Kinetics of photosensitivity in bath-PUVA photochemotherapy

Background: Bath-PUVA is used to treat a variety of dermatoses. However, the kinetics of 8-methoxypsoralen during treatment are not completely clarified. Objective: The purpose of this study was to investigate the intensity of the phototoxic response and the persistence of phototoxicity after bath-PUVA. Methods: Twelve volunteers were exposed to UVA doses ranging from 0.5 to 40 J/cm2 from 10 to 240 minutes after bath-PUVA treatment. The resulting phototoxic response of the skin was determined. Results: Irradiation 10 minutes after the psoralen bath led to the lowest assessed minimal phototoxic dose (MPD) of 1.42 J/cm2 (mean, SD ± 0.29). Thereafter, the MPD increased significantly and sharply every hour. At 4 hours after the psoralen bath, UVA doses up to 40 J/cm2 failed to induce any phototoxic erythema (MPD). Conclusion: For optimal effects, UVA irradiation has to be administered immediately after the psoralen bath; no restrictive behavior is necessary after bath-PUVA treatment. (J Am Acad Dermatol 1998;39:443-6.)