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An imaging system was used to monitor the fluorescence of disulphonated Aluminum Phthalocyanine during the PDT of tumors to check whether an index for therapy effectiveness could be found out. Two excitation wavelengths were tested to discover a possible change in the absorption spectrum of the sensitizer induced by the treatment. Two opposite phenomena take place in the tumor area: a mild photobleaching in the region directly irradiated and a marked increase in the fluorescence signal at the tumor borders. The different behavior observed with the two excitation wavelengths demonstrates that the drug absorption spectrum shifts towards shorter wavelengths as a result of the irradiation.

Time-resolved reflectance spectroscopy was used to investigate the effects of photodynamic irradiation on the in vivo absorption properties of disulphonated Aluminum Phthalocyanine (AlS2Pc) in tumor-bearing mice. Mice bearing tumors injected either intramuscularly or intradermally were administered with 5 mg/kg body weight of AlS2Pc. Uptake times of 3 h and 12 h were examined. Absorption spectra were measured from 610 to 700 nm before, during and after the irradiation. The administration of a therapeutic light dose causes a blue-shift and a reduction in the drug absorption, which are more marked 3 h than 12 h after drug injection.

The estimate of the optimal light dose is a basic step in the optimization of the protocol for the photodynamic therapy (PDT) of tumors. It depends essentially on the optical properties of the tissue to be treated and on the changes induced by the administration of the photosensitizer (i.e., essentially, the absorption increase).