exposure decreased the phototoxicity of 5-MOP and 8-MOP but had no effect on this of chlorpromazine and ketoprofen. Saccharomyces cerevisiae and Salmonella typhimurium (strains TA100 and TA102) were compared for the evaluation of 5-MOP and 8-MOP photogenotoxicity; only the yeast assay allowed to perform experiments in exposure conditions close to those encountered in environmental situations. Finally, an application of this experimental approach to the detection of traces of furocoumarins in fragrance materials was developed.

Photochemical genotoxicity: principles and test methods. Report of a GUM task force

In recent years, assessing the photogenotoxic potential of a compound became an issue for certain drugs and cosmetical products. Therefore, existing methods performed according to international guidelines (e.g. OECD guidelines) were adapted to the use of concurrent UV-visible (UV-Vis) light irradiation for the assessment of photomutagenicity/photogenotoxicity. In this review, photobiological bases of the processes occurring in the cell after irradiation with UV- and/or visible (vis)-light as well as a compilation of testing methods is presented. Methods comprise cell free investigations on naked DNA and in vitro methods, such as the photo-Ames test, the photo-HPRT/photo-mouse lymphoma assay (MLA), the photo-micronucleus test (MNT), the photo-chromosomal aberration test (CA) and the photo-Comet assay. A compilation of the currently available international literature of compounds tested on photogenotoxicity is given for each method. The state of the art of photogenotoxicity testing as well as the rational for testing are outlined in relation to the recommendations reached in expert working groups at different international meetings and to regulatory guidance papers. Finally, photogenotoxicity testing as predictor of photocarcinogenicity and in the light of risk assessment is discussed.

Photomutagenic properties of terfenadine as revealed by a stepwise photostability, phototoxicity and photomutagenicity testing approach

Administration of the second-generation antihistamine, terfenadine, is sometimes associated with photosensitivity and other skin reactions. To obtain information on its photoreactivity, we used a stepwise experimental approach involving tests for photostability, phototoxicity (PT) (mouse fibroblast cell line [3T3] neutral red uptake [NRU] test) and photomutagenicity (with standard Ames salmonella tester strains TA98, TA100 and TA102). Terfenadine was not phototoxic to cultured mammalian cells under the conditions used (i.e. 5000/161 mJ cm(-2) UVA-UVB). Natural sunlight and UV radiations caused considerable drug decomposition and formation of several photoproducts. Addition of the irradiated terfenadine solution (i.e. a mixture of photoproducts) to the tester did not significantly increase background mutation frequency. Irradiation of terfenadine coplated with the TA102 strain induced a clear-cut photomutagenic response, the magnitude of which was dependent upon the precursor compound concentration and the UV dose (212/7 to 339/11 mJ cm(-2) UVA-UVB). These findings demonstrate that in vitro terfenadine is photomutagenic in absence of PT. Further in vitro and in vivo studies are therefore needed to provide an adequate safety assessment of the photochemical genotoxicity--carcinogenicity potential of terfenadine. In the meantime, patients should be advised to avoid excessive exposure to sunlight.

Use of the photo-micronucleus assay in Chinese hamster V79 cells to study photochemical genotoxicity

Photochemical genotoxicity can be detected using appropriately adapted versions of most of the standard in vitro genotoxicity assays. The most sensitive approach to detect potentially photogenotoxic agents seems to be the investigation of DNA damage (DNA strand breakage, chromosomal aberrations, micronuclei) in mammalian cells in vitro. In a previous paper, we proposed the use of the micronucleus assay in Chinese hamster V79 cells for this purpose. This assay was found suitable to detect various photogenotoxic compounds with different photoactivation mechanisms. In order to extend the experimental experiences with this assay, we present here further data from a screening mode testing of 16 different potential photosensitizers. The photoclastogenic and photocytotoxic potential of the compounds was investigated concomitantly. So far, all substances detected in the photo-micronucleus assay as photogenotoxins also exhibited photocytotoxic properties but not vice versa. Among the compounds tested in the present study, tiaprofenic acid, 5-MOP, angelicin, nitrazepam, bendroflumethiazide, and dacarbazine were photogenotoxic and photocytotoxic. Further, 6-mercaptopurine, a metabolite of azathioprine was positive for both endpoints, whereas azathioprine was found negative. Azathioprine seems to be an example of a compound which lacks photo(geno)toxic properties in vitro but may be converted to a photosensitizer by enzymatical metabolization. With the results obtained in this study, the data base for the photo-micronucleus assay was extended to 35 compounds, which were tested using the same protocol and the same irradiation conditions. The photogenotoxicity results of all these compounds are summarized and discussed in correlation to their different photoactivation mechanisms, photocytotoxicity and photocarcinogenicity.

Photoclastogenicity-an improved protocol, its validation, and investigation of the photogenotoxicity of DMBA

An improved protocol was developed to detect light-induced clastogenic photoproducts in Chinese hamster ovary (CHO) cells. Dishes (60 mm) containing cells and the test material or vehicle control in 3 mL of phosphate-buffered saline were exposed to light using a SUNTEST CPS solar simulation unit. Importantly, cells were exposed at about 25 cm from the light source, thereby allowing a short exposure time of 2 min. With this exposure the assay was conducted with lids removed during the UV exposure with minimal risk of contamination. After preliminary experiments an exposure of 165.6 mJ/cm(2) UVA: 17.0 mJ/cm(2) UVB was selected for treatments with the different phototoxins. Under these exposure conditions about 10-15% aberrant cells were induced in vehicle control cultures with no or minimal cytotoxicity. The well-known photoclastogens 8-methoxypsoralen (8-MOP) and chlorpromazine (CLZ) were tested. In agreement with published data, 8-MOP and CLZ were clastogenic (lowest observed effect level, LOEL, was 0.0159 microg/mL and 1.03 microg/mL, respectively). In the absence of UV, 8-MOP was clastogenic at a much higher concentration (LOEL 251 microg/mL without UV vs. 0.0159 microg/mL with UV) while CLZ was negative up to a toxic concentration of 35 microg/mL. 7,12-Dimethylbenz[a]anthracene (DMBA), which is photomutagenic in bacteria, was clastogenic at > or =0.005 microg/mL with UV light (without S9) and at > or =2.53 microg/mL with S9 (without UV light). These results demonstrate the utility of the protocol for the detection of photoclastogenicity and expand the characterization of DMBA's photogenotoxic activity.

Photostability and phototoxicity studies on diltiazem

The photostability of diltiazem was studied in aqueous solutions at different pH values. Firstly, the hydrolysis of the drug to desacetyldiltiazem in alkaline medium was evaluated and then the drug photodegradation under exposure to UVA-UVB radiation (solar simulator) was monitored by HPLC methods. The main photoproduct was isolated and characterized as diltiazem-S-oxide on the basis of the NMR and mass spectra. The HPLC method was also applied to the selective analysis of diltiazem in commercial formulations. Tests on mutagenicity and photomutagenicity of the drug were also carried out using Salmonella typhimurium TA 102 strain. In this testing the drug neither was mutagenic nor toxic.

Studies on the photostability and in vitro phototoxicity of Labetalol

The purpose of this study was to obtain information on the photochemical and phototoxic properties of Labetalol, a beta-blocker drug. Preliminary information on the drug photoreactivity was achieved using a flow system with a photochemical reactor on-line with a diode array detection system. Photophysical and photochemical investigations on the drug were performed in aqueous solutions at different pH values using spectrophotometric and fluorimetric methods; the photodegradation quantum yield was found to be 2.7 x 10(-3) at pH 5.8 and 1.5 x 10(-2) at pH 11.5. Forced photodegradation of labetalol solutions under exposure to UVA--UVB radiations (xenon arc lamp) was monitored by reversed-phase liquid chromatography. The main photodegradation products were isolated and characterized by NMR and mass spectrometry; labetalol was found to give 3-amino-1-phenylbutane and salicylamide-4-carboxaldehyde as the main photoproducts. Preliminary phototoxic testings on human keratinocyte cultures were performed evaluating the viability of the cells by the neutral-red uptake assay; mutagenic and photomutagenicity tests were also carried out based on Salmonella typhimurium strains. As a result, labetalol was found to be photolabile,mainly in alkaline medium, but evidences of significant phototoxic and photomutagenic effects by the drug were not observed.

Induction of unscheduled DNA synthesis in hairless mouse epidermis by 8-methoxypsoralen plus ultraviolet A (PUVA)

Induction of unscheduled DNA synthesis (UDS) by 8-methoxypsoralen (8-MOP) plus ultraviolet A (UV-A) (PUVA) was investigated in the epidermis of female hairless mice by means of an in vivo--in vitro assay using a liquid scintillation counting method. Groups of three to five 8-week-old female hairless mice had 8-MOP applied once onto two areas of the back after stripping of the stratum corneum with adhesive tape to enhance skin penetration, and were irradiated with UV-A. Skin samples were taken and cultured in a medium containing [3H]thymidine with or without hydroxyurea (HU) for 2 hr. DNA of the epidermis was extracted, and the incorporation of [3H]thymidine into DNA and the DNA content were determined with a liquid scintillation counter and a fluorescence spectrophotometer, respectively. Induction of UDS was judged in terms of the UDS index [(the ratio of DNA synthesis in the presence of HU to that in its absence) x 100]. In a time-course study, the UDS index was increased at 1, 2 and 24 hr after 1 x 10(5) J/m2 UV-A irradiation with 0.001% 8-MOP, reaching the maximum level at 24 hr. In a dose-response study, it was significantly increased at the dose of 1 x 10(5) J/m2 of UV-A at 24 hr with 0.001% 8-MOP, but showed no significant change at the doses of 0.5 x 10(5), 2 x 10(5) and 4 x 10(5) J/m2. In a further study on the effect of varying the dose of 8-MOP, the UDS index was significantly increased at 0.001 and 0.002% 8-MOP at 24 hr after 1 x 10(5) J/m2 UV-A irradiation, reaching the maximum level with 0.002% 8-MOP. The increase of the UDS index in these studies was less than 3-fold. These results show that PUVA causes a small induction of UDS, which might be due to slow DNA excision repair over a long period.

Considerations on photochemical genotoxicity: report of the International Workshop on Genotoxicity Test Procedures Working Group

Recent toxicological observations have caused concern regarding the need to test, for example, pharmaceuticals and cosmetic products for photochemical genotoxicity. The objective of this report is to give assistance on how to adapt existing test methods to investigate the potential of light-absorbing compounds to induce genotoxic effects on photoactivation. In general, the Organization for Economic Co-Operation & Economic Development (OECD) draft guideline on in vitro phototoxicity testing served as a basis for consideration. Concomitant exposure of the cells to the test compound and solar simulated light was considered appropriate as the initial, basic test condition. Optimization of the exposure scheme, e.g., a change of the irradiation spectrum, might be indicated depending on the initial test results. Selection of test compound concentrations should be based on results obtained with the dark version of the respective test system but might have to be modified if phototoxic effects are observed. Selection of the irradiation dose has to be performed individually for each test system based on dose-effect studies. The irradiation should induce per se a small, reproducible toxic or genotoxic effect. The report includes a specification of necessary controls, discusses factors that might have an impact on the irradiation characteristics, and gives a rationale for the omission of an external metabolic activation system. It also addresses the question that physicochemical and pharmacokinetic properties might trigger the need to test a chemical for photochemical genotoxicity. Relevant experimental observations are presented to back up the recommendations. The working group did not reach a consensus as to whether a single, adequately perfomed in vitro test for clastogenicity would be sufficient to exclude a photogenotoxic liability or whether a test battery including a gene mutation assay would be needed for product safety testing regarding photochemical genotoxicity.

The application of the micronucleus test in Chinese hamster V79 cells to detect drug-induced photogenotoxicity

Recent reports on the photochemical carcinogenicity and photochemical genotoxicity of fluoroquinolone antibacterials led to an increasing awareness for the need of a standard approach to test for photochemical genotoxicity. In this study the micronucleus test using V79 cells was adapted to photogenotoxicity testing. Results of using different UVA/UVB relationships enabled us to identify a suitable irradiation regimen for the activation of different kinds of photosensitizers. Using this regimen, 8-methoxypsoralen and the fluoroquinolones lomefloxacin, grepafloxacin and Bay Y 3118 were identified to cause micronuclei and toxicity upon photochemical activation. Among the phenothiazines tested, chlorpromazine and 2-chlorophenothiazine, were positive for both endpoints, whereas triflupromazine was only slightly photoclastogenic in the presence of strong phototoxicity. Among the other potential human photosensitizers tested (oxytetracycline, doxycycline, metronidazole, emodin, hypericin, griseofulvin), only hypericin was slightly photogenotoxic. Photochemical toxicity in the absence of photochemical genotoxicity was noted for doxycycline and emodin. With the assay system described, it is possible to determine photochemical toxicity and photochemical genotoxicity concomitantly with sufficient reliability.

Photochemical genotoxicity and photochemical carcinogenesis--two sides of a coin?

The direct tumorigenic effects of ultraviolet radiation (UVR) are well known. Specifically, the premutagenic lesions of UVB (290-320 nm), are known to be the most important molecular events in UVR tumorigenicity. The less carcinogenic UVA (320-400 nm) mainly generates oxidative damage in the DNA via photodynamic generation of active oxygen species involving endogenous or exogenous photosensitizers. Several pharmaceuticals are known to act as photosensitizers. Photoinstable phenothiazines, furocoumarins and fluoroquinolones were shown to be very efficient inducers of chromosomal damage in UV-irradiated mammalian cells. Testing for photochemical carcinogenesis in hairless mice of furocoumarins and several fluoroquinolones resulted in a higher incidence and a shorter latent period for skin tumors compared to UVR alone. Overall, the correlation of experimental data between photochemical carcinogens and photochemical genotoxins is quite convincing. Therefore, testing for photochemical genotoxicity preferably in mammalian cells in vitro may be an easy hazard identification approach for photochemical carcinogens. However, further factors such as immunosuppression, irritation and dedifferentiation are to be considered for risk assessment in photochemical carcinogenesis.

Characterization of DNA damage inflicted by free radicals from a mutagenic sunscreen ingredient and its location using an in vitro genetic reversion assay

We describe an in vitro approach to assessing the potential genotoxicity of illuminated sunscreens. The photomutagenic sunscreen Padimate-O attacks DNA on illumination with simulated sunlight, producing strand breaks and lesions that are labile to N,N'-dimethylethylenediamine but few, if any, cyclobutane dimers or other direct photoproducts. The damage can be completely suppressed by the free radical quenchers Tris, ethanol, mannitol and dimethylsulfoxide, which is commonly used as a solvent in conventional photomutagenicity assays. Using a genetic reversion assay that depends on regenerating beta-galactosidase activity in photodamaged plasmids we find that GC base pairs are particularly susceptible to attack by Padimate-O.

Review of the genotoxic properties of chlorpromazine and related phenothiazines

Chlorpromazine and related phenothiazine drugs have been used in human and veterinary medications for more than 40 years, predominantly as psychotropic agents. Genotoxicity reports are in many cases of relatively antiquated test design. Overall there appears to be no genotoxic activity associated with these drugs when tested under standard conditions. Limited evidence for the potential to form mutagenic nitrosation products and some indication for the ability to modulate the genotoxic action of various mutagens have been presented in the literature. UV irradiation of chlorpromazine and other chlorinated derivatives produces reactive free radicals which possess DNA damaging properties. Induction of gene mutation and chromosomal aberrations have been observed in appropriately designed photomutagenesis experiments. Enhancement but also reduction of UV induced skin tumour formation by chlorpromazine have been found. The decisive factor for the discrepant actions has not been recognized. It is clearly advisable to avoid extensive UV exposure during therapy with these drugs.

Bacterial photomutagenicity testing: distinction between direct, enzyme-mediated and light-induced events

A bacterial photomutagenicity test system has been established according to a predetermined protocol using a light source emitting multiple wave lengths, including UV-A and UV-B, and the tester strains used for standard bacterial mutagenicity testing. 8-Methoxypsoralen (8-MOP) was used as a photomutagenic positive control showing no mutagenicity in the absence of light and borderline (Salmonella typhimurium TA1537) or clear (Salmonella typhimurium TA102 and Escherichia coli WP2) mutagenic effects in the presence of light. Using the same experimental conditions, the UV filters p-aminobenzoic acid (PABA), octyl dimethyl PABA (Eusolex 6007), and 4-methylbenzylidene camphor (Eusolex 6300) were non-mutagenic (in the absence of light) and non-photomutagenic (in the presence of light). Dihydroxyacetone (DHA), used as an active ingredient in self-tanning lotions, slightly increased the number of revertants in Salmonella typhimurium TA100 and TA102 in the absence of light. However, the relevance of these effects is equivocal, since they occurred at very high, cytotoxic concentrations (5000 micrograms/plate). Furthermore, these increases were not potentiated by light, thus demonstrating the non-photomutagenicity of DHA. In contrast, 7,12-dimethylbenz[a]anthracene (DMBA), which is non-mutagenic per se and is activated by external metabolizing systems (e.g., S9-mix) in the absence of light, was clearly mutagenic in Salmonella typhimurium TA98 and TA1537 in the presence of light (and the absence of S9-mix). Although the photomutagenic potency of DMBA, on a molar basis, was certainly lower than that of 8-MOP, the absolute mutagenic effects of DMBA in the respective bacterial strains were in a similar range under either S9-mix or photoactivation conditions. The strain specificity of the mutagenic effects were, however, different for either enzyme- or light-mediated mutagenicity. This indicates that different reactive intermediates are responsible for the mutagenicity in the tests using the two different activation systems. These results further suggest to use DMBA as a positive photomutagenic control compound alternatively to 8-MOP, since the latter is non- or only weakly photomutagenic in Salmonella typhimurium TA98 and TA1537. Furthermore, the usefulness and application of this photomutagenicity test system could be demonstrated for the testing of different photoabsorbing chemicals and cosmetic ingredients.

Sun exposure, sunscreens, and skin cancer prevention: a year-round concern

OBJECTIVE

To review the role of sunlight in skin aging and skin cancer formation, and to provide guidelines on the use of sunscreens to minimize the adverse effects of sun damage.

DATA SOURCES

A MEDLINE search of applicable articles on ultraviolet (UV) radiation, melanoma, sunscreens, and skin cancer, evaluating both human and animal studies. Published and unpublished original research as well as clinical experience were also used.

DATA SYNTHESIS

The interaction of UV radiation and skin type plays a central role in melanoma formation. Mortality from melanoma is highest in geographic locations near the equator, where UV intensity is greatest. The incidence of melanomas in light-complected individuals (skin types I-III) is several times higher than those with darker skin types (types IV-VI), even in similar geographic regions. The UVB portion of the spectrum appears to be primarily responsible for skin cancer formation and photoaging, while short wave UVA rays play a significant contributing role. Regular sunscreen use has been shown to reduce the formation of precancerous actinic keratoses (AK) lesions by 36%. A dose-response relationship has also been found between the amount of sunscreen used and AK formation.

CONCLUSIONS

Sunscreens have now been shown to reduce the carcinogenic effects of sunlight in humans. Patients should be advised of the long-term consequences of sun exposure and the benefits of regular sunscreen use.

The Galileo Data Bank on Toxicity Testing with In Vitro Alternative Methods. II. Toxicology Profiles of 20 Chemicals

The identification of the hazard of chemicals to man has relied on the use of several animal models. However, the availability of various cell toxicity models as alternatives to the use of animals has stimulated attempts to evaluate in vitro data for use in the prediction of human toxicity.

The cell toxicity models developed previously are capable of indicating a variety of endpoints susceptible to the activity of various chemical substances.

The in vitro data derived so far from testing a variety of types of chemicals, have been used to develop toxicology profiles for twenty chemicals, which are presented in this paper. Data have been selected from among those already entered in the Galileo Data Bank, a computerised data system containing all the available existing data derived using in vitro methods.

Photomutagenesis test development: I. 8-Methoxypsoralen, chlorpromazine and sunscreen compounds in bacterial and yeast assays

Two in vitro genotoxicity tests have been adapted to the evaluation of photomutagenic activity of test compounds. The study was initiated to obtain an experimental basis relating to newly proposed guidelines of the EC which request the screening of UV-absorbing compounds, for example, those employed in sunscreen preparations, for their photomutagenic potential. The well established photomutagens 8-methoxypsoralen and chlorpromazine were used to define relevant test protocols. The compounds were evaluated with the Ames test and the Saccharomyces cerevisiae D7 test for gene conversion. The influence of various parameters such as UV light sources, spectral composition, UV sensitivity of the test systems, absorbance by test materials and different exposure conditions is indicated. Two exemplary screening experiments with cosmetic ingredients are presented. Both test systems can be employed for the evaluation of compounds for photomutagenic activity although the standard excision-deficient strains of S. typhimurium pose problems because of their high UV sensitivity. The present experience in this complex field suggests that rigid test protocols and a restrictive test battery would be inadequate.

Photomutagenesis test development: II. 8-Methoxypsoralen, chlorpromazine and sunscreen compounds in chromosomal aberration assays using CHO cells

Chromosomal changes were analysed in Chinese hamster ovary (CHO) cells treated with 8-methoxypsoralen (8-MOP) or chlorpromazine (CPZ) and irradiated with either a UVA fluorescent tube (emission spectrum ranging from 350 to 400 nm) or a xenon burner (continuous emission spectrum simulating ambient sunlight). In the dark neither 8-MOP nor CPZ was genotoxic by itself. If these compounds were used in combination with UV irradiation the rate of chromosome aberrations was significantly increased. The magnitude of the clastogenic response was dependent on compound concentration and UV dose. The spectral composition also played an important role. Care must be taken to account for spectral changes caused, e.g., by passage of the light through the plastic lid of the container. The possible clastogenicity of two sunscreens was tested with two protocols: (1) cells attached to the culture dish were treated in presence of the sunscreen in the medium or (2) cells were irradiated through a layer of sunscreen solution as a filter. With this a clear UVB-absorbing effect and a decreased frequency of UVAB-induced chromosome aberration was evident with the UVB-absorbing compound Parsol HS but was absent, as expected, with the UVA-absorbing compound Parsol 1789. The presence of the sunscreens in the irradiated cell sample did not cause a significant increase in UV-induced chromosome aberrations.

Structure-activity relationships of tricyclic antidepressants and related compounds in the wing somatic mutation and recombination test of Drosophila melanogaster

Four antidepressants and one neuroleptic drug were tested for genotoxicity using the somatic mutation and recombination test (SMART) in wing cells of Drosophila melanogaster. Three-day-old larvae trans-heterozygous for two linked recessive wing hair mutations (multiple wing hairs and flare) were fed the test compounds in water or solvents mixed with a standard dry food for 48 h. Wings of the emerging adult flies were scored for the presence of spots of mutant cells which can result from either somatic mutation or mitotic recombination. The tricyclic antidepressant clomipramine, which is closely related to imipramine (previously shown to be genotoxic in somatic cells of Drosophila), was clearly genotoxic at concentrations above 10 mM. The structurally related antidepressants lofepramine and mianserin were positive only at 100 mM which is the maximum tolerated dose. The antidepressant maprotiline and the antipsychotic chlorpromazine, which are distinguished from the other compounds by a 6-membered central ring instead of a 7-membered one, were not genotoxic in the same dose range. These results lend further support for the hypothesis that an N atom in the heterocyclic 7-membered ring of the tricyclic molecule is responsible for the genotoxic property of the compounds in Drosophila.