In Vitro Characterization of the Pharmacological Properties of the Anti-Cancer Chelator, Bp4eT, and Its Phase I Metabolites

Desulfurization of thiosemicarbazones: the role of metal ions and biological implications

Thiosemicarbazones are biologically active substances whose structural formula is formed by an azomethine, an hydrazine, and a thioamide fragments, to generate a R2C=N-NR-C(=S)-NR2 backbone. These compounds often act as ligands to generate highly stable metal-organic complexes. In certain experimental conditions, however, thiosemicarbazones undergo reactions leading to the cleavage of the chain. Sometimes, the breakage involves desulfurization processes. The present work summarizes the different chemical factors that influence the desulfurization reactions of thiosemicarbazones, such as pH, the presence of oxidant reactants or the establishment of redox processes as those electrochemically induced, the effects of the solvent, the temperature, and the electromagnetic radiation. Many of these reactions require coordination of thiosemicarbazones to metal ions, even those present in the intracellular environment. The nature of the products generated in these reactions, their detection in vivo and in vitro, together with the relevance for the biological activity of these compounds, mainly as antineoplastic agents, is discussed.

Anticancer Thiosemicarbazones: Chemical Properties, Interaction with Iron Metabolism, and Resistance Development

SIGNIFICANCE

During the past decades, thiosemicarbazones were 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 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (Triapine), was intensively developed in multiple clinical phase I/II trials. Recent Advances: Very recently, two new derivatives, namely COTI-2 and di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (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 of bridging 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 in a complex and multi-faceted 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.

Disulfide-masked iron prochelators: Effects on cell death, proliferation, and hemoglobin production

The iron metabolism of malignant cells, which is altered to ensure higher acquisition and utilization, motivates the investigation of iron chelation strategies in cancer treatment. In a prochelation approach aimed at increasing intracellular specificity, disulfide reduction/activation switches are incorporated on iron-binding scaffolds resulting in intracellularly activated scavengers. Herein, this strategy is applied to several tridentate donor sets including thiosemicarbazones, aroylhydrazones and semicarbazones. The novel prochelator systems are antiproliferative in breast adenocarcinoma cell lines (MCF-7 and metastatic MDA-MB-231) and do not result in the intracellular generation of oxidative stress. Consistent with iron deprivation, the tested prochelators lead to cell-cycle arrest at the G1/S interface and induction of apoptosis. Notably, although hemoglobin-synthesizing blood cells have the highest iron need in the human body, no significant impact on hemoglobin production was observed in the MEL (murine erythroleukemia) model of differentiating erythroid cells. This study provides new information on the intracellular effects of disulfide-based prochelators and indicates aroylhydrazone (AH1-S)2 as a promising prototype of a new class of antiproliferative prochelator systems.

Comparison of metabolic pathways of different α-N-heterocyclic thiosemicarbazones

Clinical failure of novel drugs is often related to their rapid metabolism and excretion. This highlights the importance of elucidation of their pharmacokinetic profile already at the preclinical stage of drug development. Triapine, the most prominent representative of α-N-heterocyclic thiosemicarbazones, was investigated in more than 30 clinical phase I/II trials, but the results against solid tumors were disappointing. Recent investigations from our group suggested that this is, at least partially, based on the fast metabolism and excretion. In order to establish more detailed structure/activity/metabolism relationships, herein a panel of 10 different Triapine derivatives was investigated for their metabolic pathways. From the biological point of view, the panel consists of terminally dimethylated thiosemicarbazones with nanomolar IC₅₀ values, derivatives with micromolar cytotoxicities comparable to Triapine and a completely inactive representative. To study the oxidative metabolism, a purely instrumental approach based on electrochemistry/mass spectrometry was applied and the results were compared to the data obtained from microsomal incubations. Overall, the investigated thiosemicarbazones underwent the phase I metabolic reactions dehydrogenation, hydroxylation, oxidative desulfuration (to semicarbazone and amidrazone) and demethylation. Notably, dehydrogenation resulted in a ring-closure reaction with formation of thiadiazoles. Although strong differences between the metabolic pathways of the different thiosemicarbazones were observed, they could not be directly correlated to their cytotoxicities. Finally, the metabolic pathways for the most cytotoxic compound were elucidated also in tissues collected from drug-treated mice, confirming the data obtained by electrochemical oxidation and microsomes. In addition, the in vivo experiments revealed a very fast metabolism and excretion of the compound.

Graphical abstract

Lysosomal membrane stability plays a major role in the cytotoxic activity of the anti-proliferative agent, di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT)

The potent and selective anti-tumor agent, di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT), localizes in lysosomes and forms cytotoxic copper complexes that generate reactive oxygen species (ROS), resulting in lysosomal membrane permeabilization (LMP) and cell death. Herein, the role of lysosomal membrane stability in the anti-tumor activity of Dp44mT was investigated. Studies were performed using molecules that protect lysosomal membranes against Dp44mT-induced LMP, namely heat shock protein 70 (HSP70) and cholesterol. Up-regulation or silencing of HSP70 expression did not affect Dp44mT-induced LMP in MCF7 cells. In contrast, cholesterol accumulation in lysosomes induced by the well characterized cholesterol transport inhibitor, 3-β-[2-(diethyl-amino)ethoxy]androst-5-en-17-one (U18666A), inhibited Dp44mT-induced LMP and markedly and significantly (p<0.001) reduced the ability of Dp44mT to inhibit cancer cell proliferation (i.e., increased the IC(50)) by 140-fold. On the other hand, cholesterol extraction using methyl-β-cyclodextrin enhanced Dp44mT-induced LMP and significantly (p<0.01) increased its anti-proliferative activity. The protective effect of U18666A in increasing lysosomal cholesterol and preventing the cytotoxic activity of Dp44mT was not due to induced autophagy. Instead, U18666A was found to decrease lysosomal turnover, resulting in autophagosome accumulation. Moreover, preincubation with U18666A did not prevent the ability of Dp44mT to induce autophagosome synthesis, indicating that autophagic initiation via Dp44mT occurs independently of LMP. These studies demonstrate the significance of lysosomal membrane stability in relation to the ability of Dp44mT to execute tumor cell death and overcome pro-survival autophagy. Hence, lysosomal-dependent cell death induced by Dp44mT serves as an important anti-tumor strategy. These results are important for comprehensively understanding the mechanism of action of Dp44mT.