Encapsulation of Vitamin B1 and Its Phosphate Derivatives by Cucurbit[7]uril: Tunability of the Binding Site and Affinity by the Presence of Phosphate Groups

Cucurbit[7]uril-Modulated H/D Exchange of α-Carbonyl Hydrogen: Deceleration in Alkali and Acceleration in Acid Conditions

Supramolecular catalysis based on host-guest interactions has aroused much attention in past decades. Among the various strategies, modulation of the reactivity of key intermediates is an effective strategy to achieve high-efficiency supramolecular catalysis. Here, we report that by utilizing the host-guest interaction of cucurbit[7]uril (CB[7]), the reactivity of anionic enolate and cationic oxonium, the intermediates of H/D exchange of the α-carbonyl hydrogen in alkali and acid conditions, respectively, could be modulated effectively. On one hand, in alkaline conditions, both the electrostatic effect and the steric hindrance effect of CB[7] disfavored formation of the enolate anion intermediate. On the other hand, in acidic conditions, the oxonium was stabilized and the solvent effect was weakened by the electrostatic effect of CB[7]. As a result, the H/D exchange of 1-(4-acetylphenyl)-N,N,N-trimethylmethanaminium bromide is decelerated in alkaline and accelerated in acidic conditions. It is promising that the highly polar portals of CB[n] molecules together with their well-defined host-guest chemistry may be applied to modulate the reactivity of other kinds of ionic intermediates in an effective and convenient way, thus enriching the toolkit of supramolecular catalysis.

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.

Supramolecular Amphiphilic Assembly Formed by the Complexation of Calixpyridinium with Alimta

In this work, the host-guest interaction between calixpyridinium and anionic anticancer drug Alimta was studied in aqueous media. Spherical supramolecular amphiphilic assembly rather than simple complex was accidentally fabricated by the complexation of calixpyridinium with Alimta. It is the third kind of anionic guest to be discovered to form the higher-order assembly by the complexation of calixpyridinium besides polyanionic guest and anionic gemini surfactant guest. The finding of this assembly approach supplies a new idea to construct various self-assembly architectures in water via the complexation of calixpyridinium with anionic drugs. The resulting calixpyridinium-drug assemblies may also have the potential to adjust the effects of drugs.

Ratiometric DNA sensing with a host-guest FRET pair

A supramolecular host-guest FRET pair based on a carboxyfluorescein-labelled cucurbit[7]uril (CB7-CF, as acceptor) and the fluorescent dye 4',6-diamidino-2-phenylindole (DAPI, as donor) is developed for sensing of DNA. In comparison to the commercial DNA staining dye SYBR Green I, the new chemosensing ensemble offers dual-emission signals, which allows a linear ratiometric response over a wide concentration range.

Supramolecular alleviation of cardiotoxicity of a small-molecule kinase inhibitor

Small-molecule kinase inhibitors (SMKIs) have been widely used in the treatment of a variety of cancers due to their clinically demonstrated efficacy. However, the use of some SMKIs, such as sorafenib (SO), has been plagued by their cardiotoxicity that has been frequently observed in treated patients. Herein we report that the encapsulation of SO by a synthetic receptor cucurbit[7]uril (CB[7]) alleviated the inherent cardiotoxicity of SO, as demonstrated in an in vivo zebrafish model. Moreover, the anti-cancer activity of SO was well preserved, upon its encapsulation by CB[7], as demonstrated by both in vitro and in vivo cancer/angiogenesis models. This discovery may provide new insights into a novel supramolecular formulation of SMKIs for the management of their side-effects.

Fluorescence enhancement and pKa shift of a rho kinase inhibitor by a synthetic receptor

Fasudil (FSD), a selective rho kinase (ROCK) inhibitor, was found to form 1 : 1 host-guest inclusion complexes with a synthetic macrocyclic receptor, cucurbit[7]uril (CB[7]), in aqueous solutions, as evidenced by ¹H NMR, photoluminescence and UV-visible spectroscopic titrations, isothermal titration calorimetry (ITC) titration, and electrospray ionization (ESI) mass spectrometry, as well as density functional theory (DFT) molecular modeling. Upon encapsulation, whereas the UV-vis absorbance of FSD experienced a moderate decrease and bathochromic shift, the fluorescence intensity of FSD at 354 nm was dramatically enhanced for up to 69-fold at neutral pH, which could potentially be applied in fluorescent tracking of the drug delivery and release. More interestingly, the binding affinity (K_a = (4.28 ± 0.21) × 10⁶ M^-1), of FSD-CB[7] complexes under acidic conditions (pH = 2.0), is approximately three orders of magnitude higher than that (2.2∼6.6 × 10³ M^-1) under neutral pH conditions (pH = 7.0). Accordingly, UV-visible spectroscopic titration of the free and complexed FSD under various pH conditions has demonstrated that the encapsulation of FSD by CB[7] shifted the pK_a of the isoquinoline-N upward from 3.05 to 5.96 (ΔpK_a of 2.91). The significantly higher binding affinity of the complexes under acidic conditions may be applied in developing the "enteric" formulation of FSD. Furthermore, our in vitro study of the bioactivity of FSD in the absence and presence of CB[7] on a neural cell line, SH-SY5Y, showed that the complexation preserved the drug's pro-neurite efficacy. Thus this discovery may lead to a fluorescence-trackable, orally administered enteric formulation of rho kinase inhibitors that are stable under gastric conditions, without compromising bioactivity of the drugs.

Sequence-Specific Self-Assembly of Positive and Negative Monomers with Cucurbit[8]uril Linkers

The self-assembly into dynamic oligomers of Cucurbit[8]uril (CB[8]), a positive ditopic Ir(III) bis-terpyridine complex, and a negative ditopic Fe(II) bis-terpyridine complex flanked by four butyrate side chains was assessed to answer a seemingly straightforward question: does CB[8] adopt a social self-sorting pattern by encapsulating both positive and negative units into a heteroternary complex? We showed that this is indeed the case, with CB[8] linking a positive Ir unit to a neighboring negative Fe unit whenever possible. Furthermore, the solubility of the dynamic oligomers was significantly affected by their sequence; upon addition of 0.6-1.2 equiv of positive Ir oligomer to its negative Fe counterpart, the predominant assembly present in solution was a mixed oligomer with a (Fe-Ir-Ir-) _n sequence. Weak interactions between the negative butyrate side chains and the partially positive outer wall of CB[7] were also identified by two-dimensional nuclear magnetic resonance techniques, and resulted in a negative p K_a shift (0.10 p K_a unit) for the terminal carboxylic groups.

Alkaline earth cation-mediated photoluminescent complexes of thioflavin T with twisted cucurbit[14]uril

The introduction of alkaline earth metal cations into the tQ[14]–ThT interaction system leds to the precipitation of solid tQ[14]/ThT/AE²⁺ interaction products, which emitted strong blue fluorescence.

Alleviating the hepatotoxicity of trazodone via supramolecular encapsulation

In order to develop a novel strategy to alleviate the inherent hepatotoxicity of antidepressant trazodone (TZ), Cucurbit[7]uril (CB[7]) was adopted as pharmaceutical excipients and was studied for its capability to reduce the hepatotoxicity of TZ via supramolecular encapsulation. CB[7] was found to form strong 1:1 host-guest complexes with TZ and its metabolite m-chlorophenyl piperazine (mCPP), with binding constants of 1.50 (±0.13) × 10⁶ M^-1 and 6.90 (±0.49) × 10⁵ M^-1, respectively. The supramolecular complexations were examined by ¹H NMR and UV-visible spectroscopic titrations, ESI-MS and ITC. In the presence of 0.5 mM CB[7], the IC₅₀ values of TZ and mCPP on a human normal liver cell line L02 were increased from 215.5 ± 3.3 μM to 544.1 ± 51.2 μM, and from 166.8 ± 3.8 μM to 241.7 ± 6.8 μM, respectively. Evaluation on a zebrafish model demonstrated that CB[7] (0.1 mM) significantly alleviated the TZ induced liver toxicity, as shown by the level of liver degeneration, liver size and yolk sac retention. Our study may provide a supramolecular strategy to alleviate the hepatotoxicity induced by TZ and its metabolite mCPP, and this strategy may be extendable to other drugs that have inherent hepatotoxicity or other adverse effects.

Concealing the taste of the Guinness World's most bitter substance by using a synthetic nanocontainer

Upon supramolecular encapsulation by using a synthetic nanocontainer (cucurbit[7]uril), the taste of the Guinness World's most bitter substance, denatonium benzoate, was dramatically concealed during a two-bottle preference drinking test in a mammalian model, demonstrating the significant potential of cucurbit[7]uril as a taste-masking nanocontainer for the first time.

Cucurbit[7]uril: an emerging candidate for pharmaceutical excipients

Cucurbit[7]uril (CB[7]), belonging to the cucurbit[n]uril family (CB[n], n = 5-8, 10, or 13-15), may form host-guest complexes with a variety of small molecules of biomedical interest. The physical and chemical properties of the complexed drugs are often improved as a result of this complexation, suggesting the potential application of CB[7] as a pharmaceutical excipient. This review has summarized the most recent research progress reported between 2011 and early 2017 regarding the biocompatibility of CB[7] and the influence of CB[7] on the stability, solubility, biouptake, and biological activities (including therapeutic efficacies and toxicities) of guest drug molecules. Through this systemic summary and analysis, we intend to stimulate further research efforts in this area and promote the use of CB[7] as an emerging pharmaceutical excipient to improve various properties of drug molecules (or active pharmaceutical ingredients).

Acyclic cucurbit[n]uril conjugated dextran for drug encapsulation and bioimaging

An acyclic cucurbit[n]uril (CB[n]) conjugated dextran is developed as a new biomaterial for drug delivery and bioimaging. This biomaterial retains the host-guest recognition properties of acyclic CB[n]s. It is able to directly encapsulate anti-tumor drugs 5-fluorouracil and temozolomide. This polymeric material can form a supramolecular assembly with polyethyleneimine (PEI). Although there is no conventional chromophore in these supramolecular systems, they exhibit aggregation-induced emission (AIE) in water with a quantum yield of 10%. This supramolecular assembly is used for bioimaging in vitro with good biocompatibility.

Inhibition and Stabilization: Cucurbituril Induced Distinct Effects on the Schiff Base Reaction

The different effects of cucurbit[7]uril (CB[7]) on the Schiff base reactions in aqueous solution were explored by ¹H NMR spectroscopy and single X-ray crystallography. With CB[7], the condensation reaction of aldehyde and primary amine is dramatically inhibited. In contrast, the presence of CB[7] does tremendously stabilize iminium cation in water through ion-dipole interactions. A single crystal structure of the complex of iminium ion 7 with CB[7] grown in water is reported.

Molecular Encapsulation of Histamine H₂-Receptor Antagonists by Cucurbit[7]Uril: An Experimental and Computational Study

The histamine H₂-receptor antagonists cimetidine, famotidine and nizatidine are individually encapsulated by macrocyclic cucurbit[7]uril (CB[7]), with binding affinities of 6.57 (±0.19) × 10³ M(-1), 1.30 (±0.27) × 10⁴ M(-1) and 1.05 (±0.33) × 10⁵ M(-1), respectively. These 1:1 host-guest inclusion complexes have been experimentally examined by ¹H-NMR, UV-visible spectroscopic titrations (including Job plots), electrospray ionization mass spectrometry (ESI-MS), and isothermal titration calorimetry (ITC), as well as theoretically by molecular dynamics (MD) computation. This study may provide important insights on the supramolecular formulation of H₂-receptor antagonist drugs for potentially enhanced stability and controlled release based on different binding strengths of these host-guest complexes.

Bioinspired Molecular Lantern: Tuning the Firefly Oxyluciferin Emission with Host-Guest Chemistry

Fireflies generate flashes of visible light via luciferase-catalyzed chemiexcitation of the substrate (luciferin) to the first excited state of the emitter (oxyluciferin). Microenvironment effects are often invoked to explain the effects of the luciferase active pocket on the emission; however, the exceedingly complex spectrochemistry and synthetic burdens have precluded elucidation of the nature of these interactions. To decipher the effects of microenvironment on the light emission, here the hydrophobic interior of cucurbit[7]uril (CB7) is used to mimic the nonpolar active pocket of luciferase. The hydrophobic interior of CB7 induces shifts of the ground-state pKas by 1.9-2.5 units to higher values. Upon sequestration, the emission maxima of neutral firefly oxyluciferin and its conjugate monodeprotonated base are blue-shifted by 40 and 39 nm, respectively, resulting in visual color changes of the emitted light.

Influence of supramolecular encapsulation of camptothecin by cucurbit[7]uril: reduced toxicity and preserved anti-cancer activity

Encapsulation of camptothecin by cucurbit[7]uril significantly inhibited the systemic toxicities of the free drug, while maintaining its antitumor/anti-angiogenic activities.