Correlation between 8-methoxypsoralen bath-water concentration and photosensitivity in bath-PUVA treatment

Background: Bath-PUVA treatment, originally established in Scandinavia, off ers several advantages over oral PUVA and has become increasingly popular i n recent years. Outside Scandinavia 8-methoxypsoralen (8-MOP) is the prevai ling photosensitizer for this PUVA modality and is used arbitrarily in a wi de range of concentrations. Up to the present, data are lacking on the impa ct of 8-MOP bath-water concentration on UVA dosimetry. Objective: We investigated the influence of increasing 8-MOP bath-water con centrations on photosensitivity in bath-PUVA treatment. Methods: Fifteen healthy volunteers without abnormal photosensitivity or re cent exposure to ultraviolet radiation were included in an intraindividuall y controlled comparison study. In all volunteers the minimal phototoxic dos e (MPD) was determined on the volar side of their forearms after immersion for 20 minutes in 4 different 8-MOP bath-water concentrations (0.5, 1, 2.5, and 5 mg/L). The correlation between 8-MOP concentration and photosensitiv ity (defined as the reciprocal value of the MPD) was analyzed by linear reg ression analysis. In addition, the time course of erythema formation and th e UVA dose-erythema response curve was assessed for each psoralen concentra tion. Results: The median MPD and the 25%-75% interquartile were 5.7 J/cm(2) (5.7 -8), 4 J/cm(2) (4-5.7), 2.8 J/cm(2) (2.8-5.7), and 2 J/cm(2) (2-2.8) at an 8-MOP concentration of 0.5, 1, 2.5, and 5 mg/L, respectively. Linear regres sion analysis revealed a significant correlation between 8-MOP bath-water c oncentration and photosensitivity (r = 0.98; P = .019). Bath-PUVA-induced e rythema peaked after a median time interval of 3 days, with a range of 2 to 4 days. The slope of the UVA dose-erythema response curve was similar for all psoralen concentrations. Conclusion: UVA dose requirements in bath-PUVA treatment decrease linearly with increasing 8-MOP concentrations. A single MPD assessment at 72 hours a fter the UVA exposure is inappropriate for accurate determination of the pa tients' photosensitivity. The hazard of wrong UVA dosimetry is comparable a t all psoralen concentrations.