Time course of skin photosensitivity following trimethylpsoralen bath PUVA

One aspect of bath photochemotherapy (PUVA) that requires clarification is the duration of psoralen-induced cutaneous photosensitisation under conditions simulating clinical use. Using a half back comparison study technique, we investigated the persistence of trimethylpsoralen (TMP)-induced photosensitivity in skin irradiated to simulate a first PUVA exposure compared with un-irradiated skin. Baseline UVA minimal erythema dose and minimal phototoxic dose (MPD) were determined in 13 healthy volunteers. After readings at 72 h, subjects were bathed in TMP bath water for 15 min and one half of the back was immediately exposed to 40% of the MPD. Test sites (1.5 cm2) on both halves of the back were then irradiated with a UVA dose series at 15 min, 5, 10, 24, 34, 48 and 72 h after the bath. MPD readings were recorded visually at 72 h after each UVA exposure. The UVA MED was >25 J/cm2 in all the subjects. At each time point, a phototoxic index (PI) was calculated as UVA MED/MPD. In un-irradiated skin, photosensitivity returned to normal (PI=1) within 24 h after the TMP bath. In contrast, skin pre-irradiated to simulate the first PUVA treatment was still significantly photosensitive (PI=2.3; P=0.002) at 48 h. Contrary to previous recommendations, these data suggest that patients should be advised to avoid ambient or artificial sources of UVA throughout their course of TMP bath PUVA to reduce the risk of phototoxic erythema.