Cyclodextrins and Drug Membrane Permeation: Thermodynamic Considerations

Area of Science:

• Pharmaceutical Sciences
• Physical Chemistry

Background:

• Cyclodextrins (CDs) are oligosaccharides that enhance the aqueous solubility of lipophilic drugs.
• CD-drug complexes, though hydrophilic, improve drug permeation across biological membranes and oral bioavailability.
• The mechanism by which CDs enhance membrane permeation remains unclear.

Purpose of the Study:

• To elucidate the mechanism by which cyclodextrins enhance drug permeation through lipophilic membranes.
• To investigate the influence of donor medium composition on drug permeation using an artificial biomembrane.
• To determine the relationship between drug concentration, thermodynamic activity, and permeation flux.

Main Methods:

• Utilized an artificial biomembrane model (PermeaPad®) to study drug permeation.
• Investigated the behavior of cyclodextrins in aqueous solutions concerning lipophilic drug solubility and molecular mobility.
• Analyzed the effect of varying drug concentration and thermodynamic activity on drug flux.

Main Results:

• In aqueous solutions, cyclodextrins function similarly to organic cosolvents like ethanol, increasing solubility without hindering lipophilic drug partitioning into membranes.
• Drug molecule partition into the membrane is maximized when their thermodynamic activity is highest.
• Drug flux across lipophilic membranes from aqueous cyclodextrin solutions is dependent on both drug concentration and thermodynamic activity.

Conclusions:

• Cyclodextrins enhance drug permeation by increasing solubility and maintaining molecular mobility, akin to cosolvents.
• Optimal drug permeation is achieved when both the concentration and thermodynamic activity of the dissolved drug are maximized.
• Understanding these principles is crucial for optimizing drug delivery systems utilizing cyclodextrins.