Assessment of minimal phototoxic dose following 8-methoxypsoralen bath: maximal reaction on average after 5 days

An essential procedure before starting bath psoralen ultraviolet (UV) A (PUVA) photochemotherapy is the evaluation of the minimal phototoxic dose (MPD), which is traditionally assessed 3 days after irradiation. However, there are no controlled studies supporting the 72 h peak of bath-PUVA erythema. The aim of this study was therefore to determine the exact time course of the erythematous reaction in human skin following bath-PUVA. For this purpose, the skin of 10 volunteers was exposed to 0.5-3.0 J/cm2 UVA directly after a 20-min 8-methoxypsoralen bath (0.5 mg/L, 37 degrees C). At 24, 48, 72, 96, 120 and 144 h (1-6 days) after irradiation, the MPD and the erythema sum score (ESS) were determined in each subject. The results showed a maximal erythematous reaction on average 5 days after irradiation. The mean MPD gradually decreased from day 2 (> 3.0 J/cm2) to day 5 (mean +/- SD 1.15 +/- 0.63 J/cm2) and started to increase at day 6 (mean +/- SD 1.6 +/- 0.52 J/cm2). The mean +/- SD ESS correspondingly increased from day 2 (0 +/- 0) to day 5 (10.5 +/- 3. 7) with a decrease at day 6 (7.5 +/- 3.1) (difference between day 3 and beyond statistically significant at P < 0.05). As our study indicates a maximal erythematous reaction to the bath-PUVA up to 5 days after irradiation, the traditional MPD assessment at 3 days generates a risk of phototoxic side-effects within the phototherapy course by underestimating the phototoxic effect in some patients. These findings contribute towards a more defined understanding of the kinetics of the phototoxic reaction in bath-PUVA therapy.