Self-assembly of cyclodextrin complexes: detection, obstacles and benefits

Cyclodextrins (CDs) are cyclic oligosaccharides that form water-soluble inclusion complexes of lipophilic molecules. They are commonly used as pharmaceutical excipients. Recently it has been observed that CDs and CD complexes 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 is about 25, 8 and 9 mg/ml, respectively. The cac of 2-hydroxypropyl-β-cyclodextrin (HPβCD), that consists of a mixture of isomers, in pure aqueous solutions is significantly higher or about 118 mg/ml. Formation of guest/ CD complexes can increase or decrease the cac value. Due to the transient nature of the CD clusters and nanoparticles they can be difficult to detect and their presence is frequently ignored. However, formation of such particulate matter in aqueous CD solutions can lead to erroneous analytical results and product rejections during drug manufacturing. On the other hand, they have also given rise to formation of novel drug delivery systems with exceptional properties.

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.

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.

Cyclodextrin complexes of a globular protein and a lipophilic oligopeptide: the effect of structure and physicochemical properties

Cyclosporine A (CyA) is a lipophilic oligopeptide that has a very limited solubility in water of only 0.008 mg/ml at ambient temperature. It has the ability to form inclusion complexes with cyclodextrin (CD) whose complexes can self-assemble to form aggregates. We have previously developed eye drops with CyA/CD aggregates. Our aim was to study cyclodextrin complexes of lysozyme, a small polar globular protein, and to compare the results with those obtained for CyA. We also wanted to test the stabilizing effect of CDs on lysozyme. Phase-solubility studies of various CDs were performed with CyA and lysozyme. Complexation and particle size measurements were made with dynamic light scattering (DLS) and UV. Solid drug fractions were determined. Thermal and chemical stability studies were performed on lysozyme in the presence of various CDs. Recovery of lysozyme activity in the presence of various CDs after a heat shock was determined. Both CyA and lysozyme are able to form non-inclusion complexes with CD and those complexes can self-assemble and form micro sized aggregates. In case of lysozyme the forces involved are relativity weak and the lysozyme/CD complexes dissociate upon centrifuging, however for CyA the aggregates are stronger and do not dissociate upon centrifuging. CyA is therefore suitable for eye drop preparations containing CDs for sustained drug release whereas lysozyme is not. This is mainly due to the fact that CyA forms inclusion complexes with CDs, whereas lysozyme is not able to do so due to its polar surface. The lysozyme/CD non-inclusion complexes can offer some protection against lysozymes chemical and thermal denaturation. CD can, however, form complexes with unfolded lysozyme and hamper refolding of the protein after heat shock.

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.