In-vivo singlet oxygen threshold doses for PDT

Abstract

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). An in-vivo model to determine the singlet oxygen threshold dose, [1O2]rx,sh, for PDT was developed. 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. 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 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. Discussion In comparison, the [1O2]rx,sh value derived from in-vivo mouse study was reported to be 0.4 mM for mTHPC-PDT. However, the singlet oxygen required per cell is 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 is reported to be 7.9 mM for a multicell in-vitro EMT6/Ro spheroid model for mTHPC-PDT. A theoretical analysis is provided to relate the number of in-vitro singlet oxygen required per cell to reach cell killing of 1/e to in-vivo singlet 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,sh for 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 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 knowledge that factors other than singlet oxygen-mediated tumor cell killing can contribute to PDT damage in-vivo.

Macroscopic singlet oxygen modeling for dosimetry of Photofrin-mediated photodynamic therapy: an in-vivo study

“…ROS interact with substrates to cause biological damage. [8][9][10] The photosensitizer triplet may react directly with a biological substrate under hypoxic conditions. In type II interactions, the excited triplet will transfer energy to 3 O 2 to produce excited-state singlet oxygen ( 1 O 2 ), the major cytotoxic species causing biological damage.…”

“…In type II interactions, the excited triplet will transfer energy to 3 O 2 to produce excited-state singlet oxygen ( 1 O 2 ), the major cytotoxic species causing biological damage. 8 Although the precise mechanisms of PDT are not yet fully understood, it is generally accepted that 1 O 2 is primarily responsible for cell death and PDT outcome for type II photosensitizers. 11 Although the clinical use and indications of PDT are increasing slowly, an ideal dosimetric predictor for PDT efficacy is still elusive, which to some extent, has hindered the clinical application of PDT.…”

Evaluation of singlet oxygen explicit dosimetry for predicting treatment outcomes of benzoporphyrin derivative monoacid ring A-mediated photodynamic therapy

“…[9][10][11][12] Animals were under the care of the University of Pennsylvania Laboratory Animal Resources. All studies were approved by the University of Pennsylvania Institutional Animal Care and Use Committee.…”

Explicit dosimetry for 2-(1-hexyloxyethyl)-2-devinyl pyropheophorbide-a-mediated photodynamic therapy: macroscopic singlet oxygen modeling

“…In the fitting routine, the nonlinear solver starts from initial points selected based on the previous published data for the other photosensitizers (Photofrin, ALA − PpIX 2 , and mTHPC), reaching different locally optimal solutions. 7,10,11,20,21 An initial guess of ξ, σ, g, and ½ 1 O 2 rx;sh is assigned randomly within the ranges of ð10 to 100Þ × 10 −3 cm 2 mW −1 s −1 , ð1 to 10Þ × 10 −5 μM −1 , 0.7 to 1.5 μM∕s, and 0.5 to 1.0 mM, respectively. The current algorithm will encounter convergence issues when given too many free-floating parameters.…”