In vivo measurement of 5-aminolaevulinic acid-induced protoporphyrin IX photobleaching: a comparison of red and blue light of various intensities


Corresponding author:
Martin O'Dywer
Department of Physics and Astronomy
University of Glasgow
Glasgow G12 8QQ
Tel: +44 (0)141 330 6403
Fax: +44 (0)141 330 2893


Background: In recent years, 5-aminolaevulinic acid (ALA) has become an increasingly popular photosensitizing drug for use in both photodynamic therapy (PDT) and photodetection (PD) of cancers. ALA metabolizes within tissue to form the photosensitizer protoporphyrin IX (PpIX). Like most photosensitizers, PpIX is fluorescent, and this fluorescence progressively decreases during PDT. This phenomenon is referred to as photobleaching.

Aim: Our aim in carrying out this experiment was twofold: firstly, to compare the relative capacity of red and blue light to cause photobleaching; and secondly, to compare the capacity of a fixed light dose to cause photobleaching, when delivered at different intensities.

Method: In this paper, we describe the implementation of a compact fluorescence spectrometer in monitoring the photobleaching of ALA-induced PpIX in vivo on the skin of healthy volunteers.

Results: We have been able to show that blue light causes more rapid photobleaching than red light, and that under illumination with red or blue light, delivery of a fixed light dose at a lower intensity results in more photobleaching.

Conclusion: Comparison of the photobleaching rates suggests that a blue light intensity of 5 mW/cm2 gives the same rate of photobleaching as the typical red light PDT intensity of 100 mW/cm2. Further investigation of the correlation between PpIX photobleaching and PDT effect would be beneficial in interpreting the clinical significance of our findings.