Secondary reactive oxygen species production after PDT during pulmonary tumor growth in sera of nude mice

Photodynamic therapy (PDT), mediated by a sensitizer exposed to light to produce singlet oxygen (1O2), induces tumor responses varying from one person to another. Cancer growth induces oxidative stress at any step of its development from induction to treatment, which could also modify response to PDT. After the initial amount of 1O2 delivered, secondary oxidative species (SOS) are also generated inducing additional damages. Using an in vitro assay we saw variations among mice strains concerning their serum capability to generate SOS after 1O2 production. Nude mice had a higher capability to generate SOS as compared to the non mutated strain. Capability to generate SOS evolved during growth of orthotopically-grafted pulmonary cancers (A549), with either values corrected for hemolysis or not. Immediately after graft SOS production decreased, then increased again, reaching a plateau phase after 10 days which lasted for 20 days and finally increased steeply during the last phase of tumor growth, preceding cachexia and death. This profile differed profoundly from the one observed after heterotopic tumor grafts for which hemolysis induced artifacts masking important variations in SOS production. Our results demonstrate experimentally a relationship between the general health status of an individual, cancer progression and serum capability to generate SOS during PDT. These findings could explain some PDT failures as well as some unexpected successes on large tumors and should be taken into account when determining treatment parameters. They may also explain why different effects are observed on different experimental models with similar sensitizers.