Systemic photodynamic therapy with aminolaevulinic acid delays the appearance of ultraviolet-induced skin tumours in mice

Background Photodynamic therapy (PDT) with topical aminolaevulinic acid (ALA) has recently been approved by the US Food and Drug Administration for the treatment of actinic keratoses. Objectives To determine whether weekly systemic suberythemogenic ALA-PDT could prevent the appearance of ultraviolet (UV) -induced skin tumours in hairless mice. Methods One group of 20 mice received daily UV radiation from FS 20 tubes, and weekly intraperitoneal injections of ALA 40 mg kg(-1), each followed 3 h later by 12 J cm(-2) of white light (ALA-PDT). Control groups consisted of mice exposed only to UV, to UV and ALA without white light, or UV and white light without ALA, as well as untreated mice. Results The tumour-free survival was significantly longer for mice exposed to daily UV and weekly ALA-PDT as compared with the control groups. Neither the mortality nor the incidence of large skin tumours was higher in the UV/ALA-PDT group than in mice exposed only to UV. In vivo fluorescence spectroscopy showed that the 635-nm fluorescence emission within tumours was lower than in normal skin 3 h after ALA administration. This was also confirmed by quantitative fluorescence microscopy. Conclusions Systemic ALA-PDT can delay the appearance of UV-induced skin tumours in mice without increasing mortality or the incidence of large tumours.