Imaging of hypoxia, oxygen consumption and recovery in vivo during ALA-photodynamic therapy using delayed fluorescence of Protoporphyrin IX.

Background: Hypoxic lesions often respond poorly to cancer therapies. Particularly, photodynamic therapy (PDT) consumes oxygen in treated tissues, which in turn lowers its efficacy. Tools for online monitoring of intracellular pO are desirable.

Methods: The pO changes were tracked during photodynamic therapy (PDT) with δ-aminolevulinic acid (ALA) in mouse skin, xenograft tumors, and human skin. ALA was applied either topically as Ameluz cream or systemically by injection. Mitochondrial pO was quantified by time-gated lifetime-based imaging of delayed fluorescence (DF) of protoporphyrin IX (PpIX).

Results: pO-weighted images were obtained with capture-times of several seconds, radiant exposures near 10 mJ/cm, spatial resolution of 0.3 mm, and a broad dynamic range 1-50 mmHg, corresponding to DF lifetimes ≈20-2000 μs. The dose-rate effect on oxygen consumption was investigated in mouse skin. A fluence rate of 1.2 mW/cm did not cause any appreciable oxygen depletion, whereas 6 mW/cm and 12 mW/cm caused severe oxygen depletion after radiant exposures of only 0.4-0.8 J/cm and <0.2 J/cm, respectively. Reoxygenation after PDT was studied too. With a 5 J/cm radiant exposure, the recovery times were 10-60 min, whereas with 2 J/cm they were only 1-6 min. pO distribution was spatially non-uniform at (sub)-millimeter scale, which underlines the necessity of tracking pO changes by imaging rather than point-detection.

Conclusions: Time-gated imaging of PpIX DF seems to be a unique tool for direct online monitoring of pO changes during PDT with a promising potential for research purposes as well as for comparatively easy clinical translation to improve efficacy in PDT treatment.