Cyclodextrins are a group of structurally related natural products formed during bacterial digestion
of cellulose. These cyclic oligosaccharides consist of (α-1, 4)-linked α-D-glucopyranose units and contain a somewhat lipophilic central cavity and a hydrophilic outer surface. Due to the chair conformation of the glucopyranose units, the cyclodextrins are shaped like a truncated cone rather than perfect cylinders. The hydroxyl functions are orientated to the cone exterior with the primary hydroxyl groups of the sugar residues at the narrow edge of the cone and the secondary hydroxyl groups at the wider edge. The central cavity is lined by the skeletal carbons and ethereal oxygens of the glucose residues, which gives it a lipophilic character. The polarity of the cavity has been estimated to be similar to that of an aqueous ethanolic solution.
The natural α-, β- and γ-cyclodextrin (αCD, βCD and γCD) consist of six, seven, and eight glucopyranose units, respectively. The natural cyclodextrins, in particular βCD, are of limited aqueous solubility meaning that complexes resulting from interaction of lipophiles with these cyclodextrin can be of limited solubility resulting in precipitation of solid cyclodextrin complexes from water and other aqueous systems. In fact, the aqueous solubility of the natural cyclodextrins is much lower than that of comparable acyclic saccharides. This is thought to be due to relatively strong intermolecular hydrogen bonding in the crystal state. Substitution of any of the hydrogen bond forming hydroxyl groups, even by lipophilic methoxy functions, results in dramatic improvement in their aqueous solubility. Water-soluble cyclodextrin derivatives of commercial interest include the hydroxypropyl derivatives of βCD and γCD, the randomly methylated β-cyclodextrin (RMβCD), and sulfobutylether β-cyclodextrin sodium salt (SBEβCD).
Cyclodextrin n R = H or Subst.a MW b
(Da) Solubility in water c (mg/ml) Indicative bulk price ($US/kg) d
α-Cyclodextrin (αCD) 0 -H 0 972 145 45
β-Cyclodextrin (βCD) 1 -H 0 1135 18.5 5
2-Hydroxypropyl-β-cyclodextrin (HPβCD; Kleptose® HPB) 1 -CH2CHOHCH3 0.65 1400 > 600 300
Sulfobutylether β-cyclodextrin sodium salt (SBEβCD; Captisol®) 1 -(CH2)4SO3- Na+ 0.9 2163 > 500 -
Randomly methylated β-cyclodextrin (RMβCD) 1 -CH3 1.8 1312 > 500 350
γ-Cyclodextrin (γCD) 2 -H 0 1297 232 80
2-Hydroxypropyl-γ-cyclodextrin (HPγCD) 2 -CH2CHOHCH3 0.6 1576 > 500 400
Figure 1. The chemical structure and the molecular shape of β-cyclodextrin (βCD).
Table 1. Some cyclodextrins that can be found in marketed products.
a Average number of substituents per glucose repeat unit.
b MW: Molecular weight.
c Solubility in pure water at approx. 25°C.
d Approximate bulk price given as the price of 1 kg in US dollars. The price will depend on purity and technological grade of the cyclodextrin.
What are cyclodextrins used for?
About 30 different pharmaceutical products containing cyclodextrins are now on the market worldwide and numerous food products, cosmetics and other commercial products contain cyclodextrins. In these products cyclodextrins are mainly used solubilizing agents to increase water-solubility of lipophilic compounds. However, cyclodextrins can also be used to increase both chemical and physical stability of various compounds, including proteins. To enhance availability of compounds, for example to enhance taste or to enhance bioavailability of drugs. Cyclodextrins can be used to convert liquids to solid powders, to reduce local irritation, to prevent skin absorption of topically applied compounds (e.g. sunscreen agents), and to obtained sustained release of, for example, drugs or fragrances.
Are cyclodextrin safe?
The natural αCD and βCD, unlike γCD, cannot be hydrolyzed by human salivary and pancreatic amylases. However, both α- and β-cyclodextrin can be fermented by the intestinal microflora. Cyclodextrins are both large (MW ranging from almost 1000 to over 2000 Daltons) and hydrophilic with a significant number of H-donors and acceptors and, thus, are poorly absorbed from the gastrointestinal tract in their intact form. Hydrophilic cyclodextrins are considered non-toxic at low to moderate oral dosages. Lipophilic cyclodextrin derivatives, such as the methylated cyclodextrins, are to some extent absorbed from the gastrointestinal tract in to the systemic circulation and have been shown to be toxic after parenteral administration. The natural αCD and βCD, unlike γCD, cannot be hydrolyzed by human salivary and pancreatic amylases. However, both αCD and βCD can be fermented by the intestinal microflora. βCD can not be given parenterally due to its low aqueous solubility and adverse effects (e.g. nephrotoxicity). The metabolism of γCD closely resembles that of starch and linear dextrins. Oral administration of 8 g γCD or 8 g maltodextrin to humans did not reveal any differences in gastrointestinal tolerance of these two oligosaccharides. The biological half-life of HPβCD and SBEβCD in humans after intravenous injection is about 1.7 h and its apparent volume of distribution (VD) is 0.2 l/kg. After intravenous injection HPβCD and SBEβCD are almost exclusively eliminated unchanged through the kidneys via glomerular filtration.
What is the regulatory status of cyclodextrins?
αCD, βCD and γCD are listed in the generally regarded as safe (GRAS) list of the FDA for use as a food additive and HPβCD is cited in the FDA’s list of Inactive Pharmaceutical Ingredients. SBEβCD is also available in various pharmaceutical dosage forms and is also listed in the FDA’s compilation of Inactive Pharmaceutical Ingredients.
Consensus seems to be building among regulators that cyclodextrins are pharmaceutical excipients and not part of the drug substance although various opinion have been given and interpretation related to this point can be division and product-specific.
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