Developing Supramolecular metallogel derived from Pd2L4 Cage Molecule for Delivering an Anti-cancer Drug to Melanoma Cell B16-F10.

Supramolecular gels are an important class of materials that are promising for its wide range of applications including drug delivery. While supramolecular gels are intrinsically porous because of the 3D nano-matrix (gel matrix) that is being formed due to supramolecular self-assembly process involving the gelator molecules during gelation, additional nanopores can be introduced to the overall gel if the gelator molecule itself holds molecular cavity such as metal-organic-cage (MOC) molecules. A MOC having the molecular formula [(Pd2L24).4NO3].3H2O.2DMF.MeOH (Pd-cage) (L2=5-Azido-N,N'-di-pyridin-3-yl-isophthalamide) was successfully synthesized and characterized by FT-IR, 1H NMR, ESI-MS and single crystal X-ray diffraction. Stimuli-reversible supramolecular metallogel PdG could easily be formed from Pd-cage in DMSO/water mixture. The molecular cage of Pd-cage was demonstrated to be available for loading an anti-cancer drug namely doxorubicin (DOX). Subsequently, DOX was also loaded within PdG and delivered to melanoma cell line B16-F10 displaying significant anti-cancer activity as revealed by both MTT and scratch assay. Rheoreversibility of PdG and its ability to load and deliver DOX to cancer cells clearly raised hope for developing this metallogel further as topical anticancer gel.

Antibacterial Hydrogel as a Self-Drug-Delivery System Derived from Zn(II)-bis-imidazole/NSAID-Based Organic-Inorganic Hybrids.

A skin wound is prone to bacterial infection and growth. An antibacterial topical hydrogel that can act as a self-drug-delivery (SDD) system is reported here. Two bidentate ligands (L2/L1) derived from imidazole/benzimidazole derivatives when reacted with Zn(NO3)2 and a series of nonsteroidal-anti-inflammatory drugs (NSAIDs) produced crystalline products, which were characterized by single-crystal X-ray diffraction (SXRD). Simple mixing of the ingredients of the crystalline products (stoichiometry guided by the corresponding crystal structure) produced an aqueous gel (DMSO/water) when the bidentate ligand was water-insoluble L2, whereas water-soluble L1 readily produced hydrogels under similar conditions. Dynamic rheology and scanning electron microscopy (SEM) were employed to characterize the gels. Zone inhibition diameters, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and hemolysis data suggested that among the hydrogelators, L1MEC derived from L1, meclofenac and Zn(NO3)2, was found to be the best against the Gram-negative bacteria Escherichia coli. The corresponding hydrogel L1MEC_HG and a piece of a dried cloth bandage coated with the hydrogel also showed appreciable activity against E. coli. The antibacterial property of L1MEC_HG against E. coli, thus demonstrated, is relevant in developing an antibacterial SDD system because E. coli is reported to be present in infected wounds.

Designing a vehicle-free anti-bacterial topical hydrogel from Fmoc-diphenylalanine.

A supramolecular synthon-based salt formation strategy has been employed to afford an anti-bacterial topical hydrogel from Fmoc-diphenylalanine (FmocFF). The nontrivial steps (pH/solvent switch along with heat-cool protocol) required for making the hydrogel from FmocFF were successfully avoided following this strategy.

An injectable supramolecular hydrogel as a self-drug-delivery system for local chemoimmunotherapy against melanoma.

Skin-cancer melanoma caused 57k death in 2020. Some of the available therapies are: topical application of a gel loaded with an anti-skin cancer drug and intravenous injection of immune cytokines; however, both the approaches have drawbacks such as inefficient internalization of the drug in cancer cells and a short half-life with severe side effects, respectively. Interestingly, we observed for the first time that a subcutaneously implanted hydrogel designed and synthesized by coordinating NSAIDs and 5-AP with Zn(II) can effectively combat melanoma cell (B16-F10)-induced tumors in C57BL/6 mice. Both in vitro and in vivo results show that it can effectively reduce PGE2 expression, consequently upregulating IFN-γ and IL-12 that eventually engage M1-macrophages for activating T cells (CD8+), triggering apoptosis. This unique all-in-one self-drug-delivery approach, wherein the hydrogel implant is made from the drug molecules itself providing both chemotherapy and immunotherapy in combating deadly melanoma, highlights the supramolecular chemistry-based bottom-up approach in cancer therapy.

Nitrile-Containing Terpyridyl Zn(II)-Coordination Polymer-Based Metallogelators Displaying Helical Structures: Synthesis, Structures, and "Druglike" Action against B16-F10 Melanoma Cells.

An attempt has been made to develop a self-drug-delivery system against melanoma from a series of metallogelators derived from coordination polymers. Thus, a series of coordination polymers (CP1-CP6) derived from a nitrile-containing terpyridyl ligand (L) and transition metal salts (Cu(I)/Zn(II)) have been synthesized and thoroughly characterized by a number of physicochemical techniques including single crystal X-ray diffraction. Reactions of the ingredients of the coordination polymers guided by their single crystal structures produced four metallogels (CPG2-CPG5) which were characterized by dynamic rheology and TEM. The metallogelator CPG3 turned out to be the best suited for further studies as revealed from MTT assay against melanoma (B16-F10) and macrophage (RAW 264.7) cells. Various experiments (scratch, cell cycle, nuclear condensation, annexin V-FITC/PI, mitochondrial membrane potential, Ho-efflux assays) not only supported the "druglike" action against melanoma B16-F10 cells but also suggested that the mechanism of cancer cell death was via mitochondrial membrane potential depolarization-driven apoptosis. Because melanoma B16-F10 is a model cell line for human skin cancer, the metallogel CPG3 may, therefore, be further developed for such treatment.

Selective Separation of Hazardous Chemicals from Vapor Phase by an Easily Accessible Breathing Coordination Polymer Derived from Terpyridyl/terephthalate Mixed Ligands.

Coordination polymers are extensively studied materials because of their various potential applications. Amongst them, breathing coordination polymers that are capable of exchanging lattice occluded guest molecules with other guests via single-crystal-to-single-crystal (SC-SC) fashion are particularly intriguing. Herein, we disclose an easily accessible breathing coordination polymer namely DMF@Zn-CP capable of exchanging as many as 23 guest molecules of various kinds in SC-SC fashion when the crystals of the coordination polymer were exposed to the corresponding vapor of the guests. Selectivity experiments revealed that it was also capable of separating selectively hazardous chemicals such as dichloro-methane, benzene and fluorobenzene from the corresponding complex mixture of vapors of halomethanes, aromatic hydrocarbons and halobenzenes. The breathing coordination polymer could also be exploited as drug delivery vehicle; slow and sustained release of anti-bacterial agents (benzyl alcohol/phenethyl amine) as guests against both gram positive and gram negative bacteria was evident in zone inhibition assays. A mixed ligand strategy wherein a nitrile containing terpyridyl ligand (L) and terephthalate (TA) co-ligand were reacted with Co(II)/Ni(II)/Zn(II) nitrate salts was adopted herein. Three coordination polymers namely MeOH@Co-CP, DMF/H2 O@Ni-CP and DMF@Zn-CP were isolated and characterized by single crystal X-ray diffraction. Studies revealed that only DMF@Zn-CP possessed the ability to breath in response to the vapors of the guests as stimuli.

Real-time Observation of Macroscopic Helical Morphologies under Optical Microscope: A Curious Case of π-π Stacking Driven Molecular Self-assembly of an Organic Gelator Devoid of Hydrogen Bonding.

Supramolecular assemblies such as tubules/helix/double helix/helical tape etc. are usually submicron objects preventing direct observation under optical microscope. Chiral-pure form of these assemblies is important for potential applications. Herein, we report a rare phenomenon wherein a DMSO gel of a simple terpyridine derivative [(4-CNPhe)4PyTerp] produced macroscopic helical morphologies (µm length scale) which could be observed under optical microscope, formation of which could be monitored by optical videography, stable enough to withstand acidic vapour, robust enough to display reversible gel↔sol in response to acidic and ammonia vapour and sturdy enough to be maneuvered with a needle. These properties appeared to be unique to the title compound as the other related derivatives failed to display such assembly structures. SXRD and MD simulation studies suggested that weak interactions (π-π stacking) played a crucial role in the self-assembly process.

Designing coordination polymers as multi-drug-self-delivery systems for tuberculosis and cancer therapy: in vitro viability and in vivo toxicity assessment.

A proof of concept for designing multi-drug-delivery systems suitable for self-drug-delivery is disclosed. Simple coordination chemistry was employed to anchor two kinds of drugs namely isoniazid (IZ - anti-tuberculosis), various non-steroidal-anti-inflammatory-drugs (NSAIDs) namely ibuprofen-IBU, fenoprofen-FEN, naproxen-NAP, diclofenac-DIC and mefenamic acid-MEF and Zn(NO3)2 to synthesize a series of 1D coordination polymers namely IZIBU, IZFEN, IZNAP, IZDIC and IZMEF which were structurally characterized by single crystal X-ray diffraction (SXRD). The coordination polymers wherein both types of drugs were anchored to Zn(II) metal centers could easily be ground to nano-sized particles suitable for biological studies by hand grinding in a mortar and pestle. Zone inhibition studies revealed that all the coordination polymers possessed antibacterial properties against Gram positive, Gram negative and mycobacteria namely Mycobacterium tuberculosis (M.tb). Detailed studies carried out on IZDIC employing flow cytometry and confocal microscopy under various staining conditions established that such antibacterial activity was due to the generation of reactive oxygen species (ROS) such as nitric oxide (NO) and also inhibition of mycolic acid leading to incomplete cell wall formation. It was also established that IZDIC could indeed inhibit the growth of M.tb within a mouse macrophage host cell namely RAW 264.7 thereby simulating the treatment of Tuberculosis (TB) under in vitro conditions. Scratch assay and cell cycle analysis on a human lung cancer cell line (A549) revealed its anti-cancer property, thereby indicating its potential as a multi-drug-delivery system. In vivo toxicity assessment (serum parameters, histopathology, and haemolysis) carried out on BALB/c mice showed that IZDIC was safe up to a concentration of 100 mg kg-1. Finally, a reasonably high yield in bulk synthesis, stability under high temperature and humid conditions, tabletability and, slow and sustained release of the drug component of IZDIC suggested its suitability in real-life applications as multi-drug-delivery systems.

Supramolecular Gels from Bis-amides of L-Phenylalanine: Synthesis, Structure and Material Applications.

A series of small organic molecules having a bis-amide backbone containing hydrogen-bond functionalities were rationally designed, synthesized and characterized to examine their ability to act as potential low-molecular-weight gelators (LMWGs). All the bis-amides were decorated with identical 3-pyridyl amide of L-phenylalanine moieties along with variously substituted terminal benzoyl groups. Gelation studies revealed that only 4-methylphenyl substituted bis-amide (PME) was capable of gelling both aqueous (DMSO/water) and methyl salicylate (MS) (an important solvent for topical formulation for medical applications) solvents; whereas 4-chlorophenyl and 4-bromophenyl substituted bis-amides (PCL, PBR, respectively) acted as organogelator for various organic solvents. On the contrary, 4-nitrophenyl as well as 3,5-dinitrophenyl substituted bis-amides (PNI, DNI, respectively) were unable to gel any solvents studied herein. The corresponding aqueous gel namely PME-HG and three methyl salicylate gels PME-MS, PCL-MS and PBR-MS were characterized by dynamic and table top rheology followed by electron microscopy. Single crystal X-ray diffraction (SXRD) data revealed crucial insights into the supramolecular assembly of all the gelator and nongelator bis-amides. Both PME-HG and PME-MS were rheoreversible - an important property in material applications. Interestingly, PME-MS displayed remarkable material properties such as shape-sustaining, loadbearing and self-healing. Selected MS and aqueous gels loaded with nano-molar iodine were found to possess anti-bacterial property as revealed by zone inhibition assay.

Supramolecular Hydrogels Developed from Mafenide and Indomethacin as a Plausible Multidrug Self-Delivery System as Antibacterial and Anti-inflammatory Topical Gels.

Following a structural rationale, a series of simple organic salts derived from mafenide (a drug for treating burn wounds) and n-alkyl carboxylic acids (Me-(CH2)n-COOH; n = 1-3, 10-15) and various nonsteroidal anti-inflammatory drugs (NSAIDs), namely, indomethacin (IND), diclofenac (DIC), meclofenamic acid (MEC), tolfenamic acid (TOL), and flufenamic acid (FLU) (designated as salts 1-14, respectively) were synthesized as potential hydrogelators. Gelation studies revealed that mafenide n-alkyl carboxylates with n = 11-14, i.e., salts 5-8, and the indomethacin salt of mafenide, i.e., salt 10, were hydrogelators. The corresponding hydrogels, namely, 5(HG)-8(HG) and 10(HG), were characterized by table-top and dynamic rheology and high-resolution transmission electron microscopy (HR-TEM). Single-crystal structures of the nongelator salts 1-3 and the gelator salt 10 were determined by X-ray diffraction. The results obtained from various studies, which included the solubility, biostability, biocompatibility (MTT assay), and anti-inflammatory (PGE2 assay) response of salt 10, the antibacterial response (zone inhibition assay) of salt 10, its components, and 10(HG), and the release of salt 10in vitro from the corresponding hydrogel bed to the bulk solvent at 37 °C in 24 h, suggested their plausible use in developing multidrug-derived topical hydrogels for self-delivery applications.

Multi-NSAID-based Zn(II) coordination complex-derived metallogelators/metallogels as plausible multi-drug self-delivery systems.

Metallogelators/metallogels derived from a series of multi-NSAID-based Zn(II)-coordination complexes displaying anti-cancer and anti-bacterial properties were designed based on a structural rationale as plausible multi-drug self-delivery systems.

Anchoring Drugs to a Zinc(II) Coordination Polymer Network: Exploiting Structural Rationale toward the Design of Metallogels for Drug-Delivery Applications.

A new series of coordination polymers (CPs) were synthesized and crystallographically characterized by single-crystal X-ray diffraction with the aim of developing drug-delivery systems via metallogel formation. Structural rationale was employed to design such coordination-polymer-based metallogels. As many as nine CPs were obtained by reacting two bis(pyridyl)urea ligands, namely, 1,3-dipyridin-3-ylurea (3U) and 1,3-dipyridin-4-ylurea (4U), and the sodium salt of various nonsteroidal antiinflammatory drugs, namely, ibuprofen (IBU), naproxen (NAP), fenoprofen (FEN), diclofenac (DIC), meclofenamic acid (MEC), mefenamic acid (MEF), and Zn(NO3)2. All of the CPs displayed 1D polymeric chains that were self-assembled through various hydrogen-bonding interactions involving the urea N-H and carboxylate O atoms and, in a few cases, lattice-occluded water molecules. The reacting components of the CPs produced five metallogels in dimethyl sulfoxide/water. The gels were characterized by rheology and transmission electron microscopy. Three selected metallogelators, namely, 3UMEFg, 3UNAPg, and 3UMECg, showed in vitro anticancer, cell imaging, and multidrug delivery for antibacterial applications, respectively. The shear-thinning properties of 3UMECg (rheoreversibility and injectability) make it a potential candidate for plausible topical application.

Designing Metallogelators Derived from NSAID-based Zn(II) Coordination Complexes for Drug-Delivery Applications.

A crystal engineering approach has been invoked to design a new series of eight Zn(II) coordination complexes derived from various non-steroidal anti-inflammatory drugs (NSAIDs), namely diclofenac (DIC), ibuprofen (IBU), naproxen (NAP), flufenamic acid (FLU) and meclofenamic acid (MEC), and two co-ligands, namely N-phenyl-3-pyridylamide (3-Py) and N-phenyl-4-pyridylamide (4-Py), and Zn(NO3 )2 as potential supramolecular gelators. Half of the coordination complexes thus synthesized were able to form aqueous gels (MG-3-PyMEC, MG-3-PyDIC, MG-4-PyNAP and MG-4-PyMEC). Single-crystal structures of all eight complexes revealed that they possessed a gelation-inducing 1D hydrogen-bonded network including amide…amide synthon in some cases, which supported strongly the design principles based on which these complexes were synthesized. Interestingly, one such metallogelator complex, namely 3-PyMEC, showed an intriguing anticancer property against a human breast cancer cell line (MDA-MB-231), as revealed by both MTT and cell migration assays.

A supramolecular hydrogel derived from a simple organic salt capable of proton conduction.

The supramolecular hydrogel of a simple organic salt derived from a primary amine and a mono-sulfonic acid displayed a proton conductivity of 1.2 × 10-4 S cm-1. The hitherto unknown example of the supramolecular gel displaying proton conductivity provides an intriguing alternative to liquid electrolyte or polymer gel electrolytes.

Cu(II)-Metallacryptands Self-Assembled to Vesicular Aggregates Capable of Encapsulating and Transporting an Anticancer Drug Inside Cancer Cells.

Crystallographically characterized M2 L4 type cationic Cu(II)-metallacryptands [MC(X)] derived from a series of bis-pyridyl-bis-urea ligands (LX ; X = O, S, C) are self-assembled to single-layered vesicular aggregates in DMSO, DMSO/water, and DMSO/DMEM (biological media). One such vesicle is MC(O)-vesicle that is demonstrated to be able to load and release (pH responsive) an anticancer drug, namely doxorubicin hydrochloride (DOX). DOX-loaded MC(O)-vesicle is also successfully transported within MDA-MB-231 cells-a highly aggressive human breast cancer cell line. Such self-assembling behavior to form vesicular aggregates by metallacryptands (MCs) is hitherto unknown.

Design and Synthesis of ZnII -Coordination Polymers Anchored with NSAIDs: Metallovesicle Formation and Multi-drug Delivery.

A series of coordination polymers synthesized from a bis-pyridyl linker, namely 4,4'-azopyridine (L), selected non-steroidal-anti-inflammatory drugs (NSAIDs), namely diclofenac (Dic), ibuprofen (Ibu), flurbiprofen (Flu), mefenamic acid (Mefe), and naproxen (Nap), and Zn(NO3 )2 were characterized by single crystal X-ray diffraction. One of the coordination polymers, namely CP3 derived from Flu, was able to form metallovesicles in DMSO, DMSO/H2 O and DMSO/DMEM (biological media) as revealed by TEM, AFM and DLS. Metallovesicle formation by CP3 was further supported by loading a fluorescent dye, namely calcein, as well as an anti-cancer drug, doxorubicin hydrochloride (DOX), as revealed by UV-vis and emission spectra, and fluorescence microscopy. DOX-loaded metallovesicles of CP3 (DOX@CP3-vesicle) could be delivered in vitro to a highly aggressive human breast cancer cell line, namely MDA-MB-231, as revealed by MTT and cell migration assays, and also cell imaging performed under laser scanning confocal microscope (LSCM). Thus, a proof of concept for developing a multi-drug delivery system derived from a metallovesicle for delivering an anti-cancer drug to cancer cells is demonstrated for the first time.

An easy access to topical gels of an anti-cancer prodrug (5-fluorouracil acetic acid) for self-drug-delivery applications.

An easy access to topical gels (both hydro- and organogels) derived from an anti-cancer prodrug namely 5-fluorouracil acetic acid (5-FuA) achieved by exploiting a simple salt formation strategy is reported for the first time. Nearly 85% of the salts synthesized were gelators. Single crystal structures of some of the gelator salts revealed an intriguing hydrogen bonding network including double stranded 1D chains stabilized through uracil-uracil complementary interactions and the crystal structures of the gelator salts corroborated well with the hypothesis based on which the gelators were designed. Studies indicated that both the hydrogel and the methyl salicylate gel of the gelator salt FuA-15 were suitable for self-drug-delivery application.

Self-Assembly of Spherical Organic Molecules to Form Hollow Vesicular Structures in Water for Encapsulation of an Anticancer Drug and Its Release.

Developing hierarchical supramolecular structures is important for better understanding of various biological functions and possibly generating new materials for biomedical applications. Herein, we report the first examples of functional vesicles derived from cationic spherical organic molecules (C1 -C3 ) which were readily synthesized by reacting a C3 -symmetric tris-benzimmidazole derivative (possessing a 1,3,5-ethyl substituted aromatic core) with 1,3,5-substituted tris-bromomethyl benzene derivatives. Vesicle formation by C1 -C3 was probed by high-resolution microscopy (TEM and AFM), dynamic light scattering (DLS) and fluorescence microscopic imaging of calcein-loaded vesicles. One of the vesicles [Vesicle(C3 )] displayed the ability to load the anticancer drug doxorubicin (DOX). The drug was subsequently released from DOX@Vesicle(C3 ) in a stimuli-responsive manner in presence of the well-known vesicle destroyer Triton X-100, as revealed by in vitro cell migration assay carried out on a highly aggressive human breast cancer cell line (MDA-MB-231).

Designing Supramolecular Gelators: Challenges, Frustrations, and Hopes.

This article is a personal account of the author, who serendipitously entered the field of supramolecular gels nearly two decades ago. A supramolecular synthon approach in the context of crystal engineering was utilized to develop a working hypothesis to design supramolecular gelators derived from simple organic salts. The activity not only provided a way to occasionally predict gelation, but also afforded clear understanding of the structural landscape of such supramolecular materials. Without waiting for an ab initio approach for designing a gel, a large number of supramolecular gelators derived from organic salts were designed following the working hypothesis thus developed. Organic salts possess a number of advantages in terms of their ease of synthesis, purification, high yield and stability and, therefore, are suitable for developing materials for various applications. Organic salt-based gel materials for containing oil spills, synthesizing inorganic nanostructures and metal nanoparticles, sensing hazardous gas and dissolved glucose, adsorbing dyes, and facilitating drug delivery in self-delivery fashion have been developed. The journey through the soft world of gelators which was started merely by serendipity turned out to be rewarding, despite the challenges and frustrations in the field.