Fluence-rate effects upon m-THPC photobleaching in a formalin-fixed cell system

Area of Science:

• Biophysics
• Photochemistry
• Cell Biology

Background:

• Photodynamic therapy (PDT) uses photosensitizers activated by light to generate cytotoxic species.
• m-THPC (Foscan) is a photosensitizer used in PDT, but its behavior in cellular environments needs further characterization.
• Understanding photosensitizer photobleaching is key to optimizing PDT dose delivery.

Purpose of the Study:

• To investigate the photobleaching kinetics of m-THPC in individual formalin-fixed keratinocytes using micro-spectroscopy.
• To determine the influence of light dose, fluence rate, and oxygen availability on m-THPC photobleaching.
• To explore the effects of fractionated irradiation on photobleaching and potential re-oxygenation.

Main Methods:

• Application of micro-spectroscopic technique to record laser-induced fluorescence emission of m-THPC.
• Irradiation of cells with 410nm laser light to monitor fluorescence depletion.
• Analysis of fluorescence decay kinetics and plotting photobleaching rate against light dose.
• Investigating inverse fluence-rate dependence and effects of fractionated irradiation.

Main Results:

• m-THPC demonstrated high photolability within keratinocytes.
• Photobleaching followed bi-exponential decay kinetics, indicative of singlet oxygen involvement.
• Photobleaching rate showed inverse fluence-rate dependence, suggesting oxygen limitation at higher laser powers.
• Fractionated irradiation indicated intracellular re-oxygenation.

Conclusions:

• m-THPC photobleaching is significantly influenced by local oxygen partial pressure and light fluence rate.
• These findings align with previous studies highlighting the critical role of oxygen and light parameters in PDT.
• Optimizing PDT protocols requires careful consideration of these micro-environmental factors for effective treatment.