Singlet oxygen- versus nonsinglet oxygen-mediated mechanisms of sensitizer photobleaching and their effects on photodynamic dosimetry.

We report the effects of singlet oxygen (1O2) and non-1O2-mediated sensitizer photobleaching on oxygen consumption and dosimetry during photodynamic therapy (PDT) of sensitized multicell tumor spheroids. We develop a theoretical model for the description of non-1O2-mediated photobleaching resulting from irreversible reactions of the excited singlet or triplet sensitizer populations with cell substrate. We show that the fluence-dependent simple exponential decay expression of sensitizer degradation is not consistent with these mechanisms and, therefore, with any reasonable mechanism that we consider, because we have shown previously that 1O2-mediated photobleaching cannot be described by a simple exponential with a constant photobleaching coefficient (I. Georgakoudi et al., Photochem. Photobiol. 65, 135-144, 1997). Analysis of oxygen microelectrode measurements performed at the edge of Nile blue selenium (EtNBSe)- and protoporphyrin IX (PpIX)-sensitized spheroids during PDT demonstrates that the former drug photobleaches via a non-1O2-mediated mechanism, while the latter is degraded via a 1O2-mediated mechanism. Comparisons of the cytotoxic effects of EtNBSe with those of Photofrin (a drug that is degraded via a 1O2-mediated mechanism) indicate that the lower threshold 1O2 dose and the higher extinction coefficient and 1O2 yield for EtNBSe do not necessarily result in improved photodynamic effects, thus emphasizing the importance of the sensitizer photobleaching mechanism for dosimetry.