Photobleaching kinetics, photoproduct formation, and dose estimation during ALA induced PpIX PDT of MLL cells under well oxygenated and hypoxic conditions.

Fluorescence photobleaching and photoproduct formation were investigated during delta-aminolevulinic acid (ALA) induced protoporphyrin IX (PpIX) PDT of MLL cells in vitro. Cells were incubated in either 0.1 or 1.0 mM ALA for 4 h and were treated with 532 nm or 635 nm light under well oxygenated or hypoxic conditions. Fluorescence spectra were acquired during treatment. Photobleaching and photoproduct formation were quantified using singular value decomposition fitting of fluorescence spectra to experimentally determined basis spectra for PpIX, photoprotoporphyrin (Ppp), product II (peak at 655 nm), and product III (peak at 618 nm). PpIX photobleaching occurred under both normal and hypoxic conditions. The photobleaching kinetics could not be explained by purely first- or second-order photobleaching kinetics, and were attributed to differences in PpIX binding at the two ALA incubation concentrations. Ppp was the main photoproduct and accumulated in higher levels in the absence of oxygen, likely a result of reduced Ppp photobleaching under hypoxia. Increases in product II fluorescence occurred mainly in the presence of oxygen. To assess potential fluorescence based PDT dose metrics, cell viability was measured at select times during treatment using a colony formation assay. Cell survival correlated well to changes in product II fluorescence, independent of oxygenation, sensitizer concentration, and treatment wavelength, suggesting that this product is primarily a result of singlet oxygen mediated reactions and may potentially be useful to quantify singlet oxygen dose during PDT.

Characterization of Photofrin photobleaching for singlet oxygen dose estimation during photodynamic therapy of MLL cells in vitro.

A singlet oxygen dose model is developed for PDT with Photofrin. The model is based on photosensitizer photobleaching kinetics, and incorporates both singlet oxygen and non-singlet oxygen mediated bleaching mechanisms. To test our model, in vitro experiments were performed in which MatLyLu (MLL) cells were incubated in Photofrin and then irradiated with 532 nm light. Photofrin fluorescence was monitored during treatment and, at selected fluence levels, cell viability was determined using a colony formation assay. Cell survival correlated well to calculated singlet oxygen dose, independent of initial Photofrin concentration or oxygenation. About 2 x 10(8) molecules of singlet oxygen per cell were required to reduce the surviving fraction by 1/e. Analysis of the photobleaching kinetics suggests that the lifetime of singlet oxygen in cells is 0.048 +/- 0.005 micros. The generation of fluorescent photoproducts was not a result of singlet oxygen reactions exclusively, and therefore did not yield additional information to aid in quantifying singlet oxygen dose.

Calculation of singlet oxygen dose from photosensitizer fluorescence and photobleaching during mTHPC photodynamic therapy of MLL cells.

Predicting the therapeutic outcome of photodynamic therapy (PDT) requires knowledge of the amount of cytoxic species generated. An implicit approach to assessing PDT efficacy has been proposed where changes in photosensitizer (PS) fluorescence during treatment are used to predict treatment outcome. To investigate this, in vitro experiments were performed in which Mat-LyLu cells were incubated in meta-tetra(hydroxyphenyl)chlorin (mTHPC) and then irradiated with 652 nm light. PS concentration, fluence rate and oxygenation were independently controlled and monitored during the treatment. Fluorescence of mTHPC was monitored during treatment and, at selected fluence levels, cell viability was determined using a colony-formation assay. Singlet oxygen dose was calculated using four different models and was compared with cell survival. For the dose metric based on singlet oxygen-mediated PS photobleaching, a universal relationship between cell survival and singlet oxygen dose was found for all treatment parameters. Analysis of the concentration dependence of bleaching suggests that the lifetime of singlet oxygen within the cell is 0.05-0.25 micros. Generation of about 9 x 10(8) molecules of singlet oxygen per cell reduces the surviving fraction by 1/e.

Relationship between mTHPC fluorescence photobleaching and cell viability during in vitro photodynamic treatment of DP16 cells.

An implicit dosimetric model has been proposed in which biological damage caused by photodynamic therapy (PDT) is monitored through the decrease in sensitizer fluorescence during treatment. To investigate this, in vitro experiments were performed in which DP16 cells were incubated in meta-tetra(hydroxyphenyl)chlorin (mTHPC) and then irradiated with 514 nm light. Photosensitizer concentration, fluence rate and oxygenation were independently controlled and monitored during the treatment. Fluorescence of mTHPC was continuously monitored via a charge-coupled device-coupled spectrometer during treatment and, at selected fluence levels, cell viability was determined using a trypan blue exclusion assay. The relationship of cell viability to normalized fluorescence was obtained for the different treatment conditions. The relationship was independent of cell medium oxygenation, treatment fluence rate and sensitizer incubation concentration except at a high mTHPC concentration (4 microg/mL). This relationship suggests that fluorescence bleaching may be used to predict mTHPC PDT damage in vitro.