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Aminolevulinate-based photodynamic therapy (ALA-PDT) is effective for actinic keratosis (AK), but when administered in a conventional regimen, PDT can elicit pain during illumination that is very uncomfortable for patients. In conventional PDT, a 1-4 hr preincubation period occurs after ALA application, prior to the start of illumination. In a new regimen called metronomic PDT (mPDT), which simulates daylight PDT, blue light (405 nm) is delivered concurrently with ALA application and thereby provides a treatment that is not only painless but also nearly as effective as conventional PDT for AK lesion clearance. In this investigation, a murine AK model, generated by repeated UVB exposure, was treated with either mPDT or PDT. Lesion clearance was monitored via area measurements, and samples were harvested for assessment of biological mechanisms. Compared to pretreatment (100%), the average lesion area decreased to 47% and 32% in PDT, and to 57% and 40% in mPDT at 1 and 2 weeks after PDT, respectively. Relative to untreated controls, we observed enhanced cell death (by H&E staining, apoptosis, and TUNEL assay), generation of Reactive Oxygen Species (ROS, by CM-H2DCFDA staining), and autophagy (by Atg 5 and Atg 7 expression) in both the PDT and mPDT samples. Cleaved (activated) Caspase-3 was specifically observed only in PDT samples. Immunomodulation by inflammatory cells was indicated by enhanced infiltration/retention of neutrophils and macrophages in mPDT samples. Our results suggest that mPDT can be just as effective as conventional PDT for treatment of AK, but the mechanisms may be quite different.
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