Design Strategies and Medicinal Applications of Metal-Peptidic Bioconjugates

A review on the recent advances of interaction studies of anticancer metal-based drugs with therapeutic targets, DNA and RNAs

Metal-based drugs hold promise as potent anticancer agents owing to their unique interactions with cellular targets. This review discusses recent advances in our understanding of the intricate molecular interactions of metal-based anticancer compounds with specific therapeutic targets in cancer cells. Advanced computational and experimental methodologies delineate the binding mechanisms, structural dynamics and functional outcomes of these interactions. In addition, the review sheds light on the precise modes of action of these drugs, their efficacy and the potential avenues for further optimization in cancer-treatment strategies and the development of targeted and effective metal-based therapies for combating various forms of cancer.

Lignocellulosic-biomolecules conjugated systems: green-engineered complexes modified by covalent linkers

Lignocellulosic biomass represents an abundant and eco-friendly material widely explored in recent years. The main lignocellulosic fractions include cellulose, hemicellulose, and lignin. Nonetheless, the heterogeneity and complexity of these components pose challenges in achieving the desired properties. Conversely, their attractive functional groups can covalently link with other biomolecules, facilitating the creation and enhancement of material properties. Lignocellulosic molecules can form different linkages with other biomolecules through classic and modern methods. Bioconjugation has emerged as a suitable alternative to create new nuances, empowering the linkage between lignocellulosic materials and biomolecules through linkers. These conjugates (lignocellulosic-linkers-biomolecules) attract attention from stakeholders in medicine, chemistry, biology, and agriculture. The plural formations of these biocomplexes highlight the significance of these arrangements. Therefore, this review provides an overview of the progress of lignocellulosic-biomolecule complexes and discusses different types of covalent bioconjugated systems, considering the formation of linkers, applicability, toxicity, and future challenges.

Metals in Cancer Research: Beyond Platinum Metallodrugs

The discovery of the medicinal properties of platinum complexes has fueled the design and synthesis of new anticancer metallodrugs endowed with unique modes of action (MoA). Among the various families of experimental antiproliferative agents, organometallics have emerged as ideal platforms to control the compounds' reactivity and stability in a physiological environment. This is advantageous to efficiently deliver novel prodrug activation strategies, as well as to design metallodrugs acting only via noncovalent interactions with their pharmacological targets. Noteworthy, another justification for the advance of organometallic compounds for therapy stems from their ability to catalyze bioorthogonal reactions in cancer cells. When not yet ideal as drug leads, such compounds can be used as selective chemical tools that benefit from the advantages of catalytic amplification to either label the target of interest (e.g., proteins) or boost the output of biochemical signals. Examples of metallodrugs for the so-called "catalysis in cells" are considered in this Outlook together with other organometallic drug candidates. The selected case studies are discussed in the frame of more general challenges in the field of medicinal inorganic chemistry.

A cell-impermeable luminogenic probe for near-infrared imaging of prostate-specific membrane antigen in prostate cancer microenvironments

Prostate-specific membrane antigen (PSMA) imaging probes are a promising tool for the diagnosis and image-guided surgery of prostate cancer (PCa). However, PSMA-specific luminescence probes for PCa detection and heterogeneity studies with high imaging contrast are lacking. Here, we report the first near-infrared (NIR) iridium(III) complex for the wash-free and specific imaging of PSMA in PCa cells and spheroids. The conjugation of a PSMA inhibitor, Lys-urea-Glu, to an iridium(III) complex synergizes the PSMA-specific affinity and biocompatibility of the inhibitor with the desirable photophysical properties of the iridium(III) complex, including NIR emission (670 nm), high photostability and a large Stokes shift. The cellular impermeability of the probe along with its strong binding affinity to PSMA enhances its specificity for PSMA, enabling the washing-free luminescent imaging of membrane PSMA with lower cytotoxicity. The probe was successfully applied for selectively visualizing PSMA-expressing cells and for the imaging of PSMA in a multicellular PCa model with good imaging penetration, indicating its potential use in complicated and heterogeneous tumor microenvironments. Furthermore, the probe showed good imaging performance in the PCa-bearing tumor mice via targeting PSMA in vivo. This work provides a novel strategy for the development of highly sensitive and specific NIR probes for PSMA in biological systems in vitro, which is of great significance for the precise diagnosis of PCa and for elucidating PCa heterogeneity.

On-Resin Conjugation of the Ruthenium Anticancer Agent Plecstatin-1 to Peptide Vectors

Ruthenium piano-stool complexes have been explored for their anticancer activity and some promising compounds have been reported. Herein, we conjugated a derivative of plecstatin-1 to peptides in order to increase their cancer cell targeting ability. For this purpose, plecstatin-1 was modified at the arene ligand to introduce a functional amine handle (3), which resulted in a compound that showed similar activity in an in vitro anticancer activity assay. The cell-penetrating peptide TAT48-60, tumor-targeting neurotensin8-13, and plectin-targeting peptide were functionalized with succinyl or β-Ala-succinyl linkers under standard solid-phase peptide synthesis (SPPS) conditions to spatially separate the peptide backbones from the bioactive metal complexes. These modifications allowed for conjugating precursor 3 to the peptides on resin yielding the desired metal-peptide conjugates (MPCs), as confirmed by high-performance liquid chromatography (HPLC), NMR spectroscopy, and mass spectrometry (MS). The MPCs were studied for their behavior in aqueous solution and under acidic conditions and resembled that of the parent compound plecstatin-1. In in vitro anticancer activity studies in a small panel of cancer cell lines, the TAT-based MPCs showed the highest activity, while the other MPCs were virtually inactive. However, the MPCs were significantly less active than the small molecules plecstatin-1 and 3, which can be explained by the reduced cell uptake as determined by inductively coupled plasma MS (ICP-MS). Although the MPCs did not display potent anticancer activities, the developed conjugation strategy can be extended toward other metal complexes, which may be able to utilize the targeting properties of peptides.

Development and Recent Advances in Lysine and N-Terminal Bioconjugation for Peptides and Proteins

The demand for creation of protein diversity and regulation of protein function through native protein modification and post-translational modification has ignited the development of selective chemical modification methods for peptides and proteins. Chemical bioconjugation offers selective functionalization providing bioconjugates with desired properties and functions for diverse applications in chemical biology, medicine, and biomaterials. The amino group existing at the lysine residue and N-terminus of peptides and proteins has been extensively studied in bioconjugation because of its good nucleophilicity and high surface exposure. Herein, we review the development of chemical methods for modification of the amino groups on lysine residue and N-terminus featuring excellent selectivity, mild reaction conditions, short reaction time, high conversion, biocompatibility, and preservation of protein integrity. This review is organized based on the chemoselectivity and site-selectivity of the chemical bioconjugation reagents to the amino acid residues aiming to provide guidance for the selection of appropriate bioconjugation methods.

Encapsulation of Gold-Based Anticancer Agents in Protease-Degradable Peptide Nanofilaments Enhances Their Potency

We investigated the use of amphiphilic, protease-cleavable peptides as encapsulation moieties for hydrophobic metallodrugs, in order to enhance their bioavailability and consequent activity. Two hydrophobic, gold-containing anticancer agents varying in aromatic ligand distribution (Au(I)-N-heterocyclic carbene compounds 1 and 2) were investigated. These were encapsulated into amphiphilic decapeptides that form soluble filamentous structures with hydrophobic cores, varying supramolecular packing arrangements and surface charge. Peptide sequence strongly dictates the supramolecular packing within the aromatic core, which in turn dictates drug loading. Anionic peptide filaments can effectively load 1, and to a lesser extent 2, while their cationic counterparts could not, collectively demonstrating that loading efficiency is dictated by both aromatic and electrostatic (mis)matching between drug and peptide. Peptide nanofilaments were nontoxic to cancerous and noncancerous cells. By contrast, those loaded with 1 and 2 displayed enhanced cytotoxicity in comparison to 1 and 2 alone, when exposed to Caki-1 and MDA-MB-231 cancerous cell lines, while no cytotoxicity was observed in noncancerous lung fibroblasts, IMR-90. We propose that the enhanced in vitro activity results from the enhanced proteolytic activity in the vicinity of the cancer cells, thereby breaking the filaments into drug-bound peptide fragments that are taken up by these cells, resulting in enhanced cytotoxicity toward cancer cells.

Dual Modification of Artificial Protein Cage

Chemical modifications of proteins confer new functions on them or modulate their original functions. Although various approaches are developed for modifications, modifications of the two different reactive sites of proteins by different chemicals are still challenging. In this chapter, we show a simple approach for selective modifications of both interior and exterior surfaces of protein nanocages by two different chemicals based on a molecular size filter effect of the surface pores.

Photofunctional cyclometallated iridium(III) polypyridine methylsulfone complexes as sulfhydryl-specific reagents for bioconjugation, bioimaging and photocytotoxic applications

We report herein near-infrared (NIR)-emitting cyclometallated iridium(III) complexes bearing a heteroaromatic methylsulfone moiety as sulfhydryl-specific reagents; one of the complexes was conjugated to cysteine and cysteine-containing peptides and proteins for bioimaging and photocytotoxic applications.

Veni, Vidi, Vici: Immobilized Peptide-Based Conjugates as Tools for Capture, Analysis, and Transformation

Analysis of peptide biomarkers of pathological states of the organism is often a serious challenge, due to a very complex composition of the cell and insufficient sensitivity of the current analytical methods (including mass spectrometry). One of the possible ways to overcome this problem is sample enrichment by capturing the selected components using a specific solid support. Another option is increasing the detectability of the desired compound by its selective tagging. Appropriately modified and immobilized peptides can be used for these purposes. In addition, they find application in studying the specificity and activity of proteolytic enzymes. Immobilized heterocyclic peptide conjugates may serve as metal ligands, to form complexes used as catalysts or analytical markers. In this review, we describe various applications of immobilized peptides, including selective capturing of cysteine-containing peptides, tagging of the carbonyl compounds to increase the sensitivity of their detection, enrichment of biological samples in deoxyfructosylated peptides, and fishing out of tyrosine–containing peptides by the formation of azo bond. Moreover, the use of the one-bead-one-compound peptide library for the analysis of substrate specificity and activity of caspases is described. Furthermore, the evolution of immobilization from the solid support used in peptide synthesis to nanocarriers is presented. Taken together, the examples presented here demonstrate immobilized peptides as a multifunctional tool, which can be successfully used to solve multiple analytical problems.

A solid support-based synthetic strategy for the site-selective functionalization of peptides with organometallic half-sandwich moieties

The number of donor atoms available on peptides that can competitively coordinate to metal centers renders the site-selective generation of advanced metal-peptide conjugates in high purity a challenging venture. Herein, we present a transmetalation-based synthetic approach on solid support in which an imidazolium proligand can be used to selectively anchor a range of transition metal half-sandwich complexes onto peptides in the presence of multiple coordinative motifs. Amenable to solid support, a range of N-terminus and/or lysine conjugated metal-peptide conjugates were obtained in high purity after cleavage from the resin. The metalated peptides were evaluated for their anticancer properties against human cancer cell lines. While no cytotoxic activity was observed, this platform has the potential to i) provide a pathway to site-selective peptide labelling, ii) be explored as a biorthogonal handle and/or iii) generate a new strategy for ligand design in transition metal catalysts.

Luminescent rhenium(I) perfluorobiphenyl complexes as site-specific labels for peptides to afford photofunctional bioconjugates

We report herein new luminescent rhenium(I) perfluorobiphenyl complexes that reacted specifically with the cysteine residue of the π-clamp sequence (FCPF) to afford novel peptide-based imaging reagents, photosensitisers for singlet oxygen and enzyme sensors.

Solid-phase synthesis of peptide Mn(i)-carbonyl bioconjugates and their CO release upon visible light activation

A one-pot synthetic route has been developed for the assembly of peptide Mn(i)-carbonyl bioconjugates. It allows the installation of a variety of chelating agents at the late stage, and after just one purification step the TAT-MnCO complexes can be obtained. The resulting bioconjugates showed different and tunable CO releasing kinetics upon visible light activation.

Synthesis and Structural Stability of α-Helical Gold(I)-Metallopeptidesy

The synthesis of hexa- and dodecapeptides functionalized with two Au(I)–phosphine complexes is reported. The high stability of the Au(I)–phosphine bond allowed orthogonal peptide-protecting-group chemistry, even when using hard Lewis acids like boron tribromide. This enabled the preparation of an Fmoc-protected lysine derivative carrying the Au(I) complex in a side chain, which was used in standard Fmoc-based solid-phase peptide synthesis protocols. Alanine and leucine repeats in the metallododecapeptide formed α-helical secondary structures in 2,2,2-trifluoroethanol–H2O and 1,1,1,3,3,3-hexafluoroisopropanol–H2O mixtures with high thermal stability, as shown by temperature-dependent CD spectroscopy studies.

Dynamic Stereoselection of Peptide Helicates and Their Selective Labeling of DNA Replication Foci in Cells*

Although largely overlooked in peptide engineering, coordination chemistry offers a new set of interactions that opens unexplored design opportunities for developing complex molecular structures. In this context, we report new artificial peptide ligands that fold into chiral helicates in the presence of labile metal ions such as Fe^II and Co^II . Heterochiral β-turn-promoting sequences encode the stereoselective folding of the peptide ligands and define the physicochemical properties of their corresponding metal complexes. Circular dichroism and NMR spectroscopy in combination with computational methods allowed us to identify and determine the structure of two isochiral ΛΛ-helicates, folded as topological isomers. Finally, in addition to the in-vitro characterization of their selective binding to DNA three-way junctions, cell-microscopy experiments demonstrated that a rhodamine-labeled Fe^II helicate was internalized and selectively stains DNA replication factories in functional cells.

Bioconjugate Supramolecular Pd2+ Metallacages Penetrate the Blood Brain Barrier In Vitro and In Vivo

The biomedical application of discrete supramolecular metal-based structures, specifically self-assembled metallacages, is still an emergent field of study. Capitalizing on the knowledge gained in recent years on the development of 3-dimensional (3D) metallacages as novel drug delivery systems and theranostic agents, we explore here the possibility to target [Pd₂L₄]⁴⁺ cages (L = 3,5-bis(3-ethynylpyridine)phenyl ligand) to the brain. In detail, a new water-soluble homoleptic cage (C^PepH3) tethered to a blood brain barrier (BBB)-translocating peptide was synthesized by a combination of solid-phase peptide synthesis (SPPS) and self-assembly procedures. The cage translocation efficacy was assessed by inductively coupled mass spectrometry (ICP-MS) in a BBB cellular model in vitro. Biodistribution studies of the radiolabeled cage [[^99mTcO₄]^- ⊂ C^PepH3] in the CD1 mice model demonstrate its brain penetration properties in vivo. Further DFT studies were conducted to model the structure of the [[^99mTcO₄]^- ⊂ cage] complex. Moreover, the encapsulation capabilities and stability of the cage were investigated using the [ReO₄]^- anion, the "cold" analogue of [^99mTcO₄]^-, by ¹H NMR spectroscopy. Overall, our study constitutes another proof-of-concept of the unique potential of supramolecular coordination complexes for modifying the physiochemical and biodistribution properties of diagnostic species.

Metallocene QACs: The Incorporation of Ferrocene Moieties into monoQAC and bisQAC Structures

Inspired by the incorporation of metallocene functionalities into a variety of bioactive structures, particularly antimicrobial peptides, we endeavored to broaden the structural variety of quaternary ammonium compounds (QACs) by the incorporation of the ferrocene moiety. Accordingly, 23 ferrocene-containing mono- and bisQACs were prepared in high yields and tested for activity against a variety of bacteria, including Gram-negative strains and a panel of clinically isolated MRSA strains. Ferrocene QACs were shown to be effective antiseptics with some displaying single-digit micromolar activity against all bacteria tested, demonstrating yet another step in the expansion of structural variety of antiseptic QACs.

An 111In-labelled bis-ruthenium(ii) dipyridophenazine theranostic complex: mismatch DNA binding and selective radiotoxicity towards MMR-deficient cancer cells

Theranostic radionuclides that emit Auger electrons (AE) can generate highly localised DNA damage and the accompanying gamma ray emission can be used for single-photon emission computed tomography (SPECT) imaging. Mismatched DNA base pairs (mismatches) are DNA lesions that are abundant in cells deficient in MMR (mismatch mediated repair) proteins. This form of genetic instability is prevalent in the MMR-deficient subset of colorectal cancers and is a potential target for AE radiotherapeutics. Herein we report the synthesis of a mismatch DNA binding bis-ruthenium(ii) dipyridophenazine (dppz) complex that can be radiolabelled with the Auger electron emitting radionuclide indium-111 (111In). Greater stabilisation accompanied by enhanced MLCT (metal to ligand charge-transfer) luminescence of both the bis-Ru(dppz) chelator and non-radioactive indium-loaded complex was observed in the presence of a TT mismatch-containing duplex compared to matched DNA. The radioactive construct [111In]In-bisRu(dppz) ([111In][In-2]4+) targets cell nuclei and is radiotoxic towards MMR-deficient human colorectal cancer cells showing substantially less detrimental effects in a paired cell line with restored MMR function. Additional cell line studies revealed that [111In][In-2]4+ is preferentially radiotoxic towards MMR-deficient colorectal cancer cells accompanied by increased DNA damage due to 111In decay. The biodistribution of [111In][In-2]4+ in live mice was demonstrated using SPECT. These results illustrate how a Ru(ii) polypyridyl complex can incorporate mismatch DNA binding and radiometal chelation in a single molecule, generating a DNA-targeting AE radiopharmaceutical that displays selective radiotoxicity towards MMR-deficient cancer cells and is compatible with whole organism SPECT imaging.

N-Heterocyclic Carbenes: A Door Open to Supramolecular Organometallic Chemistry

The field of metallosupramolecular chemistry is clearly dominated by the use of O-, N-, and P-donor Werner-type polydentate ligands. These molecular architectures are of high interest because of their wide range of applications, which include molecular encapsulation, stabilization of reactive species, supramolecular catalysis, and drug delivery, among others. Only recently, organometallic ligands have allowed the preparation of a variety of supramolecular coordination complexes, and the term supramolecular organometallic complexes (SOCs) is gaining space within the field of metallosupramolecular chemistry. While the early examples of SOCs referred to supramolecular architectures mostly containing bisalkenyl, diphenyl, or bisalkynyl linkers, the development of SOCs during the past decade has been boosted by the parallel development of multidentate N-heterocyclic carbene (NHC) ligands. The first examples of NHC-based SOCs referred to supramolecular assemblies based on polydentate NHC ligands bound to group 11 metals. However, during the last 10 years, several planar poly-NHC ligands containing extended π-conjugated systems have facilitated the formation of a large variety of architectures in which the supramolecular assemblies can contain metals other than Cu, Ag, and Au. Such ligands are Janus di-NHCs and trigonal-planar tris-NHCs-most of them prepared by our research group-which have allowed the preparation of a vast range of NHC-based metallosupramolecular compounds with interesting host-guest chemistry properties. Although the number of SOCs has increased in the past few years, their use for host-guest chemistry purposes is still in its earliest infancy. In this Account, we describe the achievements that we have made during the last 4 years toward broadening the applications of planar extended π-conjugated NHC ligands for the preparation of organometallic-based supramolecular structures, including their use as hosts for some selected organic and inorganic guests, together with the catalytic properties displayed by some selected host-guest inclusion complexes. Our contribution describes the design of several Ni-, Pd-, and Au-based metallorectangles and metalloprisms, which we used for the encapsulation of several organic substrates, such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes. The large binding affinities found are ascribed to the incorporation of two cofacial panels with large π-conjugated systems, which provide the optimum conditions for guest recognition by π-π-stacking interactions. We also describe a series of digold(I) metallotweezers for the recognition of organic and inorganic substrates. These metallotweezers were used for the recognition of "naked" metal cations and polycyclic aromatic hydrocarbons. The recognition properties of these metallotweezers are highly dependent on the nature of the rigid connector and of the ancillary ligands that constitute the arms of the tweezer. A peculiar balance between the self-aggregation properties of the tweezer and its ability to encapsulate organic guests is observed.

Luminescent Re(I)/Au(I) Species As Selective Anticancer Agents for HeLa Cells

A series of neutral and cationic heterotrimetallic complexes of the type fac-[Re(CO)₃(bipy(CC)₂-(AuL)₂)X]ⁿ, where bipy(CC)₂ is 4,4'-alkynyl-2,2'-bipyridine; L is either triphenylphosphine (PPh₃), [1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene] (IPr), or tert-butyl isocyanide (CN^tBu); and X is a chloride (n = 0) or acetonitrile (n = 1), were synthesized and characterized together with their Re(I) precursors, i.e., fac-[Re(CO)₃(bipy(CC)₂)X]ⁿ. X-ray diffraction of complexes 1, 3, and 6 corroborated the expected octahedral and linear distribution of the ligands along the Re(I) and Au(I) centers, respectively. Luminescent studies showed that all the complexes displayed a broad emission band centered between 565 and 680 nm, corresponding to a ³MLCT from the Re(I) to the diimine derivative. The presence of the gold fragment coordinated to the diimine ligand shifted in all cases the emission maxima toward higher energies. Such an emission difference could be potentially used for assessing the precise moment of interaction of the probe with the biological target if the gold fragment is implicated. Antiproliferative studies in cancer cells, A549 (lung cancer) and HeLa (cervix cancer), showed a generalized selectivity toward HeLa cells for those heterotrimetallic species incubated at longer times (72 vs 24 h). ICP-MS spectrometry revealed the greater cell internalization of cationic vs neutral species. Preliminary fluorescence microscopy experiments showed a different behavior of the complexes in HeLa and A549 cell lines. Whereas the complexes in A549 were randomly distributed in the outside of the cell, those incubated with HeLa cells were located close to the cellular membrane, suggesting some type of interaction, and possibly explaining their cellular selectivity when it comes to the antiproliferative activity displayed in the different cell lines.

Trivalent metal complex geometry of the substrate governs cathepsin B enzymatic cleavage rate

The identity of the trivalent metal ion controls the rate of the enzymatic cleavage of a series of metal-complexed cathepsin B substrates. Increasing the distance between the metal complex and the enzyme cleavage site diminishes this effect.