Dinuclear NHC-gold(I)-thiolato and -alkynyl complexes: synthesis, anticancer activity, and catalytic activity in lactonization reactions.

A series of novel dinuclear NHC-gold-thiolato and -alkynyl complexes bearing aromatic linkers were successfully synthesized by an efficient and simple synthetic route. The catalytic activity of these complexes was tested in a lactonization reaction. The reaction proceeds in high efficiency, in short reaction time and under mild conditions, and is complementary to existing methods. Furthermore, the digold(I)-thiolato derivatives exhibit remarkable cytotoxicity towards several cancer cell lines.

Dual-mode vehicles with simultaneous thermometry and drug release properties based on hollow Y2O3:Er,Yb and Y2O2SO4:Er,Yb spheres.

Employing luminescence thermometry in the biomedical field is undeniably appealing as many health conditions are accompanied by temperature changes. In this work, we show our ongoing efforts and results at designing novel vehicles for dual-mode thermometry and pH-dependent drug release based on hollow spheres. Hereby for that purpose, we exploit the hollow Y2O3 and Y2O2SO4 host materials. These two inorganic hollow phosphors were investigated and showed to have excellent upconversion Er3+-Yb3+ luminescence properties and could be effectively used as optical temperature sensors in the physiological temperature range when induced by near-infrared CW light (975 nm). Further, doxorubicin was exploited as a model anti-cancer drug to monitor the pH-dependent drug release of these materials showing that they can be used for simultaneous thermometry and drug delivery applications.

Unveiling the Potential of Innovative Gold(I) and Silver(I) Selenourea Complexes as Anticancer Agents Targeting TrxR and Cellular Redox Homeostasis.

A series of NHC-based selenourea Ag(I) and Au(I) complexes were evaluated for their anticancer potential in vitro, on 2D and 3D human cancer cell systems. All NHC-based selenourea complexes possess an outstanding cytotoxic potency, which was comparable or even better than that of the reference metallodrug auranofin, and were also able to overcome both platinum-based and multi-drug resistances. Intriguingly, their cytotoxic potency did not correlate with solution stability, partition coefficient or cellular uptake. On the other hand, mechanistic studies in cancer cells revealed their ability to strongly and selectively inhibit the redox-regulating enzyme Thioredoxin Reductase (TrxR), being even more effective than auranofin, a well-known TrxR inhibitor, without affecting other redox enzymes such as Glutathione Reductase (GR). The inhibition of TrxR in H157 human cancer cells caused, in turn, the disruption of cellular thiol-redox homeostasis and of mitochondria pathophysiology, ultimately leading to cancer cell death through apoptosis.

A Green Synthesis of Carbene-Metal-Amides (CMAs) and Carboline-Derived CMAs with Potent in†vitro and ex vivo Anticancer Activity.

The modularity and ease of synthesis of carbene-metal-amide (CMA) complexes based on the coinage metals (Au, Ag, Cu) and N-heterocyclic carbenes (NHCs) as ancillary ligands pave the way for the expansion of their applications beyond photochemistry and catalysis. Herein, we further improve the synthesis of such compounds by circumventing the use of toxic organic solvents which were previously required for their purification, and we expand their scope to include complexes incorporating carbolines as the amido fragments. The novel complexes are screened both in vitro and ex vivo, against several cancer cell lines and high-grade serous ovarian cancer (HGSOC) tumoroids, respectively. Excellent cytotoxicity values are obtained for most complexes, while the structural variety of the CMA library screened thus far, provides promising leads for future developments. Variations of all three components (NHC, metal, amido ligand), enable the establishment of trends regarding cytotoxicity and selectivity towards cancerous over normal cells.

A simple synthetic entryway into new families of NHC-gold-amido complexes and their in vitro antitumor activity.

A simple synthetic pathway to Au-NHC amido complexes is described. Syntheses and isolation of [Au(NHC)(NR1R2)] complexes, bearing various NHC ligands and NH-containing heterocycles under mild conditions are reported. The in vitro anticancer activity of these gold-complexes was investigated on three human cancer cell lines. A number of these show comparable or even better antiproliferative activity than cisplatin. Noteworthy is the non-toxicity of most of the complexes on normal cells.

Stereodivergent Synthesis of Biologically Active Spironucleoside Scaffolds via Catalytic Cyclopropanation of 4-exo-Methylene Furanosides.

Cyclopropane fusion of the only rotatable carbon-carbon bond in furanosyl nucleosides (i.e., exocyclic 4'-5') is a powerful design strategy to arrive at conformationally constrained analogues. Herein, we report a direct stereodivergent route toward the synthesis of the four possible configurations of 4-spirocyclopropane furanoses, which have been transformed into the corresponding 4'-spirocyclic adenosine analogues. The latter showed differential inhibition of the protein methyltransferase PRMT5-MEP50 complex, with one analogue inhibiting more effectively than adenosine itself, demonstrating the utility of rationally probing 4'-5' side chain orientations.

Synthesis of Novel Nitroxoline Analogs with Potent Cathepsin B Exopeptidase Inhibitory Activity.

Nitroxoline, a well-known antimicrobial agent, has been identified in several independent studies, and on different molecular targets, as a promising candidate to be repurposed for cancer treatment. One specific target of interest concerns cathepsin B, a lysosomal peptidase involved in the degradation of the extracellular matrix (ECM), leading to tumor invasion, metastasis and angiogenesis. However, dedicated optimization of the nitroxoline core is needed to actually deliver a nitroxoline-based antitumor drug candidate. Within that context, 34 novel nitroxoline analogs were synthesized and evaluated for their relative cathepsin B inhibitory activity, their antiproliferative properties and their antimicrobial activity. More than twenty analogs were shown to exert a similar or even slightly higher cathepsin B inhibitory activity compared to nitroxoline. The implemented modifications of the nitroxoline scaffold and the resulting SAR information can form an eligible basis for further optimization toward more potent cathepsin B inhibitors in the quest for a clinical nitroxoline-based antitumor agent.

Expedient Synthesis of Lupulones and Their Derivatization to 2,8-7H-Dihydrochromen-7-ones.

A convenient and improved method for the synthesis of beta acids or lupulones, which are known to possess e. g. anti-cancer, anti-inflammatory, anti-oxidative and antimicrobial activity, has been developed successfully. Further derivatization of these complex structures to the corresponding dihydrochromen-7-ones, including the natural product machuone, was realized to simplify their analysis and to confirm their molecular structure. In addition to practical and safe laboratory procedures, the advantages associated with this new approach involve the use of water as a solvent and the direct crystallization of lupunones from acetonitrile, rendering our strategy more efficient and benign as compared to available methods.

Antitumor activity of organoruthenium complexes with chelate aromatic ligands, derived from 1,10-phenantroline: Synthesis and biological activity.

The monocationic chloro complexes containing chelating N∩N ligands: [(η6-p-cymene)Ru(L1-4)Cl]+ (1-4), where L1 = 4-methyl-1,10-phenantroline, L2 = dipyrido[3,2-a:2',3'-c]phenazine, L3 = 11-chloro-dipyrido[3,2-a:2',3'-c]phenazine, L4 = 11-nitro-dipyrido[3,2-a:2',3'-c]phenazine; p-cymene = 1-methyl-4-isopropylbenzene) have been prepared and characterized as the hexafluorophosphate salts. The biological activity of 1-4 has been investigated in selected 2D monolayer cell cultures (A549, PANC-1, MDA-MB-231, MRC-5). All investigated ruthenium complexes showed similar or even better cytotoxicity to cisplatin. However, there was no significant reduction in growth of PANC-1 cells in a 3D cell culture of multicellular tumor spheroids (MCTS) after treatment with 2-4, while the cisplatin treatment induced retardation in MCTS growth. Flow cytometry analysis of the cell cycle of PANC-1 cells shows that 3 caused changes of cell cycle phase distribution characterized by slight accumulation of cells in the G2-M phase. Absence of the Sub-G1 phase in the cell cycle of the treated cells indicated that there was no fragmentation of DNA for the analyzed time intervals (48 and 72 h treatment). Fluorescent microscopy, after acridine orange/ethidium bromide staining, revealed that the investigated ruthenium complexes induced some characteristics of apoptotic morphology (shrinking and condensation of chromatin) with notably preserved integrity of the plasma membrane. Investigation of cellular uptake and DNA - fraction accumulation performed by inductively coupled plasma mass spectrometry in PANC-1 cells with equimolar concentrations (5 µM) of 2-4 and cisplatin showed more efficient cellular uptake and DNA - fraction accumulation of complex 3 compared to complexes 2 and 4.

Preparation of 4'-Spirocyclobutyl Nucleoside Analogues as Novel and Versatile Adenosine Scaffolds.

Despite the large variety of modified nucleosides that have been reported, the preparation of constrained 4'-spirocyclic adenosine analogues has received very little attention. We discovered that the [2+2]-cycloaddition of dichloroketene on readily available 4'-exo-methylene furanose sugars efficiently results in the diastereoselective formation of novel 4'-spirocyclobutanones. The reaction mechanism was investigated via density functional theory (DFT) and found to proceed either via a non-synchronous or stepwise reaction sequence, controlled by the stereochemistry at the 3'-position of the sugar substrate. The obtained dichlorocyclobutanones were converted into nucleoside analogues, providing access to a novel class of chiral 4'-spirocyclobutyl adenosine mimetics in eight steps from commercially available sugars. Assessment of the biological activity of designed 4'-spirocyclic adenosine analogues identified potent inhibitors for protein methyltransferase target PRMT5.

Synthesis of 1,4-Thiazepane-Based Curcuminoids with Promising Anticancer Activity.

Curcumin, the main component of turmeric (Curcuma longa) is known to display an interesting bioactivity profile, including pronounced anticancer properties. However, its low bioavailability, metabolic instability and nonspecific activity are concerns that have to be addressed before curcuminoids can be considered for therapeutic applications. Within that framework, intensive research has been carried out in the last decades to develop new curcumin derivatives, generally centered on standard modifications of the sp2 curcumin framework, with the aim to augment its bioavailability while maintaining or improving its anticancer properties. To find potential hit molecules by moving away from the classical flat curcumin framework, we investigated an unexplored modification to produce novel, out-of-plane 1,4-thiazepane-based curcuminoids and assessed the impact of this modification on the biological activity. In this way, 21 new, structurally diverse thiazepane scaffolds (4-aryl-1-(7-aryl-1,4-thiazepan-5-ylidene)but-3-en-2-ones) were synthesized, as well as some biologically interesting unexpected reaction products (such as 5-aryl-6-arylmethylene-3-ethoxycyclohex-2-en-1-ones and 4-acetyl-5-aryl-2-(3-arylacryloyl)-3-methylcyclohex-2-en-1-ones). All these analogues were subsequently tested on their antioxidant capacity, their cytotoxicity properties and their ROS (reactive oxygen species) production. Many compounds demonstrated interesting activities, with ten curcuminoids, whereof eight 1,4-thiazepane-based, showing better antiproliferative properties compared to their mother compounds, as well as an increased ROS production. This unprecedented 3D curcumin modification has thus delivered promising new hit compounds with good activity profiles eligible for further exploration.

Synthesis of Non-Symmetrical Nitrogen-Containing Curcuminoids in the Pursuit of New Anticancer Candidates.

Curcumin is known to display pronounced anticancer effects and a variety of other biological activities. However, the low bioavailability and fast metabolism of this molecule present an issue of concern with respect to its medicinal applications. To address this issue, structural modifications of the curcumin scaffold can be envisioned as a strategy to improve both the solubility and stability of this chemical entity, without compromising its biological activities. Previous work in our group targeted the synthesis of symmetrical azaheteroaromatic curcuminoids, which showed better solubility and cytotoxicity profiles compared to curcumin. In continuation of that work, we now focused on the synthesis of non-symmetrical nitrogen-containing curcuminoids bearing both a phenolic and an azaheteroaromatic moiety. In that way, we aimed to combine good solubility, antioxidant potential and cytotoxic properties into one molecule. Some derivatives were selected for further chemical modification of their rather labile ß-diketone scaffold to the corresponding pyrazole moiety. In this way, thirteen new non-symmetrical aza-aromatic curcuminoids and four pyrazole-based analogues were successfully synthesized in a yield of 11-69 %. All newly synthesized analogues were evaluated for their antioxidant properties, reactive oxygen species (ROS) production, water solubility and anticancer activities. Several novel derivatives displayed good cytotoxicity profiles compared to curcumin, in combination with an improved water solubility and stability, and were thus identified as potential hit scaffolds for further optimization studies.

Revisiting tubercidin against kinetoplastid parasites: Aromatic substitutions at position 7 improve activity and reduce toxicity.

The nucleoside antibiotic tubercidin displays strong activity against different target organisms, but it is notoriously toxic to mammalian cells. The effects of tubercidin against T. brucei parasites inspired us to synthesize several C7 substituted analogs for in vitro evaluation in order to find suitable hit compounds. C7 Deazaadenosines substituted with electron-poor phenyl groups were found to have micromolar activity against T. brucei in vitro. Replacement of the phenyl for a pyridine ring gave compound 13, with submicromolar potency and much-attenuated cytotoxicity compared to tubercidin. The veterinary pathogen T. congolense was equally affected by 13in vitro. Transporter studies in T. b. brucei indicated that 13 is taken up efficiently by both the P1 and P2 adenosine transporters, making the occurrence of transporter-related resistance and cross-resistance with diamidine drugs such as diminazene aceturate and pentamidine as well as with melaminophenyl arsenicals unlikely. Evaluation of the in vitro metabolic stability of analog 13 indicated that this analog was significantly metabolized in mouse microsomal fractions, precluding further in vivo evaluation in mouse models of HAT.

New platinum(II) and palladium(II) complexes with substituted terpyridine ligands: synthesis and characterization, cytotoxicity and reactivity towards biomolecules.

A series of palladium(II) (1-3) and platinum(II) chloride complexes (4 and 5) with 2,2':6',2″-terpyridine (terpy) derivatives substituted at the 4' position, was synthesized and fully characterized. Single crystal X-ray diffraction analysis of complexes 2, 3 and 5 showed tridentate coordination of the 4'-substituted terpyridine (terpy) ligands to the metal center. Moreover, in vitro cytotoxic activity of these complexes toward a panel of human cancer cell lines (lung cancer A549, colorectal cancer HCT116, ovarian cancer IGROV-1) and toward normal cell line HDF (dermal fibroblast) was determined by Trypan Blue exclusion assay. Overall, the tested compounds manifested a relevant cytotoxicity for the selected cancer cell lines with complex 4 also showing a modest cytotoxicity on the normal cell lines. To better understand the mode of action of these metal complexes, their reactivity with three model proteins, i.e. hen egg white lysozyme (HEWL), cytochrome c (cyt c) and ribonuclease A (RNase A) were comparatively investigated through ESI-MS analysis. The results highlighted a different behavior between the two series of complexes being platinum compounds more reactive toward RNase and cyt c than palladium compounds. Based on the obtained results, it is proposed that in presence of RNase A and cyt c, the platinum complexes undergo activation through release of labile ligands followed by binding to the protein. In contrast, palladium complexes revealed a far lower reactivity implying the likely occurrence of a different mechanism of action.

The Elusive Seven-Membered Cyclic Imino Ether Tetrahydrooxazepine.

Cyclic imino ether heterocycles are used as ligands in transition metal catalysis, in various drugs and as reactive monomers in living cationic ring-opening polymerization (CROP). While five- and six-membered cyclic imino ethers, i.e. 2-oxazolines and 4,5-dihydro-1,3-oxazines, have extensively been studied in these areas, their seven-membered ring counterparts have remained unexplored. Herein, we report the synthesis of 2-phenyl-4,5,6,7-tetrahydro-1,3-oxazepine allowing reassignment of the earlier, incorrectly reported 4,5,6,7-tetrahydro-1,3-oxazepines as their N-acylated pyrrolidine isomers. Finally, we also report a comparison of the CROP reactivity of a homologous series of cyclic imino ethers with a 2-carbon, 3-carbon, and 4-carbon methylene bridge, revealing a remarkable ring size effect.

Discovery of Novel 7-Aryl 7-Deazapurine 3'-Deoxy-ribofuranosyl Nucleosides with Potent Activity against Trypanosoma cruzi.

Chagas disease is the leading cause of cardiac-related mortality in Latin American countries where it is endemic. Trypanosoma cruzi, the disease-causing pathogen, is unable to synthesize purines de novo, necessitating salvage of preformed host purines. Therefore, purine and purine-nucleoside analogues might constitute an attractive source for identifying antitrypanosomal hits. In this study, structural elements of two purine-nucleoside analogues (i.e., cordycepin and a recently discovered 7-substituted 7-deazaadenosine) led to the identification of novel nucleoside analogues with potent in vitro activity. The structure-activity relationships of substituents at C-7 were investigated, ultimately leading to the selection of compound 5, with a C-7 para-chlorophenyl group, for in vivo evaluation. This derivative showed complete suppression of T. cruzi Y-strain blood parasitemia when orally administered twice daily for 5 days at 25 mg/kg and was able to protect infected mice from parasite-induced mortality. However, sterile cure by immunosuppression could not be demonstrated.

Development of Mixed metal Metal-organic polyhedra networks, colloids, and MOFs and their Pharmacokinetic applications.

The coordination networking of discrete metal-organic polyhedra (MOPs) involving different ligands as well as metals is a challenging task due to the features of limited solubility and chemical stability of these polyhedra. An unusual approach, ligand-oriented polyhedral networking via click chemistry and further metal coordination is reported here. An alkyne decorated Cu(II)-MOP self-catalyzes the regioselective click reaction (1,3-dipolar cycloaddition) using azide-functionalized ligands under unconventional reaction conditions. Introducing new metal ions, M(II), interlinks the carboxylic groups on the MOP surfaces creating coordination networks. On the other hand, exposure of the respective individual ligand components in the presence of Cu(II) promotes an in-situ click reaction along with metal coordination generating a new 3D-framework. These materials demonstrated a high drug hosting potential exhibiting a controlled progressive release of anticancer (5-flourouracil) and stimulant (caffeine) drugs in physiological saline at 37 °C. These innovative and unconventional MOP networks provide a significant conceptual advance in understanding.

Discovery of (S)-3'-hydroxyblebbistatin and (S)-3'-aminoblebbistatin: polar myosin II inhibitors with superior research tool properties.

In search of myosin II inhibitors with superior research tool properties, a chemical optimization campaign of the blebbistatin scaffold was conducted in this paper. (S)-Blebbistatin is the best known small-molecule inhibitor of myosin II ATPase activity. Unfortunately, as a research tool this compound has several deficiencies: it is photolabile and (photo)toxic, has low water solubility, and its (fluorescent) precipitates interfere in (fluorescence) readouts. In view of obtaining tool compounds with improved properties, both enantiomers of a series of D-ring modified polar analogs were prepared. We identified (S)-3'-hydroxyblebbistatin (S)-2 and (S)-3'-aminoblebbistatin (S)-3 as two myosin II inhibitors with a 30-fold higher water solubility than (S)-blebbistatin. These molecules furthermore do not cause interference in (fluorescence) readouts. (S)-2 and (S)-3 thus are superior alternatives to (S)-blebbistatin as research tools to study myosin II.

Efficient diastereoselective synthesis of a new class of azanucleosides: 2'-homoazanucleosides.

Azanucleosides, sugar-modified nucleoside analogues containing a 4' nitrogen atom, have shown a lot of therapeutic potential, e.g. as anti-cancer and antiviral agents. We report the synthesis of a series of 2'-homoazanucleosides, in which the nucleobase is attached to the 2'-position of the pyrrolidine ring via a methylene linker. A suitable orthogonally protected iminosugar was synthesized by ring closing metathesis and dihydroxylation as key steps and further converted to a series of 8 nucleoside analogues through Mitsunobu reaction with suitably protected nucleobases. The 5' position of the adenine analogue was then further derivatized with thiols to afford 2 additional compounds. The final compounds were evaluated for biological activity.

Synthesis of Potent and Selective HDAC6 Inhibitors Bearing a Cyclohexane- or Cycloheptane-Annulated 1,5-Benzothiazepine Scaffold.

Selective inhibitors of histone deacetylase 6 (HDAC6) are an emerging class of pharmaceuticals due to the involvement of HDAC6 in different pathways related to neurodegenerative diseases, cancer, and immunology. Herein, the synthesis of ten new benzohydroxamic acids, constructed by employing the tetrahydrobenzothiazepine core as a privileged pharmacophoric unit, is described. This is the first report on the synthesis and isolation of octahydrodibenzothiazepines and octahydro-6H-benzocycloheptathiazepines, which were then used to develop a new class of HDAC6 inhibitors. Evaluations of their HDAC-inhibiting activity resulted in the identification of cis-N-(4-hydroxycarbamoylbenzyl)-1,2,3,4,4a,5,11,11a-octahydrodibenzo[b,e][1,4]thiazepine-10,10-dioxide and cis-N-(4-hydroxycarbamoylbenzyl)-7-trifluoromethyl-1,2,3,4,4a,5,11,11a-octahydrodibenzo[b,e][1,4]thiazepine-10,10-dioxide as highly potent and selective HDAC6 inhibitors with activity in the low nanomolar range, which also show excellent selectivity on the enzymatic and cellular levels. Furthermore, four promising inhibitors were subjected to an Ames fluctuation assay, which revealed no mutagenic effects associated with these structures.