Abstract
Nine photo-stable Pt(II) Schiff base complexes [Pt(O^N^N^O)] (Pt1–Pt9) containing tetradentate salicylaldimine chelating ligands have been synthesized and characterized as potential photosensitisers for photodynamic therapy (PDT). The effects of electron-withdrawing versus electron-donating substituents on their electronic spectral properties are investigated. Pt1–Pt9 show broad absorption bands between 400–600 nm, which makes them useful for green-light photodynamic therapy. The complexes showed intense phosphorescence with emission maxima at ca. 625 nm. This emission was used to track their cellular localization in cancer cells. Confocal cellular imaging showed that the complexes localized mostly in the cytoplasm. In the dark, the complexes were non-toxic to A549 human lung cancer cells, but exhibited high photo-toxicity upon low-dose green light (520 nm, 7.02 J/cm2) irradiation via photo-induced singlet oxygen generation. Thus, these photoactive Pt(II) complexes have the potential to overcome the problem of drug resistance and side effects of current clinical Pt(II) drugs, and to act as both theranostic as well as therapeutic agents.
Highlights
Photodynamic therapy (PDT) is a non-invasive method used to treat several types of cancer as well as bacterial or fungal infections, and various skin diseases [1]
The phosphorescence lifetime of Pt7 in acetonitrile and glycerol indicated (Fig. 4a) that Pt7 has a more than 40x longer excited-state lifetime time in glycerol (3277 ns) than in acetonitrile (80.7 ns). These results indicate that high viscosity and hydrophobic environments enhance the phosphorescence intensity of these Pt(II) Schiff base complexes
All the complexes were non-toxic to A549 lung cancer cells
Summary
Photodynamic therapy (PDT) is a non-invasive method used to treat several types of cancer as well as bacterial or fungal infections, and various skin diseases [1]. The PSs generate reactive oxygen species (ROS) such as singlet oxygen and hydroxyl radicals [3] These ROS cause severe oxidative damage to biomacromolecules such as proteins and DNA in cancer cells, which triggers cell death [4]. The current clinically approved PSs such as Photofrin, Chlorin e6 and Foscan share a similar tetrapyrrolic scaffold [5]. As a result, these PSs have similar limitations in clinical use, including 1) photo-instability; 2) high tissue retention times; 3) severe skin sensitivity, and 4) side effects such as hepatitis [6,7]. The Pd(II) complex, WST11 shows high absorbance at 763 nm with the 2–3Â deeper light penetration depth than
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