One of the fundamental characteristics of topical formulations is release rate. This may be measured using in vitro release testing (IVRT). The methodology for IVRT is delineated in the FDA’s “Guidance for Industry: Nonsterile Semisolid Dosage Forms: Scale-Up and Postapproval Changes: Chemistry, Manufacturing, and Controls; In Vitro Release Testing and In Vivo Bioequivalence Documentation” (May 1997), also referred to as “SUPAC-SS”. The technique, performed in a Franz cell diffusion system with

The technique, performed in a Franz cell diffusion system with synthetic membrane, was originally designed to compare different lots for the same product manufactured by the same company. Today, IVRT is also used to waive clinical endpoint studies for certain generic products (e.g., acyclovir).