Comprehensive Evaluation of Biological Effects of Pentathiepins on Various Human Cancer Cell Lines and Insights into Their Mode of Action

Pentathiepins are an understudied molecular prism of biological activities

The pentathiepin core was first synthesized in 1971, and while synthetic techniques have progressed over subsequent decades, the biological applications of this heterocycle have received less attention and are only now becoming more apparent. The first natural product, varacin, was identified in 1991, showing cytotoxicity toward a human colon cancer cell line. More recently, the pentathiepin has acted as a surrogate to replace elemental sulfur, that was discovered as a hit in neurodegenerative animal models. A variety of other medicinal chemistry applications have recently been disclosed. Here, we summarize these indications and highlight the main synthetic pathways to access the pentathiepin core. We offer a concise summary and future perspective of this unique sulfur isosteric replacement.

Structure-activity-relationships of the Stability of Six Pentathiepins Towards Glutathione: Possible Correlations with Biological Activities

The biological properties of pentathiepins have been intensively studied in recent years. Although the proposed mechanism of action requires activation by intracellular thiols, the dependence of activity on the stability of pentathiepins towards glutathione (GSH) has not been directly investigated. Here, we determined the structure-related stability of six different pentathiepins with four different scaffolds in the presence of GSH by using reversed-phase high-performance liquid chromatography (RP-HPLC) and UV-vis spectroscopy over a wide range of GSH concentrations. We found significant differences in compound stability depending on the pentathiepin scaffold; these differences were reflected in their cytotoxic activities. However, we found no substantial differences in their inhibition of glutathione peroxidase 1 (GPx-1). While the intact pentathiepin ring is necessary for the antiproliferative activity of pentathiepins, the depletion of intracellular GSH content with dl-buthionine-(S,R)-sulfoximine (BSO) led to a significant increase in cytotoxicity of the tested substances. In view of the increased cytotoxicity following artificial GSH depletion, this calls into question the sole role of GSH in the intracellular activation mechanism.

The Role of -OEt Substituents in Molybdenum-Assisted Pentathiepine Formation-Access to Diversely Functionalized Azines

1,2,3,4,5-pentathiepines (PTEs) are naturally occurring polysulfides of increasing scientific interest based on their identified pharmacological activities. Artificial PTEs with N-heterocyclic backbones are efficiently synthesized via mediation by a molybdenum-oxo-bistetrasulfido complex. A common feature of all precursor alkynes successfully used to date in this reaction is the presence of a -CH(OEt)2 group since the previously postulated mechanism requires the presence of one OEt- as the leaving group, and the second must become a transient ethoxonium moiety. This raised the question of whether there really is a need for two, maybe only one, or possibly even zero ethoxy substituents. This research problem was systematically addressed by respective variations in the precursor-alkyne derivatives and by employing one related allene species. It was found that the total absence of ethoxy substituents prevents the formation of PTEs entirely, while the presence of a single ethoxy group results in the possibility to distinctly functionalize the position on the resulting N-heterocyclic pyrrole five ring in the target compound. This position was previously exclusively occupied by an -OEt for all products of the molybdenum-mediated reaction. The allene was applied with similar success as precursor as with the related alkyne. The now-employable significant change in precursor composition gives access to a whole new PTE subfamily, allowing further modulation of (physico)-chemical properties such as solubility, and provides additional insight into the mechanism of PTE formation; it comprises a merely partial validation of the previous hypothesis. The new alkyne precursors and pentathiepines were characterized by a variety of instrumental analyses (NMR, mass spec, UV-vis) and in six cases (one alkyne precursor, one unexpected side product, and four PTEs) by single-crystal X-ray diffraction. Syntheses, isolation procedures, analytical data, and the impact of the findings on the previously proposed mechanism are described in detail herein.

Synthesis of Seven Indolizine-Derived Pentathiepines: Strong Electronic Structure Response to Nitro Substitution in Position C-9

Seven new 1,2,3,4,5-pentathiepino[6,7-a]indolizines were synthesized in which the pentathiepine moieties bear an indolizine backbone that is derivatized from C-H to F-, Cl-, Br-, I-, NO2-, and CH3-substitutions, respectively, in a meta position relative to the aza group on the pyridine moiety. Their preparation took place via two common steps: (i) a Sonogashira coupling between (4-substituted) 2-bromo- or 2-chloropyridines and propynyl 3,3-diethylacetal, and (ii) a ring closing reaction mediated by a molybdenum oxo-bistetrasulfido complex and elemental sulfur. The latter simultaneously facilitates the 1,2,3,4,5-pentathiepino chain/ring- and indolizine ring-formations. The fluoro derivative was addressed with 2-bromo-5-aminopyridine as the starting material via a Sandmeyer reaction. The iodo derivative was obtained from 5-bromo-2-alkynylpiridine using a metal-assisted variation of the Finkelstein reaction. The requirement to explore different reaction conditions and the varied respective yields of the final products are discussed. The influence of the distinct substitutions on the pyridine moieties, their electronic structures, and respective chemical properties was investigated through a set of spectroscopic/analytical characterizations. Intriguingly, in all cases, the nitro-substituted derivative exhibited a distinct behavior compared to the six other investigated derivatives, which was also addressed computationally. All seven new pentathiepines were crystallized, and their respective molecular structures were determined using single crystal X-ray diffraction. These structures are compared and discussed as are their respective packing patterns.

Liposomal formulation of model pentathiepin improves solubility and stability toward glutathione while preserving anticancer activity

The biological properties of pentathiepins have been attracting increased attention in recent years. Experiments have shown a wide range of effects of pentathiepins in vitro, such as induction of apoptosis and alteration of mitochondrial membrane potential in cancer cells, and inhibition of antioxidant enzymes, for example, glutathione peroxidase 1 (GPx1). Biological evaluation is sometimes limited due to low aqueous solubility, high lipophilicity, and poor stability toward thiols, for example, glutathione (GSH). To assess whether liposomes are suitable as drug carriers to overcome these drawbacks, a model pentathiepin was formulated in a liposomal preparation. The success of loading liposomes with pentathiepins was evaluated by using ultraviolet-visible light (UV-Vis) spectroscopy, dynamic light scattering (DLS), and high-performance liquid chromatography (HPLC). Through inclusion into 100-nm-sized 1,2-dioleoyl-sn-glycero-3-phosphocholine liposomes, the aqueous solubility of a representative pentathiepin could be increased by several orders of magnitude to ca. 400 µM. The stability of the pentathiepin in the presence of GSH was increased fourfold as determined by UV-Vis spectroscopy. In antiproliferation experiments with two human cancer cell lines, no decrease in potency in the liposomal loaded pentathiepin compared to the free pentathiepin was found. In conclusion, liposomes are a suitable carrier for pentathiepins and improve both solubility and stability in the presence of thiols without compromising anticancer activity.