The Effect of Photodynamic Therapy With Pheophorbide-a on Chemically-induced Papillary Skin Lesions in Hairless Mice

Photodynamic therapy (PDT) is an alternative treatment option for early stage oral cancer and precancerous lesions due to the limitations of conventional treatment modalities. Pheophorbide a (Pa) is a chlorine-based photosensitizer extracted from chlorophyll-a, which is known to be activated by light at a wavelength of 670nm, is rapidly metabolized, and is selectively accumulated in tumor cells over a longer duration. The aim of this study is to validate the antitumor efficacy of PDT after the injection of Pa in murine papillary skin lesions.

Hairless mice were randomly divided into two groups, namely, group I (control group, 5 mice) and group II (experimental group, 33 mice). Group I animals received topical application of acetone (100µ° per mouse) throughout the study. Group II animals received a topical application of 7,12-Dimethylbenz(a)anthracene (DMBA) 100µg in 100µ° of acetone for 2 weeks followed by a topical application of 12-O-tetradecanoylphorbol-13-acetate (TPA) 2.5µg in 100µ° of acetone for 5 weeks every second day. Animals in group II were further divided into 2 subgroups, namely group IIA (21 mice) and group IIB (6 mice). Group IIA received Pa-based laser treatment, and group IIB received Pa treatment alone. 200µg of Pa was directly injected into the 3 largest lesions over the back. After 24 hours, the animals were exposed to a light dose of 100J/µ² using a laser diode at a wavelength of 664nm. One week after the first cycle of treatment, the second cycle of treatment was performed using the same method in each group. One week after completion of the respective treatments, mice were euthanized in a CO2 chamber. In Group II, 6 mice that were found to have non-papillary skin lesions were excluded from this experiment.

After treatment, one papillary lesions was sloughed away in 8 mice, 2 papillary lesions were sloughed away in 5 mice, and 3 papillary lesions were sloughed away in 2 mice of group IIA; whereas, one papillary lesion was sloughed away in 1 mouse out of 6 mice in group IIB. As a final outcome, Pa-based PDT was effective in 54% of 63 papillary lesions, and Pa treatment alone was effective in 11.1% of 18 papillary lesions.

Results of this experiment showed that Pa does not have any remarkable effect on papillary skin lesion in the absence of illumination (11.1%) though the previous reports had demonstrated that pheophorbide a (Pa) has antitumor activity and a potential immunomodulatory effect on macrophages.PDT with Pa was observed to have an effect on early stage of tumor lesions although the success rate was relatively low (54%). More than 3 cycles of Pa-based PDT treatment can be predicted to cause more favorable treatment outcomes, especially in papillary skin lesions.Development of a dysplasia model is necessary to validate the efficacy of Pa-based PDT in premalignant lesions.

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