Phototoxicity and antipsoriatic effect of a topical methoxypsoralen solution in relation to the application time

The phototoxicity and antipsoriatic activity of a topically applied 0.075% methoxypsoralen alcohol-glycerol solution was investigated in relation to the interval between drug application and irradiation. On healthy skin, maximum UVA sensitivity for erythema development appeared 40 to 60 min after the application of the methoxypsoralen solution, but in psoriatic plaques the antipsoriatic UVA sensitization reached its maximum after 10 to 20 min. This difference between induction of antipsoriatic effect and erythema sensitivity may be explained by rapid penetration of methoxypsoralen in psoriatic lesions or by different targets at the cellular level for the erythema-producing effect and the antipsoriatic effect.