INVIVO PHOTOPRODUCT FORMATION DURING PDT WITH ALA-INDUCED ENDOGENOUS PORPHYRINS

The administration of 5-aminolevulinic acid (ALA) in tumor-bearing nude mice leads to the formation of the fluorescent, photounstable photosensitizer protoporphyrin IX in tumor tissue. On-line fluorescence spectroscopy during photodynamic therapy (PDT) shows the in vivo formation of chlorin-type photoproducts of protoporphyrin. The fluorescence of protoporphyrin as well as its photoproducts is bleached completely at the end of the PDT (100 J cm-2, 630 nm). These findings were also verified using ultrashort laser pulses and time-correlated single-photon counting. A photoinduced shortening of the decay times and decrease in the integral fluorescence intensity were measured in vivo due to the photodestruction of the endogenous photosensitizer protoporphyrin IX in the tumor.