In-vivosinglet 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 for type-II photosensitizers during photodynamic therapy (PDT). Anin-vivomodel to determine the singlet oxygen threshold dose, [1O2]rx,sh, for PDT was developed.


METHODS
Anin-vivoradiation-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.


RESULTS
The mean values (standard deviation) of thein-vivo[1O2]rx,share approximately 0.56 (0.26) and 0.72 (0.21) mM (or 3.6×107and 4.6×107singlet oxygen per cell to reduce the cell survival to 1/e) for Photofrin®and BPD, respectively, assuming that the fraction of generated singlet oxygen that interacts with the cell is 1. 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 the singlet oxygen threshold doses.


CONCLUSIONS
In comparison, the [1O2]rx,shvalue derived fromin-vivomouse study was reported to be 0.4 mM for mTHPC-PDT. However, the singlet oxygen required per cell is reported to be 9×108per cell per 1/efractional kill in anin-vitromTHPC-PDT study on a rat prostate cancer cell line (MLL cells) and is reported to be 7.9 mM for a multicellin-vitroEMT6/Ro spheroid model for mTHPC-PDT. A theoretical analysis is provided to relate the number ofin-vitrosinglet oxygen required per cell to reach cell killing of 1/etoin-vivosinglet oxygen threshold dose (in mM). The sensitivity of threshold singlet oxygen dose for our experiment is examined. The possible influence of vascular vs. apoptotic cell killing mechanisms on the singlet oxygen threshold dose is discussed by comparing [1O2]rx,shfor BPD with 3 hr and 15 min drug-light-intervals, with the later being known to have a dominantly vascular effect.


CONCLUSIONS
The experimental results of threshold singlet oxygen concentration in anin-vivoRIF tumor model for Photofrin®, BPD, and mTHPC are about 20 times smaller than those observedin vitro. These results are consistent with knowledge that factors other than singlet oxygen-mediated tumor cell killing can contribute to PDT damagein-vivo.
Timothy C
TC
Zhu
Department of Radiation Oncology, University of Pennsylvania, TRC 4W, 3400 Civic Center Blvd, Philadelphia, Pennsylvania 19104, United States.
Michele M
MM
Kim
Department of Radiation Oncology, University of Pennsylvania, TRC 4W, 3400 Civic Center Blvd, Philadelphia, Pennsylvania 19104, United States.
Xing
X
Liang
Department of Radiation Oncology, University of Pennsylvania, TRC 4W, 3400 Civic Center Blvd, Philadelphia, Pennsylvania 19104, United States.
Jarod C
JC
Finlay
Department of Radiation Oncology, University of Pennsylvania, TRC 4W, 3400 Civic Center Blvd, Philadelphia, Pennsylvania 19104, United States.
Theresa M
TM
Busch
oxygen dependence of singlet oxygen quantum yield; photodynamic therapy; singlet oxygen production