The optimal time to determine the minimal phototoxic dose in skin photosensitized by topical 8 methoxypsoralen

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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.


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.


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 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.


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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