In-vivo singlet oxygen threshold doses for PDT

Area of Science:

• Photodynamic Therapy (PDT)
• Singlet Oxygen (¹O₂) Dosimetry
• Cancer Research

Background:

• Singlet oxygen (¹O₂) is the primary cytotoxic agent in type-II photosensitizer-mediated PDT.
• Accurate dosimetry of ¹O₂ is crucial for understanding PDT's biological effects.
• An in-vivo model is needed to determine the threshold dose of ¹O₂ for PDT.

Purpose of the Study:

• To develop and validate an in-vivo model for determining the singlet oxygen threshold dose ([¹O₂]rx,sh) in PDT.
• To compare ¹O₂ dosimetry parameters for different photosensitizers, including benzoporphyrin derivative monoacid ring A (BPD).
• To correlate in-vivo experimental results with in-vitro findings and theoretical models.

Main Methods:

• Utilized a radiation-induced fibrosarcoma (RIF) tumor mouse model.
• Correlated necrosis radius with explicit PDT dosimetry, including light fluence, tissue optical properties, and photosensitizer concentrations.
• Determined photosensitizer-specific model parameters for BPD and compared them with literature data for Photofrin® and m-tetrahydroxyphenylchlorin (mTHPC).

Main Results:

• The in-vivo [¹O₂]rx,sh values were approximately 0.56 mM for Photofrin® and 0.72 mM for BPD.
• These values correspond to 3.6x10⁷ and 4.6x10⁷ singlet oxygen molecules per cell, respectively, for a 1/e reduction in cell survival.
• Preliminary photochemical parameters for BPD were obtained, with confidence in the singlet oxygen threshold dose values.

Conclusions:

• In-vivo threshold singlet oxygen concentrations for Photofrin®, BPD, and mTHPC were approximately 20 times lower than in-vitro values.
• This discrepancy highlights the contribution of factors beyond direct singlet oxygen-mediated cell killing to in-vivo PDT damage.
• The study provides a theoretical framework to relate in-vitro and in-vivo singlet oxygen requirements and discusses the influence of vascular effects.