Effect of psoriasis heliotherapy on epidermal urocanic acid isomer concentrations

A noninvasive Finn Chamber sampling method and HPLC analysis were used to determine epidermal urocanic acid (UCA) concentrations of psoriasis patients during 4 weeks of heliotherapy on the Spanish Canary Islands and a follow-up period of 8 weeks. During heliotherapy the epidermal cis-UCA concentration increased from a mean initial value of 0.2 nmol/cm2 to a mean final value of 2.9 nmol/cm2. The total UCA concentration decreased during the first week of heliotherapy from an initial value of 5.5 nmol/cm2 to a nadir of 2.0 nmol/cm2. Thereafter, a steady increase was recorded in the total UCA level, with a maximum of 10.2 nmol/cm2 in week 2 of the post-treatment follow-up period. Suberythemal sun exposures caused near-maximal UCA isomerization, and during heliotherapy the cis isomer constituted 63.7-74.3% of the total UCA concentration. Clinical response of psoriasis to heliotherapy, however, seemed to be independent of UCA isomer levels.