Effect of 1O2 quencher depletion on the efficiency of photodynamic therapy

Al(III) phthalocyanine chloride tetrasulfonic acid (AlPcS4) fluorescence and photodynamic oxygen consumption were monitored during AlPcS4-photodynamic therapy (PDT) of Mat LyLu cells in suspension. These measurements were used to calculate the PDT efficiency, which is defined as the oxygen consumption rate divided by the sensitizer concentration. As a function of the intracellular oxygen concentration consumed by PDT, the normalized PDT efficiency fell off more quickly at lower photosensitizer concentrations. The changes in PDT efficiency were compared to models of PDT in which the photosensitizer (PS) and singlet oxygen quencher (A) were either free to diffuse or were fixed. The model in which PS and A are free to diffuse did not agree with the experimental data because this model predicts that the reduction in PDT efficiency is independent of [PS]. A Monte Carlo model was written to simulate PDT when both PS and A are stationary. This model was found to describe the experimental data when the initial intracellular [A] = 90 mM and when the initial and final (i.e. after all A has been depleted) singlet oxygen lifetimes were 0.4 and 1.2 μs respectively.