Pyridocarbazole-Rhodium(III) Complexes as Protein Kinase Inhibitors

Ligand-Controlled Reactivity and Cytotoxicity of Cyclometalated Rhodium(III) Complexes

We report the synthesis, characterisation and cytotoxicity of six cyclometalated rhodium(III) complexes [CpXRh(C^N)Z]0/+, in which CpX = Cp*, Cpph, or Cpbiph, C^N = benzo[h]quinoline, and Z = chloride or pyridine. Three x-ray crystal structures showing the expected "piano-stool" configurations have been determined. The chlorido complexes hydrolysed faster in aqueous solution, also reacted preferentially with 9-ethyl guanine or glutathione compared to their pyridine analogues. The 1-biphenyl-2,3,4,5,-tetramethylcyclopentadienyl complex [CpbiphRh(benzo[h]quinoline)Cl] (3a) was the most efficient catalyst in coenzyme reduced nicotinamide adenine dinucleotide (NADH) oxidation to NAD+ and induced an elevated level of reactive oxygen species (ROS) in A549 human lung cancer cells. The pyridine complex [CpbiphRh(benzo[h]quinoline)py]+ (3b) was the most potent against A549 lung and A2780 ovarian cancer cell lines, being 5-fold more active than cisplatin towards A549 cells, and acted as a ROS scavenger. This work highlights a ligand-controlled strategy to modulate the reactivity and cytotoxicity of cyclometalated rhodium anticancer complexes.

Carbazole scaffolds in cancer therapy: a review from 2012 to 2018

For over half a century, the carbazole skeleton has been the key structural motif of many biologically active compounds including natural and synthetic products. Carbazoles have taken an important part in all the existing anti-cancer drugs because of their discovery from a large variety of organisms, including bacteria, fungi, plants, and animals. In this article, we specifically explored the literature from 2012 to 2018 on the anti-tumour activities reported to carbazole derivatives and we have critically collected the most significant data. The most described carbazole anti-tumour agents were classified according to their structure, starting from the tricyclic–carbazole motif to fused tetra-, penta-, hexa- and heptacyclic carbazoles. To date, three derivatives are available on the market and approved in cancer therapy.

Metal complexes as "protein surface mimetics"

A key challenge in chemical biology is to identify small molecule regulators for every single protein. However, protein surfaces are notoriously difficult to recognise with synthetic molecules, often having large flat surfaces that are poorly matched to traditional small molecules. In the surface mimetic approach, a supramolecular scaffold is used to project recognition groups in such a manner as to make multivalent non-covalent contacts over a large area of protein surface. Metal based supramolecular scaffolds offer unique advantages over conventional organic molecules for protein binding, including greater stereochemical and geometrical diversity conferred through the metal centre and the potential for direct assessment of binding properties and even visualisation in cells without recourse to further functionalisation. This feature article will highlight the current state of the art in protein surface recognition using metal complexes as surface mimetics.

A Rhodium(III) Complex as an Inhibitor of Neural Precursor Cell Expressed, Developmentally Down-Regulated 8-Activating Enzyme with in Vivo Activity against Inflammatory Bowel Disease

We report herein the identification of the rhodium(III) complex [Rh(phq)₂(MOPIP)]⁺ (1) as a potent and selective ATP-competitive neural precursor cell expressed, developmentally down-regulated 8 (NEDD8)-activating enzyme (NAE) inhibitor. Structure-activity relationship analysis indicated that the overall organometallic design of complex 1 was important for anti-inflammatory activity. Complex 1 showed promising anti-inflammatory activity in vivo for the potential treatment of inflammatory bowel disease.

Rhodium complexes as therapeutic agents

This perspective highlights recent examples of rhodium complexes that show diverse biological activities against various targets, including enzymes and protein–protein interactions.

An iridium(iii)-based irreversible protein-protein interaction inhibitor of BRD4 as a potent anticancer agent

Bromodomain-containing protein 4 (BRD4) has recently emerged as an attractive epigenetic target for anticancer therapy. In this study, an iridium(iii) complex is reported as the first metal-based, irreversible inhibitor of BRD4. Complex 1a is able to antagonize the BRD4-acetylated histone protein-protein interaction (PPI) in vitro, and to bind BRD4 and down-regulate c-myc oncogenic expression in cellulo. Chromatin immunoprecipitation (ChIP) analysis revealed that 1a could modulate the interaction between BRD4 and chromatin in melanoma cells, particular at the MYC promoter. Finally, the complex showed potent activity against melanoma xenografts in an in vivo mouse model. To our knowledge, this is the first report of a Group 9 metal complex inhibiting the PPI of a member of the bromodomain and extraterminal domain (BET) family. We envision that complex 1a may serve as a useful scaffold for the development of more potent epigenetic agents against cancers such as melanoma.

A rhodium(III) complex inhibits LPS-induced nitric oxide production and angiogenic activity in cellulo

Metal-containing complexes have arisen as viable alternatives to organic molecules as therapeutic agents. Metal complexes possess a number of advantages compared to conventional carbon-based compounds, such as distinct geometries, interesting electronic properties, variable oxidation states and the ability to arrange different ligands around the metal centre in a precise fashion. Meanwhile, nitric oxide (NO) plays key roles in the regulation of angiogenesis, vascular permeability and inflammation. We herein report a novel cyclometalated rhodium(III) complex as an inhibitor of lipopolysaccharides (LPS)-induced NO production in RAW264.7 macrophages. Experiments suggested that the inhibition of NO production in cells by complex 1 was mediated through the down-regulation of nuclear factor-κB (NF-κB) activity. Furthermore, complex 1 inhibited angiogenesis in human umbilical vein endothelial cells (HUVECs) as revealed by an endothelial tube formation assay. This study demonstrates that kinetically inert rhodium(III) complexes may be potentially developed as effective anti-angiogenic agents.

Antagonizing STAT3 dimerization with a rhodium(III) complex

Kinetically inert metal complexes have arisen as promising alternatives to existing platinum and ruthenium chemotherapeutics. Reported herein, to our knowledge, is the first example of a substitutionally inert, Group 9 organometallic compound as a direct inhibitor of signal transducer and activator of transcription 3 (STAT3) dimerization. From a series of cyclometalated rhodium(III) and iridium(III) complexes, a rhodium(III) complex emerged as a potent inhibitor of STAT3 that targeted the SH2 domain and inhibited STAT3 phosphorylation and dimerization. Significantly, the complex exhibited potent anti-tumor activities in an in vivo mouse xenograft model of melanoma. This study demonstrates that rhodium complexes may be developed as effective STAT3 inhibitors with potent anti-tumor activity.

Metal complexes as structural templates for targeting proteins

This article reviews recent advances in the design and discovery of inert metal complexes as protein binders. In these metal-based probes or drug candidates, the metal is supposed to exert a purely structural role by organizing the coordinating ligands in the three dimensional space to achieve a shape and functional group complementarity with the targeted protein pockets. Presented examples of sandwich, half-sandwich and octahedral d(6)-metal complexes reinforce previous perceptions that metal complexes are highly promising scaffolds for the design of small-molecule protein binders and complement the molecular diversity of organic chemistry by opening untapped chemical space.

A metal-based inhibitor of NEDD8-activating enzyme

A cyclometallated rhodium(III) complex [Rh(ppy)₂(dppz)]⁺ (1) (where ppy = 2-phenylpyridine and dppz = dipyrido[3,2-a:2′,3′-c]phenazine dipyridophenazine) has been prepared and identified as an inhibitor of NEDD8-activating enzyme (NAE). The complex inhibited NAE activity in cell-free and cell-based assays, and suppressed the CRL-regulated substrate degradation and NF-κB activation in human cancer cells with potency comparable to known NAE inhibitor MLN4924. Molecular modeling analysis suggested that the overall binding mode of 1 within the binding pocket of the APPBP1/UBA3 heterodimer resembled that for MLN4924. Complex 1 is the first metal complex reported to suppress the NEDDylation pathway via inhibition of the NEDD8-activating enzyme.

Bioactive cyclometalated phthalimides: design, synthesis and kinase inhibition

The regioselective cyclometalation of 4-(pyridin-2-yl)phthalimide was exploited for the economical design of organometallic protein kinase inhibitors. 4-(Pyridin-2-yl)phthalimide can be prepared from inexpensive 4-bromophthalimide in just three steps including one Pd-catalyzed Stille cross-coupling. The versatility of this new ligand was demonstrated with the synthesis of ruthenium(II) half-sandwich as well as octahedral ruthenium(II) and iridium(III) complexes. The regioselectivity of the C-H activation in the course of the cyclometalation can be influenced by the reaction conditions and the steric demand of the introduced metal complex fragment. The biological activity of this new class of metalated phthalimides was evaluated by profiling two representative members against a large panel of human protein kinases. A cocrystal structure of one metallo-phthalimide with the protein kinase Pim1 confirmed an ATP-competitive binding with the intended hydrogen bonding between the phthalimide moiety and the hinge region of the ATP-binding site.