Exp. 12 The influence of complexing agents on solubility in water

Exp. 12
The influence of complexing agents on solubility in water
Theory:
Cyclodextrins (CDs) are widely used as complexing agents for lipophilic and amphiphilic substances and functional excipients that have gained widespread use and attention because of their ability to solubilize, and in some instances stabilize, poorly water-soluble drug candidates enabling both oral and parenteral formulation. CD molecules are peculiar cyclic oligomers of ?-D-glucose; their shapes resemble truncated cones with primary and secondary hydroxyl groups located around their narrower and wider rim. The mostly used native cyclodextrins consist of 6, 7, and 8 glucose units, and, according to the number of monomers in the macrocycle, they are named as ?, ?, and ?-cyclodextrin, respectively. These molecules have a hydrophobic inner cavity, while the large number of hydroxyl groups on the outer surface makes them water soluble. Due to this special molecular structure, cyclodextrins are capable of forming inclusion complexes with many drugs and other compounds by taking up a lipophilic guest molecule (or its hydrophobic part) of the appropriate size into the cavity. The chemical structures of CDs are shown in bellow Figure. The ability of the CDs to form inclusion complexes with a variety of organic compounds is based on their ability to provide a hydrophobic cavity in aqueous solution for a hydrophobic guest molecule or hydrophobic moieties in the guest molecule. Moreover CDs have been used as a model for proteins and enzymes. The pharmaceutical interest in CDs extends to enhance the solubility, chemical stability, and bioavailability of poorly soluble drugs to reduce the toxicity and control the rate of release of some drugs.