Aromatic cationic
dyes have a potential as photo-chemotherapeutic agents because they are selectively concentrated into the mitochondria of cancerous cells. The mechanism of cytophototoxicity has been proposed to be primarily due to
dye sensitized photogeneration of highly toxic
singlet oxygen (1O2) at the mitochondria. We tested this hypothesis by measuring the relative
phototoxicity of a collection of aromatic cationic
dyes towards respiring rat-liver mitochondria (RLM), upon addition of 514 nm
laser light. Effectiveness of
dye photosensitization towards destruction of RLM function was assayed by its effect on the RLM membrane potential. Three physical parameters of
dye phototoxicity were independently measured and a relative
phototoxicity calculated assuming adherence of mechanism to the 1O2 hypothesis. Quantum yields of
dye sensitized 1O2 production were estimated, either from time-resolved luminescence measurements of photosensitized 1O2 formed, or by comparing rates of photobleaching of 1O2 trap; the relative partition of
dye into mitochondrial
lipid was determined gravimetrically; and the optical density of
dye was determined in a
lipid like
Triton X-100 micellar environment. Under the assumption of the 1O2 hypothesis, these parameters were used to predict a relative
phototoxicity which was compared with that observed. For 12 of the 14
dyes investigated, the observed and predicted phototoxicities were linearly correlated (r = 0.85) suggesting support of the 1O2 hypothesis.
Carbocyanines DiOC2(3) and DiSC2(3) did not correlate and were found to be 10 and 1000 times more potent than predicted, suggesting an additional factor at play in their
phototoxicity.