Tar photoxicity and phototherapy for psoriasis

Area of Science:

• Dermatology
• Photobiology

Background:

• Coal tar and ultraviolet light (UVL) therapy is a common treatment for psoriasis.
• The mechanism of action, specifically tar phototoxicity, remains unclear.

Purpose of the Study:

• To determine the phototoxicity of coal tars.
• To assess the contribution of tar phototoxicity to the efficacy of Goeckerman therapy.

Main Methods:

• Determined phototoxicity by comparing minimal phototoxic dose (MPD) with minimal erythema dose (MED).
• Calculated the phototoxic index (PI = MED/MPD).
• Tested six tars on 15 subjects using UVA and UVB light sources.

Main Results:

• All tested tars demonstrated phototoxicity to UVA light (PI > 1).
• No phototoxicity was observed with UVB light (P < 0.0001).
• Standard Goeckerman therapy likely uses insufficient UVA energy for phototoxic reactions.

Conclusions:

• Coal tars are phototoxic, but only to UVA radiation.
• The therapeutic benefits of Goeckerman therapy are unlikely due to tar phototoxicity.
• Further research may explore alternative mechanisms or optimized UVA delivery.