The partial pressure of oxygen (pO2) in biological tissues plays an important role in the mechanisms underlying photodynamic therapy (PDT) and radiotherapy. In addition, a prolonged hypoxia is associated with the presence of ulcerations. Therefore, monitoring pO2 is of high interest to predict the outcome of these therapeutic approaches and to predict the evolution of some ulcerations.
Determination of the tissue oxygenation based on the measurement of the PPIX delayed fluorescence lifetime.
Since the triplet state of most photosensitizers used in PDT is quenched by molecular oxygen and since many photosensitizers (such as for instance protoporphyrin IX (PPIX)) do not exhibit a detectable phosphorescence in the clinical context, one original approach to monitor the pO2 relies on the measurement of the lifetime of their delayed fluorescence (DF). This DF is due to a repopulation of the singlet state from the triplet state. Compared to other oxygen sensing techniques, PPIX-DF measurements have the advantage to record the oxygen concentration in close proximity to the photosensitizer and in the mitochondria.
We have developed a dedicated optical fiber-based time-resolved spectrometer to measure the PPIX DF in the chicken egg’s chorioallantoic membrane in real-time. This spectrometer can be coupled to epi-fluorescence microscopes to study pO2 in different tissue layers/compartments. We have observed an excellent correlation between therapeutic vascular damages induced by PDT and pO2 reduction. This study suggests that clinical measurement of pO2 using this simple approach may be used to optimize PDT light dose in situ and in real time, thus reducing inter- and intra-patient therapeutic outcome fluctuations.
Measurement of the tissue oxygenation by time-resolved luminescence spectroscopy of exogenous molecular probes.
The measurement of pO2 in biological tissues in vivo is also important for many fundamental applications. One approach to assess the pO2 is based on the measurement of molecular probes luminescence lifetimes. The relation between the pO2 and the lifetime is given by the Stern-Volmer equation. Unfortunately, virtually all oxygen-sensitive probes based on this principle induce some degree of phototoxicity. We are working on strategies to minimize this phototoxicity. We have assessed in vivo the oxygen sensitivity/phototoxicity of the oxygen sensor dichlorotris (1,10-phenantroline) Ruthenium (II) hydrate (Ru(Phen)32+), which presents interesting properties in this context.