Fluorescent Copper(II) Bis(thiosemicarbazonates): Synthesis, Structures, Electron Paramagnetic Resonance, Radiolabeling, In Vitro Cytotoxicity and Confocal Fluorescence Microscopy Studies

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Abstract
Copper bis(4‐ethyl‐3‐thiosemicarbazonato) acenaphthenequinone (1) and copper bis(4‐methyl‐3‐thiosemicarbazonato) acenaphthenequinone (2) are synthesized and characterized in solution, in the solid state, and radiolabeled. Serum‐protein binding radioassays show good stability in solution and about 25 % binding to protein over 1 h, which is comparable with the hypoxia selective tracer [64Cu(ATSM)]. Cyclic voltammetry shows fast and reversible reduction at redox potentials similar to the values known for hypoxia‐selective copper compounds. However, despite this, complex 1 does not show any hypoxic‐selective uptake in HeLa cells over 1‐h standard assays. Possible reasons for this are studied by using the intrinsic fluorescence of the CuII complexes to determine the cellular distributions and uptake mechanism by confocal microscopy. The complexes are found to bind to the external cell membrane and disperse evenly in the cytoplasm only after a very slow cell internalization (>1 h). No significant changes in distribution are observed by fluorescence imaging under hypoxic conditions. The rate of localization in the cytoplasm contrasts with their ZnII analogues, which are known to have fast cell uptake (up to 20 min) and a clear localization in lysosomes and mitochondria. The cytotoxicity mechanism of 1 over 24 h against a number of adherent cell lines is seen to be by membrane disruption and is of a comparable magnitude to that of [Cu(ATSM)], as demonstrated by methyl tetrazolium (MTT) and lactate dehydrogenase (LDH) assays.

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