PHOTOSTABILITY OF UV FILTERS 7 are significantly lower than the ones of the other filters in the five first tapes and significantly higher in strippings six through ten. DISCUSSION Chemical sunscreen agents are generally aromatic compounds that protect the skin from UV light by absorbing incident photons. This absorption can lead to irreversible or reversible photochemical reactions. In the first case, the filters are transformed in pho- toproducts that are usually not absorbing UV. In the second case, the molecules isom- erize. This latter reaction leads to a stationary equilibrium. The UV absorption is stabilized when this equilibrium is reached and the system stays efficient against UV radiation. Assessment of the photostability of sunscreen agents is a major concern for the deter- mination of the efficacy of a sunscreen (1), especially when irreversible photochemical reaction has occurred. Indeed, if the UV filters contained in a sunscreen are degraded by UV light, they become inefficient and the protection decreases (decreased SPF). Several publications describe the photochemical behaviour of UV filters after UV irra- diation (1). Most are in vitro methods, such as those described by Vanquerp et al. (5) and Schwarzenbach and Bertschi (6), who worked on sunscreen diluted in organic solvents. With the present in vivo method combining tape stripping with HPLC, the UVB filter 4-methylbenzilidene camphor was found photostable with 4% degradation after a 40- MED exposure, confirming the 4.4% obtained in vitro (results not shown) and suggest- ing that this filter remained efficient throughout the exposure. Concerning octyl me- thoxycinnamate, a loss of 42.3% was found with the in vivo technique (41.1% with the in vitro one). Therefore, it can be concluded that octyl methoxycinnamate is not stable under sunlight. In 1988 Deftandre and Lang (7) assessed the photostability of sunscreen agents by using quartz plates and a solar simulator. They performed a kinetic study based on spectrometric assessments, evaluating the evolution of different UV filters' absorption upon time exposure. This technique allowed them to differentiate reversible photoreactions from irreversible ones. With photo-unstable products they induced a constant decrease of the absorption spectrum with time exposure. Conversely, in the case of photoisomerizable compounds, they observed a degradation of the absorption at the beginning of the exposure followed by stabilization. Octyl methoxycinnamate and 4-methylbenzilidene camphor, two UVB filters tested in the present study, had this latter behavior, and the authors concluded that these filters were photostable. Using HPLC allowed the detection of photoproducts (appearance of new peaks) that could have been formed during the exposure. Indeed, a new peak appears on the chromatograph next to the peak of octyl methoxycinnamate, and another one (not very high) next to the peak of 4-methylbenzilidene camphor. These new peaks are probably the isomers of octyl methoxycinnamate and of 4-methylbenzilidene camphor. These results were already observed by Marginean Lazar et al. (8), who also work on UV-filter photostability with in vivo methods using the stripping method and a quantitative analysis of filters by high-performance liquid chromatography (HPLC). Several other studies have reported 4-methylbenzilidene camphor (9,10) and octyl methoxycinnamate (11) isomerization under UV light. Beck et al. (9) detected and separated the E •-• Z isomers of 4-meth- ylbenzilidene camphor using spectrophotometry, nuclear magnetic resonance (NMR),

8 JOURNAL OF COSMETIC SCIENCE and gas chromatography. Morliere eta/, (11) have also demonstrated the E •, Z pho- toisomerization of octyl methoxycinnamate by steady-state and laser-flash photolysis in aqueous and organic solutions. With their spectrometric assays Deftandre and Lang (7) assessed the absorbance of octyl methoxycinnamate. The absorbance reflects the activity of UV filters. If the absorbance doesn't show significant variation, it means that either the filter doesn't degrade or the photoproducts formed (isomer or another molecule) are as efficient as the initial filter, which is apparently the case for octyl methoxycinnamate as demonstrated in their experiment. It seems to be also the case in our study: even if there is a decrease of filter amount, the new product formed can be an isomer also active against UV radiation. In order to verify this hypothesis, an i, vitro test (results not shown) was performed to assess the efficacy of the sunscreen after 20 and 40 MED irradiations, using a modification of the draft protocol from the Colipa to test sunscreen photostability (12,13). The losses of UVB and UVA protection were found to be less than 10%. Thus octyl methoxycinna- mate is photo-unstable, but the octyl methoxycinnamate photoproduct stays efficient against UV radiation, in accordance with the results of Deftandre and Lang (7). Kammeyer eta/. (14) also assessed the photostability of sunscreen agents ], vitro using spectrometric assays on stratum corneum sheets. After applying sunscreen agents on stratum corneum sheets, they measured the absorption spectra before and after irradia- tion. By comparing the two spectra obtained, they calculated the percentage of alteration of each filter. Several UV filters were classified according to this percentage of degra- dation. They found that octyl methoxycinnamate and 4-methylbenzilidene camphor were moderately photostable, with percentages of alteration of 25% and 18%, respec- tively. In the present experiment, the percentage of degradation of octyl methoxycin- namate is the highest compared to the other filters in the preparation, as in the Kam- meyer eta/. study, but the percentage of degradation of 4-methylbenzilidene camphor is not as important. This difference could come from the solvent in which they diluted the filter for application to the stratum corneum sheets. They used liquid paraffin (non- polar) for 4-methylbenzilidene camphor, whereas we used a mix of methanol/chloroform. Moreover, their experiments did take into accounts the irregularity of the stratum corneum that could have lead to non-homogeneous spreading of the sunscreen. These factors have consequences on the spectrophotometric assays. It is interesting to note that the nature of the cosmetic vehicle in which UV filters are introduced and the presence of other filters influenced their photostability: two ], vitro studies, one based on HPLC assays (13) and the other on transmission spectroscopy assays (12), have previously demonstrated that the association of octyl methoxycinnamate and butyldibenzoylmeth- ane was very photo-unstable. Marginean Lazar eta/. (8,15) studied the photochemical behavior of butylmethoxydiben- zoylmethane, 4-methylbenzilidene camphor, and octyl methoxycinnamate in different emulsion bases using an •, v•vo method as in the present study. They applied sunscreen preparations on the backs of several volunteers and exposed the treated sites at various suberythemal doses. They collected ten sequential strippings within the same area irradiated, and they assessed, after extraction of the filters by toluene, the amount of filter (in the first tape strip and in the next sequential nine tape strippings) by HPLC assays. They also conducted a study without irradiation to determine the amount of filters collected at the skin surface. When stripping was conducted without irradiation, the entire theoretical amount of filters was recovered in the sum of the first and nine