The distirubution of a drug within a formulation can be an important factor in elucidating mechanisms of drug release, or in comparing formulations.
The drug distribution in microemulsions (in the globule vs. aqueous phases) affects the release rate in vitro and absorption rates in vivo. For instance, a lipophilic drug will preferentially partition into the oil phase, but the distribution may be affected by factors such as the pH and temperature, both of which may suddenly change when a microemulsion is administered.
Another important consideration occurs with drug release from topical formulations. Creams are oil-in-water emulsions, and the distribution of the drug can play a major role in its release. In addition, drug binding to polymers in gels can affect release rate tests, and should be taken into account when comparing formulations.
Release and leakage from liposomes is another important application. Although it is currently not possible to predict the in vivo release profile of a drug from in vitro data, it is possible to determine leakage rates and compare liposomal rupture under a variety of in vitro environmental conditions, which can be valuable information about liposomal formulations.
Physical Pharmaceutica has developed methods for the following: