Fluorescence and absorption assessment of a lipid mTHPC formulation following topical application in a non-melanotic skin tumor model

Although the benefits of topical sensitizer administration have been confirmed for photodynamic therapy (PDT), ALA-induced protoporphyrin IX is the only sensitizer clinically used with this administration route. Unfortunately, ALA-PDT results in poor treatment response for thicker lesions. Here, selectivity and depth distribution of the highly potent sensitizer meso-tetra(hydroxyphenyl)chlorin (mTHPC), supplied in a novel liposome formulation was investigated following topical administration for 4 and 6h in a murine skin tumor model. Extraction data indicated an average [± standard deviation (SD)] mTHPC concentration within lesions of 6.0(±3.1) ng/mg tissue with no significant difference (p<0.05) between 4-and 6-h application times and undetectable levels of generalized photosensitivity. Absorption spectroscopy and chemical extraction both indicated a significant selectivity between lesion and normal surrounding skin at 4 and 6h, whereas the more sensitive fluorescence imaging setup revealed significant selectivity only for the 4-h application time. Absorption data showed a significant correlation with extraction, whereas the results from the fluorescence imaging setup did not correlate with the other methods. Our results indicate that this sensitizer formulation and administration path could be interesting for topical mTHPC-PDT, decreasing the effects of extended skin photosensitivity associated with systemic mTHPC administration.