Sci‐YIS Fri ‐ 03: Calculation of singlet oxygen dose from photosensitizer photobleaching during mTHPC or Photofrin photodynamic therapy in vitro

Photodynamic therapy (PDT) is emerging as a treatment option for various malignant conditions. PDT damage is caused by the generation of singlet oxygen. This process is dependent on the complex interaction between the photosensitizer (PS), treatment light, and oxygen. Since these parameters may be highly variable among patients and may change during treatment, it is difficult to predict therapeutic outcome based on administered PS and delivered light dose alone. An implicit dose metric model has been proposed in which singlet oxygen dose is monitored by the decrease in PS fluorescence during treatment caused by reactions between PS and singlet oxygen. To investigate this, MatLyLu (MLL) rat prostate adenocarcinoma cells were incubated with Foscan (mTHPC) or Photofrin and treated with the appropriate wavelength of light. Fluorescence was monitored during treatment and, at selected fluence levels, cell viability was determined using a colony formation assay. Singlet oxygen dose models were developed based on measurements of PS fluorescence and reaction kinetics. Cell survival correlated well to calculated singlet oxygen dose, independent of initial PS concentration, treatment fluence rate, and oxygenation. These results indicate that investigation of photobleaching is warranted as an in vivo dose metric. We found 1.5 ± 0.3 and 0.45 ± 0.09 mM of singlet oxygen was required to reduce the survival fraction by 1/e for mTHPC and Photofrin respectively.