Caffeine derived platinum(II) N-heterocyclic carbene complexes with multiple anti-cancer activities

Synthesis and Medicinal Applications of N-Heterocyclic Carbene Complexes Based on Caffeine and Other Xanthines

Xanthines are purine derivatives predominantly found in plants. These include compounds such as caffeine, theophylline, and theobromine and exhibit a variety of pharmacological properties, demonstrating efficacy in treating neurodegenerative disorders, respiratory dysfunctions, and also cancer. The versatile attributes of these materials render them privileged scaffolds for the development of compounds for various biological applications. Xanthines are N-heterocyclic carbene precursors that combine a pyrimidine and an imidazole ring. Owing to their biological relevance, xanthines have been employed as N-heterocyclic carbenes in the development of metallodrugs for anticancer and antimicrobial purposes. In this conceptual review, we examine key examples of N-heterocyclic carbene complexes derived from caffeine and other xanthines, elucidating their synthetic methods and describing their pertinent medicinal applications.

Microbial metabolism of caffeine and potential applications in bioremediation

With increasing global consumption of caffeine-rich products, such as coffee, tea, and energy drinks, there is also an increase in urban and processing waste full of residual caffeine with limited disposal options. This waste caffeine has been found to leach into the surrounding environment where it poses a threat to microorganisms, insects, small animals, and entire ecosystems. Growing interest in harnessing this environmental contaminant has led to the discovery of 79 bacterial strains, eight yeast strains, and 32 fungal strains capable of metabolizing caffeine by N-demethylation and/or C-8 oxidation. Recently observed promiscuity of caffeine-degrading enzymes in vivo has opened up the possibility of engineering bacterial strains capable of producing a wide variety of caffeine derivatives from a renewable resource. These engineered strains can be used to reduce the negative environmental impact of leached caffeine-rich waste through bioremediation efforts supplemented by our increasing understanding of new techniques such as cell immobilization. Here, we compile all of the known caffeine-degrading microbial strains, discuss their metabolism and related enzymology, and investigate their potential application in bioremediation.

Metal complexes of xanthine and its derivatives: Synthesis and biological activity

Xanthine and its derivatives are considered an important class of N-heterocyclic purine compounds that have gained significant importance in medicinal chemistry. N-heterocyclic carbene (NHC) and N-coordinated metal complexes of xanthine and its derivatives have revealed a range of new possibilities for their use as therapeutic agents in addition to their established catalytic behavior. The metal complexes of xanthine and its derivatives have been designed and synthesized for the exploration of their potential therapeutic applications. These metal complexes based on the xanthine scaffold exhibited various potential medicinal applications including anticancer, antibacterial, and antileishmanial activity. The metal complexes of xanthine and its derivatives shall pave the way for the rational design and development of new therapeutic agents. In the present comprehensive review, we highlighted the recent advancements in the synthesis and medicinal applications of metal complexes based on N-heterocyclic carbene (NHC) derived from xanthine scaffolds.

Elucidating the Multimodal Anticancer Mechanism of an Organometallic Terpyridine Platinum(II) N-Heterocyclic Carbene Complex against Triple-Negative Breast Cancer In Vitro and In Vivo

Treatment of triple-negative breast cancer (TNBC) has long been a medical challenge because of the lack of effective therapeutic targets. Targeting lipid, carbohydrate, and nucleotide metabolism pathways has recently been proven as a promising option in view of three heterogeneous metabolic-pathway-based TNBC subtypes. Here, we present a multimodal anticancer platinum(II) complex, named Pt(II)caffeine, with a novel mode of action involving simultaneous mitochondrial damage, inhibition of lipid, carbohydrate, and nucleotide metabolic pathways, and promotion of autophagy. All these biological processes eventually result in a strong suppression of TNBC MDA-MB-231 cell proliferation both in vitro and in vivo. The results indicate that Pt(II)caffeine, influencing cellular metabolism at multiple levels, is a metallodrug with increased potential to overcome the metabolic heterogeneity of TNBC.

Biocatalytic production of 7-methylxanthine by a caffeine-degrading Escherichia coli strain

7-Methylxanthine, a derivative of caffeine (1,3,7-trimethylxanthine), is a high-value compound that has multiple medical applications, particularly with respect to eye health. Here, we demonstrate the biocatalytic production of 7-methylxanthine from caffeine using Escherichia coli strain MBM019, which was constructed for production of paraxanthine (1,7-dimethylxanthine). The mutant N-demethylase NdmA4, which was previously shown to catalyze N₃ -demethylation of caffeine to produce paraxanthine, also retains N₁ -demethylation activity toward paraxanthine. This study demonstrates that whole cell biocatalysts containing NdmA4 are more active toward paraxanthine than caffeine. We used four serial resting cell assays, with spent cells exchanged for fresh cells between each round, to produce 2,120 μM 7-methylxanthine and 552 μM paraxanthine from 4,331 μM caffeine. The purified 7-methylxanthine and paraxanthine were then isolated via preparatory-scale HPLC, resulting in 177.3 mg 7-methylxanthine and 48.1 mg paraxanthine at high purity. This is the first reported strain genetically optimized for the biosynthetic production of 7-methylxanthine from caffeine.

In vitro and In vivo Studies of Potential Anticancer Agents of Platinum(II) Complexes of Dicyclopentadiene and Dithiocarbamates

Three platinum(II) complexes of dicyclopentadiene (DCP) and dithiocarbamates (DTC), namely, [Pt(η4-DCP)(Me2DTC)]PF6 (1), [Pt(η4-DCP)(Et2DTC)]PF6 (2) and [Pt(η4-DCP)(Bz2DTC)]PF6 (3) [Me2DTC = dimethyldithiocarbamate, Et2DTC = diethyldithiocarbamate, and Bz2DTC = dibenzyldithiocarbamate] were prepared and characterized by elemental analysis, IR, 1H and 13C NMR spectroscopy. The spectroscopic data indicated the coordination of both DCP and dithiocarbamate ligands to platinum(II). The solution chemisty of complex 1 revealed that the complexes are stable in both DMSO and 1:1 mixture of DMSO: H2O. In vitro cytotoxicity of the complexes relative to cisplatin was tested using MTT assay, against CHL-1 (human melanoma cancer cells), MDA-MB-231 (breast cancer cells), A549 (lung cancer cells), and B16 (murine melanoma cancer cells). The antiproliferative effect of all three prepared complexes was found to be significantly higher than cisplatin. Furthermore, flow cytometric analysis of complex 1 showed that the complex induced apoptosis, oxidative stress, mitochondrial potential depolarization and cell cycle arrest in a concentration-dependent pattern in the CHL-1 cells. Confirmation of apoptosis via gene expression analysis demonstrated down-regulation of anti-apoptotic genes and up-regulation of pro-apoptotic genes in the CHL-1 cells. Wound healing assays also lent support to the strong cytotoxicity of the complexes. In vivo studies showed a significant reduction of tumor volume at the end of the experiment. In addition, the drug did not change the weight of the mice. In conclusion, complex 1 inhibited cell proliferation in vitro and reduced tumor growth in vivo.

Noble Metal Complexes of a Bis-Caffeine Containing NHC Ligand

N-Heterocyclic carbenes (NHCs) have seen more and more use over the years. The go-to systems that are usually considered are derivatives of benzimidazole or imidazole. Caffeine possesses an imidazole unit and was already utilized as a carbene-type ligand; however, its use within a tridentate bis-NHC system has—to the best of our knowledge—not been reported so far. The synthesis of the ligand is straightforward and metal complexes are readily available via silver-salt metathesis. A platinum(II) and a palladium(II) complex were isolated and a crystal structure of the former was examined. For the Pt(II) complex, luminescence is observed in solid state as well as in solution.

Facile N9-Alkylation of Xanthine Derivatives and Their Use as Precursors for N-Heterocyclic Carbene Complexes

The xanthine-derivatives 1,3,7-trimethylxanthine, 1,3-dimethyl-7-benzylxanthine and 1,3-dimethyl-7-(4-chlorobenzyl)xanthine are readily ethylated at N9 using the cheap alkylating agents ethyl tosylate or diethyl sulfate. The resulting xanthinium tosylate or ethyl sulfate salts can be converted into the corresponding PF₆^- and chloride salts. The reaction of these xanthinium salts with silver(I) oxide results in the formation of different silver(I) carbene-complexes. In the presence of ammonia, ammine complexes [Ag(NHC)(NH₃)]PF₆ are formed, whilst with Et₂NH, the bis(carbene) salts [Ag(NHC)₂]PF₆ were isolated. Using the xanthinium chloride salts neutral silver(I) carbenes [Ag(NHC)Cl] were prepared. These silver complexes were used in a variety of transmetallation reactions to give the corresponding gold(I), ruthenium(II) as well as rhodium(I) and rhodium(III) complexes. The compounds were characterized by various spectroscopic methods as well as X-ray diffraction.

Comparing the Antileishmanial Activity of Gold(I) and Gold(III) Compounds in L. amazonensis and L. braziliensis in Vitro

A series of mononuclear coordination or organometallic AuI /AuIII complexes (1-9) have been comparatively studied in vitro for their antileishmanial activity against promastigotes and amastigotes, the clinically relevant parasite form, of Leishmania amazonensis and Leishmania braziliensis. One of the cationic AuI bis-N-heterocyclic carbenes (3) has low EC50 values (ca. 4 μM) in promastigotes cells and no toxicity in host macrophages. Together with two other AuIII complexes (6 and 7), the compound is also extremely effective in intracellular amastigotes from L. amazonensis. Initial mechanistic studies include an evaluation of the gold complexes' effect on L. amazonensis' plasma membrane integrity.

Evaluation of Inhibitory Effects of Caffeine on Human Carcinoma Cells

One of the world's most consumed medications is caffeine which is available in the vast majority of beverages. Previously, some effects of caffeine have been evaluated including its inhibitory effect on cancer cells. But, the influence of caffeine on esophagus carcinoma squamous cells (CSC) and head and neck carcinoma cells still has not well understood. Herein, the relation between different amounts of caffeine with the proliferation rate of human esophagus carcinoma squamous cell line KYSE-30 as well as human head and neck carcinoma cell line HN5 was evaluated. Furthermore, concentrations of caffeine were adjusted and their effect on cells were studied. The inhibitory effects of caffeine on cells were measured using the conventional colorimetric MTT assay after 3 and 7 day of incubation. Our findings are suggested that caffeine has a significant inhibitory effect on both cell lines at the concentrations of 20, 50, and 70 milli-mol (mmol). The result shows that caffeine can prevent the proliferation of carcinoma cells and it is a perfect candidate for therapeutic applications.

A Multitarget Gold(I) Complex Induces Cytotoxicity Related to Aneuploidy in HCT-116 Colorectal Carcinoma Cells

A novel alkynyl phosphane gold(I) complex (trimethylphosphane)(3-(1,3-dimethylxanthine-7-yl)prop-1-yn-1-yl)gold(I) 1 displayed mutiple biological activites including selective proliferation inhibitory, anti-metastatic, and anti-angiogenic effects. The complex also induced effects related to aneuploidy in HCT-116 colon carcinoma cells, which might be mainly ascribed to the dysfunction of mitochondrial bioenergetics and downregulation of glycolysis. Induction of aneuploidy beyond a critical level can provide an effective strategy to target cancer, in particular colorectal tumours with a low tolerance of aneuploidy, and could be of relevance for 1 and other metallodrugs.

Platinum(ii) complexes showing high cytotoxicity toward A2780 ovarian carcinoma cells

2,6-Bis(thiazol-2-yl)pyridines functionalized with 9-anthryl (L¹), 9-phenanthryl (L²), and 1-pyrenyl (L³) groups were used for the preparation of [Pt(Lⁿ)Cl]CF₃SO₃ (1-3). The constitution of the Pt(ii) complexes was determined by ¹H and ¹³C NMR spectroscopy, HR-MS spectrometry, elemental analysis and X-ray analysis (for (1)). The electrochemical and photophysical properties of [Pt(Lⁿ)Cl]CF₃SO₃ were compared with the behaviour of the Pt(ii) complexes with aryl-substituted 2,2':6',2''-terpyridine ligands. What is noteworthy is that the coordination ability of dtpy toward the Pt(ii) centre was investigated for the first time. All complexes were tested in vitro by MTS assay on four tumor cell lines, A2780 (ovarian carcinoma), HTC116 (colon rectal carcinoma), MCF7 (breast adenocarcinoma), and PC3 (prostate carcinoma) and on normal primary fibroblasts. Compounds (1-3) showed a dose dependent antiproliferative effect in the A2780 cell line with (3) > (2) > (1) and this loss of A2780 cell viability was due to a combination of an apoptotic cell death mechanism via mitochondria and autophagic cell death. Exposure to IC₅₀ concentration of (2) induced an increase in the number of apoptotic nuclei and a depolarization of the mitochondrial membrane which is consistent with the induction of apoptosis while exposure to IC₅₀ concentration of (3) showed an increase in the apoptotic nuclei with a slight hyperpolarization of the mitochondrial membrane that might indicate an initial step of apoptosis induction. The complexes (2) and (3) induce an increase in the production of intracellular ROS which is associated with the trigger of the apoptotic pathways. The ROS production was augmented by the presence of oxidants and correlated with an increase of oxygen radicals. The IC₅₀ of (2) and (3) (4.4 μM and 2.9 μM, respectively) was similar to the IC₅₀ of cisplatin (3.4 μM) in the A2780 cell line, which together with their low cytotoxicity in normal fibroblasts, demonstrates their potential for further studies.

Reactivity of Gold(I) Monocarbene Complexes with Protein Targets: A Theoretical Study

Neutral N–heterocyclic carbene gold(I) compounds such as IMeAuCl are widely used both in homogeneous catalysis and, more recently, in medicinal chemistry as promising antitumor agents. In order to shed light on their reactivity with protein side chains, we have carried out density functional theory (DFT) calculations on the thermodynamics and kinetics of their reactions with water and various nucleophiles as a model of plausible protein binding sites such as arginine, aspartic acid, asparagine, cysteine, glutamic acid, glutamine, histidine, lysine, methionine, selenocysteine, and the N-terminal group. In agreement with recent experimental data, our results suggest that IMeAuCl easily interacts with all considered biological targets before being hydrated—unless sterically prevented—and allows the establishment of an order of thermodynamic stability and of kinetic reactivity for its binding to protein residues.

Recyclable iron(ii) caffeine-derived ionic salt catalyst in the Diels–Alder reaction of cyclopentadiene and α,β-unsaturated N-acyl-oxazolidinones in dimethyl carbonate

Iron(ii) triflate was used in combination with caffeine-derived salts as recyclable catalysts for the Diels–Alder reaction run in dimethyl carbonate (DMC) as a green solvent.

N-Heterocyclic Carbenes Derived from Guanosine: Synthesis and Evidences of Their Antiproliferative Activity

Palladium(II) and platinum(II) complexes bearing N-heterocyclic carbenes derived from guanosine are synthesized via oxidative addition, followed by protonation in the presence of acid. Cytotoxicity of the compounds is evaluated in several cell lines. Compounds 2a, 2b, and 3a are selective for glioblastoma U251 cells and are nontoxic toward healthy human embryonic kidney (HEK293) cells.

A multi-target caffeine derived rhodium(i) N-heterocyclic carbene complex: evaluation of the mechanism of action

A rhodium(i) and a ruthenium(ii) complex with a caffeine derived N-heterocyclic carbene (NHC) ligand were biologically investigated as organometallic conjugates consisting of a metal center and a naturally occurring moiety. While the ruthenium(ii) complex was largely inactive, the rhodium(i) NHC complex displayed selective cytotoxicity and significant anti-metastatic and in vivo anti-vascular activities and acted as both a mammalian and an E. coli thioredoxin reductase inhibitor. In HCT-116 cells it increased the reactive oxygen species level, leading to DNA damage, and it induced cell cycle arrest, decreased the mitochondrial membrane potential, and triggered apoptosis. This rhodium(i) NHC derivative thus represents a multi-target compound with promising anti-cancer potential.

Metal NHC Complexes with Naphthalimide Ligands as DNA-Interacting Antiproliferative Agents

Naphthalimide-based N-heterocyclic carbene (NHC) complexes of the type [(1,5-cyclooctadiene)(NHC)RhCl)] (4 a-c), [(p-cymene)(NHC)RuCl₂ )] (5 a-c), and [(NHC)CuBr] (6 a-c) were synthesized and investigated as antiproliferative agents that target DNA. The cytotoxic effects were largely driven by the naphthalimide structure, which is a DNA-intercalating moiety. Regarding the metal center, the highest activities were observed with the rhodium complexes, and cytotoxic activity was significantly lower for the ruthenium derivatives. The stable coordination of the NHC ligands of selected complexes 4 b and 5 b in solution was confirmed, and their DNA binding properties were studied by UV/Vis spectroscopy, mass spectrometry, and circular dichroism. Stable intercalative binding into the DNA for all selected naphthalimide-based complexes is indicated by high DNA binding constants. Particularly efficient binding was observed in the case of the rhodium complex 4 b. More detailed biological studies on 4 b showed promising activities against multidrug-resistant Nalm-6 cells and confirmed an important role for mitochondrial pathways in 4 b-induced apoptosis.

First Crystal Structure for a Gold Carbene-Protein Adduct

The X-ray structure of the adduct formed in the reaction between the gold N-heterocyclic carbene compound Au(NHC)Cl (with NHC = 1-butyl-3-methyl-imidazole-2-ylidene) and the model protein thaumatin is reported here. The structure reveals binding of Au(NHC)(+) fragments to distinct protein sites. Notably, binding of the gold compound occurs at lysine side chains and at the N-terminal tail; the metal binds the protein after releasing Cl(-) ligand, but retaining NHC fragment.

Metal complexes with di(N-heterocyclic carbene) ligands bearing a rigid ortho-, meta or para-phenylene bridge

Ag(i) complexes with diNHC ligands are efficient transmetalating agents towards transition metals.

Rhodium(I) N-Heterocyclic Carbene Bioorganometallics as in Vitro Antiproliferative Agents with Distinct Effects on Cellular Signaling

Organometallics with N-heterocyclic carbene (NHC) ligands have triggered major interest in inorganic medicinal chemistry. Complexes of the type Rh(I)(NHC)(COD)X (where X is Cl or I, COD is cyclooctadiene, and NHC is a dimethylbenzimidazolylidene) represent a promising type of new metallodrugs that have been explored by advanced biomedical methods only recently. In this work, we have synthesized and characterized several complexes of this type. As observed by mass spectrometry, these complexes remained stable over at least 3 h in aqueous solution, after which hydrolysis of the halido ligands occurred and release of the NHC ligand was evident. Effects against mitochondria and general cell tumor metabolism were noted at higher concentrations, whereas phosphorylation of HSP27, p38, ERK1/2, FAK, and p70S6K was induced substantially already at lower exposure levels. Regarding the antiproliferative activity in tumor cells, a clear preference for iodido over chlorido secondary ligands was noted, as well as effects of the substituents of the NHC ligand.

Human colon cancer targeted pro-apoptotic, anti-metastatic and cytostatic effects of binuclear Silver(I)-N-Heterocyclic carbene (NHC) complexes

The current mechanistic study was conducted to explore the effects of increased lipophilicity of binuclear silver(I)-NHC complexes on cytotoxicity. Two new silver(I)-N-Heterocyclic Carbene (NHC) complexes (3 and 4), having lypophilic terminal alkyl chains (Octyl and Decyl), were derived from meta-xylyl linked bis-benzimidazolium salts (1 and 2). Each of the synthesized compounds was characterized by microanalysis and spectroscopic techniques. The complexes were tested for their cytotoxicity against a panel of human cancer c as well normal cell lines using MTT assay. Based on MTT assay results, complex 4 was found to be selectively toxic towards human colorectal carcinoma cell line (HCT 116). Complex 4 was further studied in detail to explore the mechanism of cell death and findings of the study revealed that complex 4 has promising pro-apoptotic and anti-metastatic activities against HCT 116 cells. Furthermore, it showed pronounced cytostatic effects in HCT 116 multicellular spheroid model. Hence, binuclear silver(I)-NHC complexes with longer terminal aliphatic chains have worth to be further studied against human colon cancer for the purpose of drug development.

Targeting Antioxidant Pathways with Ferrocenylated N-Heterocyclic Carbene Supported Gold(I) Complexes in A549 Lung Cancer Cells

Ferrocenylated-Au(i) carbenes were designed, synthesized, and studied for their ability to generate reactive oxygen species and target antioxidant pathways via multiple mechanisms.

Ferrocene containing N-heterocyclic carbene (NHC) ligated gold(i) complexes of the type [Au(NHC)₂]⁺ were prepared and found to be capable of regulating the formation of reactive oxygen species (ROS) via multiple mechanisms. Single crystal X-ray analysis of bis(1-(ferrocenylmethyl)-3-mesitylimidazol-2-ylidene)-gold(i) chloride (5) and bis(1,3-di(ferrocenylmethyl)imidazol-2-ylidene)-gold(i) chloride (6) revealed a quasi-linear geometry around the gold(i) centers (i.e., the C–Au–C bond angle were measured to be ∼177° and all the Au–C_carbene bonds distances were in the range of 2.00 (7)–2.03 (1) Å). A series of cell studies indicated that cell proliferation inhibition and ROS generation were directly proportional to the amount of ferrocene contained within the [Au(NHC)₂]⁺ complexes (IC₅₀ of 6 < 5 < bis(1-benzyl-3-mesitylimidazol-2-ylidene)-gold(i) chloride (4)). Complexes 4–6 were also confirmed to inhibit thioredoxin reductase as inferred from lipoate reduction assays and increased chelatable intracellular zinc concentrations. RNA microarray gene expression assays revealed that 6 induces endoplasmic reticulum stress response pathways as a result of ROS increase.