In-vivo singlet oxygen threshold doses for PDT

Objective: Dosimetry of singlet oxygen (1O2) is of particular interest because it is the major cytotoxic agent causing biological effects of type-II photosensitizers during photodynamic therapy (PDT). An in-vivo model was developed to determine the singlet oxygen threshold dose, [1O2]rx,sh, for PDT. Material and methods: An in-vivo radiation-induced fibrosarcoma (RIF) tumor mouse model was used to correlate the radius of necrosis to the calculation based on explicit PDT dosimetry of light fluence distribution, tissue optical properties, and photosensitizer concentrations. Inputs to the model include five photosensitizer-specific photochemical parameters along with [1O2]rx,sh. Photosensitizer-specific model parameters were determined for benzoporphyrin derivative monoacid ring A (BPD) and compared with two other type-II photosensitizers, Photofrin® and m-tetrahydroxyphenylchlorin (mTHPC) from the literature. In order to discuss the possible influence of vascular vs. apoptotic cell killing mechanisms on the singlet oxygen threshold dose, the [1O2]rx,sh values for BPD with 3 h and 15 min drug-light intervals, with the latter being known to have a dominantly vascular effect, were compared. Results: The mean values (standard deviation) of the in-vivo [1O2]rx,sh are approximately 0.56 (0.26) and 0.72 (0.21) mm (or 3.6×107 and 4.6×107 singlet oxygen per cell to reduce the cell survival to 1/e) for Photofrin® and BPD3 h, respectively, assuming that the fraction of generated singlet oxygen that interacts with the cell is 1. The [1O2]rx,sh value for BPD15 min (0.12) was substantially lower than that for a DLI of 3 h. While the values for the photochemical parameters (ξ, σ, g, β) used for BPD were preliminary and may need further refinement, there is reasonable confidence for the values of [1O2]rx,sh. For mTHPC-PDT, the [1O2]rx,sh value derived from in-vivo mouse study was reported to be 0.4 mm. In comparison, the singlet oxygen required per cell was reported to be 9×108 per cell per 1/e fractional kill in an in-vitro mTHPC-PDT study on a rat prostate cancer cell line (MLL cells) and was reported to be 7.9 mm for a multicell in-vitro EMT6/Ro spheroid model for mTHPC-PDT. Conclusions: The experimental results of [1O2]rx,sh in an in-vivo RIF tumor model for Photofrin®, BPD, and mTHPC are about 20 times smaller than those observed in vitro. These results are consistent with the knowledge that factors other than singlet oxygen-mediated tumor cell killing can contribute to PDT damage in-vivo. © 2015 by De Gruyter.