Aminolevulinate-based photodynamic therapy (ALA-PDT) is an effective treatment for cutaneous pre-cancers (Actinic Keratoses; AK) and Basal Cell Carcinoma (BCC), the most common skin malignancies. When administered in a conventional regimen with 1-4 h of ALA preincubation prior to light exposure, ALA-PDT elicits stinging pain during illumination that patients find objectionable. To avoid this pain, we have described a new regimen called metronomic PDT (mPDT) which is similar to daylight PDT but uses blue light (Kaw et al, J Am Acad Dermatol 2019). Metronomic PDT is not only painless but also nearly as effective as conventional PDT for AK lesion clearance. In this investigation, murine models of AK induced by repeated UVB exposure were treated with mPDT, followed by time-course analyses of immune responses in the lesions harvested. Our preliminary data showed that relative to conventional PDT, cell death (apoptosis) and generation of Reactive Oxygen Species (ROS) were compromised in mPDT samples. However, relative to untreated controls, enhanced recruitment/infiltration of immune cells that mediate innate immunity [neutrophils (Ly6G+) and macrophages (F4/80+)] was observed at early times after mPDT. Just as importantly, enhanced presence of cells regulating adaptive immune responses [T cells (CD3+, CD8+ and Foxp3+)] was observed at later times post mPDT. Activation of calreticulin and HMGB1 (markers of Damage Associated Molecular Patterns, DAMPs) were also observed in mPDT treated lesions. Our results suggest that mPDT can be just as effective as conventional PDT for treatment of skin cancer and pre-cancer, and that the therapeutic mechanisms may involve immune cell responses triggered by metronomic PDT.
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.
Aminolevulinic acid based photodynamic therapy (ALA-PDT) is a popular and efficacious treatment for actinic keratosis (AK). However, standard PDT can elicit stinging pain during illumination, and hence is not always favored by patients. In a new regimen called metronomic PDT (mPDT), similar to daylight PDT but using blue light, the illumination is delivered concurrently with ALA application rather than after a 1-hour pre-incubation (conventional regimen, PDT). In the clinic, mPDT is not only painless but also nearly as effective as 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 followed by area measurement, and samples were harvested for mechanistic analyses. Compared to pretreatment (100%), the average lesion area was reduced to 47% and 32% in PDT, and to 57% and 40% in mPDT at 1- and 2-weeks post PDT, respectively. Relative to untreated controls, enhanced cell death (histomorphology by H&E staining and apoptosis by TUNEL assay), and generation of Reactive Oxygen Species (ROS; CM-H2DCFDA staining) were observed in both PDT and mPDT samples. Activation of cleaved Caspase-3 was specifically observed only in PDT samples. Immunomodulation by inflammatory cells was observed by enhanced infiltration/retention of neutrophils and macrophages in metronomic PDT samples. Our results suggest that metronomic PDT can be just as effective as conventional PDT for treatment of AK, but the mechanisms may be quite different.
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