The time-course of psoralen ultraviolet a (PUVA) erythema

The time-course for the development of ultraviolet A-induced erythema in ps oralen-sensitized skin differs from that caused by ultraviolet B or ultravi olet A but objective data are not available. During psoralen ultraviolet A therapy, the minimal phototoxic dose is determined 72 h after exposure, whe n psoralen ultraviolet A erythema is assumed to be maximal. This measuremen t is of fundamental importance in optimizing the therapeutic regimen, We ex amined a detailed time-course for development of psoralen ultraviolet A ery thema in 16 subjects. The erythemal responses to ultraviolet B, ultraviolet A and psoralen ultraviolet A were assessed visually and using a reflectanc e device, Ultraviolet B erythema was maximal 24 h after exposure compared w ith subsequent time-points. Psoralen ultraviolet A erythema was evident at 24 h, with reduction in the median ultraviolet A minimal erythema dose from 14 to 5 J per cm(2) in the presence of psoralen (p < 0.01; n = 9), Peak ps oralen ultraviolet A erythema, assessed by minimal phototoxic dose, did not occur until 96 h or later in 75% of subjects. Using individual dose-respon se curves, we determined that only 67% of mean maximum psoralen ultraviolet A erythemal intensity had developed by 72 h, Furthermore, at the time of m aximal erythema, the slope of the psoralen ultraviolet A dose-response curv e was approximately 2-fold shallower than that for ultraviolet B-induced er ythema. If assessment of psoralen ultraviolet A erythemal sensitivity had b een made at 96 h instead of the conventional 72 h time-point, peak erythema l responses would not have been missed in any of the subjects. Based on the se findings, it seems appropriate to consider whether psoralen ultraviolet A minimal phototoxic dose measurements should be performed 96 h after expos ure.