Photodynamic therapy using topically applied hypericin: Comparative effect with methyl-aminolevulinic acid on UV induced skin tumours

Photodynamic therapy (PDT) is a treatment option particularly well-suited for superficial (pre)malignant skin lesions due to the skin’s accessibility to light. In the present study, the efficacy of topical hypericin-PDT was evaluated using a mouse model for actinic keratosis. For comparison, similar experiments were conducted with methyl-aminolevulinic acid (Me-ALA). Small skin tumours (1–2 mm) were induced in hairless mice by chronic UV irradiation. After topical application of hypericin (0.1% in gelcream for 24 h) or Me-ALA (Metvix® for 4 h), the lesional/non-lesional skin surface fluorescence ratio was determined and fluorescence microscopy was used to study the skin penetration of the photosensitizers. The antitumour activity of topical PDT (20 mW cm−2, 40 J cm−2) was evaluated by measurement of the lesional diameters. Moreover, biopsies were taken at various time points after PDT for histological evaluation of the therapy.Our results demonstrate that after topical application of hypericin and Me-ALA, tumour selectivity is limited in mouse skin. The microscopic distribution of hypericin fluorescence showed an accumulation in the stratum corneum and low fluorescence levels in the rest of the lesions, whereas the distribution of PpIX in the skin was more homogenous. Topical hypericin-PDT was found to be less efficient (44% total lesional clearance) as compared to Me-ALA-PDT (80% total lesional clearance). Full lesional necrosis was observed in responsive lesions, and the atypical cells of actinic keratosis were replaced by normal keratinocytes 3 weeks later, both after hypericin-PDT and Me-ALA-PDT.