A pilot study comparing the pain sensations during PpIX build-up and clearance phases

It has been speculated that topical application of 5-aminolevulinic acid (ALA) or methyl 5-aminolevulinate (MAL) may be more painful during light exposure after longer application times of the compounds than after shorter times, even though the same levels of protoporphyrin IX (PpIX) is produced in both cases. The aim of our study was to investigate pain induction in the build-up and clearance phases of PpIX in the skin of healthy volunteers. 0.6 mmol/g of ALA (10% wt/wt) and MAL (11% wt/wt) creams were applied on the volunteers. The creams were maintained on the spots for 20- 24 hours and then wiped off. Subsequently, fresh creams were applied on the other arm of the volunteers for 4- 6 hours. Fluorescence emission spectra for all spots were measured every hour until the fluorescence levels were similar in both arms for ALA and MAL. Then the test areas were exposed to light until pain occurred. Time for pain to occur was recorded. The fluorescence of PpIX was measured before and after light exposure. PDT in the clearance phase seems to induce pain faster than in the build-up phase for ALA and MAL. Due to large interpersonal variations between volunteers further investigation is needed.