| Photodynamic therapy (PDT) is emerging as a treatment option for various malignant and non-malignant conditions. The therapeutic effect of PDT is mediated by the generation of singlet oxygen, a process 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 alternative dose metric strategy has been proposed, termed "implicit dosimetry", where changes in photosensitizer fluorescence during treatment (known as photobleaching) are used to predict treatment outcome. Since photobleaching and biological damage may be mediated by the same cytotoxic agents (i.e. singlet oxygen), photobleaching could provide a measure of PDT efficacy provided the relationship between photobleaching and singlet oxygen generation is known.; To investigate this, MatLyLu (MLL) rat prostate adenocarcinoma cells were incubated with either Foscan (mTHPC), Photofrin, or delta-Aminolevulinic acid (ALA) and treated with the appropriate wavelength of light. PDT was performed under a wide range of treatment conditions. Fluorescence was monitored during treatment and, at selected fluence levels, cell viability was determined using a colony formation assay.; For PDT with mTHPC and Photofrin, 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. 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. For ALA PDT, PpIX photobleaching could not be modeled by first or second order kinetics, and could not be used in a predictive singlet oxygen dose model. A fluorescent photoproduct with a peak at 655 nm correlated well to cell survival and may provide a useful measure of singlet oxygen dose, but this hypothesis requires further testing in vivo. |
