Ferrocene–Biotin Conjugates Targeting Cancer Cells: Synthesis, Interaction with Avidin, Cytotoxic Properties and the Crystal Structure of the Complex of Avidin with a Biotin–Linker–Ferrocene Conjugate

The avidin-theophylline complex: A structural and computational study

The interaction between avidin and its counterpart biotin is one of central importance in biology and has been reproposed and studied at length. However, the binding pocket of avidin is prone to promiscuous binding, able to accommodate even non-biotinylated ligands. Comprehending the factors that distinguish the extremely strong interaction with biotin to other ligands is an important step to fully picture the thermodynamics of these low-affinity complexes. Here, we present the complex between chicken white egg avidin and theophylline (TEP), the xanthine derivative used in the therapy of asthma. In the crystal structure, TEP lies in the biotin-binding pocket with the same orientation and planarity of the aromatic ring of 8-oxodeoxyguanosine. Indeed, its affinity for avidin measured by isothermal titration calorimetry is in the same μM range as those obtained for the previously characterized nucleoside derivatives. By the use of molecular dynamic simulations, we have investigated the most important intermolecular interactions occurring in the avidin-TEP binding pocket and compared them with those obtained for the avidin 8-oxodeoxyguanosine and avidin-biotin complexes. These results testify the capability of avidin to complex purely aromatic molecules.

Metal Complexes in Target-Specific Anticancer Therapy: Recent Trends and Challenges

Cancer is characterized by abnormal cell differentiation in or on the part of the body. The most commonly used chemotherapeutic drugs are developed to target rapidly dividing cells, such as cancer cells, but they also damage healthy epithelial cells. This has serious consequences for normal cells and become responsible for the development of various disorders. Several strategies for delivering the cytotoxic drugs to cancerous sites that limit systemic toxicity and other adverse effects have recently been evolved. Among them, biomolecule-conjugated metal complexes-based cancer targeting strategies have shown tremendous advantages in cancer therapy. This review focuses on several chemoselective biomolecules-bound metal complexes as prospective cancer therapy-targeted agents. In this review, we presented the details of the various extra- and intracellular targeting mechanisms in cancer therapy. We also addressed the current clinical issues and recent therapeutic strategies in targeted cancer therapy that may pave a way for the future direction of metal complexes-based targeted cancer therapy.

Has Ferrocene Really Delivered Its Role in Accentuating the Bioactivity of Organic Scaffolds?

Ferrocene is an important structural core in bioorganometallic chemistry because of its inherent stability, excellent redox properties, and low toxicity. Ferroquine and ferrocifen are two of the most notable contributions of ferrocene to medicinal chemistry with remarkable antimalarial and anticancer properties. The improved medicinal properties of these drug candidates highlight the impact that ferrocene can have on the molecular and biological properties of the bioactive compounds. In this Perspective, we investigate the scope and limitations of ferrocene incorporation into organic compounds/natural products on their mode of action and biological activities. We have also discussed the detailed role of ferrocene modifications in influencing the anticancer, antimalarial, and antimicrobial properties of various bioactive moieties to design safer and promising ferrocene-based drugs.

Hetero-Bis-Conjugation of Bioactive Molecules to Half-Sandwich Ruthenium(II) and Iridium(III) Complexes Provides Synergic Effects in Cancer Cell Cytotoxicity

Four bipyridine-type ligands variably derivatized with two bioactive groups (taken from ethacrynic acid, flurbiprofen, biotin, and benzylpenicillin) were prepared via sequential esterification steps from commercial 2,2'-bipyridine-4,4'-dicarboxylic acid and subsequently coordinated to ruthenium(II) p-cymene and iridium(III) pentamethylcyclopentadienyl scaffolds. The resulting complexes were isolated as nitrate salts in high yields and fully characterized by analytical and spectroscopic methods. NMR and MS studies in aqueous solution and in cell culture medium highlighted a substantial stability of ligand coordination and a slow release of the bioactive fragments in the latter case. The complexes were assessed for their antiproliferative activity on four cancer cell lines, showing cytotoxicity to the low micromolar level (equipotent with cisplatin). Additional biological experiments revealed a multimodal mechanism of action of the investigated compounds, involving DNA metalation and enzyme inhibition. Synergic effects provided by specific combinations of metal and bioactive fragments were identified, pointing toward an optimal ethacrynic acid/flurbiprofen combination for both Ru(II) and Ir(III) complexes.

Graphene quantum dot formulation for cancer imaging and redox-based drug delivery

This work develops a new multifunctional biocompatible anticancer nanoformulation to provide targeted image-guided cancer-selective therapeutics. It consists of three active covalently bound components: (1) biocompatible nitrogen-doped graphene quantum dots (GQDs) as a multifunctional delivery and imaging platform, (2) hyaluronic acid (HA) unit targeted to the CD44 receptors on a variety of cancer cells, and (3) oxidative stress-based cancer-selective ferrocene (Fc) therapeutic. The biocompatible GQD platform synthesized from glucosamine exhibits high-yield intrinsic fluorescence. It is utilized for tracking Fc-GQD-HA formulation in vitro indicating internalization enhancement in HeLa cells targeted by the HA over non-cancer HEK-293 cells not overexpressing CD44 receptor. Fc-GQD-HA, non-toxic at 1 mg/mL to HEK-293 cells, induces cytotoxic response in HeLa enhanced over time, while therapeutic ROS generation by Fc-GQD-HA is ~3 times greater than that of Fc alone. This outlines the targeted delivery, imaging, and cancer-specific treatment capabilities of the new Fc-GQD-HA formulation enabling desired cancer-focused nanotherapeutic approach.

Biotin-phenosafranin as a new photosensitive conjugate for targeted therapy and imaging

A biotinylated phenazine compound as a phenosafranin conjugate (Biot-PSF) was synthesized and reported for the first time.

Synthesis, Characterization, and Antiproliferative Activity of Novel Chiral [QuinoxP*AuCl2]+ Complexes

Herein is reported the synthesis of two Au(III) complexes bearing the (R,R)-(-)-2,3-Bis(tert-butylmethylphosphino)quinoxaline (R,R-QuinoxP*) or (S,S)-(+)-2,3-Bis(tert-butylmethylphosphino)quinoxaline (S,S-QuinoxP*) ligands. By reacting two stoichiometric equivalents of HAuCl4.3H2O to one equivalent of the corresponding QuinoxP* ligand, (R,R)-(-)-2,3-Bis(tert-butylmethylphosphino)quinoxalinedichlorogold(III) tetrachloroaurates(III) (1) and (S,S)-(+)-2,3-Bis(tert-butylmethylphosphino)quinoxalinedichlorogold(III) tetrachloroaurates(III) (2) were formed, respectively, in moderate yields. The structure of (S,S)-(+)-2,3-Bis(tert-butylmethylphosphino)quinoxalinedichlorogold(III) tetrachloroaurates(III) (2) was further confirmed by X-ray crystallography. The antiproliferative activities of the two compounds were evaluated in a panel of cell lines and exhibited promising results comparable to auranofin and cisplatin with IC50 values between 1.08 and 4.83 µM. It is noteworthy that in comparison to other platinum and ruthenium enantiomeric complexes, the two enantiomers (1 and 2) do not exhibit different cytotoxic effects. The compounds exhibited stability in biologically relevant media over 48 h as well as inert reactivity to excess glutathione at 37 °C. These results demonstrate that the Au(III) atom, stabilized by the QuinoxP* ligand, can provide exciting compounds for novel anticancer drugs. These complexes provide a new scaffold to further develop a robust and diverse library of chiral phosphorus Au(III) complexes.

Design and Synthesis of Biotinylated Bivalent Carboline Derivatives as Potent Antitumor Agents

Compound 6, a novel β-carboline comprising two 1-methyl-9H-β-carboline-3-carboxylic acids and a biotin moiety conjugated together using tris(2-aminoethyl)amine, was synthesized and tested for its cytotoxicity toward MCF-7 and HepG2 cell lines and antitumor potency in an S180 tumor-bearing mouse model. Compound 6 was delivered via biotin receptor-mediated endocytosis and exerted its therapeutic effects by intercalation binding with DNA. In vivo antitumor evaluations of 6 revealed that it is efficacious and exhibits low systemic toxicity.

Synthesis and biological evaluation of new antioxidant and antiproliferative chalcogenobiotin derivatives for bladder carcinoma treatment

Approximately 90% of bladder carcinomas are of the urothelial carcinoma type, which are characterized by high rates of recurrence and predisposition to progress to invasive tumors, representing one of the most costly neoplasms for health systems. Intravesical chemotherapy is a standard for the treatment of non-invasive bladder cancer. However, chemotherapy is usually aggressive and cytotoxic, which increases the death rates caused by cancer. Heterocyclic compounds which exhibit favorable pharmacokinetic and pharmacodynamic properties may enhance drug affinity for a target protein by targeting the treatment. Thus, this work presents the synthesis, characterization, and in vitro biological evaluation of new antioxidant (inhibition of lipid peroxidation, scavenging of free radical DPPH, and thiol peroxidase-like activity) and antiproliferative chalcogenobiotin derivatives and tests them against bladder carcinoma 5637 cells. A prominent response was obtained for the selected compounds, with tellurium biotin derivatives displaying effective antioxidant and antiproliferative activity. The effective compounds also demonstrated no toxicity in in vitro or in vivo studies.

Ferrocene-containing hybrids as potential anticancer agents: Current developments, mechanisms of action and structure-activity relationships

Cancer is one of the most fatal threatens to human health throughout the world. The major challenges in the control and eradication of cancers are the continuous emergency of drug-resistant cancer and the low specificity of anticancer agents, creating an urgent need to develop novel anticancer agents. Organometallic compounds especially ferrocene derivatives possess remarkable structural and mechanistic diversity, inherent stability towards air, heat and light, low toxicity, low cost, reversible redox, ligand exchange, and catalytic properties, making them promising drug candidates for cancer therapy. Ferrocifen, a ferrocene-phenol hybrid, has demonstrated promising anticancer properties on drug-resistant cancers. Currently, Ferrocifen is in pre-clinical trial against cancers. Obviously, ferrocene moiety is a useful template for the development of novel anticancer agents. This review will provide an overview of ferrocene-containing hybrids with potential application in the treatment of cancers covering articles published between 2010 and 2020. The mechanisms of action, the critical aspects of design and structure-activity relationships are also discussed.

Fast functionalization of ultrasound microbubbles using strain promoted click chemistry

Functionalization of microbubbles (MBs) is a difficult issue due to their unstable nature. Here we report a fast and versatile method using a strain promoted alkyne-azide cycloaddition (SPAAC) click reaction for microbubble functionalization. An azadibenzocyclooctyne (DBCO) group was first introduced onto the MB surface and then an azide group into the desired ligand. Without any initiators or catalysts, essential click ligation occurred within 1 min and a majority of the reaction completed in 5 min at 37 °C. This fast ligation shortens the microbubble reaction time and preserves essential amounts of microbubbles for further in situ imaging and delivery of therapeutics.

Chirality in metal-based anticancer agents

Chiral metal-based drugs are currently an interesting and rapidly growing field in anticancer research. Here the different chiral metal-based anticancer agents and the extent to which the chiral resolution affects their biological properties are discussed. This review will aid the design of new potent and efficient chiral metal-based anticancer drugs that exploit the unique properties combined with their potential selectivity toward targeted chiral biomolecules.

Carbon Dot Based, Naphthalimide Coupled FRET Pair for Highly Selective Ratiometric Detection of Thioredoxin Reductase and Cancer Screening

The fluorescence resonance energy transfer (FRET) mechanism has been established between carbon dots (CDs) and naphthalimide to monitor the activity of thioredoxin reductase (TrxR), which is often overexpressed in many cancer cells. The naphthalimide moiety was covalently attached to the surface of CDs through a disulfide linkage. In normal cell conditions (when devoid of high concentrations of TrxR), the CDs act as an energy donor and naphthalimide acts as an acceptor, which establishes the FRET pair as interpreted from the emission at λ_em = 565 nm, when excited at λ_ex = 360 nm. However, contrary to this, the elevated levels of TrxR cause the breakage of disulfide bonds and consequently abolishes the FRET pair through the release of the naphthalimide moiety from the surface of CDs. This process was studied by monitoring of fluorescence intensity at λ_em = 565 and 440 nm, when excited at the same wavelength (λ_ex = 360 nm). The TrxR based ratiometric quenching and enhancement of fluorescence intensity offers an interesting opportunity to monitor the enzyme activities and has many advantages over conventional monitoring of fluorescence intensity at a single wavelength to avoid interference of external factors. Fluorescence images of cancer cells in response to the nanosensor were visualized under a confocal microscope. Cytotoxicity study of nanosensor retards the growth of HeLa and MCF-7 cell lines in the presence of visible light. Therefore, the nanosensor also acts as a theranostic agent to diagnose as well as killing of cancer cells.

Biotinylated Chlorin and Its Zinc and Indium Complexes: Synthesis and In Vitro Biological Evaluation for Photodynamic Therapy

The synthesis and characterization of biotinylated chlorin photosensitizer and the corresponding zinc and indium complexes are described for potential applications in photodynamic therapy (PDT) for cancer. Phototoxicity of the biotin-chlorin conjugate and the metallated complexes was determined in colon carcinoma CT26 cell lines known to overexpress biotin (Vit B₇) receptors. Cell survival assay indicated that the biotinylated chlorin and indium complex showed increased cell growth inhibition than the zinc complex and the starting chlorin (methyl pheophorbide). Fluorescence microcopy studies revealed the generation of apoptotic cells upon light irradiation of colon cells treated with the indium complex. Targeting biotin receptors in cancer cells can improve specificity of photosensitizers for PDT applications.

Biotin Decorated Gold Nanoparticles for Targeted Delivery of a Smart-Linked Anticancer Active Copper Complex: In Vitro and In Vivo Studies

The synthesis and anticancer activity of a copper(II) diacetyl-bis(N4-methylthiosemicarbazone) complex and its nanoconjugates are reported. The copper(II) complex is connected to a carboxylic acid group through a cleavable disulfide link to enable smart delivery. The copper complex is tethered to highly water-soluble 20 nm gold nanoparticles (AuNPs), stabilized by amine terminated lipoic acid-polyethylene glycol (PEG). The gold nanoparticle carrier was further decorated with biotin to achieve targeted action. The copper complex and the conjugates with and without biotin, were tested against HeLa and HaCaT cells. They show very good anticancer activity against HeLa cells, a cell line derived from cervical cancer and are less active against HaCaT cells. Slow and sustained release of the complex from conjugates is demonstrated through cleavage of disulfide linker in the presence of glutathione (GSH), a reducing agent intrinsically present in high concentrations within cancer cells. Biotin appended conjugates do not show greater activity than conjugates without biotin against HeLa cells. This is consistent with drug uptake studies, which suggests similar uptake profiles for both conjugates in vitro. However, in vivo studies using a HeLa cell xenograft tumor model shows 3.8-fold reduction in tumor volume for the biotin conjugated nanoparticle compared to the control whereas the conjugate without biotin shows only 2.3-fold reduction in the tumor volume suggesting significant targeting.

Small Molecules for Active Targeting in Cancer

For the purpose of this review, active targeting in cancer research encompasses strategies wherein a ligand for a cell surface receptor expressed on tumor cells is used to deliver a cytotoxic or imaging cargo. This area of research is more than two decades old, but in those 20 and more years, how many receptors have been studied extensively? What kinds of the ligands are used for active targeting? Are they mostly naturally occurring molecules such as folic acid, or synthetic substances developed in campaigns for medicinal chemistry efforts? This review outlines the most important receptor or ligand combinations that have been used in active targeting to answer these questions, and therefore to address the most important one of all: is research in active targeting affording diminishing returns, or is this an area for which the potential far exceeds progress made so far?

Regulating the anticancer properties of organometallic dendrimers using pyridylferrocene entities: synthesis, cytotoxicity and DNA binding studies

Eight new heterometallic ferrocenyl-derived metal–arene metallodendrimers have been prepared and evaluated for anti-tumor activity.

Love-Hate ligands for high resolution analysis of strain in ultra-stable protein/small molecule interaction

The pathway of ligand dissociation and how binding sites respond to force are not well understood for any macromolecule. Force effects on biological receptors have been studied through simulation or force spectroscopy, but not by high resolution structural experiments. To investigate this challenge, we took advantage of the extreme stability of the streptavidin-biotin interaction, a paradigm for understanding non-covalent binding as well as a ubiquitous research tool. We synthesized a series of biotin-conjugates having an unchanged strong-binding biotin moiety, along with pincer-like arms designed to clash with the protein surface: 'Love-Hate ligands'. The Love-Hate ligands contained various 2,6-di-ortho aryl groups, installed using Suzuki coupling as the last synthetic step, making the steric repulsion highly modular. We determined binding affinity, as well as solving 1.1-1.6Å resolution crystal structures of streptavidin bound to Love-Hate ligands. Striking distortion of streptavidin's binding contacts was found for these complexes. Hydrogen bonds to biotin's ureido and thiophene rings were preserved for all the ligands, but biotin's valeryl tail was distorted from the classic conformation. Streptavidin's L3/4 loop, normally forming multiple energetically-important hydrogen bonds to biotin, was forced away by clashes with Love-Hate ligands, but Ser45 from L3/4 could adapt to hydrogen-bond to a different part of the ligand. This approach of preparing conflicted ligands represents a direct way to visualize strained biological interactions and test protein plasticity.

Sandwich complex-containing macromolecules: property tunability through versatile synthesis

Sandwich complexes feature unique properties as the physical and electronic properties of a hydrocarbon ligand or its derivative are integrated into the physical, electronic, magnetic, and optical properties of a metal. Incorporation of these complexes into macromolecules results in intriguing physical, electrical, and optical properties that were hitherto unknown in organic-based macromolecules. These properties are tunable through well-designed synthetic strategies. This review surveys many of the synthetic approaches that have resulted in tuning the properties of sandwich complex-containing macromolecules. While the past two decades have seen an ever-growing number of research publications in this field, gaps remain to be filled. Thus, we expect this review to stimulate research interest towards bridging these gaps, which include the insolubility of some of these macromolecules as well as expanding the scope of the sandwich complexes.