In vivo mTHPC photobleaching in normal rat skin exhibits unique irradiance-dependent features

Area of Science:

• Photodynamic Therapy
• Biophysics
• Pharmacology

Background:

• Photodynamic therapy (PDT) utilizes photosensitizers to generate reactive oxygen species upon light activation.
• Meso-tetrahydroxyphenyl chlorin (mTHPC) is a photosensitizer used in PDT, and understanding its photobleaching kinetics is crucial for treatment optimization.
• In vivo measurements are essential to understand photosensitizer behavior under physiological conditions.

Purpose of the Study:

• To investigate the photobleaching kinetics and mechanisms of meso-tetrahydroxyphenyl chlorin (mTHPC) in rat skin in vivo.
• To correlate photobleaching with irradiance levels during 650 nm laser irradiation.
• To compare in vivo photobleaching observations with oxygen concentration changes in mTHPC-sensitized spheroids.

Main Methods:

• In vivo fluorescence spectroscopy was used to monitor mTHPC fluorescence loss in rat skin.
• Photobleaching was assessed at different laser irradiances (5, 20, 50 mW cm(-2)).
• Oxygen (O2) concentrations were measured in mTHPC-sensitized spheroids under varying light fluences (2, 5, 20 mW cm(-2)).

Main Results:

• Two distinct phases of mTHPC photobleaching were observed in vivo.
• The first phase showed no significant irradiance dependence, while the second phase exhibited irradiance-dependent discontinuities.
• Photobleaching rates in the second phase suggested an oxygen-dependent process, but spheroid oxygen measurements did not show corresponding anomalies.

Conclusions:

• In vivo mTHPC photobleaching exhibits complex kinetics with distinct phases.
• The observed discontinuities in fluorescence loss during PDT are not directly explained by photochemical oxygen consumption.
• Fluorescence measurements reveal phenomena not captured by oxygen concentration monitoring alone, highlighting the complexity of in vivo photosensitizer photobleaching.