Role of nitric oxide and other soluble mediators in the acute inflammatory response to ALA-PDT in human skin

Acute inflammation occurs in human skin following topical aminolevulinic acid photodynamic therapy (ALA-PDT). Experimental models of PDT have identified potential mediators of this response but their involvement following PDT in human skin is little explored. We previously showed histamine to mediate the immediate ALA-PDT induced inflammatory response, but not the more prolonged erythema. We have additionally, through a series of novel studies in human volunteers, examined the involvement of proinflammatory mediators prostaglandin (PG) E2 and nitric oxide (NO) in the erythemal response. Duplicate dose-series of ALA were applied to the skin of each ventral forearm using the quantitative delivery system of iontophoresis, and exposed to 100 J/cm2 red light. In separate studies, arms were randomised within subject to receive treatment with the cyclooxygenase inhibitor indomethacin or control, or to receive treatment with NO synthase inhibitor N-nitro-L-arginine (L-NAME), or control. Following PDT, the erythemal response was quantified to determine the impact of treatment. Release of PGE2 and NO following ALA-PDT was also assessed directly using the technique of dermal microdialysis. Microdialysate was collected over 30 min periods immediately pre-irradiation, during irradiation and up to 24 h post-irradiation and mediators quantified by ELISA and chemiluminescence assay, respectively. An ALA dose-related erythema occurred by 3 h post-PDT which persisted to 48 h. Application of topical indomethacin immediately following ALA-PDT reduced the slope of the erythemal dose-response assessed at 3 h and 24 h post-PDT. Intradermal injection of L-NAME was also shown to reduce the ALA-PDT-red cell flux dose-response at 24 h post-PDT, and to reduce the red blood cell flux at sites treated with ALA-PDT from 3 to 48 h post-PDT. Analysis of dermal microdialysate confirmed NO and PGE2 to be released by PDT, with different time courses. In conclusion, topical ALA-PDT upregulates production of PGE2 and NO in human skin, both of which mediate the clinical inflammatory response. These mediators may play a role in PDT-induced acute adverse events and on PDT efficacy in human topical ALA-PDT, and could potentially be modulated to influence PDT outcomes.