Protective effect of trimethylpsoralen against UV-B erythema.

Vitiliginous patients undergoing photochemotherapy with TMP were less susceptible to a sun burn. To investigate this point, the MED for UV-B was measured before, 2 h, and 24 h after oral intake of TMP in 10 healthy volunteers. There was an increase of MED when UV exposure had taken place 2 h after TMP intake (P < 0.01). This was no longer detectable when the exposure had taken place 24 h after intake. MED increase was 11.8% for 0.19 mg/kg TMP, 21.0% for 0.37 mg/kg, and 23.7% for 0.74 mg/kg. This dose-dependence was significant (P < 0.01). The protective effect of TMP against UV-B erythema seemed related to UV absorption by the drug while it was in the blood stream.