Visible light induces skin darkening in vivo: comparative pilot studies reveal enhanced susceptibility in melasma and its mitigation by a human tyrosinase inhibitor.

Melasma is a common and recurring disorder of hyperpigmentation, characterized by dark spots that are distributed on sun-exposed areas of the skin. Whereas studies on sun-induced skin pigmentation have concentrated on implications of ultraviolet (UV) radiation, the effect of visible light (VIS) was long considered negligible. Optically opaque, tinted face care products have shown to prevent darkening of melasma spots, leading to the hypothesis that VIS plays a role in hyperpigmentation. Here, we analyzed the effects of VIS on melasma, and investigated how skin reacts to the exposure of UV and VIS irradiation, both separately and combined. Furthermore, we examined the impact of daily treatment with the tyrosinase inhibitor isobutylamido thiazolyl resorcinol (Thiamidol) on repetitive VIS irradiation. We show that melasma lesions respond more strongly to VIS than perilesional skin and were more conspicuous after irradiation. Also, in healthy skin, VIS alone induced immediate pigment darkening and delayed tanning. However, compared to vehicle treatment, the tyrosinase inhibitor Thiamidol significantly reduced the VIS-induced darkening in healthy skin.