Abstract
We evaluated how the phototoxic effect of topical PUVA application was modified by the concentration of 8-methoxypsoralen (MOP), the manner of its application, and the time between its application and ultraviolet A (UVA) irradiation. After applying 8-MOP at various conditions, a dose of UVA equivalent to double the minimal phototoxic dose, which had been preliminarily determined in each subject by applying 8-MOP solution (0.3%) 2h before irradiation, was administered to the back of seven healthy subjects with type III skin. The induced erythema was evaluated 48h after irradiation using the erythema index read by a videomicroscopic image analysis. The results were as follows: i) The intensity of erythema was the strongest when 8-MOP (0.3%) had been applied 30 min-2h before irradiation, and then reduced to 50-70% of the maximal intensity when 8-MOP was applied 15 min or 4-6h before irradiation. Slight erythema only was detected in two subjects where 8-MOP had been applied immediately before irradiation, and no erythema was detected when 8-MOP had been applied 24h before irradiation. ii) The relationship between the concentration of 8-MOP and the intensity of erythema was not linear but sigmoidal with the intensity of erythema changing abruptly at concentrations of 0.03-0.1%. iii) The intensity of erythema was unchanged when 8-MOP (0.3%) was applied repeatedly to the same test site 2h before irradiation. Although the phototoxic and therapeutic effects of PUVA do not neccessarily coincide, these results appear to provide useful information for developing a strategy in the use of topical PUVA therapy for skin diseases with a normal barrier function, such as vitiligo.
