Effect of cucurbit[n]urils on tropicamide and potential application in ocular drug delivery

Host-guest complexation of cucurbit[7]uril and cucurbit[8]uril with the antimuscarinic drugs tropicamide and atropine

Cucurbiturils are useful excipients in eye drop formulations: they can increase the water solubility of the drug, enhance drug absorption into the eye, improve aqueous stability and reduce local irritation. Effective and safe drug delivery is, however, a challenge and the information on the host (CBs)/guest (tropicamide and atropine) interactions can help improving the existing treatments and develop novel therapies not limited only to eye diseases/conditions. Since this carrier system can easily modify the properties of the drug and ensure its delivery at the targeted ocular tissue, further insight into the intimate mechanism of the host-guest recognition is crucial. The present DFT/SMD study focuses on the role of numerous factors governing this process, namely the specific position of the guest molecule in the cavity of the cucurbituril, the ionization form (non/protonated) of the antimuscarinic drug, the dielectric constant of the medium, and the size of the cavitant pore. The obtained results are in line with experimental observations and shed light on the mechanism, at atomic resolution, of recognition between the CBs and the two parasympatholytic drugs.

Cucurbituril-assisted sensitive fluorescence detection and quantitation of naproxen drug in wastewater samples: Guest-host characterization and HPLC investigation

Sensitive spectrofluorometric and liquid chromatography with fluorescence detection methods have been developed for detection and determination of naproxen drug in the presence of cucurbit7uril (CB7). Fluorescence signals have been improved with the addition of CB7 to the drug aqueous solution. Fluorescence spectroscopy, mass spectrometry, ¹H-NMR, and liquid chromatography with fluorescence detection were used to investigate the guest-host interaction of naproxen drug and cucurbiturils. Naproxen was found to form a supramolecular complex with CB7 that had a high formation constant. The optimal conditions for the interaction were discovered using spectroflurometry to be 0.2 mg/ml of CB7, 2.4 μg/ml of naproxen drug, and pH10. A 1:1 complex between naproxen and CB7 is revealed by proton NMR and tandem mass spectrometry. Using the standard addition calibration method, an HPLC with a fluorescence detector was used to detect naproxen in influent and effluent wastewater samples. Finally, it was discovered that the measured concentrations of naproxen in the influent and the effluent wastewater were 1.87 × 10^-4 ppb and 2.1 × 10^-5 ppb, respectively. This was done by sample enrichment, which reduced the 1000 mL into 1 ml.

Supramolecular Encapsulation of a Neurotransmitter Serotonin by Cucurbit[7]uril

pH-dependent host-guest complexation of a monoamine neurotransmitter, Serotonin, with cucurbit[7]uril has been thoroughly investigated. The binding phenomena were explored using steady-state and time-resolved fluorescence spectroscopy at different pH values. At lower pH, i.e., protonated Serotonin, the binding affinity with cucurbit[7]uril was significantly higher compared to higher pH. Furthermore, detailed NMR titration experiments depicted the solution structure of the host-guest complex through the complexation induced chemical shift values. A competitive binding assay with cesium ions at pD 2.8 was subsequently performed for the further manifestation of the binding. Finally, the molecular docking studies provided well-documented proof of the 1:1 inclusion complex and the geometry of the complex. We believe that understanding from such studies can be important for pH-controlled delivery of serotonin for biological applications.

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.

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.

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).

A naphthalimide derived fluorescent sensor for solid-phase screening of cucurbit[7]uril-guest interactions

A naphthalimide based fluorescent sensor displaying a significant increase in emission upon binding CB[7] with notable pH stability was developed and utilized in a surface-bound displacement assay for the rapid detection of CB[7] encapsulation of therapeutically relevant drug classes. Previously unknown binders with moderate to strong affinities were discovered.

Cucurbiturils: from synthesis to high-affinity binding and catalysis

Major developments in the synthesis of cucurbiturils and applications related to their high-affinity binding and catalysis have recently taken place.

Modulation of pKa by cyclodextrins; subtle structural changes induce spectacularly different behaviors

Supramolecular acidity tuning: it is shown that minor structural changes of guests and hosts molecules may be of great importance for the complexation-induced pK_a shifts and their temperature dependency.

Enthalpy–entropy relations in the acid–base equilibrium of warfarin and 10-hydroxywarfarin; joint experimental and theoretical studies

Warfarin and 10-hydroxywarfarin are structurally similar molecules, however, they exhibit considerably different thermodynamics of acid dissociation. Intramolecular H-bonds and solvent composition are the factors of great importance.

Protocols utilizing constant pH molecular dynamics to compute pH-dependent binding free energies

In protein–ligand binding, the electrostatic environments of the two binding partners may vary significantly in bound and unbound states, which may lead to protonation changes upon binding. In cases where ligand binding results in a net uptake or release of protons, the free energy of binding is pH-dependent. Nevertheless, conventional free energy calculations and molecular docking protocols typically do not rigorously account for changes in protonation that may occur upon ligand binding. To address these shortcomings, we present a simple methodology based on Wyman’s binding polynomial formalism to account for the pH dependence of binding free energies and demonstrate its use on cucurbit[7]uril (CB[7]) host–guest systems. Using constant pH molecular dynamics and a reference binding free energy that is taken either from experiment or from thermodynamic integration computations, the pH-dependent binding free energy is determined. This computational protocol accurately captures the large pK_a shifts observed experimentally upon CB[7]:guest association and reproduces experimental binding free energies at different levels of pH. We show that incorrect assignment of fixed protonation states in free energy computations can give errors of >2 kcal/mol in these host–guest systems. Use of the methods presented here avoids such errors, thus suggesting their utility in computing proton-linked binding free energies for protein–ligand complexes.

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.

Intermolecular interactions between cucurbit[7]uril and pilocarpine

The interactions between cucurbit[7]uril (CB7) macrocycles and pilocarpine (PIL) were investigated in aqueous solution by using (1)H NMR and circular dichroism (CD) spectroscopic techniques. The characterizations of the freeze-drying solid complex were conducted by electrospray ionization mass spectroscopy (ESI-MS), Fourier transform-infrared spectroscopy (FT-IR), thermogravimetry, and differential scanning calorimetry (DSC) techniques. The DSC and thermogravimetry confirmed the production of a thermally stable solid complex. The NMR, CD and ESI-MS measurements confirmed asymmetric induction during the complexation reaction, in which the γ-lactone ring of PIL (not the imidazole nucleus) has been fully encapsulated within the cavity of CB7. The stability of the drug has significantly enhanced as evidenced by the high-performance liquid chromatographic (HPLC) method. The results are discussed in the context of utilizing non-conventional supramolecular host-guest approaches to enhance the chemical stability in aqueous media of hydrophilic PIL drugs as model compounds. The non-classical stereospecific interactions between CB7 and PIL drugs are also highlighted.

The strategic use of supramolecular pK(a) shifts to enhance the bioavailability of drugs

Macrocyclic hosts of the cyclodextrin, sulfonatocalixarene, and cucurbituril type can be employed as discrete supramolecular drug delivery systems, thereby complementing existing supramolecular drug formulation strategies based on polymers, hydrogels, liposomes, and related microheterogeneous systems. Cucurbiturils, in particular, stand out in that they do not only provide a hydrophobic cavity to encapsulate the drug in the form of a host-guest complex, but in that they possess cation-receptor properties, which favor the encapsulation of protonated drugs over their unprotonated forms, resulting in pronounced pK(a) shifts up to 5 units. These pK(a) shifts can be rationally exploited to activate prodrug molecules, to stabilize the active form of drug molecules, to enhance their solubility, and to increase their degree of ionization, factors which can jointly serve to enhance the bioavailability of drugs, particularly weakly basic ones. Additionally, macrocycles can serve to increase the chemical stability of drugs by protecting them against reactions with nucleophiles (e.g., thiols) and electrophiles, by increasing their photostability, and by causing a higher thermal stability in the solid state. Detailed examples of the different effects of macrocyclic encapsulation of drugs and the associated pK(a) shifts are provided and discussed. Other important considerations, namely a potential lowering of the bioactivity of drugs by macrocyclic complexation, interferences of the macrocycles with biocatalytic processes, the toxicity of the macrocyclic host molecules, and problems and opportunities related to a targeted release and the rate of release of the drug from the host-guest complexes are critically evaluated.