Fluorescence Photobleaching of ALA‐induced Protoporphyrin IX during Photodynamic Therapy of Normal Hairless Mouse Skin: The Effect of Light Dose and Irradiance and the Resulting Biological Effect

Abstract

The photobleaching of 5-aminolaevulinic acid (ALA)-induced protoporphyrin IX (PpIX) was investigated during superficial photodynamic therapy (PDT) in normal skin of the SKH HR1 hairless mouse. The effects of light dose and fluence rate on the dynamics and magnitude of photobleaching and on the corresponding PDT-induced damage were examined. The results show that the PDT damage cannot be predicted by the total light dose. Photobleaching was monitored over a wide range of initial PpIX fluorescence intensities. The rate of PpIX photobleaching is not a simple function of fluence rate but is dependent on the initial concentration of sensitizer. Also, at high fluence rates (50-150 mW/cm2, 514 nm) oxygen depletion is shown to have a significant effect. The rate of photobleaching with respect to light dose and the corresponding PDT damage both increase with decreasing fluence rate. We therefore suggest that the definition of a bleaching dose as the light dose that causes a 1/e reduction in fluorescence signal is insufficient to describe the dynamics of photobleaching and PDT-induced damage. We have detected the formation of PpIX photoproducts during the initial period of irradiation that were themselves subsequently photobleached. In the absence of oxygen, PpIX and its photoproducts are not photobleached. We present a method of calculating a therapeutic dose delivered during superficial PDT that demonstrates a strong correlation with PDT damage.

Correlation between Protoporphyrin IX Fluorescence Intensity, Photobleaching, Pain and Clinical Outcome of Actinic Keratosis Treated by Photodynamic Therapy

“…The correlation, with a slope of 1, is in agreement with several studies reporting that, in well-oxygenated conditions, the PpIX and its photoproducts can be efficiently and completely bleached during the illumination [19,28,31,58,59]. It is also worth noting that this correlation is observed for all the treated lesions, independently of the anatomical place and independently of all other possible tissue parameters.…”

“…This may be due to the fact that PDT efficacy depends on several parameters, including the PS's microscopic and macroscopic distribution at treatment time, the light dose, the tissular oxygen distribution, the irradiance and the tissue's optical properties at the treatment wavelength. Therefore, it is difficult to ensure that optimal treatment parameters are applied during each individual treatment [19]. …”

Monitoring blood volume and saturation using superficial fibre optic reflectance spectroscopy during PDT of actinic keratosis

“…They may also be related to adverse events such as pain during treatment. To date, several studies have revealed the importance of using techniques that monitor these parameters during PDT to optimize individual patient dosimetry [1][2][3][4][5][6][7].…”

Protoporphyrin IX fluorescence photobleaching is a useful tool to predict the response of rat ovarian cancer following hexaminolevulinate photodynamic therapy

“…Implicit dosimetry aims to exploit a single parameter that integrates the combined effect of the photosensitizer, light, and oxygen without the need to measure them. Photosensitizer photobleaching is the most promising and the most highly developed implicit metric that has been successfully investigated [38,39]. It is defined as the light induces a loss of absorption or emission intensity and can be done monitoring the photosensitizer fluorescence or measuring fluorescence spectra of the photoproduct associated with photobleaching [18,40].…”