Cellular uptake and cytotoxicity of octahedral rhenium cluster complexes

Cellular uptake behavior of a novel class of octahedral rhenium cluster compounds, hexahydroxo complexes K₄[{Re₆S₈}(OH)₆] · 8H₂O (1) and K₄[{Re₆Se₈}(OH)₆] · 8H₂O (2), was evaluated in human cervical adenocarcinoma HeLa cells. Confocal microscopy and flow cytometry studies demonstrated that rhenium cluster 1 was not internalized into cell, while rhenium cluster 2 was. Conjugation of a polymer to rhenium cluster 1, namely the derivative K₄[{Re₆S₈}(OH)₅L] (3) (L is amphiphilic diblock copolymer MPEG550-CH₂CONH-GlyPheLeuGlyPheLeu-COO^-), considerably enhanced cellular uptake in a concentration-dependent manner and was predominantly localized in the cytoplasm and nucleus upon incubation time. The uptake of rhenium cluster 2 was mediated by energy-dependent endocytosis, whereas rhenium cluster 3 was directly ingested into cells by cell-fusion-like mechanism. According to the cytotoxicity evaluation test, both rhenium clusters 2 and 3 did not exhibit acute cytotoxic effects up to 50 μM, at the practical concentration level of biological applications. It is, therefore, expected that the rhenium cluster complexes can be promising potential candidates as diagnostic agents for medical treatment.