Correlations between photoactivable porphyrins’ fluorescence, erythema and the pain induced by PDT on normal skin using ALA-derivatives

Summary Background Photodynamic therapy (PDT) with precursors of photoactivable porphyrins is a well-established treatment modality for skin pathologies as well as hair removal. Pain is a major side effect thereof, and it affects the treatment compliance and acceptance. Methods Five male subjects underwent a PDT procedure on normal skin, either with a diode laser (635nm) or a lamp (405nm), 3 or 6h after application of various precursors of photoactivable porphyrins (ALA 1M; Metvix® 1M; ALA-DGME 1M; ALA-DGME 3.66M). Light doses ranged from 30 to 150J/cm2 and irradiances were 100 or 180mW/cm2. Fluorescence measurements were performed just before PDT, pain was quantified during PDT, and erythema was determined 24h afterwards. Results Because precursor ALA-DGME was very selective for the pilosebaceous apparatus vs. the epidermis, we solely carried out the PDTs using this precursor. In the absence of light, no pain was reported. An increase in pain was observed when increasing the irradiance. A correlation was observed between the follicular fluorescence and the maximal pain score during PDT. A correlation was observed between follicular fluorescence and skin erythema, and between pain score and skin erythema. Conclusions With our well-controlled PDT parameters and homogenous subjects’ conditions, we showed that pain could be reduced by reducing irradiance during PDT procedures. With the various correlations observed, we conclude that both pain and PaP fluorescence are useful tools to predict the post-PDT tissue effects (side effects and outcome). We suggest that A∂ nerve fibres would be the best candidate as first generators of PDT-induced pain.