Semisolid Formulations

of ill vitro release testing as a QA tool to ensure batch-to-batch uniformity has been gaining both increasing popularity and contempt in the pharmaceutical industry. Regardless of views and opinions, it is worthwhile to be prepared to conduct such studies if necessary. Currently, the most publicized methods use horizontal ("Franz or Modified Franz") diffusion cells, with an artificial support membrane to conduct in vitro release studies on a semisolid formu lations, typically creams. However, depending upon the type of formulation to be tested, and possible needs for automation, it may be desirable to investigate alternate .apparatuses and techniques. The following article gives examples of several techniques and apparatuses which could be employed in release testing of semisolid drug products. which do not impede the free diffusion of drug from the formulation. Using a membrane which is not resistant to the free diffusion of drug allows for the determination of release of drug from the semisolid formulation itself. Ln addition, it is very important to choose receptor media which maintain adequate sink conditions. Poor sink conditions andlor mixi ng will result in a large hydrodynamic boundary layer, which will both decrease the amount of drug released. and significantly lower the test's ability to determine differences between formu lations. The "goa'" in the development of such methods is to have a test which measures the "intrinsic" release of drug fTOm the formulation. In such a system. alterations in the formu lations could be measured.