Squaraine Dyes for Photodynamic Therapy: Mechanism of Cytotoxicity and DNA Damage Induced by Halogenated Squaraine Dyes Plus Light (>600 nm)¶

Halogenated squaraine dyes 1 and 2 possess favorable photophysical and in vitro photobiological properties that make these new class of molecules interesting for photodynamic therapeutic applications. For a better understanding of the mechanism of their photobiological activity, we have analyzed the DNA damage and the cytotoxicity induced by these photosensitizers in mammalian cells and cell‐free systems in the presence and absence of various additives and scavengers. Both photoactivated squaraines were found to be similar efficient in inducing single‐strand breaks (SSB) in cell‐free DNA when compared with the cellular DNA. Superoxide dismutase and catalase did not show any influence. However, the presence of tert ‐butanol and glutathione inhibited the formation of the DNA SSB, indicating an indirect (possibly squaraine radical mediated) mechanism under cell‐free conditions. Replacing H 2 O in the buffer by D 2 O resulted in a five‐ to six‐fold increase in the number of the SSB in cell‐free DNA and a significant enhancement of the photocytotoxicity in mouse lymphoma cells. The results demonstrate that singlet oxygen is the major reactive species under cell‐free and cellular conditions and confirm that squaraine‐based sensitizers 1 and 2 can have potential applications in photodynamic therapy.