In this paper, we report the photodegradation of three different chlorine photosensitizers (Photoditazine®, Radachlorin®, and Foscan®). The photosensitizer degradation was analyzed by changes in the fluorescence spectrum during illumination. The rate of fluorescence variation was normalized to the solution absorption and the photon energy resulting in the determination of the necessary number of photons to be absorbed to induce photosensitizer photodegradation. The parameter for rate of the molecules decay, the photon fluence rate and optical properties of the solution allow us to determine the photosensitizer stability in solution during illumination. The results show that the order of susceptibility for photodegradation rate is: Radachlorin® < Photoditazine® < Foscan®. This difference in the photodegradation rate for Foscan can be explained by the high proportion of aggregates in solution that inhibit the photo-oxidative process that impede the singlet oxygen formation. We hypothesize that there is a correlation between photodegradation rate and photodynamic efficacy witch is governed by the singlet oxygen formation responsible for the most relevant reaction of the cell death photodynamic induction. Then its is important to know the photostability of different types of drugs since the photodegradation rate, the photodegradation as well as the photodynamic efficacy are strong correlated to the oxygen concentration in the tissue.

Using three haematoporphyrin based photosensitizers we investigated their properties related to the phototransformation in solution, the inhibitory concentration (IC50) in cell culture and the threshold dose (D th ) in rat livers. The observation of such properties shows the existence of a strong correlation among them. The photosensitizer with the highest phototransformation rate is also the one with the lowest D th and IC50, showing more efficacy in cell killing and tissue necrosis during photodynamic therapy applications. We discuss this explanation for such correlation and the possible practical applications for this property. Threshold dose, J/cm 3The photostability parameter (η) versus the threshold dose (D TH ) for the three photosensitizers.

No abstract