1. ¹Department of Medical Biophysics, Ontario Cancer Institute/University Health Network Ontario, Canada
2. ²University of Toronto, Toronto, Ontario, Canada
3. ³Juravinski Cancer Center, Hamilton, Ontario, Canada
4. ⁴McMaster University, Hamilton, Ontario, Canada

Correspondence: BC Wilson, Department of Medical Biophysics, Ontario Cancer Institute/University Health Network, 610 University Avenue, Toronto, ON M5G 2M9, Canada. E-mail: wilson@uhnres.utoronto.ca

Received 28 June 2004; Revised 25 October 2004; Accepted 25 October 2004; Published online 18 January 2005.

Abstract

Although singlet oxygen (¹O₂) has long been proposed as the primary reactive oxygen species in photodynamic therapy (PDT), it has only recently been possible to detect it in biological systems by its luminescence at 1270 nm. Having previously demonstrated this in vitro and in vivo, we showed that cell survival was strongly correlated to the ¹O₂ luminescence in cell suspensions over a wide range of treatment parameters. Here, we extend this to test the hypothesis that the photobiological response in vivo is also correlated with ¹O₂ generation, independent of individual treatment parameters. The normal skin of SKH1-HR hairless mice was sensitised with 20% amino-levulinic acid-induced protoporophyrin IX and exposed to 5, 11, 22 or 50 J cm^-2 of pulsed 523 nm light at 50 mW cm^-2, or to 50 J cm^-2 at 15 or 150 mW cm^-2. ¹O₂ luminescence was measured during treatment and the photodynamic response of the skin was scored daily for 2 weeks after treatment. As observed by other authors, a strong irradiance dependence of the PDT effect was observed. However, in all cases the responses increased with the ¹O₂ luminescence, independent of the irradiance, demonstrating for the first time in vivo an unequivocal mechanistic link between ¹O₂ generation and photobiological response.