Background: Application of cold atmospheric plasma to medium generates “plasma-activated
medium” that induces apoptosis selectively in tumor cells and that has an antitumor effect in vivo. The underlying
mechanisms are not well understood.
Objective: Elucidation of potential chemical interactions within plasma-activated medium and of reactions of
medium components with specific target structures of tumor cells should allow to define the active principle in
plasma activated medium.
Methods: Established knowledge of intercellular apoptosis-inducing reactive oxygen/nitrogen species-dependent
signaling and its control by membrane-associated catalase and SOD was reviewed. Model experiments using
extracellular singlet oxygen were analyzed with respect to catalase inactivation and their relevance for the
antitumor action of cold atmospheric plasma. Potential interactions of this tumor cell-specific control system
with components of plasma-activated medium or its reaction products were discussed within the scope of the
reviewed signaling principles.
Results: None of the long-lived species found in plasma-activated medium, such as nitrite and H2O2, nor OCl- or
.NO seemed to have the potential to interfere with catalase-dependent control of apoptosis-inducing signaling of
tumor cells when acting alone. However, the combination of H2O2 and nitrite might generate peroxynitrite. The
protonation of peroxnitrite to peroxynitrous acid allows for the generation of hydroxyl radicals that react with
H2O2, leading to the formation of hydroperoxide radicals. These allow for singlet oxygen generation and inactivation
of membrane-associated catalase through an autoamplificatory mechanism, followed by intercellular
Conclusion: Nitrite and H2O2 in plasma-activated medium establish singlet oxygen-dependent interference
selectively with the control system of tumor cells.