cyclodextrin

A cyclodextrin is a tiny molecular bucket made of sugar. Ring-shaped, it has a water-loving outside but an oil-loving hollow inside — just the right size to swallow a small drug molecule and carry it through water, the way a doughnut hole could cradle a peg.

Cyclodextrins are cyclic oligosaccharides built from glucose units, the common types being alpha, beta and gamma with six, seven and eight units respectively. A poorly water-soluble drug slips into the lipophilic central cavity to form an inclusion complex, while the hydrophilic outer surface keeps the whole assembly dissolved. This raises apparent solubility, and can also stabilise the drug, mask taste or reduce irritation.

Chemically modified versions such as hydroxypropyl-beta-cyclodextrin and sulfobutylether-beta-cyclodextrin are far more water-soluble and less nephrotoxic than the parent beta form, making them suitable even for injections. The complex dissociates reversibly on dilution, releasing free drug. Limits include cost, a one-to-one capacity that can demand large amounts of carrier, and the fact that only molecules of the right size and lipophilicity fit the cavity well.