The optimal time to determine the minimal phototoxic dose in skin photosensitized by topical 8 methoxypsoralen |
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Authors: | Man I Dawe R S Ferguson J Ibbotson S H |
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Affiliation: | Photobiology Unit, Department of Dermatology, University of Dundee, Ninewells Hospital and Medical School, Dundee DD1 9SY, UK. iwman@yahoo.com |
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Abstract: | BACKGROUND: We recently investigated the characteristics of psoralen plus ultraviolet (UV) A erythema in skin photosensitized by topical 8-methoxypsoralen (8-MOP) in three independent studies. OBJECTIVES: In order to determine the optimal time to read the minimal phototoxic dose (MPD) after treatment with topical 8-MOP and irradiation with UVA, we assessed the overall data. METHODS: One forearm of each subject was immersed in 8-MOP solution for 15 min and test sites on the flexor surface of the forearm were immediately exposed to a UVA dose series. Erythema was assessed visually and objectively using a reflectance instrument at 24-h intervals for 7 days. RESULTS: Results were obtained from 44 subjects (predominantly Fitzpatrick skin phototype II). A broad erythemal plateau was evident beyond 72 h and the visual MPD was significantly lower at 96, 120 and 144 h than at 72 h (P < 0.01). Only 30% of subjects were at peak erythema at the conventional MPD assessment time of 72 h. The median time to reach maximal erythema was 96 h (range 48-144). Objectively, 85% of subjects were at peak erythema at or beyond 96 h. CONCLUSIONS: We recommend that (i) the optimal time to read the topical 8-MOP MPD is 4 days after UVA exposure as readings beyond this time may be difficult to interpret because of the development of pigmentation, and (ii) 40% of the topical 8-MOP MPD should be considered for the first treatment. |
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Keywords: | bath psoralen plus ultraviolet A dose–response curve 8-methoxypsoralen minimal phototoxic dose peak erythema time course |
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