The impact of guest compounds on cyclodextrin aggregation behavior: A series of structurally related parabens

Cyclodextrins: Structural, Chemical, and Physical Properties, and Applications

Due to their unique structural, physical and chemical properties, cyclodextrins and their derivatives have been of great interest to scientists and researchers in both academia and industry for over a century. Many of the industrial applications of cyclodextrins have arisen from their ability to encapsulate, either partially or fully, other molecules, especially organic compounds. Cyclodextrins are non-toxic oligopolymers of glucose that help to increase the solubility of organic compounds with poor aqueous solubility, can mask odors from foul-smelling compounds, and have been widely studied in the area of drug delivery. In this review, we explore the structural and chemical properties of cyclodextrins that give rise to this encapsulation (i.e., the formation of inclusion complexes) ability. This review is unique from others written on this subject because it provides powerful insights into factors that affect cyclodextrin encapsulation. It also examines these insights in great detail. Later, we provide an overview of some industrial applications of cyclodextrins, while emphasizing the role of encapsulation in these applications. We strongly believe that cyclodextrins will continue to garner interest from scientists for many years to come, and that novel applications of cyclodextrins have yet to be discovered.

Aggregation versus inclusion complexes to solubilize drugs with cyclodextrins. A case study using sulphobutylether-β-cyclodextrins and remdesivir

The formation of small hybrid aggregates between excipient and drug molecules is one of the mechanisms that contributes to the solubilization of active principles in pharmaceutical formulations. The characterization of the formation, governing interactions and structure of such entities is not trivial since they are highly flexible and dynamic, quickly exchanging molecules from one to another. In the case of cyclodextrins, this mechanism and the formation of inclusion complexes synergistically cooperate to favour the bioavailability of drugs. In a previous study we reported a detailed characterization of the possible formation of inclusion complexes with 1:1 stoichiometry between remdesivir, the only antiviral medication currently approved by the United States Food and Drug Administration for treating COVID-19, and sulphobutylether-β-cyclodextrins. Here we extend our study to assess the role of the spontaneous aggregation in the solubilization of the same drug, by molecular dynamics simulations at different relative concentrations of both compounds. The number of sulphobutylether substitutions in the cyclodextrin structure and two different protonation states of the remdesivir molecule are considered. We aim to shed light in the solubilization mechanism of sulphobutylether-β-cyclodextrins, broadly used as an excipient in many pharmaceutical formulations, in particular in the case of remdesivir as an active compound.

Aggregate Determination by Permeation Technique

Permeation technique is used to study molecular aggregation in aqueous solutions including formation of cyclodextrin guest/host aggregates. Since only guest molecules, host molecules and guest/host aggregates that are smaller than the pore size of a given semipermeable membrane are able to permeate through the membrane, negative deviation of permeation profiles indicates formation of guest/host aggregates or self-aggregates. This chapter describes how the method is used to detect formation of nano-sized aggregates and to determine the critical aggregation concentration (cac) from permeation profiles of a guest molecule.

Self-association of cyclodextrins and cyclodextrin complexes in aqueous solutions

Cyclodextrins (CDs) are oligosaccharides that self-assemble in aqueous solutions to form transient clusters, nanoparticles and small microparticles. The critical aggregation concentration (cac) of the natural αCD, βCD and γCD in pure aqueous solutions was estimated to be 25, 8 and 9 mg/ml, respectively. The cac of 2-hydroxypropyl-β-cyclodextrin (HPβCD), that consists of mixture of isomers, was estimated to be significantly higher or 118 mg/ml. Addition of chaotropic agents (i.e. that disrupts non-covalent bonds such as hydrogen bonds) to the aqueous media increases the cac. Formation of drug/CD complexes can increase or decrease the cac. Due to the transient nature of the CD clusters and nanoparticles they can be difficult to detect and their presence is frequently ignored. However, they have profound effect on the physiochemical properties of CDs and their pharmaceutical applications. For example, the values of stability constants of drug/CD complexes can be both concentration dependent and method dependent. Like in the case of micelles water-soluble polymers can enhance the solubilizing effect of CDs. Also, formation of drug/CD complex nanoparticles appears to increase the ability of CDs to enhance drug delivery through some mucosal membranes.

Interaction of Native Cyclodextrins and Their Hydroxypropylated Derivatives with Carbamazepine in Aqueous Solution. Evaluation of Inclusion Complexes and Aggregates Formation

A detailed comprehensive study on how the formation of soluble and insoluble carbamazepine/cyclodextrins (CBZ/CD) complexes (with consequent changes in the solid-phase composition) depends on the CD structure is not yet available. Moreover, the study of possible influence of this drug on the tendency of CDs and their complexes to self-aggregate is still lacking. Phase-solubility studies demonstrated that CDs and CBZ form a range of soluble (A_L-type: αCD, βCD, and hydroxypropylated CDs) and insoluble (B_S-type: γCD) complexes depending on CD used. HPβCD proved to be the best CD solubilizer for CBZ forming the most stable complex with highest apparent solubility, whereas γCD was shown to be the best native CD. For the native CDs, CBZ solubilization increases with increasing CD cavity diameter (αCD ≪ βCD < γCD). Solid phases collected from phase-solubility studies were characterized by Fourier-transformed infrared spectroscopy, differential scanning calorimetry, and X-ray powder diffraction to elucidate their composition and crystalline structure. They provided similar conclusions being overall supportive of phase-solubility, osmolality, and permeation studies results. Solid CBZ was the only detected component for A_L-type profiles over the CD concentration range studied, whereas precipitation of poorly soluble CBZ/γCD complexes (B_S-type) was observed (i.e., at and beyond plateau region). Osmometry and permeation studies were applied to evaluate the effect of CBZ on the aggregate formation and also to elucidate their influence on CD complex solubility and permeation profile. Permeation method was shown to be the most effective method to detect and evaluate aggregate formation in aqueous γCD and HPβCD solutions containing CBZ. CBZ did not affect the HPβCD tendency to self-aggregate but CBZ did modify the aggregation behavior of γCD decreasing the apparent critical aggregation concentration value from 4.2% (w/v) (in pure aqueous γCD solution) to 2.5% (w/v) (when CBZ was present).

Rose Bengal incorporated to α-cyclodextrin microparticles for photodynamic therapy against the cariogenic microorganism Streptococcus mutans

Rose Bengal@α-cyclodextrin (RB@α-CD) microparticles (μPs) were prepared and the RB inclusion in α-CD was experimentally demonstrated through infrared, UV-VIS absorption spectroscopy and cyclic voltammetry. The RB inclusion in α-CD was theoretically investigated using classical molecular mechanics calculations, the simulation results showing that RB can be included in both the narrow and wide apertures of the α-cyclodextrin ring with configurations exhibiting average binding energies of about 27 kcal mol^-1. The prepared RB@α-CD microparticles were characterized through Scanning Electron Microscopy (SEM) and it was demonstrated that they are highly efficient in the photodynamic therapy against a Streptococcus mutans (the main bacteria of cariogenic dental plaque) suspension, as a concentration of RB@α-CD μPs 10 times smaller than the usual concentration of pure RB is still capable to produce significant antibacterial activity.

2-Hydroxypropyl-β-Cyclodextrin Aggregates: Identification and Development of Analytical Techniques

It is extremely important for pharmaceutical formulators to have analytical methodology that provides efficient detection and quantification of HPβCD aggregates. Five different methods were then evaluated for their potential to detect these aggregates and to determine critical aggregation concentration (cac): osmometry, viscometry, tensiometry, dynamic light scattering (DLS), and permeability studies. Overall, tensiometry was an inadequate method with which to study HPβCD aggregation, since the addition of HPβCD to water resulted in only minor changes in surface tension. Osmolality and viscosity studies have shown that for HPβCD, solute⁻solvent interactions are the main contributors for the observed deviation from ideality. These deviations might be related to the presence of aggregates. The DLS method proved to be an effective method with which to detect HPβCD aggregates and estimate their hydrodynamic diameter, although it presented some limitations concerning their quantification. In terms of the assessed methods, permeation studies were shown to be the best to study HPβCD aggregation phenomena, since they were the only method where the detection of aggregates and the determination of apparent cac values was possible. Also, it was the least invasive for the HPβCD samples and the method that provided more conclusive data. Results suggested that HPβCD, as expected, has less tendency to form aggregates than βCD.

Optimization of β-cyclodextrin consolidated micellar dispersion for promoting the transcorneal permeation of a practically insoluble drug

Development of efficient ocular drug delivery system for antifungal drugs becomes a must nowadays to face and eradicate the widely spread ophthalmic fungal infections. Itraconazole, a triazole antifungal, is struggling to penetrate the cornea and subsequently, its efficacy is limited. The aim of this study was to enhance itraconazole corneal penetration through utilizing the minimum surfactant amount in presence of β-cyclodextrin which acted as a dissolution and permeation enhancer. β-Cyclodextrin consolidated micellar dispersions (CCMD) were prepared after an initial screening to select the composition of surfactant(s). The preparation was done according to a modified melt dispersion technique. The prepared CCMD were characterized through the analysis of their particle size, zeta potential and solubilization efficiency. The optimum formula was chosen based on a factorial response surface analysis and it was composed of 17:1 w/w surfactant/drug, 30:1 w/w cyclodextrin/drug ratios and 0.02% polyethylene oxide. This formula was subjected to in vitro characterization including release, imaging by transmission electron microscope, mucoadhesion, stability, in addition to the determination of the minimum inhibitory concentration. Moreover, the ex vivo/in vivo permeation, safety and efficacy profiles were determined. The optimized CCMD formula was found to be significantly safe, stable, mucoadhesive and efficient to permeate the drug through rabbits' corneas. Consequently, the optimized CCMD formulation can be a promising, safe and efficient platform for the transcorneal delivery of lipophilic drugs including most antifungals.

S-protected thiolated cyclodextrins as mucoadhesive oligomers for drug delivery

AIM

The purpose of this study was to develop a novel mucoadhesive thiolated and S-protected gamma cyclodextrin (γ-CD) with an intact ring backbone to assure a prolonged residence time at specific target sites.

METHOD

Thiolated γ-CD was generated through bromine substitution of its hydroxyl groups followed by replacement to thiol groups using thiourea. In the second step, thiol groups were protected by disulfide bond formation with 2-mercaptonicotinic acid (2-MNA).

RESULT

Thiolated γ-CD displayed 1385 ± 84 µmol thiol groups per gram of oligomer and the amount of MNA determined in the S-protected oligomer was 1153 ± 41 µmol per gram of oligomer. In-vitro screening of mucoadhesive properties of thiolated and S-protected γ-CD was done by two methods. Rheological investigation revealed the conjugates non-mucolytic with only a slight increase in viscosity of thiolated and S-protected γ-CD as compared to unmodified γ-CD, whereas mucoadhesive properties of the new thiolated and S-protected γ-CD performed on freshly excised porcine intestinal mucosa showed 44.4- and 50.9-fold improvement in mucoadhesion, respectively. The new conjugates did not show any cytotoxicity to Caco-2 cells even at a concentration of 1% (m/v) for 24 h. In addition, in-vitro studies of α-amylase degradation of γ-CD, γ-CD-SH and γ-CD-SS-MNA confirmed that all conjugates are biodegradable.

CONCLUSION

These outcomes predict that these new conjugates of γ-CD might provide a new favorable tool for drug delivery providing a prolonged residence time on mucosal surfaces.

Low density, good flowability cyclodextrin-raffinose binary carrier for dry powder inhaler: anti-hygroscopicity and aerosolization performance enhancement

BACKGROUND

The hygroscopicity of raffinose carrier for dry powder inhaler (DPI) was the main obstacle for its further application. Hygroscopicity-induced agglomeration would cause deterioration of aerosolization performance of raffinose, undermining the delivery efficiency.

METHODS

Cyclodextrin-raffinose binary carriers (CRBCs) were produced by spray-drying so as to surmount the above issue. Physicochemical attributes and formation mechanism of CRBCs were explored in detail. The flow property of CRBCs was examined by FT4 Powder Rheometer. Hygroscopicity of CRBCs was elucidated by dynamic vapor sorption study. Aerosolization performance was evaluated by in vitro deposition profile and in vivo pharmacokinetic profile of CRBC based DPI formulations.

RESULTS

The optimal formulation of CRBC (R₄) was proven to possess anti-hygroscopicity and aerosolization performance enhancement properties. Concisely, the moisture uptake of R₄ was c.a. 5% which was far lower than spray-dried raffinose (R₀, c.a. 65%). R₄ exhibited a high fine particle fraction value of 70.56 ± 0.61% and it was 3.75-fold against R₀. The pulmonary and plasmatic bioavailability of R₄ were significantly higher than R₀ (p < 0.05).

CONCLUSION

CRBC with anti-hygroscopicity and aerosolization performance enhancement properties was a promising approach for pulmonary drug delivery, which could provide new possibilities to the application of hygroscopic carriers for DPI.

The effect of 2-hydroxypropyl-β-cyclodextrin on the intestinal permeation through mucus

In addition to its important role in preventing the interaction of toxic agents with the intestinal lining, the intestinal mucus layer can impede the permeation of drugs. The purpose of this study was to evaluate whether the presence of HP-β-CD in the intraluminal environment could influence the permeation of drugs through a layer of mucus. To this end, a new artificial membrane insert system incorporating a fixed mucus layer was developed to monitor the permeation of methylparaben (log P=1.96) and heptylparaben (log P=4.83). While the transport of methylparaben remained unaffected by the mucus layer, the transport of heptylparaben was significantly impeded by the mucus layer. In presence of relatively low concentrations of HP-β-CD, however, this negative effect of mucus on the permeation of heptylparaben disappeared. Importantly, the impact of the mucus layer was found to depend on the composition of the solvent system used. The colloidal structures present in simulated intestinal media were able to neutralize the impeding effect of mucus on heptylparaben permeation observed when using simple phosphate buffers. These findings advocate the use of biorelevant media when studying the impact of the mucus layer on drug permeation.