How to avoid phototoxic reactions in photopatch testing with chlorpromazine

The purpose of this study was to determine the concentration, vehicle, and UVA dose suitable for avoiding phototoxic reactions while revealing photoallergic reactions to chlorpormazine (CPZ) in photopatch testing. Photopatch tests were performed on 27 volunteers with CPZ concentrations between 0.05% and 1% in water or Plastibase with UVA doses of 4 or 7 J/cm2. Photopatch tests were also carried out on 1 patient with photoallergic contact dermatitis to CPZ and an increased UVA sensitivity. The same concentration of CPZ caused phototoxic reactions less frequently and less intensely in Plastibase than in water. The minimal phototoxic concentration of CPZ in Plastibase was 0.5% after irradiation with 4 J/cm2 and 0.1% after irradiation with 7 J/cm2. In the patient with photoallergy to CPZ, a photoallergic reaction was elicited after 0.1% CPZ in Plastibase and a UVA dose of 2 J/cm2. In testing photoallergic reactions to CPZ a concentration of 0.1% in Plastibase and a UVA dose of 4 J/cm2 or less should be used.