Modification of fluorescent nanocrystals with 6-thioguanine: monitoring of drug delivery

The purpose of the presented work was to modify the surface of core/shell-type CdSexS1-x/ZnS quantum dots (QDs) with an anticancer drug to form bifunctional nanoconjugate for imaging and drug delivery. 6-Thioguanine (6-TG) which is used to treat acute myeloid and lymphoblastic leukemia was applied for this aim. The modification of the nanocrystals was carried out using a biphasic method, based on transferring of QDs from the organic phase to the aqueous one with the simultaneous exchange of hydrophobic ligand to the hydrophilic one. To enable a complex evaluation of 6-TG bioactivity, the surface of quantum dots was also modified using mercaptoacetic acid (MAA). Mercaptoacetic acid was chosen as a biologically neutral ligand; so, it was possible to evaluate the effect of nanocrystals toxicity without the participation of the drug (6-TG ligand versus neutral ligand). Moreover, the effect of 6-TG itself on cells was also studied. The studies were carried out on three cell lines: K562 (human myeloid leukemia), A549 (human lung cancer cells) and MRC-5 (normal human lung cells). Evaluation of the cytotoxicity of nanostructures, at various concentrations, was carried out after 24- and 48-h incubation of the cell culture with nanoparticles/drug solutions. MTT and the Alamar Blue assays were used to determine cell viability. Based on the cytotoxicity measurements, it was found that quantum dots modified with 6-TG are more toxic than the drug itself or QDs modified with 3-mercaptoacetic acid. Images taken using fluorescence and confocal microscopy allowed to define the location of examined QDs inside cells.