Thermal analysis of cyclodextrins and their inclusion compounds
Introduction
Cyclodextrins (CDs) are natural or semisynthetic cyclic oligosaccharides with a central cavity able to host foreign molecules. Their importance to pharmaceutical formulation and delivery is well documented by the increasing number of marketed or approved medicinal products containing CDs (Table 1).
The main applications are related to their solubilizing properties and to dissolution rate enhancement. A thorough solid-state characterization of these materials is of great interest considering the possible implications for patenting and marketing. In this context thermal methods are widely used to characterize CDs and their inclusion compounds.
Cyclodextrins are generally marketed as hydrates with differing water contents depending on the preparation procedures and storing conditions. TGA and DSC represent, therefore, a first choice analytical tool for an accurate physico-chemical characterization of the solid-state of these compounds in terms of water release energetics. Particular attention has also been paid to the investigation of solid–vapor equilibria of β-cyclodextrin (β-CD). Moreover, as inclusion compounds formed by drugs and CDs are generally prepared in aqueous media, they should be considered as ternary water/CD/drug systems, when the assessment of stoichiometry, i.e. the composition of the interaction product, is involved. Many excellent reviews have already been published on CDs and related topics: a prudential estimate of reviews which appeared in scientific literature in the last 20 years gives a figure of around 600 publications. Some of these are collated in Table 2. This clearly demonstrates the tremendous number of research groups operating in this field. As stated above calorimetric and microcalorimetric methods have been applied extensively in the characterization of CDs and their inclusion compounds. A review on the applications of thermal methods to these substances might therefore be of some interest when addressing topics related to CDs.
Section snippets
Chemical and physical properties of cyclodextrins
The term CDs gathers those starch derivatives characterized by a ring built with glucose units. These compounds are obtained by means of specific enzymes with a two-step process involving the cleavage of polysaccharidic chains into oligosaccharidic and resealing of the two open ends to form a cyclic structure. Natural products include cyclic compounds formed with 6–9 d-glucose units all in α(1–4) linkages, known as α-, β-, γ-, and δ-CDs (respectively, cyclo-hexa, -hepta, -octa, or
Thermal analysis of cyclodextrins
Despite the fact that thermoanalytical techniques are considered reliable and relatively fast methods [96], [97] there are only few papers on the general thermal properties of CDs. Attention has been given to investigations of the thermodynamic behavior of binary water–CD systems, especially the α-CD/water and β-CD/water systems (see relevant sections).
In addition to well-known thermoanalytical techniques routinely used for investigating the cyclodextrins and their inclusion complexes, the
Thermal analysis of water–cyclodextrin interactions
The complex relationships between CDs and water have been repeatedly investigated by a variety of thermal methods in both solution and the solid state. As a general consideration most of the research in this direction has focused on features and properties relevant to β-CD, mainly because of availability and practical reasons but also due to some specific properties (water solubility, pseudopolymorphism, etc.) of this molecule with respect to other CDs. In other words, the interaction between
Thermal characterization of inclusion compounds
In the preceding section, reports of the thermal properties and applications of thermal methods to the investigation of natural and semisynthetic CDs as single substances in the solid state were described.
This section will consider in some detail the thermal behavior and analyses of multicomponent systems containing CDs and particularly of their inclusion compounds.
As a preliminary consideration it should be pointed out that thermal methods (mainly DSC and/or TGA) represent a very popular and
Conclusions
Cyclodextrins are very popular compounds which have attracted and still attract the interest of so many researchers from different fields. Industrial applications of such compounds are found mainly in pharmaceutical and food industry, but also applications in analytical and organic chemistry should be mentioned. Thermal methods have been used and are currently employed as powerful tools in the characterization of both CDs and their inclusion compounds. Generally speaking it ought to be
Acknowledgements
The Financial Support of the Hungarian Scholarship Board, Soros Foundation and OTKA Grant No T 026 459 (Cs.N.) is gratefully acknowledged.
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Present address: Department of Pharmacy and Pharmaceutical Technology, Faculty of Pharmacy, University of Seville, c/Profesor Garcia Gonzalez s/n, 41012 Seville, Spain.