Multiple large surface photodynamic therapy sessions with topical methylaminolaevulinate in PTCH heterozygous mice.

Background: Photodynamic therapy (PDT) combines the administration of a photosensitizer with its subsequent activation by light of the appropriate wavelength. Methylaminolaevulinate (MAL) is a photosensitizer precursor, transformed by cells into protoporphyrin IX. The PTCH gene plays a central role in the genesis of basal cell carcinoma (BCC). The PTCH transgenic mouse develops microscopic BCCs when chronically exposed to ultraviolet (UV) or ionizing radiation.

Objectives: The aim of this study was to explore the ability of multiple large surface MAL-PDT to prevent BCC, using the PTCH heterozygous mouse as a model.

Methods: Thirty-five mice were exposed to UV radiation for a total of 20 weeks. Group 1 (20 mice) was exposed only to UV whereas group 2 (15 mice) was exposed to UV and weekly to MAL-PDT. At 28 weeks the mice were killed and the skin of the back processed for standard histopathology. Assessment was blind and any slide showing the presence of BCC was counted as a single BCC. The number of mice in groups 1 and 2 showing BCC were compared using Fisher's exact test.

Results: Nineteen BCCs in nine mice from group 1 were found, but no BCCs in mice from group 2. The difference was statistically significant (P = 0.001).

Conclusions: Weekly suberythematous PDT sessions with topical MAL were able to delay the development of microscopic BCCs in PTCH mice chronically exposed to UV radiation.