Comparing organic and metallo-organic hydrazone molecular cages as potential carriers for doxorubicin delivery

Molecular cages are three-dimensional supramolecular structures that completely wrap guest molecules by encapsulation. We describe a rare comparative study between a metallo-organic cage and a fully organic analogous system, obtained by hydrazone bond formation self-assembly. Both cages are able to encapsulate the anticancer drug doxorubicin, with the organic cage forming a 1 : 1 inclusion complex with μM affinity, whereas the metallo-organic host experiences disassembly by interaction with the drug. Stability experiments reveal that the ligands of the metallo-organic cage are displaced in buffer at neutral, acidic, and basic pH, while the organic cage only disassembles under acidic conditions. Notably, the organic cage also shows minimal cell toxicity, even at high doses, whilst the doxorubicin-cage complex shows in vitro anti-cancer activity. Collectively, these results show that the attributes of the pure organic molecular cage are suitable for the future challenges of in vivo drug delivery using molecular cages.

A dynamic covalent approach to [PtnL2n]2n+ cages

A dynamic covalent approach was exploited to generate a family of homometallic [PtnL2n]2n+ cage (predominantly [Pt2L4]4+ systems) architectures. The family of platinum(II) architectures were characterized using 1H nuclear magnetic resonance (NMR) and diffusion ordered spectroscopy (DOSY), electrospray ionization mass spectrometry (ESI-MS) and the molecular structures of two cages were determined by X-ray crystallography.

Water-Soluble Molecular Cages for Biological Applications

The field of molecular cages has attracted increasing interest in relation to the development of biological applications, as evidenced by the remarkable examples published in recent years. Two key factors have contributed to this achievement: First, the remarkable and adjustable host-guest chemical properties of molecular cages make them highly suitable for biological applications. This allows encapsulating therapeutic molecules to improve their properties. Second, significant advances have been made in synthetic methods to create water-soluble molecular cages. Achieving the necessary water solubility is a significant challenge, which in most cases requires specific chemical groups to overcome the inherent hydrophobic nature of the molecular cages which feature the organic components of the cage. This can be achieved by either incorporating water-solubilizing groups with negative/positive charges, polyethylene glycol chains, etc.; or by introducing charges directly into the cage structure itself. These synthetic strategies allow preparing water-soluble molecular cages for diverse biological applications, including cages' anticancer activity, anticancer drug delivery, photodynamic therapy, and molecular recognition of biological molecules. In the review we describe selected examples that show the main concepts to achieve water solubility in molecular cages and some selected recent biological applications.

Pyrenebutyrate Pt(IV) Complexes with Nanomolar Anticancer Activity

Research on platinum-based anticancer drugs continuously strives to develop new non-classical platinum complexes. Pt(IV) prodrugs are the most promising, and their activation-by-reduction mechanism of action is being explored as a prospect for higher selectivity and efficiency. Herein, we present the anticancer potency and chemical reactivity of Pt(IV) complexes formed by linking pyrene butyric acid with cisplatin. The results from cytotoxicity screening on 10 types of cancer cell lines and non-malignant cells (HEK-293) indicated IC50 values as low as 50-70 nM for the monosubstituted Pt(IV) complex against leukemia cell lines (HL-60 and SKW3) and a cisplatin-resistant derivative (HL-60/CDDP). Interestingly, the bis-substituted complex is virtually non-toxic to both healthy and cancerous cells of adherent types. Nevertheless, it shows high cytotoxicity against multidrug-resistant derivatives HL-60/CDDP and HL-60/Dox. The reactivity of the complexes with biological reductants was monitored by the NMR method. Furthermore, the platinum uptake by the treated cells was examined on two types of cellular cultures: adherent and suspension growing, and proteome profiling was conducted to track expression changes of key apoptosis-related proteins in HL-60 cells. The general conclusion points to a possible cytoskeletal entrapment of the bulkier bis-pyrene complex that could be limiting its cytotoxicity to adherent cells, both cancerous and healthy ones.

In vivo biodistribution of kinetically stable Pt2L4 nanospheres that show anti-cancer activity

There is an increasing interest in the application of metal-organic cages (MOCs) in a biomedicinal context, as they can offer non-classical distribution in organisms compared to molecular substrates, while revealing novel cytotoxicity mechanisms. Unfortunately, many MOCs are not sufficiently stable under in vivo conditions, making it difficult to study their structure-activity relationships in living cells. As such, it is currently unclear whether MOC cytotoxicity stems from supramolecular features or their decomposition products. Herein, we describe the toxicity and photophysical properties of highly-stable rhodamine functionalized platinum-based Pt₂L₄ nanospheres as well as their building blocks under in vitro and in vivo conditions. We show that in both zebrafish and human cancer cell lines, the Pt₂L₄ nanospheres demonstrate reduced cytotoxicity and altered biodistribution within the body of zebrafish embryos compared to the building blocks. We anticipate that the composition-dependent biodistribution of Pt₂L₄ spheres together with their cytotoxic and photophysical properties provides the fundament for MOC application in cancer therapy.

Effector Regulated Catalytic Cyclization of Alkynoic Acids Using Pt2 L4 Cages

Metabolic pathways are highly regulated by effector molecules that influences the rate of enzymatic reactions. Inspired by the catalytic regulation found in living cells, we report a Pt₂ L₄ cage of which the activity can be controlled by effectors that bind inside the cage. The cage shows catalytic activity in the lactonization of alkynoic acids, with the reaction rates dependent on the effector guest bound in the cage. Some effector guests enhance the rate of the lactonization by up to 19-fold, whereas one decreases it by 5-fold. When mixtures of specific substrates are used, both starting materials and products act as guests for the Pt₂ L₄ cage, enhancing its catalytic activity for one substrate while reducing its activity for the other. The reported regulatory behavior obtained by the addition of effector molecules paves the way to the development of more complex, metabolic-like catalyst systems.

Hydrazone- and imine-containing [PdPtL4]4+ cages: a comparative study of the stability and host-guest chemistry

A new [PdPtL₄]⁴⁺ heterobimetallic cage containing hydrazone linkages has been synthesised using the sub-component self-assembly approach. ¹H and DOSY nuclear magnetic resonance (NMR) spectroscopy and electrospray ionisation mass spectrometry (ESIMS) data were consistent with the formation of the [PdPtL₄]⁴⁺ architecture. The cage was stimulus-responsive and could be partially disassembled and reassembled by the addition of dimethylaminopyridine (DMAP) and p-tolenesulfonic acid (TsOH), respectively. Additionally, the stability of the hydrazone cage against hydrolysis in the presence of water and nucleophilic decomposition in the presence of guest molecules was compared to a previously synthesised imine-containing [PdPtL₄]⁴⁺ cage. It was established that the hydrazone linkage was more resistant to hydrolysis. Furthermore, the host-guest (HG) chemistry with a series of drug and drug-like molecules was examined. The hydrazone cage was shown to interact with cisplatin while the smaller imine cage was shown to interact with 5-fluorouracil and oxaliplatin in CD₃CN. No HG interactions were observed in the more polar d₆-DMSO. In vitro antiproliferative activity studies demonstrated both cages were active against the cancer cell lines tested and displayed half-maximal inhibitory (IC₅₀) values in the range of 25-35 μM. Most [PdPtL₄]⁴⁺-drug mixtures tested had higher IC₅₀ values than the hosts. However, the [PdPtL₄]⁴⁺ cages, and [PdPtL₄]⁴⁺:drug mixtures were less cytotoxic than the well established anticancer drugs cisplatin, oxaliplatin and 5-fluorouracil.

Highly-fluorescent BODIPY-functionalised metallacages as drug delivery systems: synthesis, characterisation and cellular accumulation studies

With the aim of designing new metallosupramolecular architectures for drug delivery, research has focused on porous 3-dimensional (3D)-metallacages able to encapsulate cytotoxic agents protecting them from metabolism while targeting them to cancer sites. Here, two self-assembled [Pd₂L₄]⁴⁺ cages (CG1 and CG2) featuring 3,5-bis(3-ethynylpyridine)phenyl ligands (L) exo-functionalised with dipyrromethene (BODIPY) groups have been synthesised and characterised by different methods, including NMR spectroscopy and mass spectrometry. ¹H NMR spectroscopy studies shows that the cages are able to encapsulate the anticancer drug cisplatin in their hydrophobic cavity, as evidenced by electrostatic potential (ESP) analysis based on XRD studies. The stability of the cages in an aqueous environment, and in the presence of the intracellular reducing agent glutathione, has been confirmed by UV-visible absorption spectroscopy. The luminescence properties of the cages enabled the investigation of their cellular uptake and intracellular localisation in human cancer cells by confocal laser scanning microscopy. In melanoma A375 cells, cage CG1 is taken up via active transport and endocytic trafficking studies show little evidence of transport through the early endosome while the cages accumulated in melanosomes rather than lysosomes. The antiproliferative activity of the lead cage was investigated in A375 together with two breast cancer cell lines, SK-BR-3 and MCF7. While the cage per se is non-cytotoxic, very different antiproliferative effects with respect to free cisplatin were evidenced for the [(cisplatin)₂⊂CG1·BF4] complex in the various cell lines, which correlate with its different intracellular localisation profiles. The obtained preliminary results provide a new hypothesis on how the subcellular localisation of the cage affects the cisplatin intracellular release.

High-Throughput Computational Evaluation of Low Symmetry Pd2 L4 Cages to Aid in System Design*

Unsymmetrical ditopic ligands can self-assemble into reduced-symmetry Pd2 L4 metallo-cages with anisotropic cavities, with implications for high specificity and affinity guest-binding. Mixtures of cage isomers can form, however, resulting in undesirable system heterogeneity. It is paramount to be able to design components that preferentially form a single isomer. Previous data suggested that computational methods could predict with reasonable accuracy whether unsymmetrical ligands would preferentially self-assemble into single cage isomers under constraints of geometrical mismatch. We successfully apply a collaborative computational and experimental workflow to mitigate costly trial-and-error synthetic approaches. Our rapid computational workflow constructs unsymmetrical ligands and their Pd2 L4 cage isomers, ranking the likelihood for exclusively forming cis-Pd2 L4 assemblies. From this narrowed search space, we successfully synthesised four new, low-symmetry, cis-Pd2 L4 cages.

Cavity-Containing [Fe2L3]4+ Helicates: An Examination of Host-Guest Chemistry and Cytotoxicity

Two new di(2,2′-bipyridine) ligands, 2,6-bis([2,2′-bipyridin]-5-ylethynyl)pyridine (L1) and bis(4-([2,2′-bipyridin]-5-ylethynyl)phenyl)methane (L2) were synthesized and used to generate two metallosupramolecular [Fe₂(L)₃](BF₄)₄ cylinders. The ligands and cylinders were characterized using elemental analysis, electrospray ionization mass spectrometry, UV-vis, ¹H-, ¹³C and DOSY nuclear magnetic resonance (NMR) spectroscopies. The molecular structures of the [Fe₂(L)₃](BF₄)₄ cylinders were confirmed using X-ray crystallography. Both the [Fe₂(L1)₃](BF₄)₄ and [Fe₂(L2)₃](BF₄)₄ complexes crystallized as racemic (rac) mixtures of the ΔΔ (P) and ΛΛ (M) helicates. However, ¹H NMR spectra showed that in solution the larger [Fe₂(L2)₃](BF₄)₄ was a mixture of the rac-ΔΔ/ΛΛ and meso-ΔΛ isomers. The host-guest chemistry of the helicates, which both feature a central cavity, was examined with several small drug molecules. However, none of the potential guests were found to bind within the helicates. In vitro cytotoxicity assays demonstrated that both helicates were active against four cancer cell lines. The smaller [Fe₂(L1)₃](BF₄)₄ system displayed low μM activity against the HCT116 (IC₅₀ = 7.1 ± 0.5 μM) and NCI-H460 (IC₅₀ = 4.9 ± 0.4 μM) cancer cells. While the antiproliferative effects against all the cell lines examined were less than the well-known anticancer drug cisplatin, their modes of action would be expected to be very different.

Enhanced cellular uptake of platinum by a tetracationic Pt(II) nanocapsule and its implications to cancer treatment

Despite the known limitations of cisplatin chemotherapy, the treatment of cancer by platinum-based drugs remains the method of choice for many oncologists. The advancement in drug delivery formulations and protocols of combined treatments provided effective tools to ameliorate the side effects of platinum-based therapies. Another approach to improve the pharmacological profiles of anticancer platinum drugs is to properly modify their structure and composition, which has produced numerous platinum complexes with improved therapeutic effect. Recently, we have demonstrated the strong anticancer potency of supramolecular nanocapsules that form by self-assembly of four bis-anthracene ligands with two metal ions, either Pt(II) or Pd(II). Herein, we focus our study on the Pt(II) nanocapsule and its uptake by two types of cancer cells, suspension cultures of HL-60 cells and the adherent cancer cells HT-29. Comparison of the platinum uptake by cancer cells treated with the nanocapsule and with cisplatin evidenced superior uptake of platinum caused by the nanocapsule, which in HT-29 and HL-60 cells prevails by 21 and 31 times, respectively. Morphological changes in the HL-60 cells induced by the Pt(II) nanocapsule were studied by transmission electron microscopy (TEM) which provided plausible explanation of the uptake results. These data corroborate also with the known nanocapsule's very high cytotoxicity, better selectivity, and lack of cross-resistance with cisplatin. Additionally, our estimations of the drug-drug interactions in combined treatments established the propensity of the nanocapsule to exert supra-additive cytotoxicity in combination with cisplatin against the bladder cancer T-24 cells. All these findings define the scope for more detailed pharmacological characterization of the presented Pt(II) nanocapsule.

One-/Two-Photon Excited Cell Membrane Imaging and Tracking by a Photoactive Nanocage

Cell membrane imaging by predesigned molecular and supramolecular photoluminescence probes is of great importance in understanding the nano-biointeractions for potential applications in cellular tracking, drug delivery, cancer diagnosis, and treatment. Herein, we report an effective strategy for cell membrane imaging in both living cell and tissue levels on the basis of a multifunctional nanocage (MOC-16) integrating one-/two-photon active phosphorescence, high charges, balanced hydrophobicity/lipophilicity, and proton sensitivity attributes. The intrinsic optical characters, including strong one-/two-photon excitation and pH-dependent red emission, make MOC-16 powerful optical probes for membrane imaging in living cell and tissue levels under both visible and near-infrared irradiations. Meanwhile, the highly positive charges of +28 endow MOC-16 with adequate water solubility and deprotonation ability while still maintaining its hydrophobicity, thus enabling balanced hydrophobic-lipophilic interactions at the nano-biointerface to facilitate a pH-dependent membrane absorption within the biological pH range of 5.3-7.4. However, the low-charged RuL₃ metalloligand or polyethylene glycol (PEG)-modified MOC-16^PEG with less hydrophobicity cannot offer enough nano-biointeractions for cell membrane tracking. These findings advance the fundamental understanding of nano-biointerface interactions of MOCs with cell membranes and provide further guidance in their biological applications.

A tetraphenylethene-based Pd2L4 metallacage with aggregation-induced emission and stimuli-responsive behavior

A tetraphenylethene-based Pd2L4 metallacage was self-assembled from four TPE-pyridine ligands with two Pd2+ ions. This metallacage with D4 symmetry exhibited a classical aggregation-induced emission property in different solvents and reversible stimuli-responsive behaviour with chloride ions and silver ions, successively.

Recent developments in the construction and applications of platinum-based metallacycles and metallacages via coordination

Coordination-driven suprastructures have attracted much interest due to their unique properties. Among these structures, platinum-based architectures have been broadly studied due to their facile preparation. The resultant two- or three-dimensional (2D or 3D) systems have many advantages over their precursors, such as improved emission tuning, sensitivity as sensors, and capture and release of guests, and they have been applied in biomedical diagnosis as well as in catalysis. Herein, we review the recent results related to platinum-based coordination-driven self-assembly (CDSA), and the text is organized to emphasizes both the synthesis of new metallacycles and metallacages and their various applications.

Biomedical Applications of Metal Organic Polygons and Polyhedra (MOPs)

Since the discovery and structural characterization of metal organic polygons and polyhedra (MOPs), scientists have explored their potential in various applications like catalysis, separation, storage, and sensing. In recent years, scientists have explored the potential of supramolecular MOPs in biomedical application. Pioneering works by Ehrlich, Rosenberg, Lippard, Stang and others have demonstrated that MOPs have great potential as a novel class of metallo-therapeutics that can deliver cargoes (drugs and dyes) selectively. In this article, we document the progress made over the past two decades on the biomedical applications of MOPs and discuss the future prospects of this emerging field.

Quadruply Stranded Metallo-Supramolecular Helicate [Pd2(hextrz)4]4+ Acts as a Molecular Mimic of Cytolytic Peptides

[Pd₂(hextrz)₄]⁴⁺ is a quadruply stranded helicate, a novel bioinorganic complex designed to mimic the structure and function of proteins due to its high stability and supramolecular size. We have previously reported that [Pd₂(hextrz)₄]⁴⁺ exhibited cytotoxicity toward a range of cell lines, with IC₅₀ values ranging from 3 to 10 μM. Here we demonstrate that [Pd₂(hextrz)₄]⁴⁺ kills cells by forming pores within the cell membrane, a mechanism of cell death analogous to the naturally occurring cytolytic peptides. [Pd₂(hextrz)₄]⁴⁺ induced cell death is characterized by an initial influx of Ca²⁺, followed by nuclear condensation and mitochondrial swelling. This is accompanied by progressive cell membrane damage that results in the formation of large blebs at the cell surface. This allows the efflux of molecules from the cell leading to loss of cell viability. These data suggest that it may be possible to design metallo-supramolecular complexes to mimic the cytotoxic action of pore forming proteins and peptides and so provide a new class of drug to treat cancer, autoimmune disorders, and microbial infection.

Cavity-based applications of metallo-supramolecular coordination cages (MSCCs)

This review describes cavity-based applications of cage-like SCCs such as molecular recognition and separation, stabilization of reactive species by encapsulation, as drug delivery systems and as molecular flasks.

Conformational control of Pd2L4 assemblies with unsymmetrical ligands

With increasing interest in the potential utility of metallo-supramolecular architectures for applications as diverse as catalysis and drug delivery, the ability to develop more complex assemblies is keenly sought after. Despite this, symmetrical ligands have been utilised almost exclusively to simplify the self-assembly process as without a significant driving foa mixture of isomeric products will be obtained. Although a small number of unsymmetrical ligands have been shown to serendipitously form well-defined metallo-supramolecular assemblies, a more systematic study could provide generally applicable information to assist in the design of lower symmetry architectures. Pd2L4 cages are a popular class of metallo-supramolecular assembly; research seeking to introduce added complexity into their structure to further their functionality has resulted in a handful of examples of heteroleptic structures, whilst the use of unsymmetrical ligands remains underexplored. Herein we show that it is possible to design unsymmetrical ligands in which either steric or geometric constraints, or both, can be incorporated into ligand frameworks to ensure exclusive formation of single isomers of three-dimensional Pd2L4 metallo-supramolecular assemblies with high fidelity. In this manner it is possible to access Pd2L4 cage architectures of reduced symmetry, a concept that could allow for the controlled spatial segregation of different functionalities within these systems. The introduction of steric directing groups was also seen to have a profound effect on the cage structures, suggesting that simple ligand modifications could be used to engineer structural properties.

Bent Anthracene Dimers as Versatile Building Blocks for Supramolecular Capsules

This Account provides a comprehensive summary of our 1-decade-long investigations into bent anthracene dimers as versatile building blocks for supramolecular capsules. The investigations initiated in 2008 with the design of an anthracene dimer with a meta-phenylene spacer bearing two substituents on the convex side. Using the bent polyaromatic building block, we began to develop novel supramolecular capsules from two different synthetic approaches. One is a coordination approach, which was pursued by converting the building block into a bent ligand with two pyridine units at the terminal positions. The ligands quantitatively assemble into an M₂L₄-type capsule through coordination bonding with metal ions. The other is a π-stacking approach, which was followed by utilizing the block as a bent amphiphilic molecule with two trimethylammonium groups at the spacer. In water, the amphiphiles spontaneously assemble into a micelle-type capsule through the hydrophobic effect and π-stacking interactions. Simple modification of the building block allowed us to prepare a wide variety of coordination capsules as well as π-stacking capsules, bearing different hydrophilic side-chains, terminal substitutions, connecting units, polyaromatic panels, or spacer units. The coordination capsule possesses a rigid cavity, with a diameter of ∼1 nm, surrounded by multiple anthracene panels. The spherical polyaromatic cavity binds various synthetic molecules (e.g., paracyclophanes, corannulene, BODIPY, and fullerene C₆₀) in aqueous solutions. With the aid of the polyaromatic shell, photochemically and thermally reactive radical initiators and oligosulfurs are greatly stabilized in the cavity. Biomolecules such as hydrophilic sucrose and oligo(lactic acid)s as well as hydrophobic androgenic hormones are bound by the capsule with high selectivity. In addition, long amphiphilic poly(ethylene oxide)s are threaded into the closed shell of the capsule(s) to generate unusual pseudorotaxane-shaped host-guest complexes in water. In contrast, the π-stacking capsule furnishes a flexible cavity, adaptable to the size and shape of guest molecules, encircled by multiple anthracene panels. In water, the capsule binds hydrophobic fluorescent dyes (e.g., Nile red and DCM) in the cavity. Simple grinding of the bent amphiphile with highly hydrophobic nanocarbons such as fullerenes, nanographenes, and carbon nanotubes (followed by sonication) as well as metal-complexes such as Cu(II)-phthalocyanines and Mn(III)-tetraphenylporphyrins leads to the efficient formation of water-soluble host-guest complexes upon encapsulation. Red emission from otherwise water-deactivated Eu(III)-complexes is largely enhanced in water through encapsulation. Moreover, the incorporation of pH- and photoswitches into the amphiphile affords stimuli-responsive π-stacking capsules, capable of releasing bound guests by the addition of acid and light irradiation, respectively, in water. The host functions of the coordination and π-stacking capsules are complementary to each other, which enables selection of the capsule-type depending on the envisioned target. We are convinced that continued investigation of the present supramolecular capsules featuring the bent anthracene dimer and its derivatives will further increase their value as advanced molecular tools for synthetic, analytical, material, biological, and/or medical applications.

Recent Developments of Supramolecular Metal-based Structures for Applications in Cancer Therapy and Imaging

The biomedical application of discrete supramolecular metal-based structures, including supramolecular coordination complexes (SCCs), is still an emergent field of study. However, pioneering studies over the last 10 years demonstrated the potential of these supramolecular compounds as novel anticancer drugs, endowed with different mechanisms of action compared to classical small-molecules, often related to their peculiar molecular recognition properties. In addition, the robustness and modular composition of supramolecular metal-based structures allows for an incorporation of different functionalities in the same system to enable imaging in cells via different modalities, but also active tumor targeting and stimuli-responsiveness. Although most of the studies reported so far exploit these systems for therapy, supramolecular metal-based structures may also constitute ideal scaffolds to develop multimodal theranostic agents. Of note, the host-guest chemistry of 3D self-assembled supramolecular structures - within the metallacages family - can also be exploited to design novel drug delivery systems for anticancer chemotherapeutics. In this review, we aim at summarizing the pivotal concepts in this fascinating research area, starting with the main design principles and illustrating representative examples while providing a critical discussion of the state-of-the-art. A section is also included on supramolecular organometallic complexes (SOCs) whereby the (organic) linker is forming the organometallic bond to the metal node, whose biological applications are still to be explored. Certainly, the myriad of possible supramolecular metal-based structures and their almost limitless modularity and tunability suggests that the biomedical applications of such complex chemical entities will continue along this already promising path.

Photoactive supramolecular cages incorporating Ru(ii) and Ir(iii) metal complexes

Cage compounds incorporating phosphorescent Ru(ii) and Ir(iii) metal complexes possess a highly desirable set of optoelectronic and physical properties. This feature article summarizes the recent work on cage assemblies containing these metal complexes as photoactive units, highlighting our contribution to this growing field.

Biomedical Applications of Metallosupramolecular Assemblies-Structural Aspects of the Anticancer Activity

The design and development of metallosupramolecular systems has resulted in construction of a myriad of fascinating structures with highly diverse properties and potential applications. Assessment of the biomedical applications of metallosupramolecular assemblies is an emerging field of research that stems from the recently demonstrated promising results on such systems. After the pioneering works of Therrien and coworkers on organometallic Ru-cages with promising anticancer properties, this topic has evolved to the more recent studies on bioactivity of supramolecular coordination complexes built from different metal ions and various multidentate ligands. Sufficient amount of data on the anticancer activity of metallosupramolecules has already been reported and allows outlining some general tendencies in the structural aspects of the biological activity. The main structural properties of the complexes that can be readily modified to enhance their activity are the size, the shape and charge of the formed complexes. Moreover, the intrinsic properties of the building components could predetermine some of the main characteristics of the overall supramolecular complex, such as its optical properties, chemical reactivity, solubility, etc., and could, thereby, define the areas of its biomedical applications. The unique structural property of most of the metallosupramolecular assemblies, however, is the presence of a discrete cavity that renders a whole range of additional applications resulting from specific host-guest interactions. The encapsulations of small bioactive or fluorescent molecules have been employed for delivery or recognition purposes in many examples. On the other hand, metallosupramolecules have been imbedded into target-specific polymeric nanoparticles that resulted in a successful combination of their therapeutic and diagnostic properties, making them promising for theranostic application in cancer treatment. The aim of this review paper is to mark out some key tendencies in the reported metallosupramolecular structures in relation with their biological activity and potential areas of biomedical application. In this way, a useful set of guidelines can be delineated to help synthetic chemists broaden the application areas of their supramolecular systems by few structural changes.

Visualizing Kinetically Robust CoIII4L6 Assemblies in Vivo: SPECT Imaging of the Encapsulated [99mTc]TcO4- Anion

Noncovalent encapsulation is an attractive approach for modifying the efficacy and physiochemical properties of both therapeutic and diagnostic species. Abiotic self-assembled constructs have shown promise, yet many hurdles between in vitro and (pre)clinical studies remain, not least the challenges associated with maintaining the macromolecular, hollow structure under nonequilibrium conditions. Using a kinetically robust Co^III₄L₆ tetrahedron we now show the feasibility of encapsulating the most widely used precursor in clinical nuclear diagnostic imaging, the γ-emitting [^99mTc]TcO₄^- anion, under conditions compatible with in vivo administration. Subsequent single-photon emission computed tomography imaging of the caged-anion reveals a marked change in the biodistribution compared to the thyroid-accumulating free oxo-anion, thus moving clinical applications of (metallo)supramolecular species a step closer.

Anticancer Activity and Cisplatin Binding Ability of Bis-Quinoline and Bis-Isoquinoline Derived [Pd2L4]4+ Metallosupramolecular Cages

New bis-quinoline (L q) and bis-isoquinoline-based (L iq) ligands have been synthesized, along with their respective homoleptic [Pd2(L q or L iq)4]4+ cages (C q and C iq). The ligands and cages were characterized by 1H, 13C and diffusion ordered (DOSY) NMR spectroscopies, high resolution electrospray ionization mass spectrometry (HR-ESIMS) and in the case of the bis-quinoline cage, X-ray crystallography. The crystal structure of the C q architecture showed that the [Pd2(L q)4]4+ cage formed a twisted meso isomer where the [Pd(quinoline)4]2+ units at either end of the cage architecture adopt the opposite twists (left and right handed). Conversely, Density Functional Theory (DFT) calculations on the C iq cage architecture indicated that a lantern shaped conformation, similar to what has been observed before for related [Pd2(L tripy)4]4+ systems (where L tripy = 2,6-bis(pyridin-3-ylethynyl)pyridine), was generated. The different cage conformations manifest different properties for the isomeric cages. The C iq cage is able to bind, weakly in acetonitrile, the anticancer drug cisplatin whereas the C q architecture shows no interaction with the guest under the same conditions. The kinetic robustness of the two cages in the presence of Cl- nucleophiles was also different. The C iq cage was completely decomposed into free L iq and [Pd(Cl)4]2- within 1 h. However, the C q cage was more long lived and was only fully decomposed after 7 h. The new ligands (L iq and L q) and the Pd(II) cage architectures (C iq and C q) were assessed for their cytotoxic properties against two cancerous cell lines (A549 lung cancer and MDA-MB-231 breast cancer) and one non-cancerous cell line (HDFa skin cells). It was found that L q and C q were both reasonably cytotoxic (IC50S ≈ 0.5 μM) against A549, while C iq was slightly less active (IC50 = 7.4 μM). L iq was not soluble enough to allow the IC50 to be determined against either of the two cancerous cell lines. However, none of the molecules showed any selectivity for the cancer cells, as they were all found to have similar cytotoxicities against HDFa skin cells (IC50 values ranged from 2.6 to 3.0 μM).

Selective Anion Recognition by a Dynamic Quadruple Helicate

An M₂ L₄ quadruple helicate, formed by wrapping four molecules of 1,4-bis(3-pyridyloxy)benzene (L₁ ) about two palladium(II) centers, is shown to bind anions within its internal cavity. ¹ H NMR exchange experiments provide a quantitative measure of anion selectivity and reveal a preference for ClO₄ ^- over the other tetrahedral anions BF₄ ^- and ReO₄ ^- and the octahedral anion PF₆ ^- . X-ray crystal structures of [Pd₂ (L₁ )₄ ]⁴⁺ helicates containing ClO₄ , BF₄ ^- and I^- reveal that the cavity size can dynamically change in response to the size of the guest.

Self-Assembled BODIPY-Based Iridium Metallarectangles: Cytotoxicity and Propensity to Bind Biomolecules

A new 4-ethynylpyridine 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based ligand L, which was synthesized by means of the Sonogashira coupling method, was used to obtain two new [2+2] iridium-based metallarectangles, 3 and 4. Ligand L and metallarectangles 3 and 4 were fully characterized through various analytical techniques. The structure of rectangle 4 was further confirmed by single-crystal X-ray diffraction analysis, which showed the formation of an expected [2+2] supramolecule, in which the iridium metal centers were bridged with ligand L to form the desired metallarectangle 4. In the context of the growing biological interest in metallarectangles, rectangle 4 was found to be highly active against two types of cancer cells, with IC₅₀ values almost threefold superior to those of cisplatin. Both 3 and 4 showed dose-dependent abilities to bind bovine serum albumin and salmon sperm DNA; this indicated their tendency to interact with such biomolecules as a potential mode of action.

A luminescent [Pd4Ru8]24+ supramolecular cage

A phosphorescent cage of the form [Pd₄Ru₈]²⁴⁺ is reported.

Si10Cu6N4 Cage Hexacoppersilsesquioxanes Containing N Ligands: Synthesis, Structure, and High Catalytic Activity in Peroxide Oxidations

The synthesis, composition, and catalytic properties of a new family of hexanuclear Cu(II)-based phenylsilsesquioxanes are described here. Structural studies of 17 synthesized compounds revealed the general principle underlying their molecular topology: viz., a central metal oxide layer consisting of two Cu₃ trimers is coordinated by two cyclic [PhSiO_1.5]₅ siloxanolate ligands to form a skewed sandwich architecture with the composition [(PhSiO_1.5)₁₀(CuO)₆]²⁺. In addition to this O ligation by the siloxanolate rings, two opposite copper ions are additionally coordinated by the nitrogen atoms of corresponding N ligand(s), such as 2,2'-bipyridine (compounds 1-9), 1,10-phenanthroline (compounds 10-13), mixed 1,10-phenanthroline/2,2'-bipyridine (compound 14), or bathophenanthroline (compounds 15-17). Finally, the charge balance is maintained by two HO^- (compounds 1-7, 10-13, and 15-17), two H₃CO^- (compound 8), or two CH₃COO^- (compounds 9 and 14) anions. Complexes 1 and 10 exhibited a high activity in the oxidative amidation oxidation of alcohols. Compounds 1, 10, and 15 are very efficient homogeneous catalysts in the oxidation of alkanes and alcohols with peroxides.

The Promise of Self-Assembled 3D Supramolecular Coordination Complexes for Biomedical Applications

In the supramolecular chemistry field, coordination-driven self-assembly has provided the basis for tremendous growth across many subdisciplines, spanning from fundamental investigations regarding the design and synthesis of new architectures to defining different practical applications. Within this framework, supramolecular coordination complexes (SCCs), defined as large chemical entities formed from smaller precursor building blocks of ionic metal nodes and organic multidentate ligands, resulting in intricate and well-defined supramolecular structures, hold great promise. Notably, interest in the construction of discrete 3D molecular architectures, such as those offered by SCCs, has experienced extraordinary progress because of their potential application as sensors, catalysts, probes, and containers and in basic host-guest chemistry. Despite numerous synthetic efforts and a number of inherent favorable properties, the field of 3D SCCs for biomedical applications is still in its infancy. This Viewpoint focuses on 3D SCCs, specifically metallacages and helicates, first briefly presenting the fundamentals in terms of the synthesis and characterization of their host-guest properties, followed by an overview of the possible biological applications with representative examples. Thus, emphasis will be given in particular to metallacages as drug delivery systems and to chiral helicates as DNA recognition domains. Overall, we will provide an update on the state-of-the-art literature and will define the challenges in this fascinating research area at the interface of different disciplines.

Multicavity Metallosupramolecular Architectures

Discrete metallosupramolecular systems are often macrocyclic or cage-like architectures with an accessible internal cavity. Guest molecules can reside within these cavities and much of the interest in these systems is derived from these fascinating host-guest interactions. A range of potential applications stem from the ability of these metallosupramolecular architectures to encapsulate guests. These applications include catalysis or acting as molecular reaction flasks, the molecular scavenging of pollutants, storage of reactive species, and drug delivery. Multicavity metallosupramolecular architectures combine the ability of large hollow assemblies to bind multiple guests concurrently with the binding specificity associated with small cages. A variety of different approaches to generating separate compartments within a single metallosupramolecular assembly have emerged. These include interpenetrated cages, cages with polytopic ligands that have a long backbone, and molecules that have two or more clefts. This review examines these approaches, and highlights key contributions to the field.

Self-Assembled Novel BODIPY-Based Palladium Supramolecules and Their Cellular Localization

Four new palladium metal supramolecules with triangular/square architectures derived from boron dipyrromethane (BODIPY) ligands were synthesized by self-assembly and fully characterized by ¹H and ³¹P NMR, electrospray ionization mass spectrometry, and single-crystal X-ray diffraction. These supramolecules were more cytotoxic to brain cancer (glioblastoma) cells than to normal lung fibroblasts. Their cytotoxicity to the glioblastoma cells was higher than that of a benchmark metal-based chemotherapy drug, cisplatin. The characteristic green fluorescence of the BODIPY ligands in these supramolecules permitted their intracellular visualization using confocal microscopy, and the compounds were localized in the cytoplasm and on the plasma membrane.

PdII2L4-type coordination cages up to three nanometers in size

The utilization of large ligands in coordination-based self-assembly represents an attractive strategy for the construction of supramolecular assemblies more than two nanometers in size. However, the implementation of this strategy is hampered by the fact that the preparation of such ligands often requires substantial synthetic effort. Herein, we describe a simple one-step protocol, which allows large bipyridyl ligands with a bent shape to be synthesized from easily accessible and/or commercially available starting materials. The ligands were used to construct PdII2L4-type coordination cages of unprecedented size. Furthermore, we provide evidence that these cages may be stabilized by close intramolecular packing of lipophilic ligand side chains. Packing effects of this kind are frequently encountered in protein assemblies, but they are seldom used as a design element in metallasupramolecular chemistry.