Calculation of Singlet Oxygen Dose from Photosensitizer Fluorescence and Photobleaching During mTHPC Photodynamic Therapy of MLL Cells^¶

Abstract

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 μs. Generation of about 9 × 108 molecules of singlet oxygen per cell reduces the surviving fraction by 1/e.

Photodynamic antimicrobial effect of safranine O on an ex vivo periodontal biofilm

“…The combination of laser light and PS obviously effects bacterial life of deeper biofilm layers to a greater extend than treatment with CHX. Another reason for the modest increase in bacterial reduction might be caused by photobleaching of safranine O. Photobleaching refers to an irreversible destruction of PSs during illumination with light and has been measured for a vast variety of dyes [50,51]. Throughout the present study there were no further examinations performed which observed the photobleaching of safranine O during photodynamic treatment.…”

Photophysical Parameters, Photosensitizer Retention and Tissue Optical Properties Completely Account for the Higher Photodynamic Efficacy of meso‐Tetra‐Hydroxyphenyl‐Chlorin vs Photofrin¶

“…Dysart et al (32) performed PDT on cells sensitized with mTHPC and observed oxygen-dependent photobleaching with a faster rate of bleaching under air-saturated conditions. In a subsequent study (33) the same authors examined the relationship between mTHPC photobleaching and cell survival in response to PDT and reported not only a close correlation for dose metric models based on '02-mediated photobleaching and cell survival, but also a dependence of mTHPC bleaching rate on sensitizer incubation concentration.…”

Protoporphyrin IX Fluorescence and Photobleaching During Interstitial Photodynamic Therapy of Malignant Gliomas for Early Treatment Prognosis

“…This model relies on the assumption that PDT-induced singlet oxygen causes tissue damage as well as photodegradation of the photosensitiser. The usefulness of an implicit dosimetry model has been confirmed in pre-clinical models [24,25] as well as for human skin-PDT [26,27] where a rapid photobleaching has been shown to correlate with improved treatment outcome. Speculatively, the PpIXspecific photobleaching kinetics might be employed for early treatment prognosis and to assure the application of a sufficient light dose in order to improve treatment efficacy [28].…”