Myth or Reality: Is Hypoxia Really a Problem in Photodynamic Therapy?

The mechanism of action of Photodynamic Therapy (PDT) has been assumed for decades to be closely related to the concentration of oxygen in tissue. In type II PDT, the production of singlet oxygen (1O2) from triplet oxygen (3O2, ground state) upon light activation, in the presence of a photosensitizer (PS), is considered as the main mechanism responsible for cell death. Since hypoxia is an essential feature of the tumor microenvironment, it has therefore been accepted that PDT is rather inefficient in such conditions. For this reason, there is currently ongoing research towards the development of PSs that are working without the need for the presence of oxygen like PSs that function under type I PDT or of other techniques such as photoactivated chemotherapy (PACT). In this work, we tested the toxicity (IC50 values) of five clinically approved PSs (Protoporphyrin IX, Temoporfin, Talaporfin, LUZ11 and Chlorin e6) in three human cell lines under normoxic (20% 3O2) and hypoxic conditions (2% and 1% 3O2). To our surprise, the IC50 values between normoxia and hypoxia were basically identical. Only significant differences were observed for another known PS, namely Rose Bengal (RB), which is not approved clinically. However, this difference was rationalized by the inability of RB to induce type I PDT under hypoxia, explaining thus the different IC50 values obtained. Due to the unexpected and counter-intuitive nature of these observations, an independent laboratory was contacted to reproduce these results. As anticipated, this research group reached the same conclusions. Through specific ROS detection and photocatalytic oxidation assays, we proposed three potential explanations: contribution of type I PDT, residual type II PDT activity, and additional mechanisms such as photoredox catalysis, all of which could help overcome the limitations imposed by hypoxia. Overall, our results contradict the dogma currently in place stating the need of oxygen in PDT and open up the fundamental question of the mechanisms behind PDT.