The photoisomerization of the sunscreen ethylhexyl p-methoxy cinnamate and its influence on the sun protection factor

The photoisomerization of the sun filter ethylhexyl p-methoxycinnamate (OMC) was studied under various conditions, (i) by irradiating diluted solutions using various solvents, (ii) by irradiating thin layers of concentrated solutions in non-volatile solvents, and (iii) by irradiating thinly applied commercial sunscreen products. The spectroscopic characteristics of the Z isomer were determined, along with its response factor in HPLC analysis. The ratio of the two isomers was measured under each condition of irradiation. The incidence of photoisomerization in the sun protection factor (SPF) was evaluated on the basis of calculations deduced from the concept of irregular film developed by O’Neill. The decrease in SPF, which can exceed 30%, is primarily dependent upon the rate of isomerization of OMC but also on the composition of sunscreen products. As isomerization of OMC is rapid and its rate unpredictable, it is in theory impossible to obtain good correlation between measurements of SPF in vivo and those in vitro, if measurements in vitro are not preceded by the process of irradiation precipitating isomerization of OMC.