Is the photobleaching signal a useful tool for dosimetry during photodynamic therapy?

Photosensitizing drugs (e.g. porphyrins) are likely to be degraded upon irradiation. The mechanisms of photodegradation are complex. The photodegradation behavior of photosensitizers used in photodynamic therapy (PDT) could be used as a tool for dosimetry and for protecting adjacent tissue against photodynamic effects. The photobleaching behavior of (delta) -aminolevulinic acid induced porphyrins has been studied. The results of in vitro experiments (Protoporphyrin IX in organic solvent) and the in vivo tests (SSK2-fibrosarcoma on C3H-mice) have shown the bleaching rate to be dependent on the environment of the photosensitizer. During its photodegradation a chlorine-type photoproduct is formed. Its spectrometric detection reveals on signals proposed for dosimetry in PDT.