Photoprotective effect of topical indomethacin--an experimental study.

Indomethacin applied after an erythematogenic dose of UVB light suppresses the development of erythema by inhibition of prostaglandin synthesis. In this study indomethacin was tested prior to UV exposure in humans in vivo and was shown to be a potent UVB and UVA filter. Indomethacin thus represents the first topical agent known to prevent and treat sunburn simultaneously. A 2.5% indomethacin solution was applied on test areas 1 h before UVB irradiation. Indomethacin-pretreated skin tolerated 4.3 times more UVB light and indomethacin-protected skin developed significantly less sunburn cells than untreated areas. In order to study its possible UVA screening capacity test areas were irradiated with 2, 4, 6, 8, 10 J/cm2 UVA light (PUVA) after ingestion of 8-methoxypsoralen. Indomethacin-pretreated skin tolerated 2.2 times more UVA than untreated test areas. Immediate pigment darkening induced by UVA doses up to 25 J/cm2 was less pronounced in indomethacin-pretreated skin. To investigate the mode of action of the photoprotective effect observed skin was irradiated with erythematous doses of UVB light transmitted through a quartz glass layer coated with the indomethacin solution. The skin developed no erythema thus indicating that the effect of indomethacin when applied before irradiation is based upon photoabsorption and not upon inhibition of prostaglandin synthesis. This is also confirmed by the fact that the development of sunburn cells can be prevented only when indomethacin is applied before and not after irradiation.