Synthesis and Biological Evaluation of Chromium Bioorganometallics Based on the Antibiotic Platensimycin Lead Structure

Total Synthesis of the Phenylnaphthacenoid Type II Polyketide Antibiotic Formicamycin H via Regioselective Ruthenium-Catalyzed Hydrogen Auto-Transfer [4 + 2] Cycloaddition

The first total synthesis of the pentacyclic phenylnaphthacenoid type II polyketide antibiotic formicamycin H is described. A key feature of the synthesis involves the convergent, regioselective assembly of the tetracyclic core via ruthenium-catalyzed α-ketol-benzocyclobutenone [4 + 2] cycloaddition. Double dehydration of the diol-containing cycloadduct provides an achiral enone, which upon asymmetric nucleophilic epoxidation and further manipulations delivers the penultimate tetracyclic trichloride in enantiomerically enriched form. Subsequent chemo- and atroposelective Suzuki cross-coupling of the tetracyclic trichloride introduces the E-ring to complete the total synthesis. Single-crystal X-ray diffraction analyses of two model compounds suggest that the initially assigned stereochemistry of the axially chiral C6-C7 linkage may require revision.

Metals to combat antimicrobial resistance

Bacteria, similar to most organisms, have a love-hate relationship with metals: a specific metal may be essential for survival yet toxic in certain forms and concentrations. Metal ions have a long history of antimicrobial activity and have received increasing attention in recent years owing to the rise of antimicrobial resistance. The search for antibacterial agents now encompasses metal ions, nanoparticles and metal complexes with antimicrobial activity ('metalloantibiotics'). Although yet to be advanced to the clinic, metalloantibiotics are a vast and underexplored group of compounds that could lead to a much-needed new class of antibiotics. This Review summarizes recent developments in this growing field, focusing on advances in the development of metalloantibiotics, in particular, those for which the mechanism of action has been investigated. We also provide an overview of alternative uses of metal complexes to combat bacterial infections, including antimicrobial photodynamic therapy and radionuclide diagnosis of bacterial infections.

Rational approaches towards inorganic and organometallic antibacterials

The occurrence of drug-resistant bacteria is drastically rising and new and effective antibiotic classes are urgently needed. However, most of the compounds in development are minor modifications of previously used drugs to which bacteria can easily develop resistance. The investigation of inorganic and organometallic compounds as antibiotics is an alternative approach that holds great promises due to the ability of such molecules to trigger metal-specific mechanisms of action, which results in lethal consequences for pathogens. In this review, a selection of concepts to rationally design inorganic and organometallic antibiotics is discussed, highlighting their advantages by comparing them to classical drug discovery programmes. The review concludes with a short perspective for the future of antibiotic drug development and the role metal-based compounds will play in the field.

Biological activity of manganese(i) tricarbonyl complexes on multidrug-resistant Gram-negative bacteria: From functional studies to in vivo activity in Galleria mellonella

Three new manganese(i) tricarbonyl complexes [Mn(bpqa-κ3N)(CO)3]Br, [Mn(bqpa-κ3N)(CO)3]Br, and [Mn(CO)3(tqa-κ3N)]Br as well as the previously described compound [Mn(CO)3(tpa-κ3N)]Br with bpqa = bis(2-pyridinylmethyl)(2-quinolinylmethyl)amine, bqpa = bis(2-quinolinylmethyl)(2-pyridinylmethyl)amine, tqa = tris(2-quinolinylmethyl)amine, and tpa = tris(2-pyridinylmethyl)amine were examined for their antibacterial activities on 14 different multidrug-resistant clinical isolates of Acinetobacter baumannii and Pseudomonas aeruginosa, in recognition of the current antimicrobial resistance (AMR) concerns with these pathogens. Minimal inhibitory concentrations (MIC) of the most potent tqa compound were in the mid-micromolar range and generally lower than that of the free ligand. Activity against both bacterial species increased with the number of quinolinylmethyl groups and lipophilicity in the order of tpa < bpqa < bqpa ≈ tqa, consistent with measured increases in release of ATP, a uniquely cytoplasmic biomolecule and induced permeability to exogenous fluorescent intercalating compounds. [Mn(CO)3(tqa-κ3N)]Br was also evaluated in the Galleria mellonella model of infection, and displayed a lack of host toxicity combined with effective bacterial clearance.

Metal Complexes, an Untapped Source of Antibiotic Potential?

With the widespread rise of antimicrobial resistance, most traditional sources for new drug compounds have been explored intensively for new classes of antibiotics. Meanwhile, metal complexes have long had only a niche presence in the medicinal chemistry landscape, despite some compounds, such as the anticancer drug cisplatin, having had a profound impact and still being used extensively in cancer treatments today. Indeed, metal complexes have been largely ignored for antibiotic development. This is surprising as metal compounds have access to unique modes of action and exist in a wider range of three-dimensional geometries than purely organic compounds. These properties make them interesting starting points for the development of new drugs. In this perspective article, , the encouraging work that has been done on antimicrobial metal complexes, mainly over the last decade, is highlighted. Promising metal complexes, their activity profiles, and possible modes of action are discussed and issues that remain to be addressed are emphasized.

A brief overview of metal complexes as nuclear imaging agents

Metallic radionuclides have been instrumental in the field of nuclear imaging for over half a century. While recent years have played witness to a dramatic rise in the use of radiometals as labels for chelator-bearing biomolecules, imaging agents based solely on coordination compounds of radiometals have long played a critical role in the discipline as well. In this work, we seek to provide a brief overview of metal complex-based radiopharmaceuticals for positron emission tomography (PET) and single photon emission computed tomography (SPECT). More specifically, we have focused on imaging agents in which the metal complex itself rather than a pendant biomolecule or targeting moiety is responsible for the in vivo behavior of the tracer. This family of compounds contains metal complexes based on an array of different nuclides as well as probes that have been used for the imaging of a variety of pathologies, including infection, inflammation, cancer, and heart disease. Indeed, two of the defining traits of transition metal complexes-modularity and redox chemistry-have both been creatively leveraged in the development of imaging agents. In light of our audience, particular attention is paid to structure and mechanism, though clinical data is addressed as well. Ultimately, it is our hope that this review will not only educate readers about some of the seminal work performed in this space over the last 30 years but also spur renewed interest in the creation of radiopharmaceuticals based on small metal complexes.

Bio-Organometallic Derivatives of Antibacterial Drugs

Overuse and misuse of antibacterial drugs has resulted in bacteria resistance and in an increase in mortality rates due to bacterial infections. Therefore, there is an imperative necessity of new antibacterial drugs. Bio-organometallic derivatives of antibacterial agents offer an opportunity to discover new active antibacterial drugs. These compounds are well-characterized products and, in several examples, their antibacterial activities have been studied. Both inhibition of the antibacterial activity and strong increase in the antibiotic activity of the parent drug have been found. The synthesis of the main classes of bio-organometallic derivatives of these drugs, as well as examples of the use of structure-activity relation (SAR) studies to increase the activity and to understand the mode of action of bio-organometallic antimicrobial peptides (BOAMPs) and platensimicyn bio-organometallic mimics is presented in this article.

Structure and reactivities of rhenium and technetium bis-arene sandwich complexes [M(η6-arene)2]+

Rhenium and ⁹⁹Tc bis-arene complexes for a molecule-based theranostic approach are presented. Conjugation of biovectors to benzene or substitution of naphthalene allows integration of {Re(η⁶-C₆H₆)}⁺ in pharmaceutical lead structures.

Assessment of the nematocidal activity of metallocenyl analogues of monepantel

Ferrocenyl and ruthenocenyl analogues of the nematocidal drug monepantel show organometallic-dependent activity against Haemonchus contortus and Trichostrongylus colubriformis.

In vitro and in vivo evaluation of organometallic gold(I) derivatives as anticancer agents

Alkyne gold(I) derivatives with the water soluble phosphanes PTA (1,3,5-triaza-7-phosphaadamantane) and DAPTA (3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane) were described and their anticancer potential against the colon cancer cell line Caco-2 (PD7 and TC7 clones) was studied. Strong antiproliferative effects are found, for all the new complexes, to be even more pronounced than for the reference drug cisplatin, and similar to auranofin. The interaction of these derivatives with bovine serum albumin (BSA) was studied by fluorescence spectroscopy. The types of quenching and binding constants were determined by a fluorescence quenching method. Moderate values of the binding constants are calculated for the tested derivatives indicating that these complexes can be stored and carried easily by this protein in the body. The study of the thermodynamic parameters in the case of [Au(C[triple bond, length as m-dash]CCH2Spyridine)(PTA)] points out to the presence of van der Waals interactions or hydrogen bonding between the metallic complex and the protein. In addition, the complex [Au(C[triple bond, length as m-dash]CCH2Spyridine)(PTA)] has shown inhibition in colon cancer proliferation of HTC-116-luc2 cell lines via the apoptotic pathway and S-phase arrest of the cell cycle. Intraperitoneal injection of this derivative in athymic nude mice inoculated with HTC-116-luc2 cells prolonged their survival and displayed moderate inhibition of the tumour growth with no subsequent organ (kidney and liver) damage after treatment.

An organometallic structure-activity relationship study reveals the essential role of a Re(CO)3 moiety in the activity against gram-positive pathogens including MRSA

The worrying appearance of microbial resistance to antibiotics is a worldwide problem which needs to be tackled urgently. Microbial resistance to the common classes of antibiotics involving purely organic compounds unfortunately develops very rapidly and in most cases, resistance was detected soon after or even before release of the antibiotic to the market. Therefore, novel concepts for antibiotics must be investigated, and metal-containing compounds hold particular promise in that area. Taking a trimetallic complex (1a) which contains a ferrocenyl (Fc), a CpMn(CO)3 (cymantrene) and a [(dpa)Re(CO)3] residue as the lead structure, a systematic structure-activity relationship (SAR) study against various gram-positive pathogenic bacteria including methicillin-resistant Staphylococcus aureus (MRSA) strains was performed. The [(dpa)Re(CO)3] moiety was discovered to be the essential unit for the observed antibacterial activity of 1a. The ferrocenyl and CpMn(CO)3 units can be replaced one by one or both together by organic moieties such as a phenyl ring without loss of antibacterial activity. The most potent mono-metallic complex (9c') has an antibacterial activity comparable to the well-established organic drugs amoxicillin and norfloxacin and importantly, only moderate cytotoxicity against mammalian cells. Microbiological studies on membrane potential, membrane permeabilization, and cell wall integrity revealed that 9c' targets the bacterial membrane and disturbs cell wall integrity, but shows more efficient membrane permeabilization than the lead structure 1a.

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.

Analysis of the mechanism of action of potent antibacterial hetero-tri-organometallic compounds: a structurally new class of antibiotics

Two hetero-tri-organometallic compounds with potent activity against Gram-positive bacteria including multi-resistant Staphylococcus aureus (MRSA) were identified. The compounds consist of a peptide nucleic acid backbone with an alkyne side chain, substituted with a cymantrene, a (dipicolyl)Re(CO)3 moiety, and either a ferrocene (FcPNA) or a ruthenocene (RcPNA). Comparative proteomic analysis indicates the bacterial membrane as antibiotic target structure. FcPNA accumulation in the membrane was confirmed by manganese tracing with atomic absorption spectroscopy. Both organometallics disturbed several essential cellular processes taking place at the membrane such as respiration and cell wall biosynthesis, suggesting that the compounds affect membrane architecture. Correlating with enhanced antibacterial activity, oxidative stress was induced only by the ferrocene-substituted compound. The organometallics described here target the cytoplasmic membrane, a clinically proven antibacterial target structure, feature a bactericidal but non-bacteriolytic mode of action and limited cytotoxicity within the limits of solubility. Thus, FcPNA represents a promising lead structure for the development of a new synthetic class of antibiotics.

Molecular and cellular characterization of the biological effects of ruthenium(II) complexes incorporating 2-pyridyl-2-pyrimidine-4-carboxylic acid

A great majority of the Ru complexes currently studied in anticancer research exert their antiproliferative activity, at least partially, through ligand exchange. In recent years, however, coordinatively saturated and substitutionally inert polypyridyl Ru(II) compounds have emerged as potential anticancer drug candidates. In this work, we present the synthesis and detailed characterization of two novel inert Ru(II) complexes, namely, [Ru(bipy)(2)(Cpp-NH-Hex-COOH)](2+) (2) and [Ru(dppz)(2)(CppH)](2+) (3) (bipy = 2,2'-bipyridine; CppH = 2-(2'-pyridyl)pyrimidine-4-carboxylic acid; Cpp-NH-Hex-COOH = 6-(2-(pyridin-2-yl)pyrimidine-4-carboxamido)hexanoic acid; dppz = dipyrido[3,2-a:2',3'-c]phenazine). 3 is of particular interest as it was found to have IC(50) values comparable to cisplatin, a benchmark standard in the field, on three cancer cell lines and a better activity on one cisplatin-resistant cell line than cisplatin itself. The mechanism of action of 3 was then investigated in detail and it could be demonstrated that, although 3 binds to calf-thymus DNA by intercalation, the biological effects that it induces did not involve a nuclear DNA related mode of action. On the contrary, confocal microscopy colocalization studies in HeLa cells showed that 3 specifically targeted mitochondria. This was further correlated by ruthenium quantification using High-resolution atomic absorption spectrometry. Furthermore, as determined by two independent assays, 3 induced apoptosis at a relatively late stage of treatment. The generation of reactive oxygen species could be excluded as the cause of the observed cytotoxicity. It was demonstrated that the mitochondrial membrane potential in HeLa was impaired by 3 as early as 2 h after its introduction and even more with increasing time.

Modulating the activity of short arginine-tryptophan containing antibacterial peptides with N-terminal metallocenoyl groups

A series of small synthetic arginine and tryptophan containing peptides was prepared and analyzed for their antibacterial activity. The effect of N-terminal substitution with metallocenoyl groups such as ferrocene (FcCO) and ruthenocene (RcCO) was investigated. Antibacterial activity in different media, growth inhibition, and killing kinetics of the most active peptides were determined. The toxicity of selected derivatives was determined against erythrocytes and three human cancer cell lines. It was shown that the replacement of an N-terminal arginine residue with a metallocenoyl moiety modulates the activity of WRWRW-peptides against Gram-positive and Gram-negative bacteria. MIC values of 2–6 µM for RcCO-W(RW)₂ and 1–11 µM for (RW)₃ were determined. Interestingly, W(RW)₂-peptides derivatized with ferrocene were significantly less active than those derivatized with ruthenocene which have similar structural but different electronic properties, suggesting a major influence of the latter. The high activities observed for the RcCO-W(RW)₂- and (RW)₃-peptides led to an investigation of the origin of activity of these peptides using several important activity-related parameters. Firstly, killing kinetics of the RcCO-W(RW)₂-peptide versus killing kinetics of the (RW)₃ derivative showed faster reduction of the colony forming units for the RcCO-W(RW)₂-peptide, although MIC values indicated higher activity for the (RW)₃-peptide. This was confirmed by growth inhibition studies. Secondly, hemolysis studies revealed that both peptides did not lead to significant destruction of erythrocytes, even up to 500 µg/mL for (RW)₃ and 250 µg/mL for RcCO-W(RW)₂. In addition, toxicity against three human cancer cell lines (HepG2, HT29, MCF7) showed that the (RW)₃-peptide had an IC₅₀ value of ~140 µM and the RcW(RW)₂ one of ~90 µM, indicating a potentially interesting therapeutic window. Both the killing kinetics and growth inhibition studies presented in this work point to a membrane-based mode of action for these two peptides, each having different kinetic parameters.

Inhibitors of fatty acid synthesis in prokaryotes and eukaryotes as anti-infective, anticancer and anti-obesity drugs

There is a large range of diseases, such diabetes and cancer, which are connected to abnormal fatty acid metabolism in human cells. Therefore, inhibitors of human fatty acid synthase have great potential to manage or treat these diseases. In prokaryotes, fatty acid synthesis is important for signaling, as well as providing starting materials for the synthesis of phospholipids, which are required for the formation of the cell membrane. Recently, there has been renewed interest in the development of new molecules that target bacterial fatty acid synthases for the treatment of bacterial diseases. In this review, we look at the differences and similarities between fatty acid synthesis in humans and bacteria and highlight various small molecules that have been shown to inhibit either the mammalian or bacterial fatty acid synthase or both.

Small organometallic compounds as antibacterial agents

The emergence of bacterial resistance to commercial antibiotics is an issue of global importance. During the last two decades, the number of antibacterial agents that have been discovered and introduced into the market has steadily declined and failed to meet the challenges posed by rapidly increasing resistance of the pathogens against common antibacterial drugs. The development of new classes of compounds to control the virulence of the pathogens is therefore urgently required. This perspective describes the historical development in brief and recent advances on the preparation of small organometallic compounds as new classes of antibacterial agents with potential for clinical development.

The potential of organometallic complexes in medicinal chemistry

Organometallic complexes have unique physico-chemical properties, which have been widely used in homogenous catalysis, for example, for the synthesis of lead compounds and drug candidates. Over the past two decades, a few scientists from all over the world have extended the use of the specific characteristics of these compounds (e.g. structural diversity, possibility of ligand exchange, redox and catalytic properties) for medicinal purposes. The results are stunning. A few organometallic compounds have already entered clinical trials and it can be anticipated that several more will follow in coming years. In this short review, we present the specific advantages that organometallic metal complexes have over purely organic and also coordination compounds. Furthermore, using specific examples, we illustrate how these particular properties can be put to good use in medicinal chemistry. The examples we present have an emphasis on, but are not restricted to, anti-cancer activity.

Anti-inflammatory arene--chromium complexes acting as specific inhibitors of NOD2 signalling

Inflammation is a hallmark of microbial infection in mammals and is the result of a pathogen-induced release of inflammatory effectors. In humans a variety of germ-line encoded receptors, so-called pattern-recognition receptors, respond to conserved signatures on invading pathogens, which results in the transcriptional activation of pro-inflammatory responses. Inflammation is often detrimental to the host and leads to tissue damage and/or systemic dysfunctions. Thus, specific inhibitors of these pathways are desirable for medical interventions. Herein we report on the synthesis and use of some chromium-containing compounds (arene--Cr(CO)₃ complexes) with a core structure related to anti-inflammatory diterpenes produced by the sea whip Pseudopterogorgia elisabethae. By using cell-based reporter assays we identified complexes with a potent inhibitory activity on tumour necrosis factor (TNF), Toll-like receptor (TLR), and nucleotide binding domain, leucine-rich repeat-containing receptor (NLR) pathways. Moreover, we found one complex to be a specific inhibitor of inflammatory responses mediated by the NLR protein NOD2, a pivotal innate immune receptor involved in bacterial recognition. Synthesis and characterisation of a set of derivatives of this substance revealed structural requirements for NOD2 specificity. Taken together, our studies suggest this type of arene--Cr(CO)₃ complex as a potential lead for the development of antiphlogistica and pharmacologically relevant NOD2 inhibitors.

Proteomic signature of fatty acid biosynthesis inhibition available for in vivo mechanism-of-action studies

Fatty acid biosynthesis is a promising novel antibiotic target. Two inhibitors of fatty acid biosynthesis, platencin and platensimycin, were recently discovered and their molecular targets identified. Numerous structure-activity relationship studies for both platencin and platensimycin are currently being undertaken. We established a proteomic signature for fatty acid biosynthesis inhibition in Bacillus subtilis using platencin, platensimycin, cerulenin, and triclosan. The induced proteins, FabHA, FabHB, FabF, FabI, PlsX, and PanB, are enzymes involved in fatty acid biosynthesis and thus linked directly to the target pathway. The proteomic signature can now be used to assess the in vivo mechanisms of action of compounds derived from structure-activity relationship programs, as demonstrated for the platensimycin-inspired chromium bioorganometallic PM47. It will further serve as a reference signature for structurally novel natural and synthetic antimicrobial compounds with unknown mechanisms of action. In summary, we described a proteomic signature in B. subtilis consisting of six upregulated proteins that is diagnostic of fatty acid biosynthesis inhibition and thus can be applied to advance antibacterial drug discovery programs.