Significance: During the last decades, thiosemicarbazones have been clinically developed for a variety of diseases including tuberculosis, viral infections, malaria and cancer. With regard to malignant diseases, the class of α-N-heterocyclic thiosemicarbazones, and here especially Triapine, was intensively developed in multiple clinical phase I/II trials. Recent Advances: Very recently two new derivatives, namely COTI-2 and DpC have entered phase I evaluation. Based on the strong metal-chelating/metal-interacting properties of thiosemicarbazones, interference with the cellular iron (and copper) homeostasis is assumed to play an important role in their biological activity. Critical Issues: In this review, we summarize and analyze the data on the interaction of (α-N-heterocyclic) thiosemicarbazones with iron, with the special aim to bridge the current knowledge on their mode of action from chemistry to (cell) biology. In addition, we highlight the difference to classical iron(III) chelators such as desferrioxamine (DFO) which are used for the treatment of iron overload. Future Directions: We want to emphasize that thiosemicarbazones are not solely removing iron from the cells/organism. In contrast, they should be considered as iron-interacting drugs influencing diverse biological pathways with a complex and multi-facetted mode of action. Consequently, in addition to the discussion of physicochemical properties (e.g. complex stability, redox activity), this review contains an overview on the diversity of cellular thiosemicarbazone targets and drug resistance mechanisms.

The interactions of various insulin mimetic oxovanadium(IV) compounds with serum proteins were studied in model systems and in ex vivo samples. For the modeling study, an earlier in situ method was extended and applied to the formation of ternary complexes of apotransferrin (apoTf)–V^IVO–maltol (mal) and 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone (dhp). Both systems were evaluated via simultaneous CD and EPR measurements. Determination of the formation constants of the ternary complexes allowed the calculation of more accurate stability constants for the V^IVO–apoTf parent complexes and establishment of a better model for drug speciation in serum. It was found that dhp and the synergistic carbonate are non-competitive binders. Based on the stability constants obtained for V^IVO–apoTf complexes and estimated for V^IVO–HSA (= human serum albumin), modeling calculations were performed on the distribution of V^IVO among the components of blood serum. The results were confirmed by HPLC-ICP-MS (liquid chromatography-inductively coupled plasma spectroscopy-mass spectrometry) measurements. The ex vivo interactions of the V^IVO complexes formed with mal, picolinic acid (pic) and dhp with serum protein standards and also with human serum samples were evaluated. The proteins were firstly separated by (HPLC), and the V content of each fraction was determined by ICP-MS. All the studied V^IVO compounds displayed similar chromatographic profiles, associated almost exclusively with apotransferrin as predicted by the modeling calculations. Under physiological conditions, the interactions with HSA of all of the species under study were negligible. Therefore Tf seems to be the main V^IVO transporter in the serum under in vitro conditions, and this association is practically independent of the chemical form in which V^IVO is administered.