Cellular Localisation of Antitumoral 6-Alkyl Thymoquinones Revealed by an Alkyne-Azide Click Reaction and the Streptavidin-Biotin System

The subcellular distribution and accumulation of thymoquinone 1, a natural anticancer agent, has hitherto been unknown. We prepared 6-(dec-9-ynyl) thymoquinone 3, an alkyne-labelled derivative with anticancer activity similar to that of its parent compound 1. Alkyne 3 was seen, after a Huisgen-type click reaction with 3-azido-7-hydroxycoumarin, to accumulate in distinct compartments of the nuclei of PtK2 potoroo kidney cells, and in adjoining regions that were stained with an anti-body specific for the Golgi apparatus. In contrast, a biotin-labelled thymoquinone 4 seemed to accumulate across the entire cell nucleus upon visualisation with streptavidin; but this was less easily traceable because of co-staining of other structures such as mitochondria. In conclusion, for small drug-like molecules, visualisation by alkyne-azide cycloaddition seems to be superior to conventional visualisation by the biotin-streptavidin system.