A new level of in vivo singlet molecular oxygen luminescence measurements.

Background: Singlet oxygen is known to be the main mediator of the photodynamic effect. The kinetics of its generation and deactivation allows for insights in the microenvironment and efficacy of the photodynamic effect. Therefore, it is highly desirable to perform direct and time resolved measurements of singlet molecular oxygen (O) as well as data analysis during the therapy.

Methods: In this work, tumors grown on the CAM of chicken embryos as well as blood vessels were scanned after injection of the photosensitizer Foslip®, yielding time resolved singlet molecular oxygen luminescence. Using a custom-made trifurcated fiber, it is possible to simultaneously detect time resolved NIR luminescence as well as spectrally resolved UV/VIS fluorescence.

Results: After photosensitizer application the singlet oxygen luminescence kinetics for tumors grown on the CAM of chicken embryos as well as for mixed venous and arterialized blood were recorded. Data was analyzed by traditional fitting as well as a novel and robust approach, reducing the time resolved data to a a meaningful minimum. Both approaches show the differences between blood of different oxygen saturation as well as tumor tissue.

Conclusions: This work shows for the first time the possibility of deducing the oxygen content during photodynamic therapy by measuring singlet oxygen kinetics in tissue. If more oxygen is consumed - due to chemical quenching during PDT - than is subsequently diffused, oxygen depletion occurs, resulting in inefficiency of the photodynamic effect. These results represent a major step towards live monitoring of therapy success and thus towards the possibility of direct control of PDT efficiency in real time.