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The optimal time to determine the minimal phototoxic dose in skin photosensitized by topical 8 methoxypsoralen
Author(s) -
Man I.,
Dawe R.S.,
Ferguson J.,
Ibbotson S.H.
Publication year - 2004
Publication title -
british journal of dermatology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.304
H-Index - 179
eISSN - 1365-2133
pISSN - 0007-0963
DOI - 10.1111/j.1365-2133.2004.06073.x
Subject(s) - erythema , psoralen , methoxsalen , phototoxicity , medicine , dermatology , puva therapy , skin type , psoriasis , chemistry , dna , biochemistry , in vitro
Summary 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.