Cyclodextrin

Cyclodextrins are a group of homologous cyclic oligosaccharides consisting of six, seven, and eight glucose units, namely, a-, p-, and g-cyclodextrin (Fig. 5) (24), respectively. Typically, cyclodextrins act as true carriers by keeping hydrophobic molecules in solution by their hydrophobic cores, i.e., com-plexation between hydrophobic molecules and the inner hydrophobic core of cyclodextrins. In other words, they are not capable of modifying the permeability of a biological barrier. On the other hand, drug absorption may be limited by the release of the drug from the drug-cyclodextrin com-

Figure 5 Structures of a-, ß-, and g-cyclodextrins. (From Ref. 24.)

plex. As a result, addition of a cyclodextrin in an ophthalmic formulation does not necessarily increase the ocular bioavailability. It may adversely affect drug absorption (25,26).

However, a recent study showed that a-cyclodextrin has a significant penetration-enhancing effect (10-fold) on the corneal permeability of pilocarpine (27). It was speculated that such an effect was achieved by direct interaction of the a-cyclodextrin with the corneal epithelium, which led to subsequent destabilization of the cell membrane since the same effect was also observed when the cornea was pretreated with the a-cyclodextrin before the transport study. Although a-cyclodextrin only produced minimal damaging effect on the cornea at low concentration (8%), the toxic effect at higher concentration (13% in this study) is still not known.

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