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

We report the effects of singlet oxygen (O-1(2)) and non-O-1(2)-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-O-1(2)-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 O-1(2)-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-O-1(2)-mediated mechanism, while the latter is degraded via a O-1(2)-mediated mechanism. Comparisons of the cytotoxic effects of EtNBSe,vith those of Photofrin(R) (a drug that is degraded via a O-1(2)-mediated mechanism) indicate that the lower threshold O-1(2) dose and the higher extinction coefficient and O-1(2) yield for EtNBSe do not necessarily result in improved photodynamic effects, thus emphasizing the importance of the sensitizer photobleaching mechanism for dosimetry.