Effect of self-aggregation of γ-cyclodextrin on drug solubilization

On the formulation of disulfide herbicides based on aminophenoxazinones: polymeric nanoparticle formulation and cyclodextrin complexation to combat crop yield losses

BACKGROUND

The resistance of weeds to herbicides is a significant issue in ensuring future food supply. Specific examples are Plantago lanceolata, Portulaca oleracea and Lolium rigidum, which mainly infect rice, wheat, barley and pastures, and cause high yield losses every year. In this regard, natural products and their mimics have provided new hope as a result of their different modes-of-action, activity at low concentrations and reduced pollution effects relative to conventional herbicides. However, the poor water solubility and physicochemical properties of these compounds limit their broad application. These problems can be addressed by formulation techniques, and encapsulation appears to be of great interest.

RESULTS

Disulfide herbicides inspired by aminophenoxazinones have been formulated with 2-hydroxypropyl-β-cyclodextrin (HP-β-CD), γ-CD and polymeric nanoparticles (NPs). In silico studies were employed to identify which complexes would be generated and complex formation was confirmed by nuclear magnetic resonance spectroscopy. Solubility diagrams were generated to assess any improvement in water solubility, which was enhanced 2-13-fold. Scanning electron microscopy and energy-dispersive X-ray spectra confirmed the success of the formulation process for the nanoparticles. Formulated compounds were evaluated in an in vitro wheat coleoptile bioassay, with almost 100% elongation inhibition achieved using only water for the bioassay. Specific in vitro testing on weed phytotoxicity showed that the application of core/shell NPs is highly effective in the fight against P. lanceolata seed germination.

CONCLUSIONS

The formulation of disulfide herbicides with CD complexes and NPs led to an enhancement in water solubility and bioactivity. These systems can be applied in pre-emergent mode against P. lanceolata, using only water to prepare the sample, and they showed better activity than the positive controls. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Application of γ-cyclodextrin-lysozyme as host materials for encapsulation of curcumin: characterization, stability, and controlled release properties

BACKGROUND

In this study, a safe and relatively stable γ-cyclodextrin-lysozyme (γ-CD-Lys) was synthesized using epichlorohydrin as the cross-linking agent, and curcumin was successfully encapsulated in γ-CD-Lys.

RESULTS

The successful Lys grafting onto γ-CD can be demonstrated by a high grafting ratio (79.02%) and was further confirmed by Fourier transform infrared (FTIR) band shifts and the new signal obtained at δ 2.75 in proton nuclear magnetic resonance. The encapsulation efficiency value of γ-CD-Lys was 76.74%, and the successful encapsulation of curcumin into γ-CD-Lys was confirmed by crystal structure change, increased melting point, and FTIR band shifts. The intermolecular bonds results suggested that associative forces between curcumin and γ-CD-Lys were electrostatic interaction, hydrogen bonds interaction, and hydrophobic interaction. The designed nanoparticles had excellent stability at low pH and low salt concentration. The release rate of these nanoparticles was inhibited in simulated gastric conditions, whereas it increased significantly in intestinal media. Simulated gastrointestinal digestion experiments further confirmed that nanoparticles showed higher bioaccessibility (86.05%) compared with curcumin (58.82%).

CONCLUSION

Overall, our study showed that the nanoparticles were highly promising for delivering curcumin because of their enhanced functional attributes and stabilization in acid or low salt environments. Also, it was an excellent wall material for targeting hydrophobic bioactive compounds in the intestinal tract via oral administration. © 2022 Society of Chemical Industry.

In silico formulation prediction of drug/cyclodextrin/polymer ternary complexes by machine learning and molecular modeling techniques

Ternary cyclodextrin (CD) complexes (drug/CD/polymer) can effectively improve the solubility of water-insoluble drugs with large size than binary CD formulations. However, ternary formulations are screened by a trial-and-error approach, which is laborious and material-wasting. Current research aims to develop a prediction model for ternary CD formulations by combined machine learning and molecular modeling. 596 ternary formulations data were collected to build a prediction model by machine learning. The random forest model achieved good performance with R² = 0.887 in S_T prediction and R² = 0.815 in S_T/S_B prediction. Two ternary formulations (Hydrocortisone/β-CD/HPMC and dovitinib/γ-CD/CMC) were used to validate the prediction model. Molecular modeling results showed that HPMC not only warped around hydrocortisone but also prevented CD molecules from self-aggregation to increase solubility. In conclusion, a prediction model for the ternary CD formulations was successfully developed, which will significantly accelerate the formulation screening process to benefit the formulation development of water-insoluble drugs.

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.

Cyclodextrin-based delivery systems for in vivo-tested anticancer therapies

Cyclodextrins (CDs) are naturally occurring macromolecules widely used as excipients on pharmaceutical formulations, evidencing a large spectrum of applications in the pharmaceutical industry. Their unique ability to act as molecular containers by entrapping a wide range of guest molecules in their internal cavity makes them a remarkable excipient to improve drug apparent solubility, stability, and bioavailability, and a valuable tool for the assembly of new drug delivery systems. These features are especially useful when it comes to chemotherapy, as most of the anticancer drugs present both low permeability and reduced water solubility. Therefore, guest-host inclusion complexes offer several potential advantages not only regarding the improvement of pharmaceutical formulations characteristics but also considering the reduction of drug toxic side effects. The combination of CDs with additional technologies and materials constitutes a potential strategy towards the development of advanced and multifunctional CD-based delivery systems. Paclitaxel, curcumin, camptothecin, doxorubicin, and cisplatin are among the most studied molecules with anticancer activities and have been successfully incorporated in such nanosystems. Exciting results using CDs and CD-based delivery systems have been obtained so far, paving the way towards the attainment of intelligent delivery systems to possibly address cancer therapeutics' unmet needs. In this review, a comprehensive exposition concerning in vivo-tested CD and CD-based delivery systems for anticancer therapy is undertaken. Additionally, the authors address the multivalent functionalities of CD-based delivery systems, namely the incorporation of active target ligands, stimuli-responsiveness components, surface functionalization, or further associations with other delivery systems, aiming at improved in vivo anticancer therapies. Graphical abstract.

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).

Drug Solubilization by Mixtures of Cyclodextrins: Additive and Synergistic Effects

Cyclodextrins are popular drug solubilizers, but the use of the natural cyclodextrins is hampered by their tendency to coprecipitate with the drug. To understand and overcome such problems, we have studied the solubility of dexamethasone in the presence of natural β-cyclodextrin and γ-cyclodextrin, individually and in various combinations. Equilibrium models of the phase-solubility diagrams with individual cyclodextrins revealed that dexamethasone was solubilized as 1:1 complexes, but formation of insoluble higher-order complexes set an upper limit to the concentration of solubilized dexamethasone. This limit could be raised from 8 to 17 mM by using combinations of the two cyclodextrins, as their solubilizing properties were additive in some regions of the phase-solubility diagram and synergistic in other regions. The additive effects arise from the additivity of solubilities-the same phenomenon contributes to the good solubilizing properties of many modified cyclodextrins. The synergistic effects, however, could not be explained. The results open up for an increased use of the natural cyclodextrins as an improved alternative to modified cyclodextrins.

Sliding Crosslinked Thermoresponsive Materials: Polypseudorotaxanes Made of Poly(N-Isopropylacrylamide) and Acrylamide-γ-Cyclodextrin

Novel polypseudorotaxanes (PPR) based on poly(N-isopropylacrylamide) (PNIPAAm) and acrylamide-γ-cyclodextrin (AγCD) are successfully synthesized. AγCD gives rise to sliding crosslinking systems and influences the thermoresponsive and swelling behavior of PNIPAAm hydrogels. Namely, their lower critical solution temperature (LCST) can be tuned up to 38°C, thus making the resulting materials of great interest in biomedical applications. Also, AγCD influences the thermal and mechanical properties of hydrogels, by affecting the T g and E modulus values.

How Does PHEMA Pass through the Cavity of γ-CDs to Create Mismatched Overfit Polypseudorotaxanes?

A syndiotactic-rich PHEMA oligomer ( rr = 74%, DP = 29, PDI = 1.19) was synthesized and subsequently subjected to self-assembly with a varying amount of γ-CDs in its aqueous solution to create mismatched overfit polypseudorotaxanes (PPRs). The inclusion complexation proceeded in an obvious mismatched manner between the cavity of γ-CDs and the cross-sectional area of an incoming PHEMA chain. The 2D-NOESY NMR analysis provided direct evidence indicating that two adjacent pendant hydroxyethyl groups in PHEMA preferably adopt a curled conformation to pass through the cavity of γ-CDs, giving the PPRs characteristics of a mismatched overfit instead of a matched tight-fit crystal structure. The results suggested that the mutual adaption of pendant side chains of HEMA units with the cavity geometry of γ-CDs would play a dominant role in this unfavorable overfit inclusion complexation besides the size of γ-CDs and the stereoregularity of the PHEMA chain.

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.

Solubility of Cyclodextrins and Drug/Cyclodextrin Complexes

Cyclodextrins (CDs), a group of oligosaccharides formed by glucose units bound together in a ring, show a promising ability to form complexes with drug molecules and improve their physicochemical properties without molecular modifications. The stoichiometry of drug/CD complexes is most frequently 1:1. However, natural CDs have a tendency to self-assemble and form aggregates in aqueous media. CD aggregation can limit their solubility. Through derivative formation, it is possible to enhance their solubility and complexation capacity, but this depends on the type of substituent and degree of substitution. Formation of water-soluble drug/CD complexes can increase drug permeation through biological membranes. To maximize drug permeation the amount of added CD into pharmaceutical preparation has to be optimized. However, solubility of CDs, especially that of natural CDs, is affected by the complex formation. The presence of pharmaceutical excipients, such as water-soluble polymers, preservatives, and surfactants, can influence the solubilizing abilities of CDs, but this depends on the excipients' physicochemical properties. The competitive CD complexation of drugs and excipients has to be considered during formulation studies.

Cyclodextrins: structure, physicochemical properties and pharmaceutical applications

Since their discovery over 100 years ago cyclodextrins (CDs) have been the subject of numerous scientific publications. In 2016 alone CDs were the subject of over 2200 research articles published in peer-reviewed journals and mentioned in over 2300 patents and patent applications, many of which were on pharmaceutical applications. Natural CDs and their derivatives are used as enabling pharmaceutical excipients that enhance aqueous solubility of poorly soluble drugs, increase drug permeability through biological membranes and improve drug bioavailability. Unlike conventional penetration enhancers, their hydrophilic structure and high molecular weight prevents them from penetrate into lipophilic membranes leaving biological membranes intact. The natural CDs and some of their derivatives have monographs in pharmacopeias and are also commonly used as food additives and in toiletry products. CDs form inclusion complexes with lipophilic moieties of hydrophobic drugs. Furthermore, CDs are able to form non-inclusion complexes and self-assembled aggregates; small and large complex aggregates with micellar-like structures that can enhance drug solubility. Excipients commonly used in pharmaceutical formulations may have additive or inhibiting effect on the CD solubilization. Here various methods used to investigate CD aggregate formation are reviewed as well as techniques that are used to increase the solubilizing effects of CDs; methods that enhance the apparent intrinsic solubility of drugs and/or the complexation efficacy and decrease the amount of CD needed to develop CD-containing pharmaceutical formulations. It will be explained how too much or too little CD can hamper drug bioavailability, and the role of CDs in solid dosage forms and parenteral formulations, and examples given on how CDs can enhance drug delivery after ocular, nasal and pulmonary administration.

γ-Cyclodextrin

γ-Cyclodextrin (γCD) is a cyclic oligosaccharide formed by bacterial digestion of starch and used as solubilizing agent and stabilizer in a variety of pharmaceutical and food products. γCD is a large (molecular weight 1297Da) hydrophilic molecule that does not readily permeate biological membranes and is rapidly digested by bacteria in the gastrointestinal tract. In humans γCD is metabolized by α-amylase that is found in, for example, saliva, bile fluid and tears. Thus, bioavailability of γCD is negligible. Also, γCD is readily excreted unchanged in the urine after parenteral administration. Like other cyclodextrins, γCD can form water-soluble inclusion complexes with many poorly-soluble compounds. In comparison with the natural αCD and βCD, γCD has the largest hydrophobic cavity, highest water solubility and the most favorable toxicological profile. The focus of this review is production, physiochemical properties, pharmacokinetics, toxicity and applications of γCD and its derivatives. Also, the aggregation behavior of γCD in aqueous media is discussed.

A New Approach for Quantitative Determination of γ-Cyclodextrin in Aqueous Solutions: Application in Aggregate Determinations and Solubility in Hydrocortisone/γ-Cyclodextrin Inclusion Complex

Fast and simple high-pressure liquid chromatographic (HPLC) method with charged aerosol detector (CAD) was developed for quantitation of γ-cyclodextrin (γCD) in aqueous solutions. The chromatographic system consisted of a C18 column (i.e., the stationary phase) and an aqueous mobile phase containing 7% (v/v) methanol. Calibration curve was obtained over the γCD concentration range of 0.005%-1% (w/v). The limit of detection and quantitation of γCD were 0.0001% and 0.0002% (w/v), respectively. Formation of γCD aggregates in aqueous solution and their critical aggregation concentration (cac) were determined by both conventional dynamic light scattering method and permeation method using HPLC-CAD for quantitative determination of γCD. The cac of γCD was determined to be 0.95% (w/v) and the amount of γCD self-aggregates increased with increasing γCD concentrations. Also, the developed HPLC-CAD method was used to determine the γCD phase-solubility profile in an aqueous hydrocortisone (HC)/γCD complexation medium. The maximum concentration of dissolved γCD and HC was determined to be 1.47% and 0.31% (w/v), respectively. The membrane permeation method was shown to be a reliable method for determination of metastable γCD aggregates. The HPLC-CAD method was successfully applied for quantitative determination of γCD in aqueous solutions during permeation and phase-solubility studies.

Topical drug delivery to the eye: dorzolamide

Topically applied carbonic anhydrase inhibitors (CAIs) in eye drop solutions are commonly used to treat glaucoma. However, local eye irritation and multiple daily administrations may hamper their clinical usefulness. Aqueous eye drop formulations that improve their topical bioavailability and reduce their eye irritation can improve their clinical efficacy. Earlier studies showed that dorzolamide and closely related CAIs are more effectively delivered into the eye from acidic eye drop solutions than from comparable neutral solutions. Consequently, dorzolamide was marketed as an aqueous pH 5.6 eye drop solution (Trusopt(®) , Merck). Later, it was shown that increasing the pH of the eye drops from pH 5.6 to physiologic pH significantly reduced their local irritation. Earlier attempts to use cyclodextrins (CDs) as ocular penetration enhancers in dorzolamide eye drop solutions failed since; although the CDs were able to enhance the aqueous solubility of dorzolamide, increasing the pH from 5.6 to physiologic pH reduced the ability of the drug to permeate into the eye. Later, it was discovered that formulating the drug as aqueous dorzolamide/γCD eye drop microparticle suspension resulted in significant bioavailability enhancement. The solid dorzolamide/γCD microparticles are mucoadhesive and release dorzolamide into the aqueous tear fluid for extended time period. Consequently, sustained high dorzolamide concentrations in aqueous humour and various eye tissues were observed after single administration of the aqueous dorzolamide/γCD eye drop microsuspension. The microsuspension has a potential of being developed into a once-a-day eye drop product. This article reviews the physicochemical properties of dorzolamide, its permeation characteristics and topical bioavailability.

Molecular solubilization of fullerene C(60) in water by γ-cyclodextrin thioethers

Various hydrophilic γ-cyclodextrin (CD) thioethers, containing neutral or ionic side arms were found to form molecular disperse solutions of C(60) in water reaching concentrations of 15 mg/L. Equilibrium state was approached after seven days without the use of organic cosolvents. The 1:2 stoichiometry of the C(60)/γ-CD thioether complexes was demonstrated by a parabolic phase-solubility diagram. In contrast, native γ-CD forms nanoparticles with C(60). Particle sizes of C(60) were determined by dynamic light scattering.

Cyclodextrins as functional excipients: methods to enhance complexation efficiency

Cyclodextrins have gained currency as useful solubilizing excipients with an ever increasing list of beneficial properties and functionalities. Although their use in liquid dosage forms including oral and parenteral solutions is straightforward, their application to solids can be confounded by the added bulk that is contributed to the formulation. This factor has limited the use of cyclodextrin in tablets and relates systems mainly to potent drug substances. Increasing the ability of cyclodextrins to complex with drug through a manipulation of their complexation efficiency (CE) may expand the use of these materials to the increasing list of drug candidates and marketed drugs who may benefit from this technology. This brief review assesses tools and materials that have been suggested for increasing the CE for pharmaceutically useful cyclodextrins and drugs. The relative importance of impacting the drug solubility (S(0) ) and phase-solubility isotherm slope is discussed in the context of drug ionization and salt use; the impact of polymers, charge interactions, and charge shielding; and the coincidental formation of other complex types in the media. The influence of drug form as well as supersaturation is also discussed in the context of the responsible mechanisms along with aggregation, inclusion, and noninclusion complex formation.