Cyclodextrin (CD) is a group of compounds known as cyclic oligosaccharides. They consist of at least six D-(+)-glucopyranose units linked together by α-1,4 bonds, and are produced through the enzymatic action of Bacillus macerans amylase on starch.^[1]

Types and Properties of Cyclodextrin

Cyclodextrins consist of 6, 7, and 8 units of glucose referred to as α, β, and γ cyclodextrins, respectively. They have truncated cone structure with a hydrophobic interior cavity because of the CH2 groups, and a hydrophilic exterior due to the presence of hydroxyl group. Molecules of appropriate size and stereochemistry get entrapped in the cyclodextrin cavity by hydrophobic interaction by squeezing out water from the cavity.

Cyclodextrins can enhance the solubility and bioavailability of hydrophobic compounds due to the large number of hydroxyl groups on the CDs. Cavity size is the major determinant as to which cyclodextrin is used in complexation. α-Cyclodextrins have small cavities that are not capable of accepting many molecules. γ-Cyclodextrins have much larger cavities than many molecules to be incorporated. The cavity diameter of β-cyclodextrins has been found to be the most appropriate size for most drugs. For this reason, β-cyclodextrin is most commonly used as a complexing agent. Although β-CD contains a high number of hydroxyl groups, β- CD solubility is the lowest compared to the α-CD or γ-CD. This is due to the formation of an internal hydrogen bond network between the secondary hydroxyl groups.

Partial alkylation of some of the OH groups in CD reduces the intermolecular hydrogen bonding, leaving some OH groups free to interact with water, thus increasing the aqueous solubility of CD.

Pharmaceutical Applications of Cyclodextrin

In addition to hydrophilic derivatives, hydrophobic forms of β- CD have been used as sustained release drug carriers. In addition to improving the solubility of compounds, complexation with cyclodextrin has been used to improve the stability of many drugs by inclusion of the compound and protecting certain functional groups from degradation.

Complexation with cyclodextrins has also been used to mask the bitter taste of certain drugs such as femoxetine.

Reference:

• Sinko, P. (2011). Martin’s Physical Pharmacy and Pharmaceutical Sciences. Baltimore, : Lippincott Williams & Wilkins, a Wolters Kluwer business.