Photodynamic therapy (PDT) is an evolving cancer treatment with promising results in treating malignant tumors by photoactivation of a photosensitizer with a specific wavelength. The second generation photosensitizer mono-L-aspartyl chlorin e6 (NPe6) was reported to have significant efficacy in killing cancer cells in vitro and in vivo. Though topical application might yield a higher local concentration and less systemic side effect, no data concerning topical absorption of NPe6 is available even though the drug has already been used in clinical trial for several years. To evaluate the possibility of topical delivery of NPe6 via an animal model, escalated concentrations of NPe6 were applied to BALB/c mouse skin for a different time periods after barrier disruption with tape stripping. Since NPe6 fluorescence intensity and drug concentration in tissue was well correlated, we evaluated drug penetration depth with frozen sections of treated and non-treated skin under a fluorescence microscope. An on-line fluorescence imaging system was used to monitor the NPe6 fluorescence kinetics in the skin. The fluorescence microscope confirmed successful topical delivery of NPe6 in mouse skin with or even without barrier disruption. Orange to red NPe6 fluorescence appeared at the epidermis, dermis, and even the muscular layer when using lOmg/ml NPe6 application. The fluorescence intensity peaked at 1 h and revealed a dose-dependent response pattern. NPe6 treated versus non-treated skin showed a statistically significant difference by Student's t-test (P<0.05). The results described here suggest that topical delivery of NPe6 is possible. It showed fast and deep penetration into mouse skin. This implies that NPe6 might be useful as a topical photosensitizer for PDT in treating skin cancers.
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