Dj. Robinson et al., Protoporphyrin IX fluorescence photobleaching during ALA-mediated photodynamic therapy of UVB-induced tumors in hairless mouse skin, PHOTOCHEM P, 69(1), 1999, pp. 61-70
Fluorescence photobleaching of protoporphyrin IX (PpIX) during superficial
photodynamic therapy (PDT), using 514 nm excitation, was studied in UVB-ind
uced tumor tissue in the SKH-HR1 hairless mouse. The effects of different i
rradiance and light fractionation regimes upon the kinetics of photobleachi
ng and the PDT-induced damage were examined. Results show that the rate of
PpIX photobleaching (i.e. fluorescence intensity vs fluence) and the PDT da
mage both increase with decreasing irradiance. We have also detected the fo
rmation of fluorescent PpIX photoproducts in the tumor during PDT, although
the quantity recorded is not significantly greater than generated in norma
l mouse skin, using the same light regime. The subsequent photobleaching of
the photoproducts also occurs at a rate (vs fluence) that increases with d
ecreasing irradiance. In the case of light fractionation, the rate of photo
bleaching increases upon renewed exposure after the dark period, and there
is a corresponding increase in PDT damage although this increase is smaller
than that observed with decreasing irradiance, The effect of fractionation
is greater in UVB-induced tumor tissue than in normal tissue and the damag
e is enhanced when fractionation occurs at earlier time points. We observed
a variation in the distribution of PDT damage over the irradiated area of
the tumor: at high irradiance a ring of damage was observed around the peri
phery. The distribution of PDT damage became more homogeneous with both low
er irradiance and the use of light fractionation, The therapeutic dose deli
vered during PDT, calculated from an analysis of the fluorescence photoblea
ching rate, shows a strong correlation with the damage induced in normal sk
in, with and without fractionation. The same correlation could be made with
the data obtained from UVB-induced tumor tissue using a single light expos
ure. However, there was no such correlation when fractionation schemes were
employed upon the tumor tissue.