Overcoming barriers with non-covalent interactions: supramolecular recognition of adamantyl cucurbit[n]uril assemblies for medical applications

Adamantane, a staple in medicinal chemistry, recently became a cornerstone of a supramolecular host-guest drug delivery system, ADA/CB[n]. Owing to a good fit between the adamantane cage and the host cavity of the cucurbit[n]uril macrocycle, formed strong inclusion complexes find applications in drug delivery and controlled drug release. Note that the cucurbit[n]uril host is not solely a delivery vehicle of the ADA/CB[n] system but rather influences the bioactivity and bioavailability of drug molecules and can tune drug properties. Namely, as host-guest interactions are capable of changing the intrinsic properties of the guest molecule, inclusion complexes can become more soluble, bioavailable and more resistant to metabolic conditions compared to individual non-complexed molecules. Such synergistic effects have implications for practical bioapplicability of this complex system and provide a new viewpoint to therapy, beyond the traditional single drug molecule approach. By achieving a balance between guest encapsulation and release, the ADA/CB[n] system has also found use beyond just drug delivery, in fields like bioanalytics, sensing assays, bioimaging, etc. Thus, chemosensing in physiological conditions, indicator displacement assays, in vivo diagnostics and hybrid nanostructures are just some recent examples of the ADA/CB[n] applicability, be it for displacements purposes or as cargo vehicles.

Study of Drug Delivery Using Purely Organic Macrocyclic Containers-Cucurbit[7]uril and Pillararene

An impaired immune system is the root of various human ailments provoking the urge to find vehicle-mediated quick delivery of small drug molecules and other vital metabolites to specific tissues and organs. Thus, drug delivery strategies are in need of improvement in therapeutic efficacy. It can be achieved only by increasing the drug-loading capacity, increasing the sustained release of a drug to its target site, easy relocation of drug molecules associated with facile complexation-induced properties of molecular vehicles, and high stimuli-responsive drug administration. Supramolecular drug delivery systems (SDDS) provide a much needed robust yet facile platform for fabricating innovative drug nanocarriers assembled by thermodynamically noncovalent interaction with the tunable framework and above-mentioned properties. Measures of cytotoxicity and biocompatibility are the two main criteria that lie at the root of any promising medicinal applications. This Review features significant advancements in (i) supramolecular host-guest complexation using cucurbit[7]uril (CB[7]), (ii) encapsulation of the drug and its delivery application tailored for CB[7], (iii) self-assembly of supramolecular amphiphiles, (iv) supramolecular guest relay using host-protein nanocavities, (v) pillararene (a unique macrocyclic host)-mediated SDDS for the delivery of smart nanodrugs for siRNA, fluorescent molecules, and insulin for juvenile diabetes. Furthermore, fundamental questions and future hurdles related to smart SDDS based on CB[7] and pillararenes and their future promising breakthrough implementations are also distinctly outlined in this Review.

Challenges and Opportunities of Functionalized Cucurbiturils for Biomedical Applications

Cucurbit[n]uril (CB[n]) macrocycles (especially CB[5] to CB[8]) have shown exceptional attributes since their discovery in 2000. Their stability, water solubility, responsiveness to several stimuli, and remarkable binding properties have enabled a growing number of biological applications. Yet, soon after their discovery, the challenge of their functionalization was set. Nevertheless, after more than two decades, a myriad of CB[n] derivatives has been described, many of them used in cells or in vivo for advanced applications. This perspective summarizes key advances of this burgeoning field and points to the next opportunities and remaining challenges to fully express the potential of these fascinating macrocycles in biology and biomedical sciences.

Supramolecular Activation of S8 by Cucurbiturils in Water and Mechanism of Reduction to H2S by Thiols: Insights into Biological Sulfane Sulfur Trafficking

Reactive sulfur species (RSS) play critical roles in diverse chemical environments. Molecules containing sulfane sulfur (S⁰) have emerged as key species involved in cellular redox buffering as well as RSS generation, translocation, and action. Using cucurbit[7]uril (CB[7]) as a model hydrophobic host, we demonstrate here that S₈ can be encapsulated to form a 1:1 host guest complex, which was confirmed by solution state experiments, mass spectrometry, and X-ray crystallography. The solid state structure of CB[7]/S₈ shows that the encapsulated S₈ is available to nucleophiles through the carbonyl portals of the host. Treatment of CB[7]/S₈ with thiols results in efficient reduction of S₈ to H₂S in water at physiological pH. We establish that encapsulated S₈ is attacked by a thiol within the CB[7] host and that the resultant soluble hydropolysulfide is ejected into solution, where it reacts further with thiols to generate soluble sulfane sulfur carriers and ultimately H₂S. The formation of these intermediate is supported by observed kinetic saturation behavior, competitive inhibition experiments, and alkylative trapping experiments. We also demonstrate that CB[7]/S₈ can be used to increase sulfane sulfur levels in live cells using fluorescence microscopy. More broadly, this work suggests a general activation mechanism of S₈ by hydrophobic motifs, which may be applicable to proteins, membranes, or other bimolecular compartments that could transiently bind and solubilize S₈ to promote reaction with thiols to solubilize and shuttle S₈ back into the redox labile sulfane sulfur pool. Such a mechanism would provide an attractive manifold in which to understand the RSS translocation and trafficking.

Matrix solid-phase dispersion based on cucurbit[7]uril-assisted dispersive liquid-liquid microextraction coupled with high performance liquid chromatography for the determination of benzimidazole fungicides from vegetables

A new method involving matrix solid-phase dispersion (MSPD) and dispersive liquid-liquid microextraction (DLLME) was optimized with the aid of stoichiometry and applied to the extraction, purification, and determination of benzimidazole residues in vegetables. Carbendazim, thiabendazole, and thiophanate-methyl were selectively extracted from vegetables using cucurbit[7]uril as the MSPD extractant and transferred to an aqueous solution, then further enriched using DLLME with acetonitrile and chloroform as dispersive and extraction solvents, respectively. The optimal extraction conditions of MSPD and DLLME were selected by two-level full-factorial design and central-composite design (CCD). The developed method (MSPD-DLLME-HPLC-UV) showed good linearity in the range of 0.025-5 μg/g, with R² > 0.9984. Intra- and interday precisions were 5.3-10.9% and 10.6-12.4%, respectively, and the limit of detection was between 0.004 and 0.007 μg/g of fresh weight. This method was applied to the analysis of four different types of vegetables, and the recoveries ranged from 65.4% to 124.0%. The method was environmentally friendly, easy to operate, and sensitive.

Exploration on the drug solubility enhancement in aqueous medium with the help of endo-functionalized molecular tubes: a computational approach

One major problem in the pharmaceutical industry is the aqueous solubility of newly developed orally administered drug candidates. More than 50% of newly developed drug molecules suffer from low aqueous solubility. The therapeutic effects of drug molecules are majorly dependent on the bioavailability and, in essence, on the solubility of the used drug molecules. Thus, enhancement of drug solubility of sparingly soluble drug molecules is a need of modern times. Considering the high importance of drug solubility, we have computationally shown the enhancement of drug solubility for seven class II (poorly water-soluble) drug molecules in a water medium. The uses of supramolecular macrocycles have immense importance in the same field. Thus, we have used two synthetic supramolecular receptors named host-1a and host-1b to enhance the water solubility of fluorouracil, albendazole, camptothecin, clopidogrel, indomethacin, melphalan, and tolfenamic acid drug molecules. Biomedical engagements of a supramolecular receptor commence with the formation of stable host-drug complexes. These complexations enhance the water solubility of drug molecules and sustain the release rate and bioavailability of drug molecules. Thus, in this work, we focus on the formation of stable host-drug complexes in water medium. Molecular dynamics simulation is applied to analyze the structural features and the energetics involved in the host-drug complexation process. The information obtained at the atomistic level helps us gain better insights into the key interactions that operate to produce such highly stable complexes. Thus, we can propose that these two supramolecular receptors may be used as drug solubilizing agents, and patients will benefit from this theragnostic application shortly.

Stuffed pumpkins: mechanochemical synthesis of host-guest complexes with cucurbit[7]uril

Solvent-free mechanochemical synthesis (ball-milling) was used to prepare inclusion complexes with cucurbit[7]uril and four model guest molecules (adamantane, adamantyl-1-amine hydrochloride, toluidine hydrochloride, and p-phenylenediamine dihydrochloride). Successful formation of individual inclusions was independently confirmed by one- and two-dimensional solid-state NMR techniques and differential scanning calorimetry. Mechanochemical synthesis represents an alternative path towards new types of cucurbit[n]uril/guest inclusion complexes that are not accessible due to limited solubility of the individual components.

Host-guest interaction of cucurbit[8]uril with oroxin A and its effect on the properties of oroxin A

In this study, we investigated the host-guest interactions between oroxin A (OA) and cucurbit[8]uril (Q[8]) using 1H NMR, MS, UV-vis and IR spectroscopy. The results showed that OA and Q[8] formed an inclusion compound (OA@Q[8]) with a molar ratio of 1:1 and a binding constant of 1.299 × 107 L·mol-1. In addition, the effect of Q[8] on the properties of OA was investigated through comparative experiments. The solubility of OA in water increased 22.47-fold when the concentration of Q[8] was 1 × 10-4 mol·L-1. Q[8] hardly affected the antioxidant capacity of OA, while the cumulative release of OA in gastric juice increased 2.3-fold after forming the inclusion compound with Q[8].

A General Hypoxia-Responsive Molecular Container for Tumor-Targeted Therapy

Enhanced drug delivery can improve the therapeutic efficacy of drugs and help overcome side effects. However, many reported drug-delivery systems are too complex and irreproducible for practical use. In this work, the design of a hypoxia-responsive molecular container based on calixarene, called CAC4A, which presents a significant advance in practical, hypoxia-targeted drug-delivery, is reported. CAC4A enables a wide variety of clinical drugs to be quantitatively loaded to improve their solubility and stability, as well as enable the administration of reduced doses. Furthermore, as a result of its azo functional groups, which are sensitive to reduction within a hypoxic environment, it is possible to achieve tumor-targeted drug-release with reduced side effects. CAC4A fulfils all essential requirements for a drug-delivery system in addition to multiple advantages, including facile preparation, well-defined molecular weight, and structure, and universal applicability. Such features collectively enable supramolecular prodrugs to be formulated simply and reproducibly, with potential for bench-to-bedside translation. Moreover, CAC4A is amenable to other therapy modalities and can be facilely decorated with functional groups and hybridized with nanomaterials, providing ample possibilities for its role in future drug-delivery systems.

Host-Guest Interactions between Oxaliplatin and Cucurbit[7]uril/Cucurbit[7]uril Derivatives under Pseudo-Physiological Conditions

Compared with conventional drug delivery systems (DDSs), DDSs based on host-guest interactions possess unique advantages, such as high selectivity, tunable binding ability, and controllable release of drugs. It is important to study the host-guest interactions between the carrier and drug under physiological conditions for constructing DDSs. In this work, we have studied the host-guest interaction between cucurbit[7]uril (CB[7]) and oxaliplatin (OxPt), a clinical antitumor drug, in the cell culture medium. The results show that amino acids such as phenylalanine in the 1640 culture medium can partially occupy the cavity of CB[7], which leads to the decrease of enthalpy changes of the host-guest interaction between OxPt and CB[7]. In addition, inorganic salts such as NaCl in the medium reduce the enthalpy change and increase the entropy change of the binding because of the preorganization of the portal of CB[7] and sodium cation. As a result, the binding constant of CB[7] with OxPt in the 1640 culture medium is 1/20 of that in pure water. When CB[7] is modified at the terminal of star-type PEG to construct the star-PEGylated CB[7], it is shown that the molecular weight and topological structure of the PEG polymer backbone exhibit little effect on the host-guest interactions between CB[7] and OxPt. This study enriches the host-guest chemistry of cucurbiturils and may provide guidance for constructing novel DDSs based on host-guest interactions with high loading and releasing efficiency.

Cucurbit[7]uril-driven modulation of ligand-DNA interactions by ternary assembly

The design of small organic molecules with a predictable and desirable DNA-binding mechanism is a topical research task for biomedicine application. Herein, we demonstrate an attractive supramolecular strategy for controlling the non-covalent ligand-DNA interaction by binding with cucurbituril as a synthetic receptor. With a combination of UV/vis, CD and NMR experiments, we demonstrate that the bis-styryl dye with two suitable binding sites can involve double stranded DNA and cucurbituril in the formation of the supramolecular triad. The ternary assembly is formed as a result of the interaction of macrocyclic cucurbituril with one pyridinium fragment of the bis-styryl dye, while the second pyridinium fragment of the dye is effectively associated with DNA backbones, which leads to a change in the ligand-DNA binding mode from aggregation to a minor groove. This exciting outcome was supported by molecular docking studies that help to understand the molecular orientation of the supramolecular triad and elucidate the destruction of dye aggregates caused by cucurbituril. These studies provide valuable information on the mechanisms of DNA binding to small molecules and recognition processes in bioorganic supramolecular assemblies constructed from multiple non-covalent interactions.

Applications of Cucurbiturils in Medicinal Chemistry and Chemical Biology

The supramolecular chemistry of cucurbit[n]urils (CBn) has been rapidly developing to encompass diverse medicinal applications, including drug formulation and delivery, controlled drug release, and sensing for bioanalytical purposes. This is made possible by their unique recognition properties and very low cytotoxicity. In this review, we summarize the host-guest complexation of biologically important molecules with CBn, and highlight their implementation in medicinal chemistry and chemical biology.

A Cucurbit[8]uril 2:2 Complex with a Negative pKa Shift

A viologen derivative carrying a benzimidazole group (V-P-I ²⁺ ; viologen-phenylene-imidazole V-P-I) can be dimerized in water using cucurbit[8]uril (CB[8]) in the form of a 2:2 complex resulting in a negative shift of the guest pK_a , by more than 1 pH unit, contrasting with the positive pK_a shift usually observed for CB-based complexes. Whereas 2:2 complex protonation is unclear by NMR, silver cations have been used for probing the accessibility of the imidazole groups of the 2:2 complexes. The protonation capacity of the buried imidazole groups is reduced, suggesting that CB[8] could trigger proton release upon 2:2 complex formation. The addition of CB[8] to a solution containing V-P- I³⁺ indeed released protons as monitored by pH-metry and visualized by a coloured indicator. This property was used to induce a host/guest swapping, accompanied by a proton transfer, between V-P-I ³⁺ ⋅CB[7] and a CB[8] complex of 1-methyl-4-(4-pyridyl)pyridinium. The origin of this negative pK_a shift is proposed to stand in an ideal charge state, and in the position of the two pH-responsive fragments inside the two CB[8] which, alike residues engulfed in proteins, favour the deprotonated form of the guest molecules. Such proton release triggered by a recognition event is reminiscent of several biological processes and may open new avenues toward bioinspired enzyme mimics catalyzing proton transfer or chemical reactions.

Supramolecular polymeric biomaterials

Polymeric chains crosslinked through supramolecular interactions-directional and reversible non-covalent interactions-compose an emerging class of modular and tunable biomaterials. The choice of chemical moiety utilized in the crosslink affords different thermodynamic and kinetic parameters of association, which in turn illustrate the connectivity and dynamics of the system. These parameters, coupled with the choice of polymeric architecture, can then be engineered to control environmental responsiveness, viscoelasticity, and cargo diffusion profiles, yielding advanced biomaterials which demonstrate rapid shear-thinning, self-healing, and extended release. In this review we examine the relationship between supramolecular crosslink chemistry and biomedically relevant macroscopic properties. We then describe how these properties are currently leveraged in the development of materials for drug delivery, immunology, regenerative medicine, and 3D-bioprinting (253 references).

Rational design of boron-dipyrromethene (BODIPY) reporter dyes for cucurbit[7]uril

We introduce herein boron-dipyrromethene (BODIPY) dyes as a new class of fluorophores for the design of reporter dyes for supramolecular host-guest complex formation with cucurbit[7]uril (CB7). The BODIPYs contain a protonatable aniline nitrogen in the meso-position of the BODIPY chromophore, which was functionalized with known binding motifs for CB7. The unprotonated dyes show low fluorescence due to photoinduced electron transfer (PET), whereas the protonated dyes are highly fluorescent. Encapsulation of the binding motif inside CB7 positions the aniline nitrogen at the carbonyl rim of CB7, which affects the pKa value, and leads to a host-induced protonation and thus to a fluorescence increase. The possibility to tune binding affinities and pKa values is demonstrated and it is shown that, in combination with the beneficial photophysical properties of BODIPYs, several new applications of host-dye reporter pairs can be implemented. This includes indicator displacement assays with favourable absorption and emission wavelengths in the visible spectral region, fluorescence correlation spectroscopy, and noncovalent surface functionalization with fluorophores.

A systematic evaluation of the biocompatibility of cucurbit[7]uril in mice

As one of the most water-soluble members in the macrocyclic cucurbit[n]uril (CB[n]) family, CB[7] has attracted increasing attention in pharmaceutical and biomedical fields. Despite extensive studies regarding the potential use of CB[7] for biomedical applications, its full safety and toxicity profile in a clinically relevant model is still lacking. Herein we report the full biocompatibility profile of CB[7], administered orally, peritoneally or intravenously in mice, respectively. Body-weight changes showed no significant differences among various groups of mice after they were administered with CB[7] at a single dose of 5 g/kg orally, 500 mg/kg peritoneally and 150 mg/kg intravenously, respectively. Hematology tests, as well as hepatic and renal function biochemical markers tests, of the blood collected from these mice sacrificed 21 days after CB[7] administration all exhibited normal ranges of values that were comparable with those of the control group. Moreover, histopathological analysis on the sections of major organs (including the heart, liver, spleen, lungs and kidneys) and gastrointestinal tissues revealed no detectable injuries and inflammatory cells infiltration. Taken together, these results suggest an excellent biocompatibility profile of CB[7] in mice, which provide important foundations for further investigations and even clinical applications of CB[7] in biomedical areas.

Enhancing the solubility and bioactivity of anticancer drug tamoxifen by water-soluble pillar[6]arene-based host-guest complexation

A water-soluble pillar[6]arene functions as a solubilizing agent to enhance the solubility and bioactivity of poorly water-soluble anticancer drug tamoxifen by host-guest complexation between it and tamoxifen.

Acyclic Cucurbit[n]uril-type Receptors: Preparation, Molecular Recognition Properties and Biological Applications

This article traces the development of acyclic cucurbit[n]uril-type receptors with a focus on work from the Isaacs group. First, we describe the synthesis of methylene bridged glycoluril dimers capped with aromatic sidewalls which allowed us to probe the interconversion of the S- and C-shaped dimers which is a fundamental step in CB[n] formation. The C-shaped compounds were found to undergo discrete self-assembly (dimerization) in both water and organic solvents which lead us to investigate multicomponent self-sorting systems. We supressed the self-association of 8 by electrostatic repulsion in the putative dimer which allowed expression of its innate molecular recognition properties toward methylene blue and related planar cationic dyes. Longer glycoluril oligomers (trimer - hexamer, acyclic decamer) were prepared by starving the CB[n]-forming reaction of formaldehyde. The longer oligomers (e.g. 15 and 16) bind to alkylammonium ions in water ≈ 100-fold weaker than macrocyclic CB[n] highlighting the high preorganization of the acyclic but polycyclic framework. We prepared a wide variety of acyclic CB[n] compounds (wall variants, solubilizing group variants, linker variants) based on glycoluril trimer and tetramer. In particular, 26 and 27 have been shown to possess a wide variety of chemically and biologically interesting functions. For example, 26 was used to formulate the insoluble drug Albendazole and treat mice bearing SK-OV-3 xenograft tumors. Compound 27 binds tightly to the neuromuscular blocking agents rocuronium, vecuronium, and cisatracurium and acts as an in vivo reversal agent for these compounds in anesthetized rats. Container 27 was also found to modulate the hyperlocomotive effect of rats that had been treated with methamphetamine. Finally, 38 has been used as a cross reactive component of sensor arrays that are capable of classifying and quantifying cancer related nitroamines and a range of over the counter drugs. Overall, the work demonstrates that acyclic CB[n]-type compounds are nicely pre-organized and therefore retain the essential aspects of the recognition properties of macrocyclic CB[n] but allow for more straightforward tailoring of structure and solubility that enables a variety of chemically and biologically important applications.

A combined molecular dynamics simulation and quantum mechanics study on mercaptopurine interaction with the cucurbit [6,7] urils: Analysis of electronic structure

In the current study, the probability of complex formation between mercaptopurine drug with cucurbit[6]urils and cucurbit[7]urils has been investigated. The calculations for geometry optimization of complexes have been carried out by means of DFT (B3LYP), DFT-D (B3LYP-D) and M06-2X methods. The Atoms In Molecules (AIM), Natural Bond Orbital (NBO), NMR, the density of states (DOSs) and frontier molecular orbital (MO) analyses have been done on the inclusion complexes. In addition, the UV-Vis spectra of the first eight states have been obtained by CAM-B3LYP/TD-DFT calculation. The obtained results of the complexation process reveal that CB[7]-DRG complexes are more favorable than that of CB[6]-DRG interactions. Furthermore, our theoretical results show that configurations III and I are the most stable configurations related to the CB[6]/DRG and CB[7]/DRG interactions, respectively. The positive ∇²_ρ(r) and HC values at the bond critical points indicate that exist the weak H-bonds between CB[6] and CB[7] with H atoms of the drug molecule. The obtained negative binding energy values of CB[7]-DRG interaction in solution phase show the stability of these complexes in the aqueous medium. Also, all of the observed parameters of molecular dynamics simulation such as the number of contacts, hydrogen bonding, center-of-mass distance and van der Waals energy values confirm the encapsulation of mercaptopurine molecule inside the cucurbit[7]urils cavity at about 3.2ns.

Graphene oxide based fluorescence sensor for cucurbit[7]uril

Rhodamine B (RhB) anchored reduced graphene oxide (RhBGO) offers a very sensitive and selective sensing potential for cucurbit[7]uril (CB[7]).

Nanoformulations of albendazole as effective anticancer and antiparasite agents

Initially emerging as a widely used clinical antiparasitic drug, albendazole (ABZ) has been increasingly recognized as an effective anticancer agent due to its outstanding advantage, in other words, low toxicity to normal cells but high effectiveness against parasites and some tumors. The major challenge is its poor water solubility and subsequently low bioavailability. This article thus first reviews the brief achievements in using ABZ to treat parasites and cancers, and summarizes the basic mechanisms of action of ABZ. Then this article critically reviews recent nanotechnological strategies, in other words, formulating/conjugating it with carriers into nanoformulations, in practices of improving aqueous solubility and efficacy in treatment of tumors and parasites. Our expert opinions in this field are provided for more effective delivery of ABZ to treat tumors and parasites in vivo.

Cucurbit[7]uril Enables Multi-Stimuli-Responsive Release from the Self-Assembled Hydrophobic Phase of a Metal Organic Polyhedron

Mixed self-assembly of ligands 1, 2, 1,6-hexanediamine (HDA), and Pd(NO3)2 afforded Fujita-type metal organic polyhedron MOP1 (diameter ≈ 8.2 nm), which is covalently functionalized with an average of 18 cucurbit[7]uril (CB[7]) units, as evidenced by 1H NMR, diffusion-ordered spectroscopy NMR, and transmission electron microscopy measurements. By virtue of the host-guest properties of CB[7], the inner cavity of MOP can be rendered hydrophobic by using octadecyl HDA (3) as guest during the self-assembly process. The hydrophobic cavity was successfully utilized to trap the hydrophobic dye Nile Red (NR) and the anticancer drug doxorubicin (DOX). The stimuli-responsive release of encapsulated NR or DOX occurs (1) upon addition of a competitive binder (e.g., adamantane ammonium (ADA)) for CB[7], (2) by a dual pH-chemical stimulus involving the protonation state change of adamantane carboxylate at pH 5.8, and (3) by a dual pH-photochemical stimulus involving photoisomerization of trans-6 to cis-6 at pH 5.8. NR is released from NR@MOP2 within HeLa cancer cells. This body of work suggests that the covalent attachment of cucurbit[n]uril to metal organic polyhedra constitutes a promising vehicle for the development of both diagnostic and therapeutic nanoparticles.

Impact of Dendrimers on Solubility of Hydrophobic Drug Molecules

Adequate aqueous solubility has been one of the desired properties while selecting drug molecules and other bio-actives for product development. Often solubility of a drug determines its pharmaceutical and therapeutic performance. Majority of newly synthesized drug molecules fail or are rejected during the early phases of drug discovery and development due to their limited solubility. Sufficient permeability, aqueous solubility and physicochemical stability of the drug are important for achieving adequate bioavailability and therapeutic outcome. A number of different approaches including co-solvency, micellar solubilization, micronization, pH adjustment, chemical modification, and solid dispersion have been explored toward improving the solubility of various poorly aqueous-soluble drugs. Dendrimers, a new class of polymers, possess great potential for drug solubility improvement, by virtue of their unique properties. These hyper-branched, mono-dispersed molecules have the distinct ability to bind the drug molecules on periphery as well as to encapsulate these molecules within the dendritic structure. There are numerous reported studies which have successfully used dendrimers to enhance the solubilization of poorly soluble drugs. These promising outcomes have encouraged the researchers to design, synthesize, and evaluate various dendritic polymers for their use in drug delivery and product development. This review will discuss the aspects and role of dendrimers in the solubility enhancement of poorly soluble drugs. The review will also highlight the important and relevant properties of dendrimers which contribute toward drug solubilization. Finally, hydrophobic drugs which have been explored for dendrimer assisted solubilization, and the current marketing status of dendrimers will be discussed.

Metal-Organic Polyhedron Capped with Cucurbit[8]uril Delivers Doxorubicin to Cancer Cells

Self-assembly of ligand 1 and Pd(NO3)2 delivers Fujita-type metal-organic polyhedron (MOP) 3 which bears 24 covalently attached methyl viologen units on its external surface, as evidenced by 1H NMR, diffusion-ordered spectroscopy NMR, electrospray mass spectrometry, transmission electron microscopy, and atomic force microscopy measurements. MOP 3 undergoes noncovalent complexation with cucurbit[n]urils to yield MOPs 4-6 with diameter ≈5-6 nm. MOP 5 can be fully loaded with doxorubicin (DOX) prodrug 2 via hetero-ternary complex formation to yield 7. The MOPs exhibit excellent stability toward neutral to slightly acidic pH in 10 mM sodium phosphate buffer, mitigating the concern of disassembly during circulation. The results of MTS assays show that MOP 7 is 10-fold more cytotoxic toward HeLa cells than equimolar quantities of DOX prodrug 2. The enhanced cytotoxicity can be traced to a combination of enhanced cellular uptake of 7 and DOX release as demonstrated by flow cytometry and confocal fluorescence microscopy. The confluence of properties imparted by the polycationic MOP architecture and plug-and-play CB[n] complexation provides a potent new platform for drug delivery application.

Host-Guest Complexes of Carboxylated Pillar[n]arenes With Drugs

Pillar[n]arenes are a new family of nanocapsules that have shown application in a number of areas, but because of their poor water solubility their biomedical applications are limited. Recently, a method of synthesizing water-soluble pillar[n]arenes was developed. In this study, carboxylated pillar[n]arenes (WP[n], n = 6 or 7) have been examined for their ability to form host-guest complexes with compounds relevant to drug delivery and biodiagnostic applications. Both pillar[n]arenes form host-guest complexes with memantine, chlorhexidine hydrochloride, and proflavine by ¹H nuclear magnetic resonance and modeling. Binding is stabilized by hydrophobic effects within the cavities, and hydrogen bonding and electrostatic interactions at the portals. Encapsulation within WP[6] results in the complete and efficient quenching of proflavine fluorescence, giving rise to "on" and "off" states that have potential in biodiagnostics. The toxicity of the pillar[n]arenes was examined using in vitro growth assays with the OVCAR-3 and HEK293 cell lines. The pillar[n]arenes are relatively nontoxic to cells except at high doses and after prolonged continuous exposure. Overall, the results show that there could be a potentially large range of medical applications for carboxylated pillar[n]arene nanocapsules.

Acyclic Cucurbit[n]uril-Type Molecular Containers: Influence of Linker Length on Their Function as Solubilizing Agents

Two acyclic cucurbit[n]uril (CB[n])-type molecular containers that differ in the length of the (CH2 )n linker (M2C2: n=2, M2C4: n=4) between their aromatic sidewalls and sulfonate solubilizing groups were prepared and studied. The inherent solubilities of M2C2 (68 mm) and M2C4 (196 mm) are higher than the analogue with a (CH2 )3 linker (M2, 14 mm) studied previously. (1) H NMR dilution experiments show that M2C2 and M2C4 do not self-associate in water, which enables their use as solubilizing excipients. We used phase solubility diagrams (PSDs) to compare the solubilizing capacities of M2, M2C2, M2C4, hydroxypropyl-β-cyclodextrin (HP-β-CD), and sulfobutylether-β-cyclodextrin (SBE-β-CD) toward 15 insoluble drugs. We found that M2C2 and M2C4-as gauged by the slope of their PSDs-are less potent solubilizing agents than M2. However, the higher inherent solubility of M2C2 allows higher concentrations of drug to be formulated using M2C2 than with M2 in several cases. The solubilizing ability of M2C2 and SBE-β-CD were similar in many cases, with Krel values averaging 23 and 12, respectively, relative to HP-β-CD. In vitro cytotoxicity and in vivo maximum tolerated dose studies document the biocompatibility of M2C2.

Acyclic Cucurbit[n]uril-Type Molecular Container Enables Systemic Delivery of Effective Doses of Albendazole for Treatment of SK-OV-3 Xenograft Tumors

Approximately, 40-70% of active pharmaceutical ingredients (API) are severely limited by their extremely poor aqueous solubility, and consequently, there is a high demand for excipients that can be used to formulate clinically relevant doses of these drug candidates. Here, proof-of-concept studies demonstrate the potential of our recently discovered acyclic cucurbit[n]uril-type molecular container Motor1 (M1) as a solubilizing agent for insoluble drugs. M1 did not induce significant rates of mutations in various Salmonella typhimurium test strains during the Ames test, suggesting low genotoxicity. M1 also has low risk of causing cardiac toxicity in humans since it did not inhibit the human Ether-à-go-go-Related Gene channel as tested on transfected CHO cell lines via patch clamp analysis. Albendazole (ABZ) is a widely used antihelminthic agent but that has also shown promising efficacy against cancerous cells in vitro. However, due to its low aqueous solubility (2.7 μM) and poor pharmacokinetics, ABZ is clinically limited as an anticancer agent. Here we investigated the potential of M1 as a solubilizing excipient for ABZ formulation. A pharmacokinetic study indicated that ABZ escapes the peritoneal cavity resulting in 78% absolute bioavailability, while its active intermediate metabolite, albendazole sulfoxide, achieved 43% absolute bioavailability. The daily dosing of 681 mg/kg M1 complexed with 3.2 mg/kg of ABZ for 14 days did not result in significant weight loss or pathology in Swiss Webster mice. In vivo efficacy studies using this M1·ABZ inclusion complex showed significant decreases in tumor growth rates and increases in survival of mice bearing SK-OV-3 xenograft tumors. In conclusion, we provide substantial new evidence demonstrating that M1 is a safe and efficient excipient that enables in vivo parenteral delivery of poorly water-soluble APIs.

Acyclic cucurbit[n]uril-type molecular containers: influence of aromatic walls on their function as solubilizing excipients for insoluble drugs

We studied the influence of the aromatic sidewalls on the ability of acyclic CB[n]-type molecular containers (1a–1e) to act as solubilizing agents for 19 insoluble drugs including the developmental anticancer agent PBS-1086. All five containers exhibit good water solubility and weak self-association (K_s ≤ 624 M^–1). We constructed phase solubility diagrams to extract K_rel and K_a values for the container·drug complexes. The acyclic CB[n]-type containers generally display significantly higher K_a values than HP-β-CD toward drugs. Containers 1a–1e bind the steroidal ring system and aromatic moieties of insoluble drugs. Compound 1b displays highest affinity toward most of the drugs studied. Containers 1a and 1b are broadly applicable and can be used to formulate a wider variety of insoluble drugs than was previously possible with cyclodextrin technology. For drugs that are solubilized by both HP-β-CD and 1a–1e, lower concentrations of 1a–1e are required to achieve identical [drug].

Synthesis of a Disulfonated Derivative of Cucurbit[7]uril and Investigations of its Ability to Solubilize Insoluble Drugs

Cucurbit[7]uril (CB[7]) is currently being investigated as a solubilizing agent for insoluble drugs. We recently found that acyclic CB[n]-type receptors that bear sulfonate solubilizing groups are well suited for this application. Herein, we report cucurbit[7]uril derivative (1) that bears two sulfonate groups on its convex face that we hypothesized would be a superior solubilizing excipient for insoluble drugs. Before using 1 for drug solubilization experiments we showed that 1 does not self-associate and that it retained its ability to bind to diammonium compounds as common guests for CB[7] sized cavities. X-ray crystallography shows that 1 maintains the key structural features of CB[7] with only minor ellipsoidal deformations at the equator and carbonyl portals of 1. Unfortunately, the aqueous solubility of 1 (20 mM) is slightly lower than CB[7] (20-30 mM) which limits its potential as a solubilizing excipient for insoluble drugs. We created phase solubility diagrams for the solubilization of three drugs (camptothecin, albendazole, cinnarizine) with two different containers (1 and CB[7]). CB[7] and 1 exhibit comparable solubilization abilities (e.g. K_a and maximum solubility) toward camptothecin and albendazole but 1 is an inferior solubilizing agent for cinnarizine because of the low solubility exhibited by the 1•cinnarizine complex.

Acyclic CB[n]-type molecular containers: effect of solubilizing group on their function as solubilizing excipients

We report the synthesis and X-ray crystal structures of three acyclic CB[n]-type molecular containers (2a, 2h, 2f) that differ in the charge on their solubilizing groups (SO3(−), OH, NH3(+)). The X-ray crystal structures of compounds 2h and 2f reveal a self-folding of the ArOCH2CH2X wall into the cavity driven by π–π interactions, H-bonds and ion–dipole interactions. The need to reverse this self-folding phenomenon upon guest binding decreases the affinity of 2h and 2f toward cationic guests in water relative to 2a as revealed by direct (1)H NMR and UV/Vis titrations as well as UV/Vis competition experiments. We determined the pKa of 6-aminocoumarin 7 (pKa = 3.6) on its own and in the presence anionic, neutral, and cationic hosts (2a: pKa = 4.9; 2h: pKa = 4.1; 2f, pKa = 3.4) which reflect in part the relevance of direct ion–ion interactions between the arms of the host and the guest toward the recognition properties of acyclic CB[n]-type containers. Finally, we showed that the weaker binding affinities measured for neutral and positively charged hosts 2h and 2f compared to anionic 2a results in a decreased ability to act as solubilizing agents for either cationic (tamoxifen), neutral (17α-ethynylestradiol), or anionic (indomethacin) drugs in water. The results establish that acyclic CB[n] compounds that bear anionic solubilizing groups are most suitable for development as general purpose solubilizing excipients for insoluble pharmaceutical agents.

Variational Implicit Solvation with Poisson-Boltzmann Theory

We incorporate the Poisson-Boltzmann (PB) theory of electrostatics into our variational implicit-solvent model (VISM) for the solvation of charged molecules in an aqueous solvent. In order to numerically relax the VISM free-energy functional by our level-set method, we develop highly accurate methods for solving the dielectric PB equation and for computing the dielectric boundary force. We also apply our VISM-PB theory to analyze the solvent potentials of mean force and the effect of charges on the hydrophobic hydration for some selected molecular systems. These include some single ions, two charged particles, two charged plates, and the host-guest system Cucurbit[7]uril and Bicyclo[2.2.2]octane. Our computational results show that VISM with PB theory can capture well the sensitive response of capillary evaporation to the charge in hydrophobic confinement and the polymodal hydration behavior and can provide accurate estimates of binding affinity of the host-guest system. We finally discuss several issues for further improvement of VISM.

Topical cream-based dosage forms of the macrocyclic drug delivery vehicle cucurbit[6]uril

The macrocycle family of molecules called cucurbit[n]urils are potential drug delivery vehicles as they are able to form host-guest complexes with many different classes of drugs. This study aimed to examine the utility of Cucurbit[6]uril (CB[6]) in topical cream-based formulations for either localised treatment or for transdermal delivery. Cucurbit[6]uril was formulated into both buffered cream aqueous- and oily cream-based dosage forms. The solid state interaction of CB[6] with other excipients was studied by differential scanning calorimetry and the macrocycle's transdermal permeability was determined using rat skin. Significant solid state interactions were observed between CB[6] and the other dosage form excipients. At concentrations up to 32% w/w the buffered aqueous cream maintained its normal consistency and could be effectively applied to skin, but the oily cream was too stiff and is not suitable as a dosage form. Cucurbit[6]uril does not permeate through skin; as such, the results imply that cucurbituril-based topical creams may potentially only have applications for localised skin treatment and not for transdermal drug delivery.

The delivery of triamterene by cucurbit[7]uril: synthesis, structures and pharmacokinetics study

In recent years, cucurbit[7]uril (CB[7]) has attracted great attention in drug delivery. Though the effect of CB[7] in enhancing the solubility of water insoluble drugs has been validated, the underlying mechanism remains poorly understood, particularly at a molecular level. This study is designed to evaluate a CB[7]-based pharmaceutical formulation to improve solubility and bioavailability of triamterene (a mild potassium-sparing diuretic). Two polymorphs of triamterene@CB[7] were obtained, and their crystal structures were determined by single crystal X-ray diffraction. The CB[7] molecule forms a stable host-guest complex with triamterene (Ka = 1.69 ± 0.34 × 10(4) M(-1)) in aqueous solution (pH = 1.0). The results of dissolution study demonstrate that the apparent solubility value of triamterene@CB[7] complex in 0.1 M HCl is 1.6 times as large as that of triamterene, while free triamterene was released from triamterene@CB[7] complex in phosphate buffer of pH 6.8. Pharmacokinetic studies in rats reveal that the AUC0-∞ value of triamterene@CB[7] complex increases 2.8-fold compared with that of free triamterene, and t1/2 is prolonged from 1.42 to 2.61 h (P < 0.05) after oral administration. The increased solubility and oral bioavailability are attributed to the formation of a hydrophilic capsule composed of two CB[7] molecules, in which two insoluble triamterene molecules are encapsulated. These results demonstrate that triamterene@CB[7] complex is a stable and effective pharmaceutical formulation.